UV LED Presented by Radtech— the Association for UV & EB Technology

UV LED Presented by RadTech— The Association for UV & EB Technology

eBook #1 www.radtech.org CONTENTS www.radtech.org 59 Curing: The of UV-LED State of An Investigation Chemistry and Applications This paper discusses the characteristics lamps; importance of of UV-LED properly formulating chemistries; benefits to end-users; commercial and future applications of UV-LEDs; expected developments. —By Ed Kiyoi 64 of Overview Market Not a Applications: UV-LED Approach One-Size-Fits-All technology is not a one-size- UV-LED fits-all substitute for conventional UV arc and microwave curing. What works for one application does not necessarily Businesses must work for another. invest time and resources to develop solutions for each specific UV-LED market application. —By Jennifer Heathcote 47 Measuring Outputthe Light of Ultraviolet Emitting Diodes understanding This article focuses on of UV-LEDs. and measuring the output and —By Jim Raymont Abhinav Kashyap 53 Shift Paradigm The UV-LED This paper suggests a number of questions that are fundamental to performance. measuring UV-LED The answers to these questions can determine if current measurement solutions will work, need to be modified or if a new class of UV-measurement devices will best address the needs of its users. —By Paul Mills and Jim Raymont

—the official publication of RadTech NorthAmerica RadTech of publication —the official Presented by RadTech— RadTech— by Presented Technology EB & UV The Association for 38 Curing Systems: UV-LED Equal Not Created The authors discuss some of the architectural and design trade-offs lamp makers have at their UV-LED for isn’t disposal; noting that UV-LED every application and not all UV-LED lamp systems are created equal. —By Sara Jennings, Bonnie Larson and Chad Taggard 33 Part Overview III — UV-LED and Diode Evolution Manufacturing This abridged article is the third installment in a three-part series designed to consolidate key principles and technical information regarding the science and engineering behind UV-LEDs. —By Jennifer Heathcote 21 Basics Part II — UV-LED Curing Systems This article is the second installment in a three-part series designed to technical and principles key consolidate information regarding the science and engineering behind UV-LEDs. —By Jennifer Heathcote 10 Part Overview I UV-LED and — Operation Measurement a better In an effort to provide technology, understanding of UV-LED key principlesthis article includes some regarding and technical information behind the science and engineering UV-LEDs. —By Jennifer Heathcote RadTech Report RadTech -LED

UV-LED Focus Group Co-Chairs Group Focus UV-LED JoAnn Arceneaux, Allnex Beth Rundlett, DSM At-Large Board Members Board Bean, Sun Chemical Tony Carignano, Allnex Tony Casey Cordon, DKSH North America Jennifer Heathcote, Integration Technology Steve Lapin, PCT Engineered Systems Im Rangwalla, ESI Michael Rose, Sartomer USA Mike Sajdak, INX International Red Spot Eileen Weber, President Don Duncan, Wikoff Color Corporation President-Elect Quaker Chemical Peter Weissman, Treasurer Paul Elias, Miwon North America Inc. Secretary Red Spot Eileen Weber, President Past Immediate Howard Ragin, DSM Coating Resins RadTech International N.A. RadTech Officers Susan Bailey, Brewer Science Susan Bailey, Brian Cavitt, Abilene Christian University Syed Hasan, BASF Corporation Molly Hladik, Brewer Science Mike Idacavage, Esstech Inc. Marc Jackson, Melrob U.S. Stephen Lapin, PCT Engineered Systems Sudhakar Madhusoodhanan, INX Digital Dick Stowe, Heraeus Noblelight Fusion UV FiberMark NA Huanyu Wei, 3M Corporation Sunny Ye, Jim Zawicki, Sartomer USA Editors-In-Chief Casey Cordon, DKSH North America Estron Chemical Chris Miller, Board Editorial Rick Baird, The Boeing Company Armstrong World Joshua Lensbouer, Industries Inc. Estron Chemical Chris Miller,

Thisarticles of is a compilation eBook from the UV

This paper provides a broad overview ofThis paper provides technology recent UV and visible-LED market improvements and discusses impact that developments and the may have on the these developments for systems development of UV-LED applications. UV-curing Karlicek, Jr. —By Robert F. 3 and Curing UV-LEDs Technology Applications: Developments and Market UV-LEDs and Curing Applications: Technology and Market Developments By Robert F. Karlicek, Jr. he light-emitting diode (LED) lighting, white LEDs are becoming industry is undergoing rapid bright enough to replace mercury technological and market lamps and sodium vapor lamps in Tchanges driven by the development street lighting applications. There is of efficient, white LEDs for liquid also progress in developing UV-LEDs crystal displays (LCDs) and lighting. for curing applications, but progress UV-LEDs are poised to benefit from is being made at a much slower these developments (including pace. The LEDs that are used for higher efficiency, higher output UV curing and lighting applications power and lower cost), largely are technically similar, as are the because UV and white LEDs are challenges of using them in either technically similar. However, there UV curing or lighting applications. are market-related challenges Regardless of whether LEDs emit in the UV or are used for lighting, both This article summarizes the technology and market markets are demanding the same things from LED manufacturers: trends related to LEDs and their impact on the • More light (or UV) output development of UV-LEDs for curing applications. • Higher operating efficiency (more electrical input converted to light) slowing continued improvements in • Lower cost for LEDs UV-LED performance. This article • LED system designs more suitable provides a broad overview of recent for putting the right amount of light UV and visible-LED technology (or radiation) where needed improvements and discusses market developments and the impact that These market demands are these developments may have on the driving rapid technical changes development of UV-LED systems for in LED designs; improvement in UV-curing applications. performance; and reductions in cost. Innovation at the system level is also Introduction proceeding rapidly, as lighting fixture LEDs are beginning to challenge designers and UV-system integrators existing lamps used for lighting and wrestle with how best to implement UV-curing applications. In general the visible or UV-LEDs that are

© 2013 RadTech International. 3 commercially available today. LEDs are a disruptive technology promising Figure 1 superior efficiency and reliability for creating UV and visible radiation when Approximate efficiency versus wavelength compared to conventional UV and for nitride LEDs visible lamps. Because of this LED potential, LED technology and even the business structures and supply chain models associated with LEDs and the systems that use them are evolving rapidly. This is especially true for visible-LEDs and LED system designers in the LCD display and general lighting markets where the revenue potential is huge and there is a strong focus on replacing conventional incandescent and mercury-based lighting sources. These technical and market developments (driven primarily LEDs can be made at any wavelength along the curve, and by visible-LED manufacturers and the colored markers are positioned at commercially important customers) present both opportunities wavelengths where relative size of the marker suggests market size for LEDs at those wavelengths. If UV-LED efficiencies and challenges for the development of improve to the levels of today’s 450nm LEDs, they would be UV-LED-based curing systems. This more than twice as efficient as mercury lamps and offer a wider article summarizes the technology and range of UV wavelengths. market trends related to LEDs and their impact on the development of UV-LEDs for curing applications. the wavelength to move from blue into Regardless of wavelength, the LED Technical Overview the UV. In principle, any wavelength design of an LED is extremely LEDs are made from crystalline from 250 nm (UVC) to 570 nm complex, requiring the crystal growth compound semiconductors resembling (greenish yellow) can be manufactured of many extremely thin (just a few silicon (used for conventional by adjusting the semiconductor atoms thick) layers of various alloys electronics). Unlike silicon used composition. of these nitride semiconductors in computer and memory chips, With today’s technology, the on a substrate. The design, purity compound semiconductors can emit intensity of light (visible or UV) and crystalline quality of these light when energized. LEDs are emitted by an LED depends strongly layers control not only the emission monochromatic (single color) emitters on the wavelength (Figure 1). Blue wavelength but also the output and the wavelength (color of the light) LEDs are the most efficient of all power and lifetime of the LED. The from an LED depends on the chemical the nitride LEDs. The intensity LED and LED systems supply chain composition of the semiconductor drops quickly as the wavelength gets from semiconductor to applications material. For both UV curing and shorter, especially below 365 nm is shown in Figure 2. The substrate lighting applications, the semiconductor where there are special technical and with the crystalline layers grown on material is made from alloys of AlN, manufacturing challenges related to it is typically called an LED wafer GaN and InN (aluminum nitride, growing high aluminum content nitride (Figure 2A). After it is grown, standard gallium nitride and indium nitride, materials needed for UV emission. semiconductor processing technology respectively). Increasing the indium As research continues on short- is used to convert the wafer into concentration causes the LEDs to wavelength UV semiconductors, thousands of small LED chips emit blue or green light. Reducing the these problems will be solved and (Figure 2B). These chips are tested indium concentration and increasing much higher power UVC-LEDs will at the manufacturer for wavelength the aluminum concentration causes become available. and brightness (and a host of other

© 2013 RadTech International. 4 Clearstone Technologies, Inc.). The Figure 2 visible-LED analog to UV flood cure LED arrays would include the LED- Typical supply chain for UV-LED curing equipment based light bars now used on law UV-LED system manufacturers typically integrate either bare enforcement vehicles, or LED street chips (B) or packaged chips (C) to build a UV-LED system. lights that are now beginning to appear in some areas. Applications for arrays of single packaged LEDs (visible or UV wavelength) are typically limited to flood applications. Moving beyond flood applications requires much higher optical output from individual LED chips or packages. For that reason, there is an increasing trend to put multiple LED chips in a single package to increase the concentration of optical output (W/cm2 of either UV or visible radiation). This introduces some new challenges for LEDs for thermal reasons. Unlike relatively inefficient incandescent light bulbs and high-power, mercury-arc lamps which convert a high percentage semiconductor properties), and sorted The earliest applications for single of their electrical input power to into different performance bins. Due visible or UV-LEDs include flashlights infrared radiation (IR heat), LEDs emit to the complexity of wafer and chip and fiber illuminators. (UV-LEDs are no IR radiation but still generate heat fabrication processes, the light output beginning to appear from some system and must be cooled by conducting that power (UV or visible wavelength) can integrators in spot-curing systems.) heat out of the package. Significant vary by as much as a factor of two, To get higher irradiance (for UV research efforts are focused on the even for chips coming from the same applications) or high illuminance development of advanced, high-power wafer! Binned chips are then sold to a (irradiance corrected for eye sensitivity packages and thermal management packager where the chip is placed in at visible wavelengths), many LED systems for visible-LEDs. These a protective package with optics and packages are typically combined into system-level technical developments solderable leads. System integrators a single fixture (such as in Figure will be useful for evolving high-power, then purchase the packaged 2D for the UV flood illuminator from UV-LED curing system designs as well. LEDs, adding electronics, thermal management, optics and housings to create a finished module. Figure 3

From Packaged LED to Systems A very large power UV-LED (for UV Curing or Lighting) Individual packaged LEDs are not The emitting area is 12 mm2 (rectangular bright enough for many UV curing or chip in the center of the package) and is the largest power LED on the market today. lighting applications, but have made The large copper submount is required some inroads in niche applications. for adequate thermal management. The biggest challenge for system The package is also equipped with a integrators is that while LEDs can be thermistor for thermal sensing. very efficient, a single, packaged LED doesn’t really produce much optical output (whether in the visible or UV).

© 2013 RadTech International. 5 Recently, there has also been an interest in making very large power Figure 4 LED chips (not arrays of smaller chips) for high-power visible and UV An experimental high-power, UV-LED system applications. Most visible or UV-LEDs range in size from 0.25 mm to 1.5 mm (10 to 60 mils) on an edge, but larger chips (up to many millimeters on an edge) can be manufactured (such as in Figure 3). Large, high-powered LEDs like these are manufactured using highly specialized fabrication techniques. There are both advantages and challenges in using very large, high-power LED chips. Since the main reason to use a very large LED chip is to generate a very concentrated, intense light source, these LEDs are typically operated at very high input current and high input electrical power (over 50 W per single LED The operating concept is shown in (A) and a finished chip!). These LEDs are ideal for fiber module in (B), actual operation in (C) and an intensity profile measured 80 mm from the base of the unit in (D). Operation illuminators since they can produce is based on a proprietary LED and optical design intended a very bright point source that is to emulate the power levels and emission patterns of high- easily coupled to a fiber guide for power, mercury-arc lamp systems. spot irradiation. Using very large LED chips can also be advantageous for projecting very high irradiance using systems much like those designed around high- dropped about 10 fold per decade In principle, shorter wavelength power mercury lamps. Novel optical — but recent demand for improved UV-LEDs could be as efficient as and special thermal designs are being white LEDs has produced even visible-blue LEDs, but major technical investigated using very large power faster brightness increases and cost breakthroughs will be required for LED chips so that geometric optics reductions. Continued pressure on this to happen. Over the past few (elliptical or parabolic reflectors) can improving light-emitting performance years, more research has focused be used to re-image the intense surface and reducing cost are driving on the development of UVC-LEDs irradiance from large LED chips to technology changes in all parts of the for germicidal applications, and the produce uniform high irradiance LED manufacturing supply chain, market interest for UVC-LEDs in patterns very similar to those obtained as shown in Table 1. Some of these germicidal applications is driving more from conventional mercury lamp developments will prove useful for investment in the research needed systems (Figure 4). UV-LEDs, especially the longer UVA to improve UVC-LED performance. wavelengths in which the nitride Since the nitride materials used to LED Technology Trends semiconductor compositions in the make LEDs are closely related, the Since visible-LEDs were first LED wafer are the most similar to technology leading to improvements in developed in the early 1960s, technical those of the blue LEDs which are used UVC-LED performance for germicidal improvements have led to tremendous for display and lighting applications. applications will be directly applicable increases in brightness and large The large drop in LED efficiency to making much better UV-LEDs reductions in cost. On average, LED at wavelengths at and below 365 nm at UVB and UVA wavelengths as brightness has increased about 10 (Figure 1) will require significant well. This is a normal part of the fold per decade and LED price has advancement in LED-chip technology. evolution of new LEDs operating

© 2013 RadTech International. 6 at new wavelengths. As the nitride semiconductor material Table 1 qualities improve, UV-LEDs at any wavelength needed for UV curing Technology trends in various portions of the LED supply will be available with efficiencies chain driving increased LED light output power and exceeding those of the mercury price reduction lamps used today. UV-LEDs stand to profit from LED performance and price improvements largely directed at visible-LED applications. LED Market Evolution and UV-LEDs for Curing The economies of scale needed to make LED chips and packages (visible or UV wavelength) affordable require LED chip manufacturers to make and sell hundreds of millions of LED chips per month, so LED developers tend to focus on those markets and applications that can absorb high quantities of LED chips and packages. The technical challenges of making LEDs at high yield drive manufacturers to focus on making and selling only those particular colors of LEDs needed for high-volume applications. While it 508 nm for traffic signal green, and so white” LEDs the best performing and is possible to make an LED at almost on). The greatest present demand for lowest cost nitride-based LEDs available. any wavelength, it is usually possible LEDs is for mobile display applications Because LEDs are primarily to purchase LEDs only at particular with demand for LEDs for notebook used in consumer electronics with wavelengths tied to high-volume computers and LCD TVs increasing the associated pricing sensitivity applications (450 nm blue for white, rapidly. This focus has made “blue for characteristic of those markets, the preponderance of LED manufacturing now takes place in Asia (Figure 5). Figure 5 Almost all new investment for LED LED production capacity by region* manufacturing, including R&D investment for LED performance Today, the largest LED improvement, is also in Asia. Because capacity expansion market opportunities for visible-LEDs investments are being are much larger than those for made in China, Taiwan UV-LEDs, there are many more suppliers and Korea, largely being driven by interest in LEDs of visible-LEDs than there are of UV- for displays and lighting LEDs for curing applications. While applications. most white LEDs are made from blue LED chips, some white LEDs are also made from near UV (~400 nm)-LEDs and multiple wavelength converting * Courtesy of Canacord Adams phosphors. It is possible to argue that 400 nm LEDs are available for UV-curing applications because they were first developed for lighting markets.

© 2013 RadTech International. 7 From an applications point of view, • Reduced incentives to make R&D least 10 times more expensive than both white-power LEDs for lighting investment for higher power their blue LED cousins. (referred to as solid state lighting) and shorter wavelength UV-LED The UV curing market itself and UV power LEDs for curing materials, delaying the development introduces another challenging wrinkle applications face similar technical and of higher power UV-LEDs operating for the development of UV-LEDs market challenges. Some of these are below 390 nm (and especially below and the curing systems using them— outlined in Table 2. While the history 365 nm). most coatings and inks are developed of UV curing with mercury lamps • Reduced market incentives for and coating processes optimized is considerably shorter than that manufacturers to develop and sell for the complex mix of narrow UV of electrical lighting, both markets UV-LED chips. (Today, there are wavelengths produced by mercury have evolved application designs really only two to three suppliers of lamps (among them the 254 nm, 315 and product distribution structures very high-performance UV-LED nm, 365 nm emission lines of atomic geared toward a bulb/fixture model. chips, compared to several dozen mercury). Coating formulations and This is a model not well suited for manufacturers of blue-LED processes will need to be developed manufacturers of LED systems chips for display and lighting specifically for the monochromatic (lighting or UV curing). applications.) nature of UV-LEDs if UV-LED curing The adoption of these disruptive • Inflated pricing for UV-LED curing systems are to be more widely adopted. LED-based technologies (in both systems due to high LED chip and lighting and UV curing) will require This work is already underway as packaged LED pricing as limited new market structures. In the lighting formulators explore the use of new market, this is already happening with competition provides no incentive photoinitiators and processes companies that formerly made only to reduce pricing. If one compares optimized for wavelengths other than LED chips now vertically integrating to the prices of similarly designed blue those that come from mercury lamps. become lighting companies (Cree, for and UV-LED devices on a price per The absence of very high irradiance example, with many others following optical output power (or price per UV-LED systems; insignificant suit). Similarly, most conventional photon) basis, UVA-LEDs are at irradiance at shorter wavelengths; light fixture companies are working to acquire or partner with LED companies (Philips/LumiLEDs Table 2 acquisition of Genlyte, one of the world’s largest lighting fixture Comparison of market forces and their impact on both manufacturers). The size of the visible and UV-LED developments market opportunity for LEDs in Market trends and pressures for both lighting and UV curing are quite displays and lighting is driving a lot similar, suggesting that UV-LED systems will profit from riding the of creative business development, coattails of LED-based, solid-state lighting improvements. and some of this activity will impact the availability of UV-LEDs for curing applications. Similarly, many larger UV- mercury lamp system suppliers are actively developing curing systems based on UV-LEDs, and a host of small companies are currently selling specialized UV-LED curing systems for niche applications. The smaller market opportunities for UV-curing systems relative to LCD displays and lighting markets have several effects on the development of semiconductors used to manufacture UV-LEDs:

surface curing issues; and high line history of LED development strongly Corporation, Luminus Devices, Inc. speeds will limit the deployment of suggests that UV-LEDs will continue to and Canaccord Adams are gratefully UV-LED systems until new coating develop, offering higher irradiance and acknowledged. w formulations designed to work new, shorter wavelengths. Combined specifically with LEDs are available. with improved energy efficiency, cool —Robert F. Karlicek, Jr. is president of SolidUV, Inc., in Chelmsford, Mass. operation and much longer lifetimes, Summary the evolution of UV-LED curing Visible and UV-LED technology has systems will track the development of improved tremendously in recent years, LED lighting, following the technical with improvements in the performance progress at the chip, package and and value of white LEDs for lighting system level, as well as emulating the applications now allowing LEDs to business models now being developed challenge conventional lighting sources. in the lighting industry to replace the Improvements in the performance of incumbent incandescent, fluorescent UV-LEDs has been slower because and UV-curing bulbs used today. the UV-curing market is much smaller than the display and lighting markets Acknowledgments requiring white LEDs, but even now The contributions of photographs UV-LEDs are being considered for some and data from SemiLEDs Corporation, specialized curing applications. The Clearstone Technologies, Inc., Nichia

© 2013 RadTech International. 9 © 2013RadTechInternational. understanding of the underlying technology.the underlying understanding of who are newly interested inUV-LEDs have insufficient those has now of becomethe majority oneinwhich inmanyactivity diverse markets, the challenge but new,This pro-LED climate isgreat for generating By Jennifer Heathcote Operation andMeasurement UV-LED I— Overview Part U the limitlessnumber ofenvironmental, first promotesthetechnology by touting revolved around two distinct topics. The of developmenthavepredominantly delivered duringthepreviousdecade professional societypresentations more applicationsaplausiblereality. and excitingrate,thusmakingmany continues toincreaseatanastonishing advancement andimplementation the momentumoftechnological the rightfitformanyapplications, curing. WhileUV-LEDs arestillnot forms ofelectrodeandmicrowaveUV those foundinsetupsusingtraditional production speedsandthroughcuresto we arenowachievingcomparable advancements inthetechnology, nearly adecade.With themostrecent and integratingUV-LED systemsfor been studying,trialing,promoting particularly UVdigitalinkjet,wehave to believethatinsomemarkets, the turnof21stcentury. It’s hard varying degreesofsuccess)since and productionsystems(albeitto and incorporatedintobothprototype conferences, profiledattradeshows technology havebeenhighlightedat and applicationsutilizingUV-LED Most UV-LED articlesand emitting diodes(UV-LEDs) that incorporatelight- ltraviolet-curing systems to correlateproduct andperformance UV-LED systemsorunderstandhow foundation tocompareavailable many userssimplydon’t haveasufficient provided. Throughnofaultoftheir own, or fullygrasptheinformationthey are don’t alwaysknowwhatquestionsto ask technology intotheirprocesses.They and whentoactuallyimplementthe left manyofthemunsureastohow potential usersontheconceptbutthen UV-LEDs havesuccessfullysold technology. Thoseofuspromoting understanding oftheunderlying UV-LEDs haveinsufficient those whoarenewlyinterestedin become oneinwhichthemajorityof markets, butthechallengehasnow for generatingactivityinmanydiverse curing needstoday. that UV-LEDs maynotyetfitalltheir same timetheyrealisticallyunderstand present andthefuture,whileat accept thefactthatUV-LEDs arethe and relatedtechnologiesnowreadily Most usersinvolvedinUVcuring finally safetosay, “Messagereceived.” UV-curing systems.Well, Ithinkit’s toward electrodeandmicrowave potentially disruptivenatureofLEDs of thetechnologyandtocounter effort nottooversellthecapabilities communications havebeenmoreofan development timeline.Theselatter uncertainties regardingtheUV-LED availability ofinksandcoatings, lack ofestablishedinstallations,limited cautions againstpracticallimitations, curing systems.Thesecondtopic UV-LEDs haveoverconventionalUV process andintegrationbenefitsthat This new, pro-LEDclimateisgreat

© 2013RadTechInternational. conventional UVbulbs;and(4)the output fromLEDsascomparedto junction; (3)characteristicsofUV block oftheLED;(2)LEDp-n junction, whichisthebasicbuilding include informationon(1)thep-n operation andmeasurementwill first articleismeanttocoverLED through aseriesofthreearticles.The present anelementaryfoundation behind UV-LEDs, Iwillattemptto regarding thescienceandengineering principles andtechnicalinformation dispensing partners. learn alongsideourink,coatingand while atthesametimecontinuingto educating andrefiningexpectations, spend agoodportionofourtime lines. Thismeansthatsuppliersmust information againsttraditionalcuring Figure 2 Figure 1 Figure In anefforttoconsolidatesomekey P-n junction Forward bias The P-N Junction The them againstconventionalUVsystems. compares UV-LEDs andbenchmarks intended toguidethereaderashe/she consisting ofaseriesquestions will concludewithabriefsummary manufacturing methods.Eacharticle of LEDdevelopmentandcurrentdiode curing systems,aswellthehistory integration ofUV-LED chipsintoactual two subsequentarticleswillfocuson UV outputfromLEDsources.The challenges associatedwithmeasuring direction (i.e.,fromanegativeto through thesamecircuitinreverse electrons mustsimultaneouslyflow current toflowinthisdirection, to anegativeterminal.Inorderfor current flowsfromapositiveterminal In electricalcircuits,conventional added tothediagram. Whentheanode the anodeandcathodeofjunction. when thesupplyisconnecteddirectly to negative terminalofalow-voltage supply flow fromthepositiveterminal tothe The resultisthatelectricityableto the boundaryandmovefromntop. electrons willbeabletopenetrate minimized inbothsizeandeffectiveness, junction atall.Ifthedepletionzoneis electrons willnotflowthroughthep-n of thedepletionzone,currentand that withoutalteringthecharacteristics the depletionzoneisnot.Thismeans and then-sidearerelativelyconductive, depletion zone.Whileboththep-side p-side andthen-sidemeetiscalled n-side tothep-side. Electrons, however, onlyflowfromthe it cannotflowinthereversedirection. the p-sideofdiodeton-side,but respectively. Currentisabletoflowfrom as theanode(+)andcathode(-) two sidesofthediodearereferredto a negativelychargedelectrode.The electrode whilethen-sidebecomes junction becomingapositivelycharged process thatresultsinthep-sideof sides (pandn)undergoamanufacturing into thesemiconductorandtwo Impurities areimpregnatedordoped single pieceofsemi-conductivecrystal. the diagraminFigure1. concept iscommonlyillustratedwith is lessthanhalfamicronthick.The conductive materialswhereeachlayer is madeofmanylayerssemi- is aspeciallyengineereddiodethat junction (positive-negativejunction) electricity inonlyonedirection.Ap-n property ofadiodeisthatitconducts thought ofasaswitchorvalve.Akey flow ofelectricity. Itcangenericallybe to acircuitasmeansofrestrictingthe common electricaldevicethatisadded happen withouttheother. Adiodeisa a positiveterminal).Onedoesnot In Figure2,avoltagesupplyis The junctionboundarywherethe A p-njunctionisengineeredfroma 11 © 2013RadTechInternational. electrons penetrate throughthebarrier When asufficient voltageisused,the forces oftheholesonp-side. n-side torespondtheattractive zone, causingtheelectronson the reduces thewidthofdepletion depletion zone.Thissignificantly from oppositedirectionstowardthe electrons inthen-regionarepushed holes inthep-regionandnegative forward biasvoltageisthatthepositive charged electrons.Theeffectofa the n-sidecontainsalotofnegatively of tiny, positivelychargedholeswhile the p-sideofjunctioniscomposed a forwardbiasiscreated.Imaginethat is connectedtothenegativeterminal, of thevoltagesupplyandcathode is connectedtothepositiveterminal Figure 4 Figure Figure 3 Figure Reverse biaswithsufficient voltage supply LED p-n junction (forwardLED p-njunction bias) can breakdown causingcurrentto voltage isapplied, thep-njunction In certaincaseswhereahighenough zone andinhibitstheflowofelectricity. increases thewidthofdepletion charge onthevoltagesupply. This as theyareattractedtotheopposing move awayfromthedepletionzone the negativeelectronsinn-region the positiveholesinp-regionand cathode. Theresultingeffectisthat positive terminalisconnectedto is connectedtotheanodeand negative terminalofavoltagesupply bias situation.Inthiscase,the the voltagesupplycreatesareverse called recombination. to filltheholesonp-side.Thisis Switching theconnectionson The LEDP-N Junction The are basedonthisscenario. the supply. Zeneroravalanchediodes the p-sidetonegativeterminalof across thedepletionzoneandthrough voltage supplythroughthen-side, flow fromthepositiveterminalof energies that,at anatomicleveland dopants possess varyingbandgap when thephotonisreleased.Different wavelengths emittedfromthe diode doping materialsdeterminesthe exact The selectionofsemiconductor and infrared (IR),visibleandUVlight. radiation ofallwavelengths,including that cantransportelectromagnetic is releasedintheformofaphoton of lowerenergy. Theexcessenergy and fillahole,theydropintostate electrons crossthedepletionzone the n-side(anodetocathode).As bias, currentflowsfromthep-sideto connected totheLEDwithaforward the LED.Whenavoltagesourceis for thesimplep-njunctionapplyto applications. 5 and6,areusedinawidevarietyof (365-405 nm),asshowninFigures (390-780 nm),andsomeultraviolet emit infrared(870-980nm),visible shown inthesketch.Today, LEDsthat protective outercaseorlens—are the semiconductor, wirebondand cathode (-)connections—aswellas in Figure4.Boththeanode(+)and actual LEDp-njunctionisillustrated (heat glow).Aphysicalexampleofan heated totemperaturesabove750ºC is onlyproducedwhenmaterialsare more familiarincandescencewhich room temperature—asopposedtothe electroluminescense anditoccursat spectrum. Thisisknownas throughout theentireelectromagnetic monochromatic wavelengths theoretically bedesignedtoemit semiconductor structure,could junction which,dependingonthe All oftheconceptspresented A light-emittingdiodeisap-n 12

© 2013RadTechInternational. challenge hasalways beenthatthese and trial-and-error. Theprimary identified throughexperimentation were intentionallyandunintentionally semiconductor materialsanddopants evolved. Optimalcombinations of and usefulwavelength(s)havetruly years thatLEDsemittingsustainable in 1907,itwasonlythelast50 is emittedfromanLED. determine thespecificwavelengththat not somethingcoveredinthisarticle, Figure 6 Figure 5 Figure While theLEDwasfirstobserved Examples of (a) UV (b) visible and(c)IR-LEDsExamples visible of(a) UV (b) spectrum Electromagnetic

(a)

to enterthemarket. Tables 1and2 efficiencies inthesingledigits began that UV-LED curingsystemswith Japan, anditwasn’t untilabout2002 produced inalabenvironment in with anefficiencyofaround1% was It wasn’t until1992thataUV-LED wavelength visiblelightandinfrared. successes wereachievedwithlonger occurred orwhatwascausingit.Early understand exactlyhowtheemission controlled, anditwasdifficultto experiments couldnotbeeasily (b) (b) door openers(which bothemploy ’70s and’80s,TVremotes garage telephones inthemid1960s.In the computers, circuitboardsand multiline and functionindicatorsonmainframe life. RedLEDswerefirstused asstatus standard householdgoodsintodaily considering theintroductionof of LEDsoverthelast50yearsby corresponding wavelengthregions. semiconductor materialsaswellthe provide commonexamplesofinorganic It ispossibletofollowtheevolution (c) 13 © 2013RadTechInternational.

Table 1 Table 2 Engineered semiconductor combinations Core semiconductor materials iio S)a usrt Blue(underdevelopment) Silicon carbide(SiC) Silicon (Si)assubstrate substrate Sapphire (Al2O3)as (InGaN) Indium galliumnitrate Green,emeraldgreen AlGan quantumbarrier Gallium nitrate(GaN)with Red,orange,yellow, green Gallium nitrate(GaN) Infrared Gallium phosphide(GaP) Gallium Arsenide(GaAs) (GaAsP) Gallium arsenidephosphide Diamond (C) Neartofarultraviolet Boron Nitride Aluminum nitrate(AIN) (AlGaN) Aluminum galliumnitrate nitride (AlGaInN) Aluminum galliumindium phosphide (AlGaInP) Aluminum galliumindium phosphide (AlGaP) Aluminum gallium (AlGaAs) Aluminum galliumarsenide icslnd ZS)Blue Zinc selenide(ZnSe) Lead sulfide Indium galliumarsenide Germanium Silicon Materials Materials Blue Blue ultraviolet Bluish green,blue,near Blue, white yellow Red, orangeandred,orange, Ultraviolet Ultraviolet Near tofarultraviolet,violet Ultraviolet -downto210nm yellow, green Bright orangered,orange, Green Red andInfrared 1 Wavelength 1,000-3,500 Wavelength 800-2,600 400-1,700 190-1,100 1 advances, further expansionofLED streams past.Despiteallthese can quicklychangedisplayas traffic with electronicLEDbillboards that areas haverecentlybecomepopulated dollars incomparison.Largerurban recent development,stillcostseveral Today’s blueLEDs,whichareamore dollar-per-unit priceofthe’60s. as opposedtotheseveralhundred can nowbepurchasedforpennies electronics store.RedindicatorLEDs at anybrick-and-mortaroronline point thattheyarereadilyavailable LEDs hasbeendrivendowntothe emerged andtheoverallcostofvisible underlying whitelightLEDshas crossing signals. as welltrafficandpedestrian automotive headlightsandtaillights green LEDsweredesignedforuseas in 2004,arraysofred,whiteand communication networks.Starting audio controlsaswellforlocalarea now commonlyusedforvideoand for useinsecuritycameras.Theyare century, andIR-LEDswereintroduced onto themarketaroundturnof Extremely brightLEDflashlightscame automobiles, airplanesandbuildings. for bothdecorationandfunctionin in alltypesofelectronics,aswell market andweresubsequentlyused more visiblecolorsenteredthe control ofthemanufacturingmaterials, gained greaterunderstandingand lighting. Inthelate’90s,asengineers LEDs continuedtobeusedasback on watchesandcalculators;however, crystal displays(LCDs)replacedLEDs became common. digital calculators,watchesandsigns used foralphanumericaldisplayson push-button telephones,andLEDs illuminate thedialpadsonearly electronics. GreenLEDswereusedto red indicatorLEDsonappliancesand IR-LEDs) wereintroduced,aswell In thelast10years,technology By thebeginningof1980s,liquid 14

© 2013RadTechInternational. LED chipsinits overalldesign. tens, hundredsoreventhousands of Any givenLEDarraywillincorporate known asthelampheadorirradiator. the fullcuringassembly, historically semiconductor, while LED array implies or diereferstotheindividualLED article, useofthewordschip,diode in articletwo.Forthepurposesofthis full UV-curing systemwillbecovered chips intolargerdiodeassembliesora packaging andintegrationofthese to specificapplications.Theactual ways toproducelargerarraystargeted then combinedandpackagedinvarious square. TheindividualLEDdiodesare the devicetendstobearound1mm the chipcanvarybydesignorsupplier, manufacturer. While the physical size of and capabilitiesoftheLEDchip desired wavelength,peakUVirradiance UV-LED chipordiodedependsonthe into theforeseeablefuture. challenges andwillcontinuetodoso and UVspectrumstillpresentsmany technology acrossboththevisible Figure 7 Figure The specificdesignofanindividual traditional UV system Spectral ofLEDsystems output comparedto Arc andMicrowave UV-LED asComparedto Output has beenformulated toreactwith respective peakwavelength. chips withoutputscenteredat their were emittedfromfiveseparate LED peaks towardtherighthalfof the chart whereas, thefivemonochromatic from oneconventionalUV-arc system; magenta spectraloutputinthechartis Figure 7illustratesthisdifference.The are 365,375,385,395and405nm. wavelength peaksforUV-LED systems between 200and445nm.Common emitters witharangeofoutput microwave systemsarebroadband wavelength; whereas,arcand is centeredataspecifiedpeak monochromatic bandofUVthat UV-LEDs emitarelatively conventional systems.Firstofall, very differentfromthatofmore that makethespectraloutput UV-LEDs haveuniquecharacteristics microwave systemsallemitUVenergy, Over the past 60 years, UV chemistry Over thepast60 years,UVchemistry While LED,mercuryarcand of thespectrum. curve isactuallyinthevisible portion LEDs, aportionoftheUVoutput and thefactthatfor395405 nm to thegreaterirradianceofUV-LEDs conventional UVsystems.This isdue “brighter” or“morepurple”than often commentthatthelightappears an LEDsysteminoperation,they bandwidth. Whenusersfirstview it isconcentratedinaverynarrow and mercuryarcsystems;however, the peakirradianceofmicrowave create thechartemituptofivetimes UV system.TheLEDchipsusedto of 10W/cm peak irradianceat395nmand405 also illustratedinFigure7whichshows than conventionalUVbulbs.Thisis UV irradianceattheirpeakwavelength 395 and405nm)actuallyemitmore that longerwavelengthUV-LEDs (385, to thosenewUV-LED technologyis wavelength range. slight visiblecomponentintheviolet from currentLEDsisallUVA witha resulting ozonetoaddress.TheUV (no IR)andnoharmfulUVCraysor is lessheattransfertothesubstrate compared to conventional curing, there UVC components.Asaresult,when aspect ofeliminatingtheinfraredand challenges, italsoyieldsthepositive output. Whilethisnodoubtpresents incredibly moreintensebandofLED within themorerestrictivebutalso the sameorsimilarcureresults reformulated toreactandaccomplish many cases,thechemistrymustbe work withmonochromaticLEDs.In broadband UVinkchemistrywill result, notallpreviouslyformulated photoinitiators tunedto365nm.Asa of thatchemistryreliesheavilyon for penetrationandadhesion.Much cure andthelongerwavelengths shorter wavelengthsforsurface broadband spectrumsutilizingthe 2 W/cm Secondly, whatisoftensurprising 2 at365nmforaconventional 2 fortheLEDsystemand 15 © 2013RadTechInternational. efficient range of wavelengthsand wider, moreintenseandincreasingly been discoveredandengineered, a of semiconductorsanddopants have UV region. have notyetbeenoptimizedforthe IR andvisibleLEDdevelopments dies haveonlyrecentlyevolvedoutof (365 nm).ThisisbecauseUV-LED less than10%forshorterwavelengths wavelengths (395and405nm) around 10to20%efficientforlonger technical limitationsrenderUV-LEDs for UV-LEDs. Unfortunately, present inexpensive. Ifonlythiswerethecase than normallightsandarerelatively indefinitely, are80-90%moreefficient little ifanycooling,claimtolast incorporate visibleLEDsthatrequire Seasonal holidaydecorationsnow promoted inrecentnewsfeatures. energy efficiencieshavebeenstrongly everyday societyandtheirhigh become increasinglycommonin most peopleasvisibleLEDshave LEDs. Thisisoftenasurpriseto systems aswellvisibleandIR- less efficientthanconventionalUV conventional UVsystems. and lighterthanthoseneededfor LED systemsaresignificantlysmaller appreciate isthatthepowersuppliesfor experience handlingUVsystemscan primary advantagethatanyonewhohas that willbecoveredinarticletwo,one While therearemanyadvantagestothis magnetrons toproduceadditionalUV. more energyfromphysicalballastsand and microwavesystemswhichrequire chip changes.Thisisdifferentthanarc as theforwardcurrentthrough of anLEDchipincreasesordecreases now, simplynotethattheirradiance LED andtotalpowerconsumption.For in moredetailarticletwoaswell through thechip.Thiswillbecovered based ontheamountofcurrentflowing As moreandbettercombinations Finally, UV-LED chipsarecurrently Thirdly, theoutputofaUV-LED is with air;however, lessthan4W/cm below 4W/cm chips. Ingeneral,systemsthatarerated around aheatsinkattachedtothe system isbycirculatingaliquidcoolant way toeffectivelyremoveitfromthe energy issosignificantthattheonly wasted asheat.Theamountofheat As aresult,theremainingenergyis system isactuallyconvertedintoUV. electrical energysuppliedtotheLED using aircooling.Lessthan20%ofthe higher outputarraysasopposedto chillers mustbeusedforcoolingthe inefficient istheonlyreasonthatliquid with conventionalcuring. application thanwouldbethecase necessary UVoutputforyourspecific total consumedenergytoproducethe today’s technologyitmaytakemore important tosimplyrealizethatwith and electricalefficiency. Fornow, itis UV-LEDs willincreaseinbothoutput science andmanufacturingprocess, With continuedimprovementsinthe as onemovesleftalongthechart. decreasing UVpeaksshowninFigure7 375 nm).Thisisexemplifiedbythe shorter wavelengthUV-LEDs (365and to thevisiblespectrumascompared efficient LEDsatwavelengthscloser to producemorepowerfuland it iseasierforchipmanufacturers visible technologyismoreestablished, in thevisiblespectrum.Since 405 nm)morecloselyresembleLEDs wavelength UV-LEDs (395and outputs couldbeproduced.Thelonger arrays rated at4W/cm arrays for mostcuringapplications.All is notnormallyasufficientirradiance if thatrealityis fiveor50yearsinto Unfortunately, it is difficulttoknow no additionalcoolingwillbeneeded. is evenpossiblethatonedaylittle to will eventuallybecooledwith air. It improves, thehigheroutputsystems presently becooledwithaliquidchiller. The factthattoday’s UV-LEDs are As theindividualchiptechnology 2 can be effectively cooled canbeeffectivelycooled 2 or higher must orhighermust 2

energy density. commonly usedtodescriberadiant In UVcuring,thetermdoseis also is thetimeintegrationofirradiance. a two-dimensionalsubstrate. describes theconceptofUVarrivingat however, irradiancemorecorrectly commonly usedtodescribeirradiance; In UVcuring,thetermintensityisalso Measuring UV-LEDMeasuring Output or mJ/cm square centimeter(W/cm in unitsofwattsormilliwattsper is theunitarea.Irradianceexpressed the substrateandasquarecentimeter UV curing,thesurfaceismostoften arriving atasurface-per-unit area.With energy densityaredefinedasfollows. of theRadTech Report , irradianceand have beenpublishedinpreviousissues written byJimRaymontofEITthat UV Glossaryandinmanyarticles energy density(dose).IntheRadTech definitions ofirradiance(intensity)and UV outputfromLEDs,let’s reviewthe millijoules persquarecentimeter (J/cm is expressedinunitsofjoulesor unit areaandradiantenergydensity area. Asquarecentimeterisagainthe energy arrivingatasurfaceperunit must runthenumberstobesure. the caseandsometimesitisn’t. One direct energysavings.Sometimesitis assume thatUV-LEDs translateinto to theneedforchiller. Don’t just conventional curingapplicationsdue is negligiblewhencomparedto current LEDapplications,however, curing. Theenergysavingsformost more efficientthanconventionalUV the applicationcanbeconsidered the chemistryarepreciselymatched, applications inwhichtheLEDarrayand incredibly energyefficientand,incertain UV-LEDs havethepromiseofbeing the future.Fornow, simplyknowthat Irradiance istheradiantpower Before discussinghowtomeasure Radiant energydensityisthe 2 ). Theradiantenergydensity 2 2 ormW/cm 2 2 ). 2

© 2013RadTechInternational. at thecuringsurfaceforagiven (W/cm On theotherhand,bothirradiance it doesn’t applytoLEDsystems atall. electrode andmicrowaveUVsystems, this terminologylooselyappliesto of Watts/cm orWatts/inch. While tend tocommunicateinnominalterms LED systemsalike. applies tomercuryarc,microwaveand curing surfacemultipletimes.Thisfact or bypassingtheUVsourceover UV systemsdirectedatthecuringarea line speed,increasingthenumberof and canbeincreasedbyslowingthe while radiantenergydensityisvariable irradiance istreatedasafixedvalue, of maximumirradiance.Asaresult, curing surfaceisalwaysinthespot the UVsourceandsetupsothat increases, mostapplicationsorient away fromthespecifiedlocation irradiance attenuatesasdistance underneath theUVemitter. While is concentratedataspecifiedlocation UV systematagivenpowerlevelthat maximum irradianceoutputfroma Figure 8 Figure With traditionalUVsystems,we It shouldbenotedthatthereisa Table-top integrating sphere andfloor-standing integrating sphere 2 ) andenergydensity(J/cm 2 ) powered andarranged,thelinespeed actual array, thewaychipsare the floodprofileoftraditionalsystems. LED systemsmorecloselyresemble is notcommonpractice.CurrentUV- directed toaspecifiedfocalpoint,this arrangement sothatallUVenergyis While LEDchipscanbepackagedinan typically applicableforLEDsystems. with conventionalUVsystemsisnot the focalpointcommonlyreferenced increases. Itshouldbenotedthat window andthecuringsurface the distancebetweenemitting array. Itattenuatessignificantlyas at theemittingwindowofLED manufacturers) istypicallymeasured commonly quotedbyLEDequipment W/cm full cure. or coatingchemistryinordertoobtain yield theUVrequirementsofink the curingsystemandsetupmust not necessarilyneedtobemeasured, LED. Whilethevaluesthemselvesdo emitter ismercuryarc,microwaveor wavelength areimportantwhetherthe The numberofLEDchipsinthe The irradiancevalueof2,4,8or10 2 foragivenLEDsystem(and W/cm different LEDsystemsallratedat8 application. Inotherwords,several selecting anLEDsystemforagiven currently complicatestheprocessof wide-format UVprinters.Allofthis as demonstratedbyscanninghead passes underneaththeLEDarray(s) greater output,ortheuseofrepeated or arraysthathavebeenoptimizedfor chips, theuseofmultipleLEDarrays need forLEDarrayswithmore greater energydensitywillresultinthe speeds andchemistrythatrequires line speed. variable factorbasedonchemistryand energy densityrequirements.Thisisa determined bytheapplication’s the perpendiculardirectionisthen or thelengthofLEDarray(s)in corresponding numberofLEDchips curing surfaceinonedirection.The UV-LED array(s)tocoverthedesired important tofirstselectorarrangethe at thecuringsurface.Asaresult,itis LED willaffectthetotalenergydensity and thenumberofpassesunder emitting window;however, thisrating Both applicationswithfasterline 2 willallproduce8W/cm 2 atthe 17 © 2013RadTechInternational. or theresultingenergydensity(J/cm density oroptimizationofthediodes; the numberofLEDchips;packing overall dimensionsofthecuringarea; does nothingtocommunicatethe LED dieorarrangement ofdiesinside minimized. original directionofsuchlight are other suchpointsandeffects ofthe reflections, distributedequally toall surface are,bymultiplescattering incident onanypointtheinner are showninFigure8.Lightrays Photos oftypicalintegratingspheres allows foruniformscatteringoflight. coating ontheinteriorsurfacethat hollow spherewithahighlyreflective This deviceisbestdescribedasa also knownasanUlbrichtsphere. of theLEDsinanintegratingsphere, manufacturer measurestheUVoutput UV-LED chipsordies.Each the worldcurrentlymanufacturing generated byUV-LEDs. correctly measuretheUVoutput microwave UVsystemswillnot with broadbandmercuryarcand UV radiometersdesignedforuse I cannotemphasizethisenough. measure theoutputfromUV-LEDs. broadband meterscanbeusedto it veryclearthatnoneoftheexisting cover thismaterialotherthantomake design variations.Asaresult,Iwon’t as theirproperuse,limitationsand articles writtenonthesemetersaswell UV systems.Therehavebeenmany broadband mercuryarcandmicrowave measure UVgeneratedbytraditional radiometers onthemarketdesignedto are currentlyafitforyourapplication. trials todeterminewhetherUV-LEDs supplier; and,ifatallpossible,conduct well asyourUV-LED anddispensing your inkandcoatingmanufactureras The bestadviceistoworkcloselywith for yourspecificsetupandlinespeed. Chip manufacturersplaceasingle There areonlyafewcompaniesin There areawiderangeofUV 3 2 ) area andlocatedsomewhereonthe radiated ontoadetectorofknown the sphereandenergythatis emitted UVbouncesaroundinside over itsentireviewingangle.The UV energyisreleasedfromtheLED door. Thedieispoweredandthe the integratingsphereandclose Figure 9 Figure Figure 10 Figure Typical radiometer sensitivity(UVA response curve) LED source Power representation output for a405nm energy values and measured between true 75% difference 16 nmwidthathalfintensity it’s notnecessarytounderstandall the current andpotentialusersofUV-LEDs, the totalUVoutputinW/cm. and theLEDsizeisusedtodetermine includes thesphere’s circumference and amathematicalcomputationthat Measurements aretakenin1nmbands sphere’s insidesurfaceismeasured. 10.2 W/cm Peak irradianceof measured near375nm 100% trueenergyonly 2 at405nm 395 nm about 75%at Under readsby 2 For most Formost 18

© 2013RadTechInternational. value of10.2W/cm nm. Inthiscase,theLEDhasa peak from anLEDsourcecenteredat405 representation oftheUVdistribution shown inFigure9,providesageneric skewed towardtheupperlimits. chips withinagivenbinrangetendtobe the limits.Inpractice,however, most nm orsomemixedvariationbetween and 420nmcouldallbe380nm, selected rangeofdiesbetween380nm control over the wavelength. A randomly of wavelengths.Itjustmeansless selection doesnotmeanawiderwidth guaranteed. Inotherwords,awiderbin in cheaperdies,theprocesscannotbe binning, themoreexpensivechip. diode’s irradianceandthetighter +/- 15nm.Ingeneral,thegreater tolerances are+/-5nm,10nmand the diesbecome.Typical wavelength the tolerance),moreexpensive tighter thebinwidth(i.e.,smaller within thesametolerancerange.The that haveapeakwavelengthfalls categorized orbinnedwithotherLEDs an integratingsphereandthechipis a finishedchipisdeterminedusing production. Thepeakwavelengthof something thatismeasuredafter manufacturing process,butrather something thatisengineeredintothe peak wavelength.Thetoleranceisnot LEDs basedonthetoleranceof commercial UV-LED applications. aren’t practicalmeasuringtoolsformost spheres, duetotheirshapeandsize, UV-LED irradiance and how integrating involved inaccuratelymeasuring is anappreciationofthecomplexities What youshouldtakeaway, however, how theirradianceisexactlycalculated. physics behind an integrating sphere or particular LED may providesufficient intensity fallsto2.0W/cm. of thepeak(397nmand413nm), the chip surface.At8nmoneither side A narrowbell-shapedcurve,as While looserbinselectionresults LED chipmanufacturersspecify 2 at405nmthe 2 Whilethis wavelengths emitted byUV-LEDs. radiometers are not tuned to the specific in aproductionenvironment,and most spheres arenotapracticaltool touse surface. Thisisbecauseintegrating UV thatactuallyreachesthecuring is difficulttoquantifytheamountof UV tocureagiveninkorcoating,it Figure 11 Figure Figure 12 Figure (380-400 nm) Radiometer sensitivityrequired for UV-LEDs Typical radiometer sensitivity(UVV response curve) and measuredenergyvalues 35% differencebetweentrue 35% at395nm Under readsbyabout with mercury arc and microwave curing with mercuryarc andmicrowavecuring significantly lower thanthoserecorded irradiance andenergydensity values the radiometerreadingsproduced have oftenbeendownplayedsince curing results;however, theresults UV-LED trialshaveproduceddesired In thelabandinfield,many measured near412nm 100% trueenergyonly 19 © 2013RadTechInternational. 0.2% /ºCcanalso affectreadings. In addition,temperature variationsof calibrated valueandrepeatable to+/-5%. are typicallyaccurateto+/-10% fromthe limitations. Forexample,radiometers to adequatelyunderstandthe device’s the manufacturerorreadmanual measuring error and you should contact that allinstrumentshavesomeinherent obtained. Pleasekeepinmind,however, from aUV-LED curingsystembe accurate measurementsofUVoutput using metersofthisspecificdesigncan to themarketinearly2010.Onlyby puck styletransporterwasintroduced contained inthewell-knownhockey fitted tothismeasurementprofileand single channelradiometerspecifically in the380-400nmbandwidthrange.A irradiance (W/cm are capableofproducingaccurateUV curves, asillustratedinFigure12, system byapproximately35%. the peakoutputofa395nmLED slightly above410nmandunderreads Conversely, theUVVsensoriscentered of a395nmLEDsystemby75%. and underreadsthepeakUVirradiance The UVA sensoriscenteredat375nm curves oftypicalUVA andUVVsensors. represent thesensitivityresponse The graphsinFigures10and11 concentrated inthe380-400nmrange. were neverintendedtocaptureoutput broadband UV-curing systemsand was engineeredtofitconventional sensors havearesponsecurvethat UVA, UVBandUVC).Theindividual one ofthefourUVbandwidths(UVV, sensors, eachdesignedtomeasure density arefittedwithfourdistinct measuring UVirradianceandenergy used, it’s simply a meaningless exercise. if incorrectandinappropriatetoolsare and comparingUVprocesses;however, extremely importanttoolinmaintaining systems. MeasuringUVoutputisan (J/cm Only radiometerswithflatresponse Commonly useddevicesfor 2 ) readingsfromUV-LED systems 2 ) andenergydensity aren’t yetknown. Inothercases,the learning andsometimestheanswers please keepasking.We areallstill or theanswerdoesn’t makesense, question anddon’t getananswer have alltheanswers.Ifyouask a to notethatsuppliersdon’t yet to UV-LEDs. Itisequallyimportant are nodumbquestionswhenitcomes Whatisgainedorlostinusing • Whatarethecriteriafor • CansimilarLEDperformance • Doesthebroadbandinkorcoating • Whataretherequirementsof • Whereand how wastheirradiance • Whatisthe irradiance specification • Comparing LED Comparing Technologies Whatisthe peak wavelengthand • that youshouldconsider. technology, thereareseveralquestions conduct yourownevaluationofLED It isimportanttonotethatthere price)? efficiency, operatingcost,purchase LEDs (powerconsumption,heat, restrictions)? cured, existenceandtypesofspace single ormultiplepass,areatobe array (movingorstatichead, physical installationoftheLED UV systems? throughput usedwithconventional same operationallinespeedor results beachievedatthe wavelengths? match thenarrowbandUV-LED does itneedtobereformulated chemistry curewithUV-LEDs or irradiance andenergydensity? the curingapplicationintermsof specification measured? of theLEDarray? coating chemistry? what istheimpactoninkor bin toleranceoftheLEDarray, and As youbeginorcontinueto applications moreplausible!w rate, thusmakingmoreand at anastonishingandexciting implementation continuestoincrease of technologicaladvancementand many applications,themomentum UV-LEDs arestillnottherightfitfor article andwillnowreiterate,while As Imentionedatthebeginningof may quicklyfindyourselfleftbehind. it turnsyouofftothetechnology, you Please don’t letthisdiscourageyou.If rapid advancementsinthetechnology. answers maychangequicklywith References 3. 2. 1. Integrating_sphere http://en.wikipedia.org/wiki/ January 7,2010.March15, Integrating Sphere.Wikipedia. 14-25. RadTech Report.May/June2002: Maintaining aUVProcessWindow” Raymont, Jim.“Establishingand Publications. 2009. Applications. Florida:Auerbach Emitting DiodeTechnology and Held, Gilbert.IntroductiontoLight —Jennifer Heathcoteisgeneral for IntegrationTechnology in manager, NorthAmerica, Chicago, Ill. 20

© 2013RadTechInternational. By Jennifer Heathcote SystemsCuring UV-LED — II Overview Part Figure 1 Figure UV-LED system components Interconnect Cable LED Array Power SupplyUnit Primary Secondary T communicate effectivelywithin to achievefullcomprehensionor disconnected anditcanbedifficult language, keyconceptsoftenappear Without afirmgraspofthecommon context ofthedefinedsubjectmatter. of relevantterminologywithinthe any newtopicistobeginwithastudy with PartII. want todosobeforecontinuing July/August 2010issue,youmay Operation andMeasurement”inthe If youhavenotyetread“PartI— light-emitting diodes(UV-LEDs). and engineeringbehindultraviolet information regarding thescience key principlesandtechnical The bestapproachwhentackling series designedtoconsolidate installment inathree-part his articleisthesecond or wepretendedtounderstandwhat party understoodwhatweweresaying in whichweeitherassumedtheother have eachofushadregardingUV-LEDs companies. Howmanyconversations and customersaswellwithin misunderstandings betweensuppliers their usagecanleadtoconfusionor of thewordsarenotapparent, multiple meaningsorifthedefinitions words orareusingthemincorrectly. that manyofusarenotusingthesame reference. Thisincreasesthechances industry, letaloneconsolidatedforeasy defined withrespecttotheUVcuring has notyetbeenproperlyestablishedor jargon liesinthefactthatlanguage additional challengeinlearningthe technologies suchasUV-LEDs, an the marketplace.With emerging Furthermore, ifthewordshave (Temperature &Flow) Interlock Box Chiller Liquid

© 2013RadTechInternational. UV-LED System Components UV-curing systems. of theintegrationLEDchipsinto terminology withinalargerdiscussion claims, thisarticlewilloutlinekey the truthfromcurrentmarketing UV-curing industryandseparate the languageofUV-LEDs, educatethe conversation? Inordertodocument of thewordsbeingusedin because ofinsufficientknowledge they werecommunicatingtoussimply mounting brackets andlightshielding. include material handlingequipment, Ancillary itemsnotshownwould components areprovidedinFigure 1. images oftheprimaryandsecondary and temperatureinterlock.Sample are theliquidchillerandflow currently ratedatorabove4W/cm generally necessaryforLEDarrays Two commonsecondarycomponents (PSU) andtheinterconnectcable. the LEDarray, thepowersupplyunit components aregenerallyknownas names fortheseitems,theprimary may electtouseuniquemarketing some companieswithintheindustry two secondarycomponents.While machine consistofthreeprimaryand for integrationontoanOEMorhost Figure 2 Figure All UV-LED systemsdesigned Liquid-cooled, UV-LED array 2

irradiances above4W/cm 4 W/cm maximum UVirradianceislessthan self-contained coolingfanwhenthe chips and,inmanycases,includesa UV-curing arrayhousestheLED in theUV-curing industry. Astandard has always been a frequently used term incorrectly calledalampheadasthis It isnotuncommontohearanarray or genericallyreferredtoasthehead. name “LED array” is shortened to array UV lampheadorirradiator. Oftenthe function similartothatofatraditional curing assemblythatprovidesa arrays aresupplied inawiderangeof Figure 2.Itshouldbenotedthat LED liquid-cooled LEDarrayisprovided in the driverboards.Adetailedphoto ofa other electricalcomponentssuch as system, itisspecificallyforcooling present inthearrayofaliquid-cooled cooling tubes.Ifafanis connected totheheadanditsinternal by whichtheliquidcirculationchilleris fittings ordisconnectsarethemeans LED-chip coolingfanisremoved.The to thehousingormanifold,and disconnect couplingsareattached these arrays,tubefittingsorquick cooling ismorecommonlyused.For The LEDarrayofFigure1isa 2 . Inarraysdesignedforpeak 2 , liquid and/or interconnect cable.Though elect todesigntheirownpower supply cases, integratorsorend-users may UV equipmentmanufacturer. Insome necessarily needtobesupplied bythe however, notallthreecomponents a fullyoperatingUV-LED system; interconnect cablearerequiredfor the powersupply. connection betweentheLEDarrayand equipment. Itprovidestheelectrical cable isthefinalpieceofprimary or inthePSUitself.Theinterconnect inside thearrayhousingasinFigure2 LED chipsandcaneitherbemounted the LEDArray,” directlypowerthe be discussedinmoredetail“Driving machine. Thedriverboard(s),whichwill as thecommunicationlinktoahost and controlthearray. Italsoserves contains thepartsnecessarytopower a PSU,isanelectricalenclosurethat surface oftheLEDarrayinFigure1. but itcanbeseenontheexposedtop window ishiddenfromviewinFigure2, yellowish tint.Theactualemitting can givethewindowapurplishor its opticalproperties.Thistreatment chemically treatedtofurtherenhance The emittingwindowissometimes pure quartzandlowerqualityglass. and strengthincomparisontoboth exceptional thermal-shockresistance expensive thanpurequartzandoffers of fivepercentboricoxide—isless colorless, silicaglasswithaminimum being cured.Borosilicate—whichisa the LEDchipsandmediaorpart glass locatedontheheadbetween flat pieceofquartzorborosilicate window iscommonlymadefroma chips withintheassembly. The that physicallyprotectstheLED typically includesanemittingwindow a standardarray, itisjustoneexample. Figure 2highlightscommonfeaturesof shapes, sizesanddesignswhile The array, powersupplyand The powersupplyunit,oftencalled A fullyassembledLEDirradiator 22 © 2013RadTechInternational. Packaging UV-LEDs UV outputsgreaterthan4W/cm most oftenrequiredforLEDarrayswith and buildtheirownirradiator. LED arraypackages(calledmodules) they mayevendecidetobuysmaller that theproperspecificationsaremet, great careshouldbetakentoassure that emitlightwhen currentflows to discrete,individualsemiconductors integrator orend-user. manufacturer orfurnishedbythe either besuppliedbytheLEDsystem should benotedthattheinterlockscan destroying thewirebondsisavoided.It is maintainedandthatoverheating operating temperature of the LED diode designed toensurethatthecorrect the LEDarray. Thisisasafetyfeature setting, theinterlockcircuitswitchesoff rate dropsbelowitspredetermined specified setpointoriftheoutletflow at thearrayinletandoutletexceedsa temperature ofthecirculatingcoolant the inletandoutletofarray. Ifthe meter monitorthecoolantconditionsat prematurely. the system,LEDchipswillfail correctly designedorintegratedinto decision. Ifthecoolingsystemisnot or withthesupplierbeforemakinga with theUVsystem’s operatingmanual configuration, sobesuretoconsult will varydependingonthearray own. Therequiredcoolingcapacity shelf chillerlocallyortodesigntheir or end-usertopurchaseanoff-the- more cost-effectivefortheintegrator part ofthecuringsystem,itisoften UV suppliercanfurnishthechilleras process discussed inPartI.Whilethe a by-product of the electroluminescence this byremovingunwantedheatthatis correct operatingtemperature.Itdoes chips andwirebondsremainatthe The chillerisusedtoensuretheLED A liquid chiller is the cooling system A liquidchilleristhecoolingsystem LED, chip,dieanddiodeallrefer The temperaturesensorandflow 2 . work within the size and performance work withinthe sizeandperformance also forcesUVsystemdevelopers to eliminates someoftheengineering but packaged modules,ontheother hand, final systemdesign.Purchasing pre- to bettercontrolanddifferentiate the enables theUVsystemmanufacturer modules. Purchasingindividualdiodes only providetheirownengineered modules orLEDarrayswhileothers manufacturer intocustomized fully integratedbytheUVsystem individual diodesthatcanbe larger LEDarray. can bejoinedtogethertoformaneven an LEDmodule.Severalmodules LED package,itisoftenreferredtoas occasionally, opticsareaddedtothe device. Whenaheatsinkand, electrically connectedtoanother means fortheentireassemblytobe assembled togetherandincludea that arephysicallyandelectrically terms refertoanarrangementofLEDs arranged inamatrixpattern.Both contains alargenumberofchips chips, whileanLEDarrayusually package canconsistofoneorseveral to apowersupplyunit.AbasicLED assembly orirradiatorandconnected and thenintegratedintoalamphead and microwavebulbsaresourced is similartothewayinwhichelectrode interface and,sometimes,optics.This or liquid,andequippedwithacontrols electrically powered,cooledwithair into anirradiatorassemblythatislike system designersengineerthechips tolerance andforwardvoltage.UV according toadesiredwavelength have binnedorcategorizedthem semiconductor manufacturerswho source LEDchipsdirectlyfrom operation, refertoPartI.) (For additionalinformationondiode referring tosingleormultipleLEDs. are usedinterchangeablywhether through them.Thesefourterms Some LED chip suppliers sell Some LEDchipsupplierssell In general,UVsystemdesigners process thatemploys ultrasonic hundred micronsthick.Awire bonding square andaretypicallylessthan a of amillimetertofewmillimeters LED chipsvaryinsizefromafraction a standardmercurylamp.Individual order toshiftthespectraloutputof gallium andindium(amongothers)in internal gasmixtureisdopedwithiron, additive UVlampsinwhichthe This shouldnotbeconfusedwith conductivity (asdiscussedinPartI). to producespecificn-typeandp-type doped orimpregnatedwithadditives such assiliconorgermaniumthatare tiny slicesofsemiconductivematerial producers. an alreadysmalllistofpotential are willingtosellvariesamong diode ormoduleconfigurationsthey manufacturers withrespecttothe the flexibilityofUV-LED UV-curing applications.Furthermore, interested inproducingchipsfor of thecapablemanufacturersare and securitiesindustries,notall lighting, automotive,communications targeted towardthecommercial visible andinfraredLEDmarkets small incomparisontothatofthe the UV-LED marketisextremely which marketstheypursue.Since corporate strategiesthatdetermine weaknesses, aswellestablished of UV-LED diodeshasstrengthsand with anybusiness,eachmanufacturer as Nichia,Fox,NitrideandCree.As well-known, multinationalbrandssuch these manufacturersincludelarge, in theultravioletregion.Someof producing chipsthatemitwavelengths a dozenarecapableofdevelopingand throughout theworld,onlyabouthalf with productionfacilitieslocated manufacturers ofvisibleLEDs necessary curing application in mind. necessary curingapplicationinmind. may nothavebeendesignedwiththe constraints ofanexistingassemblythat LED chipsaremadefromvery While therearemany 23

© 2013RadTechInternational. whereas, conventional microwave and whereas, conventionalmicrowaveand fabricated toanysizeandshape; LED sourcecould,inprinciple,be interconnected dies.Asaresult,an a singleLEDchiporthousandsof An LEDpackagecanconsistofjust inherent “scalability”ofthetechnology. characterized byLEDsourcesisthe cathode. made atthenegativeconnectionor terminal oranodewhiletheotheris wire bondismadeatthechip’s positive to aprintedcircuitboard(PCB).One used toelectricallyconnectLEDchips welding orthermalcompressionis Figure 4 Figure 3 Figure One of the many advantages One ofthemanyadvantages 16 LEDswithawire eachanodeandcathode bondat Cross ofa25dieLEDmodule section electroluminescence process,isnot heat, anunwantedby-productofthe sensitive andcanbedestroyedifthe noted thatthewirebondisthermally wire bondsarerequired.Itshouldbe 400 LEDchipsinanarray, then800 assembly. Forexample,ifthereare and themoreexpensivefinal involved themanufacturingprocess assembly. Themorechips,the thousands ofLEDchipsinagiven are likelytocontainhundredsoreven technology andmicrowavecavitysizes. much narrowerenvelopebasedonbulb electrode UVsourcesarerestrictedtoa In practice,mostUV-LED arrays provides examples ofplug-inmodules, for anynecessarydierepairs.Figure 5 must bereturnedtothemanufacturer heat sink.Asaresult,theentire array bonded directlytoasinglePCB and as allchipsintheassemblyare wire and cannotbetakenapartinthefield of thisnaturearenon-serviceable spans thedesiredcuringwidth.Arrays form onelarge,uniformassemblythat LED chips,PCBandmetallicheatsink sometimes designedsothattheUV- replaced. Alternatively, LEDarraysare faulty orspentmodulesneedtobe or theneedforabackuparrayasonly in thefield.Thisreducesdowntime able tomake“plug-and-play”repairs matrix, providesthebenefitofbeing modules, eachcontainingadiode or head.Theuseofmultipleplug-in to createaphysicallylargerLEDmatrix LED modulescanbearrangedtogether five inhalf. section ofthedrawingsplitsonerow diodes showninfullwhilethecross a totalof25LEDchips.Thereare10 Figure 4.Inthisassembly, thereare LED packagewithalensisshownin sectional illustrationofonetype the mediaorpartbeingcured.Across so thattheyareuniformlydirectedat sometimes usedtocontroltheUVrays dome-shaped lensmadeofsiliconeis for addedprotection.Alternatively, a LED chiporoverseveralchips or quartzisplacedoveranindividual and madeofsilicone,borosilicateglass A flatencapsulatethatistransparent away fromthediodeandwirebonds. slug. The heat sink is used to draw heat are securedtoametallicheatsinkor square pattern. 16 mountedinamatrixtoformlarger bonds are visible for each diode with all consisting of100chips.Two wire diodes takenfromalargerassembly a magnifiedimageof16separateLED sufficiently removed.Figure3shows As previouslymentioned,several The wire-bondeddiodesandPCB 24 © 2013RadTechInternational. improvements inthemanufacturing the sciencebehindLEDs, attributed togainsinunderstanding past sevenyears.Theincreasecanbe output fora395nmchipoverthe illustratestherapidgrowthinUV optics anddrivingtechniques.Figure6 can beachievedthroughtheuseof peaks. Evengreaterirradiancelevels UV-curing lampsdoattheirrespective conventional electrodeandmicrowave intensity atthesewavelengthsthan emit morethanfivetimestheUVA how LEDsratedat395nmandgreater monochromatic nature of UV-LEDs and serviceable LEDarray. a serviceableLEDarrayandnon- Figure 6 Figure 5 Figure Part Idiscussedthenearly UV-LED arrays (a)three plug-inmodules, (b) Advancement ofUV intensity for 395nmLEDs modules, and(c)1 a diodes all mounted to a single PCB and heat sink LEDarray andheat to mounted asinglePCB with100diodesall For example,anintensityof1W/cm known astheenergydensityordose. is takenoveraperiodoftime,it for completecure.WhenUVirradiance ultraviolet wavelengthsarerequired energy densityoveraspecificrangeof sufficient levelsofbothirradianceand electrode UV-curing applications, materials. combinations ofsemiconductor and qualityprocesses,new substrate allcontributesignificantlyto the distanceofarrayfrom current flowthroughthedieand manufacture oftheLEDchip, of energydensity. Thedesignand exposed for1secondisequaltoJ/cm As withtraditionalmicrowaveand b 1 LEDarray encompassing 16plug-in 2

the application, itcanbecontrolled UV-LEDs isthatifheatrequiredfor UV systems.Theadvantageof using generated byelectrodeandmicrowave This heat,however, islessthanwhat converted toheatatthesubstrate. is notabsorbedbythechemistry some oftheradiatedUVenergythat slower processspeeds,however, spectral outputofanLEDarray. For is noinfraredcomponentinthe the LEDarrayisnegligibleasthere transferred tothecuresurfaceby successful cure.Inmostcases,heat sometimes necessarytoachievea coatings, afourthvariable(heat)is tolerances ofupto±15nm. 365, 375,385,395and 405 nm with these dieshavepeakirradiancesat curing applications.Commercially, generated byLEDsforuseinUV- the rangeof350nmto430are and shortervisiblewavelengthsin the array. Today, onlylongerUV with thewavelengthsgeneratedby measurements shouldbereferenced Both irradianceandenergydensity and theend-useprocessspeed. dies, thetotalnumberofchipsused a factoroftheoutputindividual total dosedeliveredtothesubstrateis the substrate surface. Alternatively, the the array’s UVirradianceorintensityat For certaintypesofinksand c 25

© 2013RadTechInternational. Coolingthe LEDArray best systemforagivenapplication. UV-LED systemsorinselectingthe single biggestdifficultyincomparing This canbeabitmisleadingandisthe an identicalrangeofwavelengths. density andheatatthesubstrateover provide similarirradiance,energy in thesespecifications,however, will all arraysthatseemtobeidentical dimensions oftheLEDmatrix.Not window aswellthephysical maximum irradianceattheemitting simply quotethepeakwavelengthand fact thatmostUV-LED suppliers UV-LED productoriginatesinthe process specificationtrials. that shouldbeestablishedduringthe level, butwithina“processwindow” single irradianceorenergydensity processes, therefore,occurnotata cured product.MostUV-curing density thatmayresultinanoptimally combinations ofirradianceandenergy process. Fortunately, therearevarious time andtemperatureinathermal energy densityworktogetherlike wavelength source,irradianceand process speed.Foraproperlyselected coating atthedesiredapplication used inordertocureagiveninkor density, wavelengthandheatbe combination ofirradiance,energy eliminated. if itisnotrequired,essentially independently oftheUVdeviceand, on theotherhand, areintherangeof (365 nm).Visible andinfraredLEDs, less than10%forshorterwavelengths wavelengths (395and405nm) and around 10to20%efficientfor longer limitations presentlyrenderUV-LEDs into useablelightenergy. Technical to thesystemisdirectlyconverted much oftheelectricalenergysupplied semiconductor chipreferstohow Confusion inselectingtheoptimal It iscriticalthatthecorrect The overallefficiencyofanLED than 4W/cm today isthatarrayswithoutputsofless exceptions, thegeneralruleofthumb decrease. Whiletherearealways cooling requirementswillgradually efficiencies willimproveandthe part beingcured. radiate itouttowardthesubstrateor assembly willabsorbtheheatand frame, manifoldorhousingofthearray failure. Inaddition,thesurrounding around 125ºC,willsuffercatastrophic exceed asustainedtemperatureof bonds andLEDchips,whichcannot If theheatisnotremoved,wire of theelectroluminescenceprocess. energy butisinsteadaby-product not radiatedintheformofinfrared for maximumoutput.Theheatis wavelengths andmustbeliquid-cooled they generatemoreheatthanshorter currently emitthemostirradiance, wavelengths suchas395and405nm ratings. SinceLEDswithlonger than diodeswithlowerirradiance irradiance ratingsgeneratemoreheat heat. Ingeneral,diodeswithgreater in theproductionoflargequantities systems arearound25%efficient. 40 to50%efficientwhileelectrodeUV respective operating manual.Inthe sure toconsultwiththesupplier or on thearraydesignandsizeso be cooling requirementsvarydepending is alwaysimportanttonotethat exact more easilylocatedintightspaces. It typically morecompactandcanbe In addition,liquid-cooledsystemsare substrates withoutdisturbingthem. be mountedclosetothinorfragile contaminants andallowsarraysto cooling airfrequentlystirsupairborne makes foracleanerprocesssince around theUV-light source.This is thatthereminimalairmovement An advantageofliquid-cooledsystems cooled withaliquid-circulationchiller. with air;anythinggreateristypically In thenearfuture,UV-LED Inefficiencies withUV-LEDs result 2 canbesufficientlycooled and DOWFROST state canresult inthebreakdownof maintained. Maintainingade-ionized de-ionized stateoftheliquidis not be usedfordilutionaslong the De-ionized watercansometimes water canbeusedtodilutethe glycol. to water. Distilledorreverse-osmosis used proportionsare25%-30%glycol bacterial contamination.Commonly and lessthan20%glycolwillresultin than 25%glycolcanleadtocorrosion heat transfer;however, usingless in thelowerrangeresultbetter 25% and60%glycol.Concentrations its productsbemaintainedbetween recommends thatconcentrationsof locations within the closed-loop system. possible particulatebuildupatvarious in theeffectivenessofcoolantand solution. Theneteffectisareduction precipitate someinhibitorsoutof types orbrandsofglycolasthiscan is goodpracticenottomixdifferent volumes throughchillersuppliers.It concentrated ordilutedforminsmall can typicallybepurchasedineither under thebrandsDOWTHERM Both aremarketedinNorthAmerica is moreenvironmentallyfriendly. properties; however, propyleneglycol glycol hasbetterthermaltransfer concentration levelisused.Ethylene and bacteria,providedaminimum also offersprotectionagainstalgae protecting themetals.Inaddition,it and corrosionwhilesimultaneously coolant preventstheformationofscale the circulationcoolant.Aninhibited industrial inhibitedglycolandwateras recommend usingamixtureof end-user. independently bytheintegratoror UV systemmanufacturerorsourced hand, canbeprovidedbyeitherthe assembly. Liquidchillers,ontheother likely beprovidedaspartofthearray case ofair-cooled systems,thefanwill The DOWChemicalCompany Most industrialchillercompanies TM , respectively, and TM

26 © 2013RadTechInternational. the LEDArray Driving supplier tobesure. with theoperationsmanualoryour You should alwaysconsult interlock. that yoursystemisequippedwithan LED array. Donotassume,however, the interlockcircuitswitchesoff drops belowitspredeterminedsetting, set pointoriftheoutletflowrate inlet andoutletexceedsaspecified of thecirculatingcoolantatarray using liquidchillers.Ifthetemperature can andshouldbebuiltintosystems interlock isagreatsafetyfeaturethat A temperaturesensorandflowmeter failure ofthewirebondsandLEDchips. result incatastrophicandpremature cooling witheitherairorliquidwill is used.Failuretoprovidesufficient This isthetrade-offwhenaircooling cure somematerialsatfasterspeeds. less UV, making itmoredifficultto that incorporateair-cooled arraysemit array. LowerirradianceLEDsystems heat andresultinamorecompact effective waytoremoveunwanted chillers becausetheyarethemost systems tendtoincorporateliquid irradiance level.HigherLED is primarilybasedonthemaximum cool anLEDarraywithairorliquid Data Sheet. the correspondingMaterialSafety manuals, stateandlocalcodes, chiller suppliers,theirrespective consult withtheUV-LED systemand filling thereservoir, makesureyou Before selectingachillerorbefore more accessiblegivenyourlocation. from othermanufacturersandmaybe Similar inhibitedcoolantsareavailable against theuseofregulartapwater. chiller suppliersstronglydiscourage certain metalsinthesystem.Most order tovaporize themercuryand 2 to4kilovoltarcacrossthebulb in systems utilizeanignitortostrike a Today, thedecisionofwhetherto Traditional electrodeUV-curing of current passing through the device; of currentpassingthroughthedevice; is directlyproportionaltotheamount of UVirradianceemittedfromanLED between 2and4voltsDC.Theamount operate between20and500mA ignitor. EachLEDchipisdesignedto source and do not require a high-voltage connected toalow-voltage,DCpower ultraviolet lightimmediatelywhen entirely solid-statedevicesthatemit levels. LEDs,ontheotherhand,are provide forvariousUVpoweroutput flowing throughastruckbulband to stabilizetheamountofcurrent combinations arewiredintothecircuit emit UV. Ballast,chokeortransformer Figure 8 Figure 7 Figure DC power supply Four withresistors LEDswired anda inseries parallel withresistors andaDCpower supply Four LEDswithidentical voltage ratings wired in faulty LED.Parallelcircuitshavethe interrupted downstreamfromthe current flowthroughthecircuitis series installation,ifoneLEDfails, LEDs wiredintoasinglechain.In or eventhousandsofindividual arrays willtypicallyhavehundreds connection offourdiodes,UV-curing simple circuitsdemonstratethe Figures 7,8and9.Whileallthree or parallelcircuitsasillustratedin time duringoperation,thechipwillfail. exceeded foranextendedperiodof rating ofanyindividualLEDchipis however, if the design voltage or current LEDs canbeconnectedinseries 27

© 2013RadTechInternational. as resistorsgenerate heatandcannot LED circuitisactuallyquiteinefficient regulate voltageandcurrentwithin an w, x,yandzrated atdifferentvalues. illustrated inFigure9withresistors to balancethecurrentflow. Thisis be placedineachlegofthecircuit appropriately sizedresistorsmust have differentvoltageratings,then emitting thesamelightoutput.IfLEDs LEDs notlightingorall the circuitwillvaryandresultinsome current flowingthrougheachlegof same voltagerating.Otherwise, in Figure8,theymustallhavethe LEDs arewiredinparallel,asillustrated premature failureofthedies.When output, whiletoomuchcanleadto current willyieldlessthanoptimalUV each LED.Insufficientvoltageand voltage dropandcurrentflowacross are preciselysizedtoensurethecorrect sequence willdisablethecircuit. Placing twoanodesorcathodesin current willonlyflowinonedirection. This isbecauseLEDshavepolarityand cathode connectionsbealternated. it isimportantthattheanodeand LEDs fail.Ineithertypeofcircuit, functioning LEDsevenifoneormore advantage ofcontinuedoperationall Figure 9 Figure In practice,theuseofresistors to In allthreesimplecircuits,resistors DC power supply in parallel withfour different resistors anda Four LEDswithdifferent voltage ratings wired between ON and OFF. The length of the switching currenttothediodes and worksbyquicklyefficiently instantaneous responsetimeof LEDs PWM takesadvantageofthenearly an almostinfinitenumberofoutputs. power levels,PWMisusedtogenerate applications thatrequiremultiple with novariabilityinirradiance.For The LEDsareeitherONorOFF and continuouscurrentwhenpowered. always experienceanon-fluctuating enables eachandeveryLEDchipto necessary tovarytheUVoutput. Width Modulation(PWM)whenitis optional techniqueknownasPulse current driversandemployan power LEDswithregulated,constant- equipment manufacturersgenerally from eachdiode.Asaresult,UV changing theamountofUVemitted through thecircuitasameansof levels inordertovarythecurrent the useofmanydifferentresistance is alsochallengingsinceitrequires systems withmultiplepowerlevels and chiplifetimehours.Producing produce wideswingsinUVoutput variation inrunningconditionscan Eventheslightest temperatures. in die tolerances andoperating easily accommodatefluctuations The use of constant-current drivers The useofconstant-currentdrivers time orenergydensity(J/cm increase ordecreasethetotalexposure the pulsedirradiance,isvariedsoasto current and,therefore,thedurationof levels, butthedurationofpulsed (W/cm all fourdutycycles. by “P”inFigure10,isconstantacross the total cycle time or period, indicated half thetime.Itshouldbenotedthat the powerisONhalftimeandOFF 0% isfullyOFFand50%meansthat of thetime,whereas100%isfullyON, power becausetheisOFFmost A lowdutycyclecorrespondsto OFF) and is expressed as a percentage. of ONtimetothetotalcycle(ON+ Duty cycleisdefinedastheproportion levels of25,50,75and100%power. ON andOFFtimepercycle. pulsed currentvariestheproportionof every 0.25millimeters ofmediatravel. second, theUVsourceilluminates once beneath thearrayat0.5meters per 2 KHzinwhichthemediaistraveling application usingaPWMfrequency of output decreases.With respect toan dimmer inappearanceastherelative that theLEDsbecomeincreasingly human eye,anobserverwillnotice frequency cannotbedetectedbythe every second.Whilethishighofa are 2,000cycles(orcurrentpulses) 2 KHzfrequencymeansthatthere frequencies of2KHzorgreater. A of LEDsusingPWMoccursat LEDs ormoduleswithinanarray. addressability ofindividualbanks In addition,PWMprovidesforeasy and (2)prolongsthelifeofchips. variable controloftheUVoutput truly instantaneousandinfinitely resistors, arethatPWM(1)provides current throughtheLEDcircuitwith opposed tovaryingtheactualdrive drive theUV-LED powerlevels,as two mainreasonsforusingPWMto With PWM,thepeakirradiance Figure 10illustratesPWMoutput The fastcyclingorstrobing 2 ) is the same for all power ) isthesameforallpower 2 ). The

28 © 2013RadTechInternational. UV-LEDInstalling Systems interface; andensuring safeoperation array; providingameansforthe I/O between thepowersupplyand the connecting theinterconnectcable electrically poweringthePSU; connecting thechillersystem; purchasing ordesigningandthen mounting thearray(s);frequently on ahostmachinetypicallyrequire mercury vaporlamps.Installations with conventionalmedium-pressure handling anddisposalissuesassociated are used;andpossessnoneofthe near themediawhenliquidchillers create lessairbornecontamination often drawfarlesselectricalpower; special filtersforobservationwindows; attack manymaterials;donotrequire emit shortwavelengthUVwhichcan LEDs generatenoozone;donot Unlike conventionalUVsystems, some waysitisactuallyabitsimpler. a conventionalUVsystem,butin system issimilartothatofinstalling manufacturers. systems willvarysignificantlybetween the styleofdriverboardusedinLED wide varietyoffunctions.Asaresult, types ofdriversusedtoperforma reduced. Therearemanydifferent of theinterconnectcablecanbe can beminimizedandthediameter noise on thearrayisthatelectrical mounting theboards of advantage The remotely inthepowersupplyunit. be mountedonthearrayorlocated system. Thedriverboardscaneither provide thePWMcapabilityof the necessary constantcurrent; and chips ormodulesinanarray;deliver distribute theDCvoltagetoLED The purposeofthedriverboardsisto the individualdiodesarewirebonded. different PCBsthantheonestowhich (PCBs), althoughtheyareentirely essentially printedcircuitboards, The taskofinstallingaUV-LED The arraydriverboardsare moved bythehost machinetocover a staticinstallation orcanbephysically that iteitherspansthecuring width of user. Thearrayshouldbemountedso designed bytheintegratoror end- a hostmachinewithsimplebracket that allowthedevicetobesecured mounting holesorthreadedinserts Most arraysareequippedwith does varybysupplierandapplication. between 5and15mm;although,it being cured.Thisdistanceisusually as possibletothesubstrateorobject market aretypicallymountedasclose most LEDarrayscurrentlyonthe the substrateincreases.Asaresult, between theemittingwindowand decreases inmagnitudeasthedistance ancillary equipment. system, aswellthehostand for theoperatorandUV-LED Figure 10 Figure All UVoutputattenuatesor PWM of25, illustration 50, 75and100%dutycycles diodes usedincuring systemsare arc ormicrowavesystems. resemble thefloodprofileof traditional Current UV-LED systemsmoreclosely optics, thisisnotstandardpractice. common locationormanipulatedwith all theUVraysaredirectedtoa packaged inanarrangementsothat systems. WhileLEDchipscanbe is nottypicallyapplicableforLED systems employingellipticalreflectors referenced withconventionalUV mind thatthefocalpointcommonly the requiredenergydensity. Keepin arrays willneedtobeusedachieve irradiance. Insomecases,multiple density whilemaintainingaconstant adjusted toachievearangeofenergy the speedoflinecanbothbe of anarrayequippedwithPWMand a greatercuringarea.Thedutycycle The wavelengths emittedfrom 29

UV-LED T E rminoL ogy Anode—positive terminal of an LED. Doped—refers to an LED semiconductor Forward Voltage—the actual voltage material that has been impregnated with across a semiconductor diode carrying a Binning—process of sorting LED chips impurities to produce a specific n-type or forward current. into groups according to peak irradiance, p-type conductivity. wavelength tolerance and forward voltage. Interconnect Cable—electrically Driver Board—a printed circuit board connects the LED irradiator and the Borosilicate—strong, heat-resistant, (PCB) that distributes the DC voltage to power supply unit. colorless, silica glass that contains a the LED chips or modules in an array minimum of five percent boric oxide, Interlock—a temperature sensor and/or and provides the pulse width modulation exhibits exceptional thermal shock flow meter designed into the LED-cooling (PWM) capability of the system. resistance, and transmits a greater system to monitor conditions at the inlet percentage of ultraviolet energy than Duty Cycle—the proportion of ON time in and outlet of the array. If the coolant glass. Common material used for the a pulse width modulation cycle to the total temperature exceeds a specified set point emitting window on a UV-LED head. cycle time (ON + OFF) expressed as a or the flow rate drops below a specified percentage. A low duty cycle corresponds set point, the interlock circuit switches Cathode—negative terminal of an LED. to low power because the power is off off the LED array in order to avoid Chip—a fully functioning, minute slice most of the time, whereas 100% is fully overheating and destroying the individual of a semiconducting material (such as ON, 0% is fully OFF and 50% means that diodes and wire bonds. silicon, germanium and gallium arsenide) the power is ON half the time and OFF LED (light emitting diode)— doped and processed to have p-n junction half the time. semiconductive device containing a characteristics. Specifically, gallium Electroluminescence—an optical and p-n junction designed to emit specific nitride (GaN) is used to generate longer electrical phenomenon inherent to LEDs in narrow band wavelengths within the ultraviolet and blue-visible wavelengths. which a material emits light energy when electromagnetic spectrum via a process In referring to LEDs, chip is often used an electric current is passed through it. known as electroluminescence. When interchangeably with diode, die and a forward bias voltage is applied to the semiconductor. Emitting Window—flat, rectangular piece LED, current flows from the p-side to of quartz or borosilicate typically secured Coolant—liquid circulation material the n-side (anode to cathode). As the at the base of an LED head to protect the that flows over the heat sink in the electrons cross the depletion zone and dies while simultaneously transmitting LED array or module in order to (1) fill a hole, they drop into a state of lower ultraviolet wavelengths. remove wasted heat energy produced energy. The excess energy is released in by the electroluminescence process Encapsulate—a transparent material the form of a photon. The energy of the and (2) maintain the correct operating used to physically protect dies and block photon is directly related to the amount temperature of the LED chips and wire moisture. It can either be flat or shaped of excess energy, while the wavelength bonds. into a convex lens. Modern LEDs use of the photon is inversely related to encapsulates made of silicone, while older Depletion Zone—the non-conductive the excess energy. In other words, the LEDs were made of epoxy resins. Both boundary where the positive and negative higher the excess energy, the shorter the silicone and epoxy resins fully surround sides of a p-n junction meet. wavelength. the LED chip(s). Encapsulates made of LED Array—(1) packaged sub-assembly Die—a fully functioning, minute slice quartz or glass generally do not surround or module typically consisting of multiple of a semiconducting material (such as the LED chip(s), but are used as a hard LED diodes or chips that are individually silicon, germanium and gallium arsenide) protective outer cover. doped and processed to have p-n junction wire bonded to a printed circuit board and Forward Bias—occurs when the anode characteristics. Specifically, gallium then secured to a heat sink; (2) also refers of an LED is connected to the positive nitride (GaN) is used to generate longer to a full curing assembly which includes terminal of a voltage supply and the ultraviolet and blue visible wavelengths. numerous modules or LED chips, as well cathode of the LED is connected to the In referring to LEDs, chip is often used as a cooling fan or tube fittings, a manifold negative terminal. The effect of a forward interchangeably with diode, chip and block, an emitting window and a sheet bias is that the positive holes in the p semiconductor. metal or plastic outer housing. In some region and the negative electrons in the cases, a complete array assembly will Diode—common semiconductor device n region of a p-n junction are pushed also contain the driver boards. The array which is added to a circuit as a means of from opposite directions toward the is similar in concept to a lamp head or restricting the flow of electricity. It can depletion zone. This significantly reduces irradiator in traditional UV-curing systems. generically be thought of as a switch or a the width of the depletion zone causing LED Irradiator, LED Head or LED Light valve. A key property of a diode is that it the electrons on the n-side to respond Engine—a UV-curing assembly which only conducts electricity in one direction. to the attractive forces of the holes on includes multiple LED chips or modules, In referring to LEDs, diode is often the p-side. The end result is the flow of a thermal heat sink, a cooling fan or tube used interchangeably with chip, die and electricity and the emission of photons. semiconductor. fittings, a manifold block, an emitting

© 2013 RadTech International. 30 © 2013RadTechInternational. irradiator. form alargerarrayknownashead or can alsobeassembledtogetherto module isanarray, but severalmodules protection andtoblockmoisture.A encapsulate orlensoverthechipsfor sink. Amoduleoftenincludesasilicone PCB whichisthensecuredtoaheat that areindividuallywirebondedtoa consisting ofoneormultipleLEDdiodes Plug-in-module—packaged assembly the driverboard(s). interface betweentheLEDchip(s)and bonded. ThePCBprovidestheelectrical individual LEDchipsordiodesarewire the LEDmoduleorarraytowhich Printed CircuitBoard(PCB)—partof radiation fromanLEDmoduleorarray. or sometimesfocustheemittedUV Optical Device—usedtoevenlyspread a headorirradiator. together toformalargerarrayknownas several modulescanalsobeassembled moisture. Amoduleisanarray, but the chipsforprotectionandtoblock a siliconeencapsulateorlensover a heatsink.Amoduleoftenincludes circuit boardwhichisthensecuredto individually wirebondedtoaprinted of oneormultipleLEDdiodesthatare Module—packaged assemblyconsisting electroluminescence process. remove wastedheatenergyfromthe operating temperatureand(2)to remain atthecorrect bonds wire and used to(1)ensuretheLEDchips Liquid Chiller—coolingsystem See encapsulate. made ofsilicone,borosilicateorquartz. spread theemittedUVradiation.Often moisture andevenlymanipulateor physically protectLEDchips,block Lens—transparent deviceusedto entire assemblytoanotherdevice. a meansofelectricallyconnectingthe electrically assembledtogetherwith one orseveralchipsphysicallyand LED Package—anassemblycontaining driver boards. outer housingand,sometimes,the window, asheetmetalorplastic energy density(Joules/cm increase ordecreasetheexposuretime of thepulsedirradianceisvariedsoasto same forallpowerlevels,buttheduration thermal compression. and thePCBusingultrasonicwelding or electrical connection between the LED chip Bonding —the methodofmakingan Wire cathode ofthechip. bonds aremadeattheanodeand each LEDchipandthePCB.Thesewire PCB. Therearetwowirebondsbetween connection betweentheLEDchipand Wire Bond—referstotheelectrical gallium arsenideandphosphide. materials includesilicon,galliumnitride, processes. Commonsemiconductorbase created duringthemanufacturing on theimpurity(ordopant)concentration of thesemiconductorvariesdepending chemical composition.Theconductivity an electricalinsulatordependingonits be madetoconductelectricityor Semiconductor—a substancethatcan cycle. Thepeakirradiance(W/cm the proportionofONandOFFtimeper amounts ofelectricalpowerbyvarying efficient wayofprovidingintermediate Pulse WidthModulation(PWM)— mounted onthearray. Alternatively, thedriverboardscanbe unit mayalsocontainthedriverboards. for anLEDarray. Thepowersupply I/O interfaceandACpowerconnection cabinet containingtheDCvoltagesupply, term oftenusedtodescribeanelectrical Power SupplyUnit(PSU)—ageneric reverse direction. to then-side,butitcannotflowin able toflowfromthep-sideofdiode and thecathode(-),respectively. Currentis the diodearereferredtoasanode(+) semiconductor materials.Thetwosidesof can bemadefromthesameordifferent p- andn-typeregions.Theseregions into thesemiconductorlayerstocreate or dopantsareimpregnateddoped semiconductive materials.Impurities diode madebyforminglayersof junction)—a speciallyengineered Positive-Negative Junction(p-n 2 ). 2 ) isthe voltage powersupply isinstalledinthe or withthesupplier tobesure.ADC- UV-LED system’s operatingmanual you shouldalwaysconsultwith the minor variationsontheline;however, given voltage, frequency connection and automatically accommodateforthe 50 or60Hz.Solid-statepowersupplies between 100and240Volts ACateither supply power can typically be anywhere state deviceswherethemainelectrical systems aremorecommonlysolid- powered array. come incontactwiththesurfaceofa maintenance staffdoesnotunwillingly provided toensurethattheoperatoror is recommendedthatprotectionbe be hottothetouch.Asaresult,it a situationinwhichthearraymay the UV-LED equipment,itcancreate is typicallyfinefortheoperationof depending onthedesign.Whilethis heat aroundtheexteriorofarray UV-LEDs, therecanbeabuildupof electroluminescence process.With remove additionalheatfromthe substrates whenitisdesirableto working withextremelyheat-sensitive or exhaustthesystemunlessyouare a result,itisnotnecessarytoventilate than 250nminteractwithoxygen.As generated whenwavelengthsshorter ozone isproducedsinceonly the sideofcaution. review thiswiththesupplieranderron market; however, itisalwaysbestto with theUV-LEDs presentlyonthe UVC arenottypicallyasafetyconcern Potential eyeandskinissuesdueto and variousotherlow-costplastics. sheet metal,tintedpolycarbonate brightness andcanbeaddressedusing to reducevisualdiscomfortdue As aresult,shieldingisonlynecessary it intheUVBandharmfulUVCrange. visible bandwidthrangeandnoneof This rendersallofitintheUVA and currently between350nmand430nm. Power supplyunitsforUV-LED Longer wavelengthsalsomeanno 31

© 2013RadTechInternational. Arethereanyhealthorsafety • Whattypeofcoolantmixturesare • Whatisthenecessarycooling • • If supplying the chiller yourself, what Ifsupplyingthechilleryourself,what • Whatcomponentsdoyouwantto • • What components are supplied Whatcomponentsaresupplied • Comparing LED Comparing Technologies directly withthesupplier. article andshouldinsteadbediscussed specifications arenotcoveredbythis each UV-LED system.Asaresult,these while straightforward,willvarywith I/O interfaceandliquidcoolingcapacity, be higher. will thousands ofLEDs,currentdraw For someapplications more than20ampsforlargerarrays. on themarketvaryfromafewampsto main AClineforproductspresently boards andLEDs. Current drawonthe UV system’s PSUtopowerthedriver Whatterminologyisbeingusedthat • a guidethroughtheintegrationprocess. products onthemarketorcanserveas used asmeansofcomparingUV-LED concerns regardingthecoolant? and UV-LED systemsuppliers? required orrestrictedbythechiller and pressure? wattage, flowrate,temperature capacity forthechillerintermsof supplied withthearray? size tubefittingsordisconnectsare interlock, mountingbracket)? (power supplyunit,chiller, source locallyordesignyourself chiller, interlock,mountingbracket)? power supplyunit,coolingfan, (module, array, interconnectcable, by theUVsystemmanufacturer The interconnect cable connections, The interconnectcableconnections, is confusing. the suppliertoclarifyanythingthat you donotunderstand?Alwaysask The followingquestionscanbe that require that require of your competition. If you ever find of yourcompetition.Ifyouever find opportunity thatmaypropelyou ahead or youcouldcompletelymissan best foryourparticularapplication down adevelopmentpaththatisnot delayed. Worse yet,youcouldproceed progress ofyourprojectwilllikelybe grasp ofthecommonlanguage, emphasized enough.Without afirm importance ofwordusagecannotbe an appendixtothisarticle.The LED termshasbeenprovidedas a glossaryofcommonlyusedUV- the terminology. Forconvenience, make sureyouarecomfortablewith investigative studyofUV-LEDs, Whatisthesupplyvoltageand • Doyouwanta“plug-and-play” • DoestheUVsystemneedtobe • WhatistheI/Ointerface? • Atwhatdistanceshouldthe • IftheUV-LED arraymustmove • Whatlength interconnectcable • IstheUV arraypoweredwithPWM • Canthe arrayberepairedinthe • Whatare thelocalandstatecodes • As you move forward with your As youmoveforwardwithyour entire system? main ACcurrentdrawforthe requires additionalengineering? solution orapartialthat interface? it equippedwithalocaloperator controlled byahostsystemoris the substrateorpartbeingcured? UV-LED arraybemountedfrom what istheminimumbendradius? interconnect cablehigh-flexand to providefullUVcoverage,isthe does theapplicationrequire? better foryourapplication? or aconstantcurrent?Whichis manufacturer? field ormustitbereturnedtothe for useanddisposalofthecoolant? learning curveforallofus.w the technologyandwillhasten be agoodtestforthoseofuspromoting used inconversation,pleaseask.Itwill yourself uncertainofUV-LED words —Jennifer Heathcoteisgeneral for IntegrationTechnology manager, NorthAmerica, in Chicago,Ill. 32 © 2013RadTech International. By Jennifer Heathcote Manufacturing Diode Evolution and UV-LED III Overview Part Figure 1 Figure Timeline for evolution ofLEDs L improve chipstructuresandoptimize new semiconductivecompounds, effort forthelast60yearstoidentify a result,therehasbeenconcerted manufacturing methodsemployed.As to itsmaterialcompositionandthe of anygivendieisdirectlyrelated performance andoperatingefficiency is applied.Thetypeofoutput,overall (UV) wavelengthswhenaDCvoltage visit the “Members Only” area at area Only” “Members the visit please article, of this version unabridged complete, the For III. Part with continuing before so do to want 2010), may you (Sept/Oct Systems” Curing 2010) II— (July/August “Part or Measurement” and I—Operation “Part read yet not have If you UV-LEDs. behind engineering and science the regarding information technical and principles key consolidate to designed series athree-part in installment third the is article abridged This infrared, visibleorultraviolet engineered toproducediscrete ight EmittingDiodes(LEDs)are www.RadTech.org. yet totakeus. potential ofwherethetechnologyhas that onecantrulyappreciatethe is involvedinproducingLEDemitters, technology hascome,aswellwhat with anunderstandingofhowfarthe employed inchipproduction.Itisonly well as an overview of typical processes brief historyofLEDdevelopmentas purpose ofthispaperistoprovidea reliable manufacturingmethods.The core technologyaswellimplement trying tounderstandandoptimizethe developers arestillhardatwork is relativelynew, andengineers as asteadydeclineinunitcosts. increased operatingefficienciesaswell consumptions, brightercolorsand to improvedyieldrates,lowerpower infrared ranges;andhasdirectlyled the longerwavelengthvisibleand activity hasbeenconcentratedin processing techniques.Muchofthis Brief History Brief man-made semiconductor compounds aneverincreasing portfolioof with experimenting fornearly60years scientists havebeenformulating and By comparison,UV-LED technology Engineers andresearch : 33

that emit infrared, visible and, most 50 years as most of the production processing that is not generally known recently, UV wavelengths. These issues have been resolved. Yield to the public. As a result, the following compounds have been engineered to rates for white and visible spectrum sections are meant to give the reader embody specific electrochemical and high-brightness LEDs are increasing some general insight into a typical LED emissive properties, while operating rapidly; although, there is still room for manufacturing process and are by no at increasingly higher efficiencies improvement. The current quality of means meant to be fully comprehensive. and consuming less power. Since UV-LED production, unfortunately, is The Ingot each compound emits a different not very good and much optimization The manufacture of LED wavelength and/or amount of energy, work remains to be done. With UV- semiconductors starts with the an entire portfolio of materials is LEDs, there is a significant amount of production of a long, cylindrical ingot, necessary in order to produce dies scrap generated during production that also called a boule. The ingot is typically for various applications. The final has a direct impact on the final die cost formed to diameters of 2, 4, 6, 8 or 12 diode performance determines and performance. inches and can be grown to any desired which compounds are used for which Every single stage and step of the length. The material properties are applications. Some of the more widely production process—from raw material uniform throughout the ingot, and the used compounds include Aluminum selection through to packaging—is crystalline structure contains relatively Gallium Arsenide (AlGaAs), Gallium critical. Each has the potential to few impurities or contaminants. This is Arsenide Phosphide (GaAsP), and introduce defects or foreign matter, achieved through precision temperature Gallium Nitride (GaN). It should be resulting in lower efficiencies and and pressure control in a clean emphasized that research in this field insufficient performance in the final environment, and by using only the is far from exhausted, and there are dies. As a result, numerous quality highest purity of raw materials. presently complex high brightness assurance checks are performed, and Formation of the ingot begins when alloys still in development, including most processes take place in Class 1 high-grade raw materials are mixed GaN/SiC, GaN/Sapphire, GaN/ZnO and and Class 10 clean rooms. Each and together in a specially designed reactor GaN/B-GaN/pAIN. Very preliminary every step of the manufacturing chamber as illustrated in Figure 2. research into materials that emit process, therefore, is a distinct and When elevated temperatures and shorter wavelength UV-A, UV-B challenging engineering and quality pressures are applied, the materials and UV-C is also currently underway. improvement project that takes years, combine to form a uniform molten A timeline summarizing key if not decades, to perfect. solution that is covered with a layer An overly simplified description of breakthroughs in development of liquid boron oxide to prevent this process can be segmented into since the discovery of the first vaporization. A rod or chuck is then three parts that focus on the ingot, semiconductive compound, Silicon lowered toward the solution (2-a). On wafer and die. It should be noted Carbide (SiC), is illustrated in Figure 1. the end of the chuck is a tiny, purified that there are many variations to the seed that contains the exact properties Semiconductor Manufacturing production methods described; not of the mixture. When the seed reaches The spectral emission, electrical- all steps have been included in this the solution, a chemical bond begins to to-optical power-conversion efficiency, document; and each manufacturer form between the seed and the molten unit cost, quality, yield rate and likely incorporates proprietary material (2-b). lifetime hours of each die depend not only on the substrate material, but also Figure 2 on each additional processing step and manufacturing method employed. As a Forming the ingot result, significant investment has gone into developing and controlling the production of LEDs. Actual manufacturing yield rates vary significantly depending on the type of LED. Yield rates for standard infrared and visible LEDs have improved drastically over the past

© 2013 RadTech International. 34 © 2013RadTech International. involves moving the wafer over a plane the waferoveraplane moving involves Lapping isaphysical processthat damaged particlesfromthesurface. carefully removeunnecessary or and/or etchingprocessinorder to wafer issubjectedtoalapping illustrated inFigure4.Afterslicing, the microns thick(10mils).Thisis substrates thatarelessthan250 the ingotintocrystallinewafersor The Wafer ingot isgroundandpolished. cone andtheremainingcylindrical a diamondsawisusedtoremovethe moving to the next stage of production, ingots isprovidedinFigure3.Before material (2-d).Aphotoshowingtypical characteristics oftheoriginalseed result isaningotwithallthephysical are removedfromthechamber. The is reached,theseedandbondedingot of theingot.Whendesiredgrowth effect onthediameterandquality and extractionspeed,allhaveadirect and pressures,aswelltherotation maintained. Theappliedtemperatures as longtheprocessconditionsare crystals continuetogrowinlength to thedesireddiameter(2-c).The that tapersfromtheseedoutward the ingottakesshapeofacone and solidifyontheseed.Thetopof some ofthemixturebeginstocool at aslowandconstantrateofspeed, Figure 3 Figure A diamondsawisusedtoslice As therodisrotatedandwithdrawn Fully formed ingots Fully additional layersofsemiconductive Phase Epitaxyisusedtogrow remaining dirt,dustorforeignmatter. based methodsinordertoremoveany cleaned usingultrasonicorsolvent- diodes. Asaresult,allwafersare effect ontheperformanceoffinal Unclean wafersalsohaveanegative functioning ornon-functioningLED. or polished—willleadtoapoorly when thewaferislapped,etched the ingotisformedandsliced—or semiconductive material. more receptivetoadditionallayersof processing andmakethewafers efficiency andqualityofdownstream and polishedinordertoimprovethe measured; andthewafersaresorted is wafer each of thickness andflatness surface. Next,the smooth a ensure to used are unwanted material. Both intended todissolve process chemical a is etching of liquidabrasive,while Figure 5 Figure 4 Figure Next, aprocessknownasLiquid Any imperfectionscreatedwhen Wafers beingsliced from ingot Wafer processing—liquid phaseepitaxy increase thequantityofdopantsor in sequentiallayersorallatonce. either begrownonthewaferstructure produce thedesiredeffect,theycan If multipledopantsareneededto contain oneorseveraltypesofdopants. as illustratedinFigure5.Ameltcan reservoirs ofmoltenmaterialormelt passing eachwaferunderneathvarious microns thick,andtheprocessinvolves electronic density. means thateachlayerhasadifferent however, theexistenceofdopants same crystalorientationastheingot; applied. Allepitaxiallayershavethe altered outputwhenavoltageis structure ofthedieresultinginan dopants interferewiththecrystalline and efficienciesofthefinaldiodes.The improve theemissivecharacteristics added impuritiesordopantsthat layers oftencontainintentionally material ontopofeachwafer. These Sometimes itisnecessarytofurther Each epitaxiallayerisseveral 35

Figure 6 Figure 7 Wafers in front of furnace tube entrance Processed wafer

and metallic contact pattern. Figure 8 shows one LED die resting on the face of a U.S. penny. This single die would have been one of many cut from a 2-, introduce new dopants into the wafer of the semiconductor layers. It will be 4-, 6-, 8- or 12-inch diameter wafer structure after the epitaxial layers are removed in a later step. using a laser or diamond saw. grown. This step is often administered Contact metal is then evaporated After the dies are extracted from using a continuous diffusion furnace or onto the wafer in the areas where the the wafer, a forward voltage is applied furnace tube where dopants are forced resist was removed. This is done in to each and every die, and the output into the upper layer(s) using high- a high-temperature, vacuum-sealed is measured using an integrating temperature gases. A photo of one chamber flooded with a mixture of sphere. It is only at this time that such furnace is shown in Figure 6. hydrogen and nitrogen gas. High manufacturers know whether the The next step is to add conductive temperatures are used to vaporize production process was successful. metal contacts or circuitry to the the metal contact material inside the Each functioning LED is then binned wafer. This may be done all at once chamber. Once the metal is vaporized, according to wavelength, peak or built up sequentially by repeating the chamber conditions are adjusted in irradiance and forward voltage. the following photoresist and vapor order to condense the metal onto the For additional information on deposition process several times. wafer in the areas where the resist was integrating spheres and binning, First, a light-sensitive, liquid material removed. The remaining resist is then called a photoresist is applied to the stripped in a chemical bath leaving just Figure 8 top surface of the wafer while it spins. the epitaxial wafer and metal contacts. Spinning ensures that the material is Finally, the contact pattern is annealed Completed individual evenly distributed across the entire to the wafer in a high temperature die shown in surface. Next, the wafer and applied furnace over a period of several hours. proportion to a U.S. resist are baked at a low temperature The end result is a chemical bond penny in order to harden the resist surface. between the semiconductor and the A mask (also called a “reticule”) contacts. An example of processed containing the metallic contact wafer is provided in Figure 7. pattern is placed over the wafer. The entire wafer is then exposed to The Die (Chip, Diode, UV light. All exposed areas of the Semiconductor) resist material that are not blocked by Depending on the diameter, a the reticule cure underneath the light fully processed wafer is made up of and are subsequently washed away thousands or even tens of thousands of Individual die in a developer bath. The resist that individual light-emitting diodes all with was under the mask remains on top the same general material structure

© 2013 RadTech International. 36 © 2013RadTech International. Conclusion Overview PartII—UVCuringSystems.” UV-LEDs wasdetailedin“UV-LED or othersupplier. Thepackagingof by thefinalequipmentmanufacturer case, thepackagingisdonedirectly dies withoutfurtherpackaging.Inthis manufacturers willselltheindividual Alternatively, somesemiconductor the LEDstomarketasmodules. package thediesitselfbeforeselling semiconductor manufacturerwilloften to theintendedapplication.The they mustbepackagedaccording Operation andMeasurement.” refer to“UV-LED OverviewPartI— of electroluminescence;83years 104 yearssincethediscovery commercialization ofCarborundum; Silicon Carbide;118yearssincethe 187 yearssincetheinventionof After thediodesarebinned, The evolutionofLEDshasspanned These breakthroughswillnot and costswillbecomemoreattractive. increase; lifetimehourswillimprove; will begenerated;efficiencies become available;higherirradiances to improve.Morewavelengthswill UV-LED performancewillcontinue over. Duringtheforeseeablefuture, this isajourneythatstillfarfrom all theamazingaccomplishments, manufacturing methods.Despite invention andoptimizationofprecision of diversediodestructures;andthe unknown compounds;thecreation extensive developmentofpreviously remarkable journeythathasinvolved the market.Ithasbeenalongand curing systemswerefirstofferedto lab; andeightyearssinceUV-LED the firstUV-LED wasproducedina man-made compounds;19yearssince initiative toengineersemiconductive conceptualized; 56yearssincethe since thelightrelaywasfirst 4. 3. 2. 1. References possibilities. w the technology’s futureandallits make ithardnottobeexcitedabout historical contextofLEDevolution recent advancementsevaluatedinthe necessarily happenovernight,but Silicon WaferProcessing.IISME. Light-Emitting Diode.HowProducts Held, Gilbert.IntroductiontoLight Schubert, E.Fred.Light-Emitting Processing.pdf” “http://iisme.org/etp/Silicon_Wafer_ Summer 2000.April4,2011. LED.html” com/Volume-1/Light-Emitting-Diode- April 4,2011.“http://www.madehow. Are Made,Volume 1.2006-2011. Publications. 2009. Applications. Florida,USA:Auerbach Emitting DiodeTechnologyand University Press.2008. Diodes. Cambridge,UK:Cambridge —Jennifer Heathcoteisgeneral for IntegrationTechnology in manager, NorthAmerica, Chicago, Ill. 37

UV-LED Curing Systems: Not Created Equal

By Sara Jennings, Bonnie he ultraviolet (UV)-light emitting solvent-based formulation systems are Larson and Chad Taggard diode (LED) curing market has well documented. The advantages of enjoyed considerable growth UV-LED curing over UV curing using Tover the past several years as both traditional mercury lamps for specific new and existing markets recognize applications have also been well the inherent benefits of UV-LEDs. documented in previous articles. This paper will focus on the potential Most, though not all, UV-LED lamps differences in UV-LED systems. are focused on the UV-A range and the Additionally, measurement methods ability to generate high-irradiance UV that can be used to contrast and light with sufficient energy levels to compare the differences in performance cure most materials. However, due to of the various available UV-LED based its focused wavelength characteristics, systems will be discussed. The reader the typical UV-LED lamp doesn’t can then quantitatively determine generate UV-B, UV-C or even infrared the performance of a UV-LED curing emissions that are sometimes useful system and see that not all UV-LED for certain curing applications. With curing systems are created equal. the wide variety of inks, coatings and adhesives having been formulated to UV-LED Curing Advantages take advantage of UV-LED curing, Application after application across there are no substantive reasons many market segments have moved that UV-LED curing systems will not to UV curing using photopolymer continue to enjoy rapid growth in the chemistry and away from solvent- marketplace. based formulations that contain volatile organic compounds (VOCs) and require Components and Comparisons large, power-hungry furnaces. The UV-LED curing lamp systems consist advantages of UV-curable formulation of multiple sub-components which systems versus conventionally cured taken together can be used to define Table 1 UV-LED light source components

Component Purpose I LED Solid-state component that generates UV light. II Array Grouping of LEDs to maximize UV output to achieve desired curing rate. III Thermal Cooling A properly designed thermal management system for the removal of heat generated by LED array to ensure low operating temperature and long life. IV Optics The shaping, molding, reflecting and guiding of the UV-LED light to insure maximum light reaches the media.

© 2013 RadTech International. 38 the system’s overall performance. The key design subcomponents are outlined Figure 2 in Table 1. Wavelength characteristics UV-LED Light Source Components Examining these components in closer detail along with their interactions and interdependencies will provide the reader with a better understanding of how UV-LED curing lamps are not created equal.

I. LEDs—The Base Building Block Let’s start with the LED. As the base building block, this is the first choice a UV-LED lamp supplier has to make. It is a critical choice that impacts the remainder of the system’s architecture and design. A pictorial example of an LED’s construction is shown in Figure 1. Figure 3

LED Construction Wavelength comparison Simply put, an LED is a solid-state device that produces light when an electrical current is allowed to flow from the positive (anode) side of the circuit to the negative (cathode) side. Not all LEDs are built the same nor do they exhibit the same characteristics. UV-LED lamp suppliers have critical choices to make as to the quality, type, material and shape of LED for their systems. Key LED characteristics considered by each UV- LED lamp supplier include wavelength and UV output. Wavelength The wavelength emitted from an LED is controlled using differing amounts of dopants such Chemistry plays a significant role in as aluminum, gallium or this discussion. Some applications, due Figure 1 indium derivatives during the to their specific chemistry, require a LED construction manufacture of the LED. The given wavelength. However, for many general rule of thumb is that applications a small shift in the peak the shorter the wavelength, wavelength will have little impact as the lower the peak UV output the photoinitiator that kicks off the available from the die, as reaction has a broad absorption range. shown in Figure 2. For example, as you can see in The UV-LED supplier must Figure 3, three-fourths of the LED weigh the trade-offs between energy output (with a peak at 385 nm wavelength and the associated versus a peak at 395 nm) share the total energy with cure rate. same wavelength band. Material testing

© 2013 RadTech International. 39 was not the major factor in improved Figure 4 peak irradiance over time. The other three factors (arrays, Contributing factors of peak irradiance cooling and optics) significantly improvement outweigh the increase in LED capability. This answers the question asked by UV-LED naysayers, “If all LED suppliers are eating from the same bowl, then won’t all the products essentially be the same?” Therefore, let’s continue examining the other components that make up the system.

II. Array—Grouping of LEDs Arrays are the second area in which suppliers can begin to differentiate their product offerings. confirms that the difference in either vendors have improved from 2005 How the LEDs are combined; the cure rate or quality when using die to 2011 with a compound annual growth number and type of LEDs chosen; with peak outputs centered at 385 nm of 5-10%. This improvement shows the the shape of the array; the method and 395 nm is negligible. LED vendors have and will continue to of electrically connecting the LEDs; Therefore, UV suppliers will improve the output of UV-LEDs, which and even the size of the LEDs all typically select the longer wavelength only provides a better foundation for the have significant impact on the to achieve the highest UV output UV-LED curing lamps that utilize them. performance of the system. that allows for higher application While it would be tempting to jump Most applications require UV-LED throughput. to the conclusion that UV-LEDs are the curing systems that consist of more single biggest contributor to UV-LED UV Output than one LED or LED array in order The output of a single UV-LED lamp performance, Figure 4 shows to achieve not only the desired is measured in milliwatts (mW) at a that UV-LED curing system suppliers throughput but to meet the demands nominal input voltage and current. have more opportunity to differentiate for curing applications where the UV-LED output has shown considerable themselves in the areas that go beyond media can be 1-2m wide. Therefore, improvement in recent years where the base LED. A close examination of the a key question is “can the LED specifications for LEDs from various LED performance, while contributing, array be uniformly scaled?” UV-LED curing lamps can have a continuous Figure 5 scalable array that provides for better uniformity or a discrete array Typical LED array package that can be scaled, but doesn’t provide the same uniformity of output. See Figure 5. This is an area where UV-LED lamp suppliers can differentiate themselves based on the LED suppliers’ architecture, modules and their own engineering capability where two suppliers can take the same batch of LEDs and achieve very different performance in the end product.

© 2013 RadTech International. 40 toward increasing the number of Figure 6 processing cores at lower clock speeds to stay within functioning thermal UV-LED energy efficiency thresholds. UV-LED lamps face a similar challenge. As the quality of LEDs improves and the irradiance increases, so does the need to remove the heat. Original equipment manufacturers (OEMs) and end-users do not want to spend more on the cooling of the lamps than the lamps themselves. Thus, the third area of differentiation is in the cooling technologies and capabilities that suppliers choose.

IV. Optics—Guiding the Light The final component and one of the most important differentiators is optics. The science/art of optically improving the LEDs to maximize their UV output III. Thermals—Keeping it Cool It is important to note that as is key to the lamp’s final capability. The third component is cooling. the LEDs emit more light, they also Based on the end application, the As any reader knows after using a generate more heat, which must be optical engineer has to decide what notebook PC on their lap for a length managed. Thus, in the race to build shape, form and material best utilizes of time, the byproduct of solid-state even higher irradiance products, the the LED’s unique characteristics. Next, devices is heat. UV-LEDs transfer about ability of suppliers to control and they have to balance the fact that LEDs 15-25% of the received electrical energy remove heat has become more crucial are a “flood” type of light, unlike a into light with the remaining 75-85% to building reliable systems. This is focused mercury lamp where the light is transferred as heat; thus, the need to analogous to microprocessors where captured by a reflector and directed to a cool the LED arrays. See Figure 6. heat became a constraining factor (due specific-point focal length. See Figure 7. Currently, UV-LED arrays are cooled to increasing gigahertz performance) The optical engineer is challenged with either air or liquid. Table 2 lists a by increasing the number of transistors to use methods to ensure the comparison of the two most common while decreasing the trace width. maximum amount of light “escapes” methods used for cooling LED arrays. Manufacturers eventually turned at the desired irradiance through the Table 2 Air versus liquid-cooled light sources

Air-cooled Liquid-cooled Less expensive total UV light source More expensive due to need for external cooling source. solution. Lower irradiance levels as irradiance Higher irradiance levels as water’s thermal conductivity is is directly proportional to ability to cool higher than air’s (0.6 vs 0.025 W/(m·K)), which means water the LED array. Air is not as efficient at cannot only absorb more heat, but can do it faster than air. cooling. For given irradiance, larger lamp size due For given irradiance, UV source and cooling mechanism are to fan size. separated allowing a smaller lamp size as no need for a fan.

© 2013 RadTech International. 41 window/glass toward the material. Figure 7 LED lamp suppliers have used various, confidential methods to maximize the Traditional mercury lamp optics versus LED optics UV-LED light. A high level summary of optics typically used by UV-LED suppliers with their pros and cons is shown in Table 3. As shown, this is a small subset of the myriad of choices an UV-LED supplier has to make concerning the light distribution generated by their chosen LEDs, whether individual diodes or previously assembled. Hence, it is the third major area for differentiation among UV-LED suppliers.

Measuring the Differences Between UV-LED Curing Systems Regardless of the LED, array, thermal and optics design employed, the end result that matters to end-users Table 3 UV-LED optical options

Optics Pros Cons Example Macro—LED array High peak irradiance LED array cannot be scaled inside reflector optic over small area. uniformly.

Micro—Each Can be scaled uniformly. LED-to-LED spacing and, packaged LED has therefore, maximum UV output an individual optic limited by packaged LED size.

Integrated Optic— Increased optical Expensive and array is hard to Optic part of LED efficiency. scale uniformly. formation process

Directional optic Increased peak Optics configuration limits number irradiance over narrow of LEDs that can be configured in band. system, limiting total available UV output.

Scalable micro optic SLM module can be Light is not focused and diverges scaled uniformly while over distance. maintaining high peak irradiance.

Peak Irradiance Energy Density Definition Radiant power per-unit-area Radiant power per area per unit time Measurement Watt per centimeter squared (W/cm2) Joules/cm2 or mJoules/cm2 Impacted by Distance from material Material speed Emitting window size

is that their material is properly cured. method will allow OEMs and end-users specification point of reference? The two measurement parameters for to properly characterize the UV-LED 2. Over what area is the peak this are peak irradiance and energy curing system. irradiance being delivered? density (sometimes referred to as OEMs and end-users should Table 5 shows some of the typical “dose”), and are outlined in Table 4. consider two key questions when measurement locations for measuring/ These two parameters work together measuring UV-LED Lamp output: specifying peak irradiance and the pros and understanding their measurement 1. Where is the peak irradiance and cons of each approach. Table 5 UV-LED measurement options

Usefulness Location Pros Cons Example Poor At the Gives some Cannot be measured. Photon emitted at LED indication of the base No practical application. Junction LED, but this is only a small component of the performance of a UV-LED curing system.

Better At the Most relevant to Each customer’s operating Irradiance vs. Distance media end-user. distance can be different and, as noted above, the emitted UV light is divergent which means even though there is UV light, the measured peak changes with distance.

Best At the Consistent metric Where on the glass should emitting regardless of the irradiance be measured? window application. In the middle? At the edges? The corners? Average across various locations?

© 2013 RadTech International. 43 in fact, only 2 W/cm2 is delivered at Figure 8 the edges. Figure 9 shows a 3D model of Irradiance profile a wide area source and a narrow emitting source that have the same peak irradiance, but that deliver very different total energy to the material, which is the topic of the next section, energy density.

Energy Density Energy density can be a very misunderstood concept and is also variously called density, dose or exposure. Energy density is the time integral of irradiance; thus, the higher the peak irradiance and/or the longer Figure 9 the exposure time, the higher the energy density. Consequently, even Uniformity along emitting width with the same lamp unit operating at the same peak irradiance and same distance, media exposed at different belt speeds do not receive the same energy density. Conversely, even as the measured peak irradiance decreases with distance away from the media, if the media’s exposure time remains the same, the measured dose remains the same. This decreased peak irradiance is due to the divergent nature of the LEDs. OEMs or end-users could be misled by a single number that was taken Figure 10 along a single axis. Knowing the location of the measurement and how Energy density versus height (same speed) that measurement metric changes over the UV emission area will give the best overall characterization of the UV-LED curing system. Figure 8 is a thermal image which depicts a UV emission area. The center is the maximum UV irradiance and as the emission “falls” off from the center the irradiance impacting the substrate decreases, which is shown as the series of concentric circles. Each color is a lower irradiance value. The impact to the OEM or end-user is they may believe they have purchased a UV-LED system that delivers 4 W/cm2 across the entire emission area when,

© 2013 RadTech International. 44 lead to measurement errors and should Figure 11 therefore not be used to set irradiance and dose specifications. LED versus arc lamp wavelength The spectral characteristics of UV- LED lamps are significantly different than traditional systems and UV meters are just coming onto the market that will accurately measure UV-LED lamps. Even then, radiometers need to be calibrated for specific LED characteristics of the lamp manufacturers. A “generic” UV-LED radiometer that can be used between different UV-LED lamps does not currently exist. For process control, it is important for OEMs and end- users to utilize a UV-LED radiometer that is calibrated to the UV-LED lamp provider’s specifications. Otherwise, false readings and/or improper The light spreads out as the distance is There are several manufacturers conclusions are the likely results. increased, but the total amount of light that provide products to measure As shown, measuring irradiance delivered to the surface stays the same. irradiance. Most of these were and energy density is not a simple This is an important point. So said converted from mercury lamp task. The authors believe the industry, another way, for a given media speed, measurement devices and have not including UV-LED lamp manufacturers, altering the height of the UV-LED light fully comprehended the unique LED measurement device manufacturers, source from the media does not change characteristics. The sensors used in OEMs and end-users should align the total amount of light delivered radiometers have been characterized around a single industry standard that to the surface, but rather the peak and calibrated to work with the output can be used to consistently, accurately irradiance decreases. profiles of mercury lamps (Figure 11). and succinctly report irradiance and To show this graphically, see Since UV-LEDs have a very different energy density measurements. Figure 10. The red curve has a peak output profile, the sensor calibration irradiance of 8 W/cm2 while the green for a given wavelength band is the Result: UV-LED Lamps Aren’t curve shows a peak irradiance of most important characteristic. A Created Equal 5 W/cm2. The key is that the area radiometer that crops or doesn’t count UV-LED lamps are not created under the curve is equal. The peak all of the UV emission based on a equal. Suppliers of UV-LED lamps irradiance is lower but the overall normal LED wavelength tolerance can have significant architectural and energy density remains the same. implementation decisions that The quickest way for an OEM to significantly impact their product’s improve the speed of their machines is performance. The end result will be a to either (1) utilize UV-LED lamps with UV-LED curing system with optimized higher peak irradiance or (2) utilize LEDs, arrays, optics and cooling for a UV-LED lamps with larger arrays. specific application. Knowing how to Either of these will deliver more total energy density to the curing surface, and allow faster cure speeds.

Measuring Irradiance and Energy Density Lastly, what device should be used Different types of UV-LED lamps. to measure UV-LED lamp output?

© 2013 RadTech International. 45 characterize the performance allows are created equal. It is vitally important Measurement,” RadTech Report, the user to identify the best overall they consider the needs of their July/August 2010, 23-33. 3. Heathcote, Jennifer, “UV-LED system to meet their specific needs. application as well as the capabilities Overview Part II—Curing Systems,” OEMs and end-users would be wise to of their supplier. Lastly, the authors RadTech Report, September/October learn these differences and ensure their believe the UV-LED industry must band 2010, 31-42. 4. Karlicek, Robert F. Jr., “UV-LEDs and chosen suppliers are capable of not only together to create industry standards Curing Applications: Technology meeting their needs today, but have the and capabilities that simplify OEM’s and Market Developments,” RadTech technical ability to design, manufacture transition to a bright UV-LED future. w Report, November/December 2009, 17-23. and support their needs in the future. 5. Karsten, Rob, “Environmental This article has attempted to build —Sara Jennings, Bonnie Larson Benefits of UV-LED Curing,” Seventh and Chad Taggard are part of on previous work by highlighting the International Wood Coatings Congress the marketing team at Phoseon Amsterdam, The Netherlands, Oct. 12, myriad of architectural and design Technology in Portland, Ore. 2010. trade-offs UV-LED lamp makers have at 6. Mills, Paul, “UV-LEDs: Separating their disposal. More importantly, OEM Myth from Reality in UV-LED References Curing,” RadTech Webinar, January and end-users considering the transition 1. Beck, Michael, “UV-LED Lamps: 26, 2010. from mercury tubes to solid-state UV- A Viable Alternative for UV Inkjet 7. Raymont, Jim, “Establishing and Applications,” RadTech Report, LED technology must understand that Maintaining a UV Process Window,” November/December 2009, 27-33. RadTech Report, May/June 2002, 14-25. (1) UV-LED isn’t for every application 2. Heathcote, Jennifer, “UV-LED 8. Yole Developpement, UV-LED Report, and (2) not all UV-LED lamp systems Overview Part I—Operation and March 2011.

By Jim Raymont and This article is based on a submitted UV-LEDs are narrow band sources. Abhinav Kashyap paper and presentation delivered Production UV-LED sources have their at the RadTech 2010 Conference in spectral emissions somewhere in the Baltimore, Md. 365 to 405-plus nm region. UV-LEDs ultiple articles and papers are described and identified by their are available that discuss the most dominant (395 nm, etc.) spectral construction, advantages and output. If measured with a spectral Mdisadvantages of Ultraviolet (UV) Light radiometer, the user would see that Emitting Diodes (LEDs) sources. This the manufacturer has binned the article will focus on understanding and individual LED chips so that the most measuring the output of UV-LEDs. intense UV output is clustered around Medium-pressure UV lamps the dominant name line (i.e., 395 nm (microwave and arc) emit radiation if the source is a 395 nm source). It across a broad electromagnetic would be very expensive to only use spectrum. Output from these types 395 nm LED chips in this example of sources includes UV, visible and and, in reality, it is common for the infrared (IR) energy. actual spectral emission of a UV-LED to extend plus/minus 8-15 nm in either direction at the half-maximum power point from the maximum.

Autobond inkjet spot varnish system with UV-LED curing. Courtesy of Autobond Ltd.

© 2013 RadTech International. 47 Courtesy of Integration Techonology Courtesy of Integration Techonology 48 ) and energy 2 ) values expected. 2 assembled modules. TM It is also important to understand, With traditional UV sources, it is traditional UV With document and maintain your UV-LED document and maintain your UV-LED system. The spectral output of LEDs is described in nanometers (nm) such as 395 nm. The actual plus/minus range of the spectral output of the For each application, a balance needed a balance For each application, between the amount to be found other types of radiation of UV and the produced, along with (visible, IR) application, formulation, substrate, and desired needed processing speed or “process results. This balance to be found with window” also needs generated from applications using UV LED sources. document andimportant to understand, This includesmaintain your UV system. mercury-iron, bulb type (mercury, mercury-gallium); how the system is set up (focused, non-focused, additional equipment such as quartz plates); and the irradiance (W/cm density (J/cm UV-LED modules. UV-LED LEDZero Solidcure Courtesy of Integration Technology As discovered with arc Many were quick to realize lamps, increasing the applied power or amount of UV delivered to the cure surface was not always beneficial to the cure process or substrate. that comparing the power applied to the lamp is not as meaningful a measure in the curing process as measuring the amount of useful UV energy delivered to the product. Sometimes, for design and engineering reasons, a UV source with higher applied electrical power actually delivered less usable UV. and ’80s, a lamp with more “applied electrical power” certainly “had” to be better than a lamp with less applied A system with 400 electrical power. watts/inch of applied electrical power was certainly better than a system with only 200 watts/ Which inch of applied power. company would be the first to reach 600 watts/inch of applied power? 800? 1,000? of 2 . 2 of irradiance, 2 Relative UV-LED output for various wavelengths wavelengths various output for UV-LED Relative Output irradiance is usually one of A little closer to our UV world, do UV-LED sources continue to UV-LED Measuring and characterizing Figure 1 the first numbers an LED manufacturer LED an numbers first the will share with you. Do the discussions (and claims) of increased output from sound similar to discussions UV-LEDs on computer processor speeds, computer memory sizes or the number of megapixels from a digital camera? the discussions (and claims) sound similar to discussions (and claims) that were held in the ’70s and ’80s with lamps? In the ’70s traditional UV-arc with some systems going over or in the neighborhood of 10W/cm irradiance to Watts/cm increase in power. The output of UV- The output of UV- increase in power. LEDs has gone from milliWatts/cm LEDs requires an understanding of your UV source and UV measurement instrument. Instruments used to measure broadband, medium-pressure UV microwave or arc lamps may or may not be suitable for use with UV- LED sources. © 2013 RadTech International. UV LED Output Power—Déjà Vu Vu LED Output Power—Déjà UV All Again? Over of physics and photochemistry do not cease to exist when you use LEDs. They are present and lurking but can be minimized by taking some precautions: • During process design and testing, establish how you are going to measure the UV output of the LED. • Define the key process variables that need to be monitored and controlled in production. • Establish your process window in the lab and carry it over to production. • Exercise caution when you communicate radiometric values Power supplies being assembled for a UV system. either within your company or to your supply chain. LED will vary from manufacturer to surface while others measure at • Specify the process you used manufacturer. the cure surface. Ask questions and to obtain the readings and the UV-LEDs are being packaged in a evaluate the equipment carefully; instrument/bandwidth used. wide range of shapes and sizes—from making apples-to-apples comparisons. large discrete devices to hundreds of • Determine how often you need to nearly microscopic individual LED The Lab-to-Production Transition take readings based on your process. dies arranged into powerful arrays. Is Work • While it is true that LEDs will last Manufacturers of UV-LED sources How a specific UV-LED system longer than many other types of UV use a variety of techniques to assemble, performs for your application is more sources, be aware of anything in the direct and deliver the UV energy to important than the maximum power process between the LEDs and the the cure surface from the actual LED output number on a sheet of product cure surface that could change and “chip” or “die.” Manufacturers have literature. Do you get the results that alter the amount of UV delivered to proprietary processes to “bin” LEDS you are looking for at the manufacturing the cure surface. speed that you need for production? by their spectral output, forward Absolute values established during There has been impressive progress voltage and intensity. the design phase often become relative made in the development of coatings Many UV-LED systems were readings during day-to-day production. that are specifically formulated to work developed for a specific application With relative readings, you are looking with LED systems. Because LEDs are and fit into areas that will not support for day-to-day or week-to-week other types of UV technologies. relatively monochromatic, they lack changes and working to make sure that Manufacturers are concerned with the shorter UV-C wavelengths that are the UV levels stay within the process keeping the array stable over time. Ask traditionally used to establish surface window established during process questions and evaluate the equipment cure properties such as tack, scratch, design and testing. carefully. In the “more is better” stain and chemical resistance. This is approach, manufacturers of LED not the show limiter/stopper that it Measurement of UV-Arc and systems may use different techniques once was and you need to work with Microwave Sources to determine the power rating of both your formulator and the LED Radiometers used for measurement their systems. The techniques can supplier to achieve the properties of UV-arc and microwave sources have include theoretical calculations of the desired in the final cured product. bandwidths (UV-A, UV-B, UV-C, and output and measurement of the UV at You do not get a free “go directly UV-V) that match the broadband arc different points. Some manufacturers to production manufacturing” pass and microwave sources. Instrument may measure the output at the chip when working with LEDs. The laws bandwidths vary from manufacturer

© 2013 RadTech International. 49 Courtesy of Integration Techonology to manufacturer. Some instruments have “narrow” bands (UV-A classified between 320-390 nm) while others have “wide” bands (UV-A classified between 250-415 nm). Because of these differences, it is important to specify the instrument used to obtain the reading. What happens if you use these popular radiometers to measure the output of an LED source? Will you get a reading with one of the radiometer bandwidths above with a UV-LED? It depends on the type of UV-LED and the bandwidth(s) of the instrument. Just because there are values on the instrument display does not mean UV-LED arrays. that the UV-LED has been properly characterized. directions but LEDs do not follow There has been an incredible equal radiant flux rule. This is amount of research done in recent LED Intensity Measurement and because most of the LEDs have years at various public and private Challenges micro-optics built into the LED organizations for developing The same challenges that exist for packaging. measurement techniques. Due to measuring visible LEDs also exist for several variances affecting intensity measuring UV-LEDs. The light emission • LEDs do not follow the inverse measurement of an LED, the from an LED is vastly different than a square law (i.e., intensity of light Commission on Illumination (CIE) point source. This poses challenges in reduces by square of the distance) established a standard method guide quantifying its intensity. Limitations in similar to extended sources. That for LED measurement document (CIE standardizing the measuring techniques means that even for the same solid 127:1997). of LEDs include: angle, intensity measurements could vary with distance and could • A point source, by definition, RadTech UV Glossary Definition: has a constant radiant flux in all be unpredictable. Flux (radiant flux): The flow of photons, in einstein/second; one Figure 2 einstein = one mole of photons. Spectral output of H bulb and spectral response of One of the most popular ways different UV bands of radiometer of measuring radiant flux for an LED is using a photometer at a specified distance and specified area recommended by the CIE. Individual LEDs may be characterized this way under controlled laboratory conditions, but the recommended procedure cannot be easily applied to LED clusters and arrays (the arrangement of UV-LEDs used for UV production curing applications). In order to measure total radiant flux, an integrating sphere is used. CIE 127:1997 describes the placement of an

© 2013 RadTech International. 50 LED in a calibrated integrating sphere and measuring total radiant flux. When Figure 3 performing a measurement using an integrating sphere, the intention is to Spectral output of 365 nm LED source and EIT UV-A capture all energy. spectral response In real-world applications, the user might be more interested in capturing the LED radiant flux for a small solid angle that is also sometimes referred to as “useful radiant flux.” In order to make this measurement, the CIE updated their guidelines in the recently published CIE 127:2007 document to include the term “partial” radiant flux. Traditionally, intensity measurements of a point source are done using luminous intensity. As described earlier, most LEDs are not point source and do not contribute to the uncertainty of the UV-A response. The responses have follow the inverse square law. LED measurement include: been normalized. If the LED is binned intensity measurements claimed • Radiometer calibration very close to 365 nm, the EIT UV-A by a manufacturer could vary when uncertainties response does a good job of measuring the end-user performs a similar this source. • LED short-term wavelength drift measurement. When performing a But even a small drift in the LED • LED temperature drift measurement, it is always important spectral output or variations in how to know conditions and uncertainties • DC current regulation for LED the LED dies are binned can generate associated with the measurement. • Optical alignment different responses in the radiometer, Sources of uncertainties that can Industrial applications and setups which can have a pronounced impact on the measurement.

Courtesy of Integration Techonology make it more challenging to easily control and measure the above parameters. 395 nm LED As stated, LEDs have a narrow Measuring the output of a 395 nm band emission and, hence, could have LED with a radiometer utilizing an short-term or long-term wavelength EIT UV-A or EIT UV-V response can drifts due to temperature variations lead to wide variations in the reported or degradation over a period of time. irradiance values. Most integrating type radiometers The output of a 395 nm source is were originally designed for UV-arc and grouped “around the 395 nm line” with microwave sources, and have a bell- variations based on how individual shaped response curve across the UV LED dies are binned; how the array band of interest. is assembled; and the stability of the product over time. These slight 365 nm LED variations are normal. It is also normal Using a radiometer designed for to expect slight variations in each arc and microwave sources can lead to radiometer due to slight variations large errors in measurement if UV-LED in the optical components (filters, output happens to fall on the rising detectors, etc.) and electronics. or falling edge of the optical stack In Figure 4, the output from a 395 response. Figure 3 shows a 365 nm nm LED clearly falls between the EIT UV-LEDs used in UV coating. (UV-A) LED source and EIT’s UV-A and EIT UV-V response curves.

© 2013 RadTech International. 51 in the 380-410 nm regions. This region Figure 4 better covers the 395 nm LED since under normal conditions the source Spectral output of a 395 nm LED source and EIT does not fall on the steep shoulder of UV-A and EIT UV-V radiometer spectral response the response curve. Extensive testing was done to get the UV-A2 radiometer optical response to proximate a “flat top” response. A “flat top” response limits the shifts in the measured values due to slight spectral variations in the source. The UV-A2 response bandwidth inherits a Lambertian spatial response by design from early generation radiometers that further reduces measurement errors due to LED alignment. w

The steepest part of the shoulder of the LEDs themselves. Because of these Acknowledgements each optical response curve is in the variables and their combinations, it is The authors wish to thank output range of the 395 nm LED. So, hard to apply a correction “factor” to Phoseon Technology, Integration while it is possible to get a reading the UV-A reading or UV-V readings to Technology, Solid UV and Summit UV with a UV-A or UV-V bandwidth any single instrument. for their contributions to this paper radiometer, the sharp cutoff at this A better approach to measuring and for their assistance during the wavelength means that the readings LEDs in the 395 nm range is to use development of the UV-A2 bandwidth. may reflect only 5-50% of the actual an instrument with a response curve 395 nm LED. The large variation that better matches the source. EIT References can result from the output being has developed a subset of our 320-390 1. C. Cameron Miller, Yuqin Zong, Yoshihiro Ohno, LED photometric on the steep slope of the response nm UV-A bandwidth now designated calibrations at the National Institute of curve, variations between measuring as UV-A2 (Figure 5). The UV-A2 Standards and Technology and future instruments and variations between response curve is especially sensitive measurement needs of LEDs, Preprint: Proc., SPIE Fourth International Conference on Solid State lighting, Denver, CO, August 2004, 5530, Figure 5 69-79 (2004) 2. C. Cameron Miller and Yoshi Ohno, Spectral output of a 395 nm LED source and EIT Luminous Intensity Measurements of UV-A2 spectral response Light Emitting Diodes at NIST, Abstract for the 2nd CIE Expert Symposium on LED Measurement

—Jim Raymont is director of sales and Abhinav Kashyap is calibration engineer for EIT Instrument Markets in Sterling,Va.

By Paul Mills and n the game show Jeopardy!, user of the instrument still needs to Jim Raymont the program turns the tables on understand what the value means and contestants by presenting them whether or not it is the “correct” value. Owith an answer and challenging them Care needs to be exercised when to come up with the right question. communicating. Differences between Sometimes it seems that the UV-LED different brands of radiometers or market is engaged in its own version instrument types with different of Jeopardy! There are plenty of good features and responses can lead to answers—if we can only figure out different answers. Instruments have what are the right questions. evolved over the years and their With the game show, only the host combination of electronics, optics starts the dialog with the answer. and software are designed to provide In the UV-LED world, there could be solutions to the challenges of cosine 25-plus UV-LED source/array response; the nature of the mercury integrators asking questions along spectrum; and the predictable effects with formulators, machine integrators of parabolic and elliptical reflectors. and customers. If it seems as if mercury lamp Some of the questions that seem suppliers and UV-LED manufacturers important to guiding the discussion on are sometimes speaking two different proper LED measurement are: languages—that’s because they are. • What are the wavelengths of the The advent of UV-LEDs has resulted UV sources? in a paradigm shift. This shift doesn’t • What is their expected dynamic just require a new language, it also range? shatters some of the conventional wisdom and time-honored approaches • How fast should an instrument to UV measurement. Developing a new sample? prescription for how to measure LEDs • Where is the right place to measure has been made more difficult by the the LED? moving target nature of a solid-state From a measurement perspective, technology that continues to morph it feels like the clock has been turned and obsolete itself at a rapid pace. back to the late 1980s or early 1990s. It The goal of this article is to feels as if we are answering many of the suggest a number of important, but same questions for UV-LED users that as yet unresolved questions that are we have answered for arc or microwave fundamental to measuring UV-LED users. In the established world of performance. The answers to these mercury-based lamps these questions questions will determine if current have long been settled and products measurement solutions will work; need have evolved based on broad industry to be modified; or if a new class of consensus. You press a button and the UV-measurement devices will best instrument provides “a value.” The address the needs of its users.

© 2013 RadTech International. 53 What is the Spectral Output of a UV-LED Light Source? Figure 1 Any device that measures a source of irradiation—whether it’s visible Optical response of a typical photodiode sensor

light, Infrared (IR) or ultraviolet— 100 requires a sensor that is sensitive to 90 that portion of the spectrum under 80 test. These detectors must accurately 70 convert small changes in the incoming 60 Light Sensor energy into corresponding changes in 50 electrical energy while discriminating 40 against wavelengths outside the band 30 Human

Relative Sensitivity % Relative Eye of interest. 20 For example, suppose that you 10

BLUE CYAN GREEN ORANGE RED INFRARED are a camera buff who wants to 0 400 600 800 1000 1200 accurately measure light so you can Wavelength (nm) take proper photos. Your goal is to design a very accurate and sensitive photographic light meter. Since you might photograph under a range of the right size and the manufacturer of mechanical (optical) components different light sources (i.e., tungsten, provides a chart that shows its optical and electronic circuitry, you can build fluorescent, sodium vapor, sunlight characteristics. Figure 1 is, in fact, a a light meter that provides you with and candlelight), you need an fairly representative optical response reliable measurements. instrument that measures across a for many popular photoelectric diodes. Suppose one day you decide that wide band of light sources. Note that the detector’s response curve you want to start doing IR photography The sources generally emit light in a is pretty close to linear in the visible instead. This involves working range of about 380 nanometers (violet) portion of the spectrum (see inset). with sources in the 750-950 nm IR up to about 740 nanometers (red). Though the response curve is not “flat” range. There’s a problem with your In designing your light meter, you in the 380-740 nm band, it’s known old light meter. Measuring 900 nm come across a light sensor that uses and predictable. Through some clever with a detector, filters and circuitry a popular photodiode. It’s affordable, engineering and using a combination engineered for visible light causes Figure 2 Typical bandpass filters used across the UV spectrum

100

60 UV-V UV-A 50 UV-B UV-C 40 UV-A2 Normalized %T Normalized 30

20 EIT Inc. 10 2/26/09

0 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 Wave Length

of hundred milliwatts per square Figure 3 centimeter a few years ago to more than 10W per square centimeter today Example of signal clipping (and twice that much in the current development labs). This poses a problem for existing radiometers. Most conventional radiometers were intended to measure light sources up to about 10W per square centimeter. Though most users mistakenly regard LEDs as less powerful sources than arc or microwave lamps, their intensity in a narrow-band region is very potent. The latest generation of LEDs challenges the dynamic range of existing instruments. You may be familiar with clipping in audio applications where loud peaks are artificially attenuated some measurement problems when broad range detector will emerge as by amplifiers. The result of trying used with infrared. It produces a the best solution. to measure with a device that has reading, but the reading is likely to inadequate dynamic range is “clipping” be misleading since the instrument is What is the Anticipated Power that results from overpowering the intended for a different range of Output of a UV-LED? device. Clipping produces a lower light sources. The minimum and maximum irradiance measurement than the LED That’s the UV measurement problem expected amplitude of an incoming may be generating. See Figure 3. with LEDs. The existing instruments signal is an important determinant To design a proper UV-measurement were designed for light sources that for designing a measuring device. It instrument, engineers need to are in a different spectral region. They must be sensitive enough to measure anticipate the appropriate dynamic produce a reading, but the reading can the weakest signal and of sufficient range of the source. In a market be misleading when used with new LED dynamic range to accommodate the environment where the light source light sources. The chart in Figure 2 strongest signal. UV-LED output has output power doubles every couple of shows the bandwidth configurations increased steadily from only a couple years, this is an engineering challenge. developed over many years for arc and microwave-type lamps. You can see Figure 4 that a 395 nm LED would fall to the high side of the UV-A band and the Cosine effect on UV irradiance lower edge of the UV-V band. The solution has been to engineer a device optimized for 395 nm and the answer was a new combination of optics, detector and circuitry dubbed UV-A2. The chart in Figure 2 illustrates how the UV-A2 band satisfies the need for a better (more linear) response in the long wavelength LED market. But there is still much uncertainty α = 45° α = 0° about what wavelength for the LEDs will emerge from the chip manufacturers; whether a new bandwidth will be required; or which

© 2013RadTech International. Where doyou Measure anLED? error referredto ascosineerror. In from aslightangle. Thiscausesan a differentreadingthanlightcoming directly abovetheradiometer produces in theFigure4diagram.Light from detector frommanyanglesas shown from alampstrikestheradiometer’s single-point sourceseither. UVlight operate inanonabsorptivemedium. are neitherpointsourcesnordothey a big“if”sincereal-worldUV-LEDs distance fromtheobject.”Butthatis in proportiontothesquareof medium, thentheirradiancedecreases directions throughanonabsorptive source radiateslightuniformlyinall fundamental principleisthat“ifapoint from thelightsource.Infact, falls offthefartherdistanceitisaway but thisisnotthecase.UVintensity would bemuchsimplertomeasure— a sourcewereuniformeverywhere,it really justexcessenergy. process timedesired,anythingelseis density tofullycurethematerialin is adequateirradianceandenergy the chemistryreacts.Aslongasthere what’s reallyimportant—whichishow caught upinthenumbersthatwemiss Figure 5 Figure Traditional UVlampsarenotalways If theUVlightthatemanatesfrom Still, it’s importanttonotbecomeso The optical footprint:The traditional lampvs. LED correction ofunity, regardlessof combination hasanangularcosine A perfectdetectoranddiffuser components suchasadiffuser. response ofthedetectorandoptical is achievedbytinkeringwiththe light source. of theanglebetweendetectorand instrument mustcorrectfortheeffect coming fromvariousangles,the order toproperlymeasurethelight Figure 6 Figure The angularcosinecorrection The The “Where DoIMeasure?” dilemma can makeitdifficulttocomparethe the illustrationinFigure6shows,this reporting UV-LED measurements.As let aloneindustrystandards,for be awarethatthereisnoagreement, comparing manufacturer specifications, a higherirradiancespecification.When sometimes withthegoalofproducing the outputoftheirarraysagain, optical componentsthatcanalter manufacturers havebeguntodevelop output. where tobestmeasuretheirUV make itmorechallengingtodecide characteristics ofcurrentLEDsources reflectors (Figure5).Theoptical compared totraditionallampswith uniformly inalldirections,especially those tinyLEDsdonotradiatelight geometry ofmeasurementisthat distinguish tightlypackedUV-LEDs. This responseisnotalwaysableto a cosineresponseintheinstrument. are designedtoachieveorreplicate instruments, theopticalcomponents the angleofincidence.Forexisting Recently, theLEDlamp A secondcomplicationinthe

Courtesy of Integration Technology Ltd. 56

rectangles and then add them all Figure 7 together. Obviously, the narrower the rectangles, the more precisely the Computing energy density (joules) area can be measured without missing the peak irradiance value. This is how Peak Irradiance many radiometers (originally dubbed “integrating radiometers”) measure energy density. The instrument takes many irradiance (sample) measurements every second, and then adds the values Energy Density together. The push has been to faster and faster sampling rates in order to

Power / Irradiance Power achieve high resolution and accurate irradiance calculations that can lead to more consistent energy-density Time calculations. The more samples that are taken over time, the “narrower the rectangles,” so to speak. Today’s manufacturer’s specifications without both measures. Successful UV curing radiometers sample hundreds (even knowing much more about how the relies on two similar measures— thousands) of times per second. measurement was taken. Measurements irradiance (a measure of UV intensity) But what happens when you are made even a couple of centimeters and time. Without sufficient time, trying to measure something that’s apart can be dramatically different. UV curing may be incomplete and also changing at the same time? Some It’s likely that the question of the result compromised. In fact, the LED electronic control systems use a where to measure—such as always combination of irradiance and time is scheme called Pulse Width Modulation “at the glass”—can be answered so important that a measure known as (PWM). With PWM, the supply voltage without difficulty. But if various energy density (sometimes referred to is turned on and off rapidly to alter the manufacturers continue to develop as “dose”) is almost always provided output of the light source as illustrated LED sources which focus their power when specifying a UV-cure process. in Figure 8. The strobing of the LED further out from the LED array, they (Note: It is important to stress is normally so rapid that it’s invisible may be understandably reluctant that an energy-density value by to the eye and, for many curing to accept this method since these itself is not enough to properly processes, it works perfectly well. arrays will be intentionally designed define a cure specification. From While PWM may allow the to provide even greater irradiance at a UV-source standpoint, the manufacturer to vary the output power another, somewhat further distance. irradiance value; type of spectral of the array, turning the light source The question of where to make a output; and the absence/presence off and on very rapidly can create a standardized measurement of LEDs of infrared all contribute to the cure challenge when taking samples at the may, therefore, remain understandably process for a specific application. same time. It can be like trying to take unresolved as competing ideas Parameters such as the coating photos of a spinning fan blade—it’s about their optical characteristics thickness also need to be specified.) hit-or-miss. The sample energy-density are sorted out. In a mathematical sense, energy readings may not accurately reflect the density can be thought of as the area true measure and, depending on the How do Sampling Rates Affect under a curve that measures irradiance algorithm used to used to calculate the Irradiance and Energy Density over time. See Figure 7. irradiance and energy density, it may Calculations? You might remember from math be higher or lower than the true values. For thermal processes such as class that finding the area under a curing paints and baking cookies, two curve is called integration, and that What are the Right Questions? key parameters are vital—temperature one technique to find this area is The preceding questions address and time. Proper results depend on to break the area up into narrow important technical matters. The

© 2013RadTech International. in mind.TheversatilePowerPuck were designed with popular applications Many oftheexistingUVinstruments address alloftheunresolvedissues. UV-LED performance.Buttheydonot radiometers intendedformeasuring answers affectthedesignof the PalmProbe industrial conveyorizedsystems.But for instance,iswell-suiteduseon year, makingitchallengingtodesign a changes significantly fromyearto are not).Theindustryisnascent and of (andprobablyanumberthat we UV applicationsthatweareaware UV-LED sourcestargetingindustrial 25 manufacturersofcommercial different sizeorshapemaybeuseful. such asdigitalprinting,wherea LEDs alsoopenupnewapplications, measurement toolswillfitthebill.But where anLEDanalogofexisting conventional lampsinapplications Perhaps LEDswillsomedayreplace test instrumentneedtobeadapted? and howdoesthepackagingof Puck willnotphysicallyfit. into recessesandareaswhereaPower Figure 8 Figure At thistime,therearemorethan What arethelikelyusesforLED Pulse widthmodulated signal 0 0 ® V V wasdesignedtoreach Time period Time period ® , being apossiblesolutionforexisting markets toUVcuring,inaddition within ashorttimespan. moves fromundefinedtoobsolete measuring deviceforequipmentthat the rightquestions.w answers. Theproblemisagreeing on is noshortageofsolutionsandgood and expertiseatmeasuringUV, there Target applicationsthatarebest • Honestlypresenttheadvantages • Continuetooffereducational • Welcome themtoourindustry • microwave UVsources.We needto: gained overtheyearswithspot,arcor the experiencethatmanyofushave for thefirsttimeanddonothave the opportunitytouseUVsources UV applications.Manypeoplehave director ofsalesforEITInstrument UV-LEDs haveopenedupnew With yearsofprovenperformance suited forUV-LEDs and disadvantagesofUV-LEDs opportunities consultant andJimRaymont is —Paul MillsisaUVmarketing Higher average DC Higher average cycle Higher duty Lower average DC Lowercycle duty Markets inSterling, Va.

t t 58

The State of UV-LED Curing: An Investigation of Chemistry and Applications

By Ed Kiyoi ight-emitting diodes for ultraviolet- light as shown in Figure 1. Only about curing applications (UV-LEDs) 25% is in the safer UV-A range. have been commercially available A UV-LED generates UV energy in Lfor nearly 10 years. However, their an entirely different way. As an unique output characteristics require electric current (or electrons) move newly formulated UV chemistries in through a semiconductor device called order to take advantage of UV-LEDs’ a diode, it emits energy in the form of many benefits. This paper discusses photons. The specific materials in the the characteristics of UV-LED lamps; diode determine the wavelengths of the importance of properly formulating these photons and, in the case of chemistries; the benefits to end-users; UV-LEDs, the output is typically in commercial applications of UV-LEDs; a very narrow band +/- 20 nm. The and future expected developments. wavelength is dependent on the band gap between excited state and the Characteristics of UV-LED lamps ground state of the semiconductor Traditional UV arc lamps produce material. The chart in Figure 1 UV energy by generating an electric compares the output of a 395 nm, arc inside an ionized gas (typically UV-LED lamp with a typical mercury- mercury) chamber to excite atoms, arc lamp. It is important to note the which then decay and emit photons. difference in intensity and wavelength The emitted photons cover a broad of the output as both are key to range of the electromagnetic spectrum, understanding a UV-curing process. including some infrared and even visible The UV-Curing Process Figure 1 UV curing is a photopolymerization Wavelength output comparison of mercury-arc and process that uses UV energy to change UV-LED lamps a liquid to a solid. Upon absorption of the UV energy (as shown in Figure 2), the photoinitiator (PI) produces free radicals that initiate crosslinking with binders (monomers and oligomers) in a polymerization reaction to cure or solidify the ink, coating or adhesive. UV formulations also incorporate various additives such as stabilizers, wetting agents, adhesion promoters, defoamers and pigments to provide desirable characteristics or color of the cured material.

© 2013 RadTech International. 59 the monomer reacts and becomes Figure 2 part of the UV-cured material. The oligomers (and their backbone UV curing photopolymerization process structure) determine the overall properties of the material. Monomers and oligomers are generally derivatives of acrylates or methacrylates containing polyurethanes, polyesters or polyethers. The longer wavelength output— such as the UV-A range seen from UV-LEDs—penetrates through thick and pigmented systems producing through-cure of the material that developed for curing with a typical Comparison of Solvent and ensures surface adhesion and the mercury-arc lamp (shown as H-bulb) Waterborne to UV Processes ability to cure thicker screen ink Solvent and waterborne use a broad spectrum PI. While there or pigmented wood coatings. Short formulations change from a liquid to is often some absorption within the wavelength output (200-280 nm) a solid (“dry”) via evaporation of the UV-LED output range, it is clear to see is unable to penetrate very far into solvent—typically volatile organic that much of the PI absorption range a material, but provides surface compounds (VOCs) or water. This is wasted. A more efficient cure is curing which is important for surface drying process (often requiring an possible with a formulation designed properties such as scratch and oven) takes time, generates VOCs and specifically for UV-LED curing using a chemical resistance. the dried film thickness is less than PI with concentrated absorption in originally applied. UV curing happens the UV-A range such as those shown Overcoming Surface Cure Issues much faster (typically less than a in Figure 4. Surface curing due to oxygen second), does not generate VOCs The monomers in the formulation inhibition was often an issue for and the film thickness applied is what serve as the reactive diluent enabling UV-LED curing, but has largely been remains as a solid (critical for certain the formulator to control viscosity for overcome by various means. Of end-use applications). UV-curing proper application (spraying, rolling, course curing in an inert (nitrogen) processes are environmentally friendly, screen printing, etc.) of the uncured atmosphere is one option, but it adds save energy costs and floor space, and material. Rather than volatilizing, as is cost and complexity to the system. typically increase production rates typical with conventional formulations, Another option is to add in oxygen- while reducing scrap or waste streams. Figure 3 Formulating UV Chemistries for UV-LED Lamps Photoinitiator spectral absorbance compared to For efficient and effective UV traditional UV lamp output curing of an ink, coating or adhesive, the formulator seeks to overlap the UV lamp output with the spectral absorption of the PI. The amount of PI in a typical UV formulation is usually very small, less than 5%. PIs typically absorb across a range of wavelengths, not a narrow band. For example, Figure 3 shows the spectral absorption for different PIs and the wavelength output for mercury-arc UV lamps. Many existing UV formulations

© 2013 RadTech International. 60 combined with highly reactive acrylate Figure 4 oligomers and it minimizes yellowing. ITX (Type II) was more reactive, but Examples of longer wavelength absorption caused too much yellowing for OPV and photoinitiators white inks. She also noted that pigment selection is key because pigments compete with the PI for UV energy.4 Many advances have been and are being made by raw material suppliers and, in turn, more UV-LED formulations are being commercialized that result in production speeds comparable to traditional mercury lamp processes. Formulators should work closely with their suppliers to develop new UV-LED cure chemistries which are non-yellowing, will overcome surface cure issues and meet the end-use production requirements. consuming or scavenging compounds typically accounts for 20-40% of the such as amines or aminoacrylates to formulation by weight. Mono- (MAPO) Benefits UV-LED Curing Delivers to overcome oxygen inhibition.1 and bisacylphospineoxides (BAPO) End-Users Research has indicated that peak are recommended photoinitiator types The benefits of UV-LED as irradiance (W/cm2) and total UV-A for UV-LED curing. Some commercial compared to traditional mercury- energy (mJ/ cm2) delivered are more examples are IRGACURE® 2100, arc UV lamps are numerous and important than a precise wavelength LUCIRIN® TPO-L and ADDITOL® TPO.3 significant as shown in Figure 5. match on formulations developed to In September 2012, Eileen Jaranilla- UV-LEDs are more environmentally cure in the UV-A region. Peak irradiance Tran with the Rahn-Group reported on friendly because they do not generate is an important metric since intensity is her investigation of overprint varnishes ozone and contain no mercury as required to initiate the polymerization. (OPV), flexographic inks and inkjet arc lamps do. They are a cool source Higher peak irradiance (such as that inks. She found that Norrish Type I PIs compared to arc lamps, largely due to found in UV-LEDs) results in a more such as BAPO are effective to achieve no output in the infrared range. This aggressive polymerization mechanism good surface cure (and preferable to reduced heat eliminates complicated helping to overcome oxygen inhibition TPO and BDMM), especially when cooling mechanisms such as chill at the surface and achieving the rolls and external shutters, and required cure rate.2 More recently, it has been shown Figure 5 that higher functional oligomers Benefits and features of UV-LED curing can also minimize the oxygen inhibition and improve surface curing. Commercially, Cytec offers a co-resin called ADDITOL® LED 01, a mercapto-modified, polyester- acrylate resin that replaces a portion of the oligomer in a UV formulation to improve surface curing under UV-LED lamps. This co-resin is compatible with urethane acrylates, some epoxy and polyester acrylates, and acidic adhesion promoters and

© 2013 RadTech International. 61 enables applications on heat-sensitive of fiberglass composites. These Narrow Web Flexo Takes Off substrates. The electrical-to-optical early applications took advantage of More recently, UV-LED inks are conversion efficiency of UV-LEDs is UV-LEDs’ form factor (lightweight being used for screen printing (rotary, much better and the ability to instantly and small), through-cure capabilities flatbed and container) and narrow turn the unit off and on enables saving and the increased safety inherent with web flexographic printing. Flint about 50-75% on electricity. longer wavelengths. In fact, many of Group introduced the first UV-LED Table 1 shows a comparison of key the earliest applications were actually combination print inks, flexographic characteristics of UV-LEDs versus in the visible wavelength range. Today, four-color process and rotary screen traditional mercury-arc UV lamps. as the energy density has increased white inks branded as EkoCure™ at Compared to an arc lamp’s 500-2,000- and costs have decreased (especially Label Expo in September 2012. In hour life, most UV-LEDs are specified for 395 nm output), UV-LED is October, they began working closely for 10,000 hours, but can last more commercial in the graphic arts market, with a beta customer (a large label than 20,000 hours. It’s also important wood coatings, electronics, composites converter) to validate the inks and to note that over this lifetime UV-LED and others. process in a commercial setting. output only drops about 5%, compared Commercial UV-LED applications The customer had a narrow web to arc lamps that can lose about 50% in the graphic arts market (especially flexographic press running waterborne of their original output by the end of digital inkjet applications) advanced inks. They simply installed UV-LED their life. In a production environment, first owing to their form factor, low lamps to run the new EkoCure™ inks. UV-LEDs require significantly less heat and energy savings advantages. Since the UV-LED lamps run cool, there space, monitoring, maintenance and Commercial inkjet applications was no need to use chill rolls or other downtime. That translates into higher today include all inkjet segments, web cooling devices as would typically productivity rates, less scrap and including wide-web printing on a be required for mercury-arc lamps. higher quality end products. Paybacks variety of substrates for many end-use They have seen no film distortion, even for retrofitting onto existing machines applications. There are many UV-LED- on heat-sensitive, low-gauge films such or replacing existing UV arc lamps can specific inkjet inks and most inkjet as shrink films. be as low as 12 months. printing presses are available with “Our beta site customer runs Commercial Applications of UV-LEDs. For example, EFI Inkjet a variety of different commercial UV-LEDs Solutions offers a 126-inch wide, products. So, as they needed something Some of the earliest UV-LED UV-LED curing printer capable of 1,000 new, we formulated it for them,” said commercial applications were small dpi and eight-color (plus white) with Tom Hammer, product manager, area adhesive and bonding applications speeds of up to 1,200 ft2/hr. End-users Narrow Web North America, Flint such as medical device assembly; are able to print on thinner, more heat- Group. “We started with flexographic low-end thermal inkjet printing sensitive materials reducing material inks, but have also done clearcoats and applications such as marking, coding costs in half without sacrificing any adhesives, both PSAs and laminates. and variable printing; and field repair quality or speed.5 Nothing has been a problem and the customer is very impressed.” Table 1 The beta customer has been running the 10-station, 17” press since October Comparison of UV-LED to mercury-arc UV lamps on a 24/7 schedule. No matter what UV-LED Mercury Arc they’ve been running—shrink films, Life 20,000+ hrs 500-2,000 hrs four-color processes, pressure-sensitive On/Off Instant 10 Minutes labels, lamination and others—they Output Very Good. 95%+ Drops up to 50% are seeing faster line speeds than with Consistency traditional UV. They are currently on Heat Generated 60°C ~350°C Energy Efficiency Saves 50-75% track to see a payback for the press Environmental Mercury Free, Mercury Waste, retrofit in less than 12 months. The Ozone Free Generates Ozone beta customer is also adding a rotary Footprint 30-50% less head to the press and will run the opaque white ink already developed for UV-LED curing. Flint Group plans to

© 2013 RadTech International. 62 further develop rotary screen inks in a the UV-LED lamps provide consistent that is needed is an end customer variety of colors for UV-LED cure. output, require less maintenance and willing to be first, as is so often the “Formulating inks for UV-LED reduce the fire hazard. Gloss control case with “new” technologies. curing does not require starting from has been an issue when applying Richard Baird, a process engineer scratch, but it is also not as simple thick, pigmented layers (>15g/m2) via for Boeing, wrote in the fall 2011 issue as just replacing the photoinitiator roll coating or with spray application of the RadTech Report that he expects with one that has a longer wavelength (rarely used). When the customer UV-LED curing to become a viable absorption. The pigments used in the needs to produce matte finishes they option for large-scale aerospace paint flexographic and rotary inks are similar simply use a combination of arc lamps curing in the very near future.6 By all to those used in traditional UV-cured and the UV-LED. Gloss control is not indications, this and many other UV- inks as are some of the oligomers an issue for thinner topcoats. There LED curing applications will indeed be and monomers,” added Hammer. is also an EU framework project FP7 taking off soon. w “However, there are fewer choices of (http://www.fp7-uvled.eu) focused photoinitiators (longer wavelength) on researching UV-LED wood coatings. References to use and it is more challenging to Findings were expected in early 2013. 1. UV-LED Lamps: A Viable Alternative get cure performance (surface and for UV Inkjet Applications, by Michael Beck, RadTech Report, November/ through-cure) while still keeping costs Future UV-LED Applications December 2009, p. 39, http://www. for the end-user in mind.” The future for UV-LEDs looks very phoseon.com/Documentation/UV-LED- bright given the progress made to Lamps-A-Viable-Alternative-for-UV- Inkjet.pdf Wood Coating Applications date by raw material suppliers and 2. Characterizing the Efficiency of Sherwin Williams introduced its formulators. And, if the trends for UV- UV-LED Curing, by Rob Karsten and Becker Acroma™ UV-LED coatings LED development continue—namely Bonnie Larson, Phoseon Technology for wood in January 2012 and their increasing peak irradiance (77% and Kent Miller, University of Akron, presented to Radtech Europe 2009. customer BJS has been successfully compound annual improvement) and 3. UV-LED Curing ADDITOL® LED 01, using the UV-LED coatings since decreasing costs—we should see rapid CYTEC presentation, April 2011, June 2012. adoption by end-users in the near https://www.cytec.com/uv/Downloads/ “The primary driver for developing future for many new applications. ProductPresentationADDITOLLED01.pdf these UV-LED coatings was to extend According to Hammer, Flint Group 4. UV-LED Curing in Graphic Arts Applications, by EileenJaranilla-Tran, the use of UV curing to heat-sensitive is developing UV-LED inks for offset RAHN-Group, presented at Chicago wood substrates such as pine (< 45°C) and letterpress applications and he Printing Ink Production Club meeting, and other resinous woods. Most fully expects this to translate to sheet- September 2012, http://www.cpipc. org/downloads/cpipc%20led%20 end-users in Europe face strict limits fed and wide-web applications as well. 092012.pdf on VOC emissions and traditional Hammer also sees food packaging as 5. Maturing UV-Cure Technology, by Bill UV arc lamps cause problems on a growth area, once low migration Schiffner, Sign & Digital Graphics, heat-sensitive wood material,” said inks are available. Right now, most of February 1, 2012, http://sdgmag.com/ article/printing-finishing/maturing-uv- Lars Sandqvist, technical project the photoinitiators approved for food cure-technology manager at Sherwin-Williams Sweden. packaging are not appropriate for 6. UV-Curable Paints for Commercial “With the UV-LED coatings, end-users UV-LED ink formulations. Aircraft Exteriors, by Richard W. Baird, have a choice of arc or LED, or even Sherwin-Williams is developing RadTech Report, Fall 2011, p. 43, http://radtechreport.com/pdfs/RT_ a combination. Customers who have a UV-LED fillers for use by furniture fall11_baird.pdf UV line, but have heat problems, can manufacturers who want to use retrofit the line with LED in a couple of lower cost particle board, but need a —Ed Kiyoi is a technical marketing engineer at Phoseon Technology in positions to get the temperature down smooth edge after cutting and shaping. Hillsboro, Oregon. and keep the rest of the arc lamps to The clear, thick fillers are an ideal minimize investment.” application for UV-LEDs. According to BJS (which runs both arc lamps Sandqvist, another application that will and UV-LEDs on the same line) has soon be available is UV-LED coatings seen a 60% energy savings with the on wood moldings. The coatings UV-LED compared to the arc lamps. are ready and shown to cure at the In addition to energy cost savings, required speeds of 40-100 m/min. All

By Jennifer Heathcote nyone who has ever and microwave curing. What works for investigated UV-LED curing one application does not necessarily has likely encountered work for another. As a result, opposing Acontradictory statements and statements generally directed at the claims regarding the viability of the UV curing industry as a whole are technology as well as its future. Why really only credible—and much less does UV-LED technology garner such contradictory—when given correct varied support from industry experts? application context. Quite simply, it is because there is Many readers of this article are no such thing as a universal UV-LED likely familiar with the UV-LED curing solution that works for every UV-curing benefits championed by those operating application in existence in exactly the within the UV-LED supply chain. For same way. Or in other words, UV-LED convenience, a short list is provided technology is not a one-size-fits-all in Figure 1. The cost savings as well substitute for conventional UV arc as process and safety improvement benefits are often the impetus that Figure 1 leads individuals to investigate LED curing in the first place. UV-LED curing system benefits But even the strongest and most Solid-state technology persuasive list of benefits has little Easy integration to do with whether an application is Near-ambient array housing temperatures Negligible heat transfer to cure surfaces practically or economically viable. Instant on/off curing Focusing solely on a generalized list No warm-up/cool-down cycles of benefits excludes everything that No shutters needed makes an installation successful. Diode life in excess of 20,000 hours That includes hours of formulation Consistent UV output over time and engineering work to adapt the No mercury-filled UV bulbs technology to the specific process, No ozone production field tests, general technology No system exhaust improvements, safety certification and No conditioned plant makeup air No radio frequency emissions the cost analysis needed to justify the Lower total cost of ownership business case for adoption. In order to achieve a better understanding of when and why

© 2013 RadTech International. 64 • Applications that lend themselves Figure 2 to UV-LED systems with smaller form factors easily positioned within General market diffusion of UV-LED innovation a half inch of the cure surface; and • Applications where the chemistry First Movers Second Movers Later Adopters has been tweaked to react within Inkjet Pinning Inkjet Full Cure— Litho/Offset the 365-405 nm output range and Full Cure— Scanning Graphics 3-D Finishing Slower Speed emitted by most UV-LED systems. Inkjet—Under White Inline Graphics High-Speed A few more specific examples Inkjet Industrial Coatings Inkjet Marking include inkjet marking and coding of and Coding Screen High-Speed gift, security and hotel cards; inkjet for Adhesives Spot-Cure Adhesives Flexo narrow web labels; coating, bonding and Sealants UV-B Curables Wider Area and sealing electronic displays and Slow-Speed Sealants/Adhesives UV-C Curables devices as well as smaller automotive Coatings assemblies; inkjet product decoration, Photoresist particularly on heat-sensitive materials; and coatings applied to thin films. As UV-LED innovations diffuse the broader market, UV-LED first-mover UV-LED curing makes sense, it is the UV source from its cure surface applications have and continue to helpful to survey the markets that are all influence which UV configuration receive the vast majority of engineering embracing the technology today, and is needed. Just as these applications effort and comprise the most evaluate the industry and application employ very different conventional significant UV-LED revenue stream factors that are enabling successful UV solutions, they also require very for system manufacturers, formulators installations as well as the processes different UV-LED solutions. and integrators. For this portion of the that guided the evolution. Conversely, First Movers industry, listed in column one in Figure factors that hinder adoption and Generally speaking, UV-LED curing 2, UV-LED technology has increasingly penetration in other markets can also is most commonly used in slower become the preferred solution and provide notable insight regarding speed applications, including inkjet, is truly the result of a full decade of the technical issues that must yet coatings and spot-cure adhesives and development. be overcome. sealants. It also tends to be employed Second Movers It should not be any surprise that for relatively flat substrate surfaces. In other applications such as screen, a 30-second static exposure bonding While the activity across the early flexo and both scanning and industrial application over a ¼-inch or one-inch adopter markets is diverse, it is often inkjet, formulators see great near-term driven by: square area; a 10 fpm, 22-inch wide potential and have subsequently • Heat-sensitive applications that electronics coating line; a 60 fpm, invested heavily in research and cannot use conventional UV due to six-inch wide multicolor inkjet; a development in recent years to the emitted infrared; 48-inch wide screen graphic make the applications work. These application; a 250 fpm, ½-inch wide • Wavelength-sensitive applications suppliers become strong promoters single color inkjet coding application; that utilize the relatively of the technology even if there are a 650 fpm 17-inch wide eight-station monochromatic output of UV-LEDs few, if any, installations to reference flexo printer; a 2,000 fpm, 60-inch wide to avoid product-damaging and even if more development work lithography line; and a 3-D UV-curing wavelength regions; needs to be done. Many formulators chamber all have very different UV • Industrial coating and bonding and LED equipment manufacturers process and integration requirements. applications at larger manufacturing have produced successful field trials The type of ink, coating or adhesive; facilities that have additional while some integrators are leading desired post-cure functional engineering resources available the industry by developing custom properties; substrate width or part to support development and LED-curing machines and printers. All profile; line speed; and distance of integration; continue to improve and optimize the

© 2013 RadTech International. 65 technology as they hunt for end-users where the technology can be made before this point, formulators begin ready to embrace LED curing. to work, it is often not economically experimenting with LED chemistry in The greatest hurdle, however, is feasible at the desired production the lab. (3) Once formulators achieve that while the technology works or can speeds. While each application should working samples, they partner with be made to work, the business case is be evaluated individually, economics, LED-system manufacturers who have not always sufficiently demonstrated as line speed and distance are the key been working on their own technology to what is often a very price-sensitive reasons why there has not been any in parallel. Together, formulators and machine builder or end-user. While significant migration toward adoption. LED-system suppliers perform field it is typically much easier to make Nevertheless, many suppliers are trials on existing process equipment at the business case for using UV-LEDs actively engaged in these areas true production speeds. (4) Successful on new machines, field retrofits can as customers have demonstrated trials demonstrate the viability of the be a bit more challenging. The large interest in the technology based on technology and highlight weaknesses. and still functioning installed base the potential benefits, and suppliers Without a strong economic business of conventional UV; the obligation want to provide these users with a case, however, the technology hits a to requalify the curing process for solution…someday. Commercial Adoption Barrier. It is use in particular segments such as Market Evolution only when the UV-LED curing system food packaging; and the need for Figure 3 provides an illustration and the formulation material meet customized, longer or multiple arrays of the general market evolution for a sufficient level of the end-user’s on wider and faster speed machines an application. First, machine process requirements and the business make the initial capital investment builders and end-users of conventional case for adoption is economically or development more expensive. All UV-curing and drying methods become justifiable that end-users begin of this contributes to slower market aware of the benefits and successes of embracing the technology. The time penetration. UV-LED technology in early adopter duration between stages 3 and 4 can Later Adopters and second-mover markets. (1) In an be a few months, a few years or even At the far end of the spectrum are effort to determine whether UV-LEDs decades, depending on the application. the LED-lagging applications that are an option for their application, (5) But once stage 4 is reached, operate at extremely fast speeds; they begin calling formulators, it is typically not too long before handle complicated 3-D part profiles; LED system suppliers and machine commercial adoption accelerates, require UV-B and UV-C outputs; or builders. (2) When a critical mass of bringing more and more end-users and exist within machine frameworks that interest is generated, or possibly even suppliers onto the scene. prevent the curing system from getting sufficiently close to the cure surface. For these applications, today’s UV-LED Figure 3 curing technology often does not make UV-LED market evolution commercial sense from a technical or economic perspective—no matter how the numbers are run. In fact, these applications may even require another big leap in the evolution of the base chemistry or LED technology, or possibly necessitate peripheral array enhancements in order to get sufficient UV irradiance and energy density at the optimal wavelength to the cure surface. All of this takes time and money. It should also be noted that for many of these applications, some of the touted UV-LED benefits—such as lower total cost of ownership—don’t really hold up. Even in applications

© 2013 RadTech International. 66 Science of UV Curing intensities necessary for industrial the diodes increases, but it also It is often helpful to review the photopolymerization. Formulators are decreases as the junction temperature basic science behind UV curing as constrained to an existing selection rises. In addition, the relationship a means of understanding what is of raw materials developed to be between irradiance and current is not necessary to move through stages most reactive at 365 nm or shorter. a linear one and eventually it saturates. 2, 3 and 4 as illustrated in Figure 3. Some applications such as inkjet have Further increases in current lead All UV processes require a certain overcome less reactive photoinitiator to even greater inefficiencies in the combination of wavelength (nm), zones by leveraging the higher peak conversion of electricity to UV output irradiance (watts/cm2) and energy irradiances emitted by UV-LEDs at and the need for more total cooling density (joules/cm2) in order for longer wavelengths. These values capacity and AC power. Irradiance also sufficient photopolymerization to commonly exceed those emitted decreases as the array moves away occur. The material being cured does by conventional UV systems. While from the cure surface. This alone makes not care how the UV energy is supplied it is certainly possible to produce applications such as sheet metal offset (arc, microwave, fluorescent tube, increasingly higher peak irradiance decorating and 3-D finishing—where sunlight, LED, xenon pulse, electron and energy density levels for longer machine frameworks or complicated beam, etc.) as long as the formulation’s wavelength LEDs (395-405 nm), part profiles prevent the LED array minimum threshold reaction doing so leads to greater junction from being mounted close to the cure parameters are met. As a result, inefficiencies; potentially shorter diode surface—very difficult and generally the total UV energy (wavelength, life; significantly larger input power impossible with today’s technology. irradiance and energy density) requirements; and increased cooling— Energy Density required by the formulation at the all of which leads to both a larger While some UV-LED curing systems cure surface for a given process speed capital investment and generally higher use higher irradiances to generate dictates whether an LED solution is running costs. more energy density (Joules/cm2), even possible with today’s technology, energy density is more effectively Irradiance as well as how much the total solution increased by adding more diodes to an For any UV reaction, a minimum will cost. See Figure 4. array; increasing the number of LED irradiance (watts/cm2) threshold is arrays in a process; or increasing the Wavelength necessary to start the polymerization overall dwell time. The latter method UV-LED outputs are relatively process and counter oxygen inhibition is accomplished by decreasing the line monochromatic with peak intensities at the cure surface. As previously speed; increasing the number of passes between 365 and 405 nm. UV-LED mentioned, UV-LED irradiance under the UV source; or extending wavelengths shorter than 365 nm is typically higher than that of the time the cure surface is parked are not generally available on the conventional UV systems. The LED beneath the UV source. commercial market, at least not at the irradiance increases as current through Presently, the most expensive component in a UV-LED curing Figure 4 system is the diode. This means that increasing the total number of diodes UV process requirements or arrays proportionally increases the total cost of the curing and cooling systems. As a result, application speed is an important factor in determining whether a process is economically viable since speed can play such a large part in dictating how much energy density is required and, therefore, how many diodes or arrays are needed. Faster applications such as lithography simply require more total energy due to the fact that the media is under the UV source for a shorter period of time.

© 2013 RadTech International. 67 For example, if achieving proper cure for a given application at a Figure 5 specified wavelength and irradiance occurs at a maximum speed of Technology development network 100 fpm using an existing UV-LED system that costs X, in order to cure at 1,000 fpm under the same conditions, the application will typically require 10 UV-LED curing systems (or a single array with 10 times the diodes) at a total cost of 10X. Chemistry, array cooling and AC power consumption aside, this scaling investment cost is the primary factor that makes many high- speed, wide-web presses economically unattractive for LED curing today.

UV-LED Supply Chain Since financial resources for product development are finite, key suppliers focus on the markets that are most conducive to the UV-output levels delivered by today’s LED technology as well as markets where UV-LED For the sake of clarity, let’s focus encapsulate for physical protection and technology is more economically viable on the lower left hand quadrant of to seal out dirt and moisture. for the end-user. These markets are Figure 5. This area consists of four UV-LED system manufacturers most likely to produce the best rate- distinct equipment supplier segments then arrange the packaged diodes of-return for technology developers. that are further detailed in Figure 6. into a final assembly; provide a means That does not mean that suppliers are Please note that Figure 6 is not a for cooling the diodes (air or liquid); ignoring slower developing markets— fully comprehensive representation. and engineer the base controls for the activity is simply at a research- While specific companies are host interface and connection to a DC and-development stage as opposed referenced, this is for illustrative power source. Finally, other original to a commercial one. Typically, purposes only. Please also note equipment manufacturers (OEMs) this research and development is that it is not uncommon for some or system integrators purchase the spearheaded by multiyear partnerships companies to operate within entire plug-and-play, UV-LED system involving large end-users or machine multiple segments, while others elect or simply the LED array for integration builders, UV-LED system suppliers and to specialize in only one area. onto a larger machine. While all parties formulators. Within the equipment portion of collaborate as previously discussed, As the technology diffuses into the supply chain, discrete UV-LED the integrator is ultimately responsible new markets, suppliers rely on the diodes, diode packages or modules for making sure that the curing system, development network illustrated in are purchased from a finite list of formulation, formulation delivery/ Figure 5. All of the entities in this seven semiconductor manufacturers dispensing system and the material network contribute and collaborate in shown in Column 1. Before individual handling equipment all seamlessly order to propel successful applications diodes can be used in a curing system, work together for the end-user. toward more efficient evolution. It they must be properly packaged By now, it should no longer be often takes an application champion either by the semiconductor supplier a surprise that many statements to introduce, educate and focus or another company in the supply generally directed at the UV-curing co-suppliers on a new opportunity. It chain. In general, packaging diodes industry as a whole are really only also means that the markets that have includes wire bonding the anodes credible and often not so contradictory the most champions as well as the (+) and cathodes (-); securing the when given correct application most promise draw the most attention. dies to a heat sink; and providing an context. The UV-LED reality is that

UV-LED Diode UV-LED Diode UV-LED System System Integrators Manufacturers Packagers Manufacturers

Cree Diode Manufacturers (1) Integration Technology Machine Builders Fox LED System IST Branded OEMs LG Manufacturers (3) Heraeus Noblelight End-Users Nichia Specialist Electronics Honle General Integrators Nitride Manufacturers Lumen Dynamics Phillips LED Engin Luminus Devices SemiLEDs Luminus Devices Phoseon

for every successful application in successfully using UV-LED technology not a drop-in solution. As a result, operation today, there are many more right now. Various UV applications that the industry plugs away within the examples where only conventional were not possible with conventional technology development network one UV technology is employed. Businesses arc or microwave systems have also application and one market at a time, must invest time and resources to become possible with LEDs; thus, learning more and more as it anxiously develop specific UV-LED solutions expanding the total UV-curing pie. No anticipates the next big market for each market application. While matter how much LED technology breakthrough. w some markets may require years or suppliers and end-users may want —Jennifer Heathcote is general possibly decades for viable economic to utilize LED curing for specific manager, North America, solutions to mature, many others are applications, the technology is simply for Integration Technology in Chicago, Ill.