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SMT Magazine, July 2014

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SMT Magazine, July 2014 Since the EU passed the Restriction of Hazardous Substances directive in 2006, tin whiskers have become a serious threat to the assembly of PCBs. This month, our feature contributors Dr. Jennie Hwang, Andrzej Czerwinski, and Linda Woody tackle this prickly subject. 16 Tin Whiskers: Capsulization 30 Whisker Growth in Tin Alloys by Dr. Jennie S. Hwang on Glass-Epoxy Laminate by A. Czerwinski, A. Skwarek, M. Płuska, J. Ratajczak and K. Witek 40 Tin Whisker Risk Management by Conformal Coating by Linda Woody and William Fox 4 SMT Magazine • July 2014 JULy 2014 EnginEERing voLUME 29 SoLUTionS nUMBER 7 foR PCB www.smtmagazine.com MAnUfACTURing CONTENTS ARTICLE VIDEO INTERVIEWS 76 Meeting Future Stencil Printing 15 Major Changes in the Challenges with Ultrafine Last Eight Decades Powder Solder Pastes by Edward Briggs 66 A Glimpse of the Future COLUMNS 8 Electronic Assembly with Solder: an Unblinking Look at 73 DEK Deals with Automotive “The Devil We Know” Device Miniaturization by Joe Fjelstad 62 DFARS Flow Downs and Trusted Suppliers by Todd Kramer 70 More Stencil Questions (and the Answers!) by Rachel Miller-Short SHORT 88 Efficient Thermal NEWS HIGHLIGHTS Cooling and 60 Mil/Aero Heating 68 Market EXTRAS 74 Supplier/New Product 92 Events Calendar 90 SMTonline 93 Advertiser Index & Masthead 6 SMT Magazine • July 2014 gUEST CoLUMn ELECTRONIC INTERCONNECTIONS Electronic Assembly with Solder: an Unblinking Look at “The Devil We Know” by Joe Fjelstad vERDAnT ELECTRoniCS Solder is unquestionably highly practical ing electronic assemblies of every sort. The only technology for joining metals, and carries with fly in the ointment is that the EU parliament, it a long history. Its roots go back more than in a mad rush to try to look “green,” took the 2,000 years. Somewhere in the distant past, emotional and scientifically ill-advised position one of our more clever and observant ancestors that lead needed to be banned from electronic chanced to create an alloy of tin and lead that solders. melted at a low temperature. They or someone Sadly, there was never presented a credible who learned of their discovery found that this piece of scientific evidence that any user had unique combination of elements could be used ever been harmed by tin-lead solder in elec- to join pieces of metal together. This combina- tronic equipment. Nor could they prove their tion of chance observation assertion that it would be a risk to and applied imagination ground water. That has proven a key devel- said, it is true that opment in the tech- greed and com- nological history of plete mismanage- mankind. Those in the ment of electronic electronics industry of waste boarding on today are very familiar criminal behavior, with this ancient tech- has resulted in physical nology and today it illness and environ- is still the method mental harm in areas of choice for mak- of the world where 8 SMT Magazine • July 2014 We put our people first, because they are our greatest asset! When you join the Hunter team, you become part of an organization that prides itself on putting people first. If you enjoy an exciting, fast-paced work environment combined with a family-like atmosphere, the Hunter team might be the place for you! With facilities in Milpitas, CA and Atlanta, GA, we’re always looking for people to add to our 45-year history of manufacturing excellence! VIEW OUR OPEN POSITIONS » +1 (408) 957-1300 www.hunter-technology.com ELECTRoniC inTERConnECTionS ELECTRONIC ASSEMBLY WITH SOLDER continues uncontrolled recycling was being carried out b) Shorts: Solder shorts are bridges of sol- by uneducated individuals. While all is not yet der between one or more component leads on well in that regard, that “hole” in the system is an assembly. As component lead pitch con- being addressed by businesses, NGOs and gov- tinues to drop, the incidence of short circuits ernments around the world. increases. Presently, the threshold of pain for Not to be forgotten is the fact that the im- most assembly is experienced when the lead pact of the EU’s decision has been significant pitch drops below 0.5 mm. and far-reaching and it has caused the industry c) Insufficient Solder: Insufficient solder to spend needlessly many tens of billions of dol- is a condition where the amount of solder in a lars diverting the considerable talents of count- solder joint is less than desired or specified con- less talented engineers and scientists around the tractually through industry specifications or world to make products that are unfortunately customer requirements. proving less reliable and arguably less environ- d) Excessive Solder: Excessive solder is mentally friendly electronics than those built obviously the opposite of the condition of in- with tin lead solders. sufficiency and is again measured against agree- ments. It also introduces a wild card because it The Devil We Know, Disrobed is not what reliability testing is based on. While soldering (especially tin-lead solder- e) Solder Cracking: Solder cracking is ing) holds many benefits in terms of offering an obvious concern as it could result in a la- a means of mass assembly of electronic com- tent open circuit condition. Good during prod- ponents to printed circuit boards and is funda- uct test before shipping but then failing in the mentally simple, its application in the assembly field. and manufacture of electronic products of the f) Tin Whiskers: Tin whiskers are small present age is much more complex and fraught metal projections emanating from a solder joint. with opportunity for defects to be generated, They can grow up to 15 mm long and given the but it is also the a demon we have elected to live fine pitch of today’s components, they are a with, for as the old saying suggests that “Bet- significant concern. There are also challenging ter is the devil we know.” The devil we know because they are typically a latent defect that is at least familiar and the simple truth is that shows up unpredictably. Past research indicat- humans are creatures of habit and most of us ed that the addition of lead to tin solder alloys abhor change. There is a lingering question in would mitigate the formation of whiskers; how- the current situation? Is dealing with the devil ever, with the ban on lead in electronic solders we know on a daily basis really worth the price the incidence of whiskers is on the rise. we are paying? g) Poor Wetting/Dewetting: Good wet- Following is a recitation of some of the ting is manifest by the presence of uniform many types of solder and solder-related defects coat of solder on both the leads of the compo- that test and inspection is tasked with finding nent and terminations of the printed circuit to before a product reaches market. Bear in mind which they are joined. In areas of poor wetting as these defects are recounted and reviewed that or dewetting the solder thins appreciably in ar- the cost of defects rises as a product moves fur- eas leaving only a thin silvery sheen. ther from the manufacturing line. h) Voids: Voids are defect which are often difficult to detect without use of special equip- a) Opens: Opens are discontinuities gener- ment such as an X-ray apparatus. The chal- ated in the soldering process that can be mani- lenge with voids is that they represent potential fest in assembly in a number of ways. For exam- weakness in the solder joint owing to their in- ple, a bent or lifted lead on a QFP component, consistent nature. Voids can be found both in missing solder ball on a BGA, insufficient solder through-hole and surface mount components. on an LGA or the warpage of the component In the case of surface mount components the during the high-temperature, lead-free reflow voids are often extremely small and are some- process can all result in an open circuit. times referred to as champagne voids. 10 SMT Magazine • July 2014 ELECTRoniC inTERConnECTionS ELECTRONIC ASSEMBLY WITH SOLDER continues i) Blowholes: A term applied to a phenom- fect related to discrete devices such as resistors enon where a small hole is observed in a solder and capacitors, wherein solder connections are joint. Typically, the defect is found to be the not made simultaneously; the slight lag causes result of discontinuities in the plated through- the first side to reflow to pull back and rotate up, hole wall, which may absorb flux and then ex- resembling a grave marker (which is somewhat plosively out gas during the soldering process. apropos given that the assembly will likely be j) Cold Solder Joints: Solder joints that did dead if tombstones are present). not form completely a good metallurgical bond. o) Component Cracking: Component They are often the result of the joint receiving cracking can have multiple causes, one being a sufficient heat to cause complete melting and situation where there is a significant mismatch joining of the solder. Cold solder joints are of- in terms of coefficient of thermal expansion be- ten seen in cases where the component lead is tween the component and the printed circuit connected to a large thermally conductive fea- to which it is attached. It can also occur if the ture or element and insufficient heat is assembly is flexed in the area of the retained near the lead to assure a component, causing the device to good solder joint. With lead-free crack. solders, the phenomenon pro- Component cracking p) Popcorning: Popcorn- vides a greater challenge as the ing is a phenomenon mani- amount of heat which must be can have“ multiple fest when moisture entrapped supplied is much greater than causes, one being a within a component outgases it might have been with a tin during assembly, causing a lead solder, thus potentially situation where there is blister to form in the encapsu- degrading device and assem- a significant mismatch lation material.
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