SILICONE-PHOSPHOR ENCAPSULATION FOR HIGH POWER WHITE LEDS

Horatio Quinones, Brian Sawatzky, and Alec Babiarz ASYMTEK Carlsbad, CA, USA

with internal light sensitive layers which converts the blue ABSTRACT wavelength to a variety of multiple wavelengths resulting in High-brightness Light Emitter Diodes (HB-LED) market a white appearance. Typical optical radiation pattern for a has grown at an average rate of about 43% since the packaged LED emission tends to be within an angle of introduction of blue light wavelength die. HB-LEDs provide about 200 from the direction of maximum intensity. Intensity for a range of applications such as architectural lighting, of light is another parameter of importance, and has signaling, entertainment lighting, automotive lights and dependencies on the integrity of the packaging of the LED LCD backlighting. The flexibility in color-changing as well as on the volumetric accuracy of the fluid and its properties, long life time, robustness, energy efficiency composition homogeneity. Large discrepancies (up to 50%) (lumens/watt) makes this technology very attractive. There of photometric measurements of LED’s are common in the are issues and challenges including the need to improve industry. Various experiments were performed to address electronic drive circuit technology, total lumen output, potential parameters responsible for LED performance “white light problem”, color quality and reproducibility and variation. The CIE standard was used to quantify cost efficiency. HB-LEDs are not a simple die, they need to differences. be packaged in a complex structure as to maximize the effective intensity and prevent optical aberrations. HB- LEDs come in many packages, from single chip to EXPERIMENTAL WORK sophisticated multi-directional aspheric lens designs. They Test Objective include lenses, colored materials, and diffusers (one or two The main purpose of the present study is to isolate possible part silicone materials filled with phosphors) all of which sources of variability associated with LED encapsulation can alter the spatial and spectral distribution relative to the with Phosphor-Silicone mixture. Specifically we expect to basic light emitter die. Packages may include chips of investigate the influence of phosphor loading, fluid volume, different size, different types and different locations. and time-based behaviors on the photometric properties of Packages and chip locations may have different mechanical the devices. To determine the comparative performance differences of various dispensing processes including tolerances. Process for dispensing the fluids that constitute a [2] large part of both, manufacturing and packaging of HB- jetting and traditional needle dispensing both, qualitative LEDs need to be precise reliable and cost effective. Jetting and quantitatively. The test strategy consisted in the initial technology offers a five fold faster process, and higher characterization of high power (about 1 Watt) blue LEDs, precision than traditional needle dispensing. The paper see figure 1. addresses some of the challenges in the LED packaging in Bare chip spectrum particular jetting processes that lead to tighter distribution of 9.00E-03 the HB-LED color spectra (CIE, XYZ map) which in turn 8.00E-03 7.00E-03 reduces the binning of LEDs and reduces the packaging cost 6.00E-03 5.00E-03 of ownership. 4.00E-03

intensity 3.00E-03 2.00E-03 Key word: LED, jetting, dispensing, YAG, CIE, volumetric 1.00E-03 accuracy. 0.00E+00 -1.00E-03 0 200 400 600 800 1000 wavelength(nm) INTRODUCTION Figure1. HB-Blue die wavelength spectrum The introduction of blue LEDs as the base to obtain white light by means of packaging compounds, i.e., phosphors The HB-LED package for this study consisting of the wire [1] Yttrium-aluminum-garnet, YAG and silicones/epoxies bonded die and fluid dispensed (two-part silicone matrix presents several dispensing challenges. No longer can a pure and phosphor, YAG), and not including the optical lens is clear silicone compound with RGB source be used. Precise evaluated for various optical characteristics. The metrology volumetric accuracy of the phosphors needs is required to process for evaluation consisted mainly on standard assure minimum variation in the color. The white LEDs photometry. addressed in this paper are those built from a blue source

Experiment Cells Once the baseline, mass and silicone-to-phosphors ratio was The bulk of the work reported here consisted of fluid established, CIE XY and brightness were evaluated as dispensing processes with various methods that included function of various other process parameters. The effect of traditional needle dispensing and jetting process. Dow volumetric accuracy was examined using jet dispensing and Corning two part silicone material doped with YAG traditional time-pressure dispensing. Cell II experiments phosphor was used. The die used was a blue spectrum were carried by dispensing with both needle and jet. emitter diode from ELITE, 1mm2 with mean wavelength of Phosphors content was kept constant at 9.1% per weight. 460 nm (+/-2.5 nm); the die is wire bonded (1 mm high). Mass was varied from 1.2mg to 2.5 mg and CIE Figure 2 Depicts the blue HB-LED cavity and wire bonded measurements were taken. Figure 4 depicts HB-LEDs with die prior to fluid dispensing prior to fluid dispensing and various amounts of material dispensed. dicing

2mg 10wt% phosphor 2.25mg 10wt% phosphor 2. 5mg 10wt% phosphor

Figure2. HB-LED blue die showing the wire bonded die and the cavity prior to silicone and YAG dispensing. Figure4. HB-LED with various volume of silicone-YAG dispensed. Optical and electrical measurements were performed using the Labsphere System. Experiments were partitioned into As the volume dispensed was increased so did the CIE four cells. (I) To determine the appropriate mass mean value X&Y values. For the case of jetting the correlation can be dispensed on the LEDs and appropriate silicon to YAG ratio better established since its variation was tighter as depicted as to meet specific CIE X and Y values by an iterative in figure 5. For the needle dispense (time-pressure), figure 6 approach. (II) To determine correlation of material volume the CIE XY to mass correlation although has similar trend dispensed on the LED (Silicone and 9.1% by weight of as jetting, its correlation is not as high. This can be phosphor) to various optical properties of the packaged HB- attributed to larger variation of the mass dispensed by LED. (III) To determine correlation of phosphor/silicone needle. mix ratio to various optical characteristics of packaged HB- LED. (IV) To determine correlation of material (silicone + 9.1% phosphor) pot life to various optical characteristics of Jetting: CIE Spectra vs M ass the packaged HB-LED. (V) To determine the spread for 0.9 520 time-pressure needle dispense and jetting for the optimum 0.8 530 510 540 volume and silicon-to-phosphors ratio. 0.7 550

560 0.6 570 500 0.5 Experimental Results 580

590 For the first cell, optimization of volume and phosphors 0.4 600 2.5 mg 610 17.5 mg 620 content for the HB-LED, it was determined that a mass of 0.3490 2.0mg 830 about 1.75mg with a 9.1% phosphors content will give the 0.2 1.5 mg 480 best results, i.e., acceptable brightness (>30 lm/W) and 0.1 470 460 white color emission. We can see in figure 3 (a) the process 0 360 of volume given 9.1% phosphors mixture, (b) the process of 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Cx mixture ratio given a 2mg shot. Figure5. CIE X-Y values for various volumes dispensed,

CIE vs Mass for 9.1%Phosphors CIE vs. Phosphor wt% by jetting process, 9.1% phosphors per weight. 0.5

0.4 0.5 0.4 0.3 0.3 Results from Cell II clearly show trends that are directly

CIE Value CIE 0.2 0.2 CIE value CIE 0.1 related to the spread of the CIE X-Y values and that 0.1 0 1020304050 11.522.53 influence the number of bins for a given LEDs class. Cell Mass (mg) Phosphor wt% Cx Cy Target Cx Cy target III experiments were performed to quantitatively determine (a) (b) the influence of silicone-to-phosphors ratio and CIE Figure3. CIE target and results from silicone volume (a) parameters. Blue light emitter die depend on the phosphors and doping ratio (b) optimization weight of phosphors. conglomerates to produce white light spectrum. For this experiment the target was X=Y=0.3nm on the CIE map. This target was met by using the mixture of 1.75gm of silicone two part material and 9.1% YAG for each HB-LED.

Neeedle: CIE Spectra vs Mass path of the fluid flow will dictate the phosphors-to-silicone ratio being dispensed and thereby determining the CIE XY 0.9 520 values. Viscosity of the silicone has a direct correlation with 0.8 530 the time for this settling to occur, lower viscosities causes 510 540 0.7 550 faster settling. At the early stages of dispensing a good

560 0.6 mixture ratio yields good white color, however over time 570 500 the phosphors settling in various places (depending on the 0.5 2.5 mg 580 2.0mg 590 0.4 17.5 mg fluid path geometry,) yields corresponding variations in the 600 610 color map. Eventually, the phosphors settling, either 0.3490 620 1.5 mg 830 dispenses (yielding yellow rich spectra), or will permanently 0.2

480 reside on stagnation zones and low phosphors-to-silicon 0.1 470 460 ratio will be dispensed resulting in a blue spectra LED .This 0 360 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 time dependence can be depicted in figure 8. Cx

Figure6. CIE X-Y values for various volumes dispensed, Jetting: CIE vs Time with needle, time-pressure process, 9.1% phosphors per 2.0mg, 9.1%Phosphpors weight. 0.4 0.35 Hence, the distribution and amount of phosphors in the 0.3 silicone mixture is a very important parameter to control. 0.25 0.2 Scarcity of the phosphors results in blue wavelength 0.15 emission, excessive amounts of the phosphors although may 0.1 induce higher brightness, it also carries a yellowish color 0.05 0 emission. Figure 7 shows direct correlation of phosphors 0 0.1 0.2 0.3 0.4 content to the CIE chromaticity. The range for phosphors- Cx to-silicone mix varied from 5% to 90% per weight. 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th

Figure8. CX, CY map for different time after initial mixing Jetting 2.0mg,5,10, 15,20,25,30,40,50,60,70,%Phosphors: of YAG and silicone. CIE Spectra

0.9 520 Above plot can be incorporated in the CIE map and thereby

0.8 530 obtaining the corresponding variation and bin distribution. 510 540 0.7 550 Figure 9 shows this mapping. 560 0.6 570 500 50% 0.5 40% 70% Jetting 2.0mg,9.1%Phosphorsvs Time: CIE Spectra 30% 580 25% 590 0.4 20% 15% 600 610 10% 0.9 520 0.3 490 620 830

0.2 0.8 530 5% 510 540 480 0.1 0.7 550 470 460 0 360 560 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 570 500 Cx 0.5 580

590 0.4 600 Figure7. Plot of the CIE vs phosphors content in the 610 0.3 490 620 silicone matrix base, total mass dispensed was 2.0mg. 830 0.2 480 Clearly phosphor content ratio to silicone cause a large 0.1 470 460 variation in the CIE XY values, thereby increasing the 0 360 number of bins for the same die. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Cx

Cell IV experiments consisted of examining the Figure9. CIE diagram of chromaticity as function of phenomenon of phosphors settling in the silicone mixture phosphors settling in the silicone over time. over time and its influence on HB-LEDs optical characteristics. Given the properties of the mix, the Cell V addresses various dispensing processes namely phosphors conglomerates have negative buoyancy in the needle and jetting. Determination of correlations often silicone fluid and settling occurs. For a dispensing system requires precise dispensing systems capable of resolving the presence of stagnation zones and their locations in the small variations. This was accomplished by jetting

processes since it was determined that the volume accuracy jetting process exceeded target. Jetting materials for LED was better than that of needle. A targeted of 2.0mg of packages provides a dispensing method contact free and silicone-YAG mixed (9.1% phosphors per weight was volumetrically consistent. Jetting being exercised at dispensed on HB-LEDs. For the needle a time-pressure dispense gaps noticeably higher than those used by the valve was used. For the jetting a DJ-9000 DispenseJet from traditional needle dispensing, makes it surface topology free Asymtek was used. Figure 10 shows the results; there one as well as damage proof on the parts. Asymtek jetting valve can observe a larger spread in the needle dispensing process. DJ9000 is a proven technology: used extensively in the This is inherently a problem facing time-pressure dispensing underfill process and optoelectronics including several LED today, and will most likely worsen for small volumes. fluid dispensing. Jetting shows quality improvement over the traditional needle dispensing. Jetting process throughput

Jetting CIE for 2.0mg 9.1%Phosphors Needle CIE for 2mg 9.1%Phosphors about 3X to 8X higher than the needle process.

0.9 0.9 520 520

0.8 530 0.8 530 510 540 510 540 ACKNOWLEDGMENT 0.7 550 0.7 550 560 560 The authors would like to thank the ITRI center for their 0.6 0.6 570 500 500 570 0.5 580 0.5 580 metrology and suggestions on this work. We also would like Cy 590 Cy 590 0.4 0.4 600 600 to extend our thanks to Mr. G. Gibbs for his support and 610 610 0.3490 620 0.3490 620 830 830 guidance in completing this work. Last we would like to 0.2 0.2 480 480 thank the various personnel from the Asymtek Taiwan Lab 0.1 0.1 470 470 460 460 0 360 0 360 that performed the experimental work, Mr. C. Chen, B. Lin, 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Cx Cx J. Lai and P. Lee.

(a) (b) REFERENCES Figure10. (a) CIE map for jetting dispense of silicone and [1] Japanese Patent Application No. 10-194156 (194156, YAG mix. (b) CIE map corresponding to traditional time- 1998) filed on Jul. 9, 1998, Japanese Patent Application No. pressure needle dispense. 10-316169(316169/1998) filed on Nov. 6, 1998 and Japanese Patent Application No. 10-321605(321605/1998) The brightness of the HB-LED and its wavelength Spectrum filed on Nov. 12, 1998. are plotted in figure 11. Jetting yielded high brightness. [2] H. Quinones, A. Babiarz, L. Fang (EMAP, Taiwan, October 2002) Jetting Technology: A Way of the Future in Jetting (PK2) spectrum Dispensing.

lm/W=33.06 1.60E-03 1.40E-03 1.20E-03 1.00E-03 8.00E-04

intensity 6.00E-04 4.00E-04 2.00E-04 0.00E+00 0 200 400 600 800 1000 wavelength(nm) Figure11. LED wavelength spectrum after encapsulation

During the HB-LED dispensing process it was observed that the needed z-motion retraction for traditional needle dispensing resulted in longer dispensing times compare to jetting. Experiments simulating production runs showed a difference of about 5X, the jet having higher throughput.

CONCLUSIONS The CIE chromaticity spread is very much dependent on the quality of the dispensing process. The volumetric accuracy is directly related to the spread of the CIE map, i.e., the binning of LEDs. Phosphors doping variation yields spread of the CIE XY values and increased binning. Phosphor settling occurs over time. This settling results in variations of the silicone-to-phosphors ratio and thereby increasing binning as well. Jetting dispensing will result in less binning compared to that of the needle. The HB-LED brightness for