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.