Effect Of Environmental Conditions And Packaging Process On Reliability And Uniformity Of High Power LED

Solid-State lighting, which is based on high power light-emitting diodes (LEDs), iswidely recognized as the most potential candidate of next general lighting source, due to itsspecial advantage such as low power consumption, high efficiency and long life time. It hasbeen applied in landscape light, road lamp, back lighting source, and automotive headlampetc. However, reliability becomes a big challenge for LEDs further application. Andimproper packaging processes results in variation of optical performance of LED packagesand greatly affects manufacturing efficiency. These have stunted LED’s development.Therefore, it is important to improve LED products’ reliability and uniformity of opticalperformance. In this paper, the effect mechanisms of thermal and moisture, which are twovital factors in environment, on LEDs’ reliability are studied. Meanwhile, the effect of goldwire bonding process on optical uniformity of white LEDs is also discussed. This work willbe conducive to design LED packaging structure, optimize packaging process, and helpobtain better performance LEDs.A series of accelerated life tests of high power LED under different thermal andhumidity conditions have been conducted, such as storage tests and life tests. It was foundthat the moisture diffused into PC lens and entered into inner package. The defects, such asdelaminations, nigrescence of phosphor, and crack at silicone layers occurred. After thetests, the thermal resistance of LED module increased. These defects caused the decrease oflight output of LED package. Tests results indicated that lens played important role inLED’s reliability. The modules packaged with application specific packaging (ASP) lensperformed better than ones packaged with traditional hemispherical lens. The defects inpackage impacted LED module’s reliability more than the degradation of LED chip.Based on85℃/85%Relative Humidity (RH) test, a comprehensive stress model whichconsiders heat transfer, moisture diffusion, thermo-mechanical stress, hygro-mechanicalstress and vapor pressure was established for LED module. The simulation results showedthat the speed of moisture diffusion was very slow, but it needed only five hours for PC toreach90%RH at test conditions. The moisture would reduce the adhesion strength at the interfaces. PC lens and molding compound in package were inclined to deform. Thedegradation of packaging materials, defects expansion and change of optical routineresulted in light output loss of LED module.Optical models were established to estimate the effects of delaminations on lightoutput of LED package. The results showed that delamination between PC lens and siliconehad little impact, but delamination on interface between LED chip and phosphor layercaused exceeding50%light loss. For LED modules packaged with conventional chip andvertical chip, the effect mechanisms of delamination were different.The effect of surface roughness of LED chip on package’s reliability was also studied.It was found that greater unevenness caused larger stress and strain at phosphor layer nearthe chip. Also, the impurities such as moisture and dust easily existed at rough surface.These would contribute to delamination between chip and phosphor layer.From simulation and experimental research, it was found that gold wire bonding, as animportant packaging process, had great impact on the angular uniformity of correlated colortemperature (CCT) in white light-emitting diode (LED) package whose phosphor layer wascoated by freely dispersed coating process. The wire bonding process should be furtheroptimized in order to obtain LED products with better optical performance.