Effect of Thermal Properties of Die Attach Materials on Performance of High Power Blue Leds

InGaN-based High Power blue LEDs exhibit a maximum light conversion efficiency of 40%, which decreases further at high drive currents. Thus, more than 60% of the electrical input power is dissipated as heat in an LED chip, leading to a rise in the junction temperature of the LED. Junction temperature has a significant impact on the light conversion efficiency of the LEDs. Also, it reduces LED lifetime by excessive heating and results in subsequent failure of the LED chip. Thus, heat flow from junction to heat sink is important for maintaining the junction temperature as well as the light conversion efficiency and light output in a high power LED package. Heat flow can be facilitated by using high thermal conductivity die attach materials for packaging LED die on the MCPCB. Therefore, commercially available LEDs have been characterized for evaluating the affect of different die-attach materials on LED junction temperature and light conversion efficiency at different drive currents. Also, the variation of peak emission wavelength and light conversion efficiency with heat sink temperature has been determined for the packaged LEDs. The results and observations have been presented here along with recommendations for future work. INTRODUCTION The junction temperature in the LED increases with increasing drive current since, more than 60 % of electrical input power is dissipated as heat due to efficiency droop at high drive currents in InGaN LEDs. This rise in the junction temperature reduces the light output by increasing the probability of non-radiative recombination. Thus, the dissipated heat needs to be removed from the junction in order to maintain the light conversion efficiency and light output from the High Power LED package. The various components in the heat flow path in a High Power LED package as shown in Figure-1 are LED junction, substrate (Sapphire, SiC, Si), die attach material, MCPCB, thermal interface material and heat sink. The thermal conductivity of the die attach being lower than some of the other components in the heat flow path can play a significant role in determining the thermal resistance from junction to heat sink which further affects the heat flow rate and junction temperature in the LED package. Figure 1. High Power LED Thus, in order to evaluate the affect of die attach materials with different thermal conductivities on LED junction temperature, light conversion efficiency and light output, commercially available 1 W blue LED chips from SemiLED were packaged on Aluminum Metal Core PCB and characterized by junction temperature and optical measurements. LED junction temperature cannot be measured directly and therefore, forward voltage method was used for evaluating junction temperature at different drive currents, utilizing the variation in the LED forward voltage with junction temperature. Optical measurements were done on the packaged LEDs in a Labsphere Integrating Sphere for determining their light output, light conversion efficiency and peak emission wavelength.