Thermal Reliability of Low-Cost High-Power LED Package Module Under WHTOL Test

The LED issues, associated with high cost, high junction temperature, low luminous efficiency, and low reliability, have to be solved before gaining more market penetration. With special features of low-junction-temperature and low-cost design, COP (Chip-on-Plate) LED package modules with and without phosphors are evaluated in terms of their thermal resistance and reliability under wet high temperature operation life (WHTOL) test. The WHTOL test is with the condition of 85°C/85% RH and 350mA of forward current for 1008 hrs, specified in JESD22 Method A101-B. First of all, the thermal behaviors of the COP package module are investigated by experimental measurement, and a computational fluid dynamics approach. The reliability under WHTOL test is then carried out. The results show that all COP package modules with phosphors in the silicone encapsulant failed after 309 hrs at WHTOL test, but all those without phosphors passed for 1008 hrs. The failure sites are located at aluminum wire debonding to the chip and copper pads of the substrate. However, the aluminum wire bonding of the COP package modules are replaced to gold wire bonding, then all COP package modules with and without phosphors pass for 1008 hrs. For the passing package modules, their thermal resistances are found to increase more than two fold after 1008 hrs of the WHTOL test from 41°C/W to 87.1°C/W. This is due to the thermal conductivity decreasing in the die attach and thermal grease and the contact resistance increasing after the moisture absorption. Moreover, for the thermal behavior of the COP package modules under the natural and forced convections in the WHTOL test, the results show that there exists the difference of 17°C/W in the junction-to-air thermal resistances, which might result in different reliability data. In addition, it is also indicated that the junction-to-air thermal resistances are very sensitive to the flow conditions of the chamber, but not for junction-to-aluminum substrate and junction-to-heat sink thermal resistances. Therefore, the standard test of the WHTOL should specify flow conditions in the test chamber.