Thermal Resistance and Reliability of High-Power LED Packages Under WHTOL and Thermal Shock Tests

The light emitting diode (LED) packaging problems associated with high cost, high junction temperature, low luminous efficiency, and low reliability have to be resolved before the LED gaining more market acceptance. In this paper, chip-on-plate (CoP) LED packages with and without phosphors are evaluated in terms of thermal resistance and reliability under wet and high-temperature operation life (WHTOL) and thermal shock tests. The WHTOL test is with the condition of 85°C/85%RH and 350 mA of forward current for 1008 h, while thermal shock test is with 200 cycles at temperature ranging from -40°C to 125°C. The thermal behavior of the CoP packages was analyzed by 1-D thermal resistance circuit (1-D TRC) with and without spreading angle, 3-D TRC method, and 2-D axisymmetric finite element method. The feasibility of these analyses was evaluated and discussed in detail by comparing those results with experimental measurements. The reliability results indicated that all CoP packages with phosphors in the silicone encapsulant failed after 309 h in the WHTOL test, but all those without phosphors still survived after 1008 h. The failure modes were found to be the debonding of the aluminum wire from the chip or copper pad of the substrate. However, after the aluminum wire was replaced by gold wire, all the packages with and without phosphors passed after 1008 h. For these survival packages in the WHTOL test, their thermal resistances of junction-to-air and junction-to-aluminum substrate increased by about 12 and 9°C/W, respectively. Moreover, it was also found that there is a difference of 38°C/W in the junction-to-air thermal resistances for the packages between under natural and forced convections in the chamber during the WHTOL test. This might yield the different reliability data, unless the flow conditions in the test chamber are specified in this standard test. Furthermore, all the packages with and without phosphors could pass 200 cycles in thermal shock test, with minor changes in the thermal resistances. However, the degradation of luminous flux in the packages with phosphors was found to be greater than those without phosphors by 14% vs. 9%.