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The high-power Light Emitting Diode (LED) can be potentially used for general lighting to alleviate the global warming problem. However, the LED issues, associated with high cost, high junction temperature, low luminous efficiency, and low reliability, have to be resolved before becoming realized. Featuring low-junction-temperature and low-cost design, a novel high-power COP (Chip-on-Plate) LED package is proposed and its reliability under wet high temperature operation life (WHTOL) test is evaluated. The WHTOL test is with the condition of 85degC/85%RH and 350 mA (power=1.22 W) of forward current for 1008 hrs, specified in JESD22 Method A101-B. First of all, the thermal behaviors of the COP package are investigated by experimental measurement (with LED junction temperature tester and thermal couples), a thermal resistance circuit (TRC) method and a finite element method (FEM). The junction temperature (Tj) and surface temperature of the COP package are measured by the junction temperature tester and thermal couples, respectively. Furthermore, the reliability under WHTOL test is carried out. The results show that all COP packages with phosphorus in the silicone encapsulant failed after 309 hrs at WHTOL test, but all those without phosphorus passed for 1008 hrs. The failure sites are located at aluminum wire debonding to the chip and copper pad of the substrate. For the passing packages, their junction to air thermal resistances increase with the time of WHTOL by up to 12degC/W. This is due to the decrease of the thermal conductivity of the die attach, resulting from the moisture absorption. The natural and forced convections in the chamber for WHTOL test give the same the junction-to-aluminum substrate thermal resistance, but different chip junction temperatures with a 19degC difference, which might result in different reliability data. Therefore, the standard test of WHTOL should specify flow conditions in the test chamber.