Degradation Mechanisms of Mid-Power White-Light LEDs Under High-Temperature–Humidity Conditions

More and more mid-power white-light LED (MP LED) solutions have been used in outdoor illumination due to their good performance, cost attractiveness, and low energy consumption as compared with conventional lighting solutions. Hence, there is a need for MP LED manufacturers to develop more robust MP LEDs aimed at outdoor applications but still offer a significant cost benefit as compared with currently widely used high-power LEDs. This implies that MP LEDs would be operated in an environment with high humidity and high temperature. This may lead to serious degradation with different failure modes compared with an indoor operation. However, the combined effect of temperature and humidity on the MP LED reliability has not been extensively studied in literature. In this paper, MP LEDs were studied by the wet high-temperature operation life (WHTOL) test in order to understand their degradation mechanisms due to the combined effect of temperature and humidity. It is found that encapsulant discoloration (yellowing) is the major degradation mechanism in the WHTOL test, which will induce serious lumen degradation and color shift. Furthermore, it has been found that electrical degradation in terms of forward voltage increase will also affect the lumen maintenance. Finally, statistical analysis shows that lumen degradation mechanisms in the WHTOL test are similar to a failure in the LM-80-08 test, demonstrating that the WHTOL test is an efficient accelerated degradation test method, which dramatically reduces the test duration compared with the LM-80-08 test.