Investigation of lumen degradation mechanisms of mid-power LED by HAST

In this paper, the degradation modes and degradation mechanisms of mid-power LEDs were presented. For this study, commercially available mid-power LED packages were aged by method of accelerated temperature and humidity stress test (HAST). During ageing test, the ambient temperature and relative humidity was hold at 105 C and 100 %RH separately, which is so-called 105 C/100 %RH. As a result, little lumen decay was found in LED packages aged that is not stressed by current, while sharp lumen decay was observed in LED packages stressed by constant current. Physics analysis indicated that sharp lumen decay of the LED packages was mainly due to carbonization of the silicone plate, which was resulted from either the self-heating of phosphors or over-absorption of blue lights. The degradation trend observed in 105 C/100 %RH was different from that in 85 C/90%RH, indicating that different degradation mechanisms were triggered in HAST other than that in the steady-state humidity life test. Though a few researchers had demonstrated HAS T was a potential method for fast lifetime assessment of white light LED packages, experiments should be carefully designed to prevent improper lifetime prediction.