Solid-state lighting: failure analysis of white LEDs

Long life, on the order of 50,000-100,000h, is one of the key features of light-emitting diodes (LEDs) that has attracted the lighting community to this technology. White LEDs have yet to demonstrate this capability. The goal of the study described in this manuscript was to understand what affects the long-term performance of white LEDs. Different types of LEDs have different degradation mechanisms. As a starting point, this study considered a commonly available commercial package, the 5 mm epoxy-encapsulated phosphor-converted (YAG:Ce) white LED. Based on past studies, it was hypothesized that junction heat and the amount of short-wavelength emission would influence the degradation rate of 5 mm type white LEDs, mainly due to yellowing of the epoxy encapsulant. Two groups of white LEDs were life-tested. The LEDs in one group had similar junction temperatures but different amplitudes for the shortwavelength radiation, and the LEDs in the second group had similar amplitudes for the short-wavelength radiation but different junction temperatures. Experimental results showed that the degradation rate depends on both the junction temperature and the amplitude of short-wavelength radiation. However, the temperature effect was much greater than the short-wavelength amplitude effect. Furthermore, the phosphor medium surrounding the die behaves like a lambertian scatterer. As a result, some portion of the light circulates between the phosphor layer and the reflector cup, potentially increasing the epoxy-yellowing issue. To validate this theory, a second experiment was conducted with LEDs that had the phosphor layer both close to the die and further away. The results showed that the LEDs with the phosphor layer away from the die degraded at a slower rate. (C) 2004 Elsevier B.V. All rights reserved.