Robustness and reliability of high-power white LEDs under high-temperature, high-current stress

High-power white LEDs for outdoor lighting were submitted to accelerated lifetime stresses to evaluate their robustness and reliability. LEDs featured a 2 mm2 chip with 2 A absolute maximum current (Iabs) at 135 °C junction temperature. A first high-temperature, high-current robustness stress was performed at 1, 1.2, 1.4, 1.6 times Iabs for 50 hours. This stress caused a heavy decrease of optical power and a degradation of colorimetric properties of the LEDs stressed at currents exceeding Iabs. Optical analysis showed darkening of the phosphors and silicone and cracking of the lens. A second, longterm, stress was performed at 0.8 times Iabs at 45, 65, 85, 105 °C. This stress showed almost no lowering in flux for the samples stress at 45 and 65 °C, whereas samples stressed at 85 and 105 °C showed a decrease in flux from 2500 and 200 hours of stress, respectively, estimating a L90 lifetime of 5500 and 1500 hours. xy coordinates shifted proportionally to stress temperature. LEDs stressed at 85 °C and 105 °C eventually failed catastrophically, similarly to the high-current stress, with silicone and phosphors darkening and lens cracking. Raman analysis on high-current stressed LED lenses showed that poly(methyl,phenyl)siloxane was used as lens material. Stress induces higher luminescence of the silicone under Raman analysis. The cause of degradation is attributed to thermomechanical stress (cracking) and hightemperature silicone decomposition (darkening), possibly due to phosphors thermal quenching, causing a hot-spot just above the chip (confirmed by thermography), even if the junction temperature was within manufacturer specifications.