Long-term (8000 h) reliability and failures of high-power LEDs for outdoor lighting stressed at high ambient temperatures

Modern solid-state lighting systems allowed to enhance the energy efficiency of light sources, improve quality and reduce the related costs; however, the reliability of high-power light-emitting diodes (LEDs) is a critical aspect, especially in systems where high powers/small footprints are required. In this paper, the long-term reliability of high-power light emitting diodes (LEDs) for outdoor lighting is analyzed. LEDs were stressed for 8000 h near their absolute maximum current, at different ambient temperatures (45 degrees C, 65 degrees C, 85 degrees C and 105 degrees C). The devices were mounted on metal-core printed circuit boards (PCBs), 8 LEDs per PCB. LEDs were characterized individually by means of I-V characterizations and power spectral density measurements. LEDs stressed at the lowest temperatures, which junction temperature was within absolute maximum rating, showed almost no degradation, whereas LEDs stressed at 85 degrees C and 105 degrees C, with junction temperature exceeding absolute maximum, showed an initial gradual degradation, followed by a catastrophic degradation, due to silicone cracking and darkening. X-ray imaging and shear tests highlighted a solder degradation. Remarkably, negligible thermal resistance variation was measured, but junction temperature increased during the stress. This increase was attributed to gradual silicone degradation, that increased silicone lens light absorption in a positive feedback loop, leading to the cracking/darkening of the lens.