Analysis of extrinsic failure mechanisms of high-power blue, red, and white LEDs for horticulture and street lighting

Over the last decades, high-power LEDs were developed for applications in which a high irradiance source is required in a small footprint. This came at the cost of high-power dissipation in a very small footprint. We present the results of a long-term (>10000 hours) stress on commercial blue and red LEDs for horticulture applications and white LEDs for outdoor lighting. Red and blue LEDs were stressed at 1.2 times absolute maximum current (AMC), white LEDs at 0.8 AMC, at ambient temperatures ranging from 45 to 105 °C, 8 LEDs for each condition. The LEDs were characterized by means of I-V measurements, power spectral density measurements. Further analyses were performed by means of computerized tomography (CT) and several mechanical cross sections were made. Blue LEDs at 105 °C showed a catastrophic degradation just after 2500 hours of stress, with deep cracks in the silicone lens. LEDs stressed at other temperatures showed a similar, but gradual, failure mode with an onset depending on stress temperature. Red LEDs showed catastrophic degradation only at 105 °C, with darkening of the silicone, deep cracking and, in some devices, bonding wires broken due to mechanical stress. White LEDs show an initial cracking of silicone near the chip, then a catastrophic degradation with diffused cracks of the silicone lens and darkening, with a time dependence on stress temperature. The catastrophic degradation is observed only at conditions exceeding the maximum rated junction temperature. CT analysis highlighted initial ruptures on the border of the phosphor layer, which then propagate upwards, causing higher absorption, higher temperature, and a chemical decomposition of the silicone. Therefore, for all the LEDs a temperature-activated degradation of the silicone is seen, enhanced in white LEDs due to the light-conversion layer and in blue LEDs due to higher energy photons.