Analysis of delamination and darkening in high power LED packaging

Luminous emittance declination due to interfacial delamination and darkening is a fatal defect for the GaN-based high power light-emitting diodes (LEDs) under a long time service. This letter brought forward an accelerated aging test under 350 mA forward current, 25degC, 54% RH, 256 hours to testify and analysis the abnormal dark stain on the die surface of high power blue LEDs. SEM micrographs show that a delaminated layer exhibits between the die and the epoxy glue cap on the aging sample. EDAX results illuminates that the percentage of C and O elements is higher than normal. The thermal overstress on the epoxy along the interface owing to the high junction temperature is the main factor to degrade the epoxy gradually. As a result, a darkening area made up of carbon as well its oxide occurs on the LED die surface and depresses the luminous efficiency.