Effects of Ultraviolet Wavelength and Ambient Temperature on Reliability of Silicones in InAlGaN-Based Light-Emitting-Diode Package

We investigated the effects of different ultraviolet wavelengths and ambient temperatures on the optical and thermal performance of light-emitting-diode packages. After application of 500 mA at 85 °C for 1500 h, the 365 nm package sample showed a larger reduction (29.2%) in the output power than the 405 nm sample (5.3%). Further, operation at 1500 mA caused a considerably large reduction of 27.2%-60.0% of the output power of the samples. After operating at 85 °C under 1500 mA for 1500 h, the thermal resistance between the junction chip and package increased to values in the range of 1.42-2.54 K W^-1 depending on the peak wavelengths. The surface temperature of the 365 nm sample increased to 101.8 °C when operated at 85 °C under 500 mA for 1500 h. The thermomechanical stress was concentrated on the chip surface and measured as 3.32 MPa at 125 °C. Cracks were observed to initiate on the chip surface and propagate into the silicone lens via thermal stress. The reduction of light output power was attributed to the lowered extraction due to the occurrence of cracks, thereby decreasing the view angle and reducing the uniformity of illuminance.