Degradation Physics of Silicone Under UV-A Irradiation

Silicone possesses good thermal and chemical stability, hydrophobicity, and thus it has a wide range of application. It is also employed as encapsulant and housing for high power LEDs which are increasingly common for various kind of lighting applications. In outdoor applications of high-power LEDs, UV irradiation from sunlight is presence. There are also UV LEDs for other lighting applications, and these LEDs are using silicone as encapsulant to protect the semiconductor chips within. However, the effect of UV irradiation on the reliability of silicone is seldom studied. This work studied the reliability of silicone packaging in LEDs under UV irradiation, and through detailed failure analysis, we found that the silicone does degrade under the UV irradiation that affect the reliability of the LEDs. The degradation mechanisms of silicone and the degradation rate due to UV are found to depend on the type of high-power LEDs, with blue LED degrades faster than white counterpart. Ab-initio analysis using density functional theory is employed to have a deeper insight on the degradation mechanisms and the analysis results explain well our experimental observations. Basically, the silicone degradation mechanism in the presence of UV irradiation is a progression of moisture assisted hydrolysis, condensation and thermal oxidation.