Quantification of silicone degradation for LED packages using finite element analysis

Light emitting diodes (LEDs) industry has been developing rapidly. The LED package has been applied to many areas which require illumination because of its electrical efficiency and reliability. The reliability of LED packages is evaluated through several tests such as room temperature operating life test (RTOL), high temperature operating life test (HTOL), low temperature life test (LTOL), wet high temperature operating life test (WHTOL), thermal shock test, mechanical shock test, and salt atmosphere test. Among several reliability tests, the lifetime tests are time-consuming because typically more than 2000 hours is necessary. One of the major failure modes during the tests is the lens or encapsulant crack, which results to lumen depreciation

Silicone polymers are used for encapsulant and lens for LED devices because of their high transparency, manufacturability, stable thermo-mechanical properties, and controlled refractive index. However, the silicone degradation is one of the major causes for degradation of LEDs. In this study, a key assumption was made that is the degree of the silicone degradation is strongly related to creep strain rate of the siloxane. Based on the assumption, silicone degradation during this life time test of LEDs was quantitatively evaluated using finite element analysis. In order to compute creep strain rate of the silicone, the linear viscoelastic properties of the silicone was measured and used for the FE thermal mechanical simulation. The creep strain rate of the silicone polymer was computed in a pre-defined volume of interest (VOI). Furthermore, the correlation between the computed creep strain rates and lumen depreciation could be established, which lead to a lumen depreciation model using the viscosity effect of the silicone. Based on this model, the tendency of the lumen maintenance for LED packages could be estimated using numerical analysis without time-consuming tests.