Moisture Dependent Degradation Rate of Silicone in LED Optical Housing Material—Ab-Initio Modelling

Abstract

Degradation of LED housings which make up of polydimethylsiloxanes (PDMS) in outdoor applications has been one of the rising concerns as they reduce the overall lifetime of the LED leading to cost and safety issues. The primary stressors for such LED degradation were found to be humidity and light emitted from the LED dice. The degradation mechanisms are hydrolysis and condensation of PDMS which themselves are interdependent. The degradation rate was also found to be dependent on the humidity level. This work identifies these dependencies, using ab-initio density functional theory and molecular dynamics simulation, and the results agree with the reported experimental data. From the detail understanding of the degradation mechanisms and the identification of the rate determining factors which are moisture level and the level of light absorption in PDMS, acceleration models commonly used for extrapolation are questioned, and new quality indexes of the PDMS are also identified to ensure the durability of LEDs in outdoor applications. Testing method to evaluate these new quality indexes is also proposed.