Degradation of epoxy lens materials in LED systems

Due to their long lifetime and high efficacy, solid state lighting (SSL) has the potential to revolutionize the illumination industry. The long lifetime claimed by the manufacturers is often based solely on the estimated depreciation of lumen for a single LED operating at 25°C. However, self heating and high environmental temperature which will lead to increased junction temperature and degradation due to electrical overstress can shorten the life of light emitting diode. Furthermore, each SSL system includes different components such as the optical part, electrical driver and interconnections. The failure/degradation of any components will severely affects the performance and reliability of whole system and hence the weakest component will become the bottleneck for the reliability and lifetime of the module. Literature reviews of the factors influencing the life of LED lamps identified the degradation of the epoxy lens and plastic package due to the junction temperature and voltages as one of the common failure mode. In this research, a methodology to predict the degradation of the epoxy lens has been proposed. In order to correlate the mean time to failure as a function of the junction temperature and the inputted voltage, the simplified Eyring models had been proposed in this research. Since the life of a SSL system is subjected to varying loading condition, another objectives of this research is to present a methodology to predict the life of a SSL under changing condition.