Reliability and optical properties of LED lens plates under high temperature stress

• Thermal degradation of Bisphenol A Polycarbonate at 100–140 °C is studied.
• Increasing the exposure time is associated with yellowing.
• Two stages are observed in the yellowing of polycarbonate plates.
• Oxidation of plates leads to yellowing.

In this investigation the thermal degradation mechanisms of Bisphenol A Polycarbonate (BPA-PC) plates at the temperature range 100–140 °C are studied. The BPA-PC plates are currently used both in light conversion carriers in LED modules and optical lenses in LED-based products. In this study BPA-PC plates are aged at elevated temperature of 100–140 °C for a period up to 3000 h. Optical and chemical properties of the thermally-aged plates were studied using UV–Vis spectrophotometer, FTIR–ATR spectrometer, and integrated sphere. The results show that increasing the thermal ageing time leads to yellowing, loss of optical properties, and decrease of the light transmission and of the relative radiant power value of BPA-PC plates. The results also depict that there is not much discoloration within the first 1500 h of thermal ageing. The rate of yellowing significantly increases at the end of this induction period. Formation of oxidation products is identified as the main mechanism of yellowing. An exponential-based reliability model is also presented to calculate the rate of degradation reaction and to predict the life-time of BPA-PC plates.