Reliability and accelerated test methods for plastic materials in LED-based products

In this study the effects of thermal ageing on the optical properties of both lens and the remote-phosphor samples, made from Bisphenol-A polycarbonate (BPA-PC) are investigated. The BPA-PC lens and remote phosphor plates are currently widely used in light conversion carriers and optical lenses in LED-based products. Lens and the remote phosphor BPA-PC samples of 3 mm thickness were thermally aged at temperature range 100 to 140 °C. The phosphor plates, combined with a blue LED light source, produce white light with a correlated colour temperature (CCT) of 4000 K. The colour shifting due to thermal ageing was studied by Integrated Sphere. Results show that thermal ageing leads to a significant decrease in the luminous flux and chromatic properties of plates. It is also shown that by increasing the temperature, the kinetics of degradation reaction becomes faster, inferring that lumen depreciation takes place at shorter time. Lumen depreciation up to 30% reduction is extrapolated to temperatures lower than 100 °C. It is shown that the lifetime, defined as 30% lumen depreciation at 40 °C, is around 35 khrs for remote phosphor and around 100 khrs for BPAPC lens. A significant change both in the correlated colour temperature (CCT) and in the chromaticity coordinates (CIE x,y) is also observed in thermally aged specimens. Deterioration of the chromatic properties of the phosphor plates is correlated to the decrease in the luminous flux. Results also confirm the colour shifting of white light towards yellow region. Based on the observed decay of CCT and colour shifting, one could conclude that the thermal degradation of the remote phosphor plates affects both the efficiency and the colour of the LED products. The proposed thermal-ageing qualification method can be used by industries to efficiently select the proper phosphor materials and verify the product design, without many trial-error based interactions.