Failure Mechanisms Associated with Lens Shape of High-Power LED Modules in Aging Test

High-power LED modules encapsulated with different lens shapes after a thermal-aging test were studied experimentally and numerically. Samples from different manufacturers were aged at 80, 100, and 120degC under a constant driving voltage of 3.2 V. The results showed that the LED modules encapsulated with a hemispherical-shaped plastic lens exhibited a better lifetime due to their better thermal dissipation than those with cylindrical- or elliptical-shaped plastic lenses. Results also showed that the optical power of the LED modules increased after removing the plastic lens, because the degradation of the material of the plastic lens decreased the amount of light. The key module package related failure modes under thermal-aging were identified as the degradation of the plastic lens and lens material. A finite-element method (FEM) showed that the thermal and major principle stress distributions of the LED modules were significantly dependent on the aging temperature. Both experimental and FEM simulated results clearly indicated that a uniformly thermal dissipation to minimize the thermal effect along the thermal path from the LED chip to the plastic lens is essential to extend the operating life of high-power LED modules.