Thermal Management And Aging Test Of High-Power White LED

A thermal design for packaging and components of high-power white LED lamp was presented. A finite element model for LED lamp was builded. The temperature gradient was large according to the distribution of steady-state temperature field of LED device. The thermal resistance between junction and external sink was 20.7℃/W. Ceramic substrate was considered as the main part blocking heat dissipation. In order to reduce the thermal resistance, structure optimization began with ceramic substrate. Internal sink with fin structure was applied and the dies were directly bonded to it. As a result, the max junction temperature decreased from 131.5℃to 54.8℃and the thermal resistance between junction and external sink fell to 3.2℃/W. The thermal structure of LED was theoretically optimized.In order to improve the performance of LED fundamentally, the sapphire substrste was transformed to copper substrate by laser lift-off. And the luminous efficiency increased significantly. A novel package structure was designed for copper subtrate LED. And the thermal performance was proved good.The average temperature of p-n junction for LED original structure and some optimized structure was measured by voltage-method as well as thermal resistance between junction and ambient. Although the test results were a bit larger than that of simulation, the error was acceptable. The temperature of external sink was measured by thermocouple and it also agreed with simulative result.Temperature accelerated aging and current accelerated aging experiments on copper substrate high-power white LEDs were performed. Lifetime analysis was based on the results of temperature accelerated aging experiment. According to the Arrhenius relationship, the lifetime of LEDs working in normal situation of 25℃was calculated as 74700 hours. And it was 10900 hours when they worked in high temperature situation of 60℃.Before and after aging experiments, the current-voltage curves of white LEDs were measured. It was found that the reversal leakage current rose with tunnel current after aging. It was caused by the increased defect density in active area. Although the junction temperature was lower for current accelerated experiment, the chip degraded more easily. Maybe it was result of the integrated effect of high temperature and high current desity. The luminous flux of LEDs declined nearly the same for both aging experiments. This may be due to the higher junction temperature in temperature aging experiment. The phosphor and packaging resin around the chip degraded. They reduced the final light output efficiency.