The Failure Analysis Of High Power Light-emitting Diodes

This thesis mainly focuses on material related problem in the failure analysis of high power light-emitting diodes. Aiming at the core problem, we carried out aging tests towards to chip, phosphor and potting compounds separately. As is known to all, illuminant quality of LED mainly depends on these three parts. The failure analysis technique and materials science related testing methods were used to determine the LED failure mode of those samples.The work of this thesis was mainly completed on LED accelerated life & online testing platform which was developed by the optomechatronics laboratory, Research Institute of Tsinghua University at Shenzhen. The outstanding feature of this platform is in the process of accelerated life experiment of LED device, all the illuminant, electric and thermal related properties can be monitored in-situ at real time. By a comprehensive analysis on the trends of illuminant, electric and thermal parameters, we can make an accurate pre-judgment of the device failure mode. Once being promoted, the system will have a great application prospect in the LED industry.Base on this system, we carried out accelerated life tests towards to chip, phosphor and potting compounds separately. Combined with materials science related testing methods and failure analysis, we obtained the following conclusions:(1) In the aging experiment of LED naked chips, the experimental results show that horizontal and vertical type chips have stable performance, there are not failure samples found during the whole process, but meanwhile due to oxidation of the silver reflector layer, one flip chip sample failure. The specific reason is that protection of dielectric layer for silver reflector layer is not complete, which causes failure sample's illuminant, electric and thermal performance deteriorated during the whole experiment.(2) In phosphor aging experiments, the experimental results show that four kinds of industrial phosphor powder in the aging process exhibite a certain extent decline of photoelectric efficiency. Among them, the decline of orange-pink colored powder is the most obvious. The specific reason is the main components of Sr3SiO5: Eu2+ under the influence of hot humid air and blue light show the trend of amorphization, resulting in great changes in the morphology and composition, have a direct effect on the luminous efficiency.(3) In the potting compounds aging test, experimental results show that compared with the epoxy resin, silicone based materials(methyl silicone resin and phenyl silicone) exhibit a better thermal stability. In addition, the experimental results also show the phenomenon that oxidation and porosity of silver reflector cup and the microstructure's changes of the reflector cup's surface lead to the decrease of the reflection efficiency of the silver plated layer, which causes the decrease of the optical performance of the whole LED device.