Effect mechanism of moisture diffusion on LED reliability

High power light-emitting diodes (LED) are widely applied in many fields for their special advantages compared with other light sources. Many researches on LED reliability have been conducted in order to build the better LED design. However, seldom detailed reasons for the LED performance degradation are illustrated in the previous research when LED modules work under a certain thermal and moisture condition. This paper aims at studying one kind of LED failure mechanisms caused by moisture diffusion. Through the accelerated experimental test of high power LED modules, we find that the decent speeds of the optical output are different for different LED samples groups. For these tested LED samples, we detect the surfaces of LED chips by Atomic Force Microscope (AFM). The AFM micrographs show that different kinds of chips have their own characteristic surface textures. As the LED modules work under a certain condition, the thermal stress and moisture invasion may cause delaminations inside the modules. This would change the original path of light propagation and result in the degradation of optical output inevitably. For the LED modules packaged by different chips, the forms of delamination occurred at the interfaces between LED chips surface and phosphor layer are not the same. Different chip surface structure has significant impact on the optical output and reliability of LED modules.