Reliability Evaluation of Interconnection in COM Package

A novel package structure for CMOS image sensor (CIS) is proposed as COM (Chip on Module) to lower the costs. Its reliability has been verified systematically by functional tests, pull tests and micro interfacial observation after high temperature and humidity storage (THS) reliability tests at 85°C/85%RH for 240h and 1000h. These measurements are focused on the eutectic Sn-57Bi solder joint, connecting gold wire and FPC, which is the most vulnerable part to failure because of the rapid diffusion of Au. A few addition of silver dispersed in Sn-57Bi solder is also found to have a positive effect on the reliability of the joint, since silver can obstruct the diffusion of both Au and Sn atoms and lead to a less amount of IMC at the interface, less kirkendall voids and a much higher tensile strength. Furthermore, a theoretical model for Au/Sn interdiffusion is also built to qualitatively explain the growth of IMC and the influence of silver during reliability tests.