Reliability of fine pitch wafer level packages

Wafer level packaging (WLP) is regarded as one of the most potential single chip packages for its compatibility with wafer fabrication process. As the pitch size of the package becomes lower, the reliability of fine pitch WLP devices is greatly challenged. Finer pitch may result in weaker solder joints, which leads to reliability problems such as fatigue failure, creep deformation and so on. Much work has been done to investigate the reliability of wafer level package above 500 μm pitch under different conditions such as temperature cycling, thermal shock and drop test, etc. Nevertheless, there are few reports about the reliability of WLP with pitch size less than 500 μm. In this study, WLP with a size of 5×5 mm² and a pitch of 400 μm were fabricated. Each chip has 144 lead-free SAC105 solder balls in an array of 12×12. The chips are reflowed on Ni/Au pads on boards. The chips were experienced a set of reliability tests including temperature cycling, thermal shock and drop test according to JEDEC standards. Furthermore, failure analysis is carried out to study the failure mechanism. Failures are found mostly inside of the solder ball after TC and TS tests, while the drop tests cause more damage to the solder-board and solder-chip interface.