Aging effects on IMC formation and joint strength of low-Ag SAC solder/UBM (Ni (P)-Au) for WLCSP

Low-Ag solder SnAgCu (SAC) is more suitable for improving the shock performance of solder joints as reported by many papers. However the properties of low-Ag solder, with different Cu contents, under service conditions are lacking. The forming mechanism of the intermetallic compound (IMC) and the relation with the joint strength need to be investigated. In this study, the effects of high-temperature storage (HTS) aging on the joint performance of low-Ag solders/UBM (Ni(P)-Au) are examined for wafer-level chip-scale packages (WLCSP). Three types of solder balls with different Cu content are considered. The interfacial morphology and microstructure of the solder joints are investigated by SEM, EDX and deep etching techniques. The high-speed cold bump pull (HSCBP) test is used to analyse the joint strength. Results indicate that HTS aging for solder balls containing different amounts of Cu results in joint strength decreases. Solder joints with a low-Cu percentage appear to have a higher aging resistance than a solder joint with a high-Cu percentage. Too much Cu in the solder results in both a large amount of voids and the Cu3Sn layer growing near the Cu6Sn5 layer in the interface after HTS aging, leading to reduced joint strength. The IMC in the interface of solder/ Under- Bump Metallization (UBM) exhibits different morphologies and microstructures due to the solder balls with different amounts of Cu and the aging parameters. The microstructure and fracture mode of the IMC affect the pull strength of the joint. Moreover, Cu content within solder ball is a key reliability factor.