Fracture morphology and mechanism of IMC in Low-Ag SAC Solder/UBM (Ni(P)-Au) for WLCSP

In this study, the brittle fracture of intermetallic compound (IMC) between low-Ag SnAgCu (SAC) solder and under bump metallization (UBM) under high-speed pull condition is studied for wafer-level chip-scale packages (WLCSP). The fracture morphology and fracture mechanism of IMC are analyzed by SEM, EDX and deeply etching techniques. Also, the forming mechanism of IMC and the relation with joint strength are investigated. The effects of high-temperature (HTS) aging on the fracture performance of IMC are examined. Results shows that compared with keeping time, storage temperature is a much more important influencing factor on the formation of IMC and soldering joints strength. Cu content in the solder is another key influencing factor as well.