Solder joints reliability with different Cu plating current density in Wafer Level Chip Scale Packaging (WLCSP)

Electroplated copper is becoming increasingly important in UBM (under bump metallurgy). The Cu is used as a wetting material for the solder (ball). After reflow the Cu and solder joints are the main contact method for mechanical and electrical interconnection to the pads of the chip. One of the critical factors affecting solder joint reliability is the formation of intermetallic compound (IMC); that is generated at the interface of Cu/solder alloy. The formation of IMC is the merging of the plated copper and solder during the reflow process. The IMC will continue to grow when the structure is subjected to temperature. In this study, different current density (0.5ASD, 1.0ASD, 1.5ASD, 2.0 ASD, 2.5ASD, 3.0 ASD) are used on the Cu plating process, which produces different microstructure in the plated Cu layer. It will also impact IMC formation and evolution. The investigation shows that only Cu₆Sn₅ is generated in as-reflow condition; Cu₃Sn is brought forth after being stored at 150 degree C. The early failure was observed at 48 hrs, the fracture is located at the interface of Cu and Cu₃Sn in ball shear test. The analysis shows that Kirkendall void formation during aging treatment is the key factor for failure.