Effect of reflow time on shear property of two-step electroplated Sn-3.5Ag solder bumps

In this paper, the Sn-3.5Ag solder bumps were prepared successfully by two-step electroplating method. The effect of reflow time on shear property of solder bumps and Cu was investigated. After shear tests, the shear strength of Sn-3.5Ag solder bumps peaked at the reflow time of 60s, and then decreased with further increasing reflow time. It was considered that the variation of shear strength was closely related to the interfacial microstructure between the solder and Cu. Scallop-like Cu₆Sn₅ intermetallic compound (IMC) formed toward solder were observed after reflowing for 30s. As the reflow time increasing, the morphology of Cu₆Sn₅ IMCs changed from scallop-like type to the thin, long, hexagonal and rod-like type, and the grain size of Cu₆Sn₅ increased. The large hexagonal rod-like weakened the interfacial connection and made part of the IMCs exposed to the fracture surface. In addition, the bumps displayed good ductility after reflowing for a short period of time, while tended to be more brittle as the reflow time increasing.