Interfacial reaction at Cu-core heterogenous solder joints and its effects on mechanical property during isothermal aging

Ni/Cu-core + SAC0307/Cu heterogenous Cu-core solder joints(Ni-Cu Cu-core joints)were subjected to aging experiments at 100 ℃ for 0 h, 48 h, 120 h, and 360 h, respectively. The microstructure evolution of interfacial intermetallic compound(IMC), tensile properties and fracture behavior of Ni-Cu Cu-core joints were investigated. The results show that, at as-reflowed joints, the ternary (Cu,Ni)₆Sn₅ phases are formed at the Ni/Solder and Solder/Cu-core interface and the Cu₆Sn₅ phases are formed at the Solder/Cu interface in Ni-Cu Cu-core joints. Interfacial reaction occurs at Solder/Ni interfaces in Ni-Cu Cu-core joints by Cu atoms diffusing to the Ni base through solder. The IMC layers are flat, continuous, and mainly serrated at all interfaces. During the isothermal aging, the thickness of the IMC layers monotonically increase with increasing aging time and the interface growth behavior is mainly controlled by diffusion. At the same aging conditions, the thickness of the IMC layers on the near Cu base side are greater than that on the near Ni base side, respectively. The tensile strength of Ni-Cu joints is slightly higher than that of Cu/Cu-core + SAC0307/Cu (Cu-Cu) joints. The tensile strength of Ni-Cu and Cu-Cu solder joints decreases with aging time prolonged, and the fracture mode are mainly ductile brittle mixed fracture. The fracture location mainly occurs at the bulk solder with larger thickness of IMC layer near the Cu base side. With the increase of the aging time, the fracture location tends to be closer to the IMC layer near the Cu base side. Ni instead of Cu as the matrix can slow down the growth rate of the interface layer, and improve the reliability of Cu-core solder joints.