Microstructure, IMCs layer and shear property of Sn-9Zn solder joints reinforced by Cu nanoparticles during thermal cycling

The microstructure, interfacial intermetallic compounds (IMCs) and shear property of Sn-9Zn and Sn-9Zn-30Cu composite solder joints during thermal cycling were studied (Cu is the nanoparticles). The results showed that Sn-rich and Zn-rich phases exist widely in the in-situ reaction zone of the Cu/Sn-9Zn/Cu solder joints. At the later stage of the thermal cycle (800–1000 thermal cycles), Cu₆(Sn,Zn)₅ was found and the Zn-rich phase disappeared. Cu₆(Sn,Zn)₅ and Cu₃Sn phases in the in-situ reaction zone of the Cu/Sn-9Zn-30Cu/Cu solder joint remained stable during thermal cycling. The thickness of the interfacial IMCs layer of the Cu/Sn-9Zn/Cu and the Cu/Sn-9Zn-30Cu/Cu solder joints is 5.51 μm and 5.30 μm, and it is increased to 9.66 μm and 5.89 μm after 1000 thermal cycles, respectively. The shear strength of the Cu/Sn-9Zn/Cu solder joints dropped from 14.18 MPa to 5.89 MPa after 1000 thermal cycles, while the shear strength of the Cu/Sn-9Zn-30Cu/Cu solder joints decreased from 17.24 MPa to 9.47 MPa. The fracture position of the Cu/Sn-9Zn/Cu solder joints changes from the interfacial reaction zone to the in-situ reaction zone with increasing thermal cycling times. The fracture of the Cu/Sn-9Zn-30Cu/Cu solder joints always occurs in the in-situ reaction zone.