Interfacial microstructure evolution and shear behavior of Au-20Sn/(Sn)Cu solder joints bonded at 250 °C

It is of great significance to realize electronic packaging at low temperature. Au-20Sn was successfully joined with Cu substrates at 250 °C (lower than the melting point of Au-20Sn) with the aid of Sn interlayers. Interfacial microstructure evolution and shear behavior of Au-20Sn/(Sn)Cu solder joints during multiple reflows and after isothermal aging were investigated, respectively. During multiple reflows, AuSn/AuSn2/AuSn4/Sn/(Cu,Au)6Sn5/Cu3Sn layers formed initially at the interface of Au-20Sn/(Sn)Cu solder joints. As the reflowing time increased, the Sn layer was gradually consumed and the interface configuration transformed into AuSn/AuSn2/AuSn4/(Cu,Au)6Sn5/Cu3Sn layers, and finally into AuSn/AuSn2/(Cu,Au)6Sn5/Cu3Sn layers. The shear strength of Au-20Sn/(Sn)Cu solder joints first increased, and then decreased as the reflowing time increased. Shear fractures of the joints after multiple reflows always occurred at the (Cu,Au)6Sn5 layer. During the isothermal aging, the AuSn2 and (Cu,Au)6Sn5 layers gradually disappeared and the interface configuration transformed from AuSn/AuSn2/(Cu,Au)6Sn5/Cu3Sn layers to AuSn/Cu3Sn layers. The shear strength of Au-20Sn/(Sn)Cu solder joints decreased continuously as the aging time increased. Shear fracture locations transferred from the AuSn2/(Cu,Au)6Sn5 layers to the AuSn layer, and finally to the Cu3Sn/Cu interface. In addition, Au-20Sn/Cu solder joints after the same treatments and tests were analyzed for comparison.