Effects of aging time and temperature on shear properties of Sn-Zn and Sn-Ag-Cu solder joints

Sn-Zn system and Sn-Ag-Cu system solders are considered promising lead-free solders. The reliability of the joints is a key factor in evaluating the performance of the solder. The microstructure evolution of the joints under aging treatment can affect the joint properties. The temperature of shear testing also affects the fracture mechanism, thereby determining the joint reliability. This study investigated the microstructure evolution and fracture mechanism of Sn-9Zn-2.5Bi-1.5In (Sn-Zn) and Sn-3Ag-0.5Cu (SAC) pastes joints with Cu-substrate under various conditions of different aging times and shear test temperatures. It is concluded that the IMC in SAC joints are scalloped Cu6Sn5 and planar Cu3Sn, and grow along with aging duration. The IMC in Sn-Zn joints are specific Cu5Zn8 layers with a slow growth rate as aging time increases. Many Zn phases react with Cu, forming Cu5Zn8 products in the aged solder matrix. The aged Sn-Zn joints tend to fracture at the solder/IMC interface. Both increasing aging time and high operating temperature of shear test lead to an increase in the tendency of transgranular fracture and brittle fracture for the two types of joints in the solder, and therefore, joint reliability is reduced. Furthermore, as the shear test temperature increases, the plasticity of the solder material increases as well, and the interface connection strength decreases. In summary, Sn-Zn joints have better reliability performance than SAC joints under the effects of increasing aging time and high operating temperature.