Effects of Sn element on microstructure and properties of Zn-Cu-Bi-Sn high-temperature solder

The microstructures and properties of the Zn-Cu-Bi-Sn (ZCBS) high-temperature solders with various Sn contents were studied using differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicate that the increase of Sn content can both decrease the melting temperature and melting range of ZCBS solders and it can also effectively improve the wettability on Cu substrate. The shear strength of solder joints reaches a maximum value with the Sn addition of 5% (mass fraction), which is attributed to the formation of refined β-Sn and primary ɛ-CuZn5 phases in η-Zn matrix. However, when the content of Sn exceeds 5%, the shear strength decreases due to the formation of coarse β-Sn phase, which is net-shaped presented at the grain boundary.