Influence of intermetallic growth on the mechanical properties of Zn–Sn–Cu–Bi/Cu solder joints

The formation of intermetallic reaction layers and their influence on shear strength and fractography was investigated between the Zn–Sn–Cu–Bi (ZSCB) and Cu substrate during the liquid state reaction at 450 °C after 10–90 s. Results showed that reliable solder joints could be obtained at 450 °C after 15–30 s of wetting, accompanied by the creation of scallop ε-CuZn5, flat γ-Cu5Zn8 and β-CuZn intermetallic layers in ZSCB/Cu interface. However, with excess increase of soldering time, a transient intermetallic ε-CuZn4 phase was nuclear and grew at ε-CuZn5/γ-Cu5Zn8 interface, which apparently deteriorated the shear strength of solder joints from 76.5 MPa to 51.6 MPa. The sensitivity of the fracture proportion was gradually transformed from monotonic ε-CuZn5 to the mixture of ε-CuZn4 and ε-CuZn5 intermetallic cleavage. Furthermore, the growth mechanism of ε-CuZn4 intermetallic phase at the ZSCB/Cu interface was discussed. [Display omitted] •There are four interfacial intermetallic layers formed at the Zn–Sn–Cu–Bi/Cu interface.•The growth mechanism of ε-CuZn4 intermetallic phase was discussed.•The wetting time of Zn–Sn–Cu–Bi solder in contact with Cu substrate is a key parameter.