Effects of Co addition on shear strength and interfacial microstructure of Sn–Zn–(Co)/Ni joints

Sn–9Zn solder alloyed with cobalt (0, 0.5, 3 wt%) was bonded to Ni pad at 250 °C for various durations, and the effects of Co on the microstructure evolution and the shear behavior of the Sn–Zn/Ni joints were investigated by microstructural observations and shear tests. The results reveal that Co improves the shear strength and the interfacial microstructure of the Sn–9Zn/Ni joints significantly. Co is not only a diffusion barrier that impedes Zn from gathering at the interfaces to form Ni5Zn21, but it reacts with Zn in the composite solders with 3 wt% Co adding to the Sn–9Zn solder. When preferential Ni5Zn21 is transformed into Ni2Sn2Zn at the Sn–Zn–Co/Ni interfaces, ductile ruptures occur as Ni2Sn2Zn has similar elastic modulus and connects firmly with the solder. Accordingly, the strength of the Sn–Zn–Co/Ni joints reaches the highest. Moreover, the shear strength of the Sn–Zn–3Co/Ni joints maintains above 35 MPa after 90 min soldering, on account of the Ni2Sn2Zn remaining uniform ripples and fracture surfaces appearing with homogeneous Ni2Sn2Zn dimples. Whereas, at the Sn–Zn/Ni joints, the thick Ni5Zn21 layer and pores intensify the stress concentration and lead to a lower strength. In general, the addition of Co refines the interfacial microstructure that the Sn–Zn–Co/Ni joints are uniformly stressed and achieve desirable strength.