Effects of minor Bi, Ni on the wetting properties, microstructures, and shear properties of Sn–0.7Cu lead-free solder joints

In present study, minor addition of Bi and Ni were added with Sn–0.7Cu eutectic solder to investigate the effect of exotic elements on the wetting properties, microstructures as well as shear properties of the newly developed solder alloys. Wetting balance results show that the addition of 3.5 wt% Bi increased the maximum wetting force and lowered the wetting time, while 0.01 and 0.03 wt% Ni resulted in the decline of wetting force. The microstructural evolution of four types of solder/Cu joints indicate that the present of Bi suppressed the growth of Cu₃Sn phase after one time reflow and ten times reflows. This inhibiting effect was reduced with the segregation of Bi-rich phase. Besides, 0.03 wt% Ni additive caused the absence of Cu₃Sn phase upon one time reflow, ten times reflows and aging for 750 h. According to shear strength test, Bi elements enhanced the shear strength of Sn–Cu based solder/Cu joints due to the solution strengthening. This effect was reduced with the precipitation of Bi-rich phase, which weakened the shear-bearing ability of solder joints.