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

摘要

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, micro-structures 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 Cu_3Sn 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 Cu_3Sn 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.