Effect of Ni, Bi concentration on the microstructure and shear behavior of low-Ag SAC–Bi–Ni/Cu solder joints

This paper investigated the effect of Bi, Ni concentration on the microstructure and interfacial intermetallic compounds of low-Ag Sn–0.7Ag–0.5Cu–xBi–yNi/Cu solder joints by comparing with Sn–0.7Ag–0.5Cu (SAC0705)/Cu and Sn–3Ag–0.5Cu (SAC305)/Cu. Meanwhile, the shear behavior of the solder joints at both the bulk solder and soldering interface with various Bi, Ni content were also studied. Experimental results indicated that SAC0705–3.5Bi showed coarse microstructure due to the excessive growth of β-Sn dendritic crystal, which can be obviously suppressed by small amount of Ni element addition. Needle-like (Cu, Ni)6Sn5 appeared in the bulk solder of SAC–Bi–Ni/Cu, instead of the pipe-like Cu6Sn5 in SAC/Cu. Compare with SAC0705/Cu and SAC305/Cu, SAC–Bi–Ni/Cu showed higher shear strength at both the bulk solder and soldering interface. The increase of Bi content significantly increased the shear strength of Sn–0.7Ag–0.5Cu–xBi–yNi/Cu solder joints at the soldering interface. Brittle fracture appeared in the bulk solder of Sn–0.7Ag–0.5Cu–3.5Bi–0.05Ni/Cu solder joint. But this brittle failure can be suppressed by increasing the concentration of Ni in the solder alloys.