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 beta-Sn dendritic crystal, which can be obviously suppressed by small amount of Ni element addition. Needle-like (Cu, Ni)(6)Sn-5 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.