Effects of bismuth additions on mechanical property and microstructure of SAC-Bi solder joint under current stressing

Bi additions have been reported to improve the wettability and drop-impact performance. Most important of all, it improves the mechanical properties. However, few studies focus on the effect of the Bi addition on mechanical properties and microstructure of intermetallic compounds (IMCs) in solder balls, especially on those properties after current stressing. In this study, to understand the effect of Bi addition, a lead-free Sn-3Ag-0.5Cu-xBi (SAC305-Bi) solder joint with Bi additions from 1 to 3 wt% was used to investigate mechanical property of Sn-rich matrix and microstructure of IMCs in the solder joint after current stressing process. We combined nanoindentation, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) to analyze mechanical property, and observe the intermetallic compounds of Cu3Sn, and Cu6Sn5 at the interface. The mechanical properties of hardness in Sn matrix is improved by the amount of Bi, where the hardness increases from 0.095 to 0.141 GPa as increasing Bi addition from 1 to 3 wt%. In addition, the growth rate constant of intermetallic compounds of Cu3Sn is in the range between 0.175 and 0.256, that is, Cu3Sn is not sensitive to Bi addition. Furthermore, the amount of doping Bi addition enhances the growth rate Cu6Sn5, as the rate constant increases from 1.597 to 2.413 with increasing Bi addition from 1 to 3 wt%.