Effect of adding 1 wt% Bi into the Sn-2.8Ag-0.5Cu solder alloy on the intermetallic formations with Cu-substrate during soldering and isothermal aging

Intermetallic compound (IMC) formations of Sn-2.8Ag-0.5Cu solder with additional 1 wt% Bi were studied for Cu-substrate during soldering at 255 °C and isothermal aging at 150 °C. It was found that addition of 1 wt% Bi into the Sn-2.8Ag-0.5Cu solder inhibits the excessive formation of intermetallic compounds during the soldering reaction and thereafter in aging condition. Though the intermetallic compound layer was Cu₆Sn₅, after 14 days of aging a thin Cu₃Sn layer was also observed for both solders. A significant increase of intermetallic layer thickness was observed for both solders where the increasing tendency was lower for Bi-containing solder. After various days of aging, Sn-2.8Ag-0.5Cu-1.0Bi solder gives comparatively planar intermetallic layer at the solder-substrate interface than that of the Sn-2.8Ag-0.5Cu solder. The formation of intermetallic compounds during aging for both solders follows the diffusion control mechanism and the diffusion of Cu is more pronounced for Sn-2.8Ag-0.5Cu solder. Intermetallic growth rate constants for Sn-2.8Ag-0.5Cu and Sn-2.8Ag-0.5Cu-1.0Bi solders were calculated as 2.21 × 10^-17 and 1.91 × 10^-17 m²/s, respectively, which had significant effect on the growth behavior of intermetallic compounds during aging.