Influence of microstructure on quasi-static and dynamic mechanical properties of bismuth-containing lead-free solder alloys

The mechanical properties of bulk Pb-free solder alloys containing Bi have been characterized in the as-cast and aged conditions. The alloys were the commercially available Sn–3.4Ag–1.0Cu–3.3Bi and Sn–3.4Ag–4.8Bi (wt.%), which had demonstrated good reliability performance in previous circuit board-level testing programs, but have not been considered for high-volume electronic manufacturing. Room- and elevated-temperature tensile testing showed that the addition of Bi greatly reduced the loss in strength due to aging that occurs in the Sn–Ag–Cu ternary alloys. The room-temperature tensile strength of bulk SAC305 was reduced by 37% after aging at 150 °C for 336 h, but for SAC–Bi the reduction was not statistically meaningful and in SnAg–Bi the tensile strength increased. These differences are attributed to the presence of Bi and its role in solid solution strengthening and precipitation as a separate phase. The damping capacity of the alloys measured by dynamic mechanical analysis showed similar trends, with tan δ increasing in aged SAC305 but decreasing in SAC–Bi and SnAg–Bi. The reduction in damping seen in SAC–Bi and SnAg–Bi is consistent with Sn and SnPb eutectic solder, indicating that a general microstructural coarsening is responsible. The increase in damping in SAC305 suggests that the transformation of the ternary eutectic by particle coarsening leads to a continuous β-Sn matrix that contributes to damping.