How to enhance Sn-Bi low-temperature solder by alloying?

Low-temperature Pb-free solders are in demand to alleviate the warpage issue for high-density fine-pitch packaging. We designed a low-temperature tin-bismuth-silver Sn-Bi-Ag solder by CALculation of PHAse Diagram-type thermodynamic calculations using the PANDAT software and performed corresponding key experiments. The goal is to improve Sn-Bi solder’s mechanical properties while keeping their low melting temperatures. The DSC measurement shows the solidus of Sn-Bi-Ag was 135.0 °C. Higher (Sn) phase fraction, low IMC fraction, and low IMC, finer (Sn) + (Bi) eutectic spacing of Sn-Bi-Ag led to the high yield strength, high ultimate tensile strength, toughness, and better elongation in as-cast after the tensile test. Furtherly, we found Ag and Sn doping in (Bi) polycrystals induced tensile stress in the (Bi) lattice and it led to the different solution effects in each (Bi) orientation. It could alleviate the anisotropic mechanical properties of (Bi) and avoid concentrated force on the weak grains.