Novel Bi-containing Sn–1.5Ag–0.7Cu lead-free solder alloy with further enhanced thermal property and strength for mobile products

• Small amounts of Bi have been added into Sn–1.5Ag–0.7Cu (SAC157) solder.
• Bi refined the microstructure of SAC157 solder.
• Bi enhanced the elastic modulus, UTS and 0.2%YS of solder.
• Bi reduced the solidus temperature, eutectic temperature and undercooling.
• Bi changes the dominant mechanism from pipe diffusion to lattice self-diffusion.

In this study, the low Ag-content Sn–1.5Ag–0.7Cu (SAC157) lead-free solder was modified for advance electronic components with minor additions of Bi. The microstructure, melt properties and tensile behavior after and before compositional modifications were investigated. From microstructures evaluation, 1 wt% Bi addition to SAC157 significantly enhanced the solid solution effect of Bi and refined needle-like Ag3Sn and plate-like Cu6Sn5 particles as well as extended the eutectic area, which raised the mechanical strength and Young’s modulus to about 1.5 times of SAC157 solder. With increasing Bi addition to 3 wt%, the cubic-shaped Bi precipitates and enlarged eutectic area cause a dramatic increase of ultimate tensile strength, yield strength and Young’s modulus to about 2.2–2.6 times of the SAC157 alloy, although the elongation was maintained at the SAC157 level. The peak flow stress during deformation can be correlated with temperature and strain rate by a hyperbolic-sine equation. The activation energy for SAC157 alloy was increased from 58.2 kJ/mol to 101.7 and 109.0 kJ/mol when doped with 1.0 and 3.0 wt% of Bi, indicating the change of dominant deformation mechanism from dislocation pipe diffusion to lattice self-diffusion. Besides, the addition of Bi not only reduced the solidus temperature (Tonset) and eutectic temperature (Tm), but also decreased the undercooling even though the pasty rang is slightly increased.