Robust effects of In, Fe, and Co additions on microstructures, thermal, and mechanical properties of hypoeutectic Sn–Zn-based lead-free solder alloy

This study examines the influence of separate and dual minor alloying additions of indium (In) and ferrous/cobalt (Fe/Co) on the microstructure, thermal, and mechanical properties of hypoeutectic Sn–7wt.% Zn lead-free solder alloy. The results showed that the addition of In resulted in significant refinement in the microstructure and formation of new intermetallic compounds (IMCs) such as In–Sn phases which capable of extensively removing the unfavorable needle-like α-Zn phase. However, the additions of Fe/Co resulted in the creation of new coarsening flower-shaped IMCs identified as Zn–Co, Fe–Sn, and Co–Sn–Zn distributed uniformly which may have a marked effect on the mechanical and thermal properties of Sn–Zn solder alloy. Thermal analysis by a differential scanning calorimeter indicates slightly reduction in the onset temperature, melting temperature, and undercooling of the Sn–7wt.% Zn by the addition of In, while the pasty range enlarged as compared to Fe/Co additions. The tensile tests indicate that the Sn-Zn-In solder alloy exhibited a superior balance of ultimate tensile strength, yield strength, Young’s modulus, and elongation of 51.5 MPa, 44.4 MPa, 22.2GPa, and 44.5%, respectively, which are better than both those of the Sn–Zn and Sn–Zn–Fe/Co solder alloys. This can be attributed to the synergistic strengthening mechanism of refinement in the microstructure and precipitation of fine secondary particles.