Thermal Parameters, Microstructure, and Mechanical Properties of Directionally Solidified Sn-0.7 wt.%Cu Solder Alloys Containing 0 ppm to 1000 ppm Ni

Environmental concerns over the toxicity of Pb are resulting in the progressive ban of Pb-based solders as part of electrical and electronic devices. Sn-Cu alloys are becoming interesting Pb-free solder alternatives. In the case of hypoeutectic Sn-Cu alloys (<0.7 wt.% Cu), small alloying additions of Ni can prevent the growth of coarse and deleterious Cu₆Sn₅ particles. Solidification thermal parameters such as the growth rate, cooling rate, and interfacial heat transfer coefficient (h _i) determine the morphology and scale of the phases forming the resulting microstructure. In the present study, directional solidification experiments were carried out with Sn-0.7 wt.%Cu, Sn-0.7 wt.% Cu-0.05 wt.%Ni, and Sn-0.7 wt.%Cu-0.1 wt.%Ni alloys and interrelations of solidification thermal parameters, microstructure, and tensile properties have been established. The highest time-dependent h _i profile was found for the Sn-0.7 wt.%Cu-0.1 wt.%Ni alloy, which is an indication that this alloy has the highest fluidity. Constrained dendritic arrangements were observed for all alloys experimentally examined. This morphology has been associated with high cooling rates and growth rates. Cellular regions, characterized by aligned eutectic colonies, were also observed to occur for cooling rates lower than 0.9 K/s and 6.0 K/s for the unmodified Sn-0.7 wt.%Cu alloy and for both Ni-modified Sn-Cu alloys, respectively. Experimental Hall–Petch-type equations correlating the ultimate tensile strength and elongation with cell/dendritic spacings are proposed.