Effect of Indium and Nickel on Impact Toughness of Sn–0.7Cu Solder: Interplay of Solubility, Phase Configuration, and an Improved Ductile‐to‐Brittle Transition Temperature Analysis

Reliability of solder joints under sudden mechanical shocks at cryogenic conditions is critical for deep‐space electronics, where launch‐induced vibrations, pyrotechnic‐shock, micro‐meteoroid impacts, or thermally induced stress transients can compromise structural interconnect integrity. This study investigates the influence of In‐ and Ni‐alloying on impact toughness and ductile‐to‐brittle‐transition of the lead‐free Sn‐0.7 wt.% Cu (Sn‐1.3 at.% Cu) solder. Impact behaviour is correlated with microstructure and phase characteristics. In‐alloyed solders results in finer dispersion of Cu 6 Sn 5 within β‐Sn, yielding a slight reduction in impact toughness despite nearly unchanged Cu 6 Sn 5 fraction. With Ni addition, the size and morphology of the Ni‐containing IMCs evolve in a contrasting manner. Reduced toughness observed with 5 at.% Ni addition even at RT is correlated with the presence of inherently brittle (Ni, Cu) 3 Sn 4 with coarser rod‐like morphology and higher phase fraction. An anomalous trend in the DBTT curve is observed for the solder alloy with 5 at.% Ni, showing higher impact toughness at a temperature lower than RT. Within the transition range of –50 to 0 °C, In‐alloyed solders exhibited greater compositional sensitivity to toughness than Ni‐alloyed ones, owing to higher solubility in β‐Sn. A novel curvature‐based approach based on the second‐derivative of the toughness–temperature curve was proposed to determine ductile‐to‐brittle transition temperature (DBTT).