Effect of individual and synergistic alloying of In and Ni on microstructure, phase stability and thermal properties of lead-free Sn-0.7Cu solder

The study explores the individual and synergistic effect of alloying indium (In) and nickel (Ni) with Sn-0.7 wt.% Cu solder on microstructure and thermal properties. Solders consist of beta-Sn phase matrix and Cu6Sn5 intermetallic phase. In dissolves within beta-Sn, while Ni substitutes Cu atoms of Cu6Sn5 forming (Cu,Ni)6Sn5. Alloying either In or Ni with Sn-0.7Cu lowers solidus and liquidus temperatures of the alloys, with a greater reduction in the case of In. Synergistic alloying of In and Ni with Sn-0.7Cu leads to a widening of 93%. Smaller fraction and localised presence of Cu6Sn5 in In-containing alloys resulted in dual-peaked grain size distribution. In the case of Ni, a fraction of (Cu,Ni)6Sn5 increased, generating more interfaces for nucleating beta-Sn grains and eventually causing grain refinement. An increment in the fraction of low-angle grain boundaries (LAGBs) was observed in both alloying cases. The synergistic effect of In and Ni yields a more uniform grain size and lesser LAGBs.