Minor Fe, Co, and Ni additions to SnAgCu solders for retarding Cu3Sn growth

The solders used for this study are Sn2.5Ag0.8Cu doped with 0.03 wt.% Fe, Co, or Ni and 0, 0.005, 0.01, 0.06, or 0.1 wt.% Ni. Reaction conditions included multiple reflows for up to 10 times and solid-state aging at 160degC for up to 2000 hrs. In multiple reflow study, Cu₆Sn₅ was the only reaction product observed for all the different solders used. Reflows using the solder without doping produced a thin, dense layer of Cu₆Sn₅. The additions of Fe, Co, or Ni transformed this microstructure into a much thicker Cu₆Sn₅ with many small trapped solder regions between the Cu₆Sn₅ grains. In solid state aging study, both Cu₆Sn₅ and Cu₃Sn formed, but the additions of Fe, Co, or Ni produced a much thinner Cu₃Sn layer. Specifically, Ni concentration higher than 0.01 wt.% could effectively retard the Cu₃Sn growth even after 2000 hrs of aging, and accordingly 0.01 wt.% can be considered the minimum effective Ni addition. Because the Cu₃Sn growth had been linked to the formation of micro voids, which in turn increased the potential for a brittle interfacial fracture, thinner Cu₃Sn layers might translate into better solder joint strength.