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, Cu
₆
Sn
₅
was the only reaction product observed for all the different solders used. Reflows using the solder without doping produced a thin, dense layer of Cu
₆
Sn
₅
. The additions of Fe, Co, or Ni transformed this microstructure into a much thicker Cu
₆
Sn
₅
with many small trapped solder regions between the Cu
₆
Sn
₅
grains. In solid state aging study, both Cu
₆
Sn
₅
and Cu
₃
Sn formed, but the additions of Fe, Co, or Ni produced a much thinner Cu
₃
Sn layer. Specifically, Ni concentration higher than 0.01 wt.% could effectively retard the Cu
₃
Sn growth even after 2000 hrs of aging, and accordingly 0.01 wt.% can be considered the minimum effective Ni addition. Because the Cu
₃
Sn growth had been linked to the formation of micro voids, which in turn increased the potential for a brittle interfacial fracture, thinner Cu
₃
Sn layers might translate into better solder joint strength.