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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.