Effects of minor Fe, Co, and Ni additions on the reaction between SnAgCu solder and Cu

The reactions between Cu and the Sn2.5Ag0.8Cu solders doped with 0.03 wt.% Fe, Co, or Ni were studied. Reaction conditions included multiple reflows for up to 10 times and solid-state aging at 160 °C for up to 2000 h. In the multiple reflow study, Cu6Sn5 was the only reaction product noted for all the different solders used. Reflows using the solder without doping produced a thin, dense layer of Cu6Sn5. Adding Fe, Co, or Ni transformed this microstructure into a much thicker Cu6Sn5 with many small trapped solder regions between the Cu6Sn5 grains. In the solid-state aging study, both Cu6Sn5 and Cu3Sn formed, but adding Fe, Co, or Ni produced a much thinner Cu3Sn layer. 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.