Kirkendall voids formation in the reaction between Ni-doped SnAg lead-free solders and different Cu substrates

The reactions between Sn2.5Ag solder doped with different levels of Ni (0–0.1 wt.%) and two different types of Cu substrates, electroplated Cu and high-purity Cu substrates, were studied. The main objective was to investigate the effect of Cu substrate and the effect of Ni additions on the formation of Kirkendall voids within the Cu3Sn phase. Reaction conditions included one reflow and subsequent aging at 160 °C for up to 2000 h. After reflow, Cu6Sn5 was the only reaction product observed for all the different solders and substrates used. During aging, both Cu6Sn5 and Cu3Sn formed. Nevertheless, Kirkendall voids were observed only when the electroplated Cu was used, and was not observed when high-purity Cu was used. It was proposed that impurities in electroplated Cu helped the nucleation of these voids. The Ni additions made the Cu3Sn layer thinner. For the case of the electroplated Cu substrates, the amount of Kirkendall voids decreased correspondingly with the Ni additions.