Effects of Ni addition on the interfacial reactions between Sn–Cu solders and Ni substrate

This research investigates the effects of adding a small amount of Ni to solder. The interfacial reactions between Ni and the Sn–Cu solders at 250 °C are examined. Two sets of Sn–0.7 wt% Cu–xNi solders and Sn–1.0 wt% Cu–xNi solders (x = 0.02, 0.05 and 0.1 wt%) were prepared. The results reveal that the interfacial reactions are very sensitive to Ni. With increasing Ni addition, the reaction phase grain, Cu6Sn5, becomes larger and its morphology changes from a rod-like shape into a faceted structure. The reaction progressions are accelerated in comparison with those without Ni addition. The solder spreading effect on the Cu6Sn5 morphology was also investigated. Cu6Sn5 exhibits a gradual change in microstructure from the center to the rim for the case when the solder ball spreads over a Ni substrate. However, the solder does not alter the Cu6sn5 microstructure while reacting on a small Ni pad without spreading. This suggests that the Cu and Ni contents in the solder near the interface vary during the solder ball spreading process.