The eutectic 99.3Sn–0.7Cu solder (wt%, Sn–0.7Cu) is the most promising lead-free replacement for the eutectic Sn–Pb solder in wave-soldering applications. In this study, the effect of a small perturbation in the Cu concentration on the reaction between the Sn–0.7Cu solder and Ni was investigated. Specifically, four Sn–xCu solders (x = 0.2, 0.4, 0.7, and 1) were reacted with Ni at 250 °C. A slight variation in Cu concentration produced completely different reaction products. When the Cu concentration was low (x = 0.2), the reaction product was (Ni1–xCux)3Sn4. At high Cu concentrations (x = 0.7 and 1), the reaction product was (Cu1–yNiy)6Sn5. When the Cu concentration was in-between (x = 0.4), both (Ni1–xCux)3Sn4 and (Cu1–yNiy)6Sn5, formed. The above findings were rationalized using the Cu–Ni–Sn isotherm. The results of this study imply that the Cu concentration must be strictly controlled in industrial production to produce the desired intermetallic at the interface.