The reaction between Sn–xCu (x = 0.1, 0.3, 0.7, 0.9, and 1.5 wt%) solder alloys and Ni at 260, 280, and 290 °C for 60 s was studied to reveal the effect of Cu content on the composition and morphology of intermetallic compounds (IMCs) formed at the interface between solder and substrate. The results indicated that Cu concentration greatly affects both the composition and morphology of the IMC between the solder and Ni substrate. In particular, when the Cu concentration was less than or equal to0.3 wt%, (CuxNi1−x)3Sn4 IMCs were formed at the interface. When the Cu concentration was 0.7 wt%, large facets type of (CuxNi1−x)6Sn5 were mixed with (CuxNi1−x)3Sn4 in the IMC layer. At Cu concentrations higher than the eutectic one, e.g., 0.9 and 1.5 wt%, stick-shaped (CuxNi1−x)6Sn5 compounds were detected, but the (CuxNi1−x)3Sn4 IMCs disappeared. The formation and growth mechanism of the (CuxNi1−x)6Sn5 compound were analyzed. The evolution tendency of the composition and morphology of the IMCs at the three testing temperatures was found to be the same.