Interfacial reactions of Sn-Cu alloys with Ni substrate at 250 °C have been investigated by varying the Cu contents from 0.3wt%Cu to 3.0wt%Cu. The Cu6Sn5 phase is not found in the Sn-0.3wt%Cu/Ni couple. The interfacial reaction sequence of the Sn-0.6wt%Cu/Ni couple is similar to that of the Sn-0.7wt%Cu/Ni. The reaction follows Cu6Sn5 formation stage, growth stage, detachment stage, Cu6Sn5 layer continuing growth stage, Ni3Sn4 formation stage, Cu6Sn5 phase dissolution stage, Ni3Sn4 detachment stage, and Ni3Sn4 phase dissolution stage. Similar results are found for the couples prepared with higher Cu contents, 0.8wt%Cu, 1.0wt%Cu, 2.0wt%Cu, and 3.0wt%Cu, but the detachment positions are different. For the couples with higher Cu-content, the layer detaches at the Cu6Sn5/Ni interface; however, for the 0.6wt%Cu and 0.7wt%Cu couples, the Cu6Sn5 phase layer fractures and detachment occurs inside the layer. Morphologies of the Cu6Sn5 phase are also found to change with Cu contents: the pyramidal shape in the 0.3wt%Cu couple changes to the rod-shape in the 0.8wt%Cu couple, and becomes a very fine rod shape in the 2.0wt%Cu couple.