Phase Equilibria in the Sn-Rich Corner of the Sn–Cu–Ni Ternary Alloy System at 240 °C

The interfacial reactions between solders and under bump metallization (UBM) have been of great interest recently in flip chip technology. Intermetallic compounds (IMCs), i.e., (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄, usually formed between solders and UBM. To fully understand the interfacial reactions and phase transformation phenomenon, a suitable phase diagram concerning solders, IMCs, and UBM materials is required. In particular, a Sn-rich phase region in the Sn–Cu–Ni ternary diagram is very critical in determining the concentration tendency of x and y values in (Ni_1−x,Cu_x)₃Sn₄ and (Cu_1−y,Ni_y)₆Sn₅ compounds. In this study, ternary Sn–Cu–Ni alloys were prepared and annealed at 240 °C. Three equilibrium phases, Sn, Ni₃Sn₄, and Cu₆Sn₅, were identified by x-ray diffraction analysis and also showed in backscattered electron imaging. Using electron probe microanalysis quantitative analysis, three acme compositions of the ternary region in the Sn–Cu–Ni isotherm near the Sn-rich corner were determined as 98.5 at.% Sn, (Ni_0.80, Cu_0.20)₃Sn₄ and (Cu_0.59,Ni_0.41)₆Sn₅. In addition, the solubility of Cu and Ni in (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅ compounds was evaluated. Finally, the isothermal section of the ternary Sn–Cu–Ni system at 240 °C was proposed on the basis of experimental results in this study.