Journal of Materials Research
November 2005— Volume 20, Number 11

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Phase equilibria in the Sn-rich corner of the Sn-Cu-Ni ternary alloy system at 240 °C Chia-Ying Li and Jenq-Gong Duh Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China

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 quantitave 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.

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