Phase distribution and phase analysis in Cu6Sn5, Ni3Sn4, and the Sn-rich corner in the ternary Sn-Cu-Ni isotherm at 240°C

Metallurgical reactions between solders and under bump metallization (UBM) are key issues for the solder joint reliability in microelectronic packaging. A phase diagram consisting of solders and UBM materials are required to further understand the interfacial reactions and related phase transformation. In this study, series of ternary Sn-Cu-Ni alloys were designed, fabricated, and heat-treated at 240°C. Equilibrium phases of Sn, Ni3Sn4, and Cu6Sn5 were identified by XRD, and microstructure evidence in backscattered electron image (BEI) micrograph. Through detailed EPMA quantitative analysis, three acme compositions of the ternary region in the Sn-Cu-Ni isotherm near the Sn-rich corner were evaluated and determined. Furthermore, x-ray color mapping of tin, copper, and nickel were applied to study the phase distribution of the alloys with the aid of electron microprobe analysis (EPMA). According to the intensities of Sn, Cu, and Ni, collected by x-ray color mapping, special software was employed to map the corresponding concentrations on the Sn-Cu-Ni ternary isotherm. The degree of composition homogeneity and the phase distribution were further evaluated by phase-analysis techniques. Semiquantitative measurements by phase analysis can be extended to evaluate the phase boundaries with a statistical variation under 5% as compared to the quantitative analysis by EPMA. Finally, the isothermal section of the ternary Sn-Cu-Ni system near the Sn-rich corner at 240°C was constructed.

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