Phase Equilibria of the Sn-Ag-Cu-Ni Quaternary System at 210°C

Sn-Ag-Cu alloys are the most promising Pb-free solders, and Ni is a common barrier layer material for the under-bump metallurgy. Although the Sn-Ag-Cu-Ni quaternary system is important for industry, no solid-phase equilibrium information is available. This study determines the equilibrium phase relationship of the Sn-Ag-Cu-Ni system at 210°C. Quaternary alloys are prepared from the pure constituent elements and equilibrated at 210°C. The equilibrium phases formed in the quaternary alloys are determined using scanning electron microscopy, electron probe microanalysis, and x-ray diffraction (XRD) analysis. The quaternary phase relationships are constructed from the quaternary experimental results and the 210°C isothermal sections of the equilibrium phase diagrams of its constituent ternary systems. Two isoplethal sections, 90at.%Sn and 80at.%Sn, of the 210°C phase equilibria of the Sn-Ag-Cu-Ni system are determined. A Cu₆Sn₅ + Ni₃Sn₄ + Ag₃Sn + Sn four-phase region is observed in both isopleths, and no ternary and quaternary intermetallic compounds are found.