Phase equilibria of the Sn-Bi-Cu ternary system in advanced microelectronic packaging

The Sn-Bi solder is a very common lead-free solder used in electronic packaging due to its low eutectic temperature (138°C). Copper is generally used as the substrate in a printed circuit board (PBC) and a metallic pad in the integrated circuit (IC) fabrication. Cu is also used as the under-bump metallurgy (UBM) of the flip chip (FC) package. Thus, for reliability of a solder-joint, it is necessary to make a clear understanding of the phase equilibria in the the Sn-Bi-Cu ternary system. In this study, the isothermal sections of the Sn-Bi-Cu ternary system were experimentally established at 400 and 120°C. The experimental results indicated that no ternary intermetallic compounds were found at both 400 and 120°C in Sn-Bi-Cu ternary system. The isothermal sections of the Sn-Bi-Cu ternary system at 400 and 120°C consist of five single-phase areas, seven two-phase areas, and three tie-triangles, respectively. The Cu₃Sn phase is in equilibrium with most stable single phases in these two isothermal sections. This phase should be the most stable phase in the Sn-Bi-Cu ternary system.