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Sn-Pb eutectic solder was the alloy of choice since the dawn of the circuit board for creating interconnections between components in electronic devices. Under typical solder joint solidification conditions, eutectic Sn-37Pb (wt. %) solder freezes at a single temperature, usually under coupled growth conditions, thereby eliminating any “mushy zone,” providing a homogeneous microstructure, and avoiding micro-shrinkage voids. Unfortunately, lead is toxic, and as such, it has been banned in new electronic devices by various European Union (EU) regulations, notably RoHS. This is a primary driver for lead-free solder research. Additionally, increasing performance demands including rising device operating temperatures, increasing interconnection densities, and a greater demand (and consumer expectation) for reliability, also drive lead-free solder alloy research. Based on performance and processing requirements—including maximum reflow temperature limits of circuit boards—and cost, Sn based alloys have emerged as the leading lead-free solder solution. Near eutectic Sn-Ag-Cu (SAC) alloys, with a eutectic temperature of 217°C, are the leading lead-free solder alloys for surface mount technology (SMT) applications. However, SAC alloys are not without problems. Unlike Sn-Pb, which contains only ductile terminal Pb and Sn rich phases, SAC solders contain relatively brittle intermetallic compound (IMC) phases in addition to the ductile β-Sn phase. As part of a eutectic-like structure, these IMC phase particles are small and act as dispersion strengthening agents. However, primary growth of these particles from the liquid, particularly the Ag3Sn phase, may result in large faceted particles with a blade-like morphology that act as easy crack paths. Another potential problem is that SAC solder joints are often composed of only a few unique Sn grains. This combined with the anisotropic thermal expansion of Sn raises the potential for high stress at Sn/Sn boundaries under thermal strain, thus reducing thermomechanical fatigue performance. The root of these problems and others lay in the complex solidification behavior of SAC solder joints caused by the difficulty in nucleating Sn from the liquid, which results in a high undercooling, which leads to non-equilibrium solidification. To address this problem in this study, a near eutectic (NE) SAC alloy has been modified with a number of low concentration fourth element additions

Microstructure control of the Sn-Ag-Cu-X solder alloy system through nucleation catalysis of Sn