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 (referred to as X, as in SAC.X) in order to gain more microstructural control during solidification and in aging.
A novel technique, referred to as the calorimetric joint, has been developed in this project to study the nucleation and solidification of these alloys that mimics as closely as possible the solidification conditions encountered in industrial electronic assembly processes. In essence, the calorimetric joint technique replaces the typical inert DSC pan with a reactive and wetting (pre-fluxed) Cu pan, thereby taking into account the formation and effect of IMC layer formation upon solidification. This technique enables the researcher to precisely control the cooling conditions and to thermally "watch" and measure solidification events, including the nucleation of Sn, as they occur in practice. The same model solder joint samples can then be cross-sectioned and their microstructures examined and correlated with the DSC scans.
It was discovered that some X additions increase the undercooling relative to the base SAC alloy while others decrease (Al and Zn) it. The variance in undercooling also changed depending on X. A high potency catalyst will result in a low average undercooling and minimal variance. Generally, it was found that as the atomic size of X increases relative to Cu, the catalytic potency increases.
Of the selected X elements, Zn and Al resulted in the least undercooling while Mn resulted in the least variance. With Al and Mn modification at sufficient concentration, a third X rich IMC phase formed. This was possibly the active catalyst for SAC.Mn. The Al rich IMC did not form when the concentration of Al was sufficiently low, Al<0.05wt. %, while the undercooling surprisingly decreased relative to SAC.15Al. Therefore, the Al rich IMC phase was not the likely catalyst species. No new phase formed upon the addition of Zn. Instead, Zn was found concentrated within the primary η-Cu6Sn5 particles, which were frequently found embedded within Sn dendrites, indicating likely catalytic activity. XRD indicated that Zn stabilizes an orthorhombic form of η, denoted η6 in place of the slightly different monoclinic η8 modification of the η group structure, thus providing evidence of a catalysis mechanism for Zn modification.
Low undercooling did not always indicate the absence of large pro-eutectic, blade-like Ag3Sn particles, but the frequency of these blades was reduced with reduced undercooling, and sometimes eliminated. Though Mn did not result in the lowest undercooling, it suppressed Ag3Sn blade formation with an addition of only 0.10 wt. %. The oxidation and wetting behavior of SAC3595.Mn alloys reduced the attractiveness of Mn despite its effect on Ag3Sn suppression. Al and Zn did not have this problem and at the optimal concentration, 0.05 and 0.21wt. %, respectively, both eliminated Ag 3Sn blades and minimized undercooling while providing high eutectic volume solidification structures. (Abstract shortened by UMI.)