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The present study investigated interfacial reactions between Cu substrates and Bi-Ag alloys during soldering. Without forming intermetallic compounds (IMCs), the molten solder grooved and further penetrated along the grain boundaries (GBs) of the Cu substrate. An increase in Ag content enhanced GB grooving, raised the dissolution rate and also the amount of dissolved Cu in molten Bi. A stoichiometric Cu-Bi phase formed isothermally in liquid solders and considerably affected the Cu dissolution kinetics. The results also show that Bi-Ag/Cu joints possessed a better shear strength than the Pb-Sn/Cu, which implies that mechanical bonding by grain-boundary grooves was strong enough to withstand shear deformation.

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The microstructure and thermal behavior of the Sn-Ag-Ga alloy, as well as those of the Sn-Ag and Sn-Ag-Bi alloys, were examined in this study. Results show that, similarly to Bi, Ga can decrease the melting point and extend the solidus/liquidus range. Both these low-temperature elements distributed nonuniformly and resulted in a mixed normal-coarse structure. However, unlike Bi, which was only detected in the Sn matrix, Ga dissolved in Ag 3 Sn and even transformed Ag 3 Sn into a new intermetallic phase, Ag 72 Ga 28 , within the coarse eutectic cells. It is believed that this could give rise to a specific two-stage nonequilibrium eutectic solidification feature.