Interfacial reactions of eutectic Sn3.5Ag and pure tin solders with Cu substrates during liquid-state soldering

The growth behaviors of the intermetallic compounds (IMCs) formed at the eutectic Sn3.5Ag/polycrystalline Cu and pure Sn/polycrystalline Cu interfaces are comparatively studied based on an experiment in which the liquid solder is removed before the end of soldering. This removal of the solder allows for the capture and visualization of the interfacial IMCs formed during liquid-state soldering and avoids the influence of Cu ₆Sn ₅ precipitated from the solder matrix during cooling. The results show that round, scallop-type Cu ₆Sn ₅ grains with a strong texture form at the molten solder/Cu interface and that their growth is controlled more by grain boundary (GB) diffusion at the beginning of the reaction followed by volume diffusion, whereas the growth of Cu ₃Sn is only volume-diffusion-controlled. In addition, in contrast to the predictions of some studies, Ag does not inhibit interfacial IMC growth. Instead, by changing the interfacial energy between the molten solder and the interfacial IMC, the addition of Ag affects the growth orientation and coarsening behavior of interfacial Cu ₆Sn ₅ grains. These changes lead to more Cu ₆Sn ₅ GBs at the interface and therefore greater IMC formation and Cu consumption in the Sn3.5Ag/Cu reaction than in the Sn/Cu reaction under the same reflow conditions.