Evolution of Ag₃Sn intermetallic compounds during solidification of eutectic Sn-3.5Ag solder

An experimental investigation is performed to examine the effect of the cooling rate on the morphology of the Ag₃Sn intermetallic compounds (IMCs) formed during the solidification of bulk eutectic Sn-3.5Ag solder and Sn-3.5Ag/Cu joints. It is shown that the cooling rate has a significant effect on the solidification time and therefore influences both the size and the morphology of the final Ag₃Sn compounds. Specifically, the Ag ₃Sn compounds exhibit a particle-like → needle-like → needle-like with plate-like tails → plate-like → large plate-like evolution as the cooling rate is reduced. The large plate-like Ag₃Sn compounds are observed only in the Sn-3.5Ag/Cu specimens. The large plate-like Ag₃Sn formed at the interface layer due to the formation of a Cu ₆Sn₅ IMC layer at the interface. Thus, it is inferred that the Cu₆Sn₅ interfacial layer in the Sn-3.5Ag/Cu specimens leads to a local enrichment of Ag at the interface and prompts the formation of large Ag₃Sn IMCs via the Cu₆Sn₅ heterogeneous nucleation sites.