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 Ag3Sn 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 Cu6Sn5 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. (C) 2010 Elsevier B.V. All rights reserved.