Sn-Ag eutectic solder has been examined as one of the candidates of the lead-free solder comparing with Sn-Pb eutectic solder and pure Sn. All interfaces have Cu3Sn and Cu6Sn5 reaction layers. Cu6Sn5 grows into solders like “peninsula”. Kirkendall voids are formed at the Sn/Cu and Sn-Pb/Cu interfaces but not at the Sn-Ag/Cu interface. The tensile strength of the Sn-Ag/Cu joint is higher than those of the Sn/Cu and Sn-Pb/Cu joints. Fracture takes place along the Cu6Sn5 layer and the linkage of preceding microcracks at the bottom of Cu6Sn5 peninsulas is responsible for the fracture. Sn-Ag solder has fine dispersion structure of Ag3Sn particles forming subgrains. Ag3Sn particles have a specific orientation relationship with the Sn matrix as shown below.(This article is not displayable. Please see full text pdf.)
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Suganuma et al. (1995) studied this question.