Three-dimensional microstructure characterization of Ag 3Sn intermetallics in Sn-rich solder by serial sectioning

Abstract

Increasing concerns over the environmental and health hazards of Pb–Sn solders, used in electronic packaging, have prompted the need for Pb-free solder alternatives [1–3]. The eutectic Sn–3.5Ag solder system is a potential candidate for replacement of Pb–Sn solder for several reasons, including excellent mechanical properties, improved high temperature resistance compared with the Pb–Sn system, and cost effectiveness when compared with other Pb-free solder alternatives [1–8]. In the Sn–3.5Ag solder system, upon solidification, the eutectic microstructure consists of two phases: a Sn-rich matrix and Ag3Sn intermetallics that form due to the reaction between Sn and Ag [6–8]. Recently, Ochoa et al. [7,8] have determined that cooling rate has a significant effect on intermetallic size and morphology. At relatively fast cooling rates (24 jC/s), a fine distribution of spherical Ag3Sn particles was observed. At slower cooling rates (0.08 jC/s), however, the intermetallic had a needlelike morphology. Thus, to understand the physical, electrical, and mechanical properties of the solder, it is necessary to accurately characterize the intermetallic