MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF SN-IN AND SN-BI SOLDERS

This article presents information that helps to clarify the behavior of lead-free, low-melting, eutectic Sn-In and Sn-Bi solders. The Sn-Bi solder forms a well-defined eutectic microstructure at all solidification rates, while Sn-In solder forms an irregular eutectic that changes to a fine-grained two-phase mixture at high solidification rates. Both solders wet copper by forming tin-containing intermetallics; however, Sn-In forms Sn-In-Cu intermetallics with copper and Au-In intermetallics with gold. The steady-state creep behavior of Sn-In and Sn-Bi roughly resembles that of Sn-Pb with similar microstructures. Sn-Bi further resembles Sn-Pb in that its eutectic microstructure recrystallizes within inhomogeneous shear bands. This is not observed in Sn-In. Both Sn-Bi and Sn-In exhibit pronounced strain softening during shear deformation and show early transitions to tertiary creep. Shear stress-strain behavior at normal temperatures and moderate strain rates is governed by creep; the ultimate shear stress can be inferred from the steady-state creep behavior.