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

This article presents information that helps to clarify the behavior o f lead-free, low-melting, eutectic Sn-In and Sn-Bi solders. The Sn-Bi solder forms a well-defined eutectic microstructure at all solidificat ion rates, while Sn-In solder forms an irregular eutectic that changes to a fine-grained two-phase mixture at high solidification rates. Bot h solders wet copper by forming tin-containing intermetallics; however , Sn-In forms Sn-In-Cu intermetallics with copper and Au-In intermetal lics with gold. The steady-state creep behavior of Sn-In and Sn-Bi rou ghly resembles that of Sn-Pb with similar microstructures. Sn-Bi furth er 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 defor mation and show early transitions to tertiary creep. Shear stress-stra in behavior at normal temperatures and moderate strain rates is govern ed by creep; the ultimate shear stress can be inferred from the steady -state creep behavior.