Cu and Ag additions affecting the solidification microstructure and tensile properties of Sn-Bi lead-free solder alloys

Over the past few years Sn-based solders containing third and fourth elements have become of great interest to try and improve the consistency of solders during application. In most reported cases this involved the addition of either Ni in Sn-Cu or Ag in Sn-Bi solder alloys. Still there is a lack of research showing how the combination of third element additions and varying cooling rates affect the mechanical properties of Sn-Bi-X solder alloys. As such the present investigation examines the effects of minor additions of Ag and Cu on a Sn-34 wt%Bi solder alloys produced by directional solidification. Directional solidification was used as the transient regime attained during directional solidification in a water-cooled mold may allow for similar cooling rates to those found in industrial reflow soldering operations. Microstructural analysis on the Sn-Bi-X alloys was conducted using eutectic spacing (λE), Bi precipitates spacing (λp) and the secondary dendritic spacing (λ2) measurements. These measurements represented the complex eutectic growth, the solid-state precipitation of Bi within the β-Sn phase and the length-scale of the Sn-rich dendritic array respectively. In conjunction with these measurements the evolution of tensile strength and ductility as a function of λ2 was examined. Considering the Sn-34 wt%Bi, Sn-34 wt%Bi-0.1 wt%Cu, Sn-34 wt%Bi-0.7 wt%Cu and Sn-33 wt%Bi-2 wt%Ag alloys, it was found that the modified alloys containing 0.7 wt%Cu and 2.0 wt%Ag showed lower tensile properties and lower ductility. In contrast, the addition of 0.1 wt%Cu increased the ductility for λ2 < 14 µm while preserving the tensile strength, representing the best alternative of all alloys examined.