Development of solidification microstructure and tensile mechanical properties of Sn-0.7Cu and Sn-0.7Cu-2.0Ag solders

Non modified and Ag-modified eutectic Sn-0.7Cu solder alloys were directionally solidified under transient heat flow conditions. The microstructure of the Sn-0.7Cu alloy has been characterized and the present experimental results include the cell/primary dendrite arm spacing (λ₁) and its correlation with: the tip cooling rate (\(\mathop T\limits^{ \bullet }\)) during solidification, ultimate tensile strength (σ_u) and elongation to fracture (δ). Distinct morphologies of intermetallic compounds have been associated with the solidification cooling rate for both alloys examined. For the Sn-0.7Cu alloy, cellular regions were observed to occur for cooling rates lower than 0.9 K/s, being characterized by aligned eutectic colonies. On the other hand, the alloy containing 2.0 wt %Ag enabled the launch of tertiary branches within the dendritic arrangement. The comparison of results allows stating that finer solder microstructures are shown to be associated with higher ultimate tensile strengths (σ_u) for both alloys although a more complex microstructure was found for the SAC alloy. In contrast the elongation (δ) exhibited opposite tendencies. The growth of coarse Ag₃Sn fibers and platelets within interdendritic regions seems to contribute for the reduction on ductility observed for the SAC alloy.