Effects of Ni additions, trace elements and solidification kinetics on microstructure formation in Sn–0.7Cu solder

Five ternary Sn-0.7Cu-xNi alloys with nickel contents in the range 0-1000 ppm have been directionally solidified at different growth rates. The competition between primary tetragonal Sn cells/dendrites, eutectic and primary Cu 6Sn 5 is interpreted with phase diagrams available in the literature and in terms of the coupled zone concept. As for binary Sn-Cu alloys, it is found that Sn-Cu-Ni forms two different eutectic morphologies (coarse and fine) simultaneously during eutectic growth. Solidification of high-purity alloys with the intentional addition of selected trace elements usually present in tin confirms the importance of impurities in the destabilization of the eutectic interface into cellular morphologies, with the fine and coarse eutectic present at the cell centres and boundaries, respectively.