Effects of solidification kinetics on microstructure formation in binary Sn–Cu solder alloys

Three binary Sn-Cu solder alloys of near-eutectic composition have been directionally solidified at different growth rates. The competition between primary tetragonal Sn cells/dendrites and eutectic is interpreted with the coupled zone concept. It is also found that Sn-Cu is a weakly irregular eutectic system with CuSn leading the eutectic, but two different eutectic morphologies (coarse and fine) form simultaneously during eutectic growth. At higher growth rates, the eutectic interface breaks down into a cellular eutectic with the fine eutectic in the centre of the cells and the coarse one at the cell boundaries. This is explained by the segregation of Pb impurities ahead of the eutectic interface.