Evolution and Growth Mechanism of Cu2(In,Sn) Formed Between In-48Sn Solder and Polycrystalline Cu During Long-Time Liquid-State Aging

Evolution of Cu 2 (In,Sn) formed between In-48Sn solder and polycrystalline Cu during long-time liquid-state aging was systematically investigated. During aging at 160°C up to 90 min, one IMC species, Cu 2 (In,Sn) was found, which showed two different morphologies, a coarse-grained Cu 2 (In,Sn) sublayer and a fine-grained Cu2(In,Sn) sublayer. The fine Cu 2 (In,Sn) grains had and always kept a granular morphology without any growth orientation. The morphology of coarse Cu 2 (In,Sn) grains evolved from poly-facet pyramidal-type without preferential orientation into hexagonal structure preferring only one elongated direction after aging up to 90 min. Electron beam backscattered diffraction revealed that coarse-grain Cu 2 (In,Sn) compound grew along [0001] axis and exposed {11-20} crystal planes. Growth mechanism of coarse Cu 2 (In,Sn) grains related closely to thermodynamic stability of hexagonal structure, which drove by reduction of surface energy from higher to lower, and first principles calculations verified that {11-20} crystal planes had the lowest surface energy. Fine Cu 2 (In,Sn) grains had a special growth mechanism at the root of coarse Cu 2 (In,Sn) grains compared to normal fine Cu 2 (In,Sn) grains underneath coarse Cu 2 (In,Sn) grains.