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

Evolution of Cu2(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, Cu2(In,Sn) was found, which showed two different morphologies, a coarse-grained Cu2(In,Sn) sublayer and a fine-grained Cu2(In,Sn) sublayer. The fine Cu2(In,Sn) grains had and always kept a granular morphology without any growth orientation. The morphology of coarse Cu2(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 Cu2(In,Sn) compound grew along [0001] axis and exposed {11-20} crystal planes. Growth mechanism of coarse Cu2(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 Cu2(In,Sn) grains had a special growth mechanism at the root of coarse Cu2(In,Sn) grains compared to normal fine Cu2(In,Sn) grains underneath coarse Cu2(In,Sn) grains.