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A comprehensive transmission electron microscopy (TEM) study was conducted to investigate the growth mechanisms Of Cu(3)Sn on polycrystalline and single crystalline Cu substrates in solder joints. On single crystalline Cu the solder reflow process resulted in the growth of columnar Cu(3)Sn grains aligned in a thin uniform layer perpendicular to the interface, while a thick Cu(3)Sn layer formed from fine equiaxed grains on the polycrystalline substrate. In the subsequent solid state aging, columnar growth Of Cu(3)Sn continued on the single crystalline Cu before it was replaced by nucleation and growth of new triangular Cu(3)Sn grains at the triple junction sites of the Cu/Cu(3)Sn interface. On the polycrystalline Cu the solid state aging caused much more rapid growth of the Cu(3)Sn layer due to nucleation and the growth of new Cu(3)Sn grains at both the Cu/Cu(3)Sn and Cu(6)Sn(5)/Cu(3)Sn interfaces. These different growth behaviors Of Cu(3)Sn were related to the diffusive supply of reactive elements. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.