THE FORMATION AND GROWTH OF INTERMETALLICS IN COMPOSITE SOLDER

The formation and growth of intermetallics at the solder/substrate int erface are factors affecting the solderability and reliability of elec tronic solder joints. This study was performed to better understand th e diffusion behavior and microstructural evolution of Cu-Sn intermetal lics at the composite solder/copper substrate interface for eutectic s older and solder alloys containing particle additions of Cu, Cu3Sn, Cu 6Sn5, Ag, Au, and Ni. Annealing temperatures of 110 to 160-degrees-C w ere used with aging times of 0 to 64 days. The copper-containing compo site solders generally formed thinner Cu6Sn5 layers, but thicker Cu3Sn layers than were formed by the eutectic solder alone. These copper-co ntaining additions, therefore, resulted in increased activation energi es for Cu6Sn5 formation and decreased activation energies for Cu3Sn fo rmation as compared to the eutectic solder. The activation energy for Cu3Sn formation decreased relative to eutectic solder for silver and g old composite solders even though less Cu3Sn was formed at the substra te interface. Nickel and palladium drastically reduced the Cu3Sn thick ness and increased the Cu6Sn5 thickness. However, the Cu6Sn, contained a substantial volume fraction of voids close to the copper substrate. We propose two mechanisms to explain the effects of the copper-contai ning and silver particles on the kinetics of intermetallic formation. First, the particles act as tin-sinks which remove tin from the solder and decrease the amount of tin available for reaction at the solder/s ubstrate interface. Second, the particles reduce the cross-sectional a rea available for tin diffusion, which also reduces the amount of tin available at the interface for reaction.