The formation and growth of intermetallics in composite solder

Abstract

The formation and growth of intermetallics at the solder/substrate interface are factors affecting the solderability and reliability of electronic solder joints. This study was performed to better understand the diffusion behavior and microstructural evolution of Cu−Sn intermetallics at the composite solder/copper substrate interface for eutectic solder and solder alloys containing particle additions of Cu, Cu₃Sn, Cu₆Sn₅, Ag, Au, and Ni. Annealing temperatures of 110 to 160°C were used with aging times of 0 to 64 days. The copper-containing composite solders generally formed thinner Cu₆Sn₅ layers, but thicker Cu₃Sn layers than were formed by the eutectic solder alone. These copper-containing additions, therefore, resulted in increased activation energies for Cu₆Sn₅ formation and decreased activation energies for Cu₃Sn formation as compared to the eutectic solder. The activation energy for Cu₃Sn formation decreased relative to eutectic solder for silver and gold composite solders even though less Cu₃Sn was formed at the substrate interface. Nickel and palladium drastically reduced the Cu₃Sn thickness and increased the Cu₆Sn₅ thickness. However, the Cu₆Sn₅ contained a substantial volume fraction of voids close to the copper substrate. We propose two mechanisms to explain the effects of the copper-containing 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/substrate interface. Second, the particles reduce the cross-sectional area available for tin diffusion, which also reduces the amount of tin available at the interface for reaction.