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.