A kinetic study of liquid gallium diffusion in a tin-based solder alloy and its role in solder embrittlement

A first quantitative analysis of the liquid gallium (Ga) embrittlement of a tin (Sn)-based solder alloy is reported here. Ga penetration into Sn96.5-Ag3-Cu0.5 (SAC305) solder was found to follow a diffusion-controlled process using simultaneous intergranular and transgranular paths. A model of the diffusion kinetics of liquid Ga into SAC305 solder thin films was established and appears to obey an Arrhenius relationship. Diffusivity in a 1000 nm thin film was found to be of the same order of magnitude (10–10 m2 s−1) as those determined for other systems of liquid metal penetration into solid metal at similar relative temperatures, i.e., close to the liquid metal melting point and significantly below the solid metal melting point. Determined activation energies in the range of 30–37 kJ mol−1 support a mixed grain boundary and lattice diffusion process that is consistent with the observed intergranular/transgranular penetration. This exponential temperature dependence of liquid Ga diffusion and resultant SAC solder degradation suggests that a liquid metal embrittlement-based solder rework process can be established under favorable time–temperature conditions.