Geometrical size effect on the interface diffusion of micro solder joint in electro-thermal coupling aging

This paper investigates the influence of diffusion layer (solder layer) thickness (δ) on interface diffusion in both thermal aging and electro-thermal coupling aging. The different δ (δ = 60, 120 and 240 μm) of Cu/Sn–3.0Ag–0.5Cu (SAC305)/Cu butt solder joints are used. The results indicate that the geometrical size (solder layer thickness) of solder joint has significant effect on element diffusion behavior. The diffusion coefficient, time exponent, element concentrations and diffusion flux are greatly dependent upon δ. The effects of δ on the interface diffusion is different between thermal aging and electro-thermal coupling aging, due to driving force for diffusion is different. During thermal aging, concentration gradient is the main driving force of diffusion, and diffusion coefficient, time exponent and diffusion flux are relatively low for a thin solder layer. However, under electro-thermal coupling condition, the electron wind force provides the dominating driving force for diffusion, and diffusion coefficient and diffusion flux of thin δ are significantly larger than the thick ones. The Cu concentration of the area near interface is relatively high for a thin solder layer in both tests. Under the same experimental temperature, the effects of δ on the electro-thermal coupling aging are more obvious than thermal aging.