Enhancement of hardness of bulk solder by doping Cu nanoparticles at the interface of Sn/Cu solder joint

Abstract

In this study, Cu NPs prepared by chemical reduction method,were doped into flux at weight proportions (0, 0.2, 0.5, 1 and 2 wt.%) and then reflow soldering was performed for the pure Sn solder (initial diameter = 1400 μm) with Cu substrate at 250 °C for 120 s. The presence of Cu nanoparticles (NPs) in soldering flux was observed to enhance the spreading of solder on Cu substrate. Solder with larger base spread area, characterized with faster diffusion of Cu from substrate attained quicker supersaturation during reflow and larger precipitation of primary Cu6Sn5 intermetallics (IMCs) upon air cooling. The solders containing greater area proportion of primary IMCs are characterized with enhanced effective elastic modulus. Experimentally, the solders prepared with Cu nanoparticles composed flux,were characterized with the increase in Vicker's microhardness. Solder processed with flux containing 2 wt.% Cu nanoparticles possessed Vicker's hardness of 18.6 HV, whereas solder prepared with undoped flux only had hardness of 13.8 HV. Numerical computation of the Cu diffusion and the elasticity have been done using finite element method.