Formation and characterization of cobalt-reinforced Sn-3.5Ag solder

Solders under severe service environments should have enhanced mechanical properties. To achieve this goal, the approach of composite solder reinforced by second-phase particles was tried in this study. Cobalt (Co) and eutectic Sn-3.5Ag were selected as a reinforcing particle and solder matrix, respectively. Co particles and solder paste were mechanically mixed to make uniform mixing at Co weight fractions from 0.1 % to 2.0 %. For the Co-mixed Sn-3.5Ag solder pastes, melting temperature and spreading area were measured. The solder pastes were stencil printed on test substrates and reflowed to form solder bumps. Ball shear test was performed to examine shear strength of Co-reinforced Sn-3.5Ag solder bumps. As a result, small amount of Co addition did not alter melting temperature and spreadability. Maximum shear strength of Co-reinforced Sn-3.5Ag solder bumps showed 28 % increase compared to normal ones. The increase in shear strength was due to facetted needle-like (Cu,Co)₃Sn₂ intermetallic compounds (IMCs)