LAUSR

A reduction of the particle size used in solder pastes was shown to affect the electrical and mechanical properties of finely printed solder bumps. Sn–3.0Ag–0.5Cu solder nanoparticles were synthesized using a radio frequency thermal plasma system, and solder pastes were formulated for reverse-offset printing of solder bump arrays with a size of 30 µm. As the nanoparticle ratio in the paste increased, the degree of supercooling, ΔT, increased with a separation of the exothermic peaks for the solidification of β-Sn and the precipitation of intermetallic compounds (IMCs). The networks of finely precipitated IMCs formed at the boundaries of large β-Sn increased the shear strength to 73 MPa. However, insufficient flux deteriorated the electrical and mechanical properties because it delayed the solidification of primary β-Sn as well as the melting of the solder. As a result, the Sn–3.0Ag–0.5Cu solder paste containing a nanoparticle ratio of 25% exhibited an optimum printability for reverse-offset printing of solder bumps, and the resulting bumps had an electrical conductance of 0.4 mΩ and a shear strength of 73 MPa.