Characterization of reflow soldering at a peak temperature of 215 °C using a Bi-coated Sn-3.0Ag-0.5Cu solder ball

Reflow soldering using a Bi-coated Sn-3.0(wt%)Ag-0.5Cu (SAC305) solder ball was successfully performed at a low peak temperature of 215 °C. The Bi shell promptly transformed into a Sn-58Bi eutectic alloy by rapid Bi diffusion into the inner SAC305 during heating, which resulted in melting at 138 °C. The bonding area on a Cu pad produced by melting increased proportionally with the initial thickness of the Bi shell. Although the overall shear force of the solder bump was lower than those of Sn-58Bi and SAC305 bumps due to the smaller bonding area, the shear force per unit area was the highest.