Development of hypoeutectic SnBi alloy solder reinforced with WO nanoparticles for connecting Cu substrates via thermal bonding.

WO nanoparticles with different loading amounts (0.25 to 1.00 wt%) were mechanically mixed with Sn-Bi alloys (10 and 20 wt% Bi) for 45 min. The Sn-Bi nanocomposite powders were mixed with a flux mixture to form solder pastes. Solder pastes with different WO nanoparticles were deposited on the pure Cu sheets using a solder paste dispenser and heated at 275 °C for 180 s. The effect of the WO nanoparticle addition on microstructure, interface, and bonding strength of Sn-10 wt% Bi solder/Cu and Sn-20 wt% Bi solder/Cu solder joints was studied. The addition of lower amounts of WO nanoparticles (0.25 wt%) to both solder alloys improved their microstructure and wettability. The addition of small amounts of 0.25 and 0.50 wt% WO nanoparticles to the solders changed the coarse elongated Bi structure in the solder matrix to a fine spheroidal shape and a thin discontinuous interfacial layer free of cracks and/or microvoids were formed. Addition of 0.25 wt% WO nanoparticles to Sn-20 wt% Bi solder increased the shear strength to 42.25 MPa and elongation to 7.1%, depicting the increases of 31.66% and 208.70%, respectively, compared to the values of ordinary one.