High strength solders can be engineered by adding trace amounts of nanopowders into solder paste. An experimental study was conducted for a better understanding of the solidification kinetics and microstructure development of such nano-composite tin–lead solders. The composite solder pastes were prepared by mechanically mixing nanopowders of either copper or TiO2 with tin–lead powders (tens of microns in size). Solidification experiments of the composite solders were performed in an aluminum crucible with temperature variation during cooling and solidification being measured. For the titanium dioxide (TiO2) nanopowders-reinforced composite solders, the test data reveals the occurrence of melt undercooling followed by recalescence at an increased melting temperature. However, for the copper nanopowders-reinforced composite solders, a reduced melting temperature coupled with two-stages of solidification was distinctly evident. Metallographic observations of the solidified composite solders revealed a reduction in both the grain size and spacing of the eutectic lamellae. This is responsible for the observed increase in microhardness.