A Study of the Transport of Nanopowders in a Solidifying Solder Melt and its Influence on Microstructural Development

Recent experiments have clearly demonstrated an innate capability to judiciously use nanopowders to increase the strength of engineering materials. This can be achieved even for low weight percentages of the nanopowders, as low as a few percent. This research finding has provided the much desired impetus of producing high-strength materials by adding small amounts of nanopowders into a solidifying molten metal with the objective of forming nano-particle reinforced metallic composites. A practical application of such a composite is in microelectronic and/or opto-electronic devices and assemblies, which necessitate the need for high strength solders. This paper presents preliminary results of experiments aimed at understanding the kinetics governing the solidification of solders blended with nanopowders. Composite solder paste was prepared by mechanically mixing nanopowders of either copper or TiO ₂ with tin-lead powders (tens of microns in size). Solidification experiments of the composite solders were performed in an aluminum crucible and temperature variations during cooling and solidification were measured. For the TiO ₂ nanopowder reinforced composite solders, the data show a strong melt undercooling followed by recalescence to an increased melting temperature, while for the Cu nanopowder reinforced composite solders, one sees a reduced melting temperature with two-stages of solidification. Metallographic observations of the solidified composite solders find a significant reduction in both the grain size and spacing of eutectic lamellae, leading to an increase in microhardness.