New, creep-resistant, low melting point solders with ultrafine oxide dispersions

Nano-sized, nonreacting, noncoarsening oxide dispersoids have been incorporated into solder alloys to create a new, improved solder structure with an ultrafine grain size of ∼200–500 nm. The new solders exhibit significantly enhanced creep resistance combined with increased strength. The well-known thermal instability problem with ultrafine-grained structure appears to have been overcome in these solder alloys and the microstructure was seen to be quite stable upon high temperature exposure (e.g. 120°C). This is attributed to the presence of very fine dispersoid particles which impede grain boundary sliding and dislocation movement. The dispersions are seen to have a profound effect on the mechanical deformation characteristics of the solders with respect to creep. As much as three orders of magnitude reduction in the steady state creep rate has been achieved. The new solders also exhibit improved ductility under high strain rate deformation and improved strength (4–5 times higher tensile strength) at low strain rates. It is demonstrated that with a dispersion of TiO2 particles, the Pb-Sn eutectic solder with a melting point of 183°C can be made more creep-resistant than the 80Au-20Sn eutectic solder with a much higher melting point of 278°C. The new creep-resistant solders can be useful for optical and optoelectronic packaging in which dimensional stability of the assembled structure is essential.