NEW, CREEP-RESISTANT, LOW-MELTING POINT SOLDERS WITH ULTRAFINE OXIDE DISPERSIONS

Nano-sized, nonreacting, noncoarsening oxide dispersoids have been inc orporated into solder alloys to create a new, improved solder structur e with an ultrafine grain size of similar to 200-500 nm. The new solde rs exhibit significantly enhanced creep resistance combined with incre ased strength. The well-known thermal instability problem with ultrafi ne-grained structure appears to have been overcome in these solder all oys and the microstructure was seen to be quite stable upon high tempe rature exposure (e.g. 120 degrees C). This is attributed to the presen ce of very fine dispersoid particles which impede grain boundary slidi ng and dislocation movement. The dispersions are seen to have a profou nd 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 als o exhibit improved ductility under high strain rate deformation and im proved strength (4-5 times higher tensile strength) at low strain rate s. It is demonstrated that with a dispersion of TiO2 particles, the Pb -Sn eutectic solder with a melting point of 183 degrees C can be made more creep-resistant than the 80Au-20Sn eutectic solder with a much hi gher melting point of 278 degrees C. The new creep-resistant solders c an be useful for optical and optoelectronic packaging in which dimensi onal stability of the assembled structure is essential.