SUPPRESSION OF MICROSTRUCTURAL COARSENING AND CREEP DEFORMATION IN A LEAD-FREE SOLDER

Bi-43% Sn eutectic solder [melting point (m.p.)similar to 139 degrees C] undergoes extensive microstructural coarsening and severe creep def ormation under stresses at temperatures near 100 degrees C. It is show n that a relatively uniform dispersion in the solder of essentially in soluble particles such as iron can be accomplished by use of a magneti c field, and that the presence of these dispersoid particles significa ntly reduces microstructural coarsening and resultant accelerating ter tiary creep rates. The useful service ranges of this Bi-Sn lead-free s older can thus be raised to much higher homologous temperatures, allow ing its use at or exposure to elevated temperatures (near 100 degrees C) which are typical for the widely used, lead-containing, eutectic Pb -63-Sn solder alloy (m.p.similar to 183 degrees C), but with a greatly reduced creep rate.