Superplastic creep of eutectic tinlead solder joints

This paper presents experimental evidence that as-solidified eutectic Pb-Sn solder joints can exhibit superplastic behavior in shear creep loading. Stepped load creep tests of as-solidified joints show a change in the stress exponent from a high value typical of con-ventional creep at high stress and strain rate to a superplastic value near 2 at lower stress and strain rates. In addition, the change in stress exponent is accompanied by a change in the activation energy for creep from a value near that for bulk self-diffusion (20 kcal/mol) to a value near that for grain boundary diffusion (12 kcal/mol). The total shear deformation of joints in stress-rupture tests performed at 65° C are found to ex-ceed 150%. The concomitant observation that quenched solder joints creep faster than air-cooled ones is attributed to a grain, or phase, size dependence of the strain rate. The source of superplastic behavior is a fine, equiaxed microstructure. It is not yet clear whether the superplastic microstructure is present in the as-solidified joint, or develops during the early stages of plastic deformation.