Deformation characteristics of tin-based solder joints

A novel experimental configuration was devised to measure the evolution of the deformation field and the corresponding toughness in solder joints for microelectronic packaging. The utilized material system comprised a ductile layer of tin-based solder encapsulated within relatively hard copper shoulders. The experimental configuration provided pure shear state within the constrained solder layer. Different Pb/Sn compositions were tested with grain size approaching the film thickness. The in-plane strain distribution within the joint thickness was measured by a microscopic digital image correlation system. The toughness evolution within such highly gradient deformation field was monitored qualitatively through a two-dimensional surface scan with a nanoindentor. The measurements showed a highly inhomogeneous deformation field within the film with discreet shear bands of concentrated strain. The localized shear bands showed long-range correlations of the order 2-3 grain diameters. A size-dependent macroscopic response on the layer thickness was observed. However, the corresponding film thickness was approximately 100-1000 times larger than those predicted by nonlocal continuum theories and discreet dislocation.