Deformation characteristics of tin-based solder joints
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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. I. INTRODUCTION
Tin-based solder alloys of eutectic or near eutectic compositions are among the commonly used interconnect materials for microelectronic packaging. The package integrity is primarily driven by the reliability of the solder joints. From the field practice, it is usually shown that solder-joint failure is more prevalent than substrate failure under normal usage. The microstructure of a typical solder alloy (Pb37/Sn63 eutectic or Pb40/Sn60 near eutectic) is highly heterogeneous and presents strain rate sensitivity. In addition, solder joints operate at a relatively high homologous temperature (Tm ∼183 °C). Thus, the combination of microstructure and service temperature leads to a highly inhomogeneous deformation field within the solder joints,1,2 thereby severely affecting their reliability. Another reliability controlling factor is the thickness of the solder joint. When the thickness of these soft metallic solder layers approaches the micrometer range, they will be subjected to a highly nonuniform deformation field, and size-dependent hardening is likely to play a significant role in the mechanics of these joints. Such dependence will manifest itself as an apparent strength improvement of the joint. The improved joint strength is known in the literature as Friction-Hill phenomenon,3,4 a)
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J. Mater. Res., Vol. 18, No. 10, Oct 2003 Downloaded: 18 Mar 2015
due to the triaxial stress build-up when the joint is loaded normal to the interface. However, under shear loading, such strength enhancement is not known except for solder joints that possess width to thickness ratio t/Lo > 0.68.5 Moreover, when the joint thickness becomes the same order as the grain size, the dislocation mobility within
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