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A combination of various experimental techniques was coupled with threedimensional numerical simulation to study the strain distribution in anisotropic, heterogeneous lead (Pb)-free solder ball grid array interconnects used in electronic packages. An in situ full-field deformation map on the cross section of the joint showed a nonuniform strain distribution when the package was subjected to thermal loading. This nonuniformity was correlated with the locations of various grains on the cross section as obtained by orientation imaging microscopy (OIM) and optical microscopy. The solder interconnect was progressively sectioned and imaged under cross polarizers to discern the three-dimensional shapes of various grains in the solder interconnect. A methodology to replicate the three-dimensional shapes and orientations of the various grains and grain boundaries in a microstructure-based finite element model was developed. The numerical results were compared with the displacement and strain distributions obtained experimentally. The demonstrated strain localization along the grain boundaries in the case of multigrain joints and along the pad-solder interfaces in the case of the single-grain joints matched very well with the locations of plastic damage accumulation when the same interconnect was subjected to several thermal cycles.

The thermo-mechanical behavior of near eutectic lead-free SnAgCu (SAC) solder joints under Deep Thermal Cycling (DTC) and in-situ thermal loading was examined. Crossed polarizer, optical microscopy revealed that in ball grid array (BGA) solder joints, these Sn rich, Pb-free solders exhibit large grained Sn structures. After imaging, these SnAgCu solder joints were subjected to repeated thermal stresses under an inert atmosphere. Subsequent to this thermal loading, the samples were again examined with optical microscopy. Using both data sets, the intergrain strains and deformations were quantified by Digital image correlation, a full field optical measurement technique. The relations between the positions of grains as well as intermetallics compounds, their boundaries and Sn deformation fields were examined.

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