Stress-Strain Behavior of Lead Free Solder Joints Determined by Digital Image Correlation Techniques

Previous investigations on miniaturized solder joints showed a strong increase in their yield and tensile strength with decreasing the gap size. In this study using digital image correlation techniques and FEM simulations the influence of geometrical constraints on stress-strain response of leadfree solder joints was analyzed. Solder joints of Cu-Sn3.5Ag-Cu with thicknesses between 50 mu m to 500 mu m were subjected to tensile loading and the sequences of local strain build-up in the joint area was measured using a VIC3D system. With decreasing the gap size a shift of strain localization from the center of the joints towards the interface region was observed. FEM analysis showed that the interface between the solder and substrate introduces a high triaxiality of stress which reduces the deviatoric stress inside the solder. This effect was more prominent in highly constrained solder joints leading to an increase of strength and a decrease of ductility.