Because of the tremendous developments in advanced processing technologies, the dimensions of contemporary electronic devices and interconnections have become smaller and smaller. Moreover, in recent electronic designs the solder joints may not only be electrical interconnections but may also play a significant role in the mechanical stability of the joint.

Due to the strong influence of geometrical constraints and processing parameters, the macroscopic stress-strain constitutive law of lead-free solder materials must be determined in the most geometrically and physically realistic conditions. In order to identify the elasto-visco-plastic constitutive law of Sn-Ag-Cu solders, a special optical strain measurement technique based on Digital Image Correlation has been developed to study the strain field in an idealized joint during a tensile test. Experimental results of the stress-strain relationship at room temperature of geometrically constrained Sn-Ag-Cu joints have been evaluated.

The measured strain field (Digital Image Correlation) and load data have then been used in an inverse numerical identification procedure to determine the true elasto-plastic constitutive law of the solder. The effects of geometrical constraints in a real solder joint with heterogeneous stress and strain fields are then studied by comparing the apparent (constrained) and true (non-constrained) stress-strain relationships. Experimental stress-strain relations for Sn-Ag-Cu joints of 0.25 mm , 0.5 mm and 1.0 mm thickness are presented and the constraining effects are discussed.

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