A novel in-situ characterization technique combining Digital Image Correlation strain measurement and inverse numerical identification was developed to determine the plastic constitutive properties of Sn-Ag-Cu lead-free soldering material inside real joints. The effects of size and geometrical constraints on the constitutive and apparent stress-strain response of 0.1, 0.5 and 1mm joints were studied, showing clearly the important constraining and size effects in thin solder joints.