Integrity analysis of solder joints; fracture mechanics approach

Brass/solder/brass sandwich CTS (compact tension-shear) specimens were fracture tested under various loading angles. The fracture toughness vs. mode mixity relation is asymmetric and the crack often jumps from the one interface to the other as the fracture proceeds. Because the toughness is exceptionally high ( J_c=300~900J/m ²) in comparison with ordinary metal or non-metal bond, the plastic deformation of the ductile layer is so constrained due to the stiff elastic substrates. It appears to play an important role in the fracture behavior. In order to address these issues a step-by-step approach has been taken. First, analytic models, so called ´modified Dugdale´ and ´modified Irwin model´ were made to estimate the plastic zone size near the crack tip with respect to the mode mixity as well as the external load. Second, the near-tip stress field under mode-I loading, for which both models gave erroneous estimation, was analyzed by a slip line field model. Third, with aids of incremental theory of plasticity implemented in finite element code, the crack growth behavior was estimated for two kinds of fracture mechanisms