Integrity analysis of solder joints; Fracture mechanics approach

Brass/solder/brass sandwich CTS (Compact Tension-Shear) specimens were fracture tested under various loading angles. From the experiments, some peculiar phenomena have been found. For examples, 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) approximate to 300 similar to 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. And it appears to play an important role in the fracture behavior. In order to address these issues, analytic and numerical analyses have been performed. A step-by-step approach has been taken as follows; 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 the 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, i.e., brittle debonding and ductile void growth/coalescence. And also, on basis of competition between the two mechanisms, fracture toughness with remote mode mixity was evaluated and compared with experimental observations. Finally, for the purpose of comparison, experiments on fatigue crack growth along the interface were performed under load of sliding mode (mode-II), which showed some unique features sensitive to the micro structure.