Fracture, delamination and fatigue evaluation within RSM/DOE concepts of enhanced smart systems

Enhanced microelectronic assemblies as well as smart systems are basically compounds of several high precision materials with quite different thermo-mechanical behavior. Moreover, various kinds of inhomogeneity, manufacturing induced residual stresses and defects together with extreme terms of usage conditions contribute to interface delamination, cracking of smart materials and fatigue of interconnects. For that reason, numerical investigations by means of nonlinear FEA were frequently used for design optimizations and sensitivity analyses. In doing so, most of the publications utilize - classical strenght hypotheses (maximum principal stresses, peel stresses, von Mises stresses, ultimate tensile strength or strains) to estimate the cracking risk of substrates, semiconductors or encapsulations, - accumulated equivalent plastic strains to evaluate the fatigue of metals (metallizations or lead frames, for instance), and - Coffin-Manson like approaches based on accumulated equivalent creep strains or volume weighted inelastic strain energy dissipated during thermal cycling to evaluate the thermal fatigue of solder interconnects. Otherwise, it is common knowledge in mechanical engineering that cracks and delaminations starting at sharp edges have to be taken into account in order to come to a conservative evaluation of the fracture touhgness of the several present materials and interfaces.