Analysis of combined adhesive and cohesive cracking at roughened surfaces

Macroscopic delamination of polymer-metal interfaces is one of the main failure modes observed in micro-electronic components. Due to the irregularly shaped metal roughness profile, this delamination not only consists of interface separation but also bulk cracking at the micro-scale of the roughness. In fact, one of the key mechanisms that results in increased adhesion toughness at roughened interfaces is the transition from adhesive to cohesive failure. A semi-analytical approach is discussed in which the competition between adhesive and cohesive cracking is analyzed by means of the theoretical relation between interface and kinking stress intensity factors. The parameters that define this relation, the solution coefficients, are quantified by finite element (FE) simulations. Accordingly, the crack kinking location and kinking angle into the softer polymer is readily calculated. Furthermore, the geometrical effect of roughness is evaluated by means of FE simulations in which the interface topology follows from measured roughness profiles while also including interface delamination using cohesive zone elements.