Prediction of fatigue crack initiation between underfill epoxy and substrate

Delamination between underfill epoxy and a substrate is a critical reliability concern for flip-chip assemblies. Delamination often initiates from multimaterial interface corners, which are sites of elevated stresses, under cyclic loads during testing or operation. Most existing studies of delamination have focused on crack propagation; the presence of an existing crack is assumed, and the conditions under which it will propagate are studied. The related, but different, issue of crack initiation has received far less study. To date, no widely-accepted models of crack initiation from multimaterial interface corners under fatigue loading exist. Here we propose a first step toward establishing such a model. In this study we have used interface corner stress intensities under cyclic loading (ΔK) to correlate fatigue crack initiation at bimaterial interface corners with different far-field loadings and geometries. Fracture under cyclic loads usually initiates at the interface corner and propagates along an interface. The stress states at the interface corner drives the fatigue crack initiation process. In certain cases, crack initiation can be correlated using a critical value of the stress intensity that exists at the bimaterial interface corner in the context of a linear elastic stress analysis. The stress intensities uniquely characterize the stress state in an annular region surrounding the interface corner with a singular stress field. In order to demonstrate the use of critical stress intensities to correlate fracture initiation, we measured the number of cycles required to initiate a fatigue crack from an epoxy/steel interface corner. The preliminary results suggest that fatigue crack initiation can be correlated with critical values of the stress intensities. Furthermore, the proposed approach to correlate fatigue crack initiation can be coupled with existing approaches to correlate fatigue crack propagation, thus resulting in a tool for complete lifecycle analysis