A hybrid approach to characterize adhesion strength of interfaces in an organic substrate

Organic substrates used in flip-chip packaging consist of multiple layers of alternating dielectric and copper layers built around a glass-fiber Core material. These materials have widely different mechanical properties which makes the interfaces between these layers susceptible to delamination. As a result, adhesion between them is a critical mechanical property governing the reliability performance of substrate packages. Developing metrologies which can quantify the adhesive properties of an interface is an essential component of substrate process development. Peel-test is a popular metrology used to characterize adhesion of a film deposited over an underlying material. When inelastic materials are involved (like copper in this case) the peel-force measured is a convoluted function of thickness of the film, mechanical properties of the materials involved, test speed, and the true interfacial properties. Due to this, a clear understanding of the true adhesion strength based solely on experimentation is not possible. Focus of this publication is to utilize mechanical modeling (Finite element analysis) coupled with experimentation to extract the key adhesion metric - the interface fracture energy (Gc). We present our analysis in the case of copper films electroplated on a desmeared dielectric layer. We discuss in detail our findings on the effect of various parameters - peel film thickness, peel velocity, film width on the peel force, from experiments, sample fractography and modeling.