A Mechanistic Model for Plastic Metal Line Ratcheting Induced BEOL Cracks in Molded Packages

Abstract

Metal line ratcheting and passivation cracking in Back End of Line (BEOL) structures are significant reliability concerns for molded packages that are in very wide spread use at the present time. When metal lines plastically deform due to ratcheting, the passivation overcoat accumulates stress at the corner upon temperature cycling and is eventually susceptible to fracture. Since packaging materials' interaction with the die is the cause of the failure, the problem is inherently multi-scale in nature requiring bridging from package dimension to BEOL length scale. In the present paper, the mechanistic cause for the stress accumulation is elucidated. Furthermore, a global-local modeling strategy is applied to model the passivation crack initiation and growth. A global model with coarse mesh was built of the package. The local region around the interconnect metal line in the die was modeled using boundary conditions extracted from the global model. A novel load decomposition technique is developed to identify the loading mode that best correlates with the experimentally observed fracture. It is shown that shear is the dominant loading mode inducing the die cracks. Furthermore, it is demonstrated that the thermal expansion mismatch between the mold compound as well as the lead frame with the silicon die induces the shear load on the BEOL structure. Owing to the fact that mold compound is applied at a temperature that is higher than that seen during thermal cycling, the direction of the induced shear load is constant regardless of whether the package is heated or cooled. As the metal plastically yields during every temperature cycle, the plastic deformation ratchets, or, accumulates in the same direction over the course of the thermal cycling test. The yielding of metal line results in stiffness reduction, leading to steady accumulation of stress in the passivation corner, causing it to fracture eventually.