A novel experimental approach to calibrating cohesive zone elements for advanced risk analysis of interface delamination in semiconductor packages

Abstract

Interface delamination in moulded semiconductor packages, being caused by thermo-mechanical load, constitutes a major reliability risk. Delaminated units that are exposed to humidity in automotive environments are prone to moisture absorption via opened interfaces. This increases the risk of metal corrosion, which eventually leads to product failure. In order to identify the failure mechanisms behind delamination, Finite Element Modelling (FEM) based on Cohesive Zone Modelling is an indispensable tool. FEM allows one to assess the robustness of interfaces in moulded packages, e.g. between moulding compound and lead frame. On the contrary, conducting standalone stress tests is not sufficient to identify the root cause of the observed failure mode. Selecting appropriate parameters for the cohesive zone elements is a demanding task, because the set of parameters has significant effect on the failure mechanism that one observes in FE simulation. Experiments that are normally used to quantify adhesion can support calibration of cohesive elements in order to accurately predict potential reliability risks. In this article we present a novel idea that enables determining the interface stiffness and critical fracture energy in shear mode by means of the Advanced Button Shear Test Setup. To this end we sheared buttons of moulding compound off a lead frame stripe. Starting from load-displacement curves we determine the temperature dependence of measured shear forces at crack opening and calculated critical fracture energies. The respective quantities for tensile mode are then derived using phenomenological models. As a result, we obtain all parameters that are required to calibrate cohesive zone element models for FEM.