Simulation Methods for Crack Initiation and Propagation in Bulk Mold Material of Electro Mechanical Components

Abstract

Epoxy based polymers are widely used in the semiconductorindustry as thermal or/and electrical interfaces and as encapsulating material. In the automotive industry epoxy molding compounds are often used to protect not only singleIC packages but also the entire electronic control units (ECUs) or power modules. Fracture processes in mold materials used for the encapsulation of electro mechanical components are a severe phenomenon which must be considered in more detail. Once the encapsulation is fractured moisture can enter the interior domain which causes the overall failure of the entire electronic system. To this end the underlying work covers aspects of classical fracture mechanics. Hereby, the main focus of our study lies on the identification of simulation methods that can be used to model crack initiation and propagation in arbitrary three-dimensional problems subjected to thermal and/or mechanical loading conditions. As a consequence, further attention is paid to the development of experimental procedures that become necessary for the characterization of the temperature-dependent fracture mechanical parameters. In the temperature range of interest, the mold material used in the specific application exhibits a highly nonlinear behavior. Especially around the glass transition temperature, a dominant viscous characteristic of the mold material can be observed. Within the research activity, the focus lies on classical linear elastic fracture mechanics, which is not able to capture the aforementioned viscous effects. However, regarding damage initiation and crack growth in the mold material, low temperatures turn out to be the most severe loading case. Here, the material is ideal brittle and local small strains can cause a crack. On the experimental side, a procedure for the determination of fracture mechanical properties as a function of the temperature and the so called mode mix (ration of modeI and mode II loading) is established. With these parametersa simulation method is set up to model crack initiation andpropagation in thermally loaded problems. The simulationmethods are validated on specimen level which demonstrate the applicability of the theory of linear elastic fracture mechanics to the mold material under focus. The proposed methods turn out to be very powerful tools for predicting crack initiation and propagation in fracturing mold materials. The capability of this method is demonstrated by means of some representative numerical examples.