Measuring the mechanical relevant shrinkage during in-mold and post-mold cure with the stress chip

The integration of smart systems into hybrid structures is one of the challenges addressed by the project MERGE - the cluster of excellence on Technologies for Multifunctional Lightweight Structures. As a first example, a sensor system is integrated that is able to explore the thermo-mechanical conditions these systems will typically be exposed to. After briefly describing the sensor system, the paper focuses on the results of the encapsulation step as part of the fabrication process mounting the sensor chip on the test board. The sensor system measures the mechanical stresses during and after transfer molding. In particular, the in-plane components on the chip surface were recorded with high accuracy [1, 2]. Based on these informations, material parameters have been deduced by combining experimental and simulation methods within a Design of Experiment (DoE) study. During the encapsulation process, two sets of effects induce stress into the package simultaneously. On one hand, the coefficients of thermal expansion (CTE) lead to a thermal shrinkage of the materials during cooling from the curing to room temperature. On the other hand, the volume also decreases when the epoxy mold compound (EMC) is cured from its fluid into the final solid stage. This effect is called chemical cure shrinkage [3]. Separating both effects is really a challenge. The method shown in this paper allows quantifying the corresponding material parameters by combining the stress measurements with numerical parameter identification. Based on this method, the investigation on failure modes and reliability of the integrated smart systems can be improved.