Experimental Verification of FE-Models for Thermo- Mechanical Loading using Digital Image Correlation

Abstract

The usage of virtual evaluation tools based on the finite-element-method (FEM) is already widespread in the area of electronics. At the assembly level the geometries and material compositions of these models is oftentimes very complex. Here substrates, components and the joining technologies contain a large number of different materials and hence multiple interfaces occur that are thermo-mechanically relevant. The geometries often require complex modelling in order to provide the necessary level of detail. In addition, more and more complex non-linear material modelling is required for relevant system evaluation, the input parameters of which extend beyond data sheet specifications. With ever more powerful computational possibilities and more efficient FEM element technologies, very complex geometric models can be realized and calculated efficiently. Decisive efforts are also being made to reproduce the material behavior more accurately, taking into account the temperature, time and ageing history. The result quality of FE-simulations correlates with the quality of the mentioned input parameters. However the allowed degree of abstraction regarding geometries, material model and loading conditions is oftentimes not clear. Consequently a comparison of simulation results with experimental measurements is always necessary to ensure the quality and credibility of the calculation results. This verification of a FE-model is however very complex and complicated, since stress and strain fields are not easily measurable. Previous approaches examined the model assumptions in partial aspects by means of several individual experiments on individual system aspects to gradually improve the simulation result’s quality. In the presented work, the necessary verification has been carried out on the complex and heterogeneous structure itself by measuring the total deformation field of the complete system. This is experimentally realized through optical micro deformation measurements followed by image correlation algorithms. The paper presents the developed measurement setup which has been specifically designed to measure the thermo-mechanical deformation field induced in small packaging structures. Currently the setup is accurate enough to detect deformations in the range of a few micrometers within a temperature range of 20 °C to 160 °C. The comparison between simulated and measured deformation results can be used to evaluate the model quality. Based on the measurement data it becomes possible to evaluate the effectiveness of complex implementations like time- and temperature dependent material behavior. The example used here is an embedded ceramic-resistor in PCB-core. The aim of the experimental verification has been to determine whether it is necessary to take into account the viscoelastic properties of the polymer materials and the residual stress caused by the laminating process. Both of which have proven to show significant influence and hence justify the effort for the extended material characterization of the polymer and increased simulation time to establish the non-zero initial stress situation after lamination.