Optical Deformation Measurement for Validation of Thermo-Mechanical FEM Models and Material Behavior in Electronics

The application of virtual evaluation tools based on the Finite Element Method (FEM) is already widespread in the field of electronics. At the assembly level, the geometries and material compositions of these models are often very complex and predominantly non-linear. To ensure the validity of the structural mechanic simulation models, it is always advisable to compare the calculation results with real measured data of experiment. This validation allows the evaluation of the model quality, provides information about the limits of material models and thus increases the credibility of the calculation results. One possibility for experimental verification is the optical measurement of the deformations and evaluation by digital image correlation. In this paper, optical measurement setups are presented, which have been specifically designed for the measurement of the thermo-mechanical deformation field on the structures of electronics and their assembly and interconnection technology. Therefore, the measurement setup needs to detect deformations in the range of a few micrometers in a wide temperature range from 20°C to 160°C. The physical understanding of the transient heating of the test specimen is essential for the accuracy of the deformation fields to be determined. It will be shown how the development of such a setup has been accompanied by numerical flow simulation (CFD) in order to combine a good understanding of fluid mechanics with the transient thermo-mechanics of the test specimen. Deep understanding of the accuracy of deformation measurements will be shown, by elimination of erroneous external and internal influencing factors on optical light path. Based on the simulation results and measurements of test specimens, measures were derived and implemented to achieve the necessary high measurement accuracy. Furthermore, a simplified test specimen has been used to carry out the validation process exemplarily. It is a simplified deformation structure consisting of two materials, which can be used to introduce tensile and compressive forces as well as shear forces under temperature excitation. These first application measurements and the comparison of an FE-model with real deformation will be shown. Finally, based on power electronic requirements, a thick copper test specimen with polymer insulation has been particularly designed. Using the optical deformation measurement the shear and tensile specimen have been examined to analyze the deformation fields in the transition from copper to polymer to copper under different mechanical load effects. Additionally FEM calculation were conducted to evaluate the stress - strain condition at the interface regions to determine the limits of loading stress. The correlation of stress and failures at interface allows the derivation of design rules for PCB construction and layout.