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 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. The physical understanding of the transient heating of the test specimen is essential for the accuracy of the deformation fields to be determined. Two suitable setups are shown. One is for smaller samples like packages (ODU-1) while the second one ODU-2 has been designed with PCB-board size samples in mind. ODU-1 has been accompanied by numerical flow simulation (CFD) to combine a good understanding of fluid mechanics with the transient thermo-mechanics of the test specimen. Based on the simulation results and measurements of test specimens, measures were derived and implemented to achieve the necessary high measurement accuracy. Deep understanding of the accuracy of deformation measurements has shown that a temperature ramp of 1K/min is a good compromise between speed and accuracy. The measurements were carried out on samples designed for future power electronics applications and consisted of thick copper and polymer crossings with a novel technology. The measured deformation fields agree well with the calculations of the corresponding FE models at the same thermo-mechanical load. Furthermore, the shown measurements indicate crack development due to environmental loads. These cracks could be detected with DIC and were also evident in x-ray inspection. This leads to the conclusion that optical deformation measurement is a valuable tool in reliability analysis to support simulation verification and also support inspections towards interface cracking.