Finite-Element Modeling of the Visco-Plastic Shear-Dominant Deformation Behavior of a Creep-Resistant Sn-Based Solder Alloy under TMF Testing

Abstract

The tremendous growth of electric mobility has created a major impact on electronic assemblies in automotive power electronics, which are subjected to high electric currents and thus to high thermomechanical loads. Especially solder joints which realize the mechanical and electrical connection between components and the substrate such as a printed circuit board are highly exposed to these complex loading scenarios which can lead to solder joint failure due to fatigue damage. In order to address the modeling of such thermomechanical induced stress states within the solder connections, it is mandatory to derive visco-plastic material formulations which cover both the temperature- and time-dependent deformation behavior of the solder material on microstructural level.Within this work, a methodological approach is introduced to validate such a material formulation for a highly creep resistant tin-based solder alloy based on experimental results obtained from thermomechanical fatigue (TMF) tests. The TMF measurement setup is employed for in-situ-measurement of the material degradation driven by temperature cycling, which impose a thermally induced shear dominant deformation onto the solder interconnections. Three temperature profiles with variations of temperature amplitudes, duration of dwell time as well as raising times are investigated. This measurement enables online monitoring of the reaction force and the concurrent change of displacement which are used for the validation of finite element (FE) analysis results.The FE simulation is performed sequentially in two different domains: transient thermal and static structural. It is demonstrated that inhomogeneous, transient temperature changes during thermal cycling influences the deformation behavior of the solder joint specimen. In order to cover these transient thermal effects, convective heat transfer is employed as boundary condition for the FE model, which is mainly determined by the temperature and airflow conditions experienced during TMF testing. Coupling of the resulting time-dependent temperature field from the thermal transient solution as load condition for the static structural analysis enables a very good prediction of measured displacements and reaction forces for all three temperature profiles. Finally, the TMF set-up is populated with soldered samples and the measured and simulated displacements as well as the reaction forces are compared. It is thereby demonstrated, that the visco-plastic material model is able to describe the deformation behavior of the investigated solder alloy under different shear-dominant loading scenarios which enables other numerical investigations on solder joint level.