Finite strain thermomechanical material characterization of adhesives used in automotive electronics for quantitative finite element simulations

Abstract

Thermoset-based adhesives are used as thermal and electrical interfaces. These adhesives are filled with different particles in order to meet the requirements of heat transfer and electrical properties. In automotive applications, they are required to have excellent adhesion since bulk cracking and/or delamination may precipitate other electrical, thermal or mechanical failure mechanisms. With the help of finite element analysis, it is possible to calculate the behavior of the joint and to locate regions of stress and strain concentration where failure is expected to initiate. However, the accuracy of numerical calculations is dependent on the validity of the material models used in the analysis to describe the deformation behavior of the adhesive and adherents. Linear elastic (LE), elastic-plastic (EP) and linear viscoelastic (LVE) material models are frequently used in microelectronics industry. However, up to now in microelectronics industry, there is no work where the limitations of these material models are discussed. The present paper addresses the above issue. We will show the limitations of LVE models and propose a nonlinear viscoelastic (NLVE) model which is capable to describe the large strain behavior of the observed material behavior. Although the NLVE model is illustrated for an adhesive, similar behavior is also observed at other organic materials such as molding compounds and lamination foils. Thus, the suggested NLVE material model has the potential to be applied to a very wide-range of materials. The authors present LVE (between −40°C and 200°C) and NLVE (at 25°C and 100°C) characterization and modelling of the adhesive. For LVE characterization, dynamic mechanical analysis (DMA) and pressure-volume-temperature (PVT) experiments are used. Results are combined to obtain a LVE model which is described by the Prony terms and shift function. Validation of the LVE model is performed at small and large strains with the help of a newly designed dogbone geometry, which is developed in the course of this work to eliminate the disadvantages of the existing DIN EN ISO 527-2 standard. For validation, static tensile tests (STT) and static tensile tests with stress relaxation segments (STSR) up to failure are used. It is found out that the LVE model is capable of predicting the mechanical behavior of the adhesive only at small strains and fails to represent the highly nonlinear mechanical behavior. As it is crucial to predict the adhesive strength at large strains, already obtained STT and STSR results are used to fit the Bergstrom-Boyce (BB) NLVE material model. It is shown that the BB model can accurately describe the material behavior which is observed from STT and STSR experiments. In order to validate the BB model, static tensile tests with creep segments (STCR), which are not previously used for the calibration of the model, are used. A comparison of LVE and NLVE material models is also presented for the STCR simulations. In order to check the behavior of the BB model at temperatures other than the material model input temperatures (25°C and 100°C), STCR experiments at 70°C are also performed and simulated. In all cases, when compared to the LVE material model, NLVE BB model is shown to improve the predictions of the experimental results. Thus, the BB model is shown to be useful for adhesives. This will allow designers to perform quantitative FE simulations of adhesive joints.