On the nonlinear thermomechanical behavior and delamination of conductive adhesives

Abstract

Adhesives based on thermoset polymers 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. Due to the reliability requirements of automotive applications, they are required to have excellent bulk and interface properties. Finite element analysis is used to locate stress and strain concentrations and to assess where the material is expected to fail. However, the accuracy of the design calculations is dependent on the validity of the material models used in the analysis. In this thesis, the limitations of the linear viscoelastic material model are discussed and a nonlinear viscoelastic material model is proposed. Although the experiments and the nonlinear viscoelastic modeling are illustrated for an adhesive, qualitatively similar results are also obtained for commercial molding compounds. For all test cases, when compared to the linear viscoelastic material model, the nonlinear viscoelastic material model is shown to improve the prediction of the experimental results. This will allow designers to perform quantitative FE simulations of adhesive joints. As adhesives join different materials together, the interface between the adjacent materials is the place where delamination related failure is most likely to occur. Since delamination also can initiate other failure mechanisms, such as electrical, thermal or mechanical failure mechanisms, the assessment of the risk of delamination has become an integral part of the reliability approach. Only very few studies focus on delamination of adhesive bonds. In this thesis, a new methodology to produce delamination specimens from existing products is described. Although the method is illustrated for two interfaces, wherein a single step of the production process is added, the approach makes it possible to examine different interfaces, which have the same processing properties as in the real product. The LTCC/adhesive and Alloy 42/adhesive interfaces are experimentally investigated for (near) mode I loading conditions. Finite element analysis (e.g. J-integral) is used to extract the energy release rate and the established critical value is implemented for cohesive zone modeling. The presented approach will allow delamination studies of interfaces between brittle materials (such as LTCC). The increased design complexity and the demand for reduced product development times require fast pre-qualification methods to assess the reliability of an adhesive bond and in order to obtain qualitative comparisons between different adhesive choices (e.g. material changes, surface preparations, etc.). In this thesis, a novel lap shear specimen, which is obtained by optimizing the standard geometry, is tested under cyclic loading. The test results, their meaning and their reliability are discussed. Suggestions are made to further improve the approach. The presented lap-shear test approach, on test samples from genuine products, can be used to assess the stability of the adhesive joints (For example, a Cu / lamination foil / CU - connection structure (not yet published), which can be used in next-generation power modules).