LTCCs consist of a three-dimensional micro-network of metal structures embedded within a glass-ceramic substrate. The co-sintering of the ceramic substrate with metallic conducting paths at comparatively low temperatures of about 900 °C enables the use of Ag and Cu as electrode material. LTCCs provide higher thermomechanical stability (i.e. higher E-modulus, lower thermal expansion coefficient) than typical polymer circuit boards (e.g. PCB) and can withstand higher acceleration forces (up to approx. 60 g). LTCCs can be utilized in high temperature environments (until 180 °C), which facilitates their use in systems that are subjected to both thermal and mechanical loads.
An important problem during the manufacture of LTCCs is the combination of materials with different thermal expansion coefficients, which can result in high local stresses. These can lead to the formation of cracks as well as delaminations between the ceramic sheets and the metal electrodes. The main goal of this project was to identify “weak points” in LTCC design. New miniaturised testing methods were developed to determine the mechanical strength at different locations within the component. The analyses provided the foundation for quantifying the effect of humidity on the mechanical strength and life of LTCC substrates. This know-how was transferred to the companies involved in the project and has already been included in the process control of LTCC production.
A second outcome of the project was the development of a design tool based on FE models to analyse the mechanical stresses generated in specific locations of the component, i.e. near vias or between ceramic and metal electrodes. This model enables the implementation of design strategies in order to reduce the mechanical loading of certain locations within the LTCCs and thus to increase the mechanical reliability and life time of the component.
The scientific achievement consisted in the development of appropriate testing methods for locally resolved, quantitative measurement of the (fatigue) strength of LTCC substrates. The research work also provided a deeper understanding of the effects of environmental conditions on the mechanical strength of LTCCs. The generation of a parametric 2D FE model describing the stresses in special locations of the components supported the company partners (especially EPCOS OHG) in design improvement.
The project was carried out in close cooperation with the companies EPCOS OHG (Deutschlandsberg) and Continental Automotive GmbH (Regensburg, Germany) and the Institute for Structural and Functional Ceramics of the University of Leoben.