Search

Functional components are commonly fabricated combining a ceramic substrate with external and/or internal metallization (e.g. metal electrodes, vias, contact pads, etc.) using a tape casting process. Different layers are printed and/or fired (e.g. up to 800°C) onto the ceramic part to provide the component with a (certain) functionality. As a result of the combination of different materials (e.g. ceramic, glass, metal alloys) with different coefficients of thermal expansion, internal stresses may arise during the different fabrication steps. Although some of these tensile residual stresses may relax due to plastic deformation of metallic materials, stress concentrations generated in material junctions or terminations (imposed by geometrical constrains) may lead to cracks and/or reduce the component strength.In this work different architectures combining metal and glass layers on the surface of ZnO substrates were investigated experimentally and numerically in order to identify weak points in commercial components. Mechanical testing using three-point-bending was performed on samples taken after different steps. A FE model was developed to (i) calculate residual stresses generated during the manufacturing process, and (ii) simulate the propagation of initial crack/defect during manufacturing. Experimental results were compared with numerical predictions. These results in combination with fractographic analyses were used to validate the finite element model in order to assess location of failure.

In many ceramic components for electric applications, small defects in the microstructure may cause an anomalous current flow. Especially in components exhibiting nonlinear electro-thermal properties, such as varistors, PTC or NTC resistors, inhomogeneities lead to high concentrations of current and localized heat generation due to positive feedback effects. While the integral behavior of those components is often nearly not affected by these defects, the local load at these hot spot regions is considerably high and can even lead to the complete failure of the device. In this work micro-analytical tools are combined to identify the location and type of defects which may cause leakage currents, overheat or other reasons for inhomogeneous current distributions in functional ceramic components. Lock-in thermography is used to localize dominating current paths. This is even possible if the heat sources are in the micrometer range or less. Additionally, micro-sectioning techniques (e.g. Focus Ion Beam) are utilized to expose such regions to investigate features of the microstructure and causes for the enhanced current flows. Examples are shown in PTC resistor components as well as in LTCC based modules.

Functional components such as multilayer low temperature co-fired ceramics are examples of the combination of a ceramic-based substrate with internal electrodes as well as surface features (e.g. metallization, contacting pads, cylindrical vias, etc) employed to provide the component with a given functionality. Another example is that of functionalized silicon chips to be embedded into polymer circuit boards in order to enhance integration and save costs. The functionality of the system can be influenced by the mechanical reliability of the different components. Due to miniaturization and design complexity, no standard methods for mechanical testing can be applied for the characterization of these brittle components. In this work, an experimental approach is presented, which enables determining the strength distribution in functional components (e.g. rectangular plates as small as 2 × 2 × 0.1 mm3) in different environments at different temperatures. The method is based on localized biaxial testing using a ball-on-three-balls fixture. The high accuracy of the test allows quantifying the effect of surface quality, surface features and/or metallization (e.g. contact pads or cylindrical vias) on the component strength distribution. Experimental findings show that the strength distribution of ceramic components can be affected by environmental degradation, whereby subcritical crack growth phenomena can be enhanced in environments with high relative humidity. In addition, metallization at the surface subjected to tensile stresses can even raise the strength of the component, acting as a protective layer against environmental degradation, whereas cylindrical vias can become weak points in the design. It is shown that functionalized layers such as those used in silicon chips can have a significant effect on the strength parameters, thus influencing the lifetime of the device.