Crack Propagation in Low-Temperature Co-fired Ceramics(LTCC)

Low Temperature Co-fired Ceramic (LTCC) is a multi-layer, glass ceramic substrate which can be co-fired with low resistance metal conductors at low sintering temperatures (less than 900C) due to the glass content in the composite material. LTCCs are used as nowadays functional components in ceramic circuit boards (e.g. mobile phones, WLAN, Bluetooth, radar antennas etc.). LTTC technology provides reduced size and production cost and components with improved thermal, chemical and geometrical stability compared to the widely used polymer- based printed circuit board (PCB) technology. The combination of the LTCC material with metal structures and solder materials, as it is necessary in electronic applications, causes macro residual stresses due to the thermal mismatch between ceramic and metallic materials. In addition, LTCC itself is a composite material, consisting of ceramic particles embedded in a glass matrix. The co-sintering of these two materials can also lead to micro residual stresses between ceramic particles and glass. The superposition of macro and micro residual stresses can induce the propagation of surface cracks in the material. Related to crack propagation, the subcritical crack growth (SCCG) phenomenon, especially in environments with high moisture content is of high interest for reliable design of LTCC modules. Understanding the crucial role of local residual stresses in SCCG is very important for an accurate description of damage in LTCC materials. Furthermore, the influence of microstructure of LTCC (i.e. elastic properties of matrix and particles, particle fraction, particle shape, particle size and glass-ceramic interface) on SCCG should be understood. The main focus of the project is to develop a finite element model (FEM) in order to understand the effect of residual stresses and microstructure on the SCGG of surface cracks on the microscale in LTCCs. Growing in the direction of the least resistance and the highest load, the crack will repeatedly be deflected by the particles in the structure. This can lead to a clear reduction in the velocity of crack growth. For the validation of the model, in-situ SCCG measurements and crack growth resistance (R-curve) measurements will be performed. FEM model which is able to simulate the SCGG will help to optimize both the material behavior and the circuit board architecture and to develop improved LTCC materials with higher crack growth resistances by arranging the suitable microstructural parameters.