Brazing of kovar to alumina and LTCC for integration of ceramic pressure sensors

Sensors based on ceramic materials are designed to operate in corrosive environments and at high temperatures. Besides the well known application as circuit boards and housings multilayer based LTCC-ceramics offer a flexible and temperature stable platform for the development of complex sensor elements. Commercial LTCC-qualities are usually available with a matching set of metallization pastes which allow the integration of various electrical functions. For their integration into standardized steel housings it is necessary to compensate the mismatching thermal expansion behaviour between the ceramic and the metallic components. Therefore balancing elements made of Kovar® (Fe-29Ni-17Co) and alumina ceramic (Al2O3) can be used, which have coefficients of thermal expansion adjusted to LTCC. However, these elements have to be joined hermetically to each other and to the LTCC sensors in order to avoid a parasitic gas flow or leakage during operation. In this study, brazing experiments were performed for combinations of Kovar/Al2O3 and Kovar/LTCC with Ag-Cu-Ti and Ag-Cu-In-Ti based commercial braze filler metals, Cusil-ABA® and Incusil®-ABA, respectively. For both active braze filler metals optimized processing parameters were investigated to realize hermetic Kovar/Al2O3 and Kovar/LTCC joints.