Hermetic packaging for millimetre wave applications

Radio applications are utilising ever increasing frequencies. There is a pressure on the development of both semiconductor processes and module integration. For module integration low temperature co-fired ceramic (LTCC) technology has currently widely been used in e.g. RF front-end modules for portable devices but during last years it has also been applied to microwave and millimetre wave modules. The system-in-package LTCC platform is a true 3D module technology. Some components or applications require hermetic packaging. Typically AuSn solder pre-forms have been utilised in this kind of cases. High melting temperature may reduce the reliability of e.g. RF MEMS switches and other components. Therefore, processes with lower melting temperature solders (<200 °C) could be beneficial. This article reviews the current trends in LTCC materials and processing needed for hermetic millimetre wave products. LTCC materials, including some special materials, available for high-frequency applications are reviewed. Some of the key issues in the substrate processing are also reviewed. The developments in the packaging (such as low-temperature bonding and sealing processes) will also be presented. Hermetic packages with low RF losses have been characterised up to about 90 GHz.