LTCC-M: an enabling technology for high performance multilayer RF systems

Over the last several years, the customer-driven wireless market has pushed the developers of RF hardware towards more functionality in less volume, operation at ever higher RF frequencies, and greater circuit and functional integration. Additionally, the pressure to reduce costs has forced designers to look at less expensive manufacturing, assembly, and testing techniques. Multilayer and multichip system integration techniques have been realized in many different formats, each with its own strengths and weaknesses. Best performance is generally obtained using hybrid technology on high temperature ceramics, while lowest cost is often achieved with several of the organic (or "soft") board technologies. Low temperature cofired ceramic (LTCC) technology has the potential to deliver both high performance and low cost. In its basic version, however, it suffers from two major drawbacks. First, the ceramic shrinks after firing in all three dimensions. This limits the size of the boards that can be processed, imposes limitations on embedded passive components, and introduces complexity in the processing of boards with cavities. In addition, modules requiring heat removal must have a heat spreader attached after firing. In order to address many of the problems associated with conventional LTCC technology, Sarnoff has developed an improved approach, called low temperature cofired ceramic on metal (LTCC-M), in which a specially formulated multilayer ceramic structure is attached to a metal carrier or "core". The ceramic firing and core attachment process occur in one and the same step. The resulting structure exhibits virtually no shrinkage in the plane of the substrate.