A Large Area, 3-D Stackable, Mcm-D on C Signal Processor

Military applications for MCMs often require non-standard form factors tailored to fit the unique configurations of military systems. Size and weight requirements may further restrict the use of standard packaging and interconnect methods such as the typical SMT packages mounted on PWBs and backplanes. Cost and design cycle time considerations however argue in favor of standard configurations which can be used in multiple applications without incurring significant NRE costs or risks for each new application. An MCM approach will be presented which uses polymer thin film HDI deposited onto an LTCC substrate to provide a flexible form factor and a very high functional density, while maintaining the advantages of a standard, proven architecture and a fixed HDI layout. The HDI provides all internal chip-to-chip interconnect, while the LTCC acts as the package floor, redistributes primary I/O, and provides physical conformance to each target system. Hermetic sealing of the chips and HDI is accomplished by routinely brazing a Kovar seal ring onto the LTCC and subsequently seam welding a Kovar lid into place. This configuration also provides an I/O pad array interface which tends itself well to direct 3-D stacking of multiple MCMs in a "building block" fashion using only connectors but no PWB or backplane. MCMs have been successfully fabricated at two different HDI foundries, using Alumina substrates, while LTCC / HDI materials and processing techniques were matured. Testing of these MCMs has been successful with demonstrated performance superior to the PWB version of the circuit.