Area bonding conductive (ABC) adhesives for flex circuit connection to LTCC/MCM substrates

This paper describes the use, at the Harris Corporation, of a new type of Z-axis conductive epoxy adhesive for connecting four high density Multichip Modules (MCM's) together using fine pitch flex circuits. The MCM's fold together into the size of a cigarette pack. The flex circuits are copper/polyimide and the MCM substrates are made from Low Temperature Co-fired Ceramic (LTCC). The conductive pads on the flex circuit and on the substrate are in the shape of a series of parallel lines on a pitch down to 0.4 mm. The new adhesives, called area bonding conductive (ABC) epoxy adhesives, were developed under an ARPA contract at the Merix Corp., and are made by a new screening and stencilling process, to produce high strength thermosetting (dielectric) epoxies, which also contain a customized line pattern of electrically conductive epoxy adhesive. In contrast to previous random particle Z-axis adhesives, the ABC adhesives are conductive only at the desired bond pad locations and require no pressure to establish or maintain electrical contacts. This paper shows the bonding and assembly process, explains the cost and other advantages over the previous hot-bar reflow soldering method, and presents thermal shock and other reliability test results.