Internal conformal coatings for microcircuits

Inadvertent inclusion of conductive particulate contaminants in sealed hybrid microcircuit packages often results in electrical failure of the device. Particles that escape detection by post-sealing radiographic (X-ray) or particle impact noise analysis can be effectively immobilized by conformally coating the circuit with a thin, resilient film of silicone, epoxy or polyxylyene (parylene). The advantages and disadvantages of using each of these materials as a microcircuit coating are discussed, and a recommendation for the use of silicones is made, based on the objectives involved. Two silicone materials have been deposited on test structures. The methods for application of the materials are presented, including those aspects of the application process which are critical to the performance of the materials as coatings. An investigation of the two silicone materials has been conducted with respect to: chemical and mechanical compatibility with hybrid circuit components and materials, chemical purity, outgassing characteristics thermal expansion effects on wire bonds and hybrid circuit rework and repair. This report presents the results of the initial portion of the coating evaluation phase and includes a comparison of the silicone coating properties with those of the polyxylylenes (parylene).