Direct metallization process

Abstract

A method of applying a conductive carbon coating to a non-conductive surface, conductive carbon compositions for that purpose, and a printed wiring board having through holes or other nonconductive surfaces treated with such carbon compositions are disclosed. A liquid dispersion of electrically conductive carbon (for example, graphite) having a mean particle size no greater than about 50 microns is coated on the non-conductive surface to form an electrically conductive carbon coating. The conductive carbon coating is then fixed on the (formerly) nonconductive surface. Fixing may be accomplished in a variety of different ways. For example, the fixing step can be carried out by applying a fixing liquid to the carbon-coated surface. One example of a suitable fixing liquid is a dilute aqueous acid. Fixing may also be carried out by removing the excess carbon dispersion with an air knife or other source of compressed air. The fixing process removes excessive carbon composition deposits, and thus smooths the carbon coating on the recess surfaces by eliminating lumps and by making the coating more uniform. Fixing can also crosslink the first monolayer of carbon which is directly attached to the substrate or an aqueous organic binding agent associated with the coating. The resulting coating has a low electrical resistance and is tenacious enough to be plated and exposed to molten solder without creating voids or losing adhesion.