This invention arose from a continuing effort to simplify production of printed circuit boards requiring attachment of circuit components, thus reducing the overall cost of each completed assembly. It arose specifically from design of full-travel input keyboards having dedicated microprocessors, but is adaptable to any application of printed circuit technology where external circuit components are mounted to the board.
The evolution of this invention began with attempts to produce conductive traces and pads about a printed circuit board by direct printing techniques, rather than by etching clad substrate surfaces, a process which is more complicated and expensive than printing. It was found that effective multilayer traces could be produced on a single surface of an insulating substrate by silk screening alternate layers of a conductive ink and a covering dielectric. While this process of printed circuit board fabrication was amenable to conventional mounting of circuit components by soldering techniques, the expense of producing soldered joints, plus the relatively high temperatures encountered during soldering, limited the cost reductions achieved through use of such printing techniques.
A study of known surface-mounting arrangements for circuit components indicated that such components could be attached to one side of a printed circuit board by soldered terminals, with or without mechanical connection of the body portion of the circuit component to the underlying printed circuit board. However, applying solder to the relatively delicate screened traces and pads on the operational side of a printed circuit board is both expensive and a hazard to the integrity of the printed circuit, which typically cannot withstand the elevated temperatures of molten solder. Furthermore, soldered connections are rigid, and effective surface-mounting of circuit components by soldered connections therefore requires a rigid substrate to assure against relative movement between the surface-mounted body portion of the circuit component and its surface-mounted leads.
The limitations of the previously-known methods for surface-mounting circuit components on a printed circuit board have been overcome by the synergistic utilization of two different adhesives--a flexible, electrically conductive adhesive for attaching the conductive leads to underlying exposed conductive pads in the printed circuit, and a rigid mechanical adhesive for bonding the body portions of the circuit components to the board. This combination of adhesive systems assures a durable mounting arrangement for the circuit components. The flexible lead connections accommodate vibrational and bending forces to which the printed circuit board might be subjected, while the rigid mechanical bonds accommodate shock forces. The result is a secure mounting of the components, assured terminal connections to the underlying printed circuit board elements, and consistent electrically conductive connections between the printed circuit and the mounted circuit components.
As a further accessory, the assembly is capable of providing external metallic strips that can be securely mounted to the side of the printed circuit board and protectively located beneath selected circuit components. These strips can serve as power buses to overcome limitations otherwise posed by the higher resistivity levels encountered in screened circuit traces.