Universal surface finish for dca, smt and pad on pad interconnections

Abstract

A printed circuit connector terminal pad coating technique is disclosed which functions as a single universal pad surface which supports multiple electrical connection practices including wirebonding, soldering, and wear resistant, pad on pad mechanical connection. The tri-plate surface treatment includes an initial diffusion resistant coating of nickel; an intermediate layer of hard, wear resistant noble or semi-noble metal that provides pad on pad connector reliability and affords a metallurgically stable solder joints and wirebond interfaces; and a final coating of soft gold. The intermediate layer may be pure palladium having a nominal thickness of 35 microinches or a layer of gold, hardened by cobalt, nickel, iron or a combination of these dopants to effect a hardness of 200 to 250 (Knoop scale). The use of a common surface treatment for the multiple attachment processes is implemented with a single masking step, rather than a sequence of selective masking, plating and stripping operations. In the printed circuit environment, the masking is provided by the final covering that encloses, seals, and electrically insulates the conductors in a circuit board application or in the instance of a flexcable, the adhesive coated flexible coverlay the covers and seals the copper conductor elements while exposing the conductor terminal pads.