Patent GB-829789-A (GB)
Title: Process of making improved printed wiring boards
IPC: B29B11/14
Filed: 1956-06-26
Abstract: 829,789. Printed circuits. INTERNATIONAL BUSINESS MACHINES CORPORATION. June 26, 1956 [June 28, 1955], No. 19711/56. Class 37. A printed circuit is prepared from a foilcoated laminate comprising an insulating sheet coated on both sides with metal foil by covering both surfaces of the laminate with a plating resist which leaves exposed those portions of the surfaces which define the configuration of the desired circuit, electroplating the exposed portions with a metal or alloy (e.g. solder) which is resistant to etching solutions, removing the plating resist and then etching away those portions of the metal foil which are not covered by the etch-resistant coating, the printed circuit being also provided with a plurality of metal-walled holes extending through the insulating sheet and interconnecting the circuits on the two surfaces of the laminate, the holes being drilled or punched and then coated with a thin conducting layer prior to the electroplating step whereby the walls of the holes are also covered with an etch-resistant coating which is integral with those on the surfaces of the laminate and serve to anchor the circuit to the surface of the insulating sheet and hence reduce the risk of the metal coatings becoming detached from the insulating base. In the process illustrated the laminate comprising insulating base 1, copper or aluminium foil 2 and adhesive 3 is covered with insulating coatings 4 which may be applied by photoprinting, offset printing a silk-screen printed techniques to leave uncovered portions 5, 6, 7 of the metal foil 2 defining the pattern of the desired circuit. Each surface of the laminate is then covered completely with an adhesive but readily strippable film after which holes are punched or drilled through the laminate where desired; thus one is drilled or punched between the portions 6 and 7 of the circuit pattern, the diameter of the hole being less than that of the exposed portions shown. The walls of the holes are coated with a conductive layer of graphite and fine copper powder applied as a suspension in alcohol, after which the strippable layers are removed, having been provided merely to prevent the insulating coatings 4 being coated with a conductive layer during this process. The walls of the holes may alternatively be given an initial copper coating by vacuum deposition. The exposed conductive layers attached to the insulating base are then coated electrolytically with copper and finally with solder. The insulating coatings 4 are then removed, e.g. by the use of a suitable solvent and the laminate placed in a chromicacid etching both to remove the now-uncoated portion of the foil 2. Finally, the solder conductors left are cleaned to remove lead-chromate by scouring with pumice. It will be seen that the plating forming the walls of the holes is integral with that forming the surface conductors and serves to rivet the remaining portions of the foil 2 to the base 1 and hence reduce the possibilities of delamination, e.g. during subsequent dip-soldering operations. In a modification of the process, the foil-clad laminate is drilled to form the holes as a first step, the walls of the holes then being given a conductive coating. Thereafter the plating- resist coating 4 is applied, the plating steps carried out, the coating 4 removed and the exposed portion of the foil 2 etched away as in the previous process. This latter process avoids the necessity for the application of the strippable layers.