Method of making flexible printed circuits

In a method of making a flexible printed circuit wherein:

• (a) a flexible belt-like substrate strip of an insulating polymer resin film is provided with a layer of electrically conductive metal foil bonded on at least one face and is punched with a plurality of through holes;
• (b) a photopolymer film is formed on said layer of electrically conductive metal foil;
• (c) the photopolymer film is exposed in a predetermined pattern;
• (d) the photopolymer film is developed and is thereby removed according to said pattern;
• (e) the foil on the substrate, where it is now bare of photopolymer film in certain areas as a result of conducting step (d), is subjected to an etchant, for removing said layer of electrically conductive metal foil where it has thus been bared;
• (f) the photopolymer film is removed from where it remains in place upon the remaining pattern of electrically conductive metal foil that is to become electrical circuitry;
• and
• (g) a solder resist layer is formed on selected portions of the thus-bared pattern of electrically conductive metal foil that is to become electrical circuitry,
• an improvement for overcoming a problem which arises where the substrate strip of insulating polymeric resin film is subject to longitudinal shrinkage during processing subsequent to the conducting of step (a), in that, in conducting step (g), said selected portions where said solder resist layer is formed are inaccurately located relative to said through holes or said pattern of electrically conductive metal foil that is to become electrical circuitry,
• said improvement comprising:
• in conducting step (e) there is left on said substrate strip of insulating polymer resin, at least one long, narrow band of said layer of electrically conductive metal in a substantially non-etched condition, of which each such band lies near and extends parallel to a respective lateral extreme of said belt-like strip of insulating polymer resin and adjacent, but separate from and laterally outside said pattern of electrically conductive metal foil that is to become electrical circuitry, so that said at least one band is available at least until the completion of the conducting of step (g), to prevent inaccurate location of said selected portions where said solder resist layer is formed by preventing longitudinal polymer film of said substrate,
• said at least one band being formed so as to be on the order of as long as said pattern of electrically conductive metal foil that is to become electrical circuitry, at least to the extent that said pattern of electrically conductive metal is to become one flexible printed circuit.