Image Number 3 for United States Patent #4966827.
Improved printing with polymers is achieved by interposing a liquid layer conforming to rough substrate surfaces and possibly exhibiting different characteristics from an outer surface polymer layer. This is particularly advantageous in printed wiring board (PWB) manufacture requiring solder mask coating. Thus, a two layer composite polymer coating is provided. One inner adhesive photopolymer layer is applied to the PWB in the liquid state, displacing air from PWB surface. The outer layer of the composite therefore can be epoxy, dry film or liquid polymer. Dry film thus carried on a thin plastic sheet may be overlaminated onto a liquid inner layer already on the substrate, without the need for a vacuum laminator to eliminate bubbles or a plasticizing heat step to conform the dry film to the surface. The dry film solder mask so laminated is then exposed through a phototransparency to harden the light struck dry film solder mask and light struck inner layer photopolymer, thereby cojoining the dry film solder mask, inner layer and PWB surface. A solvent washout step removes unexposed dry film solder mask and unexposed inner photopolymer. This provides a faster process, requires less equipment, and improves adhesion to metal conductors. This embodiment gives a high resolution product with good solder mask qualities. The outer layer need not be photoimaged. Thus, pre-patterned epoxy solder masks may be printed on the carrier sheet, partially hardened and overlaminated in register onto liquid photopolymer-coated PWB, then exposed to light source through a phototransparency and through the pre-patterned solder mask, thereby permanently adhering the outer layer to PWB. The composite coating can be a combination of known solder mask materials, dry film, UV-curable liquid polymers and thermal-curing epoxy, chosen for desired characteristics including electrical performance, printing resolution and cost.