PCB - The Evolution of the PCB Process
Eventually processes were developed that would plate copper onto the walls of the drilled holes. That allowed PCB on both sides of the board to be connected electrically. Copper had replaced brass as the metal of choice because of its ability to carry electrical current, relatively low cost and ease of manufacturing.     

          
In 1956 the US Patent Office issued a patent for the "Process of Assembling Electrical Circuits" that was sought by a small group of scientists represented by the US Army. The patented process involved using a base material like melamine to which a layer of copper foil had been securely laminated. A drawing was made of the wiring pattern and then photographed onto a zinc plate. The plate was used to create a printing plate for an offset printing press. An acid resistant ink was printed onto the copper foil side of the board that was etched to remove the exposed copper leaving the "printed wire" behind. 

Other methods like using stencils, screening, hand printing and rubber stamping were also proposed to deposit the ink pattern. Holes were then punched in patterns using dies to match the position of the component wire leads or terminals. The leads were inserted through the non-plated holes in the laminate material and then the card was dipped or floated on a bath of molten solder. The solder would coat the traces as well as connecting the leads of the components to the traces.