Examining the Reasons behind the Rising Popularity of Printed Circuit Boards Eventually the processes of developing printed circuit boards were developed that would plate copper onto the walls of the drilled holes. That allowed circuits 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 PCB Prototype Manufacturer. 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.
Much has changed since then. With the advent of plating processes that allowed hole walls to be plated came the first double sided boards. Surface mount pad technology, something we associate with the 1980's was actually being explored twenty years earlier in the 60's. Solder masks were being applied as early as 1950 to help reduce the corrosion that was occurring to traces and components. Epoxy compounds were spread over the surface of the assembled boards similar to what we know now as conformal coating. Eventually the inks were being screen printed onto the panels before assembling the boards. Areas that were meant to be soldered were blocked out on the screens. It helped keep the boards clean, reduce corrosion and oxidation but the tin/lead coating used to coat the traces would melt during the soldering process causing the mask to flake off Rigid flex PCB.
Why is Solder Mask Green for PCB's?
Ever wonder why the solder mask on almost every circuit board made is green? Well there seems to be any number of unsubstantiated reasons. Everything from the involvement of the US government in developing and using the first circuit boards to the how easy it was on your eyes during the long hours of hand assembling the first boards.
Here's the most believable explanation I've heard. The original masks used a base resin that was a "brownish yellow" and a hardener that was a deeper muddy brown. When they were mixed together they created a honey brown color that apparently not very appetizing. They tried adding red pigments but it became a rusty adobe color and using blue simply made it a darker brown. None of them were very appealing colors. Since the laminate materials at the time had a green hue they tried adding more yellow and some blue and ended up with an acceptable green color. It became the standard color we are still using today.
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