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I was adding a ground plane to a PCB layout today, and my curiosity peaked. Virtually every EDA package uses the term "Copper Pour." Why is it called a copper pour? It doesn't really make sense considering the copper is etched away. Is it because the pour fills in all areas that aren't part of the net, or is there some historical reason?

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I suspect it's because it's as if you poured copper and it filled the empty space on the board like a liquid. See the paint bucket tool in many graphics programs.

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I'm just guessing here, but in the past, it may have actually have been done by using a liquid paint of some sort.

In the good old days, PCBs were laid out by hand, using paper tape on a transparent overlay. The layout sheet was then photographically reduced, and then used as the photo-mask when etching the copper in the PCB construction process.

I can fairly easily imagine that the way large filled areas were constructed was by creating an outline using trace-tape, and then using a liquid paint (or something like white-out) to fill in the area within the outline. I have no trouble imagining someone calling that a "pour".

I am just hypothesizing here. I actually don't know how filled areas were done in the pre-computer layout era.

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