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This gigabit Ethernet NIC has a checkerboard pattern out of copper etched on the PCB:

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Each square is electrically isolated. What's the point of adding these? I guess that the PCB isn't filled with a copper plane due to cost concerns, but why don't leave it empty then?

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That's called copper thieving. It helps balance the amount of copper on the outer layers which makes the etching process easier. Basically it helps them avoid over or under etching the board.

Usually I'll have a note on my board files that I send to the fab house like: Fab may add copper thieving at their discretion so long as it is at least 100 mil from any major feature. Which basically means theive all you want but don't get it near any of my signals.

Why not just fill the outer layers with copper pour then? It's not cost, etching a pcb is not an additive process. They start with a full sheet of copper and burn it away. Having copper pour all over the place close to my signals!!! That would cause impedance discontinuities everywhere.

Plus depending on how it's routed you don't want to unbalance your board, for example having plenty of copper pour on the bottom but little on the top. That will lead you into having your board cup and curl when it goes through the oven.

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    \$\begingroup\$ Thanks for an excellent answer! As per cost, I know how boards are etched. I heard that the removed copper (or used etching solution) can and will be recycled, especially for large-scale runs. \$\endgroup\$ – whitequark Oct 26 '12 at 13:39
  • \$\begingroup\$ Great point, I guess they can extract that copper out so it might affect cost as well. \$\endgroup\$ – Some Hardware Guy Oct 26 '12 at 13:42
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    \$\begingroup\$ This is incorrect. Copper is not just eteched away, at least in some processes. The ones I am familiar with start with a thin copper layer, which is built up with plating where traces are supposed to remain, then the remaining thin areas are etched away. It would be wasteful, expensive, and most importantly inaccurate to etch away thick copper. Imagine a board with 2 or 4 ounce finished copper thickness. \$\endgroup\$ – Olin Lathrop Oct 26 '12 at 14:07
  • \$\begingroup\$ True I didn't mention plating, I was just trying to point out that you don't start with a piece of bare (as in no copper anywhere) FR4 and then somehow add copper to that. \$\endgroup\$ – Some Hardware Guy Oct 26 '12 at 15:13
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    \$\begingroup\$ From what I understand, board houses used to discard used etchant (which contained lots of ionic copper) but that nowadays the value of the copper in used etchant is below the cost to reclaim it. The more copper is left on a board, the less will be available to reclaim. \$\endgroup\$ – supercat Oct 26 '12 at 21:03

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