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I want to add mounting holes to a PCB. The PCB is not of my design so I can't change it. There appears to be a large ground plane in the locations that are good candidates for drilling the holes.

Is it okay to drill the holes through the ground plane?

Once drilled, the holes will be fitted with metal screws and standoffs.
The circuit on the PCB does not operate at high frequencies. The PCB has two layers.

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    \$\begingroup\$ Is this a multi-layer board? You might short together internal layers while drilling. \$\endgroup\$ – Hearth Apr 28 '17 at 3:16
  • \$\begingroup\$ two layers, updated question. \$\endgroup\$ – Jet Blue Apr 28 '17 at 3:18
  • \$\begingroup\$ Drilling through a copper plane on a two layer board is not a good idea as a production process. As Felthry says, the copper from the plane layer may get kind of bent up and short to the other layer. Also, the metal screws may end up making electrical contact with the plane layer, which could also lead to problems (shorting to chassis, for example). But if you only have a small number of PCB's, and you inspect and test the holes to make sure they did not create shorts, it might be OK. For any future production, I would DEFINITELY add the holes to the PCB design. \$\endgroup\$ – mkeith Apr 28 '17 at 3:46
  • \$\begingroup\$ I think you should definitely TRY it on one board. Then carefully inspect the hole and evaluate the process. \$\endgroup\$ – mkeith Apr 28 '17 at 4:25
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If it's a two layer board then you can at least see if there are any tracks there. You still have the possibility that the designer put power on one side and ground on the other though.

If it's a multilayer board then all bets are off, on multilayer boards tracks are very often hidden under planes.

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  • \$\begingroup\$ it's a two layer board. And I'm certain it's just a ground plane. \$\endgroup\$ – Jet Blue Apr 28 '17 at 3:19
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Use a counterbore drill to make the hole and cut out a circle of copper around it at the same time. I know it's a bit of an angry looking beast but it's much more repeatable and a lot safer than spinning knives!

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A good method I've used is take an exacto knife and start spinning it in one place. I don't know how large of a hole you can drill, but it sure is awesome to drill holes into the board and cut traces. It cuts traces clean.

You could probably cut through a multilayer board, but you have to ensure that the cut is clean and that might be hard to do. If you have one layer that is not sheared off right (most drill bits leave copper behind) you risk shorting two layers.

Or you might be able to drill a hold and then clean it up with an exacto knife, I've done a lot of work uncovering traces with exacto's knifes and surgical blades work really well for scraping FR4 flat.

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