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I'm in the middle of designing a specialized communications device that uses an off-the-shelf radio module. This radio module is larger than desired and the PCB area that is not needed is easily cut away.

However, the module is built using a 10-layer PCB. Upon cutting away the undesired area, the internal conductor planes are visible and I need to make sure that the various layers do not have excess copper shards that can short to other layers.

I've used a fine-grit belt sander with the long cut edge of the PCB oriented in the direction of the sanding belt travel. In other words, the belt travel is such that it doesn't short across the various copper planes. This does clean the cut edge nicely but I still have copper smearing - the smeared copper from various internal layers is very close to the layers next to them.

I'm seeking advice on how to remove the unwanted copper from the edge of the cut PCB internal planes. My first thought was to use a high-speed down-cut router bit but, upon reflection, that will probably push the edges of the copper planes together and cause short circuits.

Another thought is to use a flat-bottomed vessel with a very shallow depth of PCB etchant, then stand the circuit board vertically with the edge of the cut PCB held just above the bottom of the vessel. I'd probably have to have some means of circulating the etchant so that the PCB edge is always exposed to fresh etchant.

Total quantity is quite small - perhaps 150 units or so.

Any thoughts?

[Edit]

The sections of PCB that I'm removing are away from the RF section. The area near the portion that I am removing contains power supply and accessory connector stuff.

One section is bare PCB with a number of test and factory-programming pads, the other section contains an accessory connector that isn't needed.

I've been very careful with my changes - remove a section, clean up the exposed layers at the PCB edge, test. Once I verified full functionality, I'd remove the next portion of circuit board and repeat the whole process.

I was fairly certain that the circuit board material did not contain important traces and my testing seems to confirm that. I have been able to remove all of the undesired board and the module appears to be completely functional.

The reason for doing the modifications in small steps was so that I would destroy at most only one module. As it is, I've lucked out and I appear to be able to remove all of the undesired circuit board and still have a fully functional module.

Now all I need to do is find an easy method of cleaning up the internal copper at the cut edges of the PCB.

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    \$\begingroup\$ If this is an RF module, then you might actually want those copper planes. The ground plane and it's size can be important to the RF performance, and the other areas could purposely be there for EMF/EMI shielding. If it's a module that was designed to be connected to a specific board and therefore has a specific shape that you don't need, then maybe the extra area is not needed. Have you tested a module with the areas cut away? Or not yet because of the possible shorting between layers? \$\endgroup\$ – DigitalNinja Jul 27 '15 at 23:32
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    \$\begingroup\$ I vote this: "Another thought is to use a flat-bottomed vessel with a very shallow depth of PCB etchant, then stand the circuit board vertically with the edge of the cut PCB held just above the bottom of the vessel. I'd probably have to have some means of circulating the etchant so that the PCB edge is always exposed to fresh etchant". You don't need to worry much about controlling etching speed, what you are removing is very thin and will etch out quickly. You can also make a mix of 1 part conc.HCl to 3 parts 3% H2O2 and apply it with a q-tip to the edge of the board. Wear gloves. \$\endgroup\$ – Oleg Mazurov Jul 28 '15 at 1:51
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    \$\begingroup\$ BTW, assuming you want to build somewhat reliable units (for some definition of reliable), all that "postprocessing" on that module could create long-term damages you cannot test for in production, e.g. PCB stresses that can develop into failures in the field after a couple of years, or more leakage due to chemicals residue that make their way into sensitive parts of the module. Did you take this into consideration? \$\endgroup\$ – Lorenzo Donati Jul 28 '15 at 1:58
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    \$\begingroup\$ Fine sand paper/emery cloth. Gently run the edge of the board against (moving in the direction of the length of the board) it and you should remove anything that could cause an issue. \$\endgroup\$ – Tom Carpenter Jul 28 '15 at 2:48
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    \$\begingroup\$ What's your budget? Laser ablation is very controllable for depth. For 150 units, it might be worth the NRE of programming a CNC laser for the repeatability that will be essential for reliability. \$\endgroup\$ – Neil_UK Sep 15 '15 at 16:42
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As copper is very malleable, grinding is probably not going to work very well. Sanding is better, however it can also leave copper fragments that could pose a problem for you. My best guess is some amount of etchant + sanding will be your best option.

I'd also seriously consider the total labor costs of your mods compared to making (or having someone else make it for you) your own pcb. Depending on the complexity & component costs, that may not be a feasible option--especially for only 150 units.

Best regards

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    \$\begingroup\$ particularly if he's talking about a 10 layer board with high frequency/RF. I know what I'd charge someone to do that wouldn't be terribly low. \$\endgroup\$ – akohlsmith Sep 21 '15 at 1:00
  • \$\begingroup\$ I understand, but perhaps the part that he is using isn't using that many layers. As we can't see it, we can't know for certain how complex the part of the device is that he's using. \$\endgroup\$ – zeffur Sep 21 '15 at 6:05
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the best way to cut PCB cleanly I've found is with a Dremel with a cutting wheel, a steady hand and very high revs. Not too sure about 10 layers however, copper traces being only a few microns I expect.

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