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I have seen so many PCB which has an un-masked (without solder mask), copper boundary across RF sections. The boundary has via-stitching and there are also few openings in the boundary. Can someone explain their purpose? Please have a look at the picture below:

enter image description here

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You meant Via Stitching.

In RF application, you have a lot of current flows happening that behave quite differently than in non-RF application, and can take some really weird ways.

This is to reduce this effect.

The via stitching is there to reduce high frequency current flow going though the internal layers, don't forget a PCB is a dielectric, but still conduct somewhat electricity, or charges, especially in high frequency application.

The traces are uncovered for the same reason, avoiding having current flow flowing above the solder mask and messing with the rest of the circuit.

Probably on top of these ring, a metallic housing might be soldered, acting as a Faraday cage and avoiding having EMI being emited by the circuit.

Like this: cage


This doesn't apply to your question, but it is an interesting addition:

These types of uncovered rings are also sometimes used in acquisition when the signal carries nA level currents.

Called Guard trace, they usually feed a follower op-amp, and the ring is placed around the signal trace and connected to the output of the op-amp, bringing the ring to the same voltage as the trace and thus avoiding leakage current. The cutout are for the same reason

guard trace

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  • \$\begingroup\$ Does current flow in solder mask? \$\endgroup\$ – abhiarora Oct 17 '18 at 16:32
  • \$\begingroup\$ You don't want solder to flow in solder mask. But electrons will take all possible paths, including thru solder mask at 1 or 3 or 20,000 electrons per second. \$\endgroup\$ – analogsystemsrf Oct 17 '18 at 16:53
  • \$\begingroup\$ @abhiarora yes it does, very tiny amount though and also depends of the frequency. Current flows in any surface, liquid, gas, just some are better than the other at conducting or resisting current flow. \$\endgroup\$ – Damien Dec 10 '18 at 3:36

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