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The below PCB belongs to a wireless access point.

Could anyone explain what are the golden-like part of the circuit that is indicated by the red circles(ish)? What material is it? What is the the functionality/use of it?

enter image description here

The left two circles have the same layout on the other side of PCB, and nothing is placed on top. The right circle is just under the wireless chipset.

P.S. I could not come up with a good title as I don't know the proper name or technique that is used on the PCB, please feel free to edit it.

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I am thinking that it might be kind of a EMC shield or something to provide a better temperature performance as the right circle is just behind the chipset, but then the right part does not make much sense to me as there is nothing on that part –  angs Aug 21 at 11:08
    
I've got this page displayed with an excellently fitting ad... –  leftaroundabout Aug 21 at 13:20

2 Answers 2

up vote 13 down vote accepted

The gold colour is just the surface finish of the PCB, called Gold Flashing.

The exposed areas could have a couple of uses:

  • As a grounding/earthing point, for example an RF screening gasket or can might make contact there.
  • Heat-sinking for a device. We have very similar footprints on boards for Allegro A4988 stepper motor driver chips and switch-mode power supply chips, but they can be anything that requires heat sinking.

There may be other reasons, those are the two I'm most familiar with.

Edit: Another reason is to do with RF performance or when RF devices such as filters or antennas are made purely using tracks on the PCB, in those cases the solder mask (and every other layer of the board) has an effect on the fabricated device, but I'm not an RF engineer so can't really expand on that other than being something I've seen in equipment we've designed.

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It looks like the exposed copper is gold plated. e.g. Wikipedia Electroless nickel immersion gold

That is a very common for SMD PCBs because it gives a lovely flat surface to solder SMD to.

Unlike other finishes, gold plating doesn't tarnish, or oxidise, so the PCB can be stored for months without it deteriorating.

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