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Do they serve any mechanical purpose? Also, there are eight dots on these rings.

enter image description here

mastermind
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Winner
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    Use your multimeter to check whether the ring is connected to GND of the circuit. If it is, then its purpose is connecting the circuit ground to the chassis ground when it's mounted in a metal enclosure to ensure both grounds are at the same potential (there are many reasons on why you want this, mainly for EMI/ESD considerations). – 比尔盖子 Oct 05 '20 at 16:59
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    It's a mounting hole AND possible grounding connection. Do what @比尔盖子 says. – StainlessSteelRat Oct 05 '20 at 17:01
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    Does this answer your question? [Mounting hole on a PCB](https://electronics.stackexchange.com/questions/137394/mounting-hole-on-a-pcb) – bobflux Oct 05 '20 at 20:53

5 Answers5

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This is a mounting hole for a screw. Smaller holes around - Vias are used to reinforce mounting point.

Copper ring around the hole together with plating increases stiffness of PCB. Vias around protect this ring from being peeled-off by screw head.

Another purpose of Vias might be to increase electrical or thermal conductivity between PCB layers and the screw.

Rusk Box
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    Do you have a reference to using vias to reinforce the mounting hole? I have seen such holes, but don't think I've ever used them, nor worked with someone who has. – TimWescott Oct 05 '20 at 16:54
  • The purpose of the vias is connecting the circuit ground to the chassis ground when the board is screwed down into an enclosure, isn't it? – 比尔盖子 Oct 05 '20 at 16:57
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    The stitching vias improve the connectivity to the other side or internal planes, especially in the case where the hole plating of the actual screw bore or its shoulder connection to surface copper might become damaged. They also mean the design would work in a fabrication process where the screw bore is too large to itself be a plated through hole. – Chris Stratton Oct 05 '20 at 16:59
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    Very hard to believe that a few mils of copper is doing what you say it is mechanically – Scott Seidman Oct 05 '20 at 18:42
  • Mechanically I believe it is not much of improvement using such holes. This is more final refinement. – Rusk Box Oct 05 '20 at 19:03
  • @RuskBox It is, in fact, a mechanical improvement. The through plating in the vias helps prevent the pad from delaminating when the screw is tightened. – Hearth Apr 08 '21 at 05:13
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This is a mounting hole where a screw can go through, to attach the circuit board to some kind of enclosure.

The screw may or may not be connected to the circuit's ground.

user253751
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Power supplies mostly have these kind of holes. because, some times you need your enclosure to be grounded. So, there's need to sold extra wire on board and body to ground the enclosure.

And via in the hole gives strength to the hole coper so that it may not be peeled-off by screw head.

mastermind
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I've done this before because I've seen it on other people's boards and thought it looked cool. 8 vias won't alter the cost at all.

You often stitch around the edges of ground planes with vias because it improves their RF properties. It stops overhanging ground plane from being a resonator. So you could have probably just had them in a square around the screw hole and it still would have helped.

Mj Beansauce
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The primary reasons that mounting hole has the vias through the pad is to connect the pad to:

  1. The pad on the opposite side of the PCB
  2. Internal copper planes of the PCB
  3. Connect to signal traces inside the PCB

These are used when the large diameter mounting hole itself is not a plated hole. The reason for that is that screw threads in the hole can shave off the plating leaving you with metal particles floating around your PCBA and electronics enclosure.

Michael Karas
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