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I need some help figuring out how to manage the mounting holes in my PCB. Searching the internet you will find a lot of information, but it is all confusing and often contradictory. I state that it is a low frequency board (MCU + some SPI and I2C) The first question is of course, how to ground them? The most popular answers are:

  • DO NOT ground them
  • Ground them all
  • Connect only one to ground and leave the others isolated
  • Connect them together but do not connect them to the ground plane
  • Ground them with a capacitor to ground

But I can't figure out what solution I need to apply. Can anyone help me?

My idea is to connect all the fixing holes to each other through a trace along the edge, as in this figure, then in the 4 corners put 4x 0603 land pattern on which, if necessary, connect a 0 ohm resistor to connect or not all the grounding holes, see figure. It could be a good idea? Or is it better to connect all holes directly to the ground id plane?

enter image description here

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    \$\begingroup\$ You are leaving out a lot of information. For example, how do you mount your PCB to something, and where, is it a metal box or plastic? Does your board have a mains power supply? Or where does it get power? Does your board have connections to external devices? Are the other devices grounded? Do you want to avoid ground loops or are they preferred? Do you have to pass EMI/EMC testing or not? \$\endgroup\$
    – Justme
    Mar 14, 2023 at 9:34
  • \$\begingroup\$ The box is metallic (1U rack unit): The power supply is 24VDC, then on the board there is a module DCDC regulator (RPMB3.3-2.0). The external connections are some serial RS485/RS422. Do you want to avoid ground loops or are they preferred? I couldn't answer this question, what is recommended for a low frequency digital bard? YES I have to pass the EMC tests. \$\endgroup\$ Mar 14, 2023 at 9:41
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    \$\begingroup\$ Yeah but that still does not tell anything about the power supply, does it have ground referenced output or floating. RS-485 and RS-422 connections have grounds or not? How did you connect them? \$\endgroup\$
    – Justme
    Mar 14, 2023 at 9:48
  • \$\begingroup\$ Input is just 2 wire, positive +24VDC e negative 0VDC. RS485 has only the 2 wire for data (no ground), RS422 have also the logic ground pin. \$\endgroup\$ Mar 14, 2023 at 9:56
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    \$\begingroup\$ Two ancillary notes: 1. Most PCB fabs will not run copper to the edge of the board; they usually require a small distance of nothing (bare) at the edge. And 2. The small vias around the mounting hole are for extra strength. Could fill them with solder for even more strength. To pass EMI, you'll have to know many details of the power supply - how it is grounded, how it is referenced/isolated, how much noise it makes, in what frequencies. It's not good enough to say "my device is 24V-powered"; they will test it under that power supply. \$\endgroup\$
    – rdtsc
    Mar 14, 2023 at 11:27

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It depends, but I'd be inclined to suggest if you're in doubt leave them unconnected. The question is what do you need them to connect to? There are a couple different points I can think of off the top of my head-

If you're not expecting to use conductive mounting hardware, then it doesn't matter at all.

If you have a chassis/EMI GND/Earth connection, and your board is inside a metal enclosure, and the mounting hardware is conductive - then you can connect to the Earth net. That can work well for ensuring a low resistance/inductance path to the Earth. If you're using this on the board then this is a good option

It's perfectly legit to use mounting hardware to connect ground to the board, particularly if you've got a stack. You might actually send 0V up one, and Vcc up another.
I actually almost always add a 0V and Vcc connection using mounting hardware to my designs where convenient, but this just gives me a testing option, when I use Nylon standoffs they don't play a part

If you're supplying power using a connector, then actually adding an alternative path for ground might not be a good idea. This is a ground loop, but also, from an EMC perspective, you create the possibility that the ground path will follow a different path if the connection through the mounting hardware has different R/L/C properties.

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