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My micro-ATX motherboard is mounted to brass standoffs inside an unpainted metal case.

What part of the motherboard is electrically bonded to the case through these standoffs?

  1. Only the metal traces around the screw holes?
  2. Or the whole ground-plane of the motherboard?

The ground-plane is the common ground for all circuit grounds on the motherboard, right? It's a copper layer inside the multilayer PCB, isn't it?

If the standoffs are bonded to the ground-plane, then i have another question.

  1. Is the case grounded to the motherboard?
  2. Or is the motherboard grounded to the case?
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    Take a multimeter and find it out? – PlasmaHH Oct 24 '16 at 19:37
  • The PSU-case is grounded to earth (by the earth-prong) and if the PSU-case is unpainted, then the whole ATX case is grounded to earth too. But i measured 0 ohm between the metal traces around the screw holes (on the motherboard) and the earth prong at the PSU. Even outside the ATX-case. This would indicate there are multiple paths from the ground-plane to the earth-prong. Wouldn't this cause unwanted ground-loops? –  Oct 24 '16 at 19:44
  • @Marty Yes, but it's a small price to pay for reliable grounding. It does tend to cause audio devices inside a computer case to suck though. An external USB audio device can be much quieter. – David Schwartz Oct 24 '16 at 19:46
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    @Marty, one thing to keep in mind about ground loops is the context of where they are. The ground loop voltages involved in a properly designed system are minuscule. For digital logic, such voltages make no difference. In analog audio, those voltages can be in a range that competes with the content and you can hear it as noise. PC designers are most concerned with reliable digital logic. The fact that you can piggyback audio onto that platform is secondary. So music mastering isn't performed on PCs, and audiophiles don't do their critical listening on their PC. – fixer1234 Oct 24 '16 at 20:43
  • Allright, thanks for the reply. Audio isn't my main concern. I am trying to figure out the ground paths in order to decide to what i will clip my anti-static wrist strap. Because often, newer cases and newer PSU-cases are painted and thus non-conductive. –  Oct 24 '16 at 21:02
  • *" to what i will clip my anti-static wrist strap"* -- That's a completely different question. And the answer is not your PC, but preferably a ground point of your home's electrical system, e.g. at the wall outlet. BTW there can be more than one ground in a piece of equipment, e.g. signal ground, chassis ground. – sawdust Oct 24 '16 at 22:48
  • The standoffs play a huge role for heat dissipation too by conducting some of the heat from the motherboard into the casing, which is a giant heat sink. Now that computers are more and more efficient heat is much less of an issue, and maybe for noise isolation nylon standoffs would make more sense. – Drunken Code Monkey Oct 28 '16 at 04:20
  • I think it grounds (the motherboard to the case) somewhat. Now that I fastened them again _with washers_ to maximize the contact to the solder, the buzz from my USB audio interface is gone. – Константин Ван Apr 10 '21 at 13:41

2 Answers2

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Nothing really. The case is grounded through the power supply's mounting screws. The motherboard is grounded through all the ground lines between the power supply and the motherboard. The standoffs probably do make some contact, but it's not particularly important.

David Schwartz
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  • Yes, 7 ground wires on the 20-pins power connector. These are paths from the ground-plane to the PSU. But if the stand-offs are also bonded to the ground-plane, then you are creating ground-loops, right? Clearly this is the way to go since it's done in almost every pc. But how exactly and why? My assumption would be that the ATX-case is grounded to the ground-plane, which is ground to earth through the ground wires from the motherboard to the PSU. –  Oct 24 '16 at 19:55
  • All of what you said is correct. There are ground loops, but so what? – David Schwartz Oct 24 '16 at 19:59
  • Ground loops can be nasty and should be avoided. That's what i learned. –  Oct 24 '16 at 20:00
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    The designers of PCs disagree with you. They think reliable grounds are more important and if that means that PC audio is a bit noisy, they can live with that. – David Schwartz Oct 24 '16 at 20:01
  • Are you saying there are paths from the ground-plane to the case (through the standoffs) to the PSU-case to earth ánd there are paths from the ground-plane to the PSU-internals (through the 20-pins connector) to earth? –  Oct 24 '16 at 20:04
  • Yes. That's correct. – David Schwartz Oct 24 '16 at 20:13
  • Thank you for your reply. So, case, PSU-case, ground-plane and earth-prong are at the same potential? One little semi-off-topic question. I can understand why there are multiple standoffs. But why would there be 7 ground wires on the 20-pins power connector? Why not 1 ground wire from the ground-plane to the PSU? –  Oct 24 '16 at 21:06
  • There are 7 ground wires because of the amount of current that needs to be conducted. The power connector pins are only rated for about 6A each. A single x16 PCIe graphics card can draw 75W through the motherboard connector, which is over 6A at 12V just for that single card. – David Schwartz Oct 24 '16 at 21:09
  • Yes, that's what i mean. Why not just one thick wire up to 42A? –  Oct 24 '16 at 21:11
  • What kind of connector would you use? I can't see how you could make it convenient. And then what about the 12V, 5V, and 3.3V supplies? Would you have different connectors of different sizes for each voltage? It's much simpler to make all the connectors the same and just use as many of each as justified for the expected current. – David Schwartz Oct 24 '16 at 21:13
  • You're probably right. To go back a little bit: case, standoffs, ground-plane and earth prong are always at the same potential? There are no resistors or coils in between? –  Oct 24 '16 at 21:23
  • Yes. There are no resistors or coils intentionally. PCs typically connect all those things together as directly as they can. This is a design choice that has some tradeoffs -- for example, if the electrical ground is faulty, the case becomes a shock hazard. – David Schwartz Oct 24 '16 at 21:52
  • note, you cannot get a 'ground loop' if all points connect to the same ground, i.e., the single mains plug. I blame US terminology for the total confusion of the simple word 'ground'. The word you want is 'earth' not ground. US uses both interchangeably yet they are not the same thing. – Tetsujin Oct 25 '16 at 11:59
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    @Tetsujin You can in the sense that you can get currents flowing in a loop through the various ground connections inside the case forming an antenna that picks up the RF generated by the various components. This is one common use of the term "ground loop". – David Schwartz Oct 25 '16 at 16:18
  • I'd call that a shield breakdown - but I'm an audio engineer, not an electronics specialist ;) – Tetsujin Oct 26 '16 at 06:51
  • Wouldn't the screws that connect motherboard directly to the case have an effect at high frequencies we're regularly seeing inside of a computer? I mean, even at 100 MHz, the wire-ground from the ATX connector will have large impedance due to cable's own inductivity. – AndrejaKo Oct 27 '16 at 11:38
  • @AndrejaKo That would just make them a less effective ground, which is find since they're not being relied on. – David Schwartz Oct 27 '16 at 15:03
  • You can just use nylon standoffs, really. The casing is not needed for proper grounding, it is already provided by the ATX connector. Connecting the ground board to the casing helps only with heat dissipation, and in fact can be detrimental to stability, since the casing also acts as an antenna, and injects noise into the power supply, independent of your 500$ 1000W's super duper regulation and noise figures. – Drunken Code Monkey Oct 28 '16 at 04:26
  • If the casing is connected to the power supply's ground, but NOT the board's ground, it will act as a Faraday cage, and considerably reduce outside interference coupling into the audio or analog video. If you connect the board to the casing, it acts as an antenna instead, and the current loop from the potential differential between the PS ground and the casing ground acts as an amplifier, and all that extra noise gets coupled into board power endpoints. This is the reason why the shield on a cable is only supposed to be connected at one end. – Drunken Code Monkey Oct 28 '16 at 23:24
  • An even better material for standoffs would be ceramic. And apparently they make them! http://www.newark.com/standoffs/standoff-material/ceramic/pg/811316990 . Ceramic is a good material because it is thermally conductive, but not electrically conductive. Keeps the grounds isolated but allows for thermal transfer from the board to the case. – Drunken Code Monkey Oct 28 '16 at 23:44
  • @Marty My understanding is that the rings are primarily intended to prevent the screw from gouging and crushing the motherboard material. – David Schwartz Nov 01 '16 at 20:08
  • You would need to check if the plated mounting holes are connected to the board ground, I haven't checked on mine. I think they are, and if they are it is only for heat dissipation, since it is not a good thing electrically for noise figures. – Drunken Code Monkey Nov 01 '16 at 20:48
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I have been an Electrics technician for over 40 years. Yes, there are ground loops, but they are miniscule. Modern motherboards come with insulating (usually red) fiber or paper washers for the screws that mount the motherboard to the stand-offs to eliminate those ground loops. The majority, practically all, of the grounding is done through the cable from the main power supply cable between motherboard and PSU. The PSU is grounded through the AC power cord to the ground lug into your Home/Office AC wiring to a copper ground rod outside. Measure the resistance from the shield around the Motherboard interface jacks in the back to the ground lug on the power cable.

Mank the old squid
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