What exactly is grounded through motherboard standoffs?
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?
- Only the metal traces around the screw holes?
- 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.
- Is the case grounded to the motherboard?
- Or is the motherboard grounded to the case?
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.
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.