Why are plated mounting holes typically grounded on a PCB? I’ve learned that connecting the board to the chassis ground at a single point is recommended. However, is it not recommended to ground four mounting holes and then screw them into the chassis? What is the reason behind this recommendation?
The answer is it depends. I just looked at 50 different answers or so and there’s clear differences in how engineers approach this. Worried about ground loops? No. Don’t tie them all to ground. Worried about safety? Yes, ground them all. Have a plastic case? No, ground one. Have a metal case? Yes, improves EMI. Etc., etc.
This has long been a subject of debate, and yes, there are some solid reasons for different opinions. I come from an RF and Microwave background and I ground everything. My rule of thumb is “more metal”. My experience in low-level analog signals also supports this. There are always new components, new architectures that affect established procedures.
The answer to almost all of your questions is “It depends”.
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As Atar says it often depends (Standard EMC answer) but more connection points is often better for this reason. If you’re in a conductive enclosure your circuit (PCB and wires) will couple on to the enclosure. This will create currents in the enclosure metal that will return to their source via the path of lowest impedance. If you only have one return point and the current must take a long path across the enclosure this increases the inductance. This causes multiple issues.
These impedances create resonances in the return paths that cause ringing that can couple to other circuits.
Other similar currents will also have this increased inductance and the fields will couple to each other creating cross talk.
The increased impedance will create larger voltage drops increasing noise voltage in the system.
These voltage drops can couple to cables going out of the enclosure and result in increased emissions or paths for outside fields to couple in causing immunity issues.
This is why you see large PC boards not only grounding on the corners but at several points along the edge and sometimes in the middle. In some instances, you can have “Ground Loops” but in general this is not a problem and if in certain cases it is the best approach is to manage it. See Eric Bogatin’s article on multiple grounds.
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