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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Grounding plated mounting holes in a PCB often serves dual purposes: enhancing shielding and providing a safety mechanism. For analog circuits, especially those sensitive to noise (e.g., audio or video), grounding at a single point minimizes ground currents that could interfere with signal integrity. In contrast, high-power circuits benefit from grounding at multiple points to distribute larger currents, reducing voltage drops and ensuring safety.
When dealing with multiple PCBs, grounding each board individually to the chassis may be beneficial for high-current applications. However, for sensitive analog designs, a single main board should connect to the chassis ground, with other boards sharing a separate ground path to minimize noise pickup.
Ultimately, the optimal strategy depends on the circuit’s requirements. High-frequency circuits may prioritize minimizing impedance paths, while low-frequency sensitive designs may focus on avoiding ground loops. The key is to balance noise suppression, safety, and practical considerations for the specific application.
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Grounding strategies are highly application-dependent. In automotive systems, for example, the metal chassis often serves as the lowest impedance ground. This approach saves cost and weight by avoiding the need for additional copper cables. For high-current components like headlights, starter motors, and windshield wipers, some ground noise is generally acceptable.
However, sensitive circuits like car audio systems require a different approach. Alternator whine is a common example of ground noise caused by pulsating DC currents from the alternator. These currents create voltage differences across the chassis ground due to its non-zero impedance (V = IR). When audio system components are grounded at different chassis points, this noise can couple into the audio signals, degrading sound quality.
To avoid this, sensitive systems often use a star grounding scheme, where all grounds connect to a single point, minimizing noise coupling. In summary, automotive designs highlight the dual grounding methodologies: general-purpose grounding anywhere on the chassis for high-current devices and single-point grounding for noise-sensitive circuits like audio systems or sensors.
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