# Why are some mounting holes connected to ground?

Why are plated mounting holes connected to ground ?

I read that if you are connecting your board to chassis ground, then it should be connected at only one point and connecting say 4 mounting holes to ground and then screwing them into the chassis seems to not be desired. Why ?

If you have 3 PCBs that fit into one large enclosure, what is the proper way to mount them ? Plated vs unplated ? Grounded ? Star configuration to chassis ?

Connecting a metal chassis to ground obviously helps with shielding. For an analog circuit where sensitivity to noise and ground currents must be avoided, such as those dealing with audio or video, you are correct that to connecting at just one point is best. But for many cases (excuse the pun), it really doesn't matter as much. Further, if it is a circuit that handles power, connection to ground at many points serves both as a safety mechanism, and offers the advantage of spreading out larger currents over more board traces or ground plane areas.

Your other question about proper physical mounting of multiple boards is mostly a design choice, and often more about convenience and fit then anything else, unless there are potential issues of EMI noise pickup between boards. But if you you are talking about multiple grounds, the same principals as the chassis discussion applies. If you are trying to distribute high current ground returns in multiple boards, you likely can connect each one to the chassis individually, at whatever point is convenient. But if there are many high sensitivity analog boards, you will likely do better to let only one main board make the chassis ground, and run grounds between boards separately and independently.

Finally, whether or not each ground needs to be run to a single point as opposed to nearest the circuit it connects to is something you'll get lots of opinions on, and no one choice is optimal in all cases.

How you ground things is application/design-dependent. In some designs the metal chassis might be the lowest impedance ground. Automotive is a great example of that. Using the chassis as ground saves a LOT of \$ and weight vs. running additional thick copper cables everywhere. And (for the most part) in a car, you don't mind if there is some noise on the ground. A little noise on the DC won't matter to the headlights, horns, starter motor, windshield wipers, etc.

One big automotive exception is car audio. A lot of you may have heard alternator whine on a car stereo at some point. That is because the pulsating rectified DC current from the alternator is causing the ground at different points (as the current flows through the chassis from the alternator back to the battery's ground) to have different voltages. Since a ground, even a chassis ground, has a non-zero impedance, current flowing through it (particularly the 10s of amps you see from an alternator), you will create a voltage (signal) across two different points of the ground. V = IR, where I = alternator current and R = resistance between two points that the alternator current is flowing through.

If one part of your audio system (maybe the receiver's ground lead) is tied to ground A, and another part (maybe the power amp or the receiver's chassis) is tied to ground B, then if there is alternator current between the two grounds, the audio system's power supply will have V(AB) (= alternator noise) across it at some point. And that signal often gets leaked into the audio you hear through various mechanisms. So for an audio system (particularly one in a car), a star ground is almost always the best choice.

A car is a good example of both grounding methodologies - you can ground anywhere on the chassis for a lot of the high-current stuff (alternator, starter motor, headlamps, etc.), but should only use one point for ground for the more sensitive circuits (audio, sensors, maybe microcontrollers).