EMC Question of the Week: March 18, 2019

When a circuit board with a plane layer devoted to "ground" is mounted on a metal surface or in a metal box, the board ground should always be connected to the exterior metal

1. at only one location away from the high-speed components
2. at only one location near the high-speed components 
3. at as many locations as possible
4. none of the above

Answer

The correct answer is "d." The key word in the problem statement was always. There are few (if any) EMC design guidelines that always apply. Whether and where connections should be made between a circuit board ground and a frame ground is highly dependent on the specific application. From an EMC standpoint, a good high-frequency connection between board ground and frame ground is often required. Nevertheless, in many applications there are safety requirements or other considerations that prohibit a direct connection between these grounds.

A couple of things to keep in mind when deciding if and where to make a connection to frame ground:

1. The frame usually makes half of a good "antenna." The board and things referenced to board ground usually make another good "antenna" half. If these are not bonded at high frequencies, it can be very difficult to keep circuits on the board from unintentionally driving these two antenna halves relative to each other resulting in radiated emissions.
2. If the product needs to meet BCI or transient immunity requirements, the injected currents usually must find their way to the system's frame ground. Good designers recognize this and provide a preferred path for those currents that keeps them away from areas where they can interfere with the device operation.

Usually, if a circuit board is going to be mounted near a large metal object, a good high-frequency bond should be established between the board's ground and the exterior metal. And usually, if the board has external wired connections, this bond needs be established near the connectors. But different applications can have very different requirements; and what is exactly the right thing to do in one situation may be exactly the wrong thing to do in another.

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