Use PCB holes to reduce EMI

Mounting holes in a PCB are an important element in electronic design. Every PCB designer will go to understand the purpose of PCB mounting holes and the basic design. And, when the installation hole is connected with the ground, it can save some unnecessary troubles after installation.

How to use PCB holes to reduce EMI?

As the name suggests, the PCB mounting holes help secure the PCB to the case. But that’s its physical mechanical purpose, and in addition to its electromagnetic function, PCB mounting holes can also be used to reduce electromagnetic interference (EMI). EMI-sensitive PCBs are usually placed in metal enclosures. To effectively reduce EMI, plated PCB mounting holes need to be connected to ground. After such a ground shield, any electromagnetic interference will be directed from the metal case to the ground.

A common question the average novice designer asks is which ground do you connect it to exactly? In common electronic equipment, there are signals, case bases, and grounds. As a rule of thumb, you cannot connect the mounting hole to signal ground. Signal ground is the reference ground for electronic components in your circuit design, and it’s not a good thing to introduce electromagnetic interference into it.

What you’re connecting to is the case chassis ground. This is the meeting point for all ground connections of the cabinet. The chassis ground should be connected at one point, preferably via a star connection. This avoids ground loops and multiple ground connections. Multiple ground connections can cause slight voltage differences and cause current to flow between the chassis grounds. The chassis ground is then connected to earth for a safety measure.

Why is it important to have a proper ground connection?

If the shell base of the PCB board is a metal shell, then the entire metal shell is the earth, the ground wire of the 220V power supply is connected to the earth, all interfaces need to be connected to the earth, and the screws are also connected to the earth. In this way, the interference entered in the EMC test is directly discharged from the ground to the ground, so as to ensure that it does not interfere with the internal system. In addition, EMC protection devices must be available for each interface and must be close to the interface.

If it’s a plastic case, it’s best to have a metal plate embedded in it. If there is no way to achieve it, then you need to think more about the wiring layout. Sensitive signal (clock, reset, crystal oscillator, etc.) lines need to be processed with protection, and filter networks (chips, crystal oscillators, power supplies) need to be added.

Connecting plated mounting holes to chassis ground is a best practice, but not the only best practice to follow. To ensure your equipment is protected, your chassis ground must be connected to a proper ground. For example, if you build an automated parking payment machine that isn’t properly grounded, you may have customers complaining about being “electricized” while paying. This can happen when a customer touches a non-insulated metal part of the enclosure.

It is also possible to receive a minor electrical shock when the computer power supply’s chassis is not properly grounded. It can also happen when the ground cable that connects the electrical outlet to the building’s ground is disconnected. This can lead to a floating ground on the respective machine.

The principles of EMI shielding rely on a proper ground connection. Having a floating ground connection not only exposes your customers to a minor electrical shock, it can jeopardize your users if your equipment shorts out. Proper grounding is important for safety and EMI shielding as shown in the diagram below.

Basic Tips for Designing PCB Mounting Holes

PCB mounting holes are often used in design. There are a few simple basic principles when working with mounting hole installations. First, pay attention to the coordinates of the mounting holes. A mistake here will directly cause your PCB not to be installed correctly in its housing. Also make sure the mounting holes are the right size for the screws you choose.

Generally don’t place the mounting holes too far on the edge of the PCB. Too little dielectric material at the edges may cause cracks on the PCB during installation or removal. You should also leave enough clearance between the mounting holes and other parts.

Great circuit design software, such as Altium Designer, Altium’s serial software, can precisely place mounting holes and define the rules for associated safe spacing.