How to reduce electromagnetic interference by designing PCB mounting holes

Mounting holes in the PCB are an important element in electronic design. Every PCB designer will understand the purpose of the PCB mounting holes and the basic design. Moreover, when the mounting hole is connected to the ground, some unnecessary troubles after installation can be saved.

How to use PCB holes to reduce EMI?

As the name implies, the PCB mounting holes help to secure the PCB to the housing. However, this is its physical and mechanical use. In addition, in terms of electromagnetic function, PCB mounting holes can also be used to reduce electromagnetic interference (EMI). PCBs that are sensitive to EMI are usually placed in a metal enclosure. In order to effectively reduce EMI, the plated PCB mounting holes need to be connected to the ground. After this grounding shield, any electromagnetic interference will be directed from the metal casing to the ground.

The common question that a typical novice designer will ask is which ground do you connect to? In common electronic devices, there are signals, housing bases and grounding. As a rule of thumb, the mounting holes cannot be connected to the signal ground. The signal ground is the reference ground for the electronic components in your circuit design. It is not a good thing to introduce electromagnetic interference into it.

The enclosure you are connecting to is grounded to the enclosure. This is the meeting point for all ground connections of the cabinet. The chassis ground should be connected at a point, preferably via a star connection. This avoids causing ground loops and multiple ground connections. Multiple ground connections can cause a slight voltage difference and cause current to flow between the chassis grounds. Then connect the chassis ground to the ground for safety measures.

Why is it important to have a proper ground connection?

If the base of the PCB board is a metal case, then the entire metal case is the earth. The ground wire of the 220V power supply is connected to the earth. All the interfaces need to be connected to the earth, and the screws should be connected to the earth. In this way, the interference entering the EMC test is directly discharged from the earth to the earth, and it is guaranteed that it does not interfere with the internal system. In addition, EMC's protection devices must be available on every interface and close to the interface.

If it is a plastic case, it is best to have a metal plate embedded in it. If there is no way to achieve it, then you need to consider more in the layout of the wiring. Sensitive signals (clock, reset, crystal, etc.) need to be grounded, and the filter network (chip, crystal, power) is added.

It is best practice to connect the plated mounting holes to the chassis, but it is not the only best practice to follow. To ensure your equipment is protected, your chassis ground must be connected to the appropriate ground. For example, if you build an automatic parking payment machine that is not properly grounded, you may have customers complaining about an “electric shock” when making a payment. This can happen when the customer touches the uninsulated metal part of the housing.

A slight electric shock may also occur when the computer's power supply chassis is not properly grounded. This can also happen when the ground cable that connects the power outlet to the floor of the building is disconnected. This can result in floating grounding on the corresponding machine.

The principle of EMI shielding relies on proper ground connections. Having a floating ground connection not only exposes your customer to a minor electric shock, but if your equipment is shorted, it can compromise your safety. As shown in the figure below, proper grounding is important for safety and EMI shielding.

Basic tips for designing PCB mounting holes

PCB mounting holes are often used in design. There are a few simple basic principles when installing mounting holes. First of all, pay attention to the coordinates of the mounting holes. An error here will directly cause your PCB to not be properly installed in its enclosure. Also make sure that the mounting holes are the right size for the screw of your choice.

Generally, do not 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 disassembly. You should also leave enough clearance between the mounting holes and other parts.

Great circuit design software, such as Altium's sequence software, AlTIum Designer, allows precise placement of mounting holes and defines rules for the associated safety spacing.