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PCB Tech

PCB Tech

The best PCB design guidelines to reduce EMI
2021-10-03
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Author:Downs

All electronic circuit boards are designed to allow or even enhance the flow of electrons to achieve certain performance goals. This action—the current passing through a closed path—produces a magnetic field that is projected outward and flows perpendicular to the current. When there are nearby electronic components or signal paths in the field, electromagnetic interference (EMI) occurs. For many PCBA designs, especially high-speed circuit boards, controlling the amount of EMI is the primary consideration that must be adequately managed. For circuit boards with heat sink classification components, the common method is to implement EMI filter design. Although filters are effective, as a circuit board designer, knowing other PCB design guidelines for reducing EMI is that you may have to often Tools used.

EMC and EMI: What is the difference?

Most PCBAs are not the only electronic or electrical equipment in the product. Therefore, before we delve into single-board EMI issues, it is helpful to have a macro or system-level understanding of EMI issues. Just as electromagnetic energy is emitted from a single component, conductor or trace, it will also radiate into the environment from the circuit board itself; if you haven't had it before, place the Gauss meter near the PCB and you will get a reading. When multiple boards are close together, it becomes important to achieve electromagnetic compatibility or EMC.

pcb board

EMC can be thought of as achieving acceptable harmony or balance between electromagnetic components so that the amount of interference is minimal or at least sufficiently low that it does not significantly hinder normal operation. Unfortunately, eliminating all EMI is not yet possible; however, obtaining EMC is. EMI is actually any interference from electromagnetic sources, usually refers to the interference on a single PCBA. This classification is sufficient to investigate the problem, because minimizing the EMI on and from the circuit board contributes to the EMC of the operating environment of the circuit board.

Where does PCB EMI come from?

Electromagnetism spans an infinite frequency range and is almost everywhere. Moreover, as shown in the figure below, it is produced by many tools, equipment and products that we use every day.

Electromagnetic spectrum

As long as there is current, there is a possibility of EMI. For PCBA, the source of EMI can be divided into one of the following categories:

Element

Electronic components and components—especially high-power devices such as processors, FPGAs, amplifiers, transmitters, and antennas—can have a significant impact on EMI. In addition, switch components can produce destructive interference.

Signal and trace

EMI can also be generated along traces or at pin and connector points. For example, unbalanced differential pair routing may cause signal attenuation and reflections along the transmission path, which may severely affect signal integrity or the ability to accurately identify signals, leading to erroneous circuit behavior. In addition, due to stray capacitance, unwanted coupling may be formed between the signal path and the ground plane.

External source

If the circuit board is too close to the radiation source (maybe another circuit board or component), EMI may be introduced into your PCBA. Vibration or movement of other equipment or equipment in the circuit board environment may also produce harmonics.

Obviously, eliminating all potential sources of EMI is a difficult task. Fortunately, PCB design guidelines to reduce EMI can be developed to help minimize noise and achieve EMC.

The best PCB design guidelines to reduce EMI

Understanding the sources of EMI that may affect your board is critical to developing strategies to mitigate this persistent threat to PCBA performance. In addition, looking at EMI from the point of view of the source, where the minimization method is aimed at a specific source, it can be a good position to design a set of PCB design guidelines to reduce EMI.

Reduce component EMI

As mentioned earlier, components may be the main source of EM radiation, which will not only affect the on-board operation, but also damage the external PCBA and electronic circuits. Therefore, defining actions to mitigate its negative effects (listed below) is essential for good EMI reduction guidelines.

How to reduce the EMI of components

Choose low-power components as much as possible

One of the largest EMI generators on a circuit board is a component that requires a lot of power. With the push to reduce power consumption, alternatives can often be found that do not sacrifice functionality or quality.

Isolate different types of components

A good design practice is to always put together components that handle the same type of signal. For example, digital components should be close to other digital components and isolated from analog equipment.

Using PCB fence

Another tool to reduce EMI is to enclose components or sub-circuits in fences; such as PCB guard rings and Faraday cages. These can also effectively reduce radiation to the environment around the circuit board.

Adopt heat dissipation technology

For electronic components, energy generates heat. Therefore, efficient heat sinks and through holes can greatly help reduce EMI.

In addition to reducing the EMI of the components, the way the traces operate will also greatly affect the EMI of the circuit board.

PCB layout design for minimizing EMI

One of the most important considerations when laying out a circuit board is pitch. This includes ensuring that the gaps and creepage distances between conductive components are adequate.

Maintaining sufficient clearance is essential to minimize EMI

How to reduce EMI from signals and planes

Leave enough gap between signal traces

The most important factor in reducing EMI between traces is the spacing or gap. Follow your CM’s recommendations, which should be based on IPC standards.

Ensure that the decoupling and bypass capacitors are grounded

Stray capacitance is difficult to avoid; however, its effect can be mitigated by placing the capacitor as close as possible to the pin to ground.

Use good EMI filtering

Most designs, especially where digital signals are used, include switching devices that produce signal distortion. In these cases, the best way to improve signal fidelity is filtering.

Minimize the length of the return path

The ground loop should be as short as possible.

Ensure that the differential traces are the same

For differential signal paths, the trace pairs must mirror each other. This includes trace length, copper weight, and constant spacing. If necessary, zigzags should be used to maintain length and spacing.

Avoid sharp angles

When wiring, please use rounded edges instead of sharp corners, which may cause reflections due to characteristic impedance modification.

Do not place conductive layers next to each other

You should never place two conductive layers side by side in a PCB stack. It is best to separate them by the ground plane.

Separate the ground plane carefully

It is best to use separate grounds for different signal types. However, if you do use a split ground plane, make sure to use a single point to combine the ground.

 PCB layout (including its stackup) is very important to promote good signal integrity and reduce EMI. However, if external EMI is not addressed, any set of PCB design guidelines to reduce EMI is incomplete.

Avoid external EMI

Minimizing external EMI is very important to the signal integrity and circuit operation on the circuit board and the EMC of the PCBA installation environment. Actions that can be taken include the following.

How to reduce EMI from external sources

Use shield

Generally, shielding is applied to specific components or sub-circuits. They differ from fences in that they are usually made of insulating material and placed on top of the parts or completely surround them.

Use the shell

The enclosure is usually regarded as a safety device. However, the enclosure can also effectively protect the circuit board from debris and EMI from external sources.

All the PCB design guidelines discussed above for components, layout, and external sources can effectively minimize EMI on the circuit board and contribute to the EMC of the circuit board operating environment. However, whether these are necessary depends on your design, its functionality, and performance goals. Therefore, you should strive to optimize your design to reduce EMI, preferably using analysis tools (such as Cadence's PSpice).