PCB Design Guidelines for EMI Reduction and EMC Optimization | Cadence

August 25, 2021

Cadence PCB Solutions

Key Takeaways

Understanding the difference between EMI and EMC.
Use low-power components, isolation techniques, PCB fencing, and heat management to reduce EMI sources.
Leverage OrCAD X tools like constraint management, signal integrity analysis, and real-time DRC updates to create EMI-optimized designs.

Composition of electromagnetic interference

All electronic circuit boards are intended to allow and even accentuate the flow of electrons to accomplish some performance objective. This action—the flow of current through a closed path—creates a magnetic field that projects outward and perpendicular to the flow of current. When there are nearby electronic elements or signal paths within this field, electromagnetic interference (EMI) occurs.

For many PCBA designs, especially high-speed boards, controlling the amount of EMI is a primary consideration that must be adequately managed. For boards with radiator classified components, a common approach is to implement an EMI filter design. Although filters are effective, as a circuit board designer, knowing additional PCB design guidelines for reduced EMI is a tool you will likely have to utilize often.

EMC and EMI: What Is the Difference?

Most PCBAs are not the only electronic or electrical devices within a product. Therefore, before we drill down into single-board EMI concerns, it is helpful to have a macro or system-level understanding of the EMI issue.

Just as electromagnetic energy emanates from a single component, conductor, or trace, it also radiates from the board itself into the environment; if you haven’t before, place a gauss meter close to a PCB and you will get a reading. When multiple boards are in close proximity, it becomes important to achieve electromagnetic compatibility or EMC.

EMC ensures minimal electromagnetic interference (EMI) between elements to maintain normal operation. While eliminating EMI entirely isn’t possible, achieving EMC is. EMI typically refers to interference on a single PCBA, and reducing it contributes to the broader EMC of the board's environment.

EMI Sources Classified

Source	Description
Components	Electronic components, especially high-power devices such as processors, FPGAs, amplifiers, transmitters, antennas, and others, may contribute significantly to EMI. Switching components also generate interference that can be disruptive.
Signals and Traces	EMI can be created along traces or at pin and connector points. Unbalanced differential pair routing may cause signal attenuation and reflections, affecting signal integrity or causing erroneous circuit behavior. Additionally, stray capacitance may lead to unwanted coupling between signal paths and ground planes.
External Sources	EMI can be introduced if the board is too close to a radiating source, such as another board or element. Harmonics may also be generated by vibrations or movement in the board’s environment from other equipment or devices.

Electromagnetism spans an infinite range of frequencies and can be found virtually everywhere. And, as indicated by the figure below, it is generated by many of the tools, appliances, and products that we use daily.

The electromagnetic spectrum

PCB Design Guidelines for Reducing EMI From Components

As discussed previously, components can be a major source of EM radiation that can not only affect onboard operation, but also disrupt external PCBAs and electronic circuitry. Therefore, defining actions to mitigate their negative impact, as listed below, is essential for good EMI reduction guidelines.

How to Reduce EMI From Components

Practice	Description
Select low power consumption parts	Choose parts with low power consumption to reduce EMI. High-power components often generate more EMI, and low-power alternatives typically maintain functionality.
Isolate different types of components	Group components processing the same types of signals together. Keep digital components near each other and separate from analog devices.
Utilize PCB fencing	Use PCB guard rings, Faraday cages, or similar fencing to enclose components or sub-circuits, reducing EMI and preventing radiation from affecting the surroundings.
Employ heat dissipation techniques	Use heat sinks and vias efficiently to manage heat generated by components, which helps minimize EMI.

PCB Layout Design for EMI Minimization

One of the most important considerations when laying out your board is spacing. That includes ensuring clearance and creepage distances between conducting elements are adequate.

Maintaining adequate clearances is critical to minimize EMI

The order and distance between conducting and ground planes are also important for multilayer boards, as indicated in the list below.

How to Reduce EMI From Signals and Planes

Practice	Description
Employ adequate clearance between signal traces	The most important factor for reducing EMI between traces is spacing or clearance. Follow your CM’s recommendations, which should be based on IPC standards.
Make sure to ground decoupling and bypass capacitors	Stray capacitance is hard to avoid; however, its effects can be mitigated by grounding capacitors as close to the pin as possible.
Employ good EMI filtering	Most designs, especially where digital signals are used, include switching devices that can create signal distortion. The best way to improve signal fidelity in these cases is by filtering.
Minimize the length of return paths	Ground returns should be as short as possible.
Make sure differential traces are identical	For differential signal paths, it is essential that trace pairs mirror each other. This includes trace lengths, copper weights, and a constant separation. If necessary, meandering should be used to maintain length and separation.
Avoid sharp angles	When routing traces, use rounded edges instead of sharp corners, which may cause reflections due to characteristic impedance modification.
Do not place conducting layers next to each other	You should never place two conducting layers next to each other in PCB stackup. It is best to separate these by a ground plane.
Be cautious with split ground planes	It is best to use separate grounds for different signal types. However, if you do use a split ground plane, make sure that a single point is used to combine the grounds.

Your PCB layout, including its stackup, is important for facilitating good signal integrity and reducing EMI. However, no set of PCB design guidelines for reduced EMI would be complete without addressing external EMI.

Avoiding External EMI

Minimizing external EMI is important for signal integrity and circuit operation on your board and contributing to EMC for the PCBA’s installation environment. Actions that can be taken include the following.

How to Reduce EMI From External Sources

Use shielding: Typically, shielding is applied to specific components or sub-circuits. These differ from fencing in that they are often made of insulating materials and are placed over the top of a part or completely enclose them.
Use enclosures: Enclosures are often viewed as safety devices. However, enclosures are also effective to protect boards from debris and EMI from external sources.

All of the PCB design guidelines discussed above for components, layout, and external sources are effective for minimizing EMI on your board and contributing to EMC for your board’s operational environment. However, implementing these guidelines efficiently often requires leveraging specialized design tools. OrCAD X offers a comprehensive suite of tools tailored for PCB design, enabling you to enforce good practices while achieving EMI and EMC mitigation.

Using OrCAD X to Create an EMI-Optimized PCB Design

OrCAD X provides functionality to analyze, validate, and optimize your PCB layout at every stage of the design process. From managing constraints and routing traces to running simulations for signal integrity and manufacturability, the platform equips you with the necessary features to address potential EMI and EMC issues. By utilizing the integrated tools and workflows within OrCAD X, alongside source-specific PCB design guidelines, you can create robust designs optimized for electromagnetic compatibility in real-world environments.

Tool	Functionality	How It Aids EMI/EMC Mitigation
Constraint Manager	Defines and manages electrical, physical, and spacing constraints.	Ensures adequate trace clearances, proper grounding, and minimized signal coupling to reduce EMI.
Routing Engine	Real-time feedback and constraint-driven routing tools.	Helps enforce controlled trace lengths, avoid sharp angles, and maintain differential pair integrity.
Cross-Section Editor	Configures PCB layer stack-ups and materials.	Optimizes dielectric layers and ground planes for effective EMI shielding and signal integrity.
DFM Wizard	Automates design-for-manufacturing rules and checks.	Ensures EMI mitigation features like shielded vias or component spacing are manufacturable.
SI Analysis Workflow	Analyzes signal integrity parameters like impedance and coupling.	Identifies and resolves issues with stray capacitance and coupling that could contribute to EMI.
EMI/EMC Filtering Tools	Integrates filter designs into layouts.	Adds EMI filters effectively to critical paths, ensuring reduced noise and improved compatibility.
Real-Time DRC Updates	Provides immediate feedback on design rule violations.	Flags EMI-critical issues such as inadequate spacing or overlapping components in real-time.

For more information on the benefits of using simulation for your PCB design guidelines for reduced EMI, refer to this E-book on electrical-thermal co-simulation.

Following PCB design guidelines for EMI and EMC is important to create reliable, high-performance circuit boards. Explore how Cadence’s PCB Design and Analysis Software and OrCAD X can streamline your workflow and enhance your designs. Talk to our experts to learn more about building robust PCBs that perform reliably in real-world conditions.

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