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The Best PCB Design Guidelines for Reduced EMI

Key Takeaways

  • Understanding the difference between EMI and EMC.

  • What are the sources of EMI on circuit boards?

  • How to design for minimal EMI.

 The relationship between electric and magnetic fields

Composition of electromagnetic interference

We know from physics that there are four basic forces in nature. These are the strong nuclear force (which binds neutrons and protons), the weak nuclear force (that allows for radioactive decay), gravity (which gives objects their weight or pull towards a larger attracting object), and electromagnetism (which is the magnetic attraction between electrically charged particles). In many cases, this attraction—which has both electrical and magnetic properties—is advantageous. For example, magnetics are used to promote stator movement around a rotor in electrical machines. However, in other cases, this naturally occurring force can pose a significant problem for desired electric circuit operation.

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 can be thought of as achieving an acceptable harmony or balance between electromagnetic elements so that the amount of interference is minimal or at least low enough not to significantly hamper normal operation. Unfortunately, the elimination of all EMI is not yet possible; however, obtaining EMC is. EMI, which is actually any interference from an electromagnetic source, is typically referring to interference on a single PCBA. This categorization is sufficient for investigating the issue, as the minimization of EMI on and from a circuit board contributes to the EMC of the environment in which the board operates.

Where Does PCB EMI Come From?

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. 

Portion of the electromagnetic spectrum

The electromagnetic spectrum

Wherever there is an electrical current, the potential for EMI exists. For PCBAs, the sources of EMI can be classified as falling into one of the following categories:

  1. Components

Electronic components and elements—especially high power devices such as processors, FPGAs, amplifiers, transmitters, antennas, and others—may contribute significantly to EMI. Additionally, switching components generates interference that can be disruptive. 

  1. Signals and Traces

EMI can also be created along traces or at pin and connector points. For example, unbalanced differential pair routing may cause signal attenuation and reflections along the transmission paths that may severely affect signal integrity or the ability to accurately identify signals, resulting in erroneous circuit behavior. Additionally, unwanted coupling may be formed between signal paths and ground planes due to stray capacitance. 

  1. External Sources

If the board is too close to a radiating source—which may be another board or element—EMI can be introduced onto your PCBA. Harmonics may also be generated by vibrations or movement in your board’s environment from other equipment or devices.

Obviously, eliminating all of the potential sources of EMI is a daunting task. Fortunately, there are PCB design guidelines for reduced EMI that can be instituted to aid in noise minimization and achieving EMC

The Best PCB Design Guidelines for Reduced EMI

Knowing the sources of EMI that may impact your board is essential in devising a strategy to mitigate this ever-present threat to your PCBA's performance. Moreover, viewing EMI from a source perspective, whereby methods for minimization are targeted to specific sources, can be a good vantage point from which to devise a set of PCB design guidelines for reduced EMI.

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 

  • Select low power consumption parts, whenever possible
  • One of the greatest generators of EMI on circuit boards are parts that require large amounts of power. With the thrust toward lower power consumption, alternatives can usually be found that will not sacrifice functionality or quality.
  • Isolate different types of components
  • A good design practice is to always place components that process the same types of signals together. For example, digital components should be near other digital parts and isolated from analog devices.
  • Utilize PCB fencing
  • Another tool for mitigating EMI is to enclose components or sub-circuits within fencing; such as PCB guard rings and Faraday cages. These are also effective to reduce radiation into the environment around your board.
  • Employ heat dissipation techniques
  • For electronic components, energy creates heat. Therefore, efficient heat sinks and vias can greatly aid in EMI minimization. 

In addition to mitigating EMI from components, how your traces are run can greatly impact your board 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. 

Clearance and creepage distances

Maintaining adequate clearances is critical to minimize EMI

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

How to Reduce EMI From Signals and Planes

  • 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 as well as 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, whether these are necessary is determined by your design, its functionality, and performance objectives. Therefore, you should strive to optimize your design for EMI reduction, which is best done using an analysis tool such as Cadence’s PSpice.

Using PSpice to Validate EMI Optimized PCB Design

PSpice is a simulation tool that can be used to analyze and validate your design’s electrical parameters prior to submission for manufacturing. One of the best uses of PSpice is to evaluate the signal integrity throughout your board. This includes evaluating heat distribution and dissipation techniques, circuit responses over a wide range of conditions, and other simulations. To aid you in creating your PCB layout, PSpice Advanced Analysis (AA) is available. By instituting these advanced capabilities, along with source-targeted good PCB design guidelines, you can optimize your board design for EMI reduction.

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.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.