Optimal EMI Filter PCB Layout Guidelines

Key Takeaways

• The importance of electromagnetic compatibility for EMI filter design.

• Design tools for optimizing EMI filter design and layout. 

• How to optimize EMC and manufacturability for your EMI filter board design.

Testing a PCBA for electromagnetism

I can vividly remember being told more than a few times as a youngster that I was putting the cart before the horse. Initially, this admonishment left me a bit bewildered. Although I never rode in one, I had read about the days when horses and carriages were common, with the horse always leading the carriage or cart. Yet, the relevance of this to whatever activity I was involved in at the time seemed elusive. What I came to realize is that this saying was meant to convey that I did not have the proper perspective to be successful in my endeavor. 

Having the proper perspective is also necessary when developing circuit boards, especially during the initial design phase. In order for a design to be successful, it must be undertaken with an understanding of the overall objective of the creation of a well fabricated and assembled board that meets its performance criteria. For virtually all PCBAs built today, that means maximizing signal integrity and designing for electromagnetic compatibility. From the perspective of achieving the best balance of electromagnetism, we can develop a set of EMI filter PCB layout guidelines to optimize the board development process and minimize the bane of PCBA performance—interference or noise.

Electromagnetic Compatibility

It is often assumed that electromagnetic radiation is only a concern for PCBAs that contain components meant to generate and transmit RF, which are classified as intentional radiators by the FCC. Though it is true these devices do require special consideration, this assumption is incomplete. In fact, there are four classes of radiators defined under the FCC Title 47 Code of Rules and Regulations.

FCC Radiator Classes

• Incidental radiators 

Components or devices that are not designed to use or generate electrical energy above  9kHz are classified as incidental radiators. Although equipment authorization is not required for these devices, they can still be the source of unwanted and disruptive interference. Common products include motors (both AC and DC), power tools, and light switches.

• Unintentional radiators

Unintentional radiators include devices that transmit electrical energy—intentionally—over wired connections. Any wireless emission is considered unintentional radiation. Most electronic products fall in this category and often include digital logic circuitry. Examples are computers, printers, phones, remote controls, and watches.

• Intentional radiators

RF designs or specified devices that emit RF are dubbed intentional radiators. This class includes all wireless devices and boards that contain Wi-Fi and Bluetooth devices.

• Industrial, scientific, and medical equipment

Other devices that produce and/or emit radiation for anything other than telecommunication purposes are classified in this group. Microwave ovens, arc welders, and fluorescent lighting fall under this classification.

In addition to the classifications above, products that are required to operate in fixed licensed spectra—such as TV transmitters, marine and aviation radios, cellular phones, and base stations—are also considered to be RF devices and must be certified.

As the listing above indicates, virtually all circuit boards today can have one or more sources of radiation that may interfere with operations for nearby electronics systems and on the board itself. Designing your PCBA to best mitigate these opportunities for interference is the goal of electromagnetic compatibility (EMC). Attaining good EMC requires coordination between all stages of development—design, manufacturing, and testing. The path to success, however, clearly begins with EMI filter design and requires the incorporation of the best EMI filter PCB layout guidelines.   

Optimizing Your EMI Filter Design

As explained in the preceding section, the likelihood that your board or the system within which it operates contains components or devices that can be classified as radiators is extremely high. Additionally, unless your board is a standalone device, its power source is connected directly or indirectly to a mains line, which is also a common source of EMI. The purpose of an EMI filter is to alleviate the introduction of interference from this source onto your board.

Basic EMI Filter Design

EMI filter design may range from a single component to a complex network with dedicated circuits for alleviating common-mode and differential mode noise. Some basic types of EMI filter designs and how they are used are listed below.

The EMI filter types listed in the table above are a good starting point for many noise sources on circuit boards, provided the right components are selected. 

EM Simulation Techniques

Once you decide on a basic design, it will be necessary to optimize it based on your circuit and/or board performance specifications. For example, if your EMI filter design is intended to drive other circuits or boards, it may be required that you augment the design with additional filtering elements for isolation, smoothing, or electrical parameter constraint—for example, fixed voltage or current. The degree to which you are able to meet these circuit demands depends upon the functionality and capabilities of the PCB design and analysis tools available. 

For the best results, your design tools, which must include EM simulation, as shown in the figure below using Cadence’s Allegro with PSpice,  should be integrated.

Analyzing a low pass EMI filter response with PSpice

Optimizing your EMI filter design typically requires the evaluation of performance over a range of electrical parameter values and frequencies. Therefore, efficiency, as well as accuracy, are premium design process attributes when analyzing your design for optimal EMI alleviation or suppression, prior to transitioning from schematic to layout. 

The Best EMI Filter PCB Layout Guidelines

As is the case for all circuit board designs, following good EMI filter PCB layout guidelines is essential for manufacturability. Again, the best design requires the adoption of the proper perspective. 

2-D PCB Layout Design Perspective

When laying out your board, the first considerations are the locations for component footprints—whether from your design package’s library or uploaded from an external source—trace routing, spacing, or board edge clearance. This is the 2-D PCB layout design perspective, which is focused on the layout of the board surface(s).

2-D PCB layout perspective

These design parameters are important and should be chosen according to guidelines, as listed below.

Surface EMI Filter PCB Layout Guidelines for EMC and Manufacturability

1. Acquire and follow your CM’s DFM rules and guidelines.

2. Ensure that pads and component libraries are matched.

3. Maximize spacing between adjacent elements—pads, traces, and annular rings—to minimize interference.

4. Partition components according to signal types.

5. Assure that trace widths and sizes are sufficient for the current capacities required.

6. Ensure that impedance matching is instituted where necessary for differential routing, maximum power transfer, etc.

7. Follow board clearance rules to facilitate depanelization. 

8. Use shielding for high radiation devices. 

9. Make good use of the silkscreen for component polarities and reference indicators to aid assembly.

Following PCB layout guidelines that include the ones above, where noise reduction and efficient board building are the focus, will help to ensure that your board meets its EMC goals and will be well constructed. However, due to the demand for smaller electronic products’ ever-increasing functionality, most boards today require a stackup. 

3-D PCB Layout Design Perspective

Multiplayer PCBA design is no longer a once-in-a-while activity as it once was. Instead, most circuit board designs today are small, densely populated, and include multiple layers. Therefore, a 3-D perspective of board layout, as illustrated in the figure below, must also be adopted.

3D PCB layout perspective

A 3-D perspective is similar to the 2-D perspective in that placement, routing, spacing, and clearances are still major considerations. However, in this case, the routes include a vertical component through vias and between layers or planes where spacing is important to achieve EMC and facilitate first-time-right (FTR) manufacturing, as included in the guidelines below. 

Stackup EMI Filter PCB Layout Guidelines for EMC and Manufacturability

1. Base the number of layers on pin density, the number of different types of signals, and provide good spacing between same type layers. For example, if possible, low and high frequency signal planes should be separate.

2. Do not place two signal layers adjacent to each other. 

3. For vias, use correct aspect ratios and use the least complex to manufacture that meets your design requirements. 

4. Apply good grounding techniques—for example, use separate planes for digital and analog signal types with a central point for board ground.  

5. Ensure there is adequate minimal spacing between signal and ground planes.

6. Choose layer material thicknesses to meet impedance requirements.

7. Radiation creates heat. Therefore, incorporate adequate thermal dissipation techniques.

In addition to the above guidelines—which are not exhaustive for PCB layout design, but are essential to achieve the best EMC—applying symmetry to your stackup is a good rule of thumb to follow. 

By adopting the proper perspectives, which include designing with an objective of attaining the best EMC possible and incorporating both a 2-D and 3-D perspective focused on manufacturability, and following good EMI filter PCB layout guidelines, you can achieve an optimal design with minimal interference on board and in the electronic system where your board is installed. 

For more information on optimizing your EMI filter design, check out this E-book on EM analysis methods. 

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