Introduction to EMC in PCBs
01-10-2020 | By Robin Mitchell
Electromagnetic Compatibility is a critical requirement for electronic products sold in most markets globally. Most EMC design considerations are found in the PCB design phase, and so in this article, we will look at common tips and tricks that can help to improve a PCBs performance.
What is EMC?
ElectroMagnetic Compatibility, or EMC, is the method in which electronic circuits are carefully designed to minimise their effect on other electronic products, as well as their ability to resist being affected by other circuits. While most circuits will function regardless of EMC considerations, product laws, and regulation imposed by most countries around the world require that commercial products follow strict EMC requirements. To understand how to reduce EMC, it is important to know what exactly electromagnetic emissions are.
EMC emissions are stray radio waves that are generated from alternating/switching currents, and any conductor with an alternating current acts as an antenna. Thus, it is the job of a designer to make all conductors in a product terrible antenna incapable of emitting strong radio waves.
Initial design considerations
Since electromagnetic emissions are radio waves, common sources of EMC issues include most high-speed switching devices, data buses, and radio circuitry. Switch-mode power supplies are notorious for causing problems EMC as well as injecting switching noise into power rails, while Wi-Fi modules can also be highly problematic. High-speed data buses such as SPI, PCIe, and memory lanes can also be sources of emissions, and these traces must be routed carefully to minimise emissions.
Before traces are routed on the PCB, the PCB design and layout must be carefully considered. Plenty of information is freely available online, but in general, it is crucial to follow the following rules.
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Components should be grouped based on function (i.e. analogue, digital etc.)
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Connectors should be put on one side of a PCB if possible.
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Distance between connected components should be minimised
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High-speed connectors should be grouped and kept separate from analogue connectors
PCB routing tricks
Routing PCB traces is an equally critical design step to determining where components and connectors should be placed. Just like the initial design phase, there are many different considerations a design can make when routing traces, and there are many free examples of good EMC practice online. Thus the information here is just a taste of what can be done.
PCB Layers
While two-layer PCBs are often the cheapest option, they have the drawback of not allowing for the use of power planes. Any PCB with more than two layers has the opportunity to use power planes whereby an entire layer is dedicated to either a power rail (such as 5V) or ground. In a four-layer PCB, it is best to use the two inner layers as the power planes, as this maximises the capacitance between the two layers, and provides minimal return paths for current loops and emitted emissions.
Differential Pairs
Differential pairs, such as those found in USB, should always be routed next to each other with no traces or planes in between. It is also critical that each of these traces is identical in length as this helps with impedance matching. If the two traces are separated too much, or have unequal impedance, not only affects the performance of the differential pair (i.e. noise and speed) but can lead to current loops which lead to radio emissions.
Guard Traces
Guard traces are specially routed ground traces that surround a trace either side and can be useful for providing both immunity from external sources as well as preventing stray emissions from the trace from escaping (the guard trace acts as a simple Faraday cage that surrounds the trace).
Stitching Via
Stitching via is via which are often connected to ground and surround particularly noisy areas. These are commonly found on PCBs, and just like guard traces, act like a Faraday cage helping to absorb stray EM emissions. The spacing between stitching via generally should be below λ/20 where λ represents the maximum frequency being guarded against. For example, if a 2.4GHz circuit was being guarded, then the minimum distance between via should be 15cm / 20 = 7.5mm. However, such small distances may not always be possible, and the inclusion of via generally improves performance regardless. The hole size of the stitching via is generally not an issue, but using the smallest drill hole offered by the PCB that does not incur additional costs is most ideal.
Reduce trace length
When reducing EM emissions, it is important to keep trace lengths as short as possible as this essentially makes them a bad antenna. However, it is also essential to ensure that the length of the trace is not a multiple of the wavelength of the expected nominal frequency of the signal that will be present in that trace. For example, if a 2.4GHz signal is to be present in a trace, and it is not a radio link (i.e. a standard data bus lane), then the length of that trace should not be a multiple or even fraction of the wavelength which is 15cm. Thus, trace lengths of 15cm, 7.5cm, and 5cm, should all be avoided, and traces over 15cm should not even be used.
Conclusion
The methods outlined in this article are but a few methods for reducing EMC, and further exploration via free online resources will reveal just how complex design for EMC is. However, the best method for reducing EMC is to understand what causes EMC and to remove such sources before trying to use methods of reducing EMC via capturing stray emissions (i.e. Faraday cages).
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