The first thing I look at when a client's project fails EMI: The Stackup!
I like to think about the stackup as the foundation of a building.
If the foundation is not done right, the whole building collapses.
Selecting the stackup is such a fundamental part of the PCB design process and of passing the EMC test that it cannot be simply overlooked and discarded because of price.
But of course, EMC design is simple, but not easy.
Most of the issues I see when a project fails the EMC test are because the stackup is totally off.
The most common issue I see is selecting a two-layer stackup where both layers are used for routing signals.
The reality is that signals and their return path, especially the digital ones, don't care about how you name your layers.
The only thing they care about is impedance, particularly the path with the least of it.
The problem with this type of stackup is that fields are left uncontained and are free to couple with whatever they find most suitable.
In most cases, this is found through parasitic coupling through the earth ground, or in cases of EMC tests, through the ground plane.
This allows them to expand and results in the PCB failing the tests simply because they are above the allowed limits.
The other part to consider is that now we have established a parasitic path that allows parasitic currents to flow through.
More EMI troubles for us.
Also, as it should be clear from this image, not only do you create Inter-system EMI, but also Intra-system EMI, meaning signal integrity issues!
Because now the signals will compromise one another.
This will result in a board that not only will fail the EMC test but also will not work properly!
Take good care of the return path, and it will take good care of you.
Show this picture to your project stakeholders next time you are asked to get rid of the "ground" planes.
EMC is NOT black magic.
Dario
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