EMI failures can happen due to small, innocent mistakes.
One specific issue, as shown in the picture, is the lack of return reference vias, also known as stitching vias.
So, what's the problem, and how can we fix it?
The problem occurs when a signal crosses multiple layers of a board, moving from horizontal to vertical without a proper return path for the current, and the reference potential for the signal voltage.
Return reference vias are needed when signals cross multiple layers in a stackup because:
- They provide a close return path for the signal, even when moving vertically.
- They keep the reference voltage close to the signal and contain the EM fields.
What happens if we don’t use return reference vias?
When a signal moves from horizontal to vertical, it loses its reference plane. This causes the EM fields of the signal to spread through the planes and potentially cause EMI.
Additionally, the return current will struggle to find a path back to the source, creating a larger loop and generating noise due to the displacement current going through the impedance of the cavity (the space between the layers).
The key point is to always provide a return reference path for signals, whether they move horizontally or vertically on a PCB.
I hope this helps,
Dario
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