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Is it ever okay to cross a plane split?

NO.
Okay, well, I guess you could say “it depends”, but in general, no, it is not okay to cross a plane split.

Why not?
Basically, you are creating a break in the current return path for the signal.  That turns your trace/plane combination into more of an antenna than a transmission line.  And if you are radiating energy, that means you are not transmitting energy to your receiver.  So, it can lead to EMI issues as well as signal integrity issues.
I discuss this more in the following article:
http://pcdandf.com/cms/component/content/article/246-2012-articles/9315-pcb-design

What about slower signals?
Generally, plane splits cause the most severe problems on faster single-ended signals, like DDR2/3/4.  Slower signals (more specifically signals with slower edge rates) have less problems crossing splits, because the return current can usually find an alternate path, and the signals are slow enough that that path can be relatively long.  But crossing splits also makes signals more susceptible to noise, so if you have a sensitive “slow” signal like a reset, this can still cause problems.

What about connecting the planes with a cap?
Bridging the plane split with a bypass capacitor can help some, but there are some caveats.  First of all, it is important to remember that a mounted capacitor is a narrowband structure – it only acts as a low impedance across a limited frequency range.  Also, you would need a bypass capacitor for every signal that crosses the split, otherwise the signals will share return paths and show a marked increase in crosstalk.

Trying to mitigate all the problems caused by crossing plane splits ends up being lot more work than just planning ahead to eliminate them from your PCB design.  So, just say no to crossing plane splits.

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About Patrick Carrier

Patrick CarrierPatrick Carrier is a Product Marketing Manager for PCB analysis tools, including signal integrity, power integrity, electromagnetic compatibility, and thermal analysis. Visit The HyperBlog

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