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I have been doing some research into the effectiveness of applying the 20H rule to 4 layer PCB designs and have been finding some very conflicting information. Some sources (including a few IEEE research papers and case studies) show experimental results that suggest that applying the 20H rule to a 2-plane stackup will actually increase edge radiation into free space, while other sources show evidence that it will cut emissions significantly. After doing some thinking about this, the pull-back on a power plane in respect to a ground plane is almost exactly the same as a microstrip. So my question is this, in a non-impedance controlled design, shouldn't adding a fairly continuous ground plane on the adjacent layer to the power plane make the power plane behave more like a stripline in regards to coupling to the sandwiching ground planes? Does anyone have a preferred method of cutting emissions in 4-layer designs apart from the usual mix of proper partitioning and slew control?

~~~~~~~~~~~~  Signal Layer Top

~~~~~~~~~~~~  Ground plane

~~~~~~~~       Power plane with 20H pullback

~~~~~~~~~~~~  Signal Layer Bottom w/Ground pour
Glorfindel
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Luke Gary
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1 Answers1

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In my experience, it doesn't make a lot of difference.

The advantage: the local electric (near) field at the edge of the PCB is somewhat reduced, useful to minimise cross-card interference (which is why I use it where there are a number of cards in close proximity). Because the near field is rather chaotic, all types of coupling are potentially there, so reducing this can be very useful.

Total radiated energy in the radiated field is slightly increased.

The resonant frequency of the PCB is raised slightly with pullback.

Details in this research paper

My methods for keeping emissions down are the usual management of signals that could be problematic, and careful shaping of the planes (not the edges, but making currents flow where I want, not where they would go otherwise).

You may find this answer useful.

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Peter Smith
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  • Thank you, this is very helpful. Would you agree in my comparison to stripline conductor? – Luke Gary Mar 22 '16 at 17:03
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    As the pull-back controls the near field (primarily), this could be considered a form of microstrip, *but*, unlike microstrip, this is resonant. – Peter Smith Mar 25 '16 at 11:53