Re: [SI-LIST] : 20-H Rule for Power Planes

Philip Gantt (pgantt@cisco.com)
Thu, 27 May 1999 15:08:03 -0700

I'll attempt to describe this rule for you. Visualize, if you will, a power plane on a board that is adjacent and parallel to a ground plane and both are of equal size. The edges of the power plane will produce fringe fields
that curve around and beyond the edge of the board, closing in upon the ground plane. The fact that this fringe field extends beyond the edge of the board presents a problem from an EMI perspective, especially if there is
digital or high frequency noise on the ground or power plane. In this case, the edge of the board becomes a radiator, perhaps even a tuned cavity. See my crude illustration below:

________________________ Power plane
( ^
( <---- fringe field h
( _____________________|___ Ground plane

The 20h rule basically states that if the power plane is smaller than the ground plane by an amount equal to 20 times the distance that separates the two planes (h), then the fringe fields will not extend beyond the board edge in
any significant amount. See my second crude illustration:

<-----20h---------> _________________________ Power plane
(
( <---- fringe field
________________(________________________________ Ground plane

The theory, then, is that if you can maintain keeping the noisier plane (power) smaller than the quieter plane (ground), you can reduce EMI. In multilayer stackups, "stitching" is use to contain the fields even more. Stitchin
is nothing more than putting vias along the edge of the board to contain the fields even more. However, as was pointed out by one contributor to this list, placing vias equally spaced along the edges of the board could produce
an array of radiators that are all tuned to the same frequency.

rbmccammon@mmm.com wrote:

> I've read every post on this subject so far. I still only have a vague
> idea of what the 20H rule is and the assumed conditions under which it is
> applied. Any one want to try a one or two paragraph description?
>
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