At 10:09 AM 7/11/01 -0500, Mark E Witherite wrote:
>At  the 1999 PCB east conference I learned about the 20H rule.  This rule 
>simply put states that EMI can be substantially reduced by keeping the 
>power planes back from the edge of the ground planes by 20 time the 
>dielectric thickness between the two planes.   This would be a lot easier 
>to implement than vias .

This rule of thumb appears to be highly controversial among experts. To my 
knowledge it has never been tested in a controlled experiment. One of its 
effects might be the change in the direction of peak radiation, so a study 
would need to be fairly sophisticated and not just make a measurement in a 
single direction.

Vias are easy to implement. If they are spaced closely enough, which 
depends on frequency, they will essentially complete a ground shield, but 
this may have little effect on radiation except at certain angles. In 
general, where there are multiple ground planes, an increased number of 
vias will be helpful, but one would want these vias not only at edges but 
also close to all potential noise sources.

I simply don't have sufficient experience to know or even make a solid 
guess, and I have never seen a study or even a sufficiently sophisticated 
analysis with a field solver. Perhaps someone else has and my understanding 
is out of date.

Attempts have been made to track back the source of the 20H rule, and it 
does not seem to be much more than a bright idea of one writer; that, of 
course, does not make it wrong.

[EMAIL PROTECTED]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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