Re: [SI-LIST] : Cross hatched planes

Fred Balistreri (fred@apsimtech.com)
Fri, 18 Jun 1999 15:24:28 -0700

fabrizio zanella wrote:
>
> I have a question regarding high frequency (over 500MHz) signals running
> over ground planes that are cross hatched, which is commonly done on flex
> circuits. The issue is how to accurately model the characteristic and
> differential impedance for the flex circuit traces. If, for example, I use
> a 2D field solver (where I assume an ideal ground plane) and am designing
> for 100 ohms differential impedance, what percent increase in impedance
> should I expect if the ground plane will actually be cross hatched?
>
> thanks and regards,
>
> Fabrizio Zanella
> EMC, Hardware Engineering
> fzanella@emc.com
> 508-435-2075, x4645
>
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This will depend on how the cross hatch is done. If the cross hatch is
square and the traces are routed directly above/below the hatch then
a 2d field solver can be applied and get good answers. However if the
cross hatch is diamond w.r.t. the traces then more work is necessary to
get a reasonable result from a 2d field solver. Electrically speaking
the square cross hatch is a better solution. However for manufacturing
and mechanical reasons flex is usually made with diamond cross hatching.
That being the case, care has to be taken to make sure the return paths
are not being increased substanually due to the hatching. And as
mentioned its much harder to model.

Best Regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com

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