Re: [SI-LIST] : Cross hatched planes

Michael E Vrbanac (mvrbanac@cisco.com)
Fri, 18 Jun 1999 13:48:50 -0700 (PDT)

Fabrizio,

Here's a simple idea. It should work....

Calculate the percent of copper remaining in the cross-hatched
reference plane compared to if it were solid. Then model that amount
of "percent of plane left" fairly consistently in the 2D model leaving
all the other variables the same and you should at least have the same
effective capacitance which should yield the characteristic impedance
fairly accurately.

If you are industrious, vary the "percent of plane left" parameter and
see how sensitive it is. I'd bet it wouldn't be much.

See if you think it's worth a shot....

Regards,

Michael E. Vrbanac
Technical Leader
Cisco Systems, Inc.

>
> 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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