Re: [SI-LIST] : trace to trace coupling

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From: Larry Smith (ldsmith@lisboa.eng.sun.com)
Date: Fri Sep 15 2000 - 16:12:56 PDT


Sean - I assume you are asking about the structure:

              ===== trace

===================================== solid copper plane

              ===== trace

With a solid copper plane, none of the electric field associated with
the top trace can pentrate the plane and disturb the bottom trace.

The magnetic field can penetrate at low frequency but not at high
frequency. Skin depth is closely related to the penetration of the
magnetic field. A perfect conductor has no magnetic field penetration
but magnetic field does penetrate into a conductor with finite
conductivity.

The skin depth for copper at 10 MHz is .82 mils, just about the
thickness of 1/2 oz copper. At 100 MHz, the skin depth is .26 mils and
the magnetic field is reduced by 1/(e^3). Only about 5% of it gets
through the plane and only a small portion of that couples with the
trace.

At higher frequency and with thicker planes (1 oz) even less field
penetrates the plane. (But watch out for thin tungsten power planes in
ceramic packages.)

regards,
Larry Smith
Sun Microsystems

> From: Sean Murray <seandm@mmspec.com>
> To: si-list@silab.eng.sun.com
>
> Does anyone know the formula in order to calculate the coupling percentage
> between two signals seperated by a solid ground plane? At what
> frequency/edge rate does one have to start thinking about staggering their
> traces so that they do not get coupling between two stripline layers..
>
> Regards,
> Sean Murray
> General Manager
>

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