[SI-LIST] Re: Thermal planes as reference planes?

  • From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
  • To: <james.f.peterson@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 15 Apr 2004 09:48:49 -0400

Signal traces will use any copper areas they see nearby.  Return switching
currents will flow through all adjacent planes, even if you didn't mean for
them to flow there.  It makes little difference to the signal what else that
plane is or is not used for, although it matters when you start looking at
how return path currents get on and off the plane in question around the
ends of the signal traces, or if traces fall off the ends of a finite plane
or cross splits.

If both driver and receiver are within the area of the copper plane, and the
whole trace stays adjacent to the plane, then it may be a good reference
plane.  But you'd want to see how the switching currents get from the driver
to this plane.


> If we have a copper plane that is used for thermal relief and is not tied
> directly to digital ground, can it be used as a reference plane? Note :
> thermal plane is tied to the chassis which has a single point connection
> digital ground (located on the backplane).
> Will an adjacent signal layer see this as a reference plane?

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