Richard,
My understanding is that the controlled impedance signals on the external
layers reference the ground plane directly under them with specific spacings
and sheet thicknesess between signal layers and planes and the core planes
themselves.
This gives the results they are looking for.
Richard Sumner wrote:
> You should question their reasons! If you have lots of things (IC's &
> connectors) which connect to all 3 planes (shorting them together with a
> via), you have effectively made one plane. Although the ground currents do
> not share (among the 3 planes) equally, the sharing will be hard to
> predict. And there will certainly be some 'filter current' on each of the
> 2 outer planes.
>
> Or, have I misunderstood what you do?
>
> At 04:45 PM 6/15/2001 Friday, you wrote:
> >Mike,
> >
> >Based on customer requirements I need 3 ground planes.
> >
> >They all share the same net name however (as they must):
> >
> >The IC's and connectors, etc. hit all three planes
> >Filter Caps only connect to the inner most ground plane
> >and any signal that must pass through the "core power/ground" pair
> >must have a return ground via that only connects to the 2 outer most
> >ground planes.
> >
> >By Manipulating the plane clearences by say a mil I get a separate Dcode
> >for the desired pads and can modify the gerbers to thermals where
> >required.
> >
> >I have been meeting these requirements for many years and their boards
> >always work so who am I to question their reasons.
> >
> >Regards,
> >
> >Jim McGrath
> >CAD Connections, Inc.
> >
> >
> >Michael Reagan wrote:
> >
> > > > > I recall you once emailing me with some details of a PCB that you had
> > > > > designed, in which you wanted various pads to connect to different
> > Power
> > > > > Plane layers in different ways (Direct Connect, Thermal Relief
> > > > Connect, or
> > > > > No Connect). Although I didn't respond to you soon afterwards, I did
> > > > > eventually send you a response in which I suggested that you
> > > > could specify
> > > > > the pads concerned to Direct Connect to the Power Plane layers,
> > > > and then add
> > > > > primitives (arcs or single layer pads) to each Power Plane layer as
> > > > > appropriate to effectively change each connection between each
> > > > pad and each
> > > > > Power Plane layer to whatever you required (i.e. Thermal Relief
> > > > Connect or
> > > > > No Connect).
> > >
> > > Jim
> > > Why would you want to do this? if I may ask
> > >
> > > Mike Reagan
> > > EDSI
> > > Frederick Md
> > >
>
> Cheesecote Mountain CAMAC
> 24 Halley Drive; Pomona, NY 10970
> 845 364 0211, www.cmcamac.com
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