Re: 3.3V/5V planes

Jeff Seeger (jseeger@appliedcad.com)
Thu, 17 Jul 1997 14:47:11 -0400

Peterson, James F (FL51) wrote:
>
> i didn't think you could do a 9 layer board - it would have to be

You are correct. In order to make a 9 layer board, you
start with 10 layers and remove all copper from one, which
will tend to warp.

> either
> 8 or 10. anyway, how about a dual-stripline approach, where you would
> sandwich 2 signal layers in between power/gnd planes? (this seems to
> keep the crosstalk level down and also might give you 1 or 2 more signal
> layers :
> 1---top
> 2---gnd
> 3---sig
> 4---sig
> 5---+5V
> 6---sig
> 7---sig
> 8---gnd
> 9---+3v
> 10-bottom

Similarly, the planes need to be distributed evenly about
the center of the board. For 10 layers I suggest the
following:

---
------------- Gnd
---
---
------------- 3.3v
------------- 5v
---
---
------------- Gnd
---

Or similar. Alternatively, switching one of the volatages
and a ground build in some level of inherent capacitance.

Going back to the return current issue,

Larry Smith wrote:

> Jeff - return current always travels on the nearest reference plane.
> That could be ground, 3.3 volts, 5.0 volts or even a floating piece of
> metal if that is nearby. If there are two power planes nearby (ie one
> over and another under the trace) current will travel on both planes
> with the majority of current traveling on the closest plane.
>
This implies to me that whether 3v or 5v, the actual path
for return is not familial and therefore not an issue
(assuming it can get where it needs to, of course!).

True?

TIA,

-- 
 
      Jeff Seeger                             Applied CAD Knowledge Inc
      Chief Technical Officer                      Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                    508 649 9800