Joe,
     Several considerations...

     You don't want to put your ground and power planes as the external
layers.  It would cause you to have to cut huge holes in your plane and
create, in effect many slot antennas.

     If you placed the planes at any other position you will have to
pick and choose what traces to route inside the planes and what outside.

     Traces which you had not considered critical or high freq (fast
switching) may carry the noise to an outer layer if they run both inside
the planes and out.  This means that you may not get the results you were
expecting.

     You will lose the benefit of distributed capacitance (small though
it may be) you get from having the planes close together.

     One other thing, any disturbance in the conductive path may, under
certain signal conditions, become a source of radiation.  Most of the time
square corners make no difference -- if, however, that corner happens to
be at length matched to the propagation delay of the waveform it becomes
a disturbance point, the signal reflects back on itself and the current
level at that point increases, thus increasing the radiation.  You want
to keep the current distribution as even as possible.  At very high
frequencies even 45 degree angles can cause problems.  You want to keep
current distributions as even as possible, high freq signals as short as
possible, noise away from signals which connect to I/O, and concentrate
your efforts on the most electrically noisy area -- keep it isolated --
shrink it -- move it away from the edges -- shield it.

L8R,
DonW...

Don Walker
Sr. CAD Designer
[log in to unmask]

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