> On this subject, why, on a four layer board (or 6 or 8 or
> 16), do the power
> planes usually go in the centre and the signal layers on the
> outside.  Isn't
> it more logical to put plane on the outside to catch EMI/RFI?
>
Planes on the outside would be better from an EMI point of view, and
this is occasionally done in bleeding edge technology.  But normally the
planes are on inner layers so that they have fewer holes in them.  If
you put your ground plane on the top and you put a big BGA on your board
you would have to make a big hole in your ground plane for all the
signal pads.

Sherpa Doug

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