On a 4 layer there isn't too many layer stack combinations.
However (just for the persons who didn't know, not Alan) layer stacks
are standardised.
You can see this here:
http://www.pcbmatrix.com/Downloads/GeneralDocuments.asp
the document is named "layer configurations"

Protecting a PCB for reverse engineering is a lost of time. The real
reason why midlle layers is good to become signal layers is to avoid
interferences and EMI radiations.
There are plenty articles about this issue on the web.
The best choice is that signal layers to be defined as mixed
signal/ground planes.

Vasile

BTW Russell, for you sanity check, this email should have TECH or EE tag ?
Please put yourself the (in)appropiate tag...if you can decide which
is the "correct" one.
:)


On 11/24/08, Alan B. Pearce <Alan.B.Pearce@stfc.ac.uk> wrote:
> Personally I would do it as follows: -
>
> Layer1: signal
> Layer2: 5v plane & 12V track as required.
> Layer3: ground
> Layer4: signal
>
> The area supplied by 12V will probably be only a small portion of the PCB,
> and I would have 12V to just that area, with the rest of that layer as a 5V
> plane.
>
> However without knowing precisely what you are setting out to do, it is
> difficult to advise you properly.
>
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