>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

Any connections to the center layers have to go through vias.  Connections
to the top layer (SM) or outside layers (through-hole) don't need a via.
Signals usually outnumber power and ground.

Having signal traces on the inside makes it harder to debug a bad board.

Tradition.

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