Dear Matthew Lamkin,

Matthew Lamkin <[log in to unmask]> asked:
>Why would the powerplane be a middle layer? if it has groundplaning/emc
>reduction effects
>wouldn't it be better nearer the outside?

In the early years, primitive etching procedures caused lots of shorted traces
(under-etched) and broken traces (over-etched), often both on the same board.
Traces on the 2 outside layers are *much* easier to fix than on an internal
layer. (And patching prototypes to fix errors in the schematic...)

So everyone doing 4 layer boards put +5V on one internal plane and GND on the
other internal power planes (since those connected to more stuff than anything
else).

Now that board houses routinely make "perfect" internal layers, that reason is
no longer relevant. But we're reluctant to change something that works :-).
(Where did I put that article on Fear, Uncertainty, and Doubt ...)


I stumbled across:
  ``we decided to put the power planes (or the ground planes) on the outside of
the board'' --
  http://www.pcdmag.com/db_area/news/2000/000304.html
but they don't have a good explaination for it.


--

David Cary

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