Very
good observations. I do not know of any specification which speaks to the number
of required layers, and I looked. The closest thing to a layer
specification is 4.3.6.1 "Routing and Layout recommendations for
Four-Layer Motherboards." And these are just recommendations not
requirements.
-rgk
Hello,
Observation re 2 layer PCI:
I can name two common devices that are sold in
the tens if not hundreds of millions of units and that are 2 layer PCI boards.
These would be ethernet cards and sound cards. These cards sell for around
$20. They need to be made dirt cheap and they need to work on a very very high
percentage of motherboards to avoid a bankruptcy inducing support nightmare.
And they must work in conjunction with whatever else is plugged into the host
PCI bus.
So my conclusion is, that in the real world
you can often ( and many do ) get away with it.
I am not qualified to answer the question from an
engineering perspective but I would like to hear what others have to
say.