> Also, in an EMI-sensitive environment, how would you recommend the layer
> stack-up?  Signal 1, Ground, Supply, Signal 2  OR Ground, S1, S2, Supply?
> OR
> something else?
>
        In an EMI sensitive environment, the rules of thumb to follow are:
        *       Put only components, pads, and short traces to vias on
exterior layers. Fill the rest of the exterior layers free area with ground
plane.
        *       Make the board as opaque as possible. Fill all free areas on
all signal layers with ground plane.
        *       Treat very high speed (that is, high energy) signals as EMI
generators. Shield them as much as possible; both from the exterior world
and from other signals. Also avoid any sharp turns with high speed signals,
as that vertex will act to radiate energy (EMI).
        *       Provide lots of connections to ground. This will reduce
ground impedance.

        The goal is to trap the electromagnetic energy within the board,
letting it be absorbed into the ground planes.
        Whether you stack signal layers within the board on top of each
other is a function of the signal levels you're dealing with, and their
susceptibility to crosstalk. Minimizing parallel trace lengths on adjacent
layers to less than an inch is a good rule and usually reasonably easily
achieved, provided you're routing each layer perpendicular or 45 degrees
with respect to adjacent layers (also, of course, providing your design
software has some way of checking this criteria for you).

 Michael Hiteshew
 Lockheed Martin Launching Systems
 [log in to unmask]
 (410) 682-1259

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