What drawbacks are there to using the top layer of the PCB for voltages? I'm considering this because...
- The top layer doesn't make for much of a routing layer anyway.
- With multiple voltages (+24, +5, +3.3, +1.5) on the board the voltage layer, wherever it is in the stackup, isn't a very good plane to use as an AC return since it will have 4 islands that signals on adjacent routing layers would need to avoid crossing the unavoidable cutouts.
- The next layer down in the stackup would then be a ground plane so this start to the stackup would seem to make an ideal plane capacitor that is as physically close to the actual components as possible to minimize via inductance in feeding current to the components (assuming top side mounting only).
I'm guessing that there may be some manufacturability concerns since there are a bunch more processing steps for the outer layer but since the voltages won't tend to need narrow traces maybe there really aren't any. This PCB will have some BGA components, at the moment it looks like 1mm pitch.
Thanks in advance for your comments.
KJ