I have a 6-layer board where the internal 4 planes are +15,GND,VCC,-15. I was wondering if there was any advantage to doing a copper pour on the top and bottom layers? I would probably leave them floating as I don't want to use micro-vias to say tie it to GND?
Is this actually a bad idea? I.e. floating copper=antenna.
Would it be just as acceptable to have 4 layer board with the top layer having copper pour to VCC and the bottom having a pour to GND and keep the two internals as +-15?
Note that this is for a fairly low speed circuit which has some analog and digital parts.
Answer
Typically outer layer pours are a bad idea. Outer layers have lots of components and traces that tend to chop up the pour. Little islands of pours lead to EMI issues.
If you do a star topology for your +5V (branch from the supply rather than creating loops) with really thick traces (0.020" min) then you could possibly do away with a couple of pour layers. It will certainly reduce board costs. Depending on your supply usage, you might be better pouring the GND and delivering one of the 15V supplies via traces.
In the end you'll have to build a board to see if it meets EMI, and performance specs.
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