What's on top, more signals? More ground/supply?
More grounding is usually a good thing, but it can be used incorrectly. As with your other EMC question -- the only way to figure out is to test, or to provide enough details that an analysis can be performed. (I'm guessing the former, since you've been quite mum on details for anything so far?)
Tim
Well the summary is that i have these motors rated for 24V. The manufacturer insists strictly 24V and that the parts inside are rated for 40V Abs-max. These motors are brushless and shit! they don't do closed loop speed control and they don't have soft start so belt straight into whatever speed and if there is too much resistance/inductance in line they fail to start or oscillate mechanically in an attempt to start.
I have a linear voltage regulator to protect the motors fram any over voltage and a basic µC based system to read a tacho feedback and change the speed drive to keep a consant speed over any voltage.
Testing has shown a lot of noise in the 80MHz range. We suspect common mode from the motor driver coupling to the chassis of the motor and system. So I put filtering on the motor, a bit of a potshot guess using CMC and some differential filtering for good measure. This removed much of the noise but there is still noise at 85-105MHz (yes it appears to have shifted while partially dissipating). So I am building a new ECU section that has basic power input fitering with a CMC followed by capacitors to the chassis ground plane and then additional filtering on a seperate voltage regulator PCB that will have further differential and CM filtering, again with capacitors to chassis ground.
It is the ECU PCB that I wantet to put the chassis ground plane on. Obviously this will create a capacitance with the negative supply rail, so will this cause a problem, maybe if it couples before the CMC rather than after it.