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say a 8-layer board just has a power and a gnd plane , six signal layers.
could i add gnd shape in some signal layers for reference plane, is ti good to EMI?
I dont think it is good idea at all to havr 8 layer board with only 2 layers of power and gnd ussually for 8 layers it recomended to have at least GND+GND+POWER. Because it is not ONLY EMC it is allos signal integrety matter
I always fill empty spaces on TOP and BOTTOM layers with GND plane and never had EMC problems
It’s also good to prevent copper over corrosion by the acid because in spaces with no copper the acid has more power and the over corrosion causes the copper trace to be jaggy, this is not good for EMC its act like small antennas
As long as they are connected to the return path of the signals that they are shielding or to the boards ground rail then it should be fine, just putting copper there where it connects to nothing is bad.
As has already been said it is not a good idea to only have one power and one gnd plane on an 8-layer board. For Best EMC and Signal Integrity you should have each signal layer adjacent to a reference plane i.e. a stack up like gnd-sig-pow-sig-sig-pow-sig-gnd or other combination. Also if you route two signal layers adjacent then the tracking on each layer should be tracked orthogonally i.e. at 90 degrees to minimise cross-talk. If your outer layers are signal then it certainly helps to ground fill these layers for both EMC/Signal integrity and PCB quality to reduce bow etc.
it is not abnormal that only has one power plane and one GND plane on a 8-Layer board.
I also suggest you to keep the GND plane to be an intact one, per se, this will provide a intact image plane for the signal layer above and under the GND plane.
High speed signals should be placed in the internal layer, close to the image plane.
you can place some copper connceting to the GND on the TOP and Bottom layer.
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