Im designing a fairly tightly packed pcb so decided to try my first 4 layer design, partly for my own learning/practice. Ive used an internal layer each for ground and 5v and the 2 outer layers are for signals.
My question is what to do with the top and bottom layer copper fills. Should they be made ground? Or left unconnected. As it stands, all component grounds are separately connected to the internal ground plane using vias but If i set the top and bottom copper fills to ground, they connect all the component grounds together which as i understand can cause EMI issues and is generally considered bad practice. If i leave them unconnected, i end up with large unconnected copper islands which, again, i understand is considered bad practice.
It won't make much difference, the internal layers are close to the surface anyway. Standard practice is power/ground internal, signal external because you can get to the
signals for any modifications that need to be made and the two sides of the board are
screened from each other.
If you have controlled impedance traces its important the ground plane is continuous
under those traces and the correct trace width and spacing is used.
If you do have ground or power fill on the surface layers they should be regularly
stitched with vias to the internal planes. Via-stitching is also important for thermal
dissipation - this is one reason you might want ground-over-ground or power-over-power.