Is it a good idea to fill free space on Top and Bottom layers in a multilayer (4-layer or more) PCB?
I usually use external layers for signal routing and internal layers for power supplies and ground.
* Top - Signals, High speed signals (if there are any) * Ground Plane * Power layer * Bottom - signals
In the stackup above there's a whole layer dedicated to ground with no traces on it. So, that means there's no need to fill the free space of external layers with copper stitched to ground (using polygon pours) but it's something I tend to do. I also see many manuacturers follow that design (e.g. Raspberry Pi 3 b+). What is the main reason of that? Is it harmfull for the design?