An unbroken copper area on one layer dedicated to GND. The single most important layout rule.
How it looks
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
Inner layer 1 of CORE3588, the whole board. Amber is copper on the GND net: the layer is one unbroken sheet. The specks are vias passing through it. Inner layer 2 looks the same.
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
Zoom: 8 x 6 mm of the same plane, drawn in grey here so the marks show: the plane is the mid-grey sheet, via rings are light grey. Every hole a signal via makes is a small gap in the sheet; many in a row can form a wall.
1A via of a signal net: the plane keeps a clear ring around it so they do not touch.
2A ground via: it joins the plane directly, no ring.
How it works
Where it fits
The ground plane is a copper pour on the GND net that covers a whole layer. On CORE3588 both inner layers are planes.
It sets the impedance of the tracks above it and gives pairs a clean return path. Each via makes a small hole in it.