A ground (GND) plane is a solid sheet of copper (a real PCB layer) which is connected to ground. Similarly, a power plane is connected to a power supply voltage. But since there are usually many bypass/decoupling capacitors between power and ground, the distinction between power and ground has no meaning for AC signals. AC signals can travel on either type of plane. This is the main reason why usually the discussions are placed around GND planes. Another reason is that in some cases the PWR plane does not exist (only a PWR structure based on tracks) but the GND plane is mandatory. For example, in high speed PCB projects, it is usually desirable that the plane be as solid as possible. Vias and through holes unavoidably put holes in the planes, but these effects are usually (but not always!) minor. Only slots and voids in planes can cause real problems and should be avoided.

 

The importance of maintaining a low impedance large area ground plane is critical to all analog and digital circuits today. The ground plane is a structure which not only acts as a low impedance return path for decoupling high frequency currents (caused by fast digital logic) but also minimizes EMI/RFI emissions. Because of the shielding action of the ground plane, the susceptibility of circuits to external EMI/RFI is also reduced. Ground planes also allow the transmission of high speed digital or analog signals using transmission line techniques (microstrip line or stripline) where controlled impedances are required.