The power and ground planes of a high-speed PCB design usually must satisfy a variety of requirements.

At dc and low frequencies, they must deliver stable reference voltages, such as V_CC and ground, to the supply terminals of integrated circuits (IC).

At high frequencies reference planes, and in particular ground planes, serve numerous purposes. For the design of controlled impedance transmission systems, the ground plane must provide strong electric coupling with the signal traces of an adjacent signal layer.

Consider a single, ac-carrying conductor with its associated electric and magnetic fields, shown in Figure 4-6. Loose or no electric coupling allows the transversal electromagnetic (TEM) wave, created by the current flow, to freely radiate into the outside environment, causing severe electromagnetic interference (EMI).

Figure 4-6 Reducing Field Fringing Through Close Electric Coupling Between Conductors

Now imagine a second conductor in close proximity, carrying a current of equal amplitude but opposite polarity. In this case, the conductors’ opposing magnetic fields cancel, while their electric fields tightly couple. The TEM waves of the two conductors, now being robbed of their magnetic fields, cannot radiate into the environment. Only the far smaller fringing fields might be able to couple outside, thus yielding significantly lower EMI.

Figure 4-7 shows the same effect occurring between a ground plane and a closely coupled signal trace. High-frequency currents follow the path of least inductance, not the path of least impedance. Because the return path of least inductance lies directly under a signal trace, returning signal currents tend to follow this path. The confined flow of return current creates a region of high current density in the ground plane, right below the signal trace. This ground plane region then acts as a single return trace, allowing the magnetic fields to cancel while providing tight electric coupling to the signal trace above.

Figure 4-7 Ground Plane Acting as a Single Return Trace

To provide a continuous, low-impedance path for return currents, reference planes (power and ground planes) must be of solid copper sheets and free from voids and crevices. For reference planes, it is important that the clearance sections of vias do not interfere with the path of the return current. In the case of an obstacle, the return current finds its way around it. However, by doing so, the current’s electromagnetic fields will most likely interfere with the fields of other signal traces introducing crosstalk. Moreover, this obstacle adversely affects the impedance of the traces passing over it, thus leading to discontinuities and increased EMI.

Figure 4-8 Return Current Paths in Solid Versus Slotted Ground Planes