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Beyond Design: The Proximity Effect

Skin effect and the proximity effect are manifestations of the same principle—magnetic lines of flux cannot penetrate a good conductor. The difference between them is that skin effect is a reaction to the magnetic fields generated by current flowing within a conductor, while proximity effect is generated by current flowing in other nearby traces or planes. The frequency at which both effects begin to occur is the same. In this month’s column, I will focus on the proximity effect. Please see “Beyond Design: Effects of Surface Roughness on High-speed PCBs” for further information on skin effect.

The Proximity Effect

In multilayer PCBs, these effects start to take hold at rather low frequencies on the order of ~30 MHz. Below that frequency, due to changing currents in the traces, the magnetic forces are too small to influence the pattern of current flow. In a low-frequency or DC circuit, the return current takes the path of least resistance filling the entire cross-sectional area of the trace. As it returns to the source via the power/ground planes, this current tends to spread throughout the wide, flat sheet of copper. However, as the frequency increases, the magnetic forces surrounding a trace become significant and the return current takes the path of least inductance. This high-frequency distribution follows a tight path directly above and/or below the trace in the reference plane(s).

As represented in Figure 1, magnetic fields distribute current to a shallow depth around the perimeter of the trace (red), increasing the apparent resistance of the trace; this is the skin effect. The magnetic fields also distribute current around the perimeter of the trace in a non-uniform manner when referenced to a plane; this is the proximity effect. This draws current toward the side of the trace facing the reference plane and forms the return current into a narrow band directly above and/or below the trace. Figure 2 shows microstrip return current density. In an asymmetric stripline configuration, the proximity effect draws current in an uneven distribution towards the near and far reference planes.

To read this entire column, which appeared in the March 2019 issue of Design007 Magazine, click here.