What is the significance of Fringe Effect in PCB?

Before going to describe about the fringe effect in PCB, let us understand about the fringe effect. Fringe Effect is something seen with the capacitors where the electric field extends beyond the capacitor parallel plates. We know that changing voltage across (potential) capacitor enables the current to flow through it. Hence, the electric field is developed between the plates. The electric field extends beyond the plates resulting in fringe effect. The fringe field indicates that there is some more additional capacitance that associated with the capacitor. If this statement is confusing, let us explain with a formula:

The above formula indicates that the capacitance is directly proportional to the distance between the capacitor plates. As the field has increased beyond plates, it means in practical are is more than theoretical formula and it also indicates that in reality the capacitance is slightly higher than the theoretically calculated value. Now,let us assume we combined two parallel capacitor plates (C1, C2), together, the equivalent capacitance is, C1+C2. However, when the area has further increased, the fringing field gets reduced and hence the actual capacitance is less than the theoretically calculated value.

As per above description, we have seen two scenarios:

• Fringe Field Capacitance caused capacitance to be higher than the theoretical value
• Fringe Field Capacitance caused capacitance to be lower than the theoretical value

Now, let us look at the case of a PCB. A trace laid out on top layer of a PCB has a ground layer (reference layer) in parallel to it. We all know that if forms a capacitance. In PCB, when current passes through, there shall be electric and magnetic fields developed and do cause fringing effect similar to the once in capacitors. Similar to the capacitors the fringing field effect gets reduced as the length of the trace is increased. So, the increase in capacitance of the trace with trace length is not seen to vary as expected. For example, doubling the capacitance of the trace when trace length is doubled.

How does fringe fields effect play a role for traces routed at the edge of the board?

When a trace is routed at the edge of the board, especially the a fast transitioning edge, and let us assume there are cables with improper termination running around the board. The fringing filed due to the traces can couple to those traces which is not a desirable scenario.