Creepage distance is the shortest path between two conductive parts measured along the surface of the insulation. An adequate creepage distance protects against tracking, a process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of the electric discharges on or close to an insulation surface.

The degree of tracking occurring depends on the comparative tracking index (CTI) of the material and the degree of pollution in the environment. Used for electrical insulating materials, the CTI provides a numerical value of the voltage that will cause failure by tracking during standard testing. IEC 112 provides a fuller explanation of tracking and CTI.

Tracking damaging the insulating material normally occurs because of one or more of the following reasons: humidity in the atmosphere, presence of contamination, corrosive chemicals, and altitude at which equipment is to be operated.

As isolation voltage levels continue to rise, it is more important than ever to have a robust PCB design that not only reduces electromagnetic interference emissions, but also reduces creepage problems. In addition to wide isolator packaging, techniques such as grooves can be used to attain a desired creepage distance (see Figure 4-2).

Figure 4-2 Groove Cutting Extends Effective Creepage Distance

For a groove (>1 mm wide), the only depth requirement is that the existing creepage distance plus the width of the groove and twice the depth of the groove must equal or exceed the required creepage distance. The groove must not weaken the substrate to a point that it fails to meet mechanical test requirements.

Also, on all layers keep the space under the isolator free from traces, vias, and pads to maintain maximum creepage distance (see Figure 4-1).