Creepage and Clearance Rules on Printed Circuit Boards

Creepage and clearance are the two distances that keep a board from arcing, and they are among the most frequently missed requirements in a design review. They are not the same thing, they are not interchangeable with conductor width, and they depend on the environment the product will operate in. This article explains how the two distances are defined and how to apply the rules without over-designing the board.

Creepage and Clearance Defined

Clearance is the shortest distance through air between two conductive parts. Creepage is the shortest distance along the surface of the insulating material between the same two parts. A straight gap between two pads has equal clearance and creepage, but as soon as a slot, a groove or a component body lies between them, the two numbers diverge.

The distinction matters because air and surface break down differently. Air can withstand a higher voltage across a short gap, while a contaminated surface can conduct at a much lower voltage than the air path would suggest, an effect that overlaps with noise immunity in mixed signal designs. Creepage is therefore usually the tighter constraint on a dirty or humid product.

Voltage, Pollution Degree and Material Group

The required distances come from tables that take three inputs: the working voltage, the pollution degree of the environment and the material group of the laminate. Pollution degree describes the expected contamination, from a clean, controlled enclosure through to conductive dust or condensation.

Material group describes how the laminate behaves when contaminated and moist, based on its comparative tracking index. Standard FR-4 usually falls into a mid range group, while high performance laminates can offer better tracking resistance and therefore allow shorter creepage for the same voltage.

Choosing the wrong pollution degree is the most common error. A product that will sit in an industrial panel with conductive dust is not the same as one in a sealed office enclosure, even if the circuit is identical.

Working Voltage and Transient Overvoltage

The working voltage is the root mean square voltage present in normal operation, but it is not the only figure that matters. Transient overvoltages, driven by switching or by the mains supply category, can be several times higher, and the insulation must withstand them without breakdown.

For mains connected products, the required clearance is often set by the transient level rather than by the working voltage. Reducing clearance because the working voltage is low ignores the event that actually causes failure, and a design that passes bench testing can still fail during a surge.

Slots, Grooves and Barriers

A slot cut through the board interrupts the surface path, so the creepage distance is measured around the slot rather than across it. That is why slots appear under optocouplers and around mains terminals: they buy distance without consuming board area.

Ribs and barriers raise the surface out of a plane and lengthen the path in a similar way. Both techniques are effective only if the geometry is wide enough to be manufactured and if the surface finish inside the slot is adequate, since a rough routed edge can trap contamination.

Coating and Potting as a Substitute

A qualified conformal coating can reduce the required creepage, because the coating excludes contamination and moisture from the surface. The reduction is only available when the coating is specified, applied and inspected to the standard the table assumes, including coverage of the relevant surfaces.

Potting goes further and effectively removes the surface path from consideration inside the encapsulation. The trade-off is thermal and mechanical, since the potting compound changes the expansion behaviour of the assembly and makes rework impractical.

Component Leads and Terminal Spacing

Distances are not only a layout concern. A connector or a relay has its own terminal spacing, and a component that does not meet the required clearance cannot be fixed by the board design around it. Component selection therefore has to respect the same rules.

Where a part must be used with inadequate spacing, the usual remedies are a slot, a coating or a change of pollution degree assumption through a sealed enclosure. Documenting which remedy applies keeps the compliance argument traceable.

Inner Layers, Vias and Coated Surfaces

Inner layers are laminated in resin, so the surface path along an inner layer is usually protected and the dominant limit becomes the clearance through the dielectric. That is why high voltage designs often route the critical spacing on an inner layer rather than on the surface.

Vias complicate the picture because they bring conductor to the surface. A via within a high voltage area has to be assessed like any other surface feature, and the solder mask over it does not count as insulation unless the coating has been qualified for that purpose.

Verifying Spacing in Layout

Design rule checks can verify spacing if the rules are entered correctly, but the check only covers what the tool understands. Component bodies, mounting hardware, heatsinks and connector shells all sit in the gap after assembly, and they have to be considered separately.

A practical review that follows the design release checklist lists the nets at different potentials, groups them by voltage class and then checks the distance between each pair with the component bodies in place. That exercise is usually done once and then maintained as the design evolves.

Documenting the Requirement

The drawing should state the pollution degree assumed, the material group, the working voltage and the standard used, and it should identify any area where a slot or a coating is required for compliance. Assembly notes should reflect the same assumptions, so the coating is not omitted by a supplier who never saw the analysis.

gopcb asks customers to confirm the pollution degree and voltage class when a design includes mains or high voltage circuitry, because the fabrication choices follow from those values. Getting them onto the drawing once prevents a compliance problem at certification and a costly re-spin afterwards, and it gives the quality record a defensible basis.

Points to Confirm at First Article

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

FAQ

Is creepage always larger than clearance? Not always, but on a contaminated surface creepage is usually the binding constraint. The two are calculated separately and the larger requirement governs.

Does solder mask count as insulation? Generally no, unless the coating has been qualified to the standard being applied and its coverage is verified. Bare laminate and bare copper both count as conductive surfaces for spacing purposes.

Can I reduce spacings with a slot? Yes, a slot interrupts the surface path and increases creepage. It must be manufacturable, and the routed edge should be clean enough not to trap contamination.