Clearance and Creepage on a PCB: Spacing Rules in Practice

Spacing rules for high voltage are not the same as design rules for signal integrity. They are derived from safety standards, they depend on the environment the product will operate in, and they are written to prevent a hazard rather than to make a circuit work.

Clearance and Creepage Are Different

Clearance is the shortest distance through air between two conductors. Creepage is the shortest distance along the surface of the insulation between the same two conductors. Both are measured along the same pair of features and they generally have different limits.

Air breaks down at a voltage that depends on the gap and on the humidity and pressure of the air. A surface contaminates, absorbs moisture and eventually tracks, so its withstand voltage depends on the material and on how clean it stays.

The two constraints interact with layout in different ways. Clearance is reduced by anything that projects between the conductors, including a component body, while creepage is reduced by anything that shortens the surface path, including a slot, a hole or a conductive contamination path.

What Sets the Required Distance

The standards derive a required distance from three inputs: the working voltage, the pollution degree of the environment and the material group of the insulation. The material group describes how readily the surface tracks, and it depends on the comparative tracking index of the laminate.

Pollution degree one describes a sealed environment; degree two a normal environment where only non-conductive pollution occurs; degree three a conductive pollution; degree four persistent conductive pollution. Most industrial electronics is degree two, and outdoor or dirty environments push it upward.

Overvoltage category, altitude and the presence of a transient source all modify the requirement further. Altitude matters because the air is thinner, and a product specified for use above two thousand metres must increase its clearances.

Material Group and the Laminate

The laminate used for the board is itself part of the insulation system, and the resin system determines its tracking index. A standard FR-4 with a high tracking index behaves differently from one filled for thermal performance, and the choice affects the permissible creepage.

Where the creepage requirement is demanding, the practical options are more distance, a better material group, or a coating that protects the surface and restores the property after the product has been in service.

Our laminate material properties notes describe the parameters that matter for insulation as distinct from those that matter for loss.

Layout Techniques That Buy Distance

A slot milled through the board between the two conductors removes the surface path and forces the creepage to follow a longer route around the slot. This can reduce the required board area compared with moving the conductors further apart.

The slot must be wide enough that it is not bridged by contamination, and its edges must be free of copper burrs. A slot in a board that flexes is also a mechanical feature that must be tolerated.

Where a slot is not possible, a routed notch or a raised barrier achieves a similar effect on the surface. The objective in each case is to lengthen the path rather than to increase the distance through air, since the air gap is usually easier to satisfy.

Coating, Potting and Conformal Coating

A conformal coating that covers the surface provides an insulating layer over the creepage path and, in the standards, can reduce the required distance where it is qualified for the purpose. The qualification requires the coating to be continuous, adherent and of a specified thickness over the whole path.

Coverage over the whole path is the difficult part. A coating that stops short of a pad, or that does not fill a corner between a component and the board, leaves an unprotected segment that the standard treats as bare surface.

Our conformal coating notes describe the coverage requirements and the methods used to verify them.

Component Bodies and Internal Gaps

A relay, an optocoupler or a transformer introduces its own internal clearance and creepage, and the component must be rated for the application. The board around the component is then designed to at least match the component rating, so that the component is not the weakest link.

Component bodies also affect the clearance on the board. Two pads that are far apart can have a small clearance if a component body with a conductive surface bridges the space between them, and some standards count the body as part of the path.

Underfilled or potted regions change the path as well, since the potting compound becomes part of the insulation and its properties must be considered.

Marking the Constraint on the Drawing

The required clearance and creepage should be stated on the fabrication or assembly drawing as a requirement with the standard referenced, rather than left to the layout. An inspector then has something to measure.

The measurement is normally made on the finished board using the actual shortest path, which means going around obstructions rather than measuring point to point. A drawing that states point to point distances is not stating the requirement.

Our fabrication notes checklist describes how such requirements are recorded so that they survive the transition from design to production.

Testing and Verification

The dielectric withstand test applies a voltage above the working voltage between the isolated circuits for a defined period and confirms that no breakdown occurs. It is a pass or fail test and it does not measure the margin.

Partial discharge testing goes further and detects the small discharges that precede breakdown, which is the technique used where the insulation must survive for many years. The measurement indicates whether the design has margin rather than whether it survives today.

For a humid environment, the test is performed after conditioning, since a dry board may pass a test that a damp one fails. Our industrial assembly notes describe the conditioning regimes used for control equipment.

Process Control and Verification

On a design of this kind, dielectric withstand is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Process Control and Verification

On a design of this kind, dielectric withstand is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

FAQ

Do I need to calculate these distances myself? The requirement comes from the applicable standard, and the calculation is a lookup once the working voltage, pollution degree and material group are known. The designer’s task is to give the layout enough room.

Does a slot always help? It shortens the surface path only if it is placed between the two conductors and is wide enough not to be bridged. A slot in the wrong place changes nothing.

What does gopcb provide for high voltage boards? We provide laminate options with their tracking data, slot routing to gain creepage, coating processes with coverage verification, dielectric withstand and partial discharge testing, and the documentation that records the requirement alongside the measurement.