High Voltage Creepage and Clearance on PCB Design

Spacing on a high voltage board is not a matter of taste. It is set by the distance the designer leaves between conductors, by the environment the product will live in, and by the standard the product must satisfy. Two distances matter, and they are measured in different ways. This guide explains creepage and clearance, how the numbers are derived, and how to hold them through fabrication.

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 gap straight across a slot is clearance, while a path that follows the surface of the board, around a pad and over a track is creepage.

The distinction matters because air and laminate behave differently. Air breaks down suddenly when the field exceeds its strength, while a surface can track progressively as contamination and moisture collect along it. Breakdown along a surface is usually the slower and more dangerous failure, which is why creepage limits are often more generous than clearance limits at high voltage.

How Standards Set the Numbers

For most products the starting point is a safety standard such as IEC 62368 or IEC 60664, which gives tables of minimum distances indexed by the working voltage, the pollution degree of the environment and the material group of the board. The designer looks up the row for the voltage and the column for the conditions, then applies the appropriate measurement.

Two quantities are usually derived first. The working voltage is the steady state voltage the insulation must withstand in normal operation, and the transient or impulse voltage is what it must survive during a surge. Clearance is generally set by the impulse withstand requirement, while creepage is set by the working voltage and the surface conditions. Our pcb design release checklist lists the spacing data that should be recorded before release.

Working Voltage, Pollution Degree and Material Group

Pollution degree describes how conductive contamination can become. Degree one assumes a clean, controlled environment, degree two covers ordinary equipment where only non conductive pollution occurs, and degree three covers conductive pollution or condensation. The higher the degree, the larger the creepage distance required for the same working voltage.

Material group describes how readily the laminate forms a conductive track when contaminated. Boards with better comparative tracking index values belong to a lower group number and are permitted smaller creepage. The group is a property of the laminate and the surface finish, so it is a material choice as much as a layout choice. Our notes on pcb laminate material properties describe how these values are specified.

Slotting and Cutouts as a Design Tool

A slot milled through the board interrupts the surface path, so the creepage distance no longer follows the board face but has to travel around the slot. If the slot is wide enough and the wall is not conductive, the creepage across it becomes the sum of the two surface paths plus the air gap, which can be far greater than the original spacing.

Slotting is a common way to reach a required creepage without consuming a large area of the board. The slot has to be routed, and its edges and any burrs count as part of the conductive geometry if they are coated with copper. Where the slot is also used to increase clearance, the cut must pass fully through, because a blind pocket does not interrupt the surface path.

Coating, Potting and Conformal Coat

A conformal coating can reduce the creepage requirement, provided it is applied to a controlled thickness, it adheres well and it is qualified as an insulating layer under the standard being applied. A thin, uneven coat that traps air at the edges is worse than no coating, because the air voids concentrate the field.

Potting goes further and can remove the surface path entirely where the compound fills the enclosure. The gain is only realised if the encapsulation is void free and if the material is specified for the temperature and the voltage. Where a coating is used to justify a reduced distance, the process must be documented and inspected, not assumed.

Component Placement and Pad Geometry

The distance that matters is between conductive parts, not between component outlines. A pad with a large annular ring reduces the gap, and a copper pour connected to one side of the isolation moves the effective boundary outward. Keep pours and planes clear of the isolation zone, and terminate them before the barrier rather than routing them up to it.

Components themselves affect the path. A resistor bridging the barrier carries its own voltage rating, and an optocoupler or a transformer carries an internal isolation distance that must exceed the requirement. Where a component body sits across the barrier, its pin spacing, its internal creepage and its package outline all become part of the calculation.

Isolation Barriers and Transformer Design

An isolation barrier is a deliberate band across the board where no copper crosses, no plane extends and no component is placed unless it is an isolating device. The barrier should be wide enough for the creepage requirement, it should be visible in the layout, and its boundary should be marked on the fabrication drawing so that it is not compromised by a late change.

The barrier is also a good place to add a slot, to route the two sides on different layers, and to keep the return currents separated so that noise does not couple across the isolation capacitance. Where the two sides share a connector, the pin order should be arranged so that the barrier runs cleanly between them rather than being broken by an interleaved pin assignment.

Verification and Testing

Verification starts with measurement on the final artwork. The spacing should be checked with a design rule that reports the minimum creepage and clearance for each net pair that crosses the barrier, using the actual pad outlines rather than the component courtyards. Manual measurement of a few critical points is a useful cross check but is not a substitute.

On the finished product a dielectric withstand test applies a specified voltage between the two sides for a specified time and confirms that no breakdown occurs. The test voltage is higher than the working voltage and is applied to every unit or to a sample, depending on the standard. The test point and the method should be defined early because they affect the layout.

Documentation and Fabrication Notes

The fabrication drawing should state the required creepage and clearance values, the working voltage and pollution degree they were derived from, the location of any slot or cutout, the surface finish and the coating specification. Where the creepage is achieved by a coating or a slot, that reasoning belongs on the drawing so that the fabricator does not treat it as cosmetic.

Spacing also interacts with cost, because a large isolation zone consumes board area. Where the product allows it, a better laminate or a controlled coating can permit a tighter layout and reduce the panel size, which is worth evaluating during the layout rather than afterwards. The trade off is discussed in our notes on pcb design manufacturing cost reduction, and the release data is covered in our pcb fabrication notes checklist.

FAQ

Is creepage always larger than clearance? Not always. At low working voltages the two requirements can be similar or the clearance can dominate, because clearance is set by the impulse withstand voltage. As the working voltage and the pollution degree rise, creepage usually becomes the larger of the two.

Does a conformal coating remove the creepage requirement? Not by itself. A coating only reduces the required distance when it is a qualified insulating layer applied to a controlled thickness and free of voids. A general purpose coating applied for moisture protection does not change the spacing calculation.

Can I measure creepage through a slot? Yes, the creepage path follows the surface and then the air gap across the slot if the wall is not conductive. The slot must pass fully through the board and must be clean, because a burr or a smear of copper would restore the original surface path.