High Voltage PCB Design: Creepage, Clearance and Materials

Above 500 V, the Design Rules Change

Most PCB guidance assumes that the limiting factors are trace geometry, impedance and layer count. Push the working voltage past roughly 500 volts and those concerns move to the background. The limiting factors become creepage distance, clearance, the tracking resistance of the laminate, insulation coordination across the whole structure, and the management of electric field concentration. In power modules, inverters, X-ray equipment, EV charging hardware and industrial power control, working voltages commonly sit between one and twenty kilovolts, and the consequence of a design or process mistake is not degraded performance but breakdown, arcing, tracking or a safety incident.

This guide walks through the design rules, material choices, layout practice, stackup insulation, manufacturing requirements, testing and assembly considerations that keep a high voltage board reliable over a long service life.

The Four Electrical Principles

Creepage is the shortest distance along the surface of the board between two conductive features. It matters because contamination and moisture form a conductive path along that surface, and tracking failures develop there.

Clearance is the shortest distance through air between the same two features. It governs whether an arc can jump the gap.

Comparative Tracking Index (CTI) describes how resistant a laminate is to forming a conductive track when contamination and voltage act together. It is a material property, and it is the reason ordinary FR-4 is a poor choice at high voltage.

Insulation coordination is the system-level planning that ties the above together with the transient overvoltages the product will see, the pollution degree of the environment and the required safety margin.

Indicative design values, which vary with pollution degree and the standard applied, give a sense of the geometry involved. At one kilovolt, minimum clearance is roughly 1.5 to 2.0 mm and creepage on FR-4 roughly 3.2 to 4.0 mm. At five kilovolts, clearance rises to about 6 to 8 mm and creepage to 10 to 12 mm. At ten kilovolts, clearance reaches roughly 12 to 15 mm with comparable creepage. Those numbers explain why high voltage boards are physically larger, more sparsely laid out and built from different materials than a conventional design of the same circuit complexity.

Material Selection: CTI Decides

Standard FR-4 tracks, carbonises and eventually breaks down under sustained high voltage and contamination. The material set for high voltage work is therefore selected for tracking resistance and dielectric strength together. High CTI FR-4, typically specified at a CTI of 600 or above, is the workhorse for boards up to a few kilovolts. Ceramic substrates handle extreme voltage and temperature together, and are common in power conversion. PTFE and other low loss fluoropolymers combine high voltage capability with high frequency performance. Polyimide brings thermal and dielectric stability where the environment is harsh. Heavy copper combined with thicker dielectric layers increases both current capability and voltage withstand.

Material choice interacts directly with design geometry, because a higher CTI laminate permits shorter creepage for the same voltage. That is why material selection is part of the electrical design, not a purchasing decision made after layout.

Layout Rules That Prevent Arcing

Layout errors are the leading cause of high voltage arcing. Spacing must be scaled to the working voltage with margin for transients, and the spacing must be maintained consistently along the whole path rather than at the obvious pinch points. Sharp copper corners concentrate the electric field and should be rounded or chamfered. Where spacing alone becomes impractical, a routed slot, isolation groove or physical barrier is far more effective than additional surface distance. High voltage and low voltage regions should be physically separated rather than interleaved. High voltage traces should be kept away from board edges, where contamination collects and where a surface flashover can reach the enclosure.

Stackup Insulation in Multilayer Boards

Multilayer designs add a vertical dimension to the insulation problem. Dielectric thickness between layers carrying high voltage must be increased beyond standard prepreg build. High voltage copper on adjacent layers should not overlap, because the field between overlapping planes is concentrated across a thin dielectric and can fail internally. Dedicated insulation layers and deliberate layer pairing allow the stackup to be planned as an insulation system rather than an accident of routing. Internal breakdown is particularly dangerous because it is invisible from the outside and may only appear after months of thermal cycling.

Preventing Tracking, Corona and Breakdown

Corona and tracking failures usually have an environmental trigger: process residues left on the surface, flux residue from assembly, accumulated moisture and dust, insufficient creepage, or incomplete solder mask coverage that leaves copper exposed between features. The engineering countermeasures are correspondingly practical. Round the copper edges to soften the field. Apply conformal coating to seal the surface against contamination and humidity. Maintain high cleanliness through fabrication and assembly. Use solder mask dams to separate closely spaced features. And verify all of it with inspection that specifically looks for protrusions, residue and mask integrity.

Special Manufacturing Requirements

High voltage boards cannot be produced on a standard process without modification. Precision routing of slots and isolation grooves requires controlled depth and clean edges, because a rough routed surface is itself a tracking path. Drilling burrs must be tightly controlled, since a burr creates a field concentration point. Cleanliness has to be managed through the entire flow rather than at the end. Surface finish selection favours a chemically stable, flat finish such as immersion silver or ENIG, because a stable finish avoids the organic residues that a hot air solder levelling process can leave. See the notes on immersion silver finishing for how those surfaces behave.

In practice, high voltage programmes run dedicated tooling programmes for slotting, a heightened inspection routine focused on insulation, and documented cleanliness checks. Those steps are what separate a board that passes initial test from one that survives years in the field.

Testing and Inspection

Four test activities are indispensable. Hi-Pot testing applies a voltage typically two to three times the working voltage to verify insulation integrity; it is a destructive-risk test and must be specified with a defined ramp, dwell and trip current. Insulation resistance measurement verifies that leakage stays within limits. Automated optical inspection looks for copper burrs, protrusions and contamination that would create a field concentration. Cross-sectioning verifies dielectric thickness in the stackup against the insulation plan.

These are supplementary to functional test, not a replacement for it. Test coverage should be agreed with the fabricator as part of the specification, because the pass criteria for Hi-Pot are as much a design decision as the geometry.

Common Mistakes

Five failures recur. Using standard FR-4 above five kilovolts. Ignoring the CTI rating when comparing laminates. Omitting slots where the layout is tight. Under-designing insulation in the stackup while over-designing it on the surface. And allowing contamination during assembly. These mistakes frequently do not show up on the bench at all; the board works, passes test, ships, and then fails months later in a humid environment.

Assembly Considerations

A large share of high voltage failures originate in assembly rather than in fabrication. Component lead spacing must satisfy creepage requirements, which constrains package selection. Flux must be completely removed, because residue is both conductive and hygroscopic. Conformal coating or potting is often mandatory rather than optional, and coverage must extend over the areas where the field concentrates. Solder mask integrity has to survive the assembly process intact.

Because of this, high voltage assemblies should be built by a team that understands insulation requirements rather than by a general purpose line. The conformal coating process deserves specific attention, since it is doing a disproportionate share of the work in a humid enclosure.

Typical Applications

Switching power supplies and power modules, inverters and converters, medical X-ray and imaging equipment, EV charging systems, industrial power control, and aerospace power electronics all share the same requirement: insulation that holds for decades. Medical imaging adds regulatory scrutiny to that list, which is why boards for those systems are usually developed with medical electronics quality expectations alongside the electrical requirements.

Cost Bands

Cost is driven by material, spacing design, slotting and test requirements. As a reference, a two layer board in high CTI FR-4 rated to one kilovolt sits in the range of roughly 80 to 150 US dollars for prototype quantities and 8 to 15 dollars per board in production. A four layer board with slotting rated to five kilovolts runs about 300 to 600 dollars for prototypes and 25 to 45 dollars in production. A six layer board on ceramic or PTFE rated to ten kilovolts runs roughly 800 to 1,500 dollars for prototypes and 90 to 160 dollars in production. Slotting, special materials, Hi-Pot testing and high cleanliness all add cost, and those are precisely the items that should not be trimmed.

Selecting a Manufacturer

Five capabilities decide the shortlist. Demonstrated high voltage project experience. Hi-Pot and insulation resistance test capability in house. Precision slotting and routing competence. Access to a high CTI material supply chain with consistent availability. And the certification framework, IPC, UL and ISO, that the end product’s safety approvals will depend on. Power and energy designs often also need heavy copper capability, which pairs naturally with high voltage work; our energy and power electronics page describes how those two requirement sets combine.

Questions Engineers Ask

What creepage does five kilovolts require? Typically 10 to 12 mm, but the correct figure depends on pollution degree, material CTI and the applicable standard.

Can FR-4 be used at high voltage? Yes, provided it is a high CTI grade and the layout uses adequate spacing and slotting.

What is Hi-Pot testing? A dielectric withstand test at two to three times the working voltage, applied to verify insulation integrity.

Why do high voltage boards arc? Almost always from insufficient creepage, contamination, sharp copper geometry or the wrong laminate.

Conclusion

Reliability at high voltage does not come from layout alone. It comes from material science, disciplined spacing rules, special fabrication processes such as controlled slotting, cleanliness control through the whole flow, and assembly practice that respects insulation requirements. Design and manufacturing have to be planned together, because the choices that make a high voltage board safe are spread across all of them.