Creepage And Clearance Rules For High Voltage PCBs

A high voltage board has to keep two conductors apart for two different reasons, and the distances are not the same. The clearance is the shortest distance through the air between two conductors, and it is set by the voltage at which the air breaks down. The creepage is the shortest distance along the surface of the board, and it is set by the voltage at which a contaminated and damp surface starts to track and carbonise. A design that satisfies one and not the other will fail, and the failure may be a flashover in dry air or a slow burn across the surface.

This article explains the two distances, the parameters that set them, the layout techniques that gain the required separation, and how the result is verified.

Why The Two Distances Differ

The clearance depends on the dielectric strength of air, which is roughly three kilovolts per millimetre in a uniform field at sea level. The figure falls sharply when the field is non uniform, as it is at the point of a conductor or at a sharp edge, and it also falls at high altitude where the air is thinner. For a given voltage the clearance is therefore a function of the geometry and of the environment rather than of the board material.

The creepage depends on the surface. A clean and dry surface has a high resistance and can hold a large voltage, while a surface that is wet and covered with dust or salt develops a leakage current that dries the surface unevenly, creates a local hot spot, and eventually carbonises a conductive track. The resistance of a material to that process is measured by its comparative tracking index, and the required creepage grows as the material becomes less resistant and as the pollution becomes more severe.

The Parameters That Set The Numbers

The working voltage is the first input, and the value used is the peak rather than the average. A rectified mains circuit that produces three hundred volts of direct current has a higher peak than its average suggests, and a circuit with switching transients has a higher peak still. The impulse withstand requirement, which comes from the overvoltage category, often produces the largest clearance figure because it is set by a transient rather than by the steady state.

The pollution degree and the material group complete the picture. A pollution degree of one describes a sealed and clean environment, degree two a typical office or laboratory, degree three an industrial environment with condensation, and degree four outdoor weather. The material group is derived from the tracking index of the laminate, with the highest group requiring the shortest creepage for a given voltage. Altitude correction has to be applied to the clearance, and the correction becomes significant above about two thousand metres.

Layout Techniques That Gain Distance

A slot in the board interrupts the surface path. The creepage is measured along the contour of the material, so a routed slot forces the path to travel around it and to pass through the air of the slot, which is a clearance rather than a creepage requirement. This is the reason a small slot is often cheaper than moving components, and the reason a gap under an optocoupler or a relay is routed rather than simply left as a wide spacing.

Coating is the other technique, with an important limitation. A conformal coating that has been qualified for the purpose, applied over a clean surface with a specified thickness and coverage, allows the creepage requirement to be reduced, because the coating prevents the surface from becoming contaminated and damp. A general production coating that is applied without that qualification does not gain the reduction, and a coating over a contaminated gap can trap the contamination in place. The coating process is described under board level protection.

The Component Interface

The separation has to be maintained through the components as well as across the board. An optocoupler, a relay, or an isolating transformer provides a barrier of its own, and the board spacing has to be consistent with the internal distances of that part, otherwise the barrier is defeated by the layout around it. A device with a specified creepage and clearance under its body, such as a relay with a wide footprint, is usually accompanied by a slot under the body in the recommended layout.

Components that bridge the barrier need particular attention. A resistor that crosses from the high voltage side to the low voltage side has a body length that sets its own clearance, and a capacitor that crosses the barrier has to be a safety rated part with the appropriate class. Fuses, varistors, and discharge paths also sit across the barrier by design, and each has a specified spacing that the layout has to respect.

Common Errors

The most common error is to measure the distances from the pad rather than from the conductor. The conductor under the mask is still a conductor, and its edge is where the field is highest, so the spacing is measured from the copper rather than from the opening in the mask. A second error is to count the soldermask as a barrier, which it is not unless it is a qualified coating applied to the specified standard.

A third is to ignore the effect of the finish, of a via, or of a component body on the path. A via that sits in the gap is a point where the copper comes closer to the surface, and a metal component body that approaches the conductor reduces the clearance to the body rather than to the conductor. A fourth is to design for the pollution degree of a clean laboratory when the product will be installed in a dusty plant, which is the error that turns a compliant design into a field failure.

Verification And Documentation

The distances are calculated at the design stage, documented on the drawing, and checked on the finished board. A dielectric withstand test applies a high voltage between the isolated circuits for a defined period and confirms that no breakdown occurs, and an impulse test reproduces the transient that the overvoltage category requires. Neither test measures the creepage, so the drawing remains the primary record and the layout the primary evidence.

The relevant separation rules and the spacing between ordinary conductors on the same net are different requirements, and both apply. The rules for a via that is close to a neighbouring trace are described under via to trace clearance, and the plane arrangement that keeps a high voltage rail separated from the rest of the board under power plane splitting.

FAQ

Does soldermask count as insulation? Not for creepage purposes unless it is a coating qualified for that role, applied over a clean surface to a specified thickness. Ordinary mask is not a barrier against tracking.

How does a slot help? It forces the creepage path to travel around the opening, which lengthens it at almost no cost in board area. It also converts part of the path into a clearance through the air of the slot.

Why is the peak voltage used rather than the average? Because breakdown occurs at the instantaneous peak. A rectified or switched waveform reaches a higher peak than its average value suggests, and the transients on top of it are higher still.