Dielectric Strength and Breakdown Voltage in PCB Insulation

Insulation on a printed circuit board is a thin sheet of material that must hold off a voltage for the life of the product, and the numbers that describe it are frequently confused with one another. Dielectric strength is a material property, breakdown voltage is a property of a specific gap, and creepage distance is a layout decision. This article separates the three and shows how they translate into design rules for high voltage boards.

Dielectric Strength, Breakdown Voltage, and What They Mean

Dielectric strength is the electric field at which an insulating material fails, expressed in kilovolts per millimetre. FR-4 typically falls between 15 and 25 kV per millimetre for a thin sample under short-term test, but the value is thickness dependent and drops as the sample gets thicker or the test duration increases. It is a material characterisation number, not a figure that can be multiplied by a layer thickness to obtain a safe working voltage.

Breakdown voltage is what a designer actually cares about: the voltage at which a specific gap, in a specific stackup, with a specific surface condition, will conduct. It depends on the layer thickness, the geometry of the electrodes, the presence of voids, the humidity, and the frequency of the applied voltage. A 100 micron layer of FR-4 does not automatically hold 2 kV, and a datasheet value of 20 kV per millimetre should be treated as a starting point for testing rather than as a design guarantee.

How Dielectric Failure Happens

Failure begins with a local defect. A void, a fibre rich region with low resin content, a conductive filament, or a sharp electrode edge concentrates the electric field. Once the field in that region exceeds the local strength, a partial discharge occurs, and the resulting heat and chemical damage widen the weak point until a conducting path forms. At high frequency the same mechanism proceeds faster because the loss heating is higher.

Time is part of the mechanism. A short duration test measures a higher strength than a long one, because the damage takes time to develop. That is why qualification programmes use a voltage well below the short-term breakdown figure, and why the working voltage in a specification is generally a small fraction of the tested limit. Partial discharge measurement is far more sensitive than a go or no-go breakdown test and is the preferred method for safety critical designs.

Creepage Distance and Clearance

Creepage distance is the shortest path along the surface between two conductors, while clearance is the shortest path through air. Both are limited by layout rather than by material, and both appear in safety standards such as IEC 60664 and its derivatives. The required distance rises with the working voltage, with pollution degree, and with altitude, because thinner air breaks down at a lower voltage.

Routing high voltage conductors means giving the surface path the space it needs. A slot milled through the board is the standard way to interrupt a creepage path, because it forces any surface tracking to travel around the slot and increases the effective distance. Conformal coating also helps, since it covers the surface with a material that resists tracking and blocks moisture and contamination from reaching the interface.

Where a design combines high voltage with high frequency, as in a high speed layout, the isolation distances and the impedance requirements have to be satisfied together, and the tighter of the two constraints normally governs the spacing.

Material Selection for High Voltage Boards

Standard FR-4 is adequate for many mains-referenced designs, but its properties vary widely between suppliers. For higher voltages, materials with higher glass transition temperature, lower moisture absorption, and controlled filler content are preferred because they hold their insulation properties better after thermal cycling. Where tracking resistance matters more than bulk breakdown, the comparative tracking index of the laminate is the relevant figure.

Thicker dielectric is not automatically better. A thicker layer reduces capacitance and raises breakdown voltage, but it also changes impedance, reduces layer count efficiency, and can make drilling and plating harder. The usual approach is to keep the dielectric thickness moderate and gain the required isolation through distance, slots, and coating rather than through brute force thickness.

Where an application also demands thermal performance or a specific dielectric constant, the material choice becomes a compromise. The controlled impedance routing requirements and the isolation requirements should be evaluated on the same stackup rather than one after the other.

Testing and Qualification

Short term dielectric strength testing applies a rising voltage to a test pattern until breakdown, typically with an electrode arrangement defined by the method. It is destructive, fast, and useful for material comparison, but it does not predict long term behaviour. Withstand tests apply a fixed voltage for a defined period to verify that a design meets a requirement, and they are normally performed on production samples rather than on coupons alone.

Partial discharge testing applies a voltage below the breakdown level and measures the small discharges that precede failure. It detects voids and weak points that a withstand test passes, which makes it valuable for boards that will operate continuously at high voltage. The setup requires a low noise environment and careful calibration, so it is usually performed at a specialist laboratory rather than on the production floor.

Test patterns matter as much as the voltage source. A pattern with sharp copper corners breaks down at a lower voltage than one with rounded edges, and a pattern with a slot behaves differently again, so the coupon should reproduce the geometry of the production board. Where a design uses several isolation distances, each should appear on the coupon so that the weakest one is found before the board is qualified.

Design Rules for High Voltage Layout

Design rules for high voltage work are mostly about distances and edges. Keep conductors of different potentials separated by at least the creepage distance required by the applicable standard, round or avoid sharp copper points where the field concentrates, and add a slot when the available surface distance is marginal. Where a conductor passes between two layers, remember that the through-thickness distance is the sum of the dielectric thicknesses.

Component placement deserves the same care as copper geometry. A connector, a relay, or an optocoupler has its own internal isolation figure, and the board must not undermine it by routing a high voltage trace close to a low voltage pin. Where an isolated supply is used, the isolation barrier should be a deliberate line on the layout with no traces crossing it, and any cross-barrier component should be placed on that line.

Document the intent. A note that marks the isolation barrier and states the working voltage and the required creepage distance prevents a later modification from quietly closing a critical gap, and it gives the fabricator the information needed to check the spacing before the board is built.

FAQ

Can I calculate breakdown voltage from dielectric strength? Only as a rough estimate. The real value depends on geometry, voids, moisture, and test duration, so a prototype should be tested at the working voltage with margin rather than assumed to be safe.

Does conformal coating improve dielectric strength? It improves surface performance by blocking moisture and contamination and by resisting tracking, which raises the practical withstand voltage of a given creepage distance, but it does not change the bulk properties of the laminate.

How much margin should a high voltage design keep? Common practice is to design for at least twice the working voltage in an accelerated test and to verify the production board with a withstand test at a level agreed with the safety authority.