High Voltage PCB Material Selection: Key Parameters

Below about 48 volts a circuit board is mostly an electrical problem. Above a few hundred volts it becomes an insulation problem as well, and the material stops being a neutral carrier of traces. A high voltage PCB has to hold off the working voltage for years in the presence of heat, humidity and contamination, and the parameters that decide whether it can do that are not the ones a low voltage design is used to considering.

Dielectric Strength and Breakdown

Dielectric strength is the field a material can withstand before it breaks down, expressed as a voltage per unit thickness. It sets the thickness needed between two conductors at different potentials and, together with the geometry, defines whether the laminate or the air gap fails first. Thicker material raises the withstand voltage, but only up to a point, because the field concentration at an edge or a void matters more than the bulk property.

The practical consequence is that voltage rating cannot be bought with board thickness alone. A void in the laminate, a sharp copper edge, a conductive path left by flux residue or a fibre path wetted by moisture will all break down far below the nominal strength of the material. That is why high voltage designs are judged on the whole construction rather than on one datasheet number.

Tracking and the CTI Value

The comparative tracking index, usually written CTI, measures how resistant a material is to the formation of a conductive carbon path across its surface when contamination and moisture are present. It is expressed as a voltage, and the bands matter: above 600 volts the material is considered strong, 400 to 600 is good, and below 250 is ordinary.

CTI sets the creepage distance the layout needs, so a higher index allows a tighter board. That makes it an economic parameter as well as a safety one, particularly in power converters and inverter drives where isolating two circuit domains on a small board is otherwise impossible. Where a material has a high CTI, the surface path can be shorter for the same safety margin, and the layout gains usable area.

Thermal and Moisture Behaviour

Temperature and insulation are linked. The glass transition temperature determines when the resin softens and its mechanical and electrical properties begin to drift, and a laminate operating above that point will not hold its dimensional or insulation performance reliably. Industrial designs commonly specify a value of 170 degrees Celsius or higher, automotive 180 to 200, and aerospace more still.

Moisture absorption is the second environmental factor and often the decisive one. Absorbed water lowers insulation resistance, increases leakage current and reduces withstand voltage, and it also creates the conditions for electrochemical migration between conductors. A low absorption material, combined with a design that keeps the surface clean and a coating that seals it, is what makes a high voltage board survive a humid service environment, and the protection options are covered in potting and conformal protection.

Base Materials Compared

A high glass transition FR4 remains the volume answer for industrial equipment in the 300 to 1000 volt range. It is inexpensive, well understood by every fabricator, mechanically strong and available with improved tracking performance, and where the layout has room for the required creepage it does the job reliably. Above that, or where heat has to be removed as well as blocked, the material changes.

Ceramic substrates offer the highest insulation and thermal performance together, with a coefficient of thermal expansion that matches a power die and no organic resin to degrade. They are the default for insulated gate bipolar transistor modules and for controllers that combine high voltage with high dissipation, and they cost accordingly. PTFE laminates combine high voltage capability with very low loss, which is why they appear in high power radio frequency equipment such as radar supplies and microwave amplifiers.

Polyimide covers the high temperature and flexible cases, with good dielectric behaviour and resistance to chemicals, and metal core boards address the thermal problem by putting an aluminium or copper plate behind the dielectric. Each material solves a different combination of the same three constraints, so the selection follows from which constraint is binding.

Design Measures for High Voltage

Creepage and clearance are the first calculation, and they are not the same thing. Clearance is the shortest path through air between two conductors; creepage is the distance measured along the surface. Humidity and contamination affect creepage far more than clearance, so polluted or wet environments demand longer surface paths, and a routed slot through the board interrupts the surface path and effectively lengthens it.

Where the voltage between two regions justifies it, an isolation slot milled through the laminate is a common and effective measure, and the routing rules for slots are the same as any other edge feature, as described in slot and edge routing rules. Rounded copper edges, a generous distance from the board edge and the absence of sharp points in the high field region all reduce the local field strength that initiates a breakdown. Where a conductor has to carry both a high voltage and a significant current, the width follows the usual thermal calculation, as set out in trace width and current.

<img src="https://www.gopcba.com/wp-content/uploads/2026/09/190-1.jpg" alt="Dielectric withstand and partial discharge test on a power PCB” />

Qualification and Testing

A high voltage board is verified by test rather than by calculation alone. A dielectric withstand test applies an elevated voltage between the isolated domains and confirms that no breakdown occurs, insulation resistance measurement confirms that leakage is low, and partial discharge testing detects small internal discharges that would eventually erode the insulation. Partial discharge is the most sensitive of the three, because a defect can pass a withstand test and still fail after a period in service.

The tests are usually applied to coupons from the same panel as the production boards, and on critical assemblies to the board itself. Thermal cycling and damp heat exposures are added to reproduce the conditions that degrade the material, and the results are compared against the original qualification to confirm that neither the material nor the process has drifted.

Standards and Documentation

High voltage design leans on the general design and acceptance standards for rigid boards, and on specific standards for the safety of the end product and for power conversion equipment. The practical value of citing them is that creepage and clearance distances, test procedures and acceptance criteria stop being a matter of opinion between customer and fabricator.

Documentation matters as much as the design. The material type, the glass transition temperature, the CTI rating, the creepage and clearance dimensions and the coating specification all belong on the fabrication drawing, along with the tests to be performed. A board that meets a high voltage requirement by accident of a material substitution is not a compliant board, and without the documentation there is no way to tell the difference.

FAQ

Can ordinary FR4 be used for a high voltage board? Yes, up to roughly 1000 volts, provided the creepage and clearance distances suit the environment and a high glass transition grade is chosen. Above that, or where heat removal matters, ceramic or PTFE based material is usually required.

Does a thicker board increase the withstand voltage? Only partly. Thickness raises the bulk breakdown voltage, but surface tracking, voids, edge field concentration and contamination usually fail first, so the layout and the material properties matter more than the thickness.

Why is partial discharge testing used? Because a small internal void can pass a simple withstand test and still erode the insulation over time. Partial discharge measurement detects those defects while the board can still be rejected.

What does a high CTI value buy? A shorter permissible creepage distance for the same safety margin, which allows a compact layout, plus better resistance to tracking in humid or contaminated conditions over the life of the product.