High Voltage PCB: Design and Fabrication Guide
A high voltage board looks like an ordinary board that has been stretched. The traces are wider, the clearances are larger and there is a lot of empty laminate, and the empty space is the point. On a high voltage PCB the distance between conductors is a design parameter in a way that it never is at low voltage, and the layout is organised around keeping those distances rather than around fitting the components in.
Voltage Classes and What Changes
The requirements change sharply as the voltage rises. Below a few hundred volts, ordinary design rules apply with extra spacing on the high voltage nets. Above a thousand volts, creepage and clearance become the dominant constraint, the dielectric strength of the laminate and the coating start to matter, and partial discharge becomes a failure mechanism that has to be designed against rather than overlooked.
Partial discharge is the effect that catches designers out. A void, a sharp point or a contaminated surface produces a local field concentration, and the air or the laminate in that region breaks down without a complete flashover. The discharge is small, but it erodes the material over time and eventually produces a complete failure, which means a board can pass every electrical test at manufacture and fail in the field after a year of operation.
Creepage and Clearance
Clearance is the distance through air between two conductors, and it is set by the peak voltage the insulation has to withstand rather than by the working voltage. Creepage is the distance along the surface of the board, and it is set by the working voltage, by the pollution degree of the environment and by the material group of the laminate. The two are separate requirements, and satisfying one does not satisfy the other.
Where the clearance requirement cannot be met, a slot milled through the board is the standard remedy, because the arc must travel around the slot rather than across it. Where the creepage requirement is the problem, the same slot helps, and the surface can also be coated with a conformal layer that changes the effective environment. Both measures have to be designed in, because a slot cannot be added after the layout is finished without moving components. Our component tolerance and reliability notes describe how those distances are verified.
Material and Coating Choices
The laminate’s dielectric strength is quoted per unit thickness, but the figure assumes a uniform field and a clean material. In practice the limit is set by voids, by the resin-to-glass interface and by contamination, which is why high voltage boards use materials with a high tracking resistance and why the assembly is cleaned thoroughly after soldering. Resin residue and flux are conductive enough at high voltage to create a leakage path that grows with time.
Conformal coating is often essential rather than optional. It excludes moisture, it prevents contamination from settling on the surface and it increases the effective creepage distance. The coating has to be applied to a clean board and cured fully, and it must cover the high voltage regions without leaving voids, because a void under the coating is worse than no coating at all. Our design release checklist places those checks in the review sequence.
Component Selection and Layout
Components on a high voltage board are chosen for their voltage rating and for their physical size rather than for their electrical parameters alone. Resistors have a voltage limit as well as a power limit, and a long resistor body is often chosen simply to spread the voltage across more distance. Capacitors have a voltage rating that must be derated, and connectors have both a voltage rating and a creepage distance between their pins.
The layout follows the voltage map rather than the signal flow. The high voltage nets are grouped together, kept as short as the function allows, and routed with the spacing their class requires, while the control electronics are placed at the other end of the board. Where the two must interact, an isolated driver crosses the boundary, and the boundary itself is a defined line that no trace crosses except through that device.
Isolation and the Control Interface
Almost every high voltage board has a low voltage control section, and the interface between them is where the design is won or lost. Optocouplers, isolated gate drivers, isolated amplifiers and isolated supplies all carry the barrier, and each of them has a rated isolation voltage and a creepage distance of its own that must be respected in the layout.
The physical arrangement matters as much as the component rating. The barrier should be a straight line across the board, with the isolated components straddling it and nothing else crossing it. The clearances on both sides are then measured from the barrier rather than from the individual component, which makes the layout auditable and makes it obvious when a later revision has violated it.
Testing and Verification
Dielectric withstand testing applies a high voltage between the isolated sections and verifies that no breakdown occurs, and it is performed on every unit rather than on a sample. Partial discharge testing is performed where the requirement is strict, because it detects the voids and field concentrations that a withstand test cannot.
Before either of those, the surface is inspected. Contamination, solder spatter and incomplete coating cure are the causes of most failures at test, and they are also the causes of most field failures. A board that is cleaned, inspected and coated properly passes the electrical tests, and one that is not will fail them intermittently in a way that is difficult to trace.
Leakage, Tracking and Long Term Degradation
At high voltage the surface of the board is a conductor rather than an insulator, because contamination, moisture and ionic residues all provide a path for leakage current. That current heats the surface, which increases the leakage, and the process accelerates until a conductive carbon track forms between the conductors. The phenomenon is called tracking, and the resistance of a material to it is a specified property that matters as much as its dielectric strength.
Design measures reduce the risk. Keeping the conductors apart, avoiding sharp points that concentrate the field, coating the surface so that contamination cannot settle on it and cleaning the assembly thoroughly after soldering all help. Where the environment is polluted, the creepage requirement is increased by the pollution degree of the installation, because the surface will not stay clean in service even if it leaves the factory clean.
Practical Layout Sequence
A high voltage layout is easier to build if it is done in a fixed order. First the barrier is drawn and the isolated devices placed across it. Then the high voltage conductors are placed with their required spacing, keeping them short and grouping them on one side. Then the low voltage control circuitry is placed on the other side, with the isolated drivers treating the barrier as an immovable line. Components that straddle the barrier are the only things that cross it.
The clearances are then checked rather than assumed, using the calculated figures for the working voltage, the pollution degree and the material group. Slots are added where a clearance cannot be met, and the barrier is marked on the silkscreen so that a later revision cannot quietly violate it. Our design release checklist places those checks in the sequence.
FAQ
What is the difference between creepage and clearance? Clearance is the shortest distance through air between two conductors; creepage is the distance along the surface of the insulating material. Both requirements apply and they are calculated separately.
Why is a slot cut into the board? To increase the distance the breakdown has to travel. An arc must go around the slot rather than across it, which raises the effective clearance without using more board area.
Why does a high voltage board fail after a year rather than immediately? Partial discharge. Small voids and field concentrations erode the insulation gradually, so the board passes at manufacture and breaks down later.
