Building High Voltage Boards: Insulation, Slots and Hi-Pot

Different Rules, Different Failure Mode

A high voltage board does not fail the way a digital board fails. A marginal signal trace produces an intermittent error; a marginal high voltage trace produces a breakdown, a tracking path along the laminate surface or an arc, and in the worst case a fire. That is why the design and the fabrication of these boards follow a separate rule set built around distances, material properties and cleanliness rather than around impedance and signal integrity.

The four concepts that govern everything are creepage, clearance, comparative tracking index and insulation coordination. Everything else is a consequence of them.

The Four Electrical Principles

• Clearance is the shortest distance through air between two conductors. It governs the air breakdown path.
• Creepage is the shortest distance along the surface of the insulator. It governs surface tracking, which is the more common failure mode in a dirty or humid environment.
• Comparative tracking index describes how well a laminate resists forming a conductive carbon path when contamination and voltage are both present. A higher index means a shorter creepage distance is acceptable.
• Insulation coordination is the system level decision about how much margin to apply at each voltage level, in each pollution degree, for the transient overvoltages the equipment must survive.

Working values used across many designs, subject to the pollution degree and the applicable standard:

• 1 kV: clearance 1.5 to 2.0 mm, creepage on FR-4 about 3.2 to 4.0 mm.
• 5 kV: clearance 6 to 8 mm, creepage about 10 to 12 mm.
• 10 kV: clearance 12 to 15 mm, with creepage in the same region and usually a change of material or construction.

The pattern to notice is that creepage grows faster than clearance. Air recovers its insulation quickly; a contaminated surface does not.

Material Selection

Ordinary FR-4 can be used, but only in a high CTI grade. A laminate with a tracking index of 600 or above behaves very differently from a standard grade once the surface is contaminated.

• High CTI FR-4, 600 and above: the default for most industrial and power designs.
• Ceramic substrates: for the most extreme voltage and temperature combinations, where the laminate itself is the weak element.
• PTFE: where a high frequency path also has to hold off voltage.
• Polyimide: where thermal stability and dielectric stability are both required.
• Thicker copper and thicker dielectric: not a material change, but a construction change that raises the internal breakdown voltage.

The material choice feeds back into the layout, because a higher tracking index allows tighter spacing for the same voltage. Selecting the laminate is therefore part of the distance calculation rather than a separate decision.

Layout Rules

• Scale the spacing to the voltage using the applicable table, then add margin for the pollution degree the product will actually see.
• Avoid sharp copper corners. A point concentrates the electric field, and field concentration is where corona starts.
• Use slots. A milled slot in the laminate interrupts the surface path and is often more effective than increasing the gap, because it removes the tracking route entirely.
• Separate the high and low voltage regions physically rather than interleaving them for convenience.
• Keep high voltage traces away from the board edge, where contamination and finger contact are both more likely.
• Watch vertical spacing as well as lateral. In a multilayer board a high voltage plane can be separated in the third dimension by a dielectric that is too thin.
• Round the copper edges. Rounded pads and smooth pours reduce field concentration compared with square corners.

Stackup Insulation

Internal breakdown is invisible from outside, which makes it the most dangerous failure mode on a multilayer high voltage board. Three rules control it: increase the dielectric thickness between layers that carry high voltage; never overlap high voltage copper on adjacent layers without an adequate dielectric between them; and consider a dedicated insulation layer where the voltage differential justifies it. The verification is a microsection, because the only way to know the finished dielectric thickness is to cut the board and measure it.

Avoiding Tracking, Corona and Breakdown

The failure paths have specific causes, and each has a specific countermeasure.

• Surface contamination left from fabrication or assembly lowers the effective creepage distance. Cleanliness during production and a proper flux cleaning step afterwards are both part of the insulation design.
• Flux residue from soldering is conductive enough to start tracking on a 5 kV board. On high voltage assemblies, cleaning is not optional.
• Moisture and dust in service create the same effect gradually. Conformal coating or encapsulation keeps the surface dry and stable.
• Sharp copper corners concentrate the field and initiate corona. Round them.
• Insufficient spacing is a design error that no process can repair.
• Poor mask coverage leaves copper exposed where tracking can start. Mask integrity over high voltage traces should be verified optically.

The practical combination that solves most of these at once is a clean process, rounded copper, adequate slots, and a conformal coating or potting compound applied over the finished assembly.

Manufacturing Requirements

A high voltage board should never be run as a routine job on an ordinary line.

• Precision milling of slots and isolation channels, since the slot depth and position are part of the insulation design.
• Strict control of drilling burrs, because a burr reduces an air gap under the mask.
• Cleanliness management through the whole process, including the handling after final finish.
• A finish chosen for insulation stability. ENIG is the usual preference, because it is flat and leaves no flux residues behind.
• Documented process control for the plating and the laminating steps, so that the internal dielectric thickness can be demonstrated on the coupon.

Testing

• Hi-Pot, or dielectric withstand. A voltage of two to three times the working voltage is applied between the isolated sections, and the leakage current is monitored. This is the acceptance test that matters most.
• Insulation resistance measured at the working voltage, to confirm the resistance between isolated circuits is high enough.
• Optical inspection for copper burrs, mask defects and contamination.
• Microsection of the coupon to verify the dielectric thickness between high voltage layers.

Hi-Pot at two to three times the working voltage is a destructive test in the sense that a marginal board will fail here rather than in the field, which is precisely the point. Boards that pass it and then fail in service usually have a contamination path added during assembly, which is why the final assembly process deserves the same attention as the board. The overall test structure carries across to the assembled product through PCBA testing, and the documentation belongs with the rest of the quality management records for the program.

Assembly Notes

A surprising share of high voltage failures originate after the board is built. Component lead spacing has to satisfy the same creepage rules as the traces, which often means using parts with a wider pitch than a low voltage design would choose. Flux must be fully removed. Conformal coating or potting has to cover the joints as well as the traces, with the connector areas masked. And the mask itself has to remain intact through the reflow, because a lifted mask edge exposes copper exactly where the field is highest.

All of this argues for assembling high voltage boards at a shop that already builds them, rather than at the lowest bidder who is otherwise competent. The process is not complicated; it is simply different, and the differences are the ones that determine whether the product survives ten years in a humid plant. A supplier who builds the board and the assembly in one PCB assembly flow can also apply the cleanliness and coating standards continuously instead of handing the boards between two sites.

Price Bands

• Two layer, high CTI FR4, 1 kV class: about 80 to 150 dollars for a prototype, 8 to 15 dollars per board in volume.
• Four layer with slotting, 5 kV class: 300 to 600 dollars for a prototype, 25 to 45 per board in volume.
• Six layer ceramic or PTFE, 10 kV class: 800 to 1,500 dollars for a prototype, 90 to 160 per board in volume.

Four factors add cost on top of an ordinary board: the slotting operation, the high tracking index laminate, the Hi-Pot and insulation resistance testing, and the clean room discipline through the process. None of them can be removed without removing the reason the board exists.

Choosing a Manufacturer

Five capabilities should be confirmed before an order: experience with high voltage programs, Hi-Pot and insulation resistance test equipment on site, precision slotting, a reliable supply of high CTI laminate, and the certification set the product needs for its market. Because the electrical behaviour depends on the manufactured geometry as much as on the design, the supplier should also be able to supply PCB manufacturing data for the dielectric thickness on the finished coupon. Where the board also passes through prototype and validation stages, the same construction should be used for every build: changing the laminate or the dielectric thickness between a prototype and production invalidates the Hi-Pot results that were used to qualify the design.

FAQ

How much creepage is needed at 5 kV? Typically 10 to 12 mm on FR-4, depending on the pollution degree and the applicable standard.

Can FR-4 be used? Yes, in a high CTI grade, combined with adequate spacing and slotting. Standard grade laminate is not appropriate above a few hundred volts.

What is a Hi-Pot test? A dielectric withstand test that applies two to three times the working voltage and monitors leakage current.

Why do high voltage boards arc? Insufficient creepage, contamination, sharp copper corners or the wrong material, usually in combination.

Is conformal coating required? Not universally, but it is the most effective single countermeasure against surface tracking in a humid or dirty environment.

Summary

High voltage board design is the discipline of distance, material and cleanliness. Creepage grows faster than clearance with voltage, so the surface path is usually the limiting factor; a high CTI laminate buys back some of that distance; slots break the surface path entirely; and rounded copper keeps the field from concentrating. Manufacturing adds precise slotting, burr control and a clean process, and the acceptance test is Hi-Pot at two to three times the working voltage with an insulation resistance check. Assembly contributes its own failures through flux residue and incomplete coating, so the coating and the cleaning standard belong in the same document as the board specification.