High Voltage PCB Design Guidelines and Materials

December 12, 2024

Cadence PCB Solutions

Key Takeaways

Proper trace spacing and creepage are vital to avoid high-voltage flashover, arcing, and insulation failures.
Choose dielectric materials with high CTI ratings to minimize creepage distances and ensure durability in high-voltage environments
Isolate high voltage components and follow IPC-2221 and IEC 60950 standards to maintain safe clearance and prevent arcing.

*NASA has experienced issues with their high voltage spacecrafts in the past.*

NASA experienced performance issues with 1.5 kV power supplies in spacecraft like the Hubble and Cassini due to initial design flaws, including narrow part parameters, insulation issues, and incorrect materials. This was because--as a 2006 report stated--“designers did not take the high voltage problems seriously in the initial design.”

High-voltage circuit design demands a more rigorous approach than typical PCB designs, especially in high-density layouts. Designers must understand insulation, board materials, clearance, creepage, altitude, and relevant regulations. Read on as we discuss high voltage pcb design guidelines.

Guideline	Details & Information
Trace Spacing: Maintain to prevent flashovers	- Follow IPC-2221 and IEC/UL 60950-1 standards to determine minimum trace spacing. - Narrow trace spacing can lead to high voltage flashover. - Higher voltages necessitate greater spacing to prevent dielectric breakdown.
Clearance: Ensure clearance between conductive elements	- Defined as the shortest distance through air between two conductive parts. - Follow IEC 60950 guidelines based on operating environment, voltage, humidity, and dust levels. - Essential to prevent arcing. - Follow IPC-2221B recommendations for minimum clearance based on operating voltage.
Creepage: Ensure appropriate creepage distance	- Measured along the surface of an insulating material. - Apply IPC-2221A standards for trace spacing. - Use slots or insulation barriers to avoid flashover. - Increase creepage distances based on voltage levels and environmental conditions. - Refer to IPC-2221B for specific creepage values. - Use slots or grooves to increase creepage without enlarging PCB size.
Sharp Corners in Layout	- Sharp edges can cause high electric fields across insulators, leading to flashover. - Prevents high electric field concentrations that can lead to insulation failure.
Material Selection: Use appropriate dielectric materials with high CTI	- Select materials with high dielectric strength, resistivity, and low power factor to prevent arcing and breakdown. - Use encapsulating resins or conformal coatings for insulation. - Materials like FR-4 may be limited for very high voltages; consider alternatives with higher CTI ratings. - Higher CTI ratings allow for shorter creepage distances.
Conformal Coatings for surface insulation	- Coatings can increase creepage distances and prevent moisture absorption. - May affect thermal characteristics. - Acrylic or silicone coatings are common for high-voltage PCBs. - Apply especially in high humidity environments. - Provides additional insulation and environmental protection. - Choose coating types that balance insulation and thermal management needs.
Copper Thickness for high currents	- Thicker copper provides better current handling and physical strength. - Ensure the copper surface is smooth to prevent arcing. - Enhances the durability and reliability of high-current paths.
Resistors and Capacitors	- High voltage resistors should have low inductance and temperature coefficients. - Select components rated above the maximum expected operating voltage. - Provides safety margins; components should exceed the highest voltage they will encounter in operation.
Circuit Design for Transients: Design to prevent transients	- Route cables and apply shielding to avoid large voltage or current transients, which can damage components. - Use shielding and proper routing to minimize transients. - Protects sensitive components from voltage spikes and electrical noise.
Slots and Cutouts & Isolation Slots	- Implement slots or cutouts between high voltage conductors to effectively increase creepage paths; use isolation slots for particularly high-voltage designs. - Use isolation slots between different nets or between high and low-voltage circuits. - Practical method to enhance creepage and isolation. - Maintain structural strength despite added slots or cutouts.
Ground Planes & Grounding Techniques: Use a solid ground	- Ground planes enhance safety and reduce electromagnetic interference (EMI). - Avoid placing ground planes under high voltage traces on adjacent layers to reduce insulation distance and prevent capacitive coupling or arcing. - Proper grounding techniques are essential for signal integrity and safety. - Prevent unintended capacitive effects and arcing by careful ground plane placement.
Surface Finish	- Use ENIG (Electroless Nickel Immersion Gold) or silver to avoid oxidation, which can lead to arcing. - Some finishes may reduce creepage distances or have lower dielectric strength.
Thermal Management:	- Ensure adequate thermal dissipation for high-voltage components, using heat sinks and thermal vias to reduce the risk of overheating. - Incorporate thermal vias to transfer heat away from critical areas. - Maintain adequate airflow and cooling pathways on the PCB layout. - Prevents thermal-induced failures and enhances the longevity of high-voltage components.
Silkscreen and Labels	- Use distinct symbols or color codes to indicate high-voltage zones. - Ensure labels are durable and remain legible throughout the PCB's lifecycle. - Enhances safety by providing clear warnings to assembly and maintenance personnel. - Reduces the risk of accidental contact with high-voltage areas.
Controlled Impedance	- Maintain consistent trace geometry to avoid signal reflections and losses. - Use impedance-matched routing techniques for high-speed signals to ensure signal integrity. - Critical for high-speed and high-frequency applications where signal integrity is paramount. - Helps in achieving reliable communication and data transmission on the PCB.

High Voltage Design Begins with the PCB Layout

In high voltage PCB design guidelines, proper trace spacing maintains signal integrity and helps with preventing the propagation of electromagnetic interference. In high voltage PCB design, trace spacing becomes even more important. If we rightfully consider the board as a series of conductive elements, the possibility of differences in potential creating high voltage flashover with narrow trace spacing becomes a certainty.

High Voltage PCB Standards

Along with the Association of Connecting Electronics Industries (IPC) IPC-2221 Generic Standard on Printed Board Design standard that establishes the design principles for interconnections on PCBs, the International Electrotechnical Commission (IEC) and the Underwriters Laboratories (UL) also produced the IEC/UL 60950-1 “Safety of Information Technology Equipment” standard that describes safety requirements for products and details minimum allowed PCB spacing requirements. As a combination, the standards also set guidelines for PCB layouts that include two important parameters called clearance and creepage.

Using the IEC 60950 definition, clearance equals the shortest distance between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured through air.

A small clearance value between two conductors establishes the environment for a high-voltage flashover or arc.
Clearance values vary according to the type of PCB material used for the circuit, the voltages, and operating environment conditions such as humidity and dust. Those environmental factors--and others--decrease the breakdown voltage of air and increase the opportunities for a high voltage flashover and a short circuit.

*While a high-voltage design doesn’t always have to be a power grid, you’ll want to be wary when dealing with any electrification designs.*

Working With Creepage Values in High Voltage

Again referring to the IEC 60950 definition, creepage represents the shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured along the surface of the insulation.

While clearance refers to the spacing through conductive elements through air, creepage considers the space between conductive elements over an insulating surface.

The design rules that we establish for trace spacing, pad-to-pad spacing, and pad-to-trace spacing for PCB designs that incorporate high voltages address creepage.

The IPC2221A standard provides clearance and creepage tables for setting design rules and performing design rule checks and electrical rule checks for minimum requirements.
PCB layouts can also include slots or vertical insulation barriers between traces. Because any metallic print pattern or printed circuit trace that has sharp edges can cause a high electric field across insulators and a flashover, the trace layout for a high-voltage power supply must avoid sharp corners and acute angles.

High Voltage Design Through Material Selection

Materials must have high dielectric strength, high resistivity to prevent arcing, and a low power factor that reduces heating effects and the possibility of thermal breakdown. Other factors to consider are:

tensile strength,
hardness,
surface breakdown strength,
thermal expansion,
chemical resistance,
and stability against aging and oxidation.

While circuits require functional insulation to operate, other types of insulation prevent high voltage problems in PCBs. Insulating materials may include encapsulating resins applied to high voltage cavities, conformal coatings, or solid insulation that surrounds conductors. Regulatory standards require additional layers of insulation if the potential for human contact with the system exists.

Selecting Dielectric Materials

When selecting dielectrics and insulators for a PCB, use the comparative tracking index (CTI) to determine which material is best. The CTI is the maximum voltage measured in volts at which a material withstands 50 drops of contaminated water without forming conductive paths because of electrical stress, contamination, or humidity.

Manufacturers use the CTI to compare the performance of insulating materials under wet or contaminated conditions.
Materials that have a high CTI value have a lower required minimum creepage distance and allow a shorter distance between two conductive parts. The shorter distance allows the use of high density circuits in a high voltage environment.

CTI Standards

UL standards divide the CTI levels for materials into the four groups shown in the table. When looking at the table, materials classified within Material Group One have the highest CTI rating.

Material Group	Rating
I	600 volts </= CTI
II	400 volts </= CTI < 600 volts
IIa	175 volts </+ CTI < 400 volts
LLb	100 volts </+ CTI < 175 volts

Design Guidelines for Dielectrics

Although FR4 laminates have a high breakdown voltage, the weaker structure and porosity of FR4 can allow the material to become prone to contamination and a lessening of the dielectric value. Because of the FR4 limitations, high voltage laminates that have a non-conductive base layer and prevent arcing serve as the gold standard for high voltage circuit design. High voltage laminates have higher levels of resin and glass than standard board materials.

*Working within high-voltage circuit boxes becomes easier when you design them properly.*

High Voltage Design Guidelines For Component Selection

High voltage circuits rely on both passive and active components:

Resistors in high-voltage circuits require low inductance and low temperature coefficients.
Ceramic capacitors must have high resistance, high-temperature coatings, and dielectrics that can withstand high voltages. These capacitors should maintain stable electrical parameters across a wide range of applied DC voltages and under various environmental conditions.
Semiconductor devices for high-voltage circuits include MOSFETs, IGBTs, MOS-controlled Thyristors, Power FETs, and SCRs, which must follow manufacturer guidelines for not exceeding values that can destroy the devices. For example, high voltage circuits may require components that have higher breakdown voltage ratings and the capability to handle higher currents.

High Voltage with ECAD

We can address clearance issues through ECAD/MCAD design principles. Since the bounding surface described in the IEC definition is the outer surface of an electrical enclosure.

We can also apply good PCB design principles by isolating high voltage circuits from low-voltage circuits.
Fabricators often recommend placing the high voltage components on the top side of a multilayer board and the low voltage circuits on the bottom side of the PCB.
Other methods involve placing the appropriate insulating materials between high voltage nodes and over any exposed high-voltage leads.

Below we’ve summarized key features of OrCAD X that aid in adhering to high voltage PCB design guidelines.

High Voltage Design Guideline	OrCAD X Features
Trace Spacing	Constraint Manager: Allows setting up custom spacing rules for high-voltage traces. Use the Constraint Manager to create spacing rules for nets and apply these for trace widths and gaps to meet high voltage standards.
Clearance	3D Visualization & Clearance Checks: OrCAD X features an interactive 3D engine to visualize clearance between parts.
Creepage	Design Rule Checks (DRC): OrCAD X performs comprehensive DRCs, including creepage distance checks based on IPC-2221B standards to prevent flashover on high voltage paths.
Layer Placement & Stackup	Cross-Section Editor: OrCAD X supports setting up a customized stack-up with specific dielectric thicknesses and layer placement to isolate high-voltage components.

When adhering to high voltage PCB design guidelines, turn to Cadence’s PCB Design and Analysis Page — and specifically OrCAD X. It has the ability to work through any voltage requirement in your electronic designs.

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