Board-level electrical insulators are key to preventing short circuits, ensuring long-term reliability, and protecting vital components in electronic assemblies.
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Effective insulation design is reliant on three related parameters: clearance, creepage, and dielectric strength. Engineers must understand these essential factors to successfully develop high-voltage or high-reliability electronics.
This article defines each term, outlines why it is important, and summarizes how to incorporate these principles into printed circuit board (PCB) and component design.
Mastering these parameters allows engineers to avoid dielectric breakdown, minimize arcing, and design safer, more reliable systems.
Clearance: Adequate Air Gaps for Safety
Clearance can be understood as the shortest distance through air between two conductive elements at different electrical potentials. The combination of rising voltage differences and insufficient air gaps can cause the electric field to ionize the air, leading to a flashover or arc.
This phenomenon causes dielectric breakdown in air, potentially leading to catastrophic circuit failure.
Standards and Guidelines
IEC 60950-1, IPC-2221, IEC 60664-1, and other international standards define minimum clearance distances based on factors such as altitude, working voltage, and pollution degree.
The dielectric strength of air decreases with increasing altitude or humidity, necessitating larger gaps to avoid arcing. Following these standards is key to ensuring compliance and safety, especially in mission-critical or high-voltage systems.
Design Considerations
It is important to maintain a clear, unobstructed line-of-sight air gap between high-voltage conductors during PCB layout. Sharp edges should be avoided because these can intensify electric fields and reduce effective breakdown voltage.
Engineers seeking robust solutions will typically exceed the minimum required clearance by 20-30 % to account for environmental variability and long-term wear. The use of protective coatings can also prevent moisture or dust accumulation, which could otherwise reduce effective clearance.
Creepage: Surface Distance to Prevent Tracking
Clearance deals with air, while creepage deals with surfaces. Creepage can be understood as the shortest path between two conductive parts along the surface of an insulating material.
Contaminants such as moisture or dust can form conductive films on insulators, creating potential leakage paths and, ultimately, surface tracking or flashover.
Influencing Factors
The required creepage distance depends on the insulating material’s Comparative Tracking Index (CTI) and the degree of environmental pollution.
Materials with higher CTI ratings resist surface tracking better, and so require shorter creepage distances. Low-CTI materials and contaminated environments require the use of longer creepage paths to maintain safety.
Design Considerations
Extending creepage without enlarging the board represents a major design challenge in dense electronics. Common approaches to addressing this include adding grooves, slots, or barriers that lengthen the leakage path.
Incorporating raised insulating ribs or milling slots beneath high-voltage components is an effective method, while the use of insulating boots or housings can further increase the surface distance, allowing compact assemblies to better meet stringent regulatory requirements.
Dielectric Strength of Insulator Materials
Dielectric strength refers to the amount of voltage a material can withstand per unit of thickness before electrical failure occurs. Dielectric strength is typically expressed in kilovolts per millimeter (kV/mm) and essentially represents the maximum electric field intensity that a material can endure without becoming conductive.
Importance of Material Selection
Choosing materials with sufficient dielectric strength is critical if reliable insulation is to be assured. For instance, a plastic with a dielectric strength of 20 kV/mm could resist 20 kV across a 1 mm layer in theory.
In practice, however, engineers generally apply substantial safety margins to accommodate surges and aging. Common materials include Kapton® (polyimide film), black aluminum foil, NOMEX®, and ceramics, with each material offering a distinct balance of dielectric performance, thermal tolerance, and flexibility.
Trade-offs and Considerations
High dielectric strength is often accompanied by mechanical or thermal trade-offs.
For example, ceramics can withstand high voltage and temperature but are brittle, while plastics are flexible but offer reduced heat resistance. It is important that the insulator thickness balances the voltage rating and the mechanical constraints.
Custom-cut insulating parts allow engineers to tailor thickness, geometry, and material to meet both clearance and creepage requirements. Instant online quoting enables options to be compared (e.g., 0.5 mm Nomex versus 1 mm FR-4) and rapid iteration during the design phase.
Designing for Reliability and Standards Compliance
Safety Standards (IEC, UL)
IEC 60664-1, UL 61010, and other key standards incorporate requirements for clearance, creepage, and dielectric testing.
High-voltage or safety-critical products are required to pass dielectric withstand (hi-pot) tests to verify insulation integrity. Exceeding minimum clearance and creepage distances is also key to improving durability and safety margins over the product’s lifetime.
Environmental Factors
Designers must account for humidity, altitude, contamination, and coating use. Thinner air reduces dielectric strength at high altitudes, necessitating larger clearances. Longer creepage paths are also required in polluted or humid environments.
Conformal coatings can reduce the minimum clearance by enhancing surface insulation. Some standards permit this approach.
Summary
The design of reliable board-level electrical insulators necessitates a careful balance between creepage, clearance, and dielectric strength.
Engineers who understand these principles are better equipped to ensure compliance, prevent arcing, and extend the lifespan of their products. It’s also important to assess voltage levels, environmental factors, and material properties as early in the design process as possible.
Materials Direct provides instant online pricing and 24-hour manufacturing for custom electrical insulators and other precision-cut insulating parts. This rapid turnaround allows engineers to test multiple insulation configurations in days rather than weeks, whether they are designing a medical device or prototyping a power supply.
Acknowledgments
Produced from materials originally authored by Universal Science UK Ltd.
This information has been sourced, reviewed, and adapted from materials provided by Materials Direct.
For more information on this source, please visit Materials Direct.