Engineering Reference
PCB Creepage, Clearance and Insulation Coordination Reference
Reference PCB creepage and clearance definitions, working and impulse voltage, pollution degree, CTI material groups, altitude, insulation classes, and layout review boundaries.
This page is a terminology and review reference, not a reproduced standards table. Final dimensions are governed by the current product-specific standard, equipment architecture, installation environment, certified material data, and conformity assessment.
Insulation Coordination Terms
Start by naming the physical path and voltage stress correctly. Creepage and clearance are complementary checks, not interchangeable labels for copper spacing.
| Term | Engineering meaning | Design boundary |
|---|---|---|
| Clearance | Shortest distance through air between two conductive parts. | Driven mainly by impulse withstand, electric field, altitude, and the applicable equipment standard. |
| Creepage | Shortest path along the surface of solid insulation between conductive parts. | Driven mainly by working voltage, pollution degree, material tracking performance, and insulation type. |
| Working voltage | Highest RMS or DC voltage that can continuously occur across the insulation under rated conditions. | Do not substitute nominal supply voltage without checking circuit and fault conditions. |
| Rated impulse voltage | Specified transient withstand level used when coordinating through-air clearance. | Usually derived from system voltage, overvoltage category, and installation context. |
| Pollution degree | Environmental classification describing contamination and whether it can become conductive. | The microenvironment at the insulation surface matters, not only the room description. |
| CTI | Comparative Tracking Index: a material test result used to classify tracking resistance. | CTI supports material grouping; it is not a voltage rating or a complete creepage rule. |
| Material group | Insulating-material classification derived from CTI bands in the applicable standard. | Use the laminate supplier's qualified value and the standard edition governing the product. |
| Altitude | Installation elevation used to account for reduced air density. | It primarily affects clearance; required correction depends on the governing standard. |
| Basic insulation | Insulation providing a basic level of protection against electric shock. | Its required spacing depends on the safety architecture and product standard. |
| Reinforced insulation | Single insulation system providing protection equivalent to double insulation. | Do not assume one universal multiplier; use the governing standard's method. |
Clearance: Through Air
Measure the shortest three-dimensional air path between conductive features. Determine the required impulse withstand and check altitude, field concentration, enclosure geometry, component leads, and manufacturing tolerance. Surface material group is not a substitute for this check.
Creepage: Along a Surface
Measure the shortest path following the insulating surface. Establish working voltage, pollution degree, material tracking group, insulation function, and whether qualified coating, grooves, or barriers may be credited.
Pollution Degree Context
Pollution degree describes the expected microenvironment around the insulation. Enclosure protection, local heating, condensation, conductive dust, process residue, and service conditions can make the PCB microenvironment different from the surrounding room.
| Class | General interpretation | Review context |
|---|---|---|
| PD1 | No pollution, or only dry non-conductive pollution | Protected or sealed microenvironments where condensation is excluded |
| PD2 | Normally non-conductive pollution; temporary conductivity from condensation may occur | Many controlled indoor electronic assemblies |
| PD3 | Conductive pollution, or dry pollution that becomes conductive through expected condensation | Industrial or exposed equipment without adequate protection |
| PD4 | Persistent conductivity caused by dust, rain, snow, or similar exposure | Severe outdoor or unprotected environments |
CTI and Material Group
CTI is a standardized material test used to group insulating materials by resistance to surface tracking. Higher tracking resistance can support a more favorable creepage classification, but CTI does not describe dielectric strength, long-term contamination, moisture absorption, thermal aging, or every finished-board process.
Use the laminate supplier's qualified CTI or material-group declaration for the exact material and construction. Generic labels such as “FR-4” do not establish one universal material group. Verify threshold bands and permitted use against the current governing standard rather than copying a remembered table into a design rule.
Engineering Review Workflow
| Step | Required decision |
|---|---|
| 1. Define the insulation boundary | Identify the two conductive nodes, accessible parts, protective earth, and whether the barrier is functional, basic, supplementary, double, or reinforced. |
| 2. Establish voltage stresses | Determine continuous working voltage, repetitive peaks, transients, mains category, fault conditions, and the required impulse withstand. |
| 3. Select environmental assumptions | Record pollution degree, expected condensation, contamination, coating or potting qualification, and installation altitude. |
| 4. Verify material data | Obtain qualified CTI/material-group and laminate data from the supplier; do not infer it from generic FR-4 labeling. |
| 5. Determine both distances | Evaluate clearance and creepage separately using the applicable standard and product-specific clauses. |
| 6. Apply manufacturing reality | Include copper registration, routing tolerance, slots, component leads, solder, contamination, board edges, and enclosure geometry. |
| 7. Validate the released design | Perform standards review, dielectric or impulse testing where required, fabrication inspection, and certification review for regulated products. |
Layout and Manufacturing Review
| Feature | Possible benefit or effect | Boundary |
|---|---|---|
| Slots and grooves | May increase the measured surface path when geometry meets the applicable standard's rules. | They do not automatically increase the shortest air path. |
| Conformal coating | Can change pollution assumptions only when the coating system and process are qualified for the application. | A coating callout alone is not permission to reduce spacing. |
| Solder mask | Usually should not be treated as certified solid insulation without explicit qualification. | Check the governing standard and manufacturing controls. |
| Board edges and cutouts | Can create shorter air or surface paths than the nominal copper-to-copper layout spacing. | Review the complete 3D assembly. |
| Component bodies and leads | Lead forming, package tolerances, terminals, heatsinks, and mounting hardware can control the minimum distance. | PCB copper spacing alone is not the system result. |
| High altitude | Reduced air density can require greater clearance above the standard's reference altitude. | Apply the prescribed correction method; do not scale creepage by the same factor automatically. |
Standards Boundary
IEC 60664-1 is a common foundation for low-voltage insulation coordination, but it is not the only requirement.
Product standards can define different voltage selection, insulation, material, test, and spacing rules.
Do not reproduce a spacing value from one table without its voltage, waveform, pollution, material, altitude, and insulation context.
IPC layout guidance does not replace electrical-safety certification requirements.
UL recognition of a material or coating does not by itself approve the finished equipment spacing.
Use the current adopted edition and regional deviations required for the target markets.
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FAQ
What is the difference between PCB creepage and clearance?
Clearance is the shortest path through air. Creepage is the shortest path along an insulating surface. They address different breakdown mechanisms and must be evaluated separately.
Does CTI affect clearance?
CTI is primarily used to classify solid insulating material for creepage assessment. It does not normally set through-air clearance, although both checks belong to the same insulation-coordination review.
Why does altitude affect PCB clearance?
Air density decreases with altitude, reducing through-air dielectric strength. Applicable standards therefore prescribe altitude correction above their reference elevation.
What voltage should be used for creepage?
Use the working-voltage definition and measurement method required by the governing standard, including relevant RMS, DC, repetitive peak, circuit, and fault conditions. Nominal supply voltage alone may be insufficient.
What voltage should be used for clearance?
Clearance coordination commonly depends on required impulse withstand as well as steady voltage. The value must be derived from system voltage, overvoltage category, installation, and the applicable product standard.
Can a PCB slot increase creepage distance?
A compliant slot or groove can lengthen the surface path, but minimum dimensions and measurement rules vary by standard. It may not increase the shortest air clearance.
Does solder mask count as insulation?
Do not assume it does. Recognition depends on the insulation system, process controls, qualification, and governing standard.
Can conformal coating reduce required spacing?
Only when the coating system, application process, environment, and relevant standard support that treatment. A generic coating note is not enough.
Are ECParts spacing calculator results certification values?
No. They are preliminary screening estimates. Final spacing must be established from the current applicable standard, product category, material data, manufacturing tolerances, and certification review.
Which standards govern creepage and clearance?
IEC 60664-1 is a common insulation-coordination foundation, but product standards for power supplies, industrial, medical, household, or information-technology equipment can modify the method. Use the standards applicable to the finished equipment.