What Is the Difference Between PCB Creepage and Clearance?
Clearance is the shortest path through air between conductive parts. Creepage is the shortest path along an insulating surface between those parts. On a PCB, both can matter at the same insulation boundary, but measuring one does not prove that the other is adequate.
Think of two exposed pads separated by board material. Air provides one possible path between them; the board surface provides another. A routed opening can change the surface route without increasing the direct air gap. That is why a board can pass a creepage check and still fail its clearance requirement.
| Question | Clearance | Creepage |
|---|---|---|
| Where is the path? | Through air | Along an insulating surface |
| What must the reviewer identify? | The closest conductive features in three dimensions | The shortest permitted surface route |
| What can a layout screenshot miss? | Raised leads, hardware or solder approaching the boundary | A shorter route around a slot end or component body |
| What is the deliverable? | Measured minimum compared with an approved requirement | Measured surface path compared with its own requirement |
This guide helps prepare that review; it does not certify a design or supply a universal mains-voltage spacing. For equipment containing hazardous voltages, have the insulation requirements and completed design assessed by a qualified safety engineer. Inspect physical boards only when isolated from all energy sources and verified safe; stored energy can remain after disconnection.
Which Inputs Must Be Known Before Choosing a Distance?
Start with the equipment’s applicable safety standard and insulation function, then document the electrical and environmental conditions. A voltage label alone is not enough to choose both distances.
Texas Instruments’ insulation-coordination seminar distinguishes the inputs for the two checks: transient conditions and altitude affect clearance decisions, while working voltage, pollution degree and material group are central to creepage decisions. Use the relevant standard’s current edition and application rules rather than treating a seminar table as an approval.
- Boundary: identify both circuits, accessible parts and the required insulation type.
- Electrical conditions: record the working voltage across that boundary and the required transient or impulse conditions, with their basis.
- Environment: specify installation altitude, pollution conditions and enclosure assumptions.
- Material: obtain the actual laminate’s tracking-related data where required. “FR-4” alone is not a complete material specification.
- Protection: state whether a qualified coating or other protective measure is part of the compliance plan.
- Acceptance: record the applicable clauses and who approved the resulting minimum distances.
For example, a purchase request saying “230 V board” leaves important questions open: which two nets are separated, what protection that boundary provides, and under which installation conditions it operates. Send those questions back before assigning a CAD rule. A PCB design review should use the approved requirements, not invent them from the board’s supply label.
How Do You Check the Paths on a Real PCB Layout?
Check the shortest actual paths at the finished assembly boundary, not just the nominal distance between trace centerlines. Review the board and the components together.
- Mark the boundary. Identify the two circuit groups and every conductor that can approach the separation, including pads, vias, leads and hardware.
- Inspect the relevant geometry. Use finished copper edges and physical component dimensions. Check both board faces and the assembly drawing; a top-view image is not a complete clearance assessment.
- Trace the air route. Look for the closest exposed conductive features, including vertical or diagonal approaches.
- Trace the surface route. Follow the shortest insulating-surface path and check whether slots or grooves may be counted under the applicable rules.
- Apply tolerances. Compare the worst credible manufactured and assembled geometry with each approved minimum.
- Record the limiting location. Save the net pair, coordinates, measured path, requirement and reviewer decision. Repeat for each relevant boundary.
Geometry example, not a safety recommendation: two equal rectangular pads have centers 5.0 mm apart, and each pad is 2.0 mm wide in the direction between them. Their nominal edge gap is 5.0 – 1.0 – 1.0 = 3.0 mm, not 5.0 mm. If the combined worst-case geometry allowance reduces that gap by 0.2 mm, the review value is 2.8 mm. Whether that passes depends on the separately approved requirement and on any closer assembled feature.
Do not confuse that simple calculation with a complete insulation analysis. A solder fillet, connector pin or mounting screw can become the limiting conductor. TI’s creepage and clearance lesson also explains why package geometry and environmental conditions belong in the assessment.
Can Slots, Solder Mask or Coating Fix a Spacing Problem?
A slot can lengthen a qualifying surface path, but none of these features should be treated as an automatic waiver of the original requirements. Confirm what the applicable standard permits before changing the design.
| Proposed change | Useful question | Do not assume |
|---|---|---|
| Add a routed slot | Does its width and geometry qualify, and where is the shortest remaining surface path? | That clearance increased with creepage |
| Move conductors apart | Did the true limiting feature move, including component leads? | That changing traces fixes a close connector footprint |
| Specify a different laminate | Is the relevant material data documented and accepted for this design? | That any higher-priced laminate improves the required classification |
| Add coating | Is the material, coverage, process and validation accepted by the compliance plan? | That an informal coating step permits reduced spacing |
| Keep ordinary solder mask | Has the safety assessment explicitly allowed its use? | That green mask automatically replaces required separation |
For a slot proposal, ask the fabricator about the finished opening, routing tolerances, remaining board strength and copper keepout. Then return that geometry to the safety reviewer. A manufacturable slot is not necessarily a compliant insulation feature. Conversely, an electrically attractive opening can create an impractical mechanical bridge.
After a change, search again for the shortest route. Lengthening one path is ineffective if another shorter route still exists around the opposite end. Keep the approved slot and copper geometry in the same controlled drawing revision.
What Should Be Checked Before Releasing the Board?
Release a documented set of requirements and evidence, not a statement that the board “looks well separated.” Every claimed pass should point to a specific boundary and verification method.
| Release item | Evidence to keep | Reason to stop |
|---|---|---|
| Requirement basis | Standard, edition, conditions and approved distances | Only an online calculator result without assumptions |
| Layout review | Boundary list and limiting geometry with tolerances | Nominal trace spacing only |
| Material and process | Approved laminate and any qualified protective process | Unapproved substitution |
| Assembly review | Relevant component dimensions and hardware positions | Parts cross the separation unexpectedly |
| Verification plan | Specified inspections and applicable electrical tests | A test result being used to excuse an unresolved geometry issue |
CAD design-rule checks are useful after the right constraints are entered. They do not select the safety standard or automatically model every surface path and assembly feature. Analog Devices’ power-management layout guidance provides application context, but its particular design should not be copied as a universal spacing rule.
Send the fabrication data, assembly drawing, critical net-pair list, slot dimensions, approved minimum distances and material requirements to the PCB manufacturing team. Ask which dimensional requirements can be inspected and how deviations will be handled. Keep product-safety approval with the responsible qualified reviewer.
FAQ
Are creepage and clearance always different numbers?
No. The physical paths can be equal in a simple geometry, but the requirements and evaluation remain separate. Equality in one drawing does not let you omit either check.
Is there one minimum spacing for every mains PCB?
No. The equipment standard, boundary function, electrical stresses, materials and environment must be established first. Do not select a safety-critical distance solely from a supply voltage.
Does a wide-body isolator guarantee enough board spacing?
No. Its package information is only one part of the assembly review. Pads, nearby copper and other components can introduce a shorter path.
Does a slot automatically solve both problems?
No. Recheck the air gap independently. Also confirm that the slot is eligible to affect the surface-path measurement under the applicable rules.
Can I assume all FR-4 has the same tracking performance?
No. Obtain the relevant data for the specified product and make substitutions subject to approval where they affect the insulation design.
Does passing a dielectric test replace a spacing review?
No. Keep the required geometry and test assessments distinct. Passing one specified test is not evidence that every other requirement has been satisfied.
Should I measure from trace centers?
No. For a conductor gap, start with the relevant finished conductive edges and inspect the assembled geometry. Center-to-center measurements can overstate the available separation.
What should procurement ask before accepting a substitution?
Ask what changed, which insulation assumptions depend on it, and who approved the updated evidence. Do not accept “equivalent FR-4” as the whole answer when material properties matter.
Ready for a manufacturing review? Contact QFPCB with the controlled drawings and approved spacing requirements. Request feedback on manufacturability and inspection scope before releasing production files.
