Clearance & Creepage
How conformal coating clearance and creepage influence IEC / IPC electrical spacing
Electrical spacing is one of the most critical factors in PCB safety, reliability, and insulation design. Engineers frequently ask how conformal coating clearance and creepage requirements interact with IEC and IPC rules. This guide explains how clearance (through-air distance) and creepage (surface distance) are defined, how conformal coating and Parylene affect these distances, and when – if ever – coating can support compliant insulation design.
Further reading: Wikipedia – Electrical Safety Testing
1. Understanding conformal coating clearance & creepage
Clearance is the shortest distance through air between two conductive parts, and creepage is the shortest distance along an insulating surface between those parts. Together they control the risk of:
- arcing and flashover
- surface tracking and carbonisation
- dielectric breakdown during transients and surges
Both IEC and IPC define spacing requirements based on:
- working voltage
- overvoltage category
- pollution degree (PD1, PD2, PD3)
- material group (CTI)
Because PCB real estate is always limited, designers often ask whether conformal coating clearance and creepage distances can be reduced compared with bare-board requirements.
In practice, the answer is sometimes for creepage, but never for clearance, and only under specific IEC rules.
2. What the standards say about conformal coating clearance and creepage
Key standards relevant to electrical spacing and how coatings influence them include:
- IEC 60664-1 – insulation coordination and PCB spacing
- IEC 61010 / 62368 – product safety standards referencing spacing
- IPC-2221B – generic PCB design spacing rules
- IPC-HDBK-830 – conformal coating handbook (guidance only)
Clearance – coating is ignored
Under all IEC product and insulation coordination standards, clearance is always measured as an air gap. Any conformal coating present is ignored. As a result, coating does not allow any reduction in clearance in IEC or IPC rules.
Creepage – coating can influence the calculation
Creepage is different. IEC 60664 allows creepage reduction only when strict conditions are met, for example:
- Pollution Degree is legitimately reduced (e.g. PD2 → PD1) by using a protective coating and controlled environment.
- The coating qualifies as “solid insulation” and passes specific dielectric and tracking tests.
Most liquid conformal coatings do not qualify as solid insulation under IEC definitions. However, Parylene can qualify when a particular grade is tested and certified as part of a defined insulation system.
3. Can conformal coating reduce electrical spacing?
Clearance (air distance)
❌ No reduction allowed.
Clearance must always be measured as if no conformal coating exists. Even a very high-dielectric coating does not change the required air gap.
Creepage (surface distance)
Coating may help protect against surface contamination and moisture; however, there are important limits:
- Liquid coatings (acrylic, urethane, silicone) cannot be used to claim reduced creepage spacing.
- Parylene may justify reduced creepage if it is tested as supplementary or reinforced insulation and approved in that role.
- Designers should assume no reduction unless there is clear, independent test evidence and a documented insulation system.
In real-world engineering, coating is primarily used to maintain creepage performance under pollution – not to achieve smaller conformal coating clearance and creepage spacings than the bare-board design would allow.
4. Coating thickness & dielectric performance
It is tempting to look at coating dielectric strength and assume that high values allow tighter spacing. In reality, the typical film thickness is so low that the total additional withstand voltage is modest.
Typical dielectric strengths:
- Acrylic: 80–150 kV/mm
- Urethane: 100–130 kV/mm
- Silicone: 20–35 kV/mm
- Parylene C: 200–250 kV/mm
- Parylene N: 240–280 kV/mm
In practice, thickness is typically only 25–50 µm for liquids and 5–20 µm for Parylene. Therefore, the extra voltage withstand added by coating is roughly:
- Liquid coatings: approximately +2–6 kV
- Parylene: approximately +1–4 kV
This additional margin improves environmental robustness and surge performance, but it is not enough to override IEC clearance and creepage tables. The standards always take precedence when deciding electrical spacing.
5. Pollution degree & insulation coordination
Conformal coating can influence the Pollution Degree that is considered in the design. For example, a well-controlled, coated assembly may behave more like PD1 than PD2 in service. This is particularly true for dense, pinhole-free coatings such as Parylene.
- PD2 → PD1 reduction is sometimes accepted for coated PCBs in controlled environments.
- Parylene-coated PCBs can emulate “sealed” or controlled-environment assemblies.
- Liquid coatings provide partial protection, but rarely enough for formal PD reduction under strict IEC rules.
Important distinction
Pollution degree reduction does not automatically allow creepage reduction. It only changes which creepage table you use when performing the calculation. Designers must still select distances from the correct IEC table and must not assume that any coating, by itself, changes the required conformal coating clearance and creepage spacing.
This nuance is often misunderstood and can lead to non-compliant spacing in high-voltage assemblies if it is overlooked.
6. Practical PCB design rules (SCH guidance)
Based on coating behaviour and industry standards, SCH recommends the following practical electrical spacing rules:
- Never reduce clearance due to coating – the compliance risk is too high.
- Design full creepage distances into the PCB without relying on coating to “make up” any shortfall.
- Use coating to maintain spacing performance under pollution and moisture, not to replace good layout practice.
- Parylene may justify reduced creepage only when tested as part of a certified insulation system (case-by-case).
- For >200 V designs, avoid sharp bends and convoluted paths in creepage routes.
- For PD3 or severe environments, treat coating as additional robustness, not as extra design margin.
The safest engineering assumption is simple: coating protects spacing; it does not create spacing. If a design meets IEC requirements with no coating, conformal coating will only improve reliability.
7. When coating truly helps high-voltage performance
Even though coatings seldom change the required distances, they provide measurable reliability benefits in high-voltage (HV) designs, including:
- reduced surface leakage currents
- protection against moisture, condensation, and airborne contamination
- lower ion migration and reduced dendritic growth risk
- improved long-term insulation resistance (IR)
- enhanced surge and overvoltage robustness
Because of these advantages, conformal coating is used extensively in:
- automotive power electronics
- EV traction and charging systems
- industrial motor drives and inverters
- defence and aerospace HV electronics
In all of these sectors, coating acts as a reliability multiplier for a design that already meets the correct clearance and creepage requirements, rather than as a way to bypass them.
8. Summary – conformal coating clearance and creepage
- Clearance cannot be reduced by any conformal coating.
- Creepage usually cannot be reduced, unless certified Parylene insulation is used within a defined system.
- IEC 60664 remains the key standard for determining electrical spacing.
- Coating supports reliability under real-world environmental stress but does not replace spacing design.
- Always design spacing into the PCB first – treat coating as a protective layer only.
By understanding how conformal coating clearance and creepage fit into insulation coordination, designers can create safer, more reliable HV assemblies while remaining fully compliant with IEC and IPC requirements.
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