Conformal Coating Defects Hub

Conformal coating defects can undermine PCB protection, reduce insulation resistance, and cause costly rework or field failures. This hub is the overview of common defect mechanisms. Use the index below to open the dedicated technical article for each defect.

This Defects Hub sits within SCH’s wider knowledge framework, linking coating failures back to process control, masking strategy, and inspection and quality requirements. Understanding defects in isolation is rarely effective — prevention depends on controlling the upstream process.

Defect Index to the Articles

Defects are grouped by mechanism to make troubleshooting faster. Use 🔗 to jump to the summary section, or ↗ to open the full technical article.

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Masking – Root Cause of Many Conformal Coating Defects

A significant proportion of conformal coating defects originate from masking decisions rather than coating chemistry or application settings. Defects typically occur at boundaries where coating is intentionally restricted, including connectors, test points, interfaces, and defined keep-out zones.

Because masking defines coating boundaries, it must be treated as a controlled process rather than a consumable choice. This is covered in detail in the Masking Hub.

Common masking-related failure mechanisms include coating ingress into prohibited areas, coating removal during de-masking, residue transfer from masking materials, and incomplete touch-up after mask removal.

• Ensure masking methods match the function (shield vs sealed barrier).
• Control fit, sealing, and placement of tapes, boots, and custom shapes.
• Define de-masking timing, technique, and mandatory post-removal inspection.
• Apply clear rules for operator touch-up versus escalation.

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Insufficient Coverage & Shadowing

Thin or missing coating occurs when tall components, dense assemblies, or approach angles prevent uniform deposition. The result is UV weak zones, missed areas (“holidays”), and under-built coverage behind components.

• Optimise spray angle, distance, and pathing around tall parts.
• Use multiple light passes to eliminate shadow zones without over-wetting.
• Verify coverage with UV inspection and defined acceptance rules.

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Coating Ingress into Keep-Out Areas

Coating enters prohibited zones such as connectors, test pads, mating faces, and contact points. This is frequently a masking boundary failure or a selective-pathing oversight, and can directly cause functional failures.

• Define keep-outs clearly and select the correct masking method (shield vs sealed barrier).
• Validate selective recipes near connectors and tight interfaces.
• Mandate post-de-mask inspection and touch-up rules.

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Bridging & Webbing

A continuous coating film spans between adjacent pins, pads, or features in tight-clearance areas. Bridging can create unwanted insulation paths, breach RF keep-outs, and thick fillets can become stress concentrators that initiate cracking.

• Avoid over-wet passes and keep-wet overlaps in fine-pitch areas.
• Control viscosity and open time so the film locks before spanning gaps.
• If cracking initiates at bridged or over-built areas, review cracking mechanisms.

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Capillary / Wicking Around Components

Capillary action draws coating into gaps/under parts and along interfaces, pulling film away from open surfaces and creating thin/bare zones or meniscus lines that can impact function.

• Reduce over-wet deposits; stabilise viscosity and “keep-wet” behaviour.
• Optimise selective valve/needle height, overlap, and pathing near capillary-prone geometry.
• Use fixtures/masking/dams to block known capillary routes; qualify with representative gaps.

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Runs, Sags & Curtains

Gravity-driven flow defects where an over-wet film moves before it can lock, leaving curtains, drips, or heavy edges and creating uneven thickness and cure.

• Use multiple light passes with defined flash-off; avoid loading vertical edges.
• Control viscosity/open time; reduce overlap and slow travel in high-risk zones.
• Hold orientation stable through flash and early cure to avoid migration.

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Pooling & Puddling

Excess coating accumulates at low points, edges, or around component features. Although coverage can look complete, thick pooled films often trap solvent, cure unevenly, and increase the likelihood of cracking and long-term reliability failures.

• Build film with multiple light coats rather than a single heavy pass.
• Control viscosity and orientation during flash-off to reduce gravity-driven migration.
• Confirm thickness at low points and validate cure profile for thick sections.

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Orange Peel

A dimpled texture driven by viscosity imbalance, spray atomisation issues, or low surface energy. Affects appearance and can correlate with poor levelling/edge definition.

• Tune solvent balance, gun distance, and fan width.
• Maintain substrate temperature and predictable surface energy.
• Use pattern boards to validate finish and recipe stability.

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Texture & Finish Defects (Router)

Texture and finish defects include roughness, dry spray, poor levelling, surface residue and other finish issues that can be cosmetic, functional, or a warning sign of film-formation problems. This router page helps you identify the pattern and then route to the most relevant mechanism.

• Use this page when the surface looks “wrong” but the mechanism is not yet clear.
• Confirm whether the issue is texture (rough/dry spray/orange peel) or surface film (haze/residue/blooming).
• Stabilise recipe and environment first (viscosity, atomisation, flash-off, RH/temperature).

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Blooming & Surface Residue

Blooming and surface residue appear as a hazy, waxy, greasy, powdery or sticky film on top of the cured coating. It is commonly linked to additive/plasticiser migration, solvent imbalance, incomplete cure, or contamination transfer.

• Check cure window stability and flash-off discipline first.
• Review contamination transfer points (masking, gloves, fixtures, compressed air).
• Confirm the issue is a surface film (not craters/pull-back or broad adhesion loss).

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Fish-Eyes & Craters

Circular pull-back defects where the wet film retracts from local contamination or low-surface-energy spots, leaving rings, craters, or “holes” in coverage.

• Eliminate silicone/oil sources and validate cleaning/handling discipline.
• Control masking materials and gloves to avoid surface-energy transfer.
• Use a controlled rework workflow if defects are local; strip/recoat if widespread.

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Dust, Fibres & FOD

Airborne or handling-related debris becomes embedded in the wet film and is locked into the cured coating. This can create leakage paths, weak zones, cosmetic rejects, or rework triggers.

• Control booth cleanliness, tack-off steps, and airflow management.
• Reduce handling after cleaning; define storage and transfer discipline.
• Inspect under UV and white light; define accept/rework rules by risk.

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Pinholes, Bubbles & Foam

Gas entrapment during application, drying and curing leaves voids that weaken protection and allow moisture ingress. Common root causes: trapped solvent, high humidity, aggressive spray settings, or insufficient flash.

• Stabilise viscosity and flash times; avoid over-wet passes.
• Manage booth RH/temperature and atomisation pressure.
• Verify drying and curing processes to prevent residual volatiles.

Many void-related defects are only identified during inspection rather than application. The Inspection & Quality Hub explains how UV inspection, thickness measurement, and acceptance criteria expose these failures.

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Tacky / Soft-Cured (Under-Cure)

Under-cured conformal coating remains soft or tacky when cure energy is insufficient or solvent remains trapped in the film. This increases contamination pickup, print-through, and long-term reliability risk.

• Confirm cure profile (time/temperature/UV dose) against the datasheet and real part temperature.
• Ensure flash-off is adequate; avoid sealing solvent beneath a skinned surface.
• Validate film build; thick sections can under-cure even if thin areas pass.

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Brittleness / Over-Cured Coating

Over-cure can leave the film overly stiff and brittle, increasing the likelihood of cracking or crazing during thermal cycling, vibration, or mechanical stress — especially at thick build and sharp edges.

• Verify cure profile and avoid exceeding time/temperature windows.
• Control thickness and edge build; thick areas see higher cure stress.
• Match chemistry (Tg/flexibility) to the duty environment and strain levels.

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Wrinkling (Recoating / Intercoat Defects)

Wrinkling is a film-formation defect where the cured surface develops ripples, ridges, or surface distortion instead of levelling smoothly. It is commonly driven by recoat timing errors, solvent attack on a partially cured layer, or cure-state mismatch between coats.

• Follow recoat windows; avoid recoating onto a skinned or partially cured film.
• Confirm solvent compatibility between layers and avoid aggressive solvent exposure.
• Where possible, use light scuff/activation steps (validated) before recoat.

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Haze, Whitening & Blushing

A cloudy or milky finish caused by moisture–solvent interactions during drying or cure. It can correlate with trapped moisture, poor levelling, and performance risk depending on severity and chemistry.

• Control RH and temperature; stabilise booth conditions.
• Use correct flash-off and avoid heavy wet films in high humidity.
• Verify cure/dry profile and confirm accept/rework rules by severity.

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Poor Adhesion on Plastics / Connector Bodies

Adhesion loss on plastics and connector bodies typically shows as peel-back, flaking, edge lift, or wipe-off. It is most commonly driven by low surface energy polymers, mould-release or silicone transfer, handling contamination, or incompatible coating chemistry.

• Confirm whether the defect is confined to plastics/connector bodies (strong clue for mechanism).
• Audit contamination transfer points (gloves, fixtures, compressed air, masking).
• Validate compatibility and (if required) controlled activation/primer steps.

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De-wetting

The wet film pulls back into “islands,” leaving bare areas due to contamination or low surface energy (silicones / oils / surfactants). It can also be triggered by substrate chemistry issues (e.g., under-cured solder resist or inks).

• Improve cleaning; validate ionic and non-ionic residues.
• Control handling/masking to avoid silicone transfer.
• Where needed, qualify surface activation/primers for difficult substrates (within process control).

De-wetting is often a symptom of upstream contamination or substrate surface-energy problems rather than coating chemistry alone.

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Delamination

Loss of adhesion between coating and substrate from poor prep, moisture, compatibility issues, or thermal/CTE stress. It often shows up first at mask edges during de-masking.

• Validate surface prep and primer/adhesion promoter compatibility.
• Control moisture (PCB/component bake where appropriate) and verify cure profiles.
• Use a controlled repair approach for local mask-edge lift; strip/recoat for widespread interface failure.

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Mask-Edge Lift & De-mask Damage

Mask-edge lift and de-mask damage occur when the coating tears, chips, peels, or lifts at masking boundaries during removal, leaving rough edges, exposed substrate, or adhesive residue. It is driven by mask selection/fit, removal timing/technique, edge stress, and adhesive interaction with the coating.

• Control de-mask timing and technique (don’t let masks “cold-weld” or over-cure into the edge).
• Use the right mask type for the boundary (barrier vs shield) and qualify adhesives.
• Define mandatory post-de-mask inspection and clear touch-up vs escalation rules.

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Cracking

Fractures form when films are too thick, too stiff, or see high thermal strain (CTE mismatch, thermal shock/cycling), creating moisture pathways.

• Keep to spec thickness; manage flash-off, solvent loss, and cure ramp.
• Select chemistries with appropriate flexibility/Tg for the duty environment.
• Validate cure profile and thermal cycling against the product requirement.

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Corrosion & Ionic Contamination

Residual ions plus moisture and bias can drive electrochemical migration and dendrites — among the most damaging failure modes.

• Qualify cleanliness (ROSE/IC/SIR as appropriate) and tighten process controls.
• Prevent moisture pathways and edge leakage; control entrapment under parts.
• Select chemistries with proven barrier performance for the environment.

Further guidance is available in the Inspection & Quality Hub, including ionic testing and verification strategy.

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Solvent Attack & Substrate Damage

Solvent attack occurs when coating solvents soften, craze, swell, or dissolve sensitive substrates such as plastics, labels, inks, and solder mask. The result can be cosmetic damage, loss of markings, adhesion problems, or functional risk.

• Check substrate compatibility and confirm solvent resistance before production.
• Control dwell time and avoid over-wet exposure on sensitive areas.
• Where required, select alternative chemistries or barrier/primer approaches (validated).

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Need Help Preventing or Solving Defects?

Whether you’re dealing with repeat coating failures, process variability, or NPI challenges, SCH can help you eliminate defects at their source — not just react to them.

🛠 Consultancy & Process Optimisation

• Root cause analysis for delamination, de-wetting, capillary wicking, contamination and cure-related defects.
• On-site or remote process audits and troubleshooting.
• NPI support — set up, validate and stabilise your process from day one.
• Improved masking, inspection and cleaning methods to reduce rework and scrap.

🎓 Technical Training for Your Team

• IPC-A-610 and IPC-CC-830 aligned inspection and best practice.
• Masking, de-masking and post-mask inspection discipline.
• Process control — thickness, viscosity, cleanliness and curing.
• Advanced modules on defect identification, RCA and repair.

→ Talk to our Consulting Team | → Explore Training Options

Why Choose SCH Services?

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Conformal Coating, Parylene & ProShieldESD Solutions, alongside equipment, materials, and training,
all backed by decades of hands-on expertise.

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