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 the most common defects and failure mechanisms. For root causes, diagnosis, and fixes for each defect type, use the index below to open the dedicated technical article.

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

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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 screen-print inks) that present as poor wetting.

• 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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

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.

↑ Back to Index · ↗

Runs, Sags & Curtains

Runs, sags and curtains occur when an over-wet coating film remains mobile and gravity pulls it downward before the film locks. This can create local over-thickness, uneven cure and thin zones upstream — even when coverage looks “complete”.

• Use multiple light passes with defined flash-off instead of heavy wet coats.
• Control viscosity and open time to keep the film stable on edges and vertical features.
• Keep orientation consistent during flash and early cure to reduce gravity-driven migration.

↑ Back to Index · ↗

Dust, Fibres & FOD in Coating

Dust, fibres and foreign object debris become trapped in the wet film during application, flash-off or handling. These inclusions can create voids, weak adhesion points and moisture pathways, and can also drive cosmetic rejects.

• Control booth airflow, filtration and housekeeping; treat contamination as a process variable.
• Reduce handling and time between coating and flash-off; protect WIP from fallout.
• Audit consumables (wipes, gloves, masking) and clothing to reduce fibre shedding.

↑ Back to Index · ↗

Fish-Eyes & Craters

Fish-eyes and craters are circular pull-back defects where the coating retracts around a local contamination site or low surface-energy spot, leaving a “ring” or exposed centre. They’re often linked to silicone transfer, oils, release agents or incompatible residues.

• Investigate contamination sources (masking, gloves, lubricants, compressed air, conveyors, cleaning residues).
• Confirm substrate surface energy and cleanliness; isolate whether the defect is localised or systemic.
• Control rework: spot-clean and recoat only if acceptance/repair rules allow; otherwise strip and recoat.

↑ Back to Index · ↗

Haze, Whitening & Blushing

Conformal coating haze, whitening and blushing describe a cloudy or milky appearance caused by moisture–solvent interactions during drying, rapid evaporation, cold substrates, or humidity conditions that push the process outside its control window.

• Control RH and temperature in the booth and flash-off zone; avoid coating “cold” assemblies.
• Stabilise solvent balance and flash-off profile; avoid overly aggressive air movement that chills the film.
• Confirm whether haze is cosmetic or correlated with under-cure, poor adhesion or insulation resistance risk.

↑ Back to Index · ↗

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?

Partnering with SCH Services means more than just outsourcing — you gain a complete, integrated platform for
Conformal Coating, Parylene & ProShieldESD Solutions, alongside equipment, materials, and training,
all backed by decades of hands-on expertise.

• ✈️ 25+ Years of Expertise – Specialists in coating technologies trusted worldwide.
• 🛠️ End-to-End Support – Selection of chemistry/process, masking strategies, inspection, and ProShieldESD integration.
• 📈 Scalable Solutions – From prototypes to high-volume production.
• 🌍 Global Reach – Responsive support across Europe, North America, and Asia.
• ✅ Proven Reliability – Consistent results across services, equipment, and materials.

📞 Call: +44 (0)1226 249019  |  ✉ Email: sales@schservices.com  |  💬 Contact Us ›