Pinholes, Bubbles & Foam Defects in Conformal Coating

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Pinholes, bubbles and foam are void-related conformal coating defects caused by trapped air, dissolved gases, or volatile release during application, flash-off, drying and cure. This page explains how they form, how to diagnose the likely mechanism, and how to prevent recurrence.

For a complete index of defect types and links to each technical article, use the Conformal Coating Defects Hub.

📄 Download:
Pinholes, Bubbles & Foam Defects bulletin (PDF)

Article Quicklinks

Topic	More
Definitions: bubbles, pinholes & foam	🔗
How they form: mechanisms & “tell-tale” signs	🔗
Root Causes: process, material, equipment	🔗
Prevention: control window & recipes	🔗
Troubleshooting & Diagnosis: isolate the mechanism	🔗
Repair: when to rework vs strip & recoat	🔗

What are Pinholes, Bubbles & Foam in Conformal Coating?

• Bubbles — trapped pockets of air or vapour within or under the coating film.
• Pinholes — small voids that break through to the PCB surface (often from bubble burst or outgassing).
• Foam — extensive micro-bubbling across an area, typically linked to air entrainment or gassing during application.

These defects can reduce insulation resistance, create pathways for moisture/contamination, and accelerate corrosion and long-term reliability failures.

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How Pinholes, Bubbles & Foam Form

• Film skinning: the surface cures/dries first, trapping solvent or vapour underneath → bubbles/pinholes as it vents.
• Outgassing: volatiles released from the PCB, components, residues, or porous materials → bubbles that appear during cure.
• Dissolved gas release: gas absorbed in coating (often via pressurisation) comes out of solution during spray/flow → “champagne” bubbles.
• Air entrapment: geometry traps air (under components, sharp steps, dense areas) which later escapes into the wet film.
• Atomisation/entrainment: spray settings or over-brushing physically introduce air → foam/micro-voids.

Pattern clue: localised defects around tall parts/underneath bodies often point to air entrapment/outgassing; uniform “sparkle” foaming across a sprayed area often points to atomisation/pressurised pot gas release.

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Root Causes of Pinholes, Bubbles & Foam

Process & Technique

• Excessive thickness / high viscosity prevents bubbles rising and clearing before cure.
• Insufficient flash-off traps solvent/volatiles; too aggressive drying can drive skinning.
• Over-brushing introduces air into viscous films.
• Dip parameters (fast entry/withdrawal, no dwell, poor wetting) trap air under parts.

Material & Condition

• Solvent imbalance (thickened material, high solids) increases air retention and slows bubble escape.
• Moisture sensitivity (some chemistries) can contribute to gassing and micro-voiding if humidity control is poor.
• Contamination can worsen wetting and create local vent paths that become pinholes.

Equipment

• Pressure pots absorbing air (especially left pressurised / partially filled) → dissolved gas release during application.
• Incorrect spray setup (head/nozzle selection, atomising vs fluid pressure imbalance, gun distance) → foam and bubbles.
• Filters/lines: restrictions and turbulence can contribute to entrainment; dirty filters also destabilise flow.

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How to Prevent Pinholes, Bubbles & Foam

Stabilise the control window

• Use multiple thin coats with defined flash-off rather than one heavy pass.
• Control viscosity: measure/record, correct solvent loss, and keep within a validated range.
• Control cure/dry profile: avoid rapid skinning; use a progressive profile appropriate to chemistry and thickness.

Stop entrainment and gas release

• Pressure pots: depressurise when idle; avoid leaving material standing under pressure; keep routines documented.
• Spray recipe: lock head/nozzle type, gun distance, fan width, and pressures to a standard recipe (then train to it).
• Brush technique: “flow it on” — minimise agitation/overworking.

Geometry-driven actions (dip / complex assemblies)

• Dip entry/withdrawal: slow down, add dwell for wetting under bodies, and optimise withdrawal to reduce voiding.
• Pre-bake / dry-out where appropriate to reduce outgassing from porous materials (validate vs components).
• Design/masking: avoid unvented cavities where possible; mask or redesign if voids recur in a known geometry.

If you’re building acceptance criteria and inspection routines, use the Inspection & Quality Hub.

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Troubleshooting & Diagnosis

1) Confirm the pattern (fastest win)

• UV inspection: map defect density and whether it correlates to geometry (under parts, edges, tall components).
• Time-to-appearance: in wet film vs during cure vs post-cure inspection.

2) Check the three big levers

• Viscosity/solids: verify the material is inside the validated range.
• Pot & line routine: pressurisation habits, purge/flush routines, filter condition, turbulence points.
• Cure/dry profile: look for skinning (too fast) or solvent trap (too thick / too little flash).

3) Validate the method

• Spray: document head/nozzle, pressures, distance, overlap, and trigger technique; compare to recipe.
• Dip: qualify entry speed, dwell, withdrawal rate, and bath health (viscosity, contamination, evaporation).

📄 If you want the condensed version for your team: download the PDF bulletin.

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Repair: When to Touch-Up vs Strip & Recoat

• Localised pinholes: if within acceptance and not bridging keep-outs, controlled touch-up may be appropriate (after cleaning/prep).
• Dense voiding / foam: typically indicates a process condition problem — stripping and recoat is often the only robust option.
• Under-component voiding: consider whether the defect is accessible/inspectable; if not, treat as a process escape and rework accordingly.

For removal workflows and best-fit methods, see the Removal & Rework Hub.

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Looking for Other Defect Types?

This page covers pinholes, bubbles and foam. For the complete index of defect types and links to each technical article:

Explore the Defects Hub ↗

Training on Conformal Coating Defects

SCH offers conformal coating training that goes beyond theory—recognising and preventing pinholes, bubbles, foam, orange peel, de-wetting, delamination, and cracking. We cover process analysis, troubleshooting, materials, and application methods.

Explore Conformal Coating Training ↗

Industry Standards We Work To

SCH Services aligns coating services, training, equipment supply and materials to relevant IPC standards, including:

• IPC-A-610 – Acceptability of Electronic Assemblies
• IPC-CC-830 – Qualification & Performance of Conformal Coatings
• IPC-HDBK-830 – Conformal Coating Handbook (guidance and best practice)

For further details on IPC standards: electronics.org/ipc-standards ↗

Why Choose SCH Services?

You gain a complete, integrated platform for Conformal Coating, Parylene & ProShieldESD—plus equipment, materials and training—backed by decades of hands-on process support.

• 🛠️ End-to-End Support – Selection, masking, inspection and troubleshooting.
• ✅ Process Discipline – Recipes, control windows and repeatability.
• 🌍 Global Reach – Support across Europe, North America and Asia.

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

Note: This article provides general technical guidance only. Final design, safety, and compliance decisions must be verified by the product manufacturer and validated against the applicable standards.