Pinholes, Bubbles & Foam Defects in Conformal Coating
Common issues like pinholes, bubbles and foam regularly occur in conformal coating application and can compromise PCB protection. Understanding how these defects form—and how to prevent them—is key to improving reliability and reducing costly rework.
Article Quicklinks
| Topic | More |
|---|---|
| Definitions: bubbles, pinholes & foam | 🔗 |
| Root Causes of bubbles/pinholes/foam | 🔗 |
| Prevention: settings, technique, control | 🔗 |
| Troubleshooting & Diagnosis | 🔗 |
| Related Defects | 🔗 |
What are Pinholes, Bubbles & Foam in Conformal Coating?
- Bubbles — trapped pockets of air under the coating film.
- Pinholes — burst bubbles that break through to the PCB surface.
- Foam — extensive bubbling across the coated area.
These defects can reduce insulation resistance, allow moisture, water and other chemicals to penetrate more easily through the coating, and can accelerate corrosion or failure of the PCB.
Causes of Pinholes, Bubbles & Foam Defects
- Skinning of the wet film traps solvent beneath, which later bursts out.
- Excessive thickness or high viscosity prevents bubbles settling before cure.
- Air under components is expelled during drying/curing and forms bubbles.
- Pressure pots absorbing air cause “champagne” bubbles on application.
- Brush over-working or viscous material introduces bubbles during application.
- Incorrect spray setup (head/pressure) creates excessive bubbles or foam.
How to Prevent Pinholes, Bubbles & Foam
- Use multiple thin coats with proper flash-off instead of one heavy pass.
- Control cure temperature to avoid surface skinning.
- Reduce air entrapment under parts: slower dip entry, dwell time, and lower viscosity.
- Manage pressure pots: depressurise when idle; don’t leave >50% standing under pressure.
- Brush technique: flow the coating; avoid working it into the board.
- Optimise spray parameters: verify head type and atomising/flow pressures.
Troubleshooting & Diagnosis
- Pattern boards/UV inspection — confirm meniscus, edge definition, and bubble density.
- Viscosity & solids checks — monitor solvent loss and restore target windows.
- Pot/line audits — verify pot pressure routines, line flush, and filter condition.
- Spray set-up validation — document head type, gun distance, fan width, and pressures.
- Dip parameters — qualify entry speed, dwell, withdrawal rate, and bath health.
- Cure profile review — ensure progressive ramping to minimise skinning/outgassing.
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Where This Sits Among Other Defects
Pinholes/bubbles/foam stem mainly from entrapped air or premature skinning. By contrast, orange peel is typically viscosity/flow related, de-wetting flags contamination/low surface energy, and delamination points to adhesion or curing issues. Tackling the right mechanism prevents rework and protects long-term reliability.
Training on Conformal Coating Defects & More
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.
Industry Standards We Work To
SCH Services follows international standards across all our coating services, training, equipment supply and materials. Our processes are aligned to the 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, visit:
electronics.org/ipc-standards ↗
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Why Choose SCH Services?
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