De-wetting in Conformal Coating

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De-wetting occurs when the wet conformal coating film refuses to wet the surface evenly and pulls back into craters, islands, fisheyes, or bare patches. In production this is most commonly driven by surface energy and contamination effects (not thickness or cure alone).

This page explains how de-wetting forms, how to isolate the likely mechanism, and how to prevent recurrence. For the complete index of defect types and links to each dedicated article, use the Conformal Coating Defects Hub.

📄 Download: De-wetting defect bulletin (PDF)

Article Quicklinks

Topic	More
Definitions: what de-wetting is and why it matters	🔗
How it forms: mechanisms & “tell-tale” signs	🔗
Root Causes: contamination, materials, handling, PCB factors	🔗
Prevention: control window & upstream discipline	🔗
Troubleshooting & Diagnosis: isolate the mechanism	🔗
Repair: when to touch-up vs strip & recoat	🔗

What is De-wetting in Conformal Coating?

• Definition: the coating pulls away from the surface during application/flow to form craters, fisheyes, islands, or exposed “windows”.
• Why it matters: exposed areas allow moisture/contamination onto the PCB, increasing the risk of corrosion and electrochemical migration in service.
• Common production reality: de-wetting is often intermittent (batch/lot/shift-related) because surface energy and contamination sources change over time.

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How De-wetting Forms (Mechanisms)

• Low surface energy “repulsion”: the wet film avoids areas contaminated with oils, silicones, surfactants, plasticisers, or release agents → craters/fisheyes.
• Scavenging / migration: the coating preferentially wets higher-energy regions and is drawn away from lower-energy regions → “islands” and bare windows (often near solder mask/ink boundaries).
• Substrate chemistry variation: solder mask/legend ink cure state (under/over-cure), additive packages, or lot variation changes wetting response → repeatable patterns that mirror PCB features.
• Dynamic surface energy: temperature, pre-heating, dwell time, solvent exposure, and humidity can shift wetting behaviour → defects that appear “random”.

Pattern clue: if de-wetting mirrors text/ink/fiducials or specific solder mask regions, suspect PCB material/cure. If it appears as isolated fisheyes or “holes” that don’t track board features, suspect local contamination (handling, masking, silicones, oils).

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Root Causes of De-wetting

Contamination & handling (most common)

• Silicones — sprays, lubricants, release agents, RTV vapours, some masking products, certain gloves/creams (classic fisheye trigger).
• Oils/grease — fingerprints, machining oils, process oils, conveyor/fixture contamination.
• Flux residues / soldering chemistry — particularly where cleaning and rinsing are marginal or inconsistent.
• Contaminated cleaning baths — contamination redistributed rather than removed; poor dry-out re-deposits residues.

PCB fabrication factors

• Solder mask cure state — under-cure can leave unreacted resins/plasticisers/surfactants; over-cure can leave a glassy low-energy surface; both reduce wetting.
• Legend inks / screen print — incompatible binders, cure variation, or additives can repel coating and create defects that mirror print patterns.
• Fab residues — processing aids, surfactants, and release agents from PCB manufacture (often lot-related).

Process interactions that make it worse

• Pre-heat / temperature sensitivity — can mobilise contaminants or change wetting response, making defects appear after “harmless” changes.
• Solvent exposure / dwell time — wiping, partial cleaning, or prolonged dwell can redistribute low-energy contamination into a wider area.
• Masking as a trigger — masking choices frequently introduce low-surface-energy contamination or transfer agents. See masking causes most conformal coating defects.

Sanity check (look-alikes):

• If you see voids/bubbles/pits rather than coating pull-back, route to pinholes, bubbles & foam.
• If you see lifting/peeling after cure, route to delamination.
• If the surface is uniformly textured (not bare patches), route to orange peel.

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How to Prevent Conformal Coating De-wetting

Eliminate contamination sources

• Segregate silicones: ban silicone sprays/RTV handling near coating; control maintenance products; define “no-silicone” zones.
• Glove discipline: consistent glove type, no powders/hand creams; control handling steps between clean and coat.
• Masking discipline: qualify masking products for transfer risk; avoid unknown tapes/adhesives; standardise approved materials.

Stabilise cleaning & dry-out

• Validated cleaning process with effective rinsing and controlled dry-out (avoid “cleaned but still wet/solvent-rich” boards).
• Bath control: monitor chemistry, contamination loading, change intervals, and rinse performance so residues are removed, not redistributed.
• Witness coupons: run known-clean coupons/boards alongside production to catch upstream contamination shifts early.

Control PCB-related contributors

• Supplier escalation if defects mirror solder mask/ink features: request cure verification, lot traceability, and material/cure window review.
• Surface energy checks (where appropriate): dyne pens or contact-angle testing on suspect solder mask/ink regions.

For upstream controls that prevent multiple defect types, see Surface Preparation & Cleanliness for Reliable Coating.

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Troubleshooting & Diagnosis

1) Confirm the pattern (fastest win)

• Map where it occurs: solder mask vs copper vs legend ink. Good wetting on copper but not mask/ink strongly indicates surface energy rather than coating chemistry.
• Does it mirror PCB features? If it tracks text, fiducials, or mask regions, suspect ink/mask cure or fab residues.
• Wet-film observation: de-wetting is often easier to see during application/flash than after cure.

2) Test the likely drivers

• Surface energy check on suspect regions (dyne/contact angle) to confirm low-energy behaviour.
• Thermal sensitivity: lightly pre-heat and observe changes (contamination mobilisation and wetting shifts are common).
• Masking audit: review tapes/boots/dots/adhesives and nearby silicone sources; de-wetting “sudden onset” is often a product/substitution issue.
• Cleaning audit: verify bath chemistry, rinse effectiveness, drag-out control, and dry-out (avoid “cleaning re-deposition”).

3) Separate board vs process

• Witness coupons: run known-clean coupons alongside production. If coupons wet well but boards don’t, the root cause is usually incoming PCB/components.
• Lot traceability: tie defects to PCB lot/date/supplier process changes; de-wetting often correlates strongly once tracked properly.

Inspection note: de-wetting is frequently missed if relying on UV fluorescence alone. White-light inspection during wet film and early flash-off is often more revealing. See the Inspection & Quality Hub.

📄 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

• Small isolated craters: if contamination source is removed and the surface can be re-cleaned/activated, local repair may be possible (validate acceptance and inspect boundaries).
• Feature-tracking de-wetting (ink/mask pattern): treat as a systemic wetting failure—local touch-up often repeats; stripping and recoat after corrective action is usually required.
• Unknown contamination source: assume recurrence risk is high; stripping and recoat after root-cause removal is typically the robust option.

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

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Looking for Other Defect Types?

This page covers de-wetting (a surface energy/contamination defect). For the complete index of defect types and links to each technical article:

Explore the Defects Hub ↗

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.