Surface Craters & Fish-Eyes in Conformal Coating

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Fish-eyes and craters are circular surface defects where the wet film pulls away from a small local contamination source (often silicone or oils), leaving a round void or thin “ring” edge. They can look similar to de-wetting, but are often more circular, localised and repeatable when driven by point-source contamination or surface-energy hotspots.

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

Article Quicklinks

Topic	More
Definitions: fish-eye vs crater vs de-wetting	🔗
Mechanisms: surface energy and pull-back rings	🔗
Root Causes: silicone, oils, aerosols, handling	🔗
Prevention: contamination control and process discipline	🔗
Troubleshooting & Diagnosis	🔗
Repair: local rework vs strip and recoat	🔗

What are Fish-Eyes and Surface Craters?

• Fish-eye — a small, circular pull-back defect with a defined rim, typically caused by a local low surface-energy contaminant (often silicone/oil).
• Crater — a larger circular depression where coating retreats away from a point source and leaves a thin/void centre or a ring edge.

These defects are especially risky near keep-outs and fine pitch because the “ring edge” can leave local thin zones and leakage paths.

Difference vs de-wetting: de-wetting often presents as broader “islands” and patchy pull-back across an area; fish-eyes/craters are commonly round, localised and repeatable (point-source driven), although both share surface-energy mechanisms.

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How Fish-Eyes and Craters Form (Mechanisms)

• Surface-energy hotspot: a tiny contamination spot (silicone/oil) creates a low-energy region the coating cannot wet.
• Wet film pull-back: the coating retreats away from the hotspot, forming a circular rim and leaving a thin/void centre.
• Levelling can worsen visibility: as the film levels, the defect becomes a cleaner ring if the contaminant remains active.

Pattern clue: if defects appear in the same locations board-to-board (near a specific connector, handling point, or masking edge), suspect repeatable point-source contamination rather than bulk chemistry.

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Root Causes of Fish-Eyes / Craters

Contamination sources (most common)

• Silicone transfer — gloves, wipes, release agents, lubricants, sealants, RTVs, and silicone-containing products in the area.
• Oils/greases — handling residues, tooling lubricants, compressor oils, skin oils.
• Aerosols — sprays (cleaners, lubricants, air fresheners) that can deposit trace silicones/oils.

Process contributors

• Masking residues — adhesive transfer or latex residues at edges.
• Rework contamination — polish compounds, flux residues, marker inks, label adhesives.
• Inadequate cleaning validation — non-ionic residues can still drive wetting defects even when ionic tests are acceptable.

Sanity check (look-alikes): If you see broader patchy pull-back or bare “islands,” route to de-wetting. If the defect is a void inside the film rather than a surface ring, route to pinholes/bubbles/foam. If debris is embedded (not a ring), route to dust/fibres/FOD.

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How to Prevent Fish-Eyes and Craters

Contamination discipline

• Control silicone sources — treat silicones as “critical contaminants” in coating areas.
• Standardise gloves/wipes — validated, low-transfer consumables only.
• Ban aerosols near coating — even trace deposition can trigger fish-eyes.

Cleaning and handling controls

• Validate cleaning for both ionic and non-ionic residues where risk exists (process-appropriate methods).
• Control handling points and avoid touching coated areas before coating.
• Segregate rework tools/materials that may introduce oils or silicone transfer.

For standards-aligned inspection and defect calling under UV/white light, use the Inspection & Quality Hub.

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Troubleshooting & Diagnosis

• Confirm the geometry: circular rings are a strong fish-eye/crater indicator.
• Map repeatability: same locations suggest point-source contamination.
• Trace the source: gloves/wipes, masking residues, lubricants, aerosols, fixtures.
• Run a controlled wipe test on sacrificial coupons (clean vs suspected source contact) to see if rings appear (only if appropriate to your process controls).

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Repair: When to Touch-Up vs Strip & Recoat

• Small isolated rings: if accessible and acceptance allows, local removal and recoat may be possible — but only after eliminating the contamination source.
• Multiple/repeating craters: treat as contamination control failure — stripping and recoat is often the only robust outcome.
• Near keep-outs/fine pitch: treat as high risk due to thin ring edges; escalate per repair rules.

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

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Looking for Other Defect Types?

This page covers fish-eyes and surface craters. For the complete index of defect types and links to each technical article:

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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.