Fish-eye defects in UV conformal coatings — circular, craterlike depressions in the cured coating surface where the coating has receded from a central point and left a thin or absent film — are among the most difficult defects to eliminate without understanding their cause. They appear as small, distinct circles ranging from a few millimeters to centimeters in diameter. The center of the fish-eye typically has a very thin coating or no coating at all, surrounded by a ring of normal coating thickness. The cause is almost always surface contamination, but identifying and removing the specific contaminant requires systematic investigation.
The Mechanism of Fish-Eye Formation
Fish-eyes form through a surface tension phenomenon called Marangoni flow. When a low-surface-energy contaminant is present on the board surface at a specific location, it creates a local region of lower surface energy — typically silicone, fluoropolymer residue, or oil. When the conformal coating is applied over this contaminated area, the coating — which has a higher surface tension than the contaminant — cannot wet the contaminated zone.
As the coating is applied and spreads, it flows away from the low-surface-energy contamination site. The coating film over the contaminated area thins as coating is displaced radially outward. A circular depression forms centered on the contamination site, with normal or slightly thicker coating at the perimeter. The result is the characteristic fish-eye pattern — thin or absent center, ring of thicker coating at the edge.
The fish-eye does not necessarily expose bare board at the center — for mild contamination, the coating may be merely thinned rather than completely displaced. But even thinned coating at the fish-eye center fails to provide the barrier protection the conformal coating is intended to deliver.
Silicone Contamination: The Most Common Cause
Silicone compounds are the most common cause of fish-eye defects in conformal coating. Silicone has exceptionally low surface energy — typically 20–25 mN/m, well below the surface tension of most conformal coatings (30–40 mN/m). Even trace amounts of silicone on a board surface are sufficient to create fish-eye defects.
Sources of silicone contamination on PCBs:
Silicone-based lubricants and greases. Machine operators, maintenance personnel, or automated equipment that uses silicone lubricants near the assembly process can introduce silicone to board surfaces. Even silicone spray used in adjacent areas can contaminate boards through aerosol drift.
Silicone-bodied components. Some electronic components use silicone polymer housings, silicone-tipped test probes, or silicone gaskets. These can transfer silicone to the board surface through contact during handling or assembly.
Silicone-containing mold release. If components are molded with silicone-containing release agents, the release agent can be present on component bodies. Components handled and placed on the board transfer silicone from their bodies to the board surface and surrounding area.
Silicone outgassing from adjacent materials. Silicone-based potting compounds, silicone rubber seals, and silicone adhesive in adjacent assemblies outgas low-molecular-weight silicone cyclic compounds (D4, D5, D6 siloxanes) that deposit on nearby surfaces. Boards stored near silicone-containing materials accumulate silicone contamination over time.
Silicone in cleaning agents. Some cleaning solvents and degreasers contain silicone-based surfactants or release aids that deposit a silicone residue on the cleaned surface.
Identifying the Contamination Source
The pattern of fish-eye defects on the board helps identify the contamination source:
- Random, scattered fish-eyes: suggests airborne contamination (aerosol drift, outgassing from nearby materials) or operator hand contact
- Fish-eyes concentrated at specific component locations: suggests silicone on those specific component types or component-adjacent areas
- Fish-eyes on all boards from a specific batch: suggests contamination introduced at a specific process step or from a specific incoming material (component lot, cleaning agent lot)
- Fish-eyes on boards stored for an extended period before coating: suggests outgassing contamination during storage
If you need help identifying the source of fish-eye contamination in your conformal coating process, Email Us and an Incure applications engineer can assist with the diagnostic approach.
Other Causes of Fish-Eye-Like Defects
While silicone is the most common cause, other contaminants produce similar defects:
Fluoropolymer contamination. PTFE from threads, tape residue, or fluoropolymer-coated handling tools can cause fish-eye patterns. Fluoropolymers have even lower surface energy than silicone.
Oil contamination. Machining lubricants, hydraulic fluid, fingerprint oils, and other hydrocarbon oils create fish-eye patterns, though typically less severe than silicone because their surface energy, while lower than conformal coating surface tension, is not as dramatically low as silicone’s.
No-clean flux residues. Some no-clean flux formulations have surface energy characteristics that interfere with conformal coating wetting, producing fish-eye patterns particularly at and near soldered joints where flux residue concentrations are highest.
Eliminating Fish-Eye Defects
Source elimination. The most effective fix is eliminating the contamination source. Replace silicone-based lubricants in the assembly environment with silicone-free alternatives. Handle boards with clean nitrile gloves (not vinyl gloves, which often contain silicone-based release). Store boards in sealed bags away from silicone-containing materials.
More aggressive cleaning. If contamination cannot be fully eliminated at the source, more aggressive cleaning before conformal coating may remove sufficient silicone to allow coating without fish-eye. Aqueous alkaline cleaning or plasma cleaning can remove silicone contamination more effectively than IPA wiping. Confirm the cleaning method is compatible with the board and components.
Surfactant-modified coating formulations. Some conformal coating formulations include leveling additives or low-surface-energy additives that improve coating ability to wet over marginal contamination. These are not a substitute for source elimination but can improve coating performance on boards with low-level contamination.
Coating formulation surface tension. A conformal coating with lower surface tension is less susceptible to fish-eye formation from a given contamination level, because the surface energy difference between the coating and the contamination is smaller. Evaluate whether a formulation with better wetting characteristics reduces fish-eye occurrence.
Contact Our Team to discuss conformal coating fish-eye elimination and surface preparation for your UV conformal coating process.
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