Voids and Bubbles in Conformal Coating

What Coating Voids Are

A void is a gap in a conformal coating where the film does not cover the surface, or where an air pocket is trapped within or beneath the film. Bubbles are the visible form: a raised blister in the coating, or a pinhole where one has burst. Both defeat the purpose of the coating, which is to form a continuous barrier against moisture, contamination, and ionic species.

The defect is often localised. A coating that is intact over most of the board can have a bubble at one end of a connector or a void under a tall component, and that single break in the barrier is where corrosion will begin. This is why coating quality is judged by the weakest point rather than by an average.

Voids also matter for coating adhesion. A region where the coating has lifted is a place where moisture can travel along the interface, spreading corrosion underneath a film that looks intact from the outside. That hidden path makes the failure hard to find and hard to diagnose.

How Bubbles Form

Three mechanisms account for most bubbles. The first is air trapped as the coating is applied, either under a component or at a sharp edge where the liquid cannot displace the air before it cures. The second is solvent or volatile release: as the coating dries or cures, solvent evaporates, and if the film has already skinned over, the vapour is trapped and forms a bubble. The third is outgassing from the board itself, including moisture released from the laminate, flux residue volatilising, and air escaping from vias and plated holes.

Vias are a common source. A plated through hole that has not been filled or tented can hold air that expands during cure and pushes through the coating. The same applies to connectors with internal cavities and to any part where air is trapped against the board surface.

Thermal effects amplify all three. A cure profile that warms the assembly too quickly drives volatiles out faster than the film can release them, and a coating applied to a substrate that is not at the specified temperature can behave unpredictably.

Substrate and Residue Effects

The coating can only adhere to what it can wet, and residue prevents wetting. Flux residue, handling oils, mould release agents, and silicone contamination all reduce surface energy so that the coating pulls back into droplets or leaves an unbonded film. Even where a film forms, a contaminated interface is a path for moisture to travel along.

Cleaning before coating is therefore the most important single control, and it must be matched to the flux chemistry. A no-clean process that leaves residue in place can be perfectly acceptable for the electrical function of the board and still be a problem for coating adhesion. Where coating is required, the residue question has to be answered with a specific coating qualification rather than assumed.

Surface treatment, such as plasma activation, raises surface energy and improves wetting where cleaning alone is not sufficient. The effect decays over time, so the interval between treatment and coating should be short and defined.

Application Methods and Voiding

Spray application, whether manual or automated, produces a thin film and generally fewer trapped bubbles, but it struggles to coat under low-standoff components and can leave shadowed areas. Because the film is thin, solvent escape is easier and bubbling is less common.

Dipping produces a more uniform film and covers complex geometry well, but it can trap air under components if the immersion or withdrawal speed is wrong, and it can build thickness where the coating drains. The withdrawal rate and the viscosity have to be matched to the geometry, and a slow, controlled withdrawal with a dwell at the top of the dip reduces entrapment.

Selective coating combines the two and is the most common approach for mixed assemblies. It gives control over where the material goes, which reduces the risk of coating connectors and test points. Regardless of method, the coating should be applied in a way that lets air escape ahead of the liquid, which usually means starting at one side and letting the flow front advance rather than spraying into the middle of a cavity.

Cure and Outgassing

Most coating bubbles form during cure rather than during application, because that is when solvent leaves and when the film solidifies. A staged cure that allows solvents to escape before the film skins over is the standard remedy. That means a period at a lower temperature, sometimes at ambient, before the final cure, so the volatile fraction can leave without being trapped.

Humidity-curing materials add another variable. They need moisture to react, and too much moisture causes a fast surface skin with unreacted material beneath it, while too little leaves the film soft. The cure environment should be specified and controlled rather than left to the ambient conditions of the workshop.

UV-curing coatings create their own void risk, because the light cures the surface quickly while shadowed areas rely on a secondary cure. A bubble that forms in a shadowed area is harder to detect and slower to resolve, and the coating may look fully cured while a soft, uncured pocket remains beneath.

Detection and Prevention

Detection of voids uses visual inspection under magnification, often with ultraviolet light if the coating fluoresces, and cross-sectioning or optical inspection for critical areas. Some programs add a glass slide inspection standard that defines the acceptable film thickness and coverage so that operators are judging against the same criterion.

Prevention is mostly a checklist. Bake the boards to remove moisture before coating. Clean and, where necessary, plasma-treat the surface. Bring the board and the coating material to the specified application temperature. Use a staged cure so volatiles can escape. Control the application method to avoid trapping air under components, and define which areas must be masked and which must be coated.

Where bubbles persist, the cause can usually be isolated by process of elimination. Bubbles at a connector point to trapped air; bubbles spread across the board point to a solvent or cure problem; bubbles at vias point to outgassing. The right corrective action follows the location.

FAQ

Why do bubbles appear in conformal coating? Usually trapped air, solvent release during cure, or outgassing from the board, especially from unfilled vias and from moisture in the laminate. The location of the bubbles usually identifies the cause.

Do I need to clean before coating? Yes. Residue and oils prevent the coating from wetting and adhering. Where no-clean residue is present, a specific coating qualification should confirm that the coating still bonds.

Does spraying produce fewer bubbles than dipping? Spray produces a thinner film and generally fewer trapped bubbles, but it covers under low-standoff parts less reliably. Dipping covers complex geometry better but needs a controlled withdrawal to avoid air entrapment.

How do I see voids under a component? Visual inspection cannot see under a package. Cross-sections, optical inspection of accessible areas, and sectioning of critical locations are used, along with a defined inspection standard.

Can a bubbled coating still protect the assembly? Only if the void is isolated and does not breach the barrier. A void or a lifted film is a place where moisture and contamination can travel along the interface, so it should be treated as a defect.

Conclusion

Coating voids and bubbles are a barrier failure, and they are almost always caused by something the process can control: trapped air, solvent leaving too quickly, outgassing from the board, or contamination on the surface. Baking before coating, cleaning and activating the surface, applying the material at the right temperature, and staging the cure to let volatiles escape removes most of them. For the surrounding process, read our notes on conformal coating, PCB assembly, quality management, and PCB capabilities to see how coating quality is controlled in 2026.