Conformal Coating Defects and Prevention

Conformal coating protects a board against moisture, contamination and ions that would otherwise promote corrosion and electrochemical migration. The coating only works if it is continuous and adherent, and most defects are not caused by the coating material itself but by the state of the surface it is applied to, by the application method and by the way the coating cures.

Why the Surface Decides

A coating spreads only where its surface energy is lower than that of the board. Contamination left from soldering raises the surface energy in some areas and lowers it in others, so the coating pulls away from the very places that need protection, such as joints, pad edges and the flanks of components.

Flux residue is the usual culprit. Rosin residues and ionic activators are both capable of changing wetting, and the effect survives a coating that looks acceptable at the time and fails in service. Cleaning before coating is therefore a process step that must be specified, monitored and verified rather than treated as optional.

Other contamination includes mould release agents from fixtures, skin oils from handling, silicone from packaging and residues from the previous process step. Silicone in particular is difficult to remove and produces crawling that no adjustment to the coating equipment can correct.

Common Defect Modes

Conformal coating defects fall into a small number of families. Dewetting and crawling appear as areas where the coating has pulled back to leave exposed metal, often with a raised edge. They are surface energy defects and they are corrected by cleaning or by a plasma treatment rather than by applying more material.

Bubbles and voids form where solvent cannot escape or where air is trapped under the film. Thick application, a high solvent content, a short flash off time and a coating applied over a via or under a component all encourage them. The bubbles matter because a burst bubble leaves a pinhole that is a direct path for moisture.

Orange peel and a rough finish indicate that the coating has begun to cure before it has levelled, or that the solvent has evaporated too quickly. The film may still be continuous, but its thickness varies and the thin areas are where the protection fails first.

Application Methods

Spray application gives the greatest control over placement and thickness and suits selective coating of a fully assembled board. The trade is overspray, which must be managed so that connectors and test points are not coated, and the need for masking and a program.

Dipping gives a uniform film on both sides in a single step and is fast, but it coats everything, so masking becomes a large part of the work. The withdrawal speed controls the thickness more than any other variable, and the viscosity of the bath changes with use and must be controlled.

Brush and dispense methods are used for repair and for small areas. They are operator dependent, and the thickness is difficult to control, so they are best limited to touching up a coated assembly rather than to production coverage.

Thickness and Coverage

The specification is usually a thickness range rather than a single figure, and the range must be verified on the assembly rather than on a test coupon. The measurement is made with an eddy current gauge on a known reference or by a section, and the practice of measuring only the flat areas overstates the protection.

Coverage of the difficult features is what determines performance. The tops of components, the underside of a package, the inside of a via and the shadowed side of a tall part are all places where the film is thinner or absent, and they are also the places where condensation collects.

The coating should extend over the board edge and a defined distance from the connector interface, and the boundary should be drawn on the assembly drawing so that it is applied consistently. A boundary that is decided by the operator will vary between shifts.

Cure and Its Control

The cure is a chemical reaction or a solvent removal, and both need time, temperature and, for some chemistries, moisture or ultraviolet light. A coating that is handled before it has cured will be damaged, and the damage may be invisible until the unit is in the field. A film that is only partly cured can pass a visual check and still fail a solvent rub, so the verification method matters as much as the cure schedule itself.

Ultraviolet cured coatings need light to reach the material, and the shadowed areas under components cure only by a secondary mechanism, if at all. Where the product relies on full cure, the shadowed areas should be treated as a known weakness and covered by a second application or a different material.

Cure verification should be part of the process record. The usual checks are a tack test, a solvent rub or a measurement of the cured thickness, and whichever is chosen should be applied to the same features in the same way each time so that the results can be compared.

Inspection and Rework

Inspection under ultraviolet light reveals the coverage and the pinholes that are invisible in white light, since most coatings contain a tracer. It is the fastest way to see a dewetted area, an uncoated joint or a bubble, and it should be used on the first article and at intervals through the run.

Where a defect is found, the rework is to remove the coating locally, clean the surface, and reapply. Removal must not damage the board or the components, since the coating cannot be stripped from a fine pitch assembly without risk to the solder joints.

The reworked area should be recoated with the same material and to the same thickness, and the record should note the rework so that a pattern can be seen. A board that has been reworked several times around the same feature is telling you something about the design rather than about the operator.

Practical Rules

Clean and verify the surface before coating, choose the application method for the shape of the assembly, and control the thickness with a measurement rather than a setting. Flash off the solvent before cure and verify the cure on the parts of the board that are hardest to reach.

Record the surface cleanliness, the thickness, the cure and the inspection results with the build records and the defect history, and treat the coating specification as a design decision rather than a finishing touch.

Process Control and Verification

On a design of this kind, conformal coating defects is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

FAQ

Why does a coating pull away from joints? Flux residue changes the surface energy locally. The coating spreads only where its surface energy is lower than the board, so contamination is the usual cause.

What causes bubbles under a coating? Trapped solvent or air from thick application, a short flash off time or coating over a via. A burst bubble leaves a pinhole that lets moisture through.

How should coating thickness be verified? On the assembly, with an eddy current gauge against a known reference or by a section. Measuring only flat areas overstates the protection.