Coating Thickness: 6 Rules for Conformal Film

Coating thickness is the number that decides whether a conformal coating does its job, because the film has to be thick enough to be continuous and thin enough to cure without trapping solvent. A specification that names a material and a process but no thickness leaves the most important variable undefined.

The measurement is harder than it looks. A cured coating varies over the topography of the board, and it is thinnest exactly where protection matters most: on the edges of traces, at the base of tall components and in the shadowed areas behind a connector body.

Why Coating Thickness Is Specified at All

Below a certain thickness the film can be discontinuous over an edge or a sharp corner, and a pinhole lets moisture reach the surface anyway. Above a certain thickness the coating can crack during thermal cycling or trap solvent that later forms bubbles.

Standards therefore quote a range rather than a single value, and the range comes with a test method. A coating thickness callout that does not name a method cannot be verified at all, and a value quoted from one method cannot be compared with a value produced by another one.

Wet Film and Dry Film Measurements

The wet film measurement is taken immediately after application, using a comb gauge or an optical method, and it allows a fast correction before the board is cured. The dry film value is what the specification usually names, and the ratio between them depends on the solids content of the material.

That ratio is the reason a wet film reading is a guide rather than a record. A coating with fifty percent solids will lose roughly half its thickness as solvent leaves, so a wet film figure has to be converted before it can be compared with the dry film requirement.

Methods: Micrometer, Eddy Current, Ultrasonic, Optical

A micrometer on a coated coupon is the simplest method and also the least useful on the board, because it measures total thickness and cannot separate coating from laminate. Eddy current instruments work over metal and are quick, but they need a metallic reference beneath the film.

Ultrasonic and optical methods measure the film on the board and can resolve the individual layer, at the cost of calibration and operator skill. Cross sectioning a sample gives the reference value that the other methods are calibrated against, and it is the only way to see what happened at an edge or under a component.

Where the Coating Is Thinnest

Spray and dip processes leave less material on sharp edges and on the upper faces of tall parts. Surface tension pulls the liquid away from an edge as it cures, so the thinnest point on a coated board is usually a trace corner or a component shoulder.

Measuring the middle of a large flat area therefore reports the best case on the board and says nothing at all about the worst case. The sample positions should be chosen where the coating is expected to be thin, and the same positions should be measured each time so that the trend is comparable, as it is in other quality measurements.

Coverage Under Components and in Shadowed Areas

Coverage is a separate requirement from thickness, and the two are often confused. A film can be thick on the open board and missing entirely behind a component body, because the spray could not reach the surface at a grazing angle.

The remedy is process based: change the spray angle, apply the material from more than one direction or add a second pass, then confirm the coating thickness at the same shadowed position afterwards. A coverage gap is almost never cured by spraying more material at the same angle. The keepout and masking rules in coating masking define where the coating is required, and the coverage check should be planned from that drawing rather than from the board layout.

Thickness Against the Standard and the Customer Callout

The acceptance range usually comes from a customer specification or from a standard such as the IPC coating requirements, which define both the range and the method for a given material type. Quote the document and the revision on the traveler so that the operator knows which limits apply.

Where the product is used in a harsh environment, the customer may name a higher minimum for specific areas and a lower one elsewhere. The drawing should show that zoning, because a single number cannot describe a board with both critical and non critical surfaces.

Process Variables That Move Thickness

Viscosity, spray pressure, nozzle distance, pass speed and the number of passes all move coating thickness from one panel to the next. So does the board temperature, because a warm surface wets differently from a cold one and the film that forms is thinner at the same settings.

The variables should be recorded with the result, and the process should be requalified when any of them changes. A coating line that has been stable for months can drift when a new batch of material arrives with slightly different rheology, and the thickness record is what reveals it.

Curing, Solvent Loss and the Final Dimension

The final thickness is set during cure, when solvent leaves and the film shrinks. A cure that is too fast skins the surface and leaves solvent below, which produces bubbles and soft patches that measure thick while performing badly.

Coating thickness and cure therefore have to be qualified together, and a change in either one should trigger a recheck of the other before production resumes. Boards that will be coated on a surface that has already been characterised for cleanliness, as set out in ionic contamination testing, give the coating process a stable starting point.

Records, Coupons and Routine Checks

Keep a coated coupon with each lot, measured by the reference method, and record the wet film, the dry film and the cure conditions. The coupon is what allows a later question about coating thickness to be answered without cutting up a production board, and it costs almost nothing to prepare at the start of a lot.

Routine checks should include the thinnest expected position and a coverage inspection at the shadowed area, and the results should be linked to the board record, following the same logic as traceability for any other process. Where a board is cleaned before coating, the no-clean decision should also be documented so that the coating is applied to the surface it was qualified against.

FAQ

Can a wet film reading be used as the acceptance record? It can guide the operator, but the specification is nearly always written as a dry film value. Convert using the solids content, and confirm the result on a cured sample with the reference method.

How thick should coating thickness be for a conformal film? The range comes from the standard and from the material, and common values sit between about 25 and 130 microns depending on the chemistry and the method. The requirement should be quoted per area where the board has critical and non critical zones.

Does a thicker coating give better protection? Only up to a point. Beyond the recommended range the film can crack under thermal cycling and can trap solvent, which is why the upper limit exists and why a coating thickness above specification is also a non conformance that has to be dispositioned rather than accepted.