A conformal coating acts as a barrier between the circuit board and its operating environment. The thickness of that barrier directly determines how well it performs. If the layer is too thin, the board may not survive humidity cycling or chemical exposure. If the layer is too thick, the coating can develop stress cracks during thermal expansion, trap solvents that outgas later, or pool around connectors and cause electrical issues.
Most acrylic, silicone, and urethane conformal coatings perform well within a dry-film range of roughly 25 to 75 microns (about 1 to 3 mils), though some applications call for thicker builds. Hitting that window consistently across every board in a production batch is what separates a reliable conformal coating pcb process from an unpredictable one.
Several standards define both the coating materials and the acceptance criteria for coated assemblies. The most commonly referenced documents include:
| Standard | Scope |
|---|---|
| IPC-CC-830 | Qualification and performance of conformal coatings, including thickness and dielectric requirements |
| IPC-A-610 | Acceptability of electronic assemblies, including visual inspection criteria for coating coverage and defects |
| UL 94 | Flammability rating of coating materials |
| MIL-I-46058C | Military specification for electrical insulating compounds (legacy, still widely referenced) |
IPC-A-610 is particularly important for PCBA manufacturers because it defines what a compliant coating looks like under visual inspection — covering sharp edge coverage, absence of bubbles, masking of keep-out areas, and uniform appearance. Understanding how to check conformal coating against these visual criteria complements the quantitative thickness measurements described below.
Dry film measurement is performed after the coating has fully cured. It gives a definitive reading of the final coating thickness on the board or on a test coupon processed alongside it.
Eddy current instruments measure the distance between a probe and the conductive copper beneath the non-conductive coating. The probe generates an alternating magnetic field; when placed near a metal substrate, induced eddy currents change the probe impedance in proportion to the coating thickness. Modern eddy current gauges can resolve thickness to within approximately one micron and are fast enough for production-line use.
Ultrasonic gauges send a high-frequency sound pulse through the coating. The pulse reflects off the interface between the coating and the substrate, and the instrument calculates thickness from the time-of-flight and the known speed of sound in the coating material. This method is useful when the substrate is non-metallic (for example, ceramic or FR-4 areas without copper) where eddy current probes cannot operate.
Optical profilers and laser displacement sensors can map coating topography without contacting the surface. These are typically laboratory-grade tools used for process development rather than routine production checks. They are valuable when you need to characterise coating uniformity across an entire board or study how coating behaves around component edges and corners.
Wet film measurement is performed immediately after coating application, before curing. It lets operators verify thickness while the coating is still workable, so adjustments to spray pressure, conveyor speed, or viscosity can be made on the spot rather than discovered after a full batch has cured.
The most common tool is a wet film gauge — a notched comb or wheel with teeth of known depth. The gauge is pressed into the wet coating and withdrawn; the coating wets the teeth up to a point that corresponds to the wet film thickness. The dry film thickness can then be estimated by multiplying the wet film reading by the volume solids percentage of the coating material.
A test coupon is a small bare board or metal panel that travels through the same coating line as the production boards. Because it has a flat, predictable surface, it provides a clean reference for thickness measurement without the complications of component geometry.
Coupons are typically processed at the start, middle, and end of a production run to verify that the coating line remains stable throughout the batch. They can also be retained as physical records of the coating thickness applied on a given date, which supports traceability requirements in automotive, medical, and industrial sectors.
| Challenge | How to Address It |
|---|---|
| Uneven coating on populated boards | Use test coupons for repeatable readings; supplement with visual inspection of the populated board |
| Coating on non-copper areas | Switch to ultrasonic measurement where eddy current probes cannot detect a conductive substrate |
| Variation across a single board | Measure multiple points — corners, centre, and near high components — and track the range, not just the average |
| Solvent entrapment in thick coatings | Verify thickness against the coating manufacturer's maximum recommended build; adjust spray parameters if readings trend high |
Farway Electronic operates an automated conformal coating line at its Shenzhen production facility, designed to protect circuit boards from moisture, leakage, shock, dust, corrosion, ageing, and harsh temperature environments. The line supports boards up to 550 mm by 470 mm, handles dense and high-pin-count assemblies, and offers selective masking, double-sided spraying and baking, and both fan and needle spray modes with average spray times of 0.5 to 3 minutes per board.
Thickness control is integrated into a broader quality system that also includes AOI optical inspection, X-ray inspection, thermal imaging, and high- and low-temperature reliability testing. For customers who need full traceability, Farway combines its conformal coating capability with smt pcb assembly and pcba testing services, so that coating thickness data sits alongside solder inspection results and functional test records within a single manufacturing flow.
The company holds ISO 9001, ISO 13485, IATF 16949, and ISO 14001 certifications, and works to IPC-A-610 acceptance criteria for PCBA assembly. This means coating thickness is not measured in isolation but as one checkpoint in a documented, standardised process that spans from PCB fabrication through finished-product assembly.
Selecting a measurement method depends on where you are in the process and what level of precision you need:
| Situation | Recommended Method |
|---|---|
| In-process check during spraying | Wet film gauge on a test coupon |
| Post-cure verification on copper areas | Eddy current gauge |
| Post-cure verification on non-copper areas | Ultrasonic gauge |
| Detailed uniformity mapping for process development | Optical profilometry |
| Production traceability record | Retained test coupon with documented eddy current readings |
In practice, most production lines use a combination: wet film gauges for real-time control during application, and eddy current or ultrasonic readings on cured coupons for verification and record-keeping. Understanding what is conformal coating used for in your specific end product will also help you determine the acceptable thickness range and the inspection rigour required.
Consistent coating thickness comes from a controlled, automated line paired with systematic measurement and inspection. Farway Electronic combines automated spraying, test-coupon verification, and IPC-A-610-based inspection to keep your boards protected batch after batch.
Whether you need conformal coating as a standalone service or as part of a full turnkey PCBA build — from SMT assembly through functional testing and box-build — Farway's engineering team in Shenzhen is ready to review your requirements.
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