Conformal coating is a thin polymeric film — typically between 25 and 125 micrometres — applied across a printed circuit board assembly to shield it from moisture, leakage, shock, dust, corrosion, ageing, corona, and harsh temperature environments. The protective value of that film depends almost entirely on how uniformly and how accurately it is deposited. When the layer falls below the recommended range, exposed copper traces and solder joints remain vulnerable to electrochemical migration and condensation damage. When it exceeds the upper limit, the cured film can develop micro-cracks, blister during thermal cycling, or prevent connectors and test points from making reliable contact.
This is why leading PCB conformal coating providers treat thickness verification as a mandatory production control rather than an afterthought. The IPC-A-610 assembly standard, which Farway Electronic follows on every production line, references coating quality acceptance criteria, and most acrylic, silicone, urethane, and epoxy chemistries carry a manufacturer-specified target range. Measuring the actual deposited thickness is the only objective way to confirm that the process has delivered what the design intended.
There is no single universal technique for checking coating thickness. The right choice depends on whether the coating is still wet or already cured, whether the board can be contacted or must remain untouched, and whether a single point or the entire board surface needs to be verified. The four methods below cover the overwhelming majority of production scenarios.
A wet film thickness gauge — a small notched comb or wheel — is pressed into the freshly applied, still-liquid coating before it enters the curing oven. The teeth that touch the wet coating reveal the wet film thickness, which can then be converted to an expected dry film thickness using the solid-resin content percentage supplied by the coating manufacturer. This method is fast, inexpensive, and allows operators to correct spraying parameters in real time before the coating cures. Its limitation is that it contacts the wet surface, so it is best used on test coupons or designated measurement areas rather than on functional circuitry.
After the coating has cured, a non-contact eddy-current probe can measure dry film thickness by placing the probe flat on a coated area over a conductive copper plane. Instruments such as the PosiTest system can resolve thickness to within approximately one micrometre across the 25–50 micrometre range, delivering near-instantaneous, repeatable readings. Because the probe does not damage the coating, this method is widely used for incoming inspection and production sampling on boards where a flat copper-backed reference area is available.
For the highest-accuracy laboratory verification, a coated sample or test coupon is sectioned and examined under a microscope. The cured film is visible as a distinct layer, and its thickness can be measured directly to sub-micrometre precision. Cross-sectioning is destructive, slow, and typically reserved for initial process qualification, failure analysis, or calibration of other measurement methods. It is not practical for routine production screening but serves as the reference standard against which other techniques are validated.
Most conformal coatings contain a fluorescent additive that glows blue under ultraviolet light. Because the emitted fluorescence intensity correlates with the number of fluorescent molecules in the light path — and therefore with the coating thickness — calibrated UV camera systems can map thickness distribution across an entire board without contacting the surface. This approach, first proposed in the 1970s and refined into production-grade equipment over the past decade, enables full-board, non-destructive screening. It is especially valuable for automotive and medical electronics, where every coated region must be verified rather than only sampled at a few points.
Test coupons — small blank or copper-clad panels processed alongside production boards through the same spraying and curing cycle — are one of the most practical tools for thickness control. They can be measured destructively or non-destructively without touching the actual product, and they provide a physical record of the coating application at the time each batch was produced. When a customer or auditor questions long-term reliability, a retained coupon with documented thickness data is a far stronger answer than a verbal assurance.
Farway Electronic processes coating test coupons through the same how to apply conformal coating workflow used for production boards — same spray parameters, same baking cycle, same selective masking rules — so coupon thickness readings accurately represent what was deposited on the delivered assemblies.
Thickness alone does not guarantee protection. Even when average readings fall within specification, visual and UV inspection should confirm that the coating is free of the following common defects, any of which can create a localised failure point on an otherwise acceptable board.
A robust inspection regime combines UV-light visual checks for coverage and defects with point or full-board thickness measurement for quantitative verification. Farway's coating line supports boards up to 550 mm × 470 mm, handles dense and high-pin-count assemblies, and uses selective masking to keep connectors and test points clean — all of which reduce the defect modes that thickness measurement alone cannot catch.
Target thickness is not the same for every material. The table below summarises commonly referenced dry-film ranges for the principal coating chemistries. Always confirm the exact specification with the coating manufacturer and the end-customer's reliability requirements.
| Coating Type | Typical Dry Film Range | Key Characteristics |
|---|---|---|
| Acrylic (AR) | 25 – 75 µm | Easy to rework, fast drying, good moisture resistance |
| Silicone (SR) | 50 – 200 µm | High flexibility, excellent thermal cycling performance |
| Urethane (UR) | 25 – 75 µm | Strong chemical and abrasion resistance, harder to rework |
| Epoxy (ER) | 30 – 80 µm | Very high chemical resistance, rigid, difficult to remove |
Farway Electronic operates an Anda automatic conformal-coating spraying line in its 2,000-square-metre LongGang, Shenzhen facility. The line supports fan and needle spraying, double-sided coating and baking, and selective masking, with average spraying times of 0.5 to 3 minutes per board. Thickness verification is woven into the process rather than bolted on afterward: wet-film checks during application confirm that spray parameters are on target before curing, and post-cure inspection under UV light verifies full coverage and flags defects for rework.
This production discipline is backed by ISO 9001, ISO 13485, IATF 16949, and ISO 14001 certifications, making Farway's coating service suitable for automotive, medical, new energy, security, and communications electronics where coating thickness directly affects field reliability and regulatory acceptance. The company has served more than 100 industry customers across more than 20 countries and regions, combining prototype flexibility with the documentation and traceability that regulated industries demand.
When specifying and verifying conformal coating thickness, keep the following principles in mind:
Whether you need acrylic, silicone, or urethane conformal coating on a prototype run or a production batch, Farway Electronic combines automated spraying, UV-based coverage inspection, and IPC-A-610-aligned quality control to deliver boards you can trust in the field. Contact the engineering team at sales@farway.hk or visit the conformal coating service page to discuss your thickness specification, masking requirements, and production schedule.
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