The Protection You Cannot See
Conformal coating is a thin polymeric film — typically 25–250µm — applied to a PCB assembly to protect it from moisture, dust, chemicals, and temperature extremes. Acrylic, silicone, polyurethane, epoxy, and parylene are the common materials, each chosen for specific environmental exposures.
But applying coating is only half the job. Verifying it was applied correctly is the other half — and it's not as simple as looking at the board under room light.
Most conformal coatings are transparent or translucent when cured. A board that looks "coated" under ambient light may have pinholes, thin spots, bubbles, or completely missed areas — any of which defeats the purpose of coating. Conformal coating inspection is the QC station that ensures the protective layer is continuous, the right thickness, and free of defects before the board is cleared for shipment.
At Superb Automation, conformal coating inspection is a mandatory step for every coated assembly. Here's what we check and how.
What Conformal Coating Protects Against
Before understanding the inspection criteria, it helps to know what the coating is fighting:
| Threat | Effect on Uncoated PCB | How Coating Protects |
|---|---|---|
| Humidity / condensation | Electrochemical migration, dendritic growth, corrosion | Hydrophobic barrier prevents moisture film formation |
| Salt spray / marine atmosphere | Accelerated corrosion of exposed copper and solder | Impermeable coating isolates metal surfaces |
| Dust and conductive particles | Short circuits between closely spaced conductors | Physical barrier prevents particulate contact |
| Chemical exposure (fuels, cleaning solvents, industrial atmosphere) | Metal corrosion, solder joint degradation | Chemical-resistant polymer selected for specific exposure |
| Fungal growth | Mycelial bridges causing shorts, organic acid corrosion | Fungus-resistant coating (polyurethane, parylene) |
| Thermal shock (condensation during rapid temperature changes) | Water droplets causing instantaneous shorts | Coating prevents condensation nucleation on metal |
Given these failure modes, a coating that is 95% complete is not a coated board — the 5% gap is where failure starts.
The Three-Part Inspection Protocol
1. UV Light Inspection — Visual Coverage Verification
Most conformal coatings include a UV tracer — a fluorescent dye that glows brightly under ultraviolet (365nm) light. Under UV illumination, coated areas fluoresce (typically blue-white for acrylic, yellow-green for silicone), while uncoated areas remain dark.
What we look for:
Uncoated areas (voids): Dark spots where coating should be. Most common around tall components that shadow the spray pattern, under large QFP packages, or on connector bodies that were masked.
Pinholes: Tiny dark dots indicating a bubble that burst after application, leaving the substrate exposed. Pinholes as small as 0.1mm are visible under UV with magnification.
Bubbles: Bright rings with dark centers — trapped air that formed a dome in the coating. Bubbles near high-voltage nodes or fine-pitch leads are defects; bubbles in open board areas may be acceptable depending on size and quantity.
Incomplete edge coverage: Dark lines along board edges or around mounting holes where coating pulled back during curing (dewetting). Edges are the most common moisture ingress point.
Overspray onto keep-out zones: Fluorescence on connector pins, test points, edge connectors, or thermal pads that were supposed to be masked. Conformal coating is an electrical insulator — coating a connector pin creates an open circuit.
Equipment: Superb uses a UV-A inspection lamp (365nm peak, 100W) in a darkened inspection booth. Each board is examined systematically, zone by zone, at 3× to 10× magnification.
2. Coating Thickness Measurement
Coverage without correct thickness is not protection. Too thin and the coating is porous — moisture and contaminants penetrate. Too thick and it cracks during thermal cycling, traps heat, or interferes with component clearance in tight enclosures.
Measurement method: Wet-film gauge during application, or dry-film eddy-current probe after curing. For acrylic and silicone coatings on PCBs:
| Coating Type | Target Thickness (Dry) | Minimum | Maximum |
|---|---|---|---|
| Acrylic (AR) | 30–130µm | 25µm | 150µm |
| Silicone (SR) | 50–210µm | 50µm | 250µm |
| Polyurethane (UR) | 30–130µm | 25µm | 150µm |
| Epoxy (ER) | 30–130µm | 25µm | 150µm |
| Parylene (XY) | 12–50µm | 5µm | 75µm |
Measurements are taken at 5-10 locations per board — corners, center, and near tall components where coating tends to thin. All measurements must fall within the specified range. A single measurement outside range triggers rework.
3. Coverage Under Components (Shadowing Check)
The most difficult area to coat — and inspect — is the space under tall components and connectors. Spray coating relies on line-of-sight from the nozzle; areas shadowed by component bodies may receive little or no coating.
How we verify: A borescope or angled UV light with a mirror probe examines under connectors (USB, RJ45, pin headers), large capacitors, and transformers. If coating is absent under a component that needs protection (especially in high-humidity applications), the board is reworked or the coating method is changed — selective robotic dispensing with a needle applicator can reach under low-clearance components that spray cannot.
Common Coating Defects and Root Causes
| Defect | Appearance | Root Cause | Action |
|---|---|---|---|
| Dewetting | Coating pulls back from board surface in islands or craters | Surface contamination — flux residue, finger oils, mold release on PCB | Clean board before coating |
| Orange peel | Rough, dimpled surface texture | Coating viscosity too high; solvent evaporating too fast | Adjust thinner ratio or spray pressure |
| Blisters / bubbles | Raised domes, often clustered | Outgassing from PCB or components during curing; moisture trapped under coating | Pre-bake boards at 100°C for 2-4 hours before coating |
| Cracking / delamination | Fine cracks visible under UV as dark lines | Coating too thick; CTE mismatch between coating and substrate; thermal shock | Reduce thickness; select coating with matching CTE |
| Fisheyes | Small circular voids with raised edges | Surface contamination — silicone oil, mold release, or fingerprint residue | Clean with IPA; wear gloves during handling |
| Sag / runs | Uneven thickness with visible drip patterns | Excessive coating application; viscosity too low | Reduce spray time or increase viscosity |
When Conformal Coating Inspection Is Mandatory
Not every PCB needs conformal coating, and not every coated board needs the full three-part inspection. The level depends on the application:
| Application | Coating Required? | Inspection Level |
|---|---|---|
| Consumer indoor electronics | No (unless specified) | N/A |
| Industrial control (factory floor) | Optional | UV visual check |
| Automotive (cabin electronics) | Recommended | UV + thickness |
| Automotive (under-hood, chassis) | Mandatory | UV + thickness + shadowing |
| Marine / offshore | Mandatory | UV + thickness + shadowing + adhesion test |
| Aerospace / avionics | Mandatory | Full per IPC-CC-830 |
| Medical (non-implantable) | Mandatory | Full per IPC-CC-830 |
| Military / defense | Mandatory | Full per MIL-I-46058C or IPC-CC-830 |
At Superb Automation, we default to UV + thickness inspection for all coated boards. Customers with higher reliability requirements can specify full IPC-CC-830 including adhesion (cross-hatch tape test per IPC-TM-650 2.4.1.6) and dielectric withstand voltage testing.
Superb Automation's Coating Capabilities
Coating materials: Acrylic, silicone, polyurethane, UV-cure — sourced from HumiSeal and Electrolube
Application methods: Selective robotic spray (PVA or Asymtek), manual spray for prototypes, dip coating for small high-volume boards
Masking: Custom-cut Kapton tape and latex peelable masking for connectors, test points, and keep-out zones
Inspection: UV-A lamp, eddy-current thickness gauge, stereomicroscope at 10-40×
Rework: Localized coating removal with micro-abrasive blasting or solvent pen for component replacement; re-coating after rework verified by UV re-inspection
This article is part of Superb Automation's PCBA Quality Control & Testing series. Previous: FCT Functional Test | Next: Thermal Stress Test
