Every conformal coating chemistry protects a circuit board from moisture, and every one of them has a board it should never be used on. The selection question is rarely “which coats best” — it is which chemistry survives your thermal range, your rework plan, and your production rate. These are the questions that decide it.
Q: What is a conformal coating?
A: A conformal coating is a thin polymer film — typically 25–250 µm — applied over an assembled PCB to protect it from moisture, dust, chemicals, and temperature extremes while conforming to component contours. It prevents corrosion, leakage current, and dendrite growth between closely spaced conductors. The main chemistries are acrylic, silicone, urethane, epoxy, parylene, and UV-curable formulations, applied by brush, spray, dip, or selective dispensing.
Q: How thick should it be?
A: Thickness is set by chemistry and inspected against IPC-A-610’s published ranges: roughly 30–130 µm for acrylic, urethane, and epoxy; 50–210 µm for silicone; 10–50 µm for parylene. Too thin leaves pinholes and uncovered edges; too thick traps stress, cracks at component leads under thermal cycling, and bridges connector contacts. Measure wet film during application and dry film after cure — the two are not the same number.
Q: When is acrylic the right choice?
A: When rework matters and the environment is moderate. Acrylics dry by solvent evaporation, apply easily, and dissolve in common solvents, so a coated board can be reworked with minimal effort. Their limits are chemical resistance and continuous service temperature, which are lower than silicone or urethane.
Q: When is silicone the right choice?
A: When the board sees wide thermal swings or continuous high temperature. Silicone stays flexible from far below freezing to well above 150°C, absorbs the expansion mismatch between components and laminate, and damps vibration. Its trade-offs are lower abrasion and solvent resistance and a soft surface that picks up contamination. Incure’s Pyra-Sil™ silicone conformal coatings cover this class across a range of viscosities and cure paths.
Q: When is urethane the right choice?
A: When chemical and abrasion resistance dominate — fuel vapor, hydraulic fluid, solvents, or a handling environment that would scuff a softer film. Urethanes are tough and hard; that same hardness makes them stressful on leaded components under thermal cycling and difficult to rework.
Email Us with the board’s thermal range, the fluids it meets, the rework plan, and the production volume, and Incure’s engineers can identify which chemistry — and which grade — fits before a sample is coated.
Q: When is UV-cure the right choice?
A: When throughput is the constraint. Solvent-based coatings need minutes to hours to dry; a UV-curable coating cures in seconds under 365–405 nm light, with no solvent emissions and no drying oven. Incure’s Ultra-Illumina™ line illustrates the range within one chemistry: 3511 at 100–200 cP flows into fine gaps for uniform, low-profile general coverage; 3552 at 300–600 cP is an aromatic urethane-acrylate with roughly 45% elongation built to absorb thermal-expansion mismatch around power components and BGA corners; 5454F is a non-sag gel at 11,000–22,000 cP for vertical boards and tall components. Grade selection is covered in Incure’s Ultra-Illumina™ conformal coating guide and the broader introduction in What Is UV Conformal Coating?. The one design constraint is optical access: UV cure needs line of sight, so shadowed areas under tall components need a secondary cure mechanism or a coating angle that reaches them.
Q: How is coverage verified?
A: Most conformal coatings carry a UV-fluorescent tracer so coverage can be inspected under blacklight — gaps, thin spots, and masked areas show immediately. Fluorescence inspection is a coverage check, not a thickness check; pair it with a dry-film measurement on a witness coupon.
Q: What has to be masked before coating?
A: Connectors, test points, switches, heat-sink mating faces, and any surface that will be soldered or bonded later. Masking is a process step with its own cost — tape, boots, or a peelable UV-curable maskant applied and removed per board — and it scales with the number of keep-out areas. A selective-dispense process reduces masking by applying coating only where it belongs, which is one reason UV-curable coatings, dispensed in a controlled bead and cured in place, suit high-mix boards with many keep-outs.
Q: Which chemistry fails on which board?
A: Acrylic fails on boards with continuous high temperature or solvent exposure. Silicone fails where abrasion or solvent wipe-down is routine, and it contaminates surfaces that must later be bonded or soldered. Urethane fails on boards with tall leaded components cycled through wide temperature ranges, where its hardness cracks at the leads. UV-cure fails in shadow when specified without a secondary cure. Matching the failure mode to the environment is a faster selection method than comparing benefits.
A One-Line Selection Rule
Start from the harshest condition the board will meet — temperature range, chemical exposure, rework frequency, or line speed — and choose the chemistry whose known failure mode is not that condition. Thickness, application method, and inspection follow from the chemistry, not the other way around.
Contact Our Team to match a conformal coating chemistry and grade to your board, environment, and production rate.
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