Conformal coating — when you need it and how to specify it.

What conformal coating does

An assembled PCB in a humid environment fails primarily through two mechanisms: (1) electrochemical migration — ionic contaminants (flux residue, skin oils, salts) dissolved in surface moisture form an electrolytic path between adjacent conductors, leading to leakage current and eventually dendritic growth that bridges the gap; and (2) corrosion of conductor surfaces, solder joints, and pad metallisation.

Conformal coating doesn’t seal the board hermetically. At 25–75 µm thickness, it slows moisture vapour transmission but doesn’t stop it. What it does is: raise the humidity threshold at which condensation-related failures occur, dilute ionic contaminants so their effective concentration is below the electrochemical-migration threshold, provide a physical barrier against chemical splash, and reduce mechanical abrasion of delicate components. That’s enough for most non-hermetic applications.

When it’s required vs. optional

Required (specify on drawing):

• IPC Class 3 (high-reliability: aerospace, military, medical equipment where failure risk to life).
• Automotive: any board exposed to the under-hood environment (condensation, oil, cleaning chemicals). ISO 26262 ASIL C/D designs frequently require it. Automotive PCB design standards reference conformal coating as standard practice.
• Outdoor installations: solar inverters, outdoor LED drivers, EV charging equipment, weather stations.
• Industrial environments with condensing humidity, chemical vapour, or salt fog (ASTM B117 salt-spray test environments).
• Medical devices that may contact bodily fluids (in combination with appropriate biocompatibility-rated coating materials).

Optional but advisable:

• Consumer electronics rated for high-humidity environments (IPXX ratings even without full immersion).
• Any board with fine-pitch, high-density components where creepage distance is tight and humidity excursions are possible.

Skip it:

• IPC Class 2 consumer/commercial boards in climate-controlled environments with adequate creepage spacing. Adding coating to these adds cost, complicates rework, and provides little real-world benefit.
• Boards assembled in volumes where selective masking and application would cost more than the reliability improvement justifies.

The four main types

IPC-CC-830 defines five types by material chemistry. Four are routinely used:

Acrylic (AR) — the default choice for most designs

Fast-drying, single-component, solvent-evaporation cure (air-dry); UV-cure grades also available. Moderate moisture barrier, moderate dielectric strength, easy to apply and easy to rework (dissolves in acetone or ketones). Shelf life > 12 months. Good protection for Class 2 and moderate-environment Class 3. Degraded by hydrocarbon solvents and strong acids.

Use when: you need reliable moisture protection with the option to rework, reasonable cost, and the environment doesn’t involve sustained chemical exposure or temperatures above ~125 °C.

Silicone (SR) — best temperature range

Flexible, maintains properties from −65 °C to +200 °C (some grades to +260 °C). Excellent moisture barrier, excellent dielectric properties, but notoriously difficult to rework (requires specialty solvent or mechanical stripping). Silicone can migrate to adjacent surfaces during cure, contaminating connectors and bonding surfaces — requires careful masking. Two-part RTV or UV-cure grades available.

Use when: temperature extremes are the primary concern (under-hood automotive, industrial ovens, power electronics with high ambient Tj). Accept that rework will be destructive or very difficult.

Urethane/Polyurethane (UR) — best chemical resistance

Excellent resistance to fuels, oils, hydraulic fluid, and cleaning agents. Good moisture barrier and abrasion resistance. Very hard to remove — requires strong solvents that may damage components or the PCB substrate. Suitable for applications with chemical splash exposure where the board can’t be hermetically sealed.

Use when: the board will be exposed to hydrocarbons, lubricants, or industrial chemicals. Appropriate for sealed power modules and industrial drives where rework is rare.

Epoxy (ER) — best mechanical and chemical protection

Rigid after cure, excellent mechanical protection, chemical resistance comparable to UR. Essentially irreversible once cured — rework requires grinding or pyrolysis. High adhesion; acts like a structural element over the board. Risk of stress on SMD components during thermal cycling due to CTE mismatch.

Use when: maximum mechanical protection is required (vibration environments, potting alternatives) and rework is not expected. Less common than the other three.

Application methods

• Brush coating: manual, selective. Lowest cost, highest variability. Acceptable for small volumes or repair.
• Dip coating: the entire board is masked then dipped in liquid coating. Uniform coverage, relatively fast, but coat-everywhere approach means thorough masking is critical. Not selective by nature.
• Spray coating: manual or automated aerosol spray. Reasonable uniformity, faster than brush, requires masking for exclusion zones.
• Selective conformal coating (SCC) machine: CNC-controlled needle or valve dispenser applies coating precisely to programmed areas, eliminating masking for most features. Adds ~$0.20–2.00 per board depending on complexity and volume. The right answer for medium-to-high volume production with complex exclusion zones.
• UV cure: faster cycle time than thermal cure; requires UV-opaque masking where cure-through would be a problem.

How to specify it on the drawing

IPC-CC-830 type designation: e.g., “Apply IPC-CC-830 Class AR (Acrylic) to all board surfaces. Minimum dry film thickness: 25 µm. Maximum: 75 µm.” Then a list of exclusion zones (connector bodies, switch actuation surfaces, test points, heat-sink mating pads, potentiometer wiper areas, any area requiring press-fit or soldering after coating).

For selective application, provide a separate drawing view or CAD layer indicating coating coverage vs. exclusion in contrasting colours. Include a note on whether through-hole barrel interiors should be coated (yes for Class 3 moisture-critical applications, no if barrel conductivity is needed for test access).

Specify inspection per IPC-A-610 Section 10: conformal coating coverage, voids, delamination, and thickness if critical. UV-fluorescent dye is routinely added to most coatings to enable inspection under UV light (365 nm).

Rework considerations

Plan for coating removal before routing a board through rework. Acrylic (AR): solvent removal is clean and component-safe with ketones or proprietary removers. Silicone (SR): expensive specialty solvent (e.g., Dow Corning DS-1000) required, or mechanical stripping. Urethane (UR) and Epoxy (ER): often requires mechanical removal (micro-abrasion or grinding) which risks pad and component damage. The rework cost multiplier is a real engineering trade-off in the material selection.

The most common conformal coating error is forgetting to mask connectors. The second most common is specifying coating without specifying the exclusion list — leaving the CM to guess, and usually guessing wrong on at least one critical connector.