Conformal Coatings: Comparison of Protective Coatings for Printed Circuit Boards
Electronic assemblies are exposed to numerous environmental influences during operation: moisture, dust, chemicals, temperature changes, and mechanical stress. Conformal coatings—thin protective layers that adapt to the geometry of the circuit board—form the first line of defense against these influences. But which material is best for which application? This technical article compares the five common material classes, explains application methods, and provides practical decision-making aids for developers and production managers.
What are conformal coatings?
Conformal coatings are thin polymer layers (typically 25 to 75 micrometers) that are applied to assembled printed circuit boards to protect electronic components from environmental influences. The term "conformal" means that the coating adapts to the three-dimensional geometry of the assembly—it follows the contours of components, solder joints, and conductor tracks.
Difference from casting compounds
Unlike potting compounds, which completely encapsulate electronics and can be several millimeters thick, conformal coatings form only a thin protective layer. This has significant advantages:
- Lower weight: Critical for aerospace applications and mobile devices
- Better heat dissipation: The thin layer has little effect on heat dissipation.
- Repairability: Coatings can usually be removed to replace defective components.
- Visual inspection: Components remain visible for optical quality control
- Cost efficiency: Lower material consumption for large-scale assemblies
protective functions
Conformal coatings fulfill several protective functions simultaneously:
- Moisture barrier: Prevention of corrosion and electrochemical migration
- Insulation: Increase in creepage voltage between adjacent conductors
- Mechanical protection: shielding against abrasion and light impacts
- Chemical resistance: Protection against solvents, oils, and aggressive gases
- Dust protection: Prevention of short circuits caused by conductive particles
- Biological protection: Defense against mold and microorganisms in humid environments
Comparison of the 5 material classes
Acrylic (AR) – The universalist
Acrylic-based coatings are single-component systems that cure through solvent evaporation. They offer a good balance between protective effect, workability, and cost-effectiveness. Acrylic coatings are transparent and allow components to be inspected even after coating. A key advantage is that they can be removed with solvents, which makes repairs easier.
Typical applications: Consumer electronics, household appliances, non-critical industrial electronics, prototypes
Polyurethane (UR) – The all-rounder
Polyurethane coatings combine high mechanical strength with excellent chemical resistance. These mostly two-component systems cure through chemical reaction and form a hard, resistant layer. They offer better protection than acrylic, but are more difficult to remove—repairs require sanding or aggressive solvents.
Typical applications: Automotive electronics (engine compartment), industrial controls, mining equipment, outdoor lighting
Silicone (SR) – The temperature pro
Silicone coatings such as the Bluesil Conformal Coating series are characterized by exceptional temperature resistance. They remain flexible and functional from -60°C to +200°C. Silicone coatings offer excellent moisture protection and low mechanical stress on components—ideal for temperature-sensitive components. Their flexibility makes them resistant to vibrations and thermal cycles.
Typical applications: Automotive (under the hood), LED lighting, high-temperature sensors, aerospace, military electronics
Epoxy (ER) – The Resilient One
Epoxy coatings offer the highest mechanical strength and best chemical resistance of all conformal coatings. These two-component systems form a hard, glass-like layer after curing. The disadvantage is that epoxy coatings are virtually impossible to repair without damaging the assembly. They are therefore primarily used for high-reliability applications where repairs are unlikely.
Typical applications: Military and aerospace electronics, medical technology (implantable devices), oil and gas exploration
Parylene (XY) – The Specialist
Parylene is a high-performance coating that is applied by chemical vapor deposition (CVD). The gaseous starting material penetrates into the smallest gaps and polymerizes to form an absolutely uniform, pinhole-free layer. Parylene offers excellent barrier properties against moisture, is biocompatible according to USP Class VI, and is extremely thin (typically 5-30 µm). The high processing costs limit its use to special applications.
Typical applications: Medical implants, high-frequency electronics, MEMS sensors, mission-critical aerospace
Comparison table of coating types
| property | Acrylic (AR) | Polyurethane (UR) | Silicone (SR) | Epoxy (ER) | Parylene (XY) |
|---|---|---|---|---|---|
| temperature range | -40°C to +125°C | -40°C to +130°C | -60°C to +200°C | -40°C to +150°C | -200°C to +220°C |
| moisture protection | Good | Very good | Excellent | Very good | Excellent |
| chemical resistance | Limited | Very good | Good | Excellent | Very good |
| Mechanical strength | means | High | Flexible/soft | Very high | means |
| repairability | Simple (detachable) | Difficult | Medium (can be cut) | Very difficult | Difficult |
| order method | Spraying, dipping, brushing | Spraying, dipping | Spraying, dipping | Spraying, dipping | Chemical vapor deposition (CVD) |
| Curing time (23°C) | 30-60 min. (touch dry) | 4-24 hours | 6-24 hours | 24-72 hours | 4-8 hours (process) |
| Dielectric constant (1 MHz) | 3.2-3.8 | 3.5-4.2 | 2.7–3.5 | 3.5-4.5 | 2.6-3.1 |
| Typical layer thickness | 25–75 µm | 25–75 µm | 50–100 µm | 25–75 µm | 5–30 µm |
| Relative costs | € (low) | €€ (medium) | $$$ (medium-high) | €€ (medium) | €€€€ (very high) |
| IPC-HDBK-830 Type | AR | UR | SR | ER | XY |
Application methods for conformal coatings
The choice of application method has a significant impact on coating quality, production speed, and cost-effectiveness. The following methods have become established in practice:
spray coating
Manual spray gun: Flexible method for prototypes and small series. The worker applies the coating to the masked assembly using a spray gun. Advantages: low investment costs, high flexibility. Disadvantages: dependent on worker skill, limited reproducibility, high overspray loss (30-50%).
Automated spraying: Robot-controlled spraying systems follow programmed paths and ensure reproducible layer thicknesses. Ideal for medium to high quantities. Modern systems with ultrasonic atomization reduce material loss to 10-20%.
Dip coating
The assembly is completely immersed in a coating bath and pulled out at a controlled speed. The layer thickness is determined by viscosity, pulling speed, and angle. Advantages: uniform coating of complex geometries, high throughput, minimal material loss. Disadvantages: connectors and test points must be laboriously masked, large bath volumes required.
selective coating
Computer-controlled dosing systems apply the coating precisely only to defined areas. The assembly passes under a dosing nozzle that dispenses the material in a targeted manner. Advantages: no masking required, minimal material consumption, different materials can be used in a single process. Disadvantages: slower than dipping or spraying, higher investment costs, primarily suitable for medium quantities.
Vapor deposition (CVD for parylene)
A special process exclusively for parylene: The solid starting material (dimer) is vaporized, pyrolyzed into monomers, and condensed onto the assembly at room temperature to form a polymer. The entire process takes place under vacuum. Advantages: Absolutely uniform coating of all surfaces, pinhole-free, penetrates microscopic gaps. Disadvantages: very high investment costs (from CHF 150,000), only contract coating is economical, batch process with several hours of cycle time.
Practical tip: Inspection with UV light
Many conformal coatings contain fluorescent additives that become visible under UV light (365 nm). This enables fast, non-destructive quality control: uneven coating, missing areas, or bubbles are immediately visible. Automated UV inspection systems are available for series production, which check and document every coated area with camera systems.
Norms and standards
Conformal coatings for professional applications must meet defined standards. An overview of the most important standards:
IPC-CC-830C
The central standard for conformal coatings, published by the Institute for Printed Circuits. It defines the five coating types (AR, ER, SR, UR, XY) and specifies test procedures and minimum requirements: insulation resistance, dielectric strength, moisture resistance, thermal shock, fungal resistance, and flame resistance. Manufacturers indicate compliance with this standard in data sheets.
IPC-A-610
"Acceptability of Electronic Assemblies" – the most commonly used standard for quality assessment of electronic assemblies. Section 10 deals with conformal coatings and defines three acceptance classes: Class 1 (General Electronics), Class 2 (Dedicated Service Electronics), and Class 3 (High Performance/Reliability). The standard specifies which coating defects (bubbles, uneven thickness, missing areas) are acceptable for which class.
MIL-I-46058C (obsolete, but referenced)
Military specification of the US Department of Defense. Officially replaced by MIL-STD-202 and MIL-PRF-55110, but still frequently cited in tenders. Defines particularly stringent requirements for temperature cycles (-65°C to +125°C), salt spray testing, and fungal resistance.
UL94 – Flame resistance
Underwriters Laboratories standard for the flammability of plastics. Conformal coatings are typically classified according to UL94 V-0 (self-extinguishing, no burning droplets) or UL94 V-1 (self-extinguishing within 30 seconds). Important for applications with high safety requirements.
EN 45545 (railway applications)
European standard for fire and smoke behavior of materials in rail vehicles. Particularly relevant for rolling stock electronics. Tests smoke development, toxicity, and flame spread under realistic conditions.
Areas of application by industry
automotive
Modern vehicles contain over 100 electronic control units (ECUs) that must withstand extreme conditions: temperature fluctuations from -40°C (cold starts in Scandinavia) to +125°C (engine compartment in summer), humidity, salt spray, fuels, oils, and vibrations. Polyurethane and silicone coatings dominate here. Typical applications: engine control units, ABS/ESP modules, battery management systems (BMS) in electric vehicles, LED headlight electronics.
Aerospace and Military
Highest reliability requirements under extreme environmental conditions: pressure fluctuations, cosmic radiation, temperature shocks, aggressive fuels. Silicone coatings and parylene are preferred. Examples: flight control systems, satellite electronics, radar and communication systems, military night vision devices, drone avionics.
industrial automation
PLC controllers, frequency converters, and sensors in factories are exposed to dust, cooling lubricants, cleaning agents, and mechanical vibrations. Acrylic and polyurethane coatings offer the best cost-benefit ratio here. Applications: Robot controllers, industrial HMI panels, process measurement technology, welding controllers.
Consumer electronics
Smartphones, wearables, smart home devices: Here, IP protection (Ingress Protection) against water and dust is paramount, combined with low weight and low cost. Acrylic and thin-film silicone coatings are standard. Examples: Waterproof smartphones (IP67/IP68), fitness trackers, Bluetooth speakers for outdoor use, smart door locks.
Marine and offshore
Saltwater atmospheres are the harshest environments for electronics: electrochemical corrosion threatens unprotected circuit boards after just a few weeks. Silicone and polyurethane coatings with high moisture resistance are indispensable. Areas of application: marine navigation and radar, offshore wind controls, ship engine monitoring, underwater ROV electronics.
medical technology
Biocompatibility according to ISO 10993 and FDA approval are key here. Parylene is the preferred material for implantable electronics (pacemakers, neurostimulators), while silicone and acrylic coatings are used for non-implantable devices. Other applications: patient monitors, portable infusion pumps, diagnostic devices.
Conformal coating vs. encapsulation: When to use which?
The decision between conformal coating and potting compound is one of the most important in the protection concept for electronic assemblies. Both technologies have their merits—the optimal choice depends on the specific requirements.
Decision criteria for conformal coating
- Repairability required: Assemblies must be serviceable in the field
- Weight-critical: aerospace, mobile devices
- Heat dissipation important: power electronics, LED drivers
- Visual inspection required: Quality assurance must be able to see components
- Large assemblies: Material costs play a role
- Moderate environmental protection sufficient: moisture and dust, but no complete immersion
Decision criteria for casting
- Maximum protection required: Persistently high humidity, immersion, high pressure
- Mechanical stresses: Strong vibrations, shock loads
- No repair planned: replacement of the entire unit in the event of a fault
- High voltages: Additional insulation and leakage current protection required
- Tamp protection: Protection against manipulation and reverse engineering
- Compact modules: Encapsulation provides mechanical stabilization and enables compact design
Combination of both methods
In practice, conformal coating and potting are often combined: The entire assembly receives a coating as basic protection, while particularly critical areas (high-voltage sections, exposed connectors, sensitive ICs) are additionally potted. This hybrid strategy combines the advantages of both technologies:
- The coating protects the main surface with minimal weight and cost.
- The encapsulation provides maximum protection for critical areas.
- Repairs are still possible in non-critical areas.
- Optimal material utilization: Casting only where really necessary
Practical example: Automotive control unit for the engine compartment: The printed circuit board is coated with silicone (temperature resistance, flexibility). The high-voltage area with ignition coil drivers is additionally encapsulated with epoxy potting compound. The connector area remains free for servicing.
Processing tips for optimal results
Preparation and masking
Cleaning is crucial: flux residues, fingerprints, and grease prevent adhesion. The assembly should be cleaned with isopropanol or special defluxers and dried completely. Manual cleaning with a brush and lint-free cloths is more thorough than spray cleaning.
Masking: Areas that must remain coating-free are protected with removable masks, Kapton tape, or liquid masking lacquers: connectors, test points, heat sink contact surfaces, buttons, switches, battery compartments, screw domes. For series production, there are silicone masking tools that are placed over the assembly like stencils.
Application and curing
Check coating thickness: Too thin (below 25 µm): insufficient protection, pinholes possible. Too thick (above 100 µm): stress cracks, longer curing time, higher costs, impaired heat dissipation. Wet film thickness gauges allow for checking immediately after application.
Accelerating curing: Most coatings cure at room temperature, but elevated temperatures significantly accelerate the process. Typically: 60-80°C for 30-60 minutes instead of 24 hours at 23°C. Important: Ramp heating (slow heating/cooling) avoids thermal stress. Moisture-curing systems (some silicones and polyurethanes) benefit from 50-60% relative humidity.
Inspection and quality control
Visual inspection: Check under white light and UV light for irregularities, bubbles, missing areas, and flux residues under the coating (appearing as dark spots under UV light).
Coating thickness measurement: Non-destructive using ultrasonic thickness gauges or eddy current sensors (only on metallic substrates). For random samples: Cross sections under the microscope.
Function test: Electrical tests after coating ensure that no areas that should remain free have been accidentally coated. High-voltage tests check the insulation effect.
Rework and repair
Acrylic: Dissolve with acetone, isopropanol, or special coating removers, then remove with a brush or swab.
Polyurethane: Scrape off mechanically with a scalpel or grinding pin, supported by aggressive solvents (MEK, NMP). Caution: Components may be damaged.
Silicone: Can be cut with a sharp knife or peeled off. Thermal method: Local heating to 250°C (hot air) makes silicone brittle and peelable.
Epoxy: Practically impossible to remove. Micro-milling or micro-sandblasting required – high risk for components.
Parylene: With plasma etching or aggressive solvents. Contract services are usually required.
Common mistakes and how to avoid them
- Bubble formation: Cause: trapped air, too fast application, outgassing of flux residues. Prevention: thorough cleaning, slow dipping/pulling process, vacuum degassing before coating.
- Orange peel effect (rough surface): Cause: excessive viscosity, incorrect spray pressure, incorrect nozzle size. Prevention: dilute according to data sheet, optimize spray parameters.
- Cracking: Cause: layer too thick, curing too fast, mechanical stress. Prevention: apply several thin layers instead of one thick layer, controlled temperature ramps.
- Delamination (peeling): Cause: poor adhesion due to contamination, unsuitable substrate. Prevention: thorough cleaning, use primer, adhesion tests before series production.
- Creepage currents despite coating: Cause: layer too thin, pinholes, contamination on surface. Prevention: check layer thickness, UV inspection, optimize cleaning
Frequently asked questions (FAQ)
Yes, but the effort involved depends heavily on the coating material. Acrylic coatings can be easily removed with solvents—the repair area is recoated after soldering. Silicone can usually be removed mechanically (cutting, peeling). Polyurethane requires more aggressive solvents or mechanical sanding. Epoxy coatings are practically impossible to repair without damaging the assembly.
Practical tip: Always use acrylic for prototypes and small series, even if polyurethane or silicone would be technically better—the repairability saves a lot of time during development.
The standard recommendation is a dry film thickness of 25-75 micrometers, which complies with IPC-HDBK-830 specifications. Layers that are too thin (less than 25 µm) offer insufficient protection and may have pinholes. Layers that are too thick (more than 100 µm) are prone to stress cracks, impair heat dissipation, and result in higher material costs.
Exception: Parylene. Due to its perfect uniformity and freedom from pinholes, 5-30 µm is sufficient for excellent protection.
Important: Manufacturers usually specify the wet film thickness in data sheets. Depending on the solids content, the dry film thickness is only 30-70% of this. A coating with 50% solids content therefore requires 100-150 µm wet film for 50-75 µm dry film.
Same coating type: Yes, no problem. Two thin layers are often better than one thick layer—better wetting, fewer bubbles, more even overall thickness. The first layer should be completely cured before applying the second layer.
Different coating types: Possible, but with restrictions. Chemical compatibility is important. Proven combinations: Acrylic as base coat + polyurethane as top coat (better mechanical strength). Silicone as base coat + parylene as top coat (optimal barrier).
Not recommended: polyurethane over silicone (poor adhesion), acrylic over polyurethane (solvent can dissolve polyurethane). If in doubt, carry out adhesion tests or consult the manufacturer's recommendations.
No. Despite its excellent properties, parylene also has disadvantages that make it unsuitable for some applications:
- Repair almost impossible: impractical for prototypes and development projects
- Batch process: Long throughput times (8+ hours per batch), unsuitable for rapid production
- Limited thickness: often too thin for mechanical protection
- Temperature-sensitive components: The CVD process requires a vacuum and, in some cases, elevated temperatures.
- Chemical resistance: Less resistant to some organic solvents than polyurethane or epoxy
- No on-site coating: Contract service always required
Conclusion: Parylene is ideal for high-reliability applications with extreme requirements for moisture protection and biocompatibility (medical technology, implants, MEMS). For most industrial and automotive applications, silicone or polyurethane offer better value for money.
Conclusion: Making the right choice
Conformal coatings are essential for protecting electronic assemblies in demanding environments. Choosing the right material and the optimal application method requires careful consideration of environmental conditions, reliability requirements, repairability, and cost-effectiveness.
Rule of thumb for material selection:
- Acrylic: For consumer electronics, prototypes, and non-critical applications requiring repair
- Polyurethane: For industrial electronics, automotive (interior), and chemically demanding environments
- Silicone: For high-temperature applications, automotive (engine compartment), high vibration loads
- Epoxy: For maximum chemical and mechanical resistance without the need for repairs
- Parylene: For medical technology, MEMS, mission-critical aerospace with the highest reliability requirements
The combination of conformal coating with targeted encapsulation of critical areas often offers the optimal solution for complex protection requirements.
