Conformal Coating Process in PCBA

In PCB manufacturing and PCB assembly, long-term reliability is often influenced by the operating environment. Moisture, dust, chemicals, and temperature cycling can affect solder joints, component leads, and exposed conductors on a printed circuit board assembly (PCBA). When a product needs additional environmental protection, conformal coating is a commonly used finishing step.

Highleap Electronics is an electronics manufacturer offering PCB fabrication services and PCB assembly services for prototype and production builds. Depending on the design and application requirements, the assembly process may include Surface Mount Technology (SMT), through-hole assembly, inspection, and functional testing. When the operating scenario involves humidity, contamination, or corrosive exposure, conformal coating can be considered as part of the PCBA finishing process.

What is Conformal Coating?

A conformal coating is a thin protective polymer film applied to a PCBA to help reduce damage from moisture, corrosive agents, chemicals, dust, and other environmental stressors. The coating “conforms” to the board surface and component geometry, creating a barrier while maintaining electrical insulation where required.

Conformal coating was originally developed for high-reliability electronics used in harsh or mission-critical environments. Today, it is used across many industries whenever the end-use conditions include humidity, contamination risk, or exposure to corrosive atmospheres.

Standards for Conformal Coatings

Conformal coatings are typically evaluated against industry references that define inspection criteria and performance expectations, such as coating coverage, thickness, adhesion, and electrical characteristics. Standards commonly referenced in electronics manufacturing include IPC-A-610 and IPC-CC-830. In practice, coating requirements are also determined by the end-use environment, reliability targets, and customer specifications.

Material for Conformal Coatings

Conformal coating material selection depends on factors such as operating temperature range, chemical exposure, dielectric needs, rework considerations, and required mechanical robustness. Several materials are commonly used, each with distinct properties and typical use cases.

Acrylic

Acrylic coatings are often selected for ease of application and fast drying. They provide practical moisture resistance and electrical insulation, and are frequently used in consumer electronics and general industrial products where reworkability is beneficial.

Silicone

Silicone coatings are known for flexibility and high-temperature stability. They can tolerate thermal cycling and temperature extremes, so they are often used in demanding environments such as automotive and aerospace-related electronics.

• Low dissipation factor, suitable for high-impedance circuitry.
• Broad operating temperature capability (commonly referenced around -55°C to 200°C depending on formulation).
• Good resistance to UV, thermal stress, humidity, moisture, and salt spray.
• Can withstand vibration stress, though abrasion resistance may be limited.
• Often used for assemblies operating in high-humidity environments.
• Frequently referenced alongside IPC-CC-830 and (legacy) MIL-I-46058C depending on project requirements.

Urethane

Urethane conformal coatings are valued for durability and chemical resistance. They provide strong protection against abrasion and solvents, making them common in industrial equipment and certain ruggedized applications.

Parylene

Parylene is deposited via a vapor process and forms a thin, uniform, pinhole-resistant film. It offers strong protection against moisture and chemicals and is often used where consistent coverage is critical, such as some medical devices and high-reliability electronics.

Epoxy

Epoxy coatings provide robust mechanical protection and strong adhesion. They are typically chosen for applications that need a tougher, more permanent protective layer, with good solvent resistance and dielectric properties.

Polyurethane

Polyurethane coatings balance flexibility and protection and are often used when moisture resistance and UV stability are required, such as in outdoor or semi-exposed operating conditions.

How to Apply Conformal Coating

• Manual Spraying:In this method, our skilled technicians use handheld spray guns or aerosol cans to apply the coating. We typically employ this technique for low-volume production runs or prototypes. The advantage is that it’s a hands-on process, ensuring consistency and quality on every board it’s applied to.
• Automated Spraying: For larger production volumes, we utilize automated spray systems or programmed equipment. These systems are integrated into a conveyor line and feature spray heads that precisely apply the coating to each PCB as it moves through the line.
• Dipping: This method involves immersing the component horizontally, vertically, or at an angle to achieve complete coverage. The dipping process can be automated or manual, depending on the specific requirements, guaranteeing a uniform coating across the board.

Attention During Conformal Coating

Executing the conformal coating process requires meticulous attention to detail:

• Surface Preparation: Thoroughly clean and inspect the PCBA to ensure there are no contaminants or defects.
• Masking: Certain components, connectors, or areas may not require coating. Proper masking prevents over-coating.
• Coating Application: Choose the appropriate method (spraying, dipping, brushing) and ensure even coverage with the correct thickness.
• Curing: Proper curing (drying) of the coating material is vital for adhesion and performance.
• Inspection and Testing: Rigorous inspection and testing, including electrical testing, verify the coating’s integrity.

Masking for Selective Conformal Coating on PCBAs

In PCB assembly, conformal coating is often used to protect a printed circuit board assembly (PCBA) in environments where humidity, contamination, or corrosive exposure may affect reliability. At the same time, many PCBAs include coating keep-out areas where the coating must not be applied to avoid electrical, mechanical, or thermal issues. Masking is the standard method used to keep these areas free of coating and enable selective conformal coating.

Masking is the process of temporarily covering defined regions of a PCBA before coating. Typical masking methods use tapes, dots, caps/plugs, or peelable/liquid masking compounds to block coating from reaching specific components or interfaces. Proper masking helps maintain intended function while still allowing protective coverage where it is required.

Why Masking Matters

Conformal coating can interfere with contact, mating, measurement access, or heat transfer if applied to the wrong location. Common areas that are typically kept uncoated include:

• Connectors and edge fingers: Contact surfaces must remain clean to ensure reliable mating and low contact resistance.
• Test points and test pads: Electrical measurements, ICT, and troubleshooting access require exposed pads.
• Press-fit pins and mechanical contact areas: Coating may affect mechanical fit or contact performance.
• Thermal interfaces: Heat-generating components and heatsink/thermal pad contact zones may require a keep-out area to maintain heat dissipation pathways.

Common Masking Materials

Masking material selection depends on geometry, required precision, coating type, and production method. Common options include:

• Masking tapes and dots: Suitable for flat areas and well-defined keep-out zones; widely used in both manual and automated lines.
• Caps, boots, and masking plugs: Used for connectors, headers, terminals, and through-holes that must remain free of coating.
• Peelable or liquid masking compounds: Applied to complex shapes or dense areas where tape is impractical; removed after curing.

Typical Masking Workflow

1. Define keep-out areas: Requirements are identified during design review or pre-production based on drawings, specs, and end-use conditions.
2. Apply masking: Tapes/plugs/liquid mask are placed to fully cover the keep-out zones and prevent coating ingress.
3. Apply conformal coating: Coating is applied by spraying, dipping, or brushing depending on process selection.
4. Cure and inspect: After curing, the board is inspected for coverage, thickness, and any coating leakage into masked zones.
5. Remove masking and re-check: Masking materials are removed and critical areas (connectors/test pads) are verified to be clean and functional.

Key Benefits of Proper Masking

• Maintains functional interfaces: Ensures contact points, mating surfaces, and measurement pads remain usable.
• Reduces rework risk: Prevents coating-related failures caused by blocked connectors, contaminated pads, or unintended insulation.
• Supports consistent coating quality: Helps achieve controlled, repeatable selective coating coverage based on defined requirements.

Masking is a practical step that enables selective conformal coating while preserving critical electrical, mechanical, and thermal features of a PCBA. Masking requirements are typically documented in drawings or build notes so production can apply and verify keep-out areas consistently.

Conformal Coating for PCBAs: Final Notes

Conformal coating is a widely used protective process for printed circuit board assemblies (PCBAs) when electronics must operate under humidity, moisture, dust, chemical, or corrosive exposure. Coating selection is typically based on the specified operating conditions and reliability requirements, with common materials including acrylic, silicone, urethane, epoxy, polyurethane, and parylene.

In PCB manufacturing and PCB assembly projects, conformal coating is usually implemented as a customer-specified PCBA step. Coating scope and coating keep-out areas are defined in drawings or specifications, and the process is followed by curing and inspection to confirm coverage, workmanship, and that functional interfaces (such as connectors and test pads) remain free of coating.