Conformal coating is a protective chemical coating or polymer film applied to printed circuit boards (PCBs) and critical electronic components to protect them from environmental factors such as moisture, dust, chemicals, and temperature extremes.
This coating, typically 25 to 300 µm thick, enhances the product’s resistance to moisture, dirt, dust, and chemical contamination. Additionally, it prevents solder joints and conductors from continuing to corrode by blocking contact with air. By incorporating specific ingredients, conformal coatings can also provide shielding against electromagnetic interference and serve as an insulator.
Since conformal coating can block air from contacting solder joints and provide moisture protection, it can also indirectly prevent the growth of “tin whiskers” and “electrochemical migration (ECM).”
This conformal coating has an additional benefit: it can increase the wear and solvent resistance of components and help reduce stress release caused by temperature fluctuations.
Due to these advantages, conformal coatings are often used in products exposed to harsh environments, such as chemically-rich environments, high pollution or dusty areas, high humidity, and extreme temperatures. In consumer electronics, conformal coatings are frequently used to protect or insulate electrical and electronic components, such as motors, generators, transformers, and solenoid switches.
Can Conformal Coating be Waterproof?
Some people think that since conformal coating has so many advantages, it should also be waterproof. This idea is not entirely correct. Conformal coating primarily provides “moisture protection” rather than “waterproofing.” When a product is exposed to a high-humidity environment and condensation occurs, conformal coating offers a certain level of protection. However, if excessive water enters the product, conformal coating cannot provide absolute protection. This is because conformal coating cannot be applied to certain components that require electrical contact, such as connector pins and button contacts. If these areas get wet, it can cause functional issues. Therefore, conformal coating mainly offers “moisture protection” rather than being fully “waterproof.”
Related Reading:
What do IP67 or IP68 ratings mean for waterproof and dustproof protection? Explanation and considerations for IP rating testing methods.
Products used in gas stations or aviation are required to comply with ATEX regulations, and applying conformal coating is one of these requirements. The goal is to prevent any potential short circuits or sparks in special environments that could cause fires or explosions, which could lead to significant human casualties.
According to the IPC-CC-830 standard, there are seven common types of conformal coating materials: Silicone, Acrylic, Urethane, Epoxy, Polyurethane (PUR, PU), Styrene Block-Copolymer (SBC), Paraxylylene (XY), and Ultra-Thin (UT).
Among these, Silicone, Acrylic, and Urethane are more commonly used due to their transparency. Epoxy is also used by some, but it is opaque and requires mixing two components, making it more cumbersome to use, so it is less popular.
Conformal coatings can be cured using either room temperature curing or heat curing methods.
Here are the characteristics and considerations for the seven types of conformal coatings:
-
Silicone: Once cured, silicone usually forms a transparent and flexible rubber-like coating. It effectively absorbs shocks and can withstand significant temperature fluctuations (from -40°C to 200°C). Silicone provides excellent waterproofing but may leave gaps at the coating interface, making it less suitable for environments with high moisture or corrosive concentrations.
-
Acrylic: When cured, acrylic forms a transparent and hard coating with low moisture absorption and quick curing time. It can air dry at room temperature or be heat-cured to speed up the process. It offers good abrasion resistance and insulation. However, acrylic is less solvent-resistant, making it possible to remove for rework with solvents.
-
Urethane: Cured urethane forms a transparent and hard coating with excellent abrasion resistance and good moisture protection. It performs well in low-temperature environments but is less resistant to high temperatures.
-
Epoxy: Epoxy-based coatings are strong and usually opaque. They offer good moisture and humidity protection, as well as excellent chemical resistance and abrasion resistance. Epoxy also has good dielectric properties. Epoxy coatings are typically two-part systems, making thickness control challenging, and they are often used to cover most components.
-
Polyurethane: By adjusting the formulation, polyurethane can cure to be either very soft and flexible or very hard and brittle. It can also vary in chemical resistance, making it a versatile material. Early applications included protecting printed text on keyboards with PU sprays. PU usually requires a hardener for curing, leading to an irreversible reaction. This provides better protection but makes the process more complex and rework more difficult.
-
Paraxylylene (XY): This is a special vapor deposition material that provides a very uniform coating. However, areas that do not require coating must be meticulously masked. Due to the need for vacuum operations, mass production is challenging and time-consuming. Rework with this material is inconvenient, so alternative materials might be considered for rework purposes.
-
Ultra-Thin (UT): This is a new type introduced in the CC-830 standard, requiring a coating thickness of less than 12.5µm. Due to its thinness, application methods might include liquid deposition, vapor deposition, or vacuum plasma deposition. Ultra-thin coatings are not suitable for abrasion resistance or high dielectric strength due to their thinness. They primarily provide moisture protection and are more suitable for consumer products like mobile earbuds (such as TWS) or microphones. The nano-coating that Workingbear previously introduced falls into this category.
Conformal Coating Application Considerations:
When applying conformal coating, it’s important to avoid coating components that require electrical contact or circuit connection during subsequent processing. This includes power jacks, connectors, and button contact circuit traces, as the insulating nature of conformal coating can cause electrical issues if applied to these parts. Additionally, components like buzzers and speakers, which have open holes, should not be coated as the coating can interfere with their vibration and sound production. LEDs should also be kept free of coating to prevent dimming, refraction, or color changes.
Safety Precautions for Conformal Coating Application:
Based on Workingbear’s experience, conformal coatings are generally volatile and have a strong odor, with some potentially harmful to humans. Therefore, it’s best to apply the coating in a dedicated area with good ventilation. Extraction systems, similar to kitchen range hoods, are often used to minimize the spread of fumes. Automatic coating machines typically feature strong extraction and minimal openings. Most conformal coatings have a low flash point, so if a powerful ventilation system isn’t feasible, ensure the area is well-ventilated to avoid the risk of flash fires due to high concentrations of fumes.
Methods of Applying Conformal Coating:
The method chosen for applying conformal coating depends on board design, coating material type, cycle time, existing production line and equipment, and quality requirements. Here are the four main methods:
-
Dipping: Economical for large-scale equipment. This method coats both sides of the board simultaneously and quickly, saving time and providing a uniform coating thickness. However, areas that don’t need coating must be fully masked, and the process’s consistency is influenced by dipping temperature, duration, withdrawal speed, dripping time, and air knife use. The equipment is more expensive than brushing, and the masks need regular cleaning or replacement to prevent leaks.
-
Spraying: Suitable for medium to small-scale equipment. Similar to spray painting, the coating’s uniformity depends on the speed of relative movement, spray position, pressure, and the presence of taller components. Additional steps may be needed to coat the underside of components.
-
Brushing: Ideal for small-scale equipment. This manual method is cost-effective for low-volume production as it requires minimal tooling. Even if masking jigs are needed, they don’t have to be highly precise. However, the coating can be inconsistent due to operator skill or fatigue, and brush hairs may shed. Coating the underside of components is also challenging.
-
Selective Coating: Although coating can be labor-intensive, it’s possible to apply it only where needed. Methods include robotic spraying or manual brushing, offering flexibility in application.
By understanding these application methods and safety considerations, you can ensure effective and safe use of conformal coatings in electronic manufacturing and assembly.
Inspection of Conformal Coating
Since most conformal coatings are transparent or have a faint color after spraying, it’s hard to inspect the coating with the naked eye. Therefore, many conformal coating materials contain a small amount of UV fluorescent agent, allowing for inspection of the coating coverage and uniformity using a UV inspection light.
When selecting a conformal coating material, consider the curing conditions. Some require high-temperature curing while others cure at room temperature. High-temperature cured coatings are typically harder and more wear-resistant, whereas room temperature cured coatings are more flexible.
▼When using the spraying method, it’s essential to have vacuum extraction equipment to prevent the sprayed material from dispersing into the air, which can be harmful to health.
▼You can also use fixtures to cover areas and components that shouldn’t be coated to prevent the insulating material from being sprayed onto parts that require electrical contact.
▼Non-residue stickers can be used to mask components that shouldn’t be coated.
▼After spraying, UV light can be used to inspect the coating coverage on the PCB. For comparison, Workingbear intentionally included an uncoated PCB in the images.
Removing Conformal Coating
Typically, conformal coatings can be removed by heating. For localized removal, such as for rework soldering, use a heat gun to heat the area. The coating will become brittle and peel off like a plastic film, allowing for soldering. After rework, reapply the conformal coating to complete the process.
For complete removal from a board, consult the conformal coating supplier to see if there are solvents like xylene that can be used. Workingbear’s method involves running the entire board through a reflow oven. Test the temperature settings for different coatings, then remove any residual coating.
If you have better methods for removing conformal coating, feel free to share and discuss.
Measuring Conformal Coating Thickness
The thickness of conformal coating is typically defined by customer requirements. If not specified, refer to the IPC-A-610 standard for thickness guidelines. Use a film thickness gauge for measurement, starting with the bare board, then measure again after coating. Subtracting the two measurements gives the true coating thickness.
Other Options for Insulation and Moisture Protection. Apart from conformal coating, other options include:
- Nano-coating: Can it prevent water or only moisture?
- Safe, simple, effective corrosion and moisture protection for electronics
- Insights on evaluating two industrial-grade moisture-proof and waterproof coatings
Leave a Reply