In the PCB industry, many friends will come into contact with conformal coatings. So, what exactly is this conformal coating? Today, let’s take a closer look.

Conformal coating is a special formulation of paint primarily used to protect circuit boards and related equipment from environmental erosion. (Conformal coating refers to moisture-proof, salt spray-proof, and mildew-proof)

Conformal coatings have excellent low-temperature performance. When cured into a transparent protective film, they exhibit superior insulation, moisture resistance, leakage prevention, shock resistance, dust resistance, corrosion resistance, aging resistance, and corona resistance.

In reality, circuit boards that do not use conformal coatings may experience corrosion, mold growth, and short circuits in environments containing chemicals (such as fuels, coolants, etc.), vibrations, humidity, salt spray, and high temperatures, leading to circuit failures. Using conformal coatings can protect circuits from damage, thereby improving the reliability of circuit boards, increasing their safety factor, and ensuring their lifespan. This is the importance of conformal coatings.

Additionally, conformal coatings have the advantage of preventing leakage, allowing for higher power and closer spacing between printed circuit boards, thus meeting the miniaturization goals of components.

Types of Conformal Coating Materials

There are various types of conformal coatings available in the market to meet different needs. The most common types include epoxy resin conformal coatings, acrylic conformal coatings, and polyurethane conformal coatings.

Epoxy resin conformal coatings have high electrical insulation properties and mechanical strength, widely used in electronic products. Their coating process includes surface treatment, paint formulation, coating, drying, and curing steps.

First, the PCB needs to undergo surface treatment to remove impurities and grease to ensure the adhesion of the coating. Next, the epoxy resin paint is mixed with a diluent solvent in a specific ratio and stirred evenly. Then, the paint is uniformly applied to the PCB using spraying or dipping methods. After coating, the film needs to dry for an appropriate time to form a strong protective layer. Finally, it is cured through drying or curing agents to enhance the hardness and corrosion resistance of the film.

Acrylic conformal coatings are known for their good insulation, moisture resistance, and chemical corrosion resistance. Their coating process is similar to that of epoxy resin conformal coatings, requiring surface treatment, paint formulation, coating, drying, and curing.

Polyurethane conformal coatings have high insulation strength and corrosion resistance. The coating process differs slightly from the previous two. Before coating, color formulation is required because polyurethane paints come in a variety of colors. Coating can be done using spraying or dipping methods. After coating, thorough drying and curing are necessary.

Different types of conformal coatings share some common processes in coating, such as surface treatment, paint formulation, coating, and drying/curing. However, due to the differences in materials, some details in the coating process may vary.

Classification of Conformal Coating Processes

The first type is solvent-based conformal coatings, which can be applied by hand brushing, hand spraying, or machine spraying, followed by air drying at room temperature or heat curing. Solvent-based conformal coatings contain a large amount of solvent, which has the disadvantages of being volatile, having high VOC (Volatile Organic Compounds) content, and low flash points, resulting in strong odors, inability to be directly discharged, and being flammable and explosive. It is best to work in a closed space with good ventilation and to prohibit unrelated personnel from entering. Raw materials also need to be managed to reduce usage risks. Additionally, emission treatment equipment should be increased to meet environmental protection requirements. The thickness of the coated film is generally controlled at about 150um wet film and 30um dry film; too thick is wasteful, and too thin is ineffective for protection.

The second type is UV (Ultraviolet) conformal coatings, which can be applied by hand brushing, hand spraying, or machine spraying, and can cure rapidly under UV light. Its characteristics include almost no volatility, no VOCs, non-flammable and non-explosive, meeting UL94 flame retardant requirements, and having built-in fluorescence for easy inspection. The thickness of the coated film is generally controlled at about 80-100um wet film and 60-80um dry film.

The third type is Parylene vacuum conformal coatings, which use unique vacuum deposition equipment to coat PCBA, allowing small active molecules to “grow” a fully conformal polymer film coating on the substrate surface, resulting in longer curing times. Its characteristics include no dead corners in coating, good protective performance, but low coating efficiency. The thickness of the coated film is about 20um.

Requirements and Specifications for Conformal Coating Processes

Spraying Requirements:

1. Thickness: The film thickness should be controlled between 0.05mm-0.15mm. The dry film thickness should be 25um-40um.

2. Secondary Coating: To ensure the thickness for high protection requirements, a secondary coating can be applied after the film has cured (whether to perform a secondary coating is determined by demand).

3. Inspection and Repair: Visually inspect the coated circuit board to ensure it meets quality requirements and repair any issues. For example, if pins and other protected areas are contaminated with conformal coating, they can be cleaned with tweezers and a cotton ball dipped in cleaning solution, taking care not to remove the normal coating.

4. Component Replacement: After the film has cured, if components need to be replaced, the following steps can be taken:

(1) Use an electric soldering iron to directly remove the component, then clean the surrounding area with a cotton cloth dipped in cleaning solution.

(2) Solder the replacement component.

(3) Use a brush dipped in conformal coating to coat the soldered area and allow the film to surface dry and cure.

Operational Requirements:

1. The conformal coating work area must be dust-free and clean, with no dust flying around, and must have good ventilation measures, prohibiting unrelated personnel from entering.

2. During operation, wear masks or respirators, rubber gloves, and chemical protective goggles to avoid bodily harm.

3. After work, promptly clean the tools used and securely close the containers holding the conformal coating.

4. Ensure anti-static measures for circuit boards, do not stack circuit boards, and keep them horizontal during the coating process.

Quality Requirements:

1. The surface of the circuit board must not have paint flow or dripping. When brushing paint, care must be taken not to drip onto isolated areas.

2. The conformal coating layer should be smooth, shiny, and evenly thick, protecting pads, surface-mounted components, or conductor surfaces.

3. The surface of the coating and components must be free of bubbles, pinholes, ripples, shrinkage holes, dust, and other defects and foreign materials, with no powdering or peeling. Note: Do not touch the coating before it has surface dried.

4. Isolated components or areas must not be coated with conformal coating.