What Are The Three Defenses Of Conformal Coating On PCBA?

In reality, in an environment containing chemicals (such as fuel, coolant, etc.), vibration, moisture, salt spray, humidity and high temperature, circuit boards without conformal coating may be corroded, mold grows and produce Problems such as short circuits lead to circuit failures. The use of conformal coating can protect the circuit from damage, thereby improving the reliability of the circuit board, increasing its safety factor, and ensuring its service life. This is the importance of conformal coating.

In addition, another advantage of Conformal Coating is that it can prevent leakage, thus allowing higher power and closer printed board spacing, thereby meeting the purpose of component miniaturization.

Conformal Coating Material
According to different needs, there are many types of Conformal Coating on the market to choose from. Among them, the most common types are epoxy resin conformal coating, acrylic conformal coating and polyurethane conformal coating.
Epoxy resin conformal coating has high electrical insulation properties and mechanical strength and is widely used in electronic products. Its coating process includes surface treatment, paint preparation, coating, drying and curing.
First, the PCB board needs to be surface finished to remove impurities and grease to ensure the adhesion of the paint film. Next, mix the epoxy resin paint and the diluting solvent in a certain proportion to make the paint, and stir evenly. Then, use spray or dip coating to evenly apply the paint on the PCB board. After the coating is completed, the paint film needs to be allowed to dry for an appropriate period of time to allow it to form a strong protective layer. Finally, it is dried or cured with a curing agent to enhance the hardness and corrosion resistance of the paint film.
Acrylic conformal coating is a conformal coating with good insulation, moisture resistance and chemical resistance. The coating process is similar to that of epoxy resin conformal coating, which also requires surface treatment, paint preparation, coating, drying and curing.
Polyurethane Conformal Coating is a conformal coating with high insulation strength and corrosion resistance. The coating process is slightly different from the previous two. Before coating, the color needs to be adjusted because polyurethane paint has a wide range of color options. When coating, spray coating or dip coating can be used. After the coating is completed, it needs to be fully dried and cured.
Different types of Conformal Coating have some common features in the coating process, such as surface treatment, paint preparation, coating, drying and curing. However, due to different materials, some details in the coating process will be different.

Conformal Coating process classification
The first is solvent-based Conformal Coating, which can be applied by hand brushing, hand spraying or machine spraying, and then dried at room temperature or cured by heating. Solvent-based conformal coating contains a large amount of solvent, which has the disadvantages of being volatile, high in VOC (Volatile Organic Compounds), and low in flash point. Therefore, it has a strong smell, cannot be directly discharged, and is flammable and explosive. It is best for the work workshop to be a closed space and to provide personal protection. Raw materials also need to be better managed to reduce usage risks. At the same time, it is necessary to increase emission treatment equipment to meet environmental protection requirements. The film thickness after coating is generally controlled at about 150um for wet film and 30um for dry film. If it is too thick, it is a waste, and if it is too thin, the protective effect will be poor.
The second type is UV (ultraviolet light) Conformal Coating, which can be applied by hand brushing, hand spraying or machine spraying, and can be quickly cured under UV light. Its characteristics are that it is almost non-volatile, contains no VOC, is non-flammable and non-explosive, meets UL94 flame retardant requirements, and has its own fluorescence for easy inspection. The film thickness after coating is generally controlled at about 80-100um for wet film and 60-80um for dry film.
The third type is Parylene vacuum conformal coating, which uses unique vacuum vapor deposition equipment to dip-coat PCBA. Active small molecules "grow" a fully conformal polymer film coating on the surface of the substrate, so the curing time is longer. It is characterized by no dead ends in coating, good protective performance but low coating efficiency. The film thickness after coating is about 20um.
Conformal Coating process requirements and process specifications

Painting requirements:
1. Thickness: The paint film thickness is controlled at 0.05mm-0.15mm. Dry film thickness 25um-40um.
2. Secondary coating: In order to ensure the thickness of products with high protection requirements, you can wait for the paint film to solidify before performing secondary coating (determine whether to perform secondary coating according to demand).
3. Inspection and repair: Visually inspect whether the coated circuit board meets the quality requirements, and repair the problems. For example, if pins and other protected areas are stained with conformal coating, you can use tweezers to hold absorbent cotton balls or clean cotton balls dipped in washing water to scrub them clean. When scrubbing, be careful not to wash off the normal paint film.
4. Component replacement: After the paint film is cured, if you want to replace components, proceed as follows:
(1) Use electric ferrochrome to directly solder the components, and then use cotton cloth dipped in board cleaning water to clean the material around the solder pad
(2) Soldering replacement components
(3) Use a brush dipped in conformal anti-paint to coat the welded parts, and allow the paint film to dry and solidify.
Operational requirements:
1. The Conformal Coating workplace is required to be dust-free and clean, with no flying dust. Good ventilation measures must be provided, and irrelevant personnel are prohibited from entering.
2. Wear protective equipment such as masks or gas masks, rubber gloves, chemical protective glasses, etc. during operation to avoid harm to the body.
3. After the work is completed, the used tools should be cleaned in time, and the container containing Conformal Coating should be closed and tightly covered.
4. Anti-static measures should be taken for circuit boards. Circuit boards should not be placed on top of each other. During the coating process, circuit boards should be placed horizontally.
Quality requirements:
1. There should be no paint flow or dripping on the surface of the circuit board. When applying paint with the brush, be careful not to drip onto the locally isolated parts.
2. The Conformal Coating layer should be smooth, bright, and uniform in thickness to protect the surface of the pads, chip components or conductors.
3. There should be no bubbles, pinholes, ripples, shrinkage holes, dust and other defects and foreign objects on the surface of the paint layer and components. There should be no powdering or peeling. Note: Do not touch the paint at will before the paint film is dry. membrane.
4. Partially isolated components or areas cannot be coated with Conformal Coating.