Three-proof paint operation process

Conformal coatings can be divided into acrylate, silicone, and polyurethane conformal coatings according to their chemical composition; From the perspective of curing methods, there are solvent-based curing, room temperature curing, thermal curing and ultraviolet curing. The choice of process is usually determined by the viscosity of the product (different concentrations correspond to different solids). Each conformal coating manufacturer generally has its own diluent, which is used to dilute and configure products with different concentrations, so as to determine the suitable for different process requirements such as spraying, brushing, and dipping. Conformal coatings are available in a variety of types, including acrylic, silicone, and polyurethane conformal coatings with different chemical compositions, as well as solvent-based curing, room temperature curing, thermal curing, and UV curing conformal coatings with different curing methods.

<!--[if !supportLists]-->1.<!--[endif]-->Solvent-containing acrylic resin conformal coating (the most widely used popular product in the market) features: surface drying, fast curing speed, good conformal performance, low price, transparent color, flexible texture and easy repair. It is used to protect the circuit board and its related equipment from environmental erosion, so as to prolong the service life and ensure the safety and reliability of use. Under realistic conditions, such as chemical environment (fuel, coolant, etc.), vibration, high dust, salt spray, humidity and high temperature, the circuit board may be corroded, softened, deformed, mildewed and other problems, resulting in circuit failure. Conformal coating is applied to the surface of the circuit board to form a thin, flexible film about 25 to 50 microns thick that protects the circuit from damage under these harsh conditions.

<!--[if !supportLists]-->2.<!--[endif]-->Solvent-free acrylic resin conformal coating features: through UV curing, it can be dried in a few seconds to more than ten seconds, the color is transparent, the texture is hard, and the chemical corrosion resistance and wear resistance are excellent. This conformal coating initially cured with UV light is solvent-free, non-combustible, free of volatile organic compounds and fully complies with current and proposed environmental regulations. It has good chemical and heat resistance, good flexibility, glossy surface, strong adhesion and good coverage. The curing time is short, only a fraction of that of other products (measured in seconds), and the product cost is significantly reduced by increasing productivity. It is used for the spot coating and bonding of thermoplastic materials, such as PMMA (acrylic, plexiglass), PC, ABS, PS, PVC, PCB and most other engineering plastics, and can also be used for the bonding of organic materials such as plastic and glass, plastic and metal, and is typically used in the fixing of plastic parts of circuit boards, mobile phone button coating and other industries.

<!--[if !supportLists]-->3.<!--[endif]-->Polyurethane conformal coating features: brittle texture, excellent solvent resistance. In addition to superior moisture-proof performance, it has stable performance in low temperature environments. As a heat transfer medium, it has good electrical insulation and thermal conductivity, high and low temperature resistance, and can work for a long time in the temperature range of -60° - 250°, and will not cause cirrhosis or melting.

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<!--[if !supportLists]-->4.<!--[endif]-->Silicone conformal coating is a soft elastic coating material that relieves pressure well, is resistant to high temperatures of 200 degrees, and is easy to repair. Silicone conformal coatings are available in solvent resistant and solvent removal types, which are used at high temperatures and have good chemical resistance and salt spray resistance. Although solvent-resistant products can be welded, they are difficult to remove by organic solvents. Its most important feature is that the chemical bonds are bonded more and more strongly over time. For optimal performance, heat curing can be performed if necessary. Silicone products have significant electrical insulation characteristics, and are ideal products for high electric energy and high integration circuit boards, with excellent waterproof, moisture-proof, dust-proof, anti-corrosion, anti-aging and other characteristics, and are widely used in the insulation protection of circuit boards and electronic components.

The brushing method of the conformal coating operation process The brushing method is the simplest coating method, which is usually used for local repair and repair, and can also be used in laboratory environments or small batch trial production/production, and is generally suitable for occasions with low requirements for coating quality. The advantages of the brushing method are: almost no investment in equipment fixtures; Saving on coating materials; Generally, there is no need for a masking process. The disadvantages are: narrow scope of application; Minimal efficiency; There is a masking effect when the whole board is brushed, and the coating consistency is poor, and because it is a manual operation, it is easy to produce bubbles, ripples, uneven thickness and other problems; It takes a lot of manpower. Precautions: Before brushing, first make sure that the diluted product is fully stirred, and it is advisable to leave it for 2 hours. Containers for conformal coating should be made of metal or plastic, and wood and PVC should not be used, as chemical reactions can occur, corrode the container and destroy the product. To clean and dry the plates, remove moisture and moisture. Dust, moisture and oil must be removed from the surface of the object to be coated, and thorough cleaning will ensure that the corrosive residue is completely removed, so that the conformal coating adheres well to the surface of the circuit board. The choice of brush is also very important, because the product is not insulating before curing, so choose a brush that is anti-static and does not shed, such as a high-quality natural fiber brush. When brushing, the plate should be placed as flat as possible, and there should be no dew after brushing, and the brushing should be flat, and there should be no exposed parts, and 0.1 - 0.3mm is appropriate. The circuit board after brushing should not be placed upside down, otherwise the coating will be thin at the top and thick at the bottom, and it still needs to be placed flat.

The dipping method of the conformal coating operation process The dipping coating method has been widely used since the early stage of the coating process, and is suitable for occasions that require complete coating; In terms of coating effect, dip coating is one of the most effective methods. Advantages of dip coating: Coating can be done manually or automatically. Among them, the manual operation is simple and the investment is small; High material transfer rate, which can completely coat the entire product without masking effect; Automated dip coating equipment can meet the needs of high-volume production. Disadvantages of dip coating method: if the coating material container is open, impurities will occur with the increase of coating times, the material needs to be changed regularly and the container cleaned, and the solvent volatilization also needs to be continuously replenished; The coating thickness is too large, and a lot of material is wasted due to trickle after the board is extracted; The area needs to be covered; Masking/removing coverings requires a lot of manpower and material resources; The coating quality is difficult to control and the consistency is poor; Too much manual handling can cause unnecessary physical damage to the product. Key points of the dip coating method: use a density meter to monitor solvent loss at any time to ensure a reasonable ratio; Control the immersion and extraction speed to obtain a satisfactory coating thickness and reduce defects such as air bubbles; It should be operated in a clean and temperature/humidity controlled environment to avoid affecting the adhesion of the material; Adhesive-free and anti-static masking tape should be chosen, and if normal tape is chosen, a deionized blower must be used. Precautions for dip coating method: The PCB assembly should be immersed vertically in the paint tank. Do not immerse the connector unless it is carefully covered, the circuit board should be immersed for 1 minute until the air bubbles disappear, and then slowly take it out. A uniform film will form on the surface of the PCB, and most of the paint residue should be allowed to flow back to the impregnation machine from the PCB. The immersion speed of the circuit board or components should not be too fast to avoid excessive air bubbles. If there is crusting on the surface after dipping, you can continue to use it after removing the epidermis.

The spraying method of the conformal coating operation process The spraying method is the most commonly used coating method in the industry, and it is divided into two types: automatic spraying by machine and manual spraying. Among them, machine automatic spraying is suitable for customers with large-scale production, large quantities or long-term production. The advantages are: selective spraying, high precision, uniformity, high efficiency, and automatic detection. The disadvantages are: high cost, high requirements for operational accuracy, and the possibility of shading (where the lower part of the component is not covered with conformal coating). Manual spraying generally uses a combination of hand-held spray gun + air hose + air compressor (+ exhaust fan). The advantages are: low cost, suitable for medium-sized customers. However, it takes more than 5KG pressure to form a fog barrier, so the requirements for air pressure adjustment are higher, if the air pressure is not high, the conformal paint will not be atomized, it will be drawn, and the manual spraying has higher requirements for operation accuracy. Hand can spraying is also a type of manual spraying, using spray can type products, suitable for maintenance and small-scale production, the biggest advantage is convenience. In short, manual spraying has a small investment and is easy to operate; Machine automatic spraying coating has good consistency, the highest production efficiency, easy to achieve online automatic production, suitable for large and medium batch production, consistency and material cost are usually better than dip coating method, although also need a masking process but the requirements are not as high as dip coating method. The corresponding disadvantages are: the need for a masking process; There is a lot of waste of materials; Requires a lot of manpower; Poor coating consistency, possible masking effect, and difficulty in coating underneath narrow-pitch components.

The coating method of the conformal coating operation process The coating method is a less commonly used conformal coating method. The conformal coating, which is stored in a high tank, is poured down through a nozzle or narrow slit and drenched on the coated object driven by the conveyor in the form of a curtain to form a uniform coating film. The excess conformal coating flows back into the container and is pumped into a high-level tank for recycling. In small batch production, manual operation is generally used to directly pour conformal paint on the product, so the spraying method is also called the paint pouring method. In large-scale production, automatic assembly line operation is generally adopted, and the thickness of the conformal coating film on the product is controlled by adjusting the nozzle size or narrow slit width. If the coating film is thick, bubbles will appear on the coating film on the product coming out of the conveyor belt through the drying box; If the coating film is thin, the product will be exposed and unevenly coated. Therefore, the coating method is also called curtain coating. According to the characteristics of different products, in order to increase the adhesion of the conformal coating film on the product, it is sometimes necessary to carry out sulfurization; In order to increase the thickness of the product, the product needs to be sprayed and vulcanized once or several times.

• The so-called sagging of conformal coating operation process refers to the phenomenon of flowing when spraying conformal coating by hand, due to improper operation and unskilled technology, etc., the conformal coating sprayed on the surface of the circuit board produces a flow phenomenon. Sag not only affects the appearance of the product, but also prolongs the drying time of the sagging point, so it must be controlled during production operations. Suzhou Tuotai company produces a self-spraying conformal coating 120, using environmentally friendly high volatile solvent, especially suitable for electronic equipment, for the majority of customers to provide good conditions. Based on an acrylic resin, the conformal coating forms a shiny, stretchable protective film that resists harsh environmental conditions such as acids, salts, mold, corrosive vapors, thermal stress, mechanical abuse of alkalis, alcohol, water vapor, etc., retains effectiveness over a wide temperature range of -70°C to +120°C, adheres to a wide range of materials such as metal, plastic, wood, cardboard, glass, etc., does not flow and can be soldered directly on this layer. In the specific operation process, only the following aspects need to be paid attention to, and a very ideal paint film can be produced.

<!--[if !supportLists]-->1.<!--[endif]-->Ensure that the surface being sprayed is dry and clean;

<!--[if !supportLists]-->2.<!--[endif]-->Adjust the spray distance and moving speed according to the ambient temperature, humidity and ventilation;

<!--[if !supportLists]-->3.<!--[endif]-->Move at a constant speed, do not stay when rotating;

<!--[if !supportLists]-->4.<!--[endif]-->On the premise of ensuring complete coverage, the paint film should be thin and uniform;

<!--[if !supportLists]-->5.<!--[endif]-->The previous film must be fully cured before the next spray can be applied. The quality of the paint film has a great impact on the "three prevention" effect of the conformal coating, and it is hoped that customers will determine the strict production process in combination with the actual production situation in the use process, so as to achieve the best effect of protection, efficiency and saving.

Notes on the operation process of conformal coating Due to the wide variety of conformal paint, different characteristics, and the products for which they are targeted also have their own characteristics, the operation process is naturally becoming more and more diversified, but it is always changing, and there are always some commonalities worth paying attention to.

<!--[if !supportLists]-->1.<!--[endif]-->The temperature of conformal coating should be between 15 °C - 30 °C, and the relative humidity should not be higher than 85%. In order to make the conformal coating fully protect and eliminate the hygroscopic phenomenon of dullness or whitishness, the circuit board can be preheated and baked and vacuum dried to remove most of the moisture.

<!--[if !supportLists]-->2.<!--[endif]-->Conformal coatings generally contain flammable solvents and should be avoided from high temperatures and open flames; Conformal paint generally has a slight pungent odor, and the operation workshop should ensure sufficient ventilation to facilitate the volatilization and rapid curing of solvents in conformal coating; Avoid prolonged inhalation of vapors and prolonged contact with the skin; The cured processed parts are basically harmless to the human body, but the coating operation should pay attention to safety and protection, the environment should be ventilated, and employees should wear protective masks.

<!--[if !supportLists]-->3.<!--[endif]-->If you want a thicker coating, it is best to get it by applying two thinner coats, and it is required that the first coat must be completely dried before applying the second coat.

<!--[if !supportLists]-->4.<!--[endif]-->When coating the conformal coating on the circuit board, the general connectors, sockets, switches, sensitive devices, plug-in board areas, etc. are not allowed to have coating materials, and the corresponding masking process must be used.

<!--[if !supportLists]-->5.<!--[endif]-->Used brushes, spray guns and other utensils should be washed with special thinner in time for the next use.

<!--[if !supportLists]-->6.<!--[endif]-->Do not pour the conformal coating back into the original packaging to avoid contamination of the unused conformal coating. Unused conformal coating should be sealed and stored in a cool place for next use.

<!--[if !supportLists]-->7.<!--[endif]-->To repair a device that has been coated with conformal coating, simply remove the component by directly touching the soldering iron with the coating, then install a new component, and then clean the area with a brush or special thinner; After drying, re-coat with conformal paint.

To sum up, this paper introduces in detail the characteristics of different types of conformal coatings, such as solvent-containing acrylic resin conformal coatings are widely used, cheap and have good conformal coatings; Solvent-free acrylic resin conformal coating can be UV-cured, environmentally friendly and has a short curing time; Polyurethane conformal coating has a brittle texture and excellent solvent resistance; Silicone conformal coatings are soft, flexible, resistant to high temperatures and easy to repair. Then, the operation process of brushing, dipping, spraying, spraying, and anti-sagging of conformal paint was expounded, including their respective advantages and disadvantages, key points, and precautions. The brushing method is simple, but inefficient and inconsistent; The dipping method is effective, but the quality is difficult to control; The spraying method is divided into automatic machine spraying and manual spraying, each with its own advantages and disadvantages; The coating method is rarely used, and the thickness of the coating film needs to be paid attention to during operation. In terms of anti-sagging, Suzhou Tuotai's self-spraying conformal coating and its operation precautions were mentioned. Finally, the notes mentioned the commonalities of the conformal coating operation process, such as operating temperature and humidity requirements, safety protection, coating thickness acquisition, covering process, cleaning and preservation of utensils, and repair of coated devices.