What is conformal coating?
PCB conformal coating is the process of applying a 25-75 μm thick (typically 50 μm) polymer film to protect components in a printed circuit board, to protect circuit boards and related equipment from environmental erosion. Three-proof paint “three-proof” refers to moisture, salt spray and mildew. In addition, the conformal coating prevents dendritic growth and / or inhibits the occurrence of electromigration.
To correct any environmental problems that would affect the proper operation of the circuit, PCB manufacturers will typically add a clear layer of conformal coating, instead of potting, to the PCB and associated assemblies.
Why PCB or PCBA must need it?
Based on the definition and brief introduction on conformal coating, it is easy to summarize that the key role conformal coating plays in PCB manufacturing process lies in its protective function as moisture resistance, waterproof, insulation etc.
Moisture is the most common destructive factor that causes PCB board damage. Too much moisture will greatly reduce the insulation resistance and Q value, accelerate high-speed decomposition, and cause conductor corrosion. Similar to humidity or high humidity, contaminants contaminated on circuit boards can also lead to undesirable consequences, such as electrical failures, corrosion of conductors, and even not recovered shortcuts. Most of the contaminants left on the board come from residues from the manufacturing process, including fluxes, solvent release agents, wire clips, and marking inks. In addition, when the operator touches the circuit board, some residues are caused by artificial residues, including human oil, fingerprints, cosmetics and food residues. In addition, many pollutants even come from the working environment, such as salt spray, sand, fuel, acids and other corrosive vapors and mold.
When use conformal coating on the surface of circuit board or components, if negative factors in operation environment take the leading role, conformal coating is capable of reducing or eliminating electronic operating deterioration. Even if a conformal coating is only a thin layer, it can Excellent resistance to mechanical vibration, thermal shock and high temperature operation.
There are five types of conformal coating materials available commonly offered in the market.
This type of coating is usually solvent based and is very easy to repair. Acrylic resins are typically a type of solvent-soluble preformed acrylic polymer. Acrylic paint is easy to repair because it is easily soluble in many organic solvents. It has good anti-fungal and moisture-proof properties, is easy to dry, and has a longer service life. Although acrylic coatings have the advantages of being hard, durable, and transparent, they do not provide good stress relief.
Paraxylene is a polymer-based compound consisting of linear and crystalline polymers. The coating is applied in a vacuum environment that is spread over the material as a gas, forming a transparent polymer film. This protective coating can penetrate spaces as narrow as 0.01mm. Due to its good heat resistance, the coating has an operating temperature range of -200ºC and 150ºC.
This type of coating is highly uniform and can achieve high PCB pin coverage. However, it is expensive and sensitive to contaminants and must be used in a vacuum.
Urethane coatings have enhanced chemical resistance and excellent dielectric properties. The coating provides excellent adhesion in a variety of climatic conditions and has a high dielectric value.
This type of coating has high strength, hardness, and high solvent resistance. Polyurethane coatings also have excellent abrasion resistance and low levels of moisture permeability. In addition, they have good low temperature adaptability. Their limitations are their poor performance in high temperature environments and most of them cannot be repaired.
Epoxy coating materials are very hard and opaque and are good at resisting the effects of moisture and solvents. It is usually supplied as a two-part thermoset mixture and shrinks during curing, making it difficult to repair the film. Epoxy coatings have good abrasion resistance, chemical resistance and moisture resistance. Coatings can work at temperatures up to 150 ° C, but can put pressure on PCB components at extreme temperatures. Epoxy is very easy to use, but it is almost impossible to remove without damaging the components.
Silicone resin is a soft, transparent and soft coating with excellent corrosion resistance, chemical resistance, good moisture resistance, etc.
Due to its flexibility, it has good thermal shock resistance and is easy to apply and maintain. Its moisture resistance is similar to urethane and acrylic, and its dielectric resistance is lower than other coatings (1100 volts / mil).
Compared with similar acrylic or polyurethane coatings, the coating’s flexibility makes the coating thicker.
Silicone coating can work in the temperature range of 55 ° C to 200 ° C. It has good heat resistance, UV resistance and moisture resistance, which helps to protect delicate parts.
With excellent light transmission properties, it is very suitable for solar applications.
The low dissipation factor of the silicone coating makes it suitable for high impedance circuits board.
Where is conformal coated PCB and PCBA used?
Initially, conformal coatings were only used for PCBs belonging to the high-tech sector. In recent years, the development of materials and new processes has also enabled consumer electronics products to be coated. As circuits and electronic components continue to shrink in size and size, this will become more common and become the norm. The application of conformal coatings plays a vital role in helping to effectively improve product quality and reduce the cost of handling failures
Typical applications for conformal coated or conformal coated PCBs include:
1.Consumer electronics products.
Conformal coatings protect circuits used in home appliances, enabling them to overcome the following issues:
• Water and detergent (washing machine, dishwasher, sanitary products, outdoor LED)
• Harsh external environment (surveillance, security and fire alarm systems)
• Chemical substances (air conditioners, dryers)
• Contaminants in offices and homes (computers, microwave ovens) • Others
Electronic systems have always been required, so the application of conformal coatings can meet the basic requirements for automotive long-term reliability. The automotive industry requires the use of conformal coatings to prevent the following hazards: gasoline evaporation, salt spray or brake fluid.
Due to the special nature of the application environment, the aerospace environment places extremely stringent requirements on electronic equipment, especially when it comes to rapid compression and decompression. Therefore, the pressure stability of conformal coatings has been widely used.
Both fresh and brackish water can easily damage circuits in sailing equipment. The application of conformal coatings can maximize the protection of equipment on and under seawater.
Conformal coatings protect electronic devices from corrosion caused by external chemicals or special application environments, allowing them to operate consistently and smoothly.
Before coating a printed circuit board, it must be cleaned and de-moisturized within 8 hours of conformal coating. De-moisturizing can be done in an oven with temperature set to 88 °C to 98 °C for 4 hours. Methods of coating include spraying, brushing or dipping.
During conformal coating application, some components can never receive conformal coating including high-power components with heat delivery surface or heat sink, power resistor, power diode, cement resistor, DIP switch, adjustable resistor, buzzer, battery holder, fuse holder, IC holder, tact switch etc.
As discussed, pcb conformal coating services help protect PCB assemblies and improve their overall quality. 86PCB is a professional manufacture with more than 14 years experience in PCB and PCBA industry, we can provide one-stop service include Components Sourcing,PCB Manufacturing, PCB Reverse Engineering, SMT & DIP Assembly, X-ray Test etc.
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