General Aspects You Should Know About Conformal Coating Applied on PCB

This Gold Phoenix PCB Knowledge Center article explains conformal coating — what it is, why it matters, and how to prepare for it in PCB design.

What Is Conformal Coating?

Conformal coating is a type of coating with special ingredients, used to protect circuit boards and related equipment from erosion in order to prolong product shelf life and guarantee the security and reliability of printed circuit boards (PCBs). Conformal coating features high capability to withstand high and low temperatures. It becomes a transparent protective film after solidification, featuring excellent merits including insulation, moisture resistance, anti-creeping, vibration resistance, dust resistance, anti-corrosion, anti-aging, and corona resistance.

The reason for the existence of conformal coating lies in the fact that PCB boards may suffer from issues such as erosion, softening, deformation, or mold in practical environments that may include chemical exposure, vibration, high dust, salt fog, moisture, and extremely high temperatures — all of which can lead circuit boards to suffer from defects or failure.

Why Is Conformal Coating a Must for PCBs?

Based on the definition and brief introduction of conformal coating, the key role it plays in the PCB manufacturing process lies in its protective function as moisture resistance, waterproofing, and insulation.

a. Moisture

Moisture is the most common and destructive factor causing damage to PCB boards. Too much moisture will dramatically decrease insulation resistance and the Q value, accelerate high-speed decomposition, and cause conductor erosion. It is common to see metal parts assembled on PCB boards covered with a layer of green copper patina, simply because a chemical reaction takes place between copper, water vapor, and oxygen.

b. Pollutants

Hundreds of pollutants can be found on the surface of printed circuit boards when conformal coating is not applied. These pollutants are equally hazardous to the reliability and functionality of PCBs. Similar to moisture or high humidity, pollutants contaminated on circuit boards can lead to electronic failure, conductor erosion, or even irreparable short circuits. The majority of contaminants left on circuit boards derive from residues during the manufacturing process, including flux, solvent parting agent, clipped wire, and marking ink. Moreover, some contaminants result from manual handling, including human oil, fingerprints, cosmetics, and food residue. Additionally, many pollutants come from the operating environment, such as salt spray, sand, soil, fuel, acid, corrosive vapor, and mold.

These pollutants can be greatly reduced by effective measures during the PCB fabrication process. New technologies can be used to optimize the manufacturing process to reduce the number of chemical residues on circuit boards. Rigorous operating principles can be established and followed, and operation engineers can be trained to meet all requirements of scientific manufacturing. For example, when operators are required to wear gloves when handling boards, no fingerprints will be left on them.

When conformal coating is used on the surface of circuit boards or components, if negative factors in the operating environment take the leading role, conformal coating is capable of reducing or eliminating electronic operating deterioration. As long as conformal coating can maintain its effectiveness for a time period longer than the lifetime of the final product, it will fulfill its protective responsibilities. Even though conformal coating is just a thin layer, it can excellently withstand mechanical vibration, thermal shock, and high-temperature operations.

It should be noted that conformal coating is not capable of making components stronger mechanically or providing sufficient conductivity. Fixation of components depends on mechanical methods with joint adhesive, while conformal coating does not contribute to this.

Where Is Conformal Coating Used?

At first, conformal coating was only used for PCBs in high-tech fields. As electronic products have been increasingly applied in daily life, consumers have started to pay more attention to the quality and reliability of electronic products. As a result, the application of conformal coating plays an essential role in improving product quality and decreasing expenses related to breakdowns.

Typical application fields include:

a. Consumer Electronics. Conformal coating provides protection for circuits in home appliances to address the following issues:

• Water and detergent (washing machines, dishwashers, sanitary products, outdoor LEDs)

• Bad external environments (monitors, security and fire alarm systems)

• Chemical substances (air conditioners, dryers)

• Contaminants in office and home (computers, microwave ovens)

b. Automotive Industry. Electronic systems have been constantly increasing in vehicles, and the application of conformal coating can meet fundamental demands for long-term reliability. Automotive applications require conformal coating to protect against hazards such as gasoline vaporization, salt fog, and brake fluid.

c. Aerospace. Due to the specificity of the application environment, aerospace sets extremely rigorous requirements for electronic devices, especially when rapid compression and decompression are involved. Pressure stability of conformal coating has enabled wide applications in this field.

d. Navigation. Both fresh water and salty seawater tend to cause damage to circuits in sailing equipment. Conformal coating can protect equipment on the surface and below the water to the greatest extent.

e. Medical Care. Conformal coating is capable of protecting electronic devices from erosion caused by external chemical agents or special application environments, ensuring consistent and smooth operation.

How to Prepare for Conformal Coating in the PCB Design Phase

To obtain optimal conformal coating on PCBs or PCBAs, the following preparations can be made during the PCB design phase:

• Conformal coating PCB boards that go through assembly must contain a technical rail of at least 5 mm.

• The maximum size of conformal coating PCB boards for assembly is 410 mm × 410 mm, and the minimum is 10 mm × 10 mm.

• The maximum height of components assembled on conformal coating PCB boards is 80 mm.

• The shortest distance between areas with conformal coating and without conformal coating is 3 mm.

• Complete cleaning enables the elimination of eroded contaminants and makes conformal coating adhere firmly to the surface of circuit boards. The optimal thickness of conformal coating is within the range of 0.1 mm to 0.3 mm, and the board should be baked for 10 to 20 minutes at 60°C.

During conformal coating application, certain components should never receive conformal coating, including high-power components with heat dissipation surfaces or heat sinks, power resistors, power diodes, cement resistors, DIP switches, adjustable resistors, buzzers, battery holders, fuse holders, IC holders, and tactile switches.