Conformal coating is a protective chemical coating of polymeric material applied in thin layers to printed electronic components (PCB), usually, the thickness of the cured coating is 25-250 μm. Its primary purpose is to protect electronics from environmental elements and corrosion. This coating is "attached" to the circuit board. The coating acts as both a protective shield and insulative material for a PCB. The protection and covering effects are very good, which can extend the working life of the printed circuit board.
Type
1. Polyurethane Coatings
Urethane conformal coatings are known for their excellent moisture and chemical resistance. They are both abrasion-resistant and solvent-resistant, which results in coatings that are very difficult to remove. Urethane resins can be either single- or double-part substances.
- Good dielectric properties.
- Good moisture resistance.
- Solvent resistance.
- Less reversion potential.
- Abrasion resistance.
2. Acrylic Coatings
Acrylic resins are preformed acrylic polymers that have been dissolved in a solvent. They have high dielectric strength, and fair moisture and abrasion resistance. Acrylic coatings are easily and quickly removed by a variety of solvents, often without the need of agitation. They are typically one-part substances.
- Ease of rework.
- Simple drying process.
- Good moisture resistance.
- High fluorescence level.
- Ease of viscosity adjustment.
3. Epoxy Coatings
Epoxies provide good humidity resistance and high abrasive and chemical resistance. They are a tough coating, however extremely difficult to remove chemically for rework, is almost impossible to remove once it is cured.
- Useful to about 150°C [302℉].
- Harder durometer, abrasion resistance.
- CTE closer to epoxy PCB substrate.
- Higher Tg (Glass transition).
- Good dielectric properties.
4. Silicone Coatings
Silicone conformal coatings are single-component compounds that are often chosen for electronics that will be subjected to extreme temperature ranges. Silicone conformal coating is not abrasion resistant because of its rubbery nature, but that property makes it resilient against vibrational stresses.
- Stable over wide temperature range (in general, -50°C to 250°C)[-58℉ to 482℉].
- Flexible, provides dampening and impact protection.
- Good moisture resistance.
- High dielectric strength.
- Low surface energy for better wetting.
5. Fluorinated or non Fluorinated - Poly-Para-Xylylene (Parylene)
Parylene coatings are applied through a process called chemical vapor deposition. Parylene becomes a gas when heated. After cooling, it is put into a vacuum chamber, where it polymerizes and turns into a film. The film is then placed over the electronics.
- Excellent uniformity regardless of part geometry.
- Chemical inertness.
- The minimal added mass and low outgassing.
- Low environmental impact process.
- Low dielectric constant.
6. Amorphous Fluoropolymers
Amorphous fluoropolymers exhibit low refractive index, low coefficient of optical dispersion and good lamination properties. Same excellent chemical, thermal, electrical and surface properties as conventional fluoropolymers like PTFE. Fluoropolymers are like transparent PTFE.
- Low dielectric constant.
- High glass transition temperature.
- Low surface energy.
- Low water absorption.
- Solvent resistance.
Each has its own characteristics, the best type of resin to choose depends on the required protection needed for the electronic device and the particular environment in it will be located. The following aspects should be considered when choosing the type of coating material:
1. What is being protected against? (e.g., moisture, chemicals)
2. What temperature range will the electrical device encounter?
3. What are the physical, electrical, and chemical requirements for the coating material itself?
4. Electrical, chemical, and mechanical compatibility with the parts and substances to be coated (for instance, does it need to match the coefficient of expansion of chip components?)
5. How easily can the material be reworked once applied?
6. How fast does the material dry (cure)?
7. What type of process and equipment is necessary to achieve the required coating quality (uniformity and repeatability)?
How To Use
Coatings material can be applied in a number of ways, including brushing, spraying, dispensing and dip coating. Different methods of curing and drying are available depending on the conformal coating material.
Manual spraying - For low volume production when capital equipment is not available, the conformal coating can be applied by an aerosol can or handheld spray gun. This can be time-consuming and may need to be masked. Also, the quality and consistency of outcome are operator-dependent, so variations are common from board to board.
Automated spraying - Programmed spray systems can move PCBs on a conveyor under a reciprocating spray head.
Selective coating – An automated spray system can actually be programmable, with a robotic spray nozzle applying the conformal coating to specific areas on the PCB. This is popular for high-volume assembly because it eliminates the laborious masking process.
Dipping – This is a popular conformal coating method for high-volume assembly. Masking is usually necessary before PCBs are coated. Dipping is only practical when coating both sides of the board. Immersion speed, withdrawal speed, immersion time, and viscosity determine the resulting film formation.
Brushing - Brushing is used mostly for repair and rework. Conformal coating is brushed onto specific areas on the board, not generally the entire PCB. While low-cost, brushing is both a labor-intensive and highly variable method, so it more appropriately suits small production runs.
Is Conformal Coating Waterproof?
Typically, conformal coatings are not waterproof. These coatings are semi-permeable, so do not fully water-proof or seal the coated electronics. However, the use of conformal coating can improve the adhesion of silicone potting compounds and electronic components, thereby improving the waterproof level of silicone potting compounds. conformal coating can also be used as an isolation layer to prevent the poisoning reaction of platinum-cured silicone.
Bad Defect
1.De-Wetting
De-wetting is when a conformal coating will not evenly coat the surface it is being applied to. De-wetting usually happens because of nonionic contamination, Common contaminants are: flux, grease, oil or cutting fluid. The solution is to clean the substrate thoroughly until no contaminants remain. Select low-residue materials to control the process more effectively.
2. Orange Peel
Orange peel happens when the coating is uneven and textured, often appearing dull and very similar to the skin of an orange. This issue stems from the spray technique – namely, setting the spray pressure too low, which provides insufficient atomization, or from using the wrong thinner.
3.Bubbled Surface (Bubbles, Pinholes, Foam)
Bubbles occur when pockets of air get trapped under a conformal coating layer. They commonly occur when the coating fails to level and adhere to the substrate. Applying conformal coating with a brush may also create bubbles in the hardening surface.
Coatings that are too viscous or too thick are the main causes of blistering. To avoid air bubbles, pinholes and foam, avoid thick coatings and instead increase film thickness by laminating successive thin layers.
4.Delamination
Delamination happens when a conformal coating lifts away from the substrate. This is characterized by an area of the coating losing adhesion to the substrate, which can cause the coating to lift from the surface. In most cases, delamination is not immediately observed and only noticed once the part is in use. The main reason for delamination is that the surface of electronic components is contaminated, resulting in decreased adhesion.
5.Cracking
Cracking happens when a smooth surface of coating fractures into sections. Cracking can compromise a conformal coating film, exposing a substrate or PCBA to potential contaminants.
This is usually caused by excessive film thickness or insufficient time between overcoats. If curing with heat acceleration, exposing the wet film to excessively high temperatures can also cause this to occur. Because there is a higher risk of moisture, water, and debris reaching the board level, cracking could lead to other conformal coating defects.
6.Fish Eyes
Fish eyes are normally caused by a contaminant on the surface prior to the application of the conformal coating. Oily spots or silicone particles deposited on a surface while spraying conformal coating create small, crater-like openings known as fish eyes. Usually, that contaminant is a substance like a silicon contaminant. If the surface of the circuit board has been properly cleaned and degreased, the fish eye can usually be prevented completely.
