Conformal coating has a very important role to play when it comes to protecting circuit boards against unfavorable environmental conditions. In environments that have extremes of temperature, moisture, dust and more, circuit boards can often suffer from defects, compromising the shelf life of the product. Conformal coating acts like a protective film that keeps the PCB moisture proof, dust proof, protects it against temperature variances, keeps it free from corrosion & more. Another common aspect that impacts printed Circuit Boards are a wide variety of pollutants. They can lead to corrosion and even failure of PCBs. While a number of pollutants could accrue from the operating environment, often times there are a lot of contaminants on account of residues during the manufacturing process.
The application fields of conformal coating include, but are not limited to:
- Consumer Electronics – Consumer electronics particularly have to face harsh external conditions. Be it water and detergent in case of washing machines and dishwashers or contaminants as in the case of computers, microwaves and others.
- Automobiles- With break fluids, gasoline vaporization & more, there are enough and more reasons for the use of conformal coatings in automotive.
- Aerospace – With compression and decompression being the norm, it only stands to reason that the aerospace industry should see a wide spread application of conformal coating
While the benefits of using conformal coating are well established, however to reap the benefit of conformal coating, it is imperative that some preparations be made in the design phase itself.
You need to make sure that there are no contaminants on the board as contaminants can be sealed in and can create issues in the long run. In fact the boards need to be thoroughly cleaned as well as dried. This will go a long way in ensuring that long term performance isn’t impacted.
It is also important to remember that some components cannot receive conformal coating. Some such components include:
- High power components
- Adjustable resistor
- Battery holders
- DIP Switch and more
Conformal coating is also not effective in case of applications which are exposed to water significantly. In such cases it is advisable to offer protection through the process of encapsulation.
Ideally conformal coating works best for applications which have:
- Good electrical properties
- Chemical resistance
- Are durable mechanically
Also conformal coating does not work well over sharp edges as it results in edge crawling where the coating isn’t present on such edges. To prevent edge crawling it is necessary to check the viscosity of the coating as also use multiple coats. Specialized coating of 100 micron works best for improved edge cover.
Some of the tests that might be carried out on conformal coated boards include:
- Damp heat testing
- Thermal cyclic performance
- Chemical resistance test
- Flammability test
Conformal coating is normally applied by the process of dipping or spraying. Typically the thickness of the coating is 20-50 micron, even though specialist coatings can be applied at up to 100 micron thickness. In case of PCB Prototypes or small batch production, it is also possible to apply the coating by the process of brushing.
Conformal coatings can be of several broad types:
- Acrylic coatings – This comprises of thermoplastic polymer dissolved in organic solvents. Typically low boiling solvents are used that in turn leads to rapid touch dry. However these solvents are flammable and hence have environmental hazards associated with them.
- Epoxy coatings- They offer resistance to both moisture and chemicals. Also they rate highly in terms of abrasion resistance. The flip side however is that they are very hard to remove from the board.
- Urethane coatings- These are known for their moisture resistance however their removal and hence board repair is extremely difficult.
- Silicones cover- These can vary from tough abrasion resistant material to soft coatings. These coatings are especially known for their high temperature performance.
- Specialist coatings – These are of various kinds including UV cure coatings. These are especially useful where the production is high volume.
- Water based conformal coatings- These are largely developed in response to the use of organic solvents. However these take time to dry and with high coating thickness cracks can develop.
- Parylene coating process- This comes in handy in evenly coating components as also coating the under side of board components. The process, however, is expensive compared to conventional coating.