PCBA conformal coating
A Conformal coating is a protective chemical coating or polymer film 25-75µm thick (50µm typical) that ‘conforms’ to the circuit board topology. Its purpose is to protect electronic circuits from harsh environments that may contain moisture and or chemical contaminants. By being electrically insulating, it maintains long-term surface insulation resistance (SIR) levels and thus ensures the operational integrity of the assembly. It also provides a barrier to air-borne contaminants from the operating environment, such as salt-spray, thus preventing corrosion.
Some typical application examples include high-reliability defense, automotive and aerospace, where coatings are used to protect against various combinations of moisture, aggressive chemicals and vapours, salt sprays, large temperature variations, mechanical vibration, and even organic attack (e.g. fungus). The protective nature of conformal coatings also means that they not only protect, but also serve to enhance product reliability and thereby reduce the potential cost and damaging effects of early field failures.
How do conformal coatings work?
A conformal coating is a protective barrier that shields sensitive electronic components against harsh environmental conditions such as moisture, chemicals and debris. They are not designed to be a total sealant. Conformal coatings are a breathable protective layer that will protect against the particular environment requirement but will also allow any moisture trapped in the circuit board to escape.
The particular advantages of conformal coatings can be summarized as follows:
- Insulating properties allow a reduction in PCB conductor spacing of over 80%
- Can help eliminate the need for complex, sophisticated enclosures Light weight
- Completely protect the assembly against chemical and corrosive attack
- Eliminate potential performance degradation due to environmental hazards
- Minimize environmental stress on a PCB assembly
Ideally, conformal coatings should exhibit the following characteristics:
- Simple application
- Easy removal, repair and replacement
- High flexibility
- Protection against thermal and mechanical shock
- Protection against environmental hazards including: moisture, chemicals and other corrosive elements
How do I apply Conformal Coating
There are four main types of application method used for conformal coatings.
- Selective Coating by Machine
- Spray Coating
Products are available in bulk, aerosol and small packaging sizes, therefore the correct method and conditions should be assessed for each application. Careful consideration of the advised humidity and temperature conditions for the selected coating should be taken for both application and curing stages.
Conformal Coating Method
This method as the name suggests uses a brush to apply the coating onto the circuit board.The benefits of using this application method are primarily that it can be more cost effective for small scale production, and it is easy to select which areas to coat.
However, we generally advise against brush coating application as it is not always easy to apply an even coating. This can lead to either a lack of adequate protection if the coating is too thin, or if the coating is too thick, can lead to the coating cracking (especially under thermal cycling). Another disadvantage of this method is that you can only coat one side of the PCB at a time
With this method, the PCB is ‘dipped’ into the coating by machine, immersing the entire board and allowing the coating to easily get into gaps and under components otherwise hard to reach.
Most coatings can be used for this process, however dipping generally requires higher viscosity/higher solids content materials and any coating which reacts to the environment such as moisture-curing coatings are difficult to use since the tank cannot be sealed from moisture contained on the boards that are being immersed.
Disadvantages of this method are:
Accurate masking is essential for connectors and other areas that must not be coated. Masking is often a labour intensive process , and is 100% waste. The material (liquid or tape) must be applied, dried, and removed, and often the coating must be touched up if areas are damaged during demasking, and then the masking material must be thrown away. The use of custom made rubber masking boots, designed to form fit the components being masked, can save time, but is still an added process and additional expense that is difficult to automate.
Selective Coating by Machine
Selective coating is a method by which you ‘select’ which part of the PCB you would like to coat. Usually this method uses a machine which you program to coat only the areas which you want.
This method is suitable for all levels of manufacture and has the benefit over dip coating of needing minimum masking, and if the board is designed well for coating, masking can be avoided entirely. Being applied by machine also means you are guaranteed an even, uniform and repeatable coating application to the recommended thickness. It can usually be applied more quickly and is suitable for in-line, one-piece flow production.
The downside to this method is that it requires a more sophisticated operator to run the machine, and it is not always easy to get penetration under components. Programming can also be time intensive, and require machine downtime, which can limit the appeal of this method in low-volume, high-mix production environments. It also can also lead to ‘cobwebbing’ or blooming
Spray coating is, as the name suggests a method where the coating is sprayed onto the board, usually applied by hand in a spray booth or by aerosol, although it can be automated as in the selective coating process.
This is one of the most cost effective and convenient ways to apply a coating as it can be done on the benchtop if necessary and is therefore a good choice for rework or repair items, or small scale projects.
Most coatings can be used for this application method, however it requires a low viscosity, so solvent based coatings may need to be diluted to the required viscosity using the appropriate thinners.
Finally, when selecting the coating application method you should review the coatings curing instructions. As is mentioned above, some application methods are not suitable for certain curing types. More information on the types of cure
What else should I consider while applying a conformal coating?
There are several environmental factors which should be considered when applying coatings.
- Temperature – this effects the viscosity of the coating which in turn can affect the application effectiveness.
- Humidity – Humidity in the atmosphere can contaminate dip coat tanks. It can also cause blooming in hand spray and affects curing / pot life.
- Ventilation – Inadequate ventilation can lead to a build up of vapours which can affect the health of operators. Excessive ventilation can lead to problems applying the coating and excessive cob-webbing.
- Air Filtration – Factory air can contain debris which can contaminate the coating during drying, leading to a poor cosmetic finish, and potential reliability concerns, depending upon the nature of the debris.
Each product has a detailed outline of the best application method within the TDS (technical data sheet) provided on each product page.
At Electrolube we pride ourselves on the care and attention we provide our customers and our technical team are more than happy to help you with choosing the correct application for you. We work closely with a number of machine suppliers, both locally and internationally to provide the best possible support throughout your contact with us.