The conformal coating of a PCB helps to protect it from the short circuiting of components, high voltage arcing, rusting, etc, thus extending its life. This, in turn, brings down the costs of servicing, component replacement, or even replacing the PCB.
By Baishakhi Dutta
A conformal coating offers a huge array of benefits, including protecting the circuitry from moisture, fungus, dust and corrosion under extreme climatic conditions. It also protects the circuitry from the damage caused by board mishandling during construction, installation and use; reduces the mechanical stress on components, and protects them from thermal shock, apart from other benefits. The right coating enhances performance and allows greater component density due to the increased dielectric strength between conductors.
“Conformal coatings protect electronics from damage caused by high moisture levels, salt spray, contamination from dust, etc, and even from harsh chemical environments. The main aim is to extend the reliability of devices operating in harsh environments. A conformal coating allows an 80 per cent reduction in circuit board track spacing, which is very important in today’s electronics, as systems and components are becoming smaller. Safety systems such as airbag systems or avionic systems need guaranteed reliability over a known period,” explains Chris Palin, EMEIA manager, Humiseal.
“Conformal coatings are designed to protect printed circuit boards (PCBs) and related equipment from the environment. Typically applied at 25-75μm, these coatings ‘conform’ to the contours of the board allowing for excellent protection and coverage, ultimately extending the working life of the PCB. It is due to conformal coatings that the life of the electronic product is extended. The coating protects the circuit from humidity and other contaminants in the atmosphere,” informs Padmanabha Shakthivelu, general manager, Electrolube.
Listed below are the common methods of applying the conformal coating materials.
- Manual spraying: Aerosol cans or handheld spray guns can be used for applying conformal coatings. This method is generally used for low volume production when machines are not available. This is a time consuming method, since the areas for which coating is not required need to be masked. It is also operator dependent; so variations are common from board to board.
- Brushing: 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.
- Dipping: In this method, the PCB is ‘dipped’ into the coating with the help of a machine. Immersing the entire board allows the coating to get into gaps and under the components, areas which are otherwise hard to reach.
- Selective coating: In this method, you ‘select’ which part of the PCB you would like to coat. Usually, this method uses a machine that is programmed to coat only the areas that you want. Suitable for manufacturers of all strata, selective coating is more popular than dip coating.
- Spray coating: Spray coating is, as the name suggests, a method by which the coating is sprayed onto the board, usually by hand in a spray booth or using an aerosol can. This process can also be automated as with selective coating, and is considered to be one of the most cost-effective and convenient ways of applying a coating.
- Chemical vapour deposition: This application procedure is specific to parylene conformal coatings. When heated, parylene is transformed into a gaseous state, which is cooled and introduced to a vacuum chamber where it polymerises and becomes a film. The process requires specialised equipment and training, so is often outsourced.
Traditional and emerging sectors where conformal coatings are being used are:
- Industrial engineering
- White goods
- Solar panels
- Marine and coastal environments
Other facts to keep in mind
Several environmental factors should be taken into account before applying conformal coatings.
- Temperature: This affects the viscosity of the coating which, in turn, can impact the effectiveness of the application.
- Humidity: Humidity in the atmosphere can contaminate dip coat tanks.
- Ventilation: Inadequate ventilation can lead to a build-up of vapours, which can affect the health of operators. At the same time, excessive ventilation can lead to problems when applying the coating and cause excessive cob-webbing.
- Air filtration: The air in a factory 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 market sector has a distinct set of requirements in terms of standards, operating environments as well as other factors. Previously, a conformal coating was applied only to military and safety critical products like medical devices, since the cost of the process was high. In the last couple of years, developments in the materials used and new processes have led to the entry of coatings into the consumer electronics domain as well. This will become more common and possibly the norm, as circuitry and electronic components continue to shrink in size and dimension,” says Dr Lee Hitchens, technical director, SCH Technologies.
“At present, liquid conformal coatings and parylene are being adapted. They are being made with less solvents and are applied in a thick manner to give better protection. As far as new technology is concerned, ultra-thin coatings like molecular vapour deposition (MVD) are being used of late. These are highly cost-effective compared to the traditional materials but require some compromises on other fronts,” Hitchens adds. He feels that the new coating technologies are really making a difference and would be great for the Indian market.
Palin elaborates, “Coatings are evolving in several areas. They can now withstand higher operating temperatures like a 150°C constant. There are also rapid curing systems such as UV light cure that conform to safer standards with regard to solvent systems, like the elimination of toluene, for example. Coatings are being developed to survive more extreme testing, like thermal shock for up to 1,000 hours+, elevated temperature and humidity testing for 1,000 hours+, etc. Other innovations are in the area of low outgassing coatings for optical and space applications.”
“Acrylic coatings are the most popular because they can be easily reworked in case of any repair of the PCB. Silicone coatings are used when better flexibility is required, in cases when the PCB is used in equipment subjected to heavy vibrations. Epoxy and urethane coatings are used where high chemical and mechanical resistance is required. Parylene coatings are used in aerospace applications,” says Chetan Uchil, partner, Aalpha Conformal Coatings.
“ As far as the latest technology is concerned, 2K coating is the new trend. 2K coatings are two-part coatings for special applications and for harsh environments. This new technology is picking up very fast in the electronics and automotive electronics industries,” states Shakthivelu.
Conformal coating resin types
Conformal coatings can be divided into five major categories:
- AR – Acrylic resin
- SR – Silicone resin
- UR – Polyurethane (urethane) resin
- ER – Epoxy resin
- XY – Parylene
Suggestions to buyers
Opinions from experts have helped us consolidate the crucial factors that are to be kept in mind to identify the best conformal coating materials. These are listed below.
- Operating temperature range: Conformal-coated assemblies will often be exposed to a wide range of operating temperatures during their service life. It is important to be aware of the maximum and minimum temperatures that the unit will be exposed to.
- Environmental considerations: A conformal coated PCB gets exposed to different types of corrosive environments. In order to prevent damage in the future, common dangerous environmental conditions should be factored in when selecting a coating.
- Suppliers’ performance: Choosing a conformal coating material is a long and exhaustive process, with many considerations and conflicting choices and tradeoffs to be made. If the suppliers are unwilling or unable to help you through the selection process, then they are unlikely to be of much help with any subsequent issues that may need to be resolved. Getting adequate support and backup prior to qualification is an indicator that the supplier knows the business well enough to become a value-added supplier during production.
- Price: Often, materials are selected on the basis of price alone. Although this is a valid point, considering all the other factors is arguably as important, if one wants to make the best choice.