Conformal coating protects components from mechanical and chemical damage.
The state of a component, particularly cleanliness, impacts how conformal coating works.
Masking is a useful way to prepare a PCB or component for the conformal coating process.
Conformal coating dispensing system.
Electronic components, large or small, are vulnerable to wear and tear after prolonged use. Consumers expect electronics to last for a long time, so manufacturers have developed ways to protect their components from damage and keep them functional for extended periods of time. One of these methods is called conformal coating. This process protects PCBs from mechanical and chemical damage, extending their lifespan and reliability in multiple ways.
What is a Conformal Coating?
A conformal coating is a specialized polymer film that protects components and PCB boards from mechanical or chemical damage. It is essential to protecting aerospace components from extreme conditions and was originally developed for mission-critical applications. In the modern digital age, the conformal coating process has become commonplace in many applications, including some used by the general public, like mobile devices.
Conformal coatings are usually a form of resin mixed with water or various solvents, which can be applied in a variety of ways. Conformal coatings that are purely resin-based are semi-permeable, but coatings that are combined with other substances, such as epoxy, are waterproof and impermeable. Typically, the conformal coatings used on electronic components are the semi-permeable variety. Electronics with conformal coatings can be easily repaired with minimal hassle needed to touch up the coating.
Most resin-based conformal coatings are made from one of three different resin types: acrylic, silicone, or (poly)urethane. Each resin has specific strengths in harsh environments. For example, acrylic resins have a high dielectric strength, while silicone resins are resistant to salt spray. Polyurethane is resistant to abrasion, moisture, and chemicals. Depending on their strengths, each conformal coating type must go through specific processes if they are to be removed. Acrylic coatings can be removed comparatively easily with solvents, but silicone and polyurethane coatings require specialized solvents, agitation from an ultrasonic bath, and a long soak time.
There is also a type of coating called a non-conformal coating. These coatings are made similarly to conformal coatings, but they don’t conform as closely to the surface features of an electronic component, hence their name. They also tend to be thicker than conformal coatings, making repairs much more difficult.
Essential Preparation for the Conformal Coating Process
Conformal coating only works well if a component is clean and free of physical defects. There must be no impurities on the component’s surface, including dirt, debris, or moisture, and the coating itself must be uniform. There are many different ways that uneven coating due to a dirty or defective component can cause failures, including corrosion, short circuits, and physical damage. Cracks can appear within the coating where the board and the coating meet, a defect called blistering.
There are also a number of other adhesion problems that can result from a badly prepared board, including discoloration or an uneven finish. One adhesion problem with a memorable name is “fish eyes”, which happens when little craters form on the surface of the coating due to the coating material running unevenly across the board as it sets.
Many conformal coatings are required to meet strict international standards, so creating a conformal coating process that adheres to those standards is essential. The main standard for conformal coatings is IPC-CC-830C, and it contains sections for every variant of conformal coating, along with specific concerns for a variety of environmental issues. These issues include water, fungus, and flammability. The standard also includes guidelines for electrical appearance and functionality, along with guidelines for using different materials and processes when making electronics. The standard was last updated in January of 2019 and is periodically updated to reflect the needs of the electronics industry. Other international standards exist for specialized use cases, including electronics assemblies and soldered electronics. In total, three different international electronics standards deal wholly or partly with the conformal coating process.
An example of how one should not apply a conformal coating.
Improving Conformal Coatings through Masking
When placing conformal coatings on a component, there are certain areas that it’s best to leave uncoated. These areas can include connectors, holes, LEDs, and certain types of integrated circuits. Preventing these areas from being coated involves a process called masking.
There are multiple masking techniques, including shields, tapes, fluids, and gels. There is no specific technique that is always best for a particular masking scenario, and designers must consider many factors before masking. The shape, geometry, and function of the masked area, along with the level of protection given by the masking, are major factors that should influence how a designer decides to apply masking. Designers must also take into account the expected lifespan of the masking and how difficult it would be to remove the masking if necessary.
Some masking techniques are permanent in preventing adhesion of conformal coatings, while some are temporary. Permanent liquids and gels are common, but some new technologies have opened up more possibilities in temporary masking. Temporary masking liquids that can be cured with ultraviolet light are a recent addition. They are easy to apply and remove without requiring physical media like shields or tape. In addition, they don’t have the same restrictions as solid masking materials on board geometry. Despite these advantages, these temporary liquids require skilled application and removal techniques—although some automation is possible, a skilled technician must direct the application tools, and the process to do that is not trivial.
A PCB protected by conformal coating.
Consider including conformal coating in your manufacturing processes to produce reliable and durable electronic components. The PCB Design and Analysis overview page at Cadence Design Systems provides a wide variety of information about the conformal coating process. Learn what’s new in OrCAD and use the OrCAD PCB Designer to keep your PCB design processes both time and cost efficient. If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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