How to Choose Between Potting and Conformal Coating

When safeguarding printed circuit boards (PCBs), manufacturers often turn to potting or conformal coating. The choice between the two depends on the PCB’s intended use and the level of protection needed. While potting provides a sturdy shield, it comes with operational drawbacks that can outweigh its advantages. On the other hand, conformal coatings create a trustworthy protective barrier that often avoids the issues associated with potting. This is especially true with Parylene, a conformal coating that is not in liquid form.

Potting Properties

Potting is a two-step process, wherein a PCB is first protected from the surrounding environment by insertion (embedding) into a case or shell (the “pot”). A liquid potting compound is then poured over and around the containerized PCB, encasing it completely within.

Materially, certain epoxy resins make suitable potting compounds; thermo-setting plastics or silicone rubber gels are also frequently used. More specifically, a PCB employing surface-mount technology benefits from the application of lower glass transition temperature (T_g) potting compounds like urethane or silicone. High T_g potting compounds may break solder bonds as they harden and shrink at low temperatures.

Most potting applications use epoxy because of its adhesive, electrical, mechanical and thermal properties. Selection of the wrong potting for your application can generate unwanted stresses or heat. If necessary, colored compounds can effectively obscure the PCB’s visual appearance in cases where competitors or others seek to reverse-engineer the item and its function.

Benefits
Potting techniques are particularly adaptable for high-speed, automated line production, where the objective is to manufacture a certain quantity of units per hour. Potting cases are secure and offer dielectric protection while preventing contaminants from penetrating the device. Prominent potting compounds like epoxy are also chemically inert, safeguarding the PCB from the polluting effects of acids, bases, corrosion, salt and most solvents.

In this respect, potting is often the method of clear choice for impact resistance, vibration dampening, heat dissipation and conditions where assembly privacy and security are necessary for product performance.

Disadvantages
Despite potting’s demonstrated protective-utility for PCB assemblies, it lacks the sufficient flexibility for many component applications, limiting its overall use. Other disadvantages of potting include:

• Difficult to rework
• Inability to overcome restrictions to coating thickness or assembly weight due to encasement
• Housing design flaws that generate a surface susceptible to material-strain during compound shrinkage, causing PCB damage
• Utilizing underfills to enhance shock-robustness can encourage failure under thermal cycling

Properties of Conformal Coatings 

Unlike potting, conformal coatings add a protective film specifically to the substrate surface. The much thinner surface coatings are far lighter than potting encasements and can be adapted to numerous applications. Coating that conforms to the contours of the substrate generates non-conductive, dielectric protection that increases a PCB’s electrical performance.

Wet conformal coatings — acrylic, epoxy, silicone and urethane — are applied via liquid methods, brushing, dipping or spraying the coating directly onto the substrate surface. Each wet coating offers a range of uses, specific to its particular properties:

• Acrylics are popular because they are easy to apply and rework, cure quickly and are fungus/moisture resistant
• Epoxy is exceptionally durable but difficult to remove or rework
• Silicone is also difficult to rework but is moisture resistant and remains chemically stable at 200°C, providing good protection in environments typified by chemicals, solvents or vibration
• Urethane’s dielectric properties insulate signal traces from circuits situated in close proximity, making it useful for MEMS/nano applications. It inhibits tin whisker growth while generating abrasion, chemical and humidity resistance.

Parylene not only differs from other conformal coatings but is also very different from potting in process and result. Applied in a gaseous state via chemical vapor deposition (CVD), its protective film penetrates deep into the product’s surface, creating a protective coat that covers it entirely — even under components and inside gaps. Unlike the block-coating outcome common to potting, Parylene’s protective film is exceptionally thin — usually measured in microns — and essentially invisible. Parylene is chemically inert protecting against acid, bases, corrosion, salt and most solvents, offers good dielectric protection, blocks humidity and moisture, withstands extremes of physical stress and temperature and is pinhole-free and ultra-thin, making it very suitable for MEMS/nano applications.

Conclusion

PCBs used in environments characterized by persistent exposure to fluids, corrosion, fungus, humidity, salt/sand-spray or temperature extremes, experience performance issues. Encapsulating or conformally-coating PCBs helps mitigate these problems. Potting and conformal coating processes lessen operational fatigue, enhancing PCB-performance. Both are effective when used in the right situation, thus it may be advisable to seek professional assistance to develop solutions that coincide with your project, budget and timeline.