There is a fascinating scene in the iconic movie Apollo 13 when the astronauts are repowering the electronics of their capsule after a few days of freezing temperatures when all the electronics have developed a nice coat of frost. There is a tense moment as the control panels warm up with water dripping off them before the spacecraft successfully comes back to life. Without knowing the details of Apollo 13’s electronics, it is safe to say they probably had some form of conformal coating, allowing them to operate in such moist conditions.
Photo Credit: NASA (via Wikimedia Commons) - This photo from training for the Apollo 16 mission shows some of the electronics. Pictured is Ken Mattingly (right foreground), during extravehicular activity training at NASA's Manned Spacecraft Center. Mattingly was scheduled to fly on Apollo 13 but was replaced by Jack Swigert (see below) after he was potentially exposed to German Measles.
Fast forward to 2022 as the Internet of Things technology mega-trend continues, we are seeing PCB assemblies being deployed in an exponentially increasing variety of locations, often exposing them to harsh environments, such as those encountered in automotive, consumer electronics and medical applications. Conformal coating is a popular option for improving the performance of PCBAs and prolonging their life span in such conditions.
Conformal coatings protect PCBAs from a variety of environmental hazards such as moisture, dirt, fungi, salt, acids, most solvents, vibration and even extreme temperatures. The coatings form a thin film, typically around 50 µm thick, “conforming” easily to the irregular contours of the substrate. Considering the thinness of the film conformal coatings are very effective in providing a cost-effective alternative to potting the PCB in resin, without any significant additions to the weight or thickness of the device.
There are several types of resin to choose from including acrylic, silicone, epoxy, urethane and parylene. Acrylic is the most popular as it's easy to apply, does not shrink and is most affordable. It is easily removed with solvents to facilitate rework but that makes them unsuitable for environments where they may encounter unwanted solvents. For chemically harsh environments, polyurethane coatings provide better protection. Epoxy coatings are ridged and generally used in physically harsher applications. Silicone has better insulating properties so better for exposure to high temperatures above 150°C.
There are several different methods for applying the conformal coating. The most rudimentary approach is an application with a brush, but for most low volumes the easiest method is spraying, typically in a booth to evacuate the fumes. The spray is applied from two directions on both sides of the board to ensure an even coating.
Dipping can be used when increasing volumes justify the wastage of compound from the dipping vessel at the end of the run. Dipping ensures complete coverage as PCBAs are withdrawn from the dip at an even rate, ensuring smooth coverage. With large boards, unsightly streaks can appear during the drying process, so dipping is most typically used for small boards.
For higher volumes, there are a variety of automated options. Generally, these are dedicated to a particular customer and so can be customised to requirements during production contract negotiations.
In all of these methods, it is important to clean the board thoroughly ensure contaminants are not trapped under the coating, potentially defeating the purpose of applying it in the first place.
A key question is when to use conformal coating versus potting or no coating at all?
Potting is generally the preferred option when the operating environment may include the likelihood of impact and abrasions, high levels of vibration, extreme heat, water, electrical arcs or situations where security of the device is a concern. With the right potting compound, potting can also help heat dissipation.
Most product developers choose not to use conformal coating at all. This is a reflection of the many benign environments PCBAs are deployed in and the easy availability of IPxx rated enclosures that do a good job of protecting boards. However, if your board is being applied in any application where there is a likelihood of corrosion or exposed to any of the extremes listed above, conformal coating is a good option.
The astronauts of Apollo 13 probably owed their lives to someone who paid attention to conformal coating and today we need to take the same attitude to any product in harsh environments. For those who want to see a triumph of engineering ingenuity, watch the Apollo 13 movie and look out for that power-up scene.