The Romans were the first to pioneer a protection method for their troops during a battle that is still in use by law enforcement today. By interlocking their shields together, the front, sides, and rear lines of a formation of soldiers presented a strong barrier of protection. At the same time, the remaining troops held their shields over the heads of the group, protecting them from incoming arrows. This configuration of shields was known as a testudo, or “tortoise” formation, and it provided a protective coating that was difficult for their enemies to penetrate.
The strategy of using a protective covering is not limited to combat tactics and can also help in protecting electronics. For years, mission-critical circuit boards operating in extremely stressful and harsh environments have benefited from a protective coating. Now, more consumer electronics are considering applying a conformal coating to their PCBs. Here we’ll look at what PCB conformal coating is and when you should consider using it to protect your sensitive electronics.
The Purpose of PCB Conformal Coating
PCB conformal coating is a protective material applied to a fully assembled circuit board, providing a thin transparent covering. The coating is made of non-conductive materials and can cover the entire board and its components, protecting exposed metal areas such as component leads and solder joints. The conformal coating also gives extra dielectric strength to the circuit board, allowing for reduced spacing between circuits, and will help protect the board against structural and thermal stresses.
However, conformal coating is not the same as circuit board encapsulation, where a circuit board is encased in a hardened material shell. Although it provides more protection, encapsulation increases the weight of a PCB considerably, and its manufacturing process is more complicated and expensive than conformal coating.
Conformal coatings are made from the following materials, which have different properties and are used in different applications:
- Acrylic resin: This is the easiest conformal coating material to apply and remove, and is the least expensive. It also provides the least protection from abrasion, solvents, and chemicals but has high dielectric strength.
- Urethane resin: Urethane has long cure times and is difficult to remove, but it provides great protection against humidity and chemicals, and
- Silicone resin: This coating provides good protection against high temperatures, humidity, chemicals, and corrosion and adheres well to most PCB materials. However, these capabilities make silicon resins the most difficult to remove, requiring aggressive abrasion and chemical solvents.
- Epoxy resin: Epoxy is also a difficult conformal coating to remove, but it provides excellent protection against moisture, chemicals, and abrasion.
There are multiple methods of applying a conformal coating to a circuit board, starting with manual brushing or spraying. These methods are usually used during rework operations to re-apply the coating, or in the case of spraying, giving technicians an economical method to quickly apply it to a new circuit board. Conformal coating can also be applied by automated spraying machines, with some having the capabilities to be configured for selective spraying in specific areas of the board. And lastly, the most common method of applying the conformal coating in a production environment is to dip the boards to cover both sides quickly.
Conformal coatings should be applied at a thickness of 25 to 127 microns, and in this case, “thicker” does not necessarily equate to “better.” A coating thicker than its specified tolerance does not necessarily offer any better protection and usually ends up merely wasting the material.
Thick layers of conformal coating can corrode and fail, especially with cracks or peeling. Curing conformal coating varies depending on the chemistry used and can take hours or days to cure fully. In many cases, curing can be accelerated with applications of heat or UV. Let’s look next at whether or not your board should be coated.
When Should Printed Circuit Boards be Coated?
Conformal coatings should be used on circuit boards subjected to difficult operational conditions. Depending on the type, conformal coatings will help protect the board from thermal stress, abusive or rough handling, and different solvents and chemicals. The coatings will also protect the board from the following common contaminants:
- Condensation and moisture
- Salt spray
- Dirt and dust
The conformal coating will also help seal up the potentially delicate components on the board, preventing the possible leakage of their materials and eventual corrosion.
Because of their protective properties, conformal coatings should be used on circuit boards used in harsh environments. Some examples of these applications would include:
Even more common consumer applications, such as wearable electronics, will benefit from being conformally coated. Wearable IoT devices in regular contact with your skin run the risk of greater moisture exposure due to sweat, which can cause problems for delicate electronics.
Which coating to use will depend on the circuit board’s application. For instance, acrylics provide only basic protection and are not recommended for solvent, vapor, or chemical resistance such as gas or oil pumping equipment. On the other hand, urethane has excellent chemical resistance and is often used in aerospace applications where protection against fuel vapors is required. The flexible nature of silicone makes it ideal for outdoor applications where protection against vibration is essential. The hard covering provided by epoxy makes it very resistant to moisture and chemicals.
How Your Contract Manufacturer Can Help with PCB Conformal Coating
To get the best protection possible for your circuit board, you need to understand what the board needs protection from and which coating will be the best choice for your application. Remember, PCB rework is crucial when evaluating which PCB conformal coating should be used. If your circuit board is expected to be upgraded during its life cycle, you should avoid using a coating that requires a lot of effort to remove. Digging out the coating material to change the board or even test a circuit takes time and could potentially result in collateral damage that requires even further rework.
The best thing you can do is work with your PCB contract manufacturer to fully understand all the options and considerations that may impact the board. This process is important and may even influence how you lay out your design to lessen the cost of production conformal coating. At VSE, we have been helping our customers with these decisions for a long time, and we know what information you need to build the best board possible.