Potting vs Conformal Coating of PCB Assemblies
Conformal coatings are a protective, non-conductive dielectric layer that are added to a circuit board or electronic device.
A coating “conforms” to the object being coated (often referred to as the “substrate”), allowing protection without significant additions to the weight or thickness of the device.
Potting is also designed to protect a device from the surrounding environment. But instead of a surface coating, potting employs a “pot” or box encase the board assembly.
Once the assembly or substrate is placed inside the “pot”. A liquid compound is then poured into the enclosure filling it and covering the assembly. The liquid hardens, encasing the device inside it. The pot and the hardened compound surrounding the device become part of the final product.
Similarities Between Potting and Conformal Coating
The objectives of both processes are the same:
- Prevent the substrate from failing in harsh environments or from vibrations or other potentially-damaging uses.
- Increase the electrical performance of a circuit board through shielding.
Either method can be effective when used in the right situation.
Potting– Impact resistance.If the product needs to withstand potential impacts or rough mechanical abrasion, potting is likely to provide a higher level of protection. Also, if you need protection from vibration/dampening, heat dissipation, privacy or security.
Potting compounds can be colored and thereby obscure the circuit board or device. This can provide protection against someone else reverse engineering your product. The biggest drawback of potting is that it creates an extremely thick monolithic block as compared to conformal coating.
Conformal Coating Conformal coating is thin making it a good choice for circuit boards and devices, especially ones with weight or thickness considerations.
| Type |
Full Name |
Thickness when applied |
- Type AR
- Type ER
- Type UR
- Type SR
- Type XY
|
- Acrylic Resin
- Epoxy Resin
- Polyurethane Resin
- Silicone Resin
- Parylene Resin
|
- 0.00118 to 0.00512 in.
- 0.00118 to 0.00512 in.
- 0.00118 to 0.00512 in.
- 0.00197 to 0.00827 in.
- 0.000394 to 0.00197 in.
|
As with any engineering decision, the needs of your device will determine which tyoe of coating is best. Both potting and conformal coating provide a degree of dielectric protection, and both can protect against:
- Corrosion
- Salt
- Acids
- Bases
- Most solvents
Potting is a popular choice because it is fast and easy to apply on assembly lines or in high-volume production environments.
A device that has been potted, however, is much more difficult to work with than a device that has been coated. Potted devices are extremely difficult to rework, since removing the potting often destroys the circuit board or device underneath.
Because they are so thin, conformal coatings are the clear choice when tolerances are tight. The invisibility of the coatings also makes them the right choice when an item needs to be visible. Potting a device with an indicator light, for example, defeats the purpose of having the light in the first place.
Types of Conformal Coatings
There are five types conformal coating. Four of the five—acrylic, epoxy, polyurethane, and silicone—are applied by either brushing, spraying, or dipping the coating on the substrate, then letting it dry. The fifth coating type—Parylene—is applied using a unique vapor-phase polymerization process.
Acrylic Resin (Type AR)
Acrylic conformal coatings are fungus resistant and can easily be applied. They dry to the touch at room temperature in minutes and have excellent electrical and physical properties. Acrylic coatings are typically applied at 0.002 to 0.005 inches thick. Most variations cure in as little as 30 minutes, making them a great choice when you need a short turnaround time.
Most popular acrylic coatings are not good choices for high-temperature environments. They can usually only withstand temperatures of up to 125 degrees Celsius.
Epoxy Resin (Type ER)
Epoxy conformal coatings are “two-component” compounds. They deliver a rugged coating with good resistance to damage from humidity, high abrasion, or chemicals.
Epoxy coatings are known for their extreme hardness, making them a good choice when you need toughness and durability.
Their extreme hardness means rework and repair are difficult. Extreme temperatures also tend to reduce the stress resistance properties of epoxy coatings.
Polyurethane Resin (Type UR)
Polyurethane (also called “urethane”) conformal coatings deliver excellent humidity and chemical resistance. Polyurethane is often the optimal choice for devices that will be exposed to chemical solvents. Its dielectric properties also promote miniaturization because it insulates signal traces from circuits that are close together.
Polyurethane provides humidity, abrasion, and chemical resistance. It retains high dielectric properties over time and is one of a few methods to prevent against tin whisker growth.
Polyurethane’s resistance to solvents means it can be difficult to remove or rework. It also doesn’t do well in high-vibration or high-heat environments.
Silicone (Type SR)
Silicone conformal coatings perform well in high-temperature environments, even up to 200 degrees Celsius. That makes silicone a popular choice for automotive applications. It also has a resistance to humidity and corrosion and can be applied in thicker layers than other coatings, promoting vibration damping.
Silicone is less resistant to abrasion and solvents than other coatings. It also requires more care to apply correctly.
Parylene (Type XY)
Parylene is often considered the “gold standard” of conformal coatings. Unlike the other coatings, Parylene conformal coatings are applied using a unique vapor phase polymerization process.Parylene’s application begins with a raw parylene dimer. The dimer is placed in a loading boat inside a vaporizer.The powdery dimer is heated to 100-150 degrees Celsius, converting it from a solid into a gas. The gas is then heated to 680 degrees Celsius. At the higher temperature, the parylene gas splits from a polymer into a monomer, causing a single molecule vapor to be formed.
The gas is pulled through a vacuum into an attached coating chamber, where it evenly coats the surface of the circuit board or device placed there by the operator.
Parylene-coated surfaces are exceptionally resilient, withstanding extremes in temperature and physical stress. The unique coating process makes Parylene coating the thinnest coating available, and it ensures a pinhole free application.
Parylene can be applied to virtually any surface and objects of any shape, including glass, metal, paper, resin, plastics, ceramics, ferrite, and silicon. Parylene is also completely inert, making it an excellent choice for implants and biomedical devices.
Rework is difficult with Parylene due to its unique application process. Operators must also ensure the object to be coated is completely clean and that any areas not to be coated are meticulously masked.
Both potting and conformal coating are complex processes with a number of variables that will determine cost, effectiveness, cost, and turnaround time. Consulting a team of experts will help you choose the right protection for your project, budget, and timeline.