The Definitive Guide to Conformal Coating

Conformal coatings are a thin film of material applied to a completed circuit board that “coats” the board and its assembled parts with a protective substance for additional protection. When first introduced, these coatings were initially used only on circuit boards exposed to extreme environmental conditions due to the expense. The circuit boards included in these categories included industrial uses, marine & automotive applications, and military & aerospace electronics.

However, with the cost of their application dropping, conformal coatings are now used to protect circuit boards that operate in various environments that aren’t always as extreme. One example of products that are now coated is wearable IoT devices. With the dense circuitry of wearable electronics and the challenging conditions they have to operate in, including vibration, moisture, and shock, the conformal coating has become necessary to ensure their uninterrupted performance. This definitive guide will examine PCB conformal coating, what it is, how it’s used, and its potential concerns and benefits to give you the information you need when designing your next electronics project.

Printed Circuit Board Conformal Coating

Before we get into the details of conformal coating, we’ll first lay out some of its basic information. This section will look at what conformal coating is and what it is not. We’ll also look at what conformal coatings are designed to do and what circuit boards will benefit the most from their use.

What is PCB conformal coating?

Conformal coatings are a thin film of protective material that covers or “coats” an assembled circuit board. Coatings are made from non-conductive materials and can cover part or all of a PCB. Conformal coatings will cover areas of metal that normally aren’t covered with solder mask giving those areas additional protection from their operating environment. These areas include exposed traces, leads and other metal on components, and solder joints. Conformal coatings will also help protect a circuit board from stresses, rough treatment, chemicals, and corrosion and protect the components.

Circuit board conformal coating is different from, and should not be confused with, PCB surface finishing, which is part of the fabrication process. A surface finish, such as hot air solder leveling (HASL), will cover and protect the exposed metal of a bare board until its components are soldered on during the assembly process. Once the assembly process is completed, the conformal coating can be applied to provide long-term protection for the PCB.

What will a conformal coat protect a PCB from, and does your board need it?

Conformal coating will primarily protect a circuit board from the following:

Condensation and moisture
Dust and dirt
Fungus
Salt-spray

Conformal coatings will protect the board from mechanical stress from regular device usage or rough handling and help manage stress from changing thermal conditions. They are also resistant to many different chemicals and solvents. Finally, the conformal coating will also protect the circuit board by sealing delicate components and preventing any leakage and potential corrosion.

The question now is whether or not your circuit board will need a conformal coating applied to it. This answer will depend on the circuit board’s operating environment, how durable the product is expected to be, and how sensitive the components and circuitry are on the board. IPC Class 3 products performing mission-critical services are most likely to be conformally coated to ensure their continued operation no matter what their environment.

Applications for PCB Conformal Coating

Now that we’ve outlined what PCB conformal coatings are, let’s take a deeper look into what goes into those coatings. These details will include the different types of conformal coatings available for circuit boards and the differences between conformal coating and circuit board encapsulation. We’ll also look at how conformal coatings are applied to a circuit board and what industry standards provide guidance on using them.

Types of conformal coatings

Conformal coatings are made up of non-conductive dielectric materials, and their different types of resins are used to classify them by type:

Acrylic resin (AR): This material provides basic entry-level protection and is the most affordable coating to use. It is also easily applied and removed and is known for its high dielectric strength. However, it provides only minimal protection from chemicals and solvents and does not hold up well in high temperatures or harsh environments, making it a poor choice for extreme conditions.
Urethane resin (UR): This coating provides excellent protection against chemicals, moisture, and abrasions, but at a cost. Urethane is challenging to apply with long curing times, and its resilience makes it hard to remove. Aerospace applications often use urethane conformal coatings for protection against fuel vapors and other harsh conditions.
Silicone resin (SR): Silicon is resistant to moisture, chemicals, corrosion, and salt spray. Additionally, silicon is very flexible, can tolerate high vibration stresses, and adheres well to most circuit board materials and components. However, silicon is the most difficult conformal coating to remove and requires long soak times in specialized solvents, brush agitation, or ultrasonic baths.
Epoxy resin (ER): This material differs from other more traditional coatings in that it generally isn’t permeable. Epoxies are usually applied as two-part compounds that create a tough protective layer. This strength gives epoxy great humidity, chemical, moisture, and abrasion resistance and works well in harsh environments. The downside of epoxies is that removal of the coating is complex, and they may shrink during the curing process.
Parylene: This coating has high dielectric strength, offers the best resistance to solvents, moisture, and excessive temperatures, and is transparent. Parylene forms at room temperature and will cure very fast but requires specialized vapor phase deposition equipment for its application on the board. This coating is also challenging to remove and must be scraped off.

Conformal coating or encapsulation, what’s the difference?

Both conformal coating and encapsulation provide thermal, moisture, chemical resistance, and electrical insulation, and the difference between them is in how much protection they provide. While the conformal coating is a thin covering over the circuit board, an encapsulation or “potting” surrounds and seals the PCB instead of merely coating it. A casing or “pot” is built around the PCB to encapsulate a circuit board, and the encapsulating fluid is poured in, completely covering the circuit board with a thick protective shell. Here are some of the differences between conformal coating and encapsulation you should be aware of:

Conformal coating	Encapsulation
Thinner coating allows flexibility Usually transparent Minimal PCB weight increase Less expensive Easier application	Thicker material is better protection Darker colors hide proprietary circuits Large PCB weight increase More expensive Additional process steps

While encapsulation has its detractors, such as its expense, weight, and increased process steps, it provides greater protection for those circuit boards subjected to extreme abuse. An excellent example of this is the electronics controlling automobile engines that experience extreme heat and vibration abuse. However, conformal coatings protect most electronics in the environments they are targeted for, and we’ll look next at how they are applied.

How conformal coatings are applied to a circuit board

Conformal coatings are usually between one and five mils thick (25 to 127 microns) to provide the best protection possible while minimizing the amount of circuit board operational heat trapped within the coating. There are several methods for applying a conformal coating, with the following being the most common:

Automated spraying: PCB production runs often employ automated spraying where reciprocating spray heads apply a consistent coating on the boards as they are run through the process on a converter belt.
Selective automated spraying: This process is very similar to automated spraying, except programmable sprayer heads are used in place of reciprocating. With this level of control, the conformal coating can be applied in specific regions on the boards.
Manual spraying: Using spray guns or aerosol cans, technicians can quickly apply conformal coatings to low-volume runs of circuit boards. Manual spraying also is ideal when masking areas of a board to restrict the application of the coating for unique requirements.
Dipping: This is the standard method of applying a conformal coating to circuit boards in high-volume production runs to coat both sides of the board quickly.
Brushing: This is a slow procedure and is usually used during rework or repairs of a circuit board with its conformal coating stripped away for the work.

As you can see, conformal coatings are usually applied in a liquid form. It is important not to apply conformal coatings too thick as it will increase the curing time and potentially lead to problems such as stress shrinkage and cracking. For the best results and maximum protection, conformal coatings should be applied at the thickness stated in their specifications. Not only will this give better results, but it is less wasteful of the coating material. It is better to use multiple thin coatings for those coats that require a thicker application instead of applying it all at once.

Many conformal coatings will need to cure after being applied to the circuit board. Some coatings will cure through the evaporation of their liquid carrier, leaving behind the coating resin, although it may take days to cure fully. Other coatings can accelerate their curing with heat or UV, and in many cases, a combination of methods is used to fully cure the coating around all of the components on the circuit board.

Industry standards that govern the application of conformal coatings

Many protective materials are available for electronics and engineered conformal coatings explicitly designed for printed circuit board applications. As you would expect, there are also different manufacturer and user specifications detailing the use of these products. However, there are specific industry standards that apply directly to the use and application of conformal coatings for circuit boards:

IPC-CC-830C: Replacing the deactivated MIL-I-46058C, 830C is the primary standard for conformal coating of printed circuit boards. This standard contains sections on materials, curing, shelf life, viscosity, chemical, and appearance requirements and can be used to qualify conformal coating products. 830C also contains specifications on moisture, fungus, flammability resistance, and electrical specifications such as dielectric withstanding voltage.
IPC-A-610G: This standard contains conformal coating coverage and thickness requirements as part of its acceptability of electronic assemblies.
IPC J-STD-001H: Within this standard’s requirements for soldered electrical and electronic assemblies is a section detailing the application, materials, thickness, and uniformity of conformal coating and encapsulation.

Designers should investigate other industry standards such as UL94 and UL746. These two standards cover the ability of conformal coatings to self-extinguish flames on a circuit board and the test procedures for materials such as conformal coating used in PCBs. Next, we’ll look in more detail at what PCB designers need to know to design a circuit board that will successfully use a conformal coating.

Conformal Coating and PCB Design: What You Need to Know

Now that we’ve laid out the basics of conformal coatings and examined some of their more intricate details, let’s look at what you need to know when designing a PCB to be coated. This includes setting up the design for conformal coating and communicating the details of what you want to your contract manufacturer. We’ll also examine how conformal coating can be removed from circuit boards, coating application defects, and other potential problems.

Setting up a layout for conformal coating

The first step in designing a circuit board that will be conformally coated is to familiarize yourself with the requirements and the applicable standards. We’ve already listed the most common industry standards, and you may find more specific specifications in the coatings that you decide to use. To ensure compliance with all of the proper requirements, it is best to work ahead with your PCB contract manufacturer to understand the standards fully. Here at VSE, we can advise you on your options and give you recommendations on how best to lay out your board for the conformal coating you are using.

Here are some general PCB layout recommendations to keep in mind as you design a circuit board that will be conformal coated:

It is helpful to place components that shouldn’t be coated along one edge of the board to help simplify the application of the coating.
Tall components may shadow smaller components during the conformal coating application, and components that must be coated should be placed with some extra clearance to these larger parts.
The dense placement of discrete components can also create a challenge for applying conformal coating due to the liquid’s tendency to collect in these areas due to capillary forces. Conformal coatings are more likely to delaminate and crack if applied thicker than their specifications.

Specifying the conformal coating

To communicate the coating requirements to your manufacturer, designers should add the details of the coat and its coverage to the manufacturing assembly drawings. These drawings should include the type of coating material to be used and the areas of the board to be coated. Although most circuit boards are entirely covered with conformal coating, it is essential to detail any non-coated areas on the drawings if so required. These areas could include connectors or interfacing devices, shields, batteries, and heat sinks.

Another detail to include in the manufacturing drawing is the final thickness of the conformal coating on the finished circuit board. Coating thicknesses will vary depending on the chemistry of the coating used, and it is important to specify the correct thickness for the type of coating you are using. For PCB layouts with dense circuitry, it can be hard to find a flat area of the board for measuring coating thickness. In these cases, it is common to measure the coating thicknesses on the board’s process control coupons.

Even though you will have worked out the coating details with your contract manufacturer before it is built, it is still important to include them in the drawings. This step will save you additional effort in the future should your manufacturing plans change or the board goes through re-designs.

Conformal coating removal

When circuit boards have to be reworked or repaired, any applied conformal coating will first have to be removed. Depending on the type of coating used, removing it can be a challenge, and rework technicians will use one or more of these techniques to get the job done:

Solvent: Many coatings will dissolve using different chemicals, but care has to be taken to avoid damaging other nearby components on the board.

Stripping: Coatings such as silicone or other flexible materials can be peeled off the board.

Burning: In many cases, heat application through a soldering iron will burn through the coating while the board is reworked.

Microblasting: Compressed air combined with a soft abrasive is often used to remove epoxy coatings.

Grinding: Technicians often have to grind and scrape the material off the board for harder conformal coatings such as polyurethanes.

Although some coatings can be removed easier than others, it is still an extra operation that will add time and complexity to circuit board rework. Most contract manufacturers will advise waiting until a PCB has gone through all its expected changes during prototyping and is ready for full production before applying conformal coatings.

Conformal coating defects and problems

Conformal coating is an important part of the assembly of printed circuit boards. However, if not applied correctly, the coating can exhibit the following manufacturing defects:

Delamination: Coating will lift off the surface of the circuit board if it loses its adhesion. This effect is usually due to contaminations on the surface, and proper cleaning and tack time between coats will solve the problem.

Bubbles: Air can penetrate the coating through brushing or the spray system. Bubbles can also be formed by trapped solvent vapors coming to the surface.

Dewetting: Like delamination, this is primarily due to contamination making the board’s surface unsuitable for the coating.

Fisheyes: These small circular craters are usually due to contaminants in the spraying system and can be resolved through proper air filtration.

Cracking: If the coating is applied too thick or too high of heat is used for curing, cracking may appear in the coating.

Orange peel: If the sprayer is not set up correctly, differences in how the layers of coatings evaporate while being applied will create a marked texture resembling an orange peel.

To avoid problems with conformal coating, designers need to remember the following:

Some circuit board areas may need to be masked off from being coated to maintain accessibility to connectors and other interfacing parts.
Most conformal coatings are water-resistant but not waterproof. Moisture that is trapped on the board may eventually create problems.
Conformal coatings that have not been fully cured will not provide the full protection intended.
Trying to cover sharp edges of components and parts on the board with conformal coating can be difficult.
Like those associated with a no-clean process, some flux residues can interfere with the curing of some conformal coatings.

Conformal Coating at VSE

As you can see, there is a lot to consider when specifying conformal coating. You need to choose the best type and application methods to ensure your board will have the protection you expect and avoid potential manufacturing problems. Here is where the guidance of an experienced PCB contract manufacturer like VSE can help you make the right choices.

Making the best choice

Choosing the right conformal coating involves understanding your board’s needs and what kind of coatings will provide the protection you seek. For instance, the type of conformal coating you choose will depend partly on your board’s operating temperature. Temperatures greater than 160℃ would suggest using silicone instead of polyurethane coating. In the end, you must choose the conformal coating that is right for your circuit board and not a default choice made for some other product. Another very important consideration is rework, field repairs, and upgrades. Although the coating may offer many advantages, these can be quickly swallowed up by expensive and time-consuming repairs. If your circuit board is expected to change and be upgraded regularly, choosing a coating that will help facilitate these changes is imperative.

Choose VSE as Your Electronics Contract Manufacturing Company

We at VSE understand the concerns surrounding conformal coating application on circuit boards. We can help you make the right coating choice and show you how to prepare your design to best work with the coating. Our technicians work with conformal coating during rework and apply replacement coatings manually. For those boards that require a production application of a conformal coating, we will work with you to choose the best vendor for the job.

If you are looking for a CM that understands all aspects of PCB manufacturing, including the application and management of conformal coating, to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.