Conformal Coating: Protecting Electronics in Harsh Environments

Understanding the Basics of Conformal Coating

At its most basic level, a conformal coating is simply a very thin layer of polymers. These coatings get their name from their ability to conform to the shape of whatever they're protecting [1]. The typical conformal coat is anywhere from 25 to 250 micrometers in width. 

It repels water and other damaging materials, yet the unique polymers in conformal coats are still very breathable. This creates a helpful coating that protects circuitry without allowing temperature fluctuations to trap moisture underneath the coating. 

Conformal coats can be applied in a variety of ways such as spraying, brushing, or dipping the circuit board, and they are also available in a variety of types of materials.

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Types of Conformal Coatings

There are several different types of conformal coats [1][2][3][4] for engineers and electricians to choose from:

• Acrylic: Acrylic coatings are a popular choice for hobbyists with a low volume of conformal coats to apply. They're affordable, easy to apply, and easy to remove. Acrylic has a decent dielectric strength, a moderate level of thermal conductivity, and excellent moisture resistance. However, an acrylic conformal coating is not good at protecting from solvents.
• Silicone: Silicone has good dielectric strength and a slightly higher thermal conductivity than acrylic coatings. It performs well in a variety of extreme temperatures and rates highly for chemical resistance, moisture resistance, and salt resistance. The main downside of a silicone conformal coating is that it is very vulnerable to abrasion and has poor thermal resistance.
• Urethane: Also called polyurethane conformal coatings, these coatings are known for their toughness and quality adhesion. Urethane has a slightly higher dielectric strength than acrylic and silicone, and it has excellent thermal conduction and moisture resistance. Unfortunately, urethane is a little harder to work with. Urethane coatings require a lengthy curing process to affix the coating to the circuitry properly, and they're very hard to remove.
• Epoxy: Epoxy has excellent dielectric strength and high moisture and abrasion resistance. It's especially popular in industrial environments because it can withstand even harsh solvents and has good thermal conductivity. Like urethane, epoxy coatings are a curable coating that requires a very specific drying method, so they don't work well for hobbyists. Furthermore, they are inflexible and can cause issues if the circuit board moves around.
• Parylene: Parylene is a good choice for certain, highly-specialized situations. It has high dielectric strength, good thermal conductivity, and very low thermal expansion. Unfortunately, this specialized coating also requires a somewhat unusual application method with high-end equipment, so it's a somewhat rare material for conformal coats.

Pros and Cons of Conformal Coat Materials

How to Apply Conformal Coating

There are a variety of methods for applying conformal coats, and processes will change depending on which materials you use. However, regardless of the method you select, you'll need to follow the same basic steps every time.

Step-by-Step Guide to Application

• Carefully clean your circuit board with isopropyl alcohol or a degreasing agent and allow it to dry.
• Read the manufacturer's instructions for your coating and dilute it or mix it as necessary. 
• Prepare your application tools and work area.
• Mask all areas of the board that you do not wish to get a coating.
• Apply a thin, even layer of the coating.
• Let the layer set for a few moments and then apply another layer or two if desired.
• Cure your coating with UV light or a heat cure if required by the manufacturer.
• Allow the coating to dry.

What's the Difference Between Application Styles?

• Brush: Using a brush to apply the coating is often a popular choice with hobbyists who want a cheap and straightforward way of applying a conformal coat. Brushes are unlikely to leave gaps, so they provide excellent protection. However, it can be hard to get thin layers without drips when you're using a brush, and this can make it harder to remove the coating later if necessary.
• Aerosol Coating: A spray-on coating with an aerosol gun is often the most effective way for getting a thin coating by hand. It performs well on high-volume projects, but the lengthy setup means it can be time-consuming for small projects. Aerosolized coating results may have small spots without coverage if the application wasn't thorough enough.
• Dip Coating: Here, you lower the board into a vat of solution to provide even coverage. They ensure that every part of your board is very thoroughly protected from the elements. However, this can get messy if you don't properly mask the board before dipping it. It could potentially cover areas of the board you still need to work on.
• Machine Coating: Machine coatings can be more efficient and accurate for large-scale production. These coatings consist of a machine programmed to selectively apply a coat on specific parts of a circuit board. They provide excellent coverage for the most part, but there can be some small issues like a lack of coverage around low-profile connectors. The other downside of machine coatings is that they require costly equipment, so they're usually only popular in commercial settings.[5]

How to Find the Right Method for Your Situation?

When choosing your application method, you'll need to carefully consider your needs and your technical abilities. Here are some things to think about before selecting a method:

1. How many items do you plan on coating in one batch?
2. Do you have plans to coat more materials in the future?
3. What coating material are you going to use?
4. Do you need to leave part of the circuit board uncoated?
5. What's your budget for equipment like?
6. Are you patient or do you prefer a faster application?
7. Do you have an area large enough to set up an application environment?

Advancements in Conformal Coat Technology

The conformal coat field has gone through a lot of changes lately [6][8]. As conformal coats become more popular in everyday industries, there have been some shifts in how they're designed and structured. Here are some of the most exciting changes in recent times.

Nanotechnology

Nanotechnology focuses on manipulating tiny particles of matter, so it has some interesting applications for conformal coats. Nano-conformal films are typically made of materials like aluminum silicate nanoparticles, which provides them with additional strength and durability. Options like NanoFlowX have very fast curing and application compared to options like perylene, and they also offer these benefits:

• Extremely hydrophobic and water-resistant
• More robust than many other coating materials
• Repels oil and other highly viscous materials
• More lightweight than other materials [7]

Bio-Based Coatings

The interest in environmentally friendly engineering has also led to some novel new coating materials. In 2021, Electrolube launched their exciting new line of UV Cure Coating Xtra with bio-based material. This coating uses organic content and is solvent-free. Bio-based coatings have a lot of potential perks for engineers including:

• Less potentially dangerous solvents to handle during application
• More options for renewable production
• Fewer toxic byproducts created during manufacturing
• Unusual formulas that may be especially fast to dry

Recommended reading: 7 Trends Driving the Future of Consumer Electronics Manufacturing

Conformal Coatings in Real-World Engineering Applications

This type of protective coating can be useful in all sorts of situations. Any time someone needs to ensure their circuit board is waterproof, a conformal coat is an excellent choice. It can also be helpful for protecting circuits from other risk factors like chemical solvents, oil, corrosive chemicals, electric shocks, and rough handling. To better understand the wide array of applications for protective coatings, take a look at some of the most common engineering fields that use conformal coats.

Automotive Applications

The growing number of vehicles with complex electrical systems has resulted in a huge need for automotive conformal coats. These coatings ensure a printed circuit board (PCB) can keep functioning for years even when exposed to wet weather. They also provide protection from abrasion and vibration as the movements of the car shake the PCBs around. Choosing the right coating for automotive electronics depends on where the PCB will be placed. Silicone is a popular choice for ones exposed to weather since it's very resistant to the salt spray that cars encounter. However, if the main concern is corrosive materials or abrasion, manufacturers may prefer a durable urethane coating instead. Spray application is common for automotive applications since it's so fast and efficient.

Medical Applications

A variety of different types of medical electronic devices use protective coatings. Electronic equipment used in operating rooms may be coated so they can withstand exposure to liquids and cleaners. Coatings are even more important for implants that go inside the body. If a patient needs a pacemaker for their heart or a cochlear implant to help them hear, they'll need a protective coating to ensure their device can withstand heat and moisture. Conformal coats for medical applications need special coatings that won't be toxic when they're inside the human body. Right now, urethane and parylene coatings are the only options approved by the FDA for medical implants. Sturdy, thorough dip coating applications are a popular choice for these types of electronics.

Industrial Electronic Applications

The electronics used to manage machinery in factories often require a conformal coat to help them avoid damage. A conformal coat is also non-conductive, so it can help prevent shocks from damaging this pricey equipment. Many of these electronics use urethane or parylene coatings because they're the best materials for resisting solvents. Since they're often manufactured in an industrial environment themselves, these coatings are frequently applied with selective machine coating technology. 

Personal Device Applications

Personal device use is one of the newest applications for conformal coats. The growing interest in wearable fitness trackers and smartwatches has resulted in a huge need for conformal coats. These coatings ensure that an accidental spill or a bit of sweating won't permanently damage a pricey device. They're especially important if the device is meant to be a waterproof model that users can wear while swimming or showering. One of the biggest considerations when designing these conformal coats is finding a material with a stable coefficient of thermal expansion. For many manufacturers, silicone can be an ideal option for conformal coats on personal devices.

Testing and Inspection of Conformal Coatings

An expert inspecting PCB coating

Whether you're mass-producing objects in an industrial setting or just creating a few conformal coats for a personal project, testing and inspection are an important part of the process. They help to catch potential issues, so you can be confident that your final product truly protects your circuitry.

Potential Problems to Be Aware Of

Conformal coats can be an excellent way of protecting a circuit board, but they only work properly if they're applied correctly. Even a few minor errors during application can lead to issues such as:

• Delamination: The coating fails to adhere to the underlying surface. This can occur due to improper drying time or a failure to clean the board before application. 
• Dewetting: The coating retracts from part of the circuit board, leaving a blank space behind. This is usually caused by oil or other residue on the circuit board.
• Air bubbles: Small, circular bumps will appear on the surface of the coating. This can be caused by improper surface cleaning or hasty brushing during application.
• Orange peel: The coating has an uneven, bumpy surface. It can be caused by low air pressure in a spray gun or a poorly mixed thinner in your coating material.
• Cracking: Thin lines appear on the surface of your circuit board. This can be caused by overly thick film, improper curing, or a lack of proper dry times.[10]

Methods for Testing Circuit Board Coatings

To catch issues before they lead to circuit board damage, it's important to inspect each circuit board after it's coated. Some manufacturers rely on automated inspection systems which can be a very consistent and accurate way of checking boards. Others prefer manual inspections that may be less consistent but are more likely to catch minute errors. Whichever method you prefer, it's essential to set up a routine protocol for examining new coatings. Following each of these steps can help to catch many common issues.

Visual Inspection

This simple step involves looking over a coating. Ideally, it should be smooth and even without any noticeable bumps or lines. Pay close attention to any changes in color or gaps that could indicate adhesion issues. During a visual inspection, you may want to lightly touch the coating to see if it moves or feels uneven.

Thickness Measurement

An easy and affordable for testing thickness is to use a micrometer. All you need to do is subtract the coating thickness from the uncoated measurement to find the overall thickness of the coating. In cases where large volumes of circuit boards are being produced, it might be worthwhile to invest in an ultrasonic thickness gauge or eddy current probes. These send waves through the circuit board and then calculate how thick it is.

Adhesion Testing

Adhesion testing usually involves applying a conformal coat to a discardable piece of circuit board at the same time as you coat your actual circuit board. After letting it dry, you then cut a line through the coat and apply tape. When you pull up the tape, the coating should remain behind. If it sticks to the tape instead, you know there is some issue in your process that's impacting adhesion. For a less destructive option, some manufacturers like to use special equipment that can affix to a circuit board coating and test whether or not the coating can be manually peeled off.[11]

Electrical Testing

Electrical testing ensures that the circuitry was not damaged in any way during the coating process. It often consists of using a simple multimeter tool to read the electrical currents that pass through a circuit board. Some engineers may also have a test device they can insert the circuit board into to ensure it performs correctly.

Removal and Repair of Conformal Coats

Conformal coats aren't always meant to be a permanent solution. There are many cases where it might be necessary to remove a previously applied coating. If you need to rework your circuit board or make adjustments, you'll have to take off the coating to access the underlying components. You might also need to remove a coating if the previous coating was improperly applied or if you want to try out a different material or application method. In some cases, it's not necessary to remove the entire coating. Sometimes, conformal coats end up with just a small section of damage that needs to be repaired. 

Dealing with repairs or removal can be tricky. Some types of conformal coats can be very challenging to remove, and there's always a risk you can damage the underlying components. Furthermore, it's often important to maintain an even, smooth surface, so it can take some time to get a seamless effect after a repair. Fortunately, there are several very effective options available to engineers who need to remove or repair a conformal coating.

Solvent Removal

Solvent removal doesn't work for all types of conformal coats, but if you have a conformal coat made of acrylic, silicone, or urethane, you're in luck. Solvent removals are quite simple. You just apply a solvent like acetone, toluene, or xylene, and then gently wipe away the conformal coat once it starts to break down. Just keep in mind that solvents are often very harsh and toxic, so you need proper ventilation and personal protection equipment.

Peeling Removal

For silicone conformal coats, you can often manually remove the coating. After cutting a fine line in the coating, you can carefully peel it off of the circuit board. Just keep in mind that this can damage components on the board, so this method isn't advisable for particularly delicate circuitry. 

Thermal Removal

All types of conformal coats besides thicker silicone coatings are somewhat heat sensitive. Extreme heat can help to loosen their grip on the circuit board, so you can remove the conformal coat with a manual or solvent method. Check your circuit board construction before trying thermal removal though since it can harm the board substrate or components.

Abrasion Removal

This method of removing conformal coats takes more work, but it can succeed whether other methods fail. There are multiple options such as grinding, scraping, or microblasting. Some use simple tools like sandpaper while others may require specialized microblasting equipment. All of these options manually grind away at the conformal coat bit by bit to remove it from the circuit board. They typically work best when used alongside solvent or thermal removal methods.[13]

Brush Touch-Ups

If your conformal coat has a small gap in it, a brush touch-up can be quite effective. This consists of dipping a brush in your coating and applying it to the desired spot. Some manufacturers even create touch-up pens that combine the brush and coating into one convenient, small package. Before any touch-up, make sure to clean and degrease the conformal coat very thoroughly. This helps to ensure the repair adheres to the surface properly.

Selective Spray Coating Touch-Ups

For hard-to-reach areas, a spray coating can be an effective way of touching up a scratch or gap in your conformal coating. The benefit of this method is that it provides very fine layers. You're less likely to have to deal with a thick border around the patch, and it can blend with the rest of the coating more effectively. To prepare for a selective spray touch-up, make sure to clean the circuit board and mask areas you don't want to get sprayed.

Conclusion

Ultimately, conformal coats play an important role in designing electronics. They help to protect electronics from corrosion, moisture, and other issues, so they can expand a circuit board's lifespan even more than other protective methods can. With options ranging from affordable acrylic to durable urethane, engineers have a lot of exciting options to choose from. As long as you pay close attention to application, inspection, and testing, it's easy to ensure you end up with reliable, effective conformal coats.

The conformal coat field has made a lot of exciting advancements lately. New materials have made conformal coats more versatile than ever while new applications have helped to advance the medical and electronic industries. If you're interested in engineering, it's worthwhile to keep up with all these advancements. By learning about the fascinating new features of conformal coats, you can find ways to improve your own projects.

Frequently Asked Questions

If you're looking for information on conformal coats, here are a few essential things you should know.

1. What material is used for conformal coating?

Professional conformal coats are usually made out of acrylic, silicone, epoxy, urethane, or parylene. There are also some experimental coating options made from cutting-edge materials like nanotechnology.

2. How long does a conformal coat take to dry or cure?

Depending on your material, it can take anywhere from a few minutes to 72 hours. Keep in mind that many coats, like acrylic and urethane, are dry to the touch within minutes but won't be fully cured for hours. Some materials will cure faster if you expose the coat to heat, moisture, or UV light.[12]

3. What are the disadvantages of conformal coating?

While the water resistance and other protective features of conformal coats make them quite useful, they have downsides too. Using the wrong material for the wrong application can damage your electrical components. These coats also make it harder to alter existing circuit boards. 

4. What's the best type of conformal coating?

This greatly depends on your situation. Acrylic conformal coats are often a well-rounded, versatile choice that works great for a broad range of situations. Meanwhile, epoxy and urethane coatings are popular for industrial settings and silicone is ideal for circuit boards that will be in extreme temperatures. Parylene is less common but is the best choice for certain specialized devices like medical equipment.

5. Can you solder after conformal coating?

If you want to solder after you coat your circuit board, you can use a soldering iron to burn through the conformal coat. However, this can be messy and unsightly, so many engineers prefer to solder before coating.

References

[1] Altium. Conformal Coating Types. Link

[2] Chemtronics. Ultimate guide to conformal coating. Link

[3] Parylene Conformal Coating. Best Coating For Dielectric Strength. Link

[4] Mgchemicals. How to use conformal coating. Link

[5] Electrocube. How do I apply conformal coating?. Link

[6] Verifiedmarketreports. Top Conformal Coating Trends. Link

[7] Paryleneconformalcoating. Nano coating vs conformal coating. Link

[8] Electrocube. Current and future trends in conformal coatings. Links

[9] Proto-electronics. Wearable PCB design guidelines. Link

[10] Techspray. Identifying and curing top 7 conformal coating defects. Link

[11] Defelsko. Test methods for coating adhesion. Link

[12] Conformalcoating. Conformal coating curing and drying. Link

[13] Techspray. Conformal coating identification and removal. Link