Conformal Coating for PCBs: 7 Materials

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Conformal Coating for PCBs: 7 Materials

Conformal coating is a cost-effective way to protect electronic products against water, chemicals, and dust, which could harm the PCB.

 

 

In some cases, you can save yourself a lot of money and anguish by applying waterproof conformal coating on a  PCB rather than going through the substantial effort of creating an IP65 waterproof housing

 

In this article, we will look at the different types of conformal coating and the various application methods. When to use a conformal coating? Which conformal coating method will work best?  What are the pros and cons of each method?

 

We’ll also discuss, conformal coating inspection, conformal coating removal, and electronic coating.

 

Read this and you’ll be a conformal coating expert!

 

1- Waterproof Conformal Coating vs Waterproofing the Housing

 

Making a plastic housing waterproof generally requires a lot of skill and experience. Even then quite a few iterations of testing and mold modifications will be needed to get it right.

 

It generally involves adding a soft material such as TPR between the mating surfaces. The cheapest way to do this is to friction-fit or glue a TPR o-ring in place, but this is a laborious and error-prone process.

 

Better is to combine the soft and hard plastic into a single part, using overmolding. For high quantity products, such as toothbrushes 2K molding is used technology, where a mold…. molds and injection machines.

 

The molds and injection machines needed for this technology tends to be roughly 3 times as expensive as for a single plastic part.

 

For order quantities under say 50K it is much more advantageous to use the “sneaker method” where an ABS part is taken out of one mold and walked over to the next machine where the TPR is injected over it.

 

Best is if the ABS part is still warm so that the second material will bond better to the first.

 

Conformal coating basically bypasses all those additional investments in injection molds, engineering, and endless testing and mold modifications.

 

2- The reasons to use a conformal coating

 

During the electronics product development process comes the embedded electronic design stage, especially the core of electronic products: the PCB.

 

In consumer electronics, the failure of an electronic device such as a mixer simply means a costly RMA. But the reliability is even more important in B2B industrial applications. Many electronic systems are critical to be able to run production or service, while in some cases electronic failures can be life-threatening.

 

So protecting the circuit is important, especially so when it is exposed to an aggressive environment (Solvents, high temperature, humidity/water spills, etc). Waterproof conformal coating can help with all of this.

 

In fields such as automotive, aeronautics, or medical, it is essential to provide PCBs with a suitable conformal coating in order to increase reliability.

 

When applying a conformal coating, there are four factors to be checked and optimized:

 

  • The conformal coating material
  • The conformal coating application method
  • The conformal coating curing and drying method
  • The conformal coating inspection method

 

Your best pick for each of these factors will depend on the production volume and lead times, the fixtures available, the environment, the complexity of the PCB, etc.

 

3- Why masking is important for conformal coating

 

Some parts of the PCBA should not be coated because their functionality will be degraded, so therefore these areas should be masked before applying the coating. The main masked components are:

 

  • The electromechanical and opened components

 

Electromechanical components such as potentiometers, actuators, motors, switches are used to convert an electrical signal into a mechanical process or vice versa. Most of the time the mechanism isn’t hermetic so therefore, the conformal coating will penetrate it, act against the mechanical movement and crack.

 

  • The capacitance and electromagnetic sensitive components

 

Depending on the coating material applied, the capacitance or electromagnetic PCB behavior can be highly impacted. Then it is necessary to apply a masking aver components such as RF filter or electromagnetic shield.

 

  • Passive or active optical components

 

Optical components such as photodiodes, photovoltaic cells, laser, cameras, or any displays need to be protected from the conformal coating in order to avoid any default.

 

  • Testing point and connector PCB areas

 

Areas of PCB can be used in order to inject test signals or monitor the circuit states such as grounding or testing points, connectors or mating sockets. Therefore, the conformal coating will block the signals.

 

  • Mounting surface areas

 

Much of conformal coatings have poor mechanical friction properties. Then, each area designed to receive screws, spacers, or fasteners needs to be mask from the coating.

 

  • Flexible electronic components

 

Most of the conformal coatings are not as flexible as flex circuits. Therefore, if applied on a flexible part, there is a high risk of coating cracking causing a crack path initiation.

 

4- Conformal coating Materials overview

 

Conformal coating can be done with a wide variety of materials. They are selected according to the longevity required and the environment your PCBs will have to endure.

 

Acrylics

 

Acrylic is the most common material used for conformal coating. It has excellent moisture resistance, making it the perfect conformal coating waterproof, which is easy to apply and easily repairable.

 

It is suitable for most applications as long as the PCB will not be exposed to solvent splashes or excessive heat.

 

Main uses Aircraft, automotive, communication equipment, engine management.

 

Thickness required: 25 to 75 µm / 0.001″ to 0.003″.

 

Polyurethane

 

The polyurethane conformal coating is easy to apply and resistant to solvents. However, it is difficult to repair and requires a long curing time.

 

Main uses: Salt spray protection, applications where resistance to solvents is required.

 

Thickness required: 25 to 75 µm / 0.001″ to 0.003″.

 

Silicone

 

The silicone coating has the great advantage of being resistant to high temperatures. However, it is very sensitive to solvents, difficult to apply, and very expensive. In terms of conformal coating removal, it’s almost impossible to remove.

 

Main uses: Equipment subject to high heat and mining equipment.

 

Thickness required: 50 to 200 µm / 0.002″ to 0.008″.

 

Below, the so-called “traditional” coatings, less used but existing.

 

Epoxy

 

The epoxy conformal coating provides good resistance to moisture, chemical attack, and high temperatures. However, it has poor flexibility and conformal coating removal is difficult.

 

Thickness required: 25 to 75 µm / 0.001″ to 0.003″.

 

Paraxylylene

 

The paraxylylene conformal coating has excellent resistance to mold, chemical, dielectric, and high-temperature attacks. However, the process is very expensive and difficult to implement.

 

Thickness required: 12 to 17 µm / 0.0005″ to 0.0007″.

 

Fluoropolymer nano-coating

 

A fluoropolymer Nano-coating is an ultra-thin film comprised of fluorocarbons. This has good resistance to mold and high temperatures. It has excellent dielectric resistance and low process cost.

 

However, it has a low resistance to abrasion and chemicals. In addition, the tooling set up cost is high.

 

Thickness required: 12 to 17 µm / 0.0005″ to 0.0007″.

 

Comparison table of Conformal Coating materials

 

 

5- Conformal coating application methods

 

There are many ways to apply a conformal coating. The choice will depend directly on the desired PCBs volume and the production yield.

 

Brush coating

 

This coating method is only suitable for small production series or touch-up. It is cheap in terms of tooling. However, this method is prone to defects, excessive thickness, or application irregularity and so it can look rather amateurish.

 

Manual or automated spraying coating application

 

This coating method is one of the most cost-effective and is suitable for low to large volume series if automated. Spraying provides a very clean finish when automated but varies from board to board when the operators are humans. Moreover, this method may not be so suitable when your PCB design is rather “3D” in nature, because the masks used in this method are 2D.

 

Masking requirements are more of a shield nature rather than a barrier since there is less penetration. The lack of penetration can be a problem where the coating is desired to penetrate beneath devices.

 

Coating dipping

 

This coating method is suitable for large production series. This technique is very efficient. However, if the PCB-design is not properly optimized for dipping, there is a risk of leakage.

 

Selective coating by machine

 

This coating method is the safest choice for large production series.

 

This technology consists of applying a coating only to localized areas by spraying. The selective coating is a fast and precise method and offers the best protection.

 

Here again, it is necessary that the PCB is well-designed to avoid capillary effects around low profile connectors that may suck in the coating and cause uneven finish or even uncoated area. This means that it is necessary to have an experienced operator to perform the operation.

 

Chemical Vapour Deposition

 

Also known as CVD, this coating method is specific to Parylene and nano conformal coatings. CVD is efficient, more uniform than the other methods, and suitable for medium to large production series. Moreover, the material in vaporous form can penetrate the hardest to reach areas of your PCBA.

 

Comparison table of Conformal Coating application methods

 

 

6- Conformal coating curing and drying methods

 

There are few methods to cure the coating, and it takes from a few seconds to days depending on the type of resin, and thickness of it. Solventless UV curing is most suitable for high quantity production but requires investing in UV curing equipment.

 

Solventless UV conformal coating

 

This curing method is preferred because of its high production speed. In addition, it offers greater processability, superior thermal resistance, and environmental friendliness.

 

However, this method is difficult to apply manually. UV coatings are more difficult to repair and rework.

 

Evaporative curing

 

The coating is appearing by the evaporation of a liquid. However, the operation needs to be repeated two times at least to accumulate enough coating. This is a slow conformal coating curing process.

 

Moisture curing

 

Silicone and Urethane resins get cured by this method. The moisture in the atmosphere cures the resin and forms a polymer. Boards are handled between a few minutes to an hour but take a few days to reach its final properties.

 

Heat curing

 

Heat cure improves wetting and lowers viscosity, and it is used as a secondary cure for moisture and UV cure. The additional heating will speed up the process but may also damage some components.

 

Two-part systems

 

This curing method works for epoxy systems. If the ratio between the two substances is not strictly respected, your PCB will not be protected.

 

Comparison table of Conformal Coating Curing methods

 

 

7- Conformal coating inspection methods

 

Traditionally, for conformal coating inspection, an inspector was manually inspecting the conformal coatings using a lamp.

 

Recently automated optical inspection (AOI) has begun to replace manual inspection.

 

Automated Inspection Systems can be camera- or scanner-based, hence the technology can be matched to the project.

 

8- How to do a Conformal Coating Removal?

 

Sometimes conformal coating removal from a PCB to allow for repair or rework process is necessary. The conformal coating removal is directly dependent on the material and the size of the area.

 

Solvent Removal

 

Acrylic is susceptible to solvents and, hence they are removed easily. Also, Epoxies, Urethanes, and Silicones are less affected by the solvent but can be removed with it.

 

Peeling

 

Only Silicone coatings can be peeled off from the board. However, it is not an easy process.

 

Thermal/Burn‐through

 

It is a common technique for removing most types of conformal coating. However, there is a risk of burning the components if this removal method is not correctly processed.

 

Micro blasting

 

It uses a concentrated mix of soft abrasive and compressed air to remove the coating. It is most commonly used when removing Parylene and Epoxy coatings.

 

Grinding/Scraping

 

The conformal coating is removed by scraping from the circuit board. It is used to remove harder coatings like Parylene, Epoxy, and Polyurethane. This method isn’t preferred as damage can occur to the PCB.

 

10- Conformal Coating Standards

 

IPC CC-830 revision B is a conformal coating qualification standard mostly used by board fabricators, OEM design engineers, and coatings suppliers.

 

UL746E is the Underwriters Laboratories (UL) tests, and it is the certification that is also commonly required for consumer goods.

 

 

11- Potting, the absolute PCB protection?

 

As we have seen, the different materials using conformal coatings are never more than 0.2mm thick. Even if they can be a water semi-protection, they do not make the PCB waterproof and do not protect it from a mechanical-sensitive environment.

 

In order to do so, the only solution will be the PCB potting, often based on thermoset, epoxy resin, or silicone. Potting, more than provide a perfect waterproofing, gives the PCB excellent mechanical based properties such as resistance to vibration and shock.

 

However, the PCB potting is more expensive, bigger, and sometimes much heavier than a conformal coating.

 

 

To learn more about PCBs, read also:

Design for reliability of electronics – 5 practical steps

Key to quality electronic product design

ENIG vs OSP vs HASL: Which PCB finish for manufacturing?

Elias Houmadi

As far back as I can remember, I have always been fascinated by the complex mechanisms, the computers ingeniosity, or even the fact that a million of well-designed parts could have sent men to the moon! This is why once graduated engineer, I wanted to devote myself immediately to this vocation. Today, working for Titoma, I try to bring my engineering and passionate point of view to the articles we write.

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