Three key points you should know about polyurethane conformal coatings when using them for protecting electronic circuit boards


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A polyurethane (urethane for short and designated UR by IPC) conformal coating is part of the organic family of coating materials that also includes the acrylic and epoxy coatings.

Here are three key facts to consider when examining polyurethane conformal coatings:

  1. Most conformal coatings provide good humidity and moisture protection although some are slightly better in performance than others. UR type coatings are just as good on average as acrylic materials.
  2. A polyurethane coating has traditionally been used to protect electronic circuit boards against chemical attack due to their excellent chemical resistance. This protection allows electronic circuit boards to survive in highly aggressive environments and atmospheres such as the aerospace, military and industrial sectors. However, it does make repair a little more difficult as chemical resistance to a coating means more difficult to remove.
  3. Times are changing and whereas acrylic conformal coatings used to dominate 70-80% of the market, there is a shift in emphasis towards alternative materials due to higher specifications for protecting electronics. Many new conformal coatings (UV cure, two part thin film coatings) now comprise of urethane resin bases and are becoming more popular in high volume sectors such as automotive electronics. This is because the urethane resin lends itself to this type of technology more easily than the acrylic based resins.

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What is a fluoropolymer nano coating and how can it protect my circuit board?


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A fluoropolymer Nano-coating is an ultra-thin film comprised of fluorocarbons and characterised by carbon-fluorine bonds.

Chemically inert, fluorocarbons are not susceptible to Van der Waals force. This means that films formed using these materials are non-stick (hydrophobic and water repellent) and friction reducing.

Also, due to the fluorine bonds, these Nano-coatings demonstrate a high level of chemical resistance to acids, bases and most solvents.

This makes them interesting materials for protecting electronic circuits. 


What properties of the Nano-coatings areused to protect circuit boards?

Fluoropolymer coatings have very specialised properties.

However, for electronics the main properties that are useful include:

  • Being highly hydrophobic (water repellent)
  • Having a high moisture barrier
  • Having a high chemical resistance
  • Having high dielectric properties
  • Providing high corrosion resistance
  • Providing good abrasion / wear resistance

These properties are excellent for protecting circuit board assemblies in harsh environments.


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What is a hydrophobic acrylic conformal coating?


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There are several reasons for using a conformal coating or Parylene to protect a printed circuit board (PCB).

They include:

  • High insulation protection
  • High moisture and humidity protection
  • Chemical and temperature resistance
  • Ruggedisation
  • Improve dielectric properties
  • Barrier protection against particulates

However, one property that most liquid conformal coatings do not possess that is extremely useful to have is being water repellent or hydrophobic.

A coating that is hydrophobic repels water from the surface by changing the surface energy of the coating.

This makes it energetically unfavourable for the water to wet the surface. Hence, the water “balls up” and rolls off the surface.

This occurs in the fluorinated nano-coatings that are hydrophobic and is an extremely useful property when protecting a circuit board where water may be present.

However, typical conformal coatings are not hydrophobic.

Materials like acrylics and polyurethane coatings are excellent against moisture but not so good as water repellent materials. They do not have a low surface energy.

Conformal coatings like acrylics and urethanes do not have water repellent properties (left). A hydrophobic conformal coating repels the water and does not allow it to wet the surface (right).
Conformal coatings like acrylics and urethanes do not have water repellent properties (left). A hydrophobic conformal coating repels the water and does not allow it to wet the surface (right).

A hydrophobic acrylic conformal coating – the best of both coatings

Now, there exists a hybrid acrylic coating that has both the properties of an acrylic conformal coating at the same thickness plus the benefits of a hydrophobic nano-coating.

These benefits include:

  • Hydrophobic (water repellent) surface
  • High Insulation protection
  • High moisture and humidity protection
  • Ruggedisation
  • Improved dielectric properties
  • Barrier protection against particulates

This new range of conformal coatings can now offer the best properties of both without any cost penalty.

The water wets the surface (left) on a normal acrylic conformal coating. A hydrophobic acrylic conformal coating, with a low surface energy, makes water de-wet from the surface (right) and still has all the benefits of the acrylic conformal coating.
The water wets the surface (left) on a normal acrylic conformal coating. A hydrophobic acrylic conformal coating, with a low surface energy, makes water de-wet from the surface (right) and still has all the benefits of the acrylic conformal coating.

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The ABCs of ultra-thin fluoropolymer coatings for electronic circuit boards


 

Nano coatings are no mask conformal coatings with great water repellent properties

What is a fluoropolymer coating?

A fluoropolymer coating is typically comprised of fluorocarbons and characterised by carbon-fluorine bonds.

They have many interesting properties and especially for printed circuit boards.

However the three key properties for electronics are that the coatings are:

  • Hydrophobic
  • Chemically resistant
  • No masking required

These properties can be key to protecting the electronics and providing a highly cost effective production process.

Hydrophobic coating

Fluorocarbons are not susceptible to Van der Waals force.

This gives the coatings their signature characteristics. That is they are non-stick, hydrophobic and friction reducing.

Therefore, water does not like to wet the surface of the circuit board and this gives the circuit excellent protection.

Chemically Resistant

These fluorinated coatings are chemically inert.

Owing to the fluorine bonds, fluoropolymer coatings demonstrate a high level of durability as well as resistance to acids, bases and most solvents.

This gives the circuit board a high degree of protection from chemical attack.

No masking required

Finally, what is really interesting is that these properties are exhibited at ultra-thin film thicknesses.

Typically a dry film can be 1-2um or even less.

This means that masking generally is not required for circuit boards before application.  Therefore, you can dip the whole product into the liquid and there is no issue with electrical contact.

This can lead to significant cost savings in production.


What other properties do the fluoropolymer coatings have that may be relevant in electronics?

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As already mentioned these hydrophobic coatings have very specialised properties.

They can include:

  • Being highly hydrophobic (water repellent)
  • Having a high moisture barrier
  • Requiring no masking before application
  • Being highly oleophobic (oil repellent)
  • Having a high chemical resistance
  • Having a high lubricity
  • Having high dielectric properties
  • Providing high corrosion resistance
  • Providing good abrasion / wear resistance

Note, not all fluoropolymer coatings have all of the above properties. But, some coatings can in fact have almost all of the properties.

The fluoropolymer coatings are extremely flexible coatings and becoming more prolifically used throughout engineering.


What sectors of industry are fluoropolymer coatings being used in protecting electronics?

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Fluorinated coatings are used to protect electronics in almost all industrial sectors.

They include:

  • Aviation
  • Aerospace
  • Defence
  • Automotive
  • Industrial
  • Oil & Gas
  • LEDs
  • Medical
  • Optics
  • Telecommunications
  • White goods / Commercial

This list is limited and there are a lot more areas that they are used.


What are the major differences between a fluoropolymer coating and a conformal coating for protecting an electronic printed circuit board or assembly?

There are several key differences between a conformal coating and a fluoropolymer coating.

They include:

  • Hydrophobic Properties – A fluoropolymer coating is generally hydrophobic in nature. It repels water when the water is on the surface of the coating.
  • Extremely thin coating – The fluoropolymer coating is normally applied a lot thinner than a typical liquid conformal coating. This is due to its superior performance when repels liquids
  • No masking – Due to the extremely thin fluoropolymer coating applied (<1-2um), the components that normally require protecting (connectors, switches etc) from the insulating liquid conformal coating may not need to be masked for the fluoropolymer. The circuit board can be completely submerged in the liquid with no masking applied without fear of damaging the connections.
  • Simple process – No masking means an extremely fast application process
  • Fast drying – due to the thin nature of the fluoropolymer coating and the solvents normally used the coating dries extremely quickly.

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What are the alternative materials to liquid conformal coatings?


There are several alternative coatings available to the traditional conformal coating materials.

These alternative coatings include:

  • Parylene and other Chemical Vapour Deposition (CVD) films
  • Fluorinated ultra-thin and thin film coatings
  • Molecular Vapour Deposition (MVD) coatings
  • Atomic Layer Deposition (ALD) coatings

They can provide extremely high protection to circuit boards if used correctly for the right product.

There are several new and old alternative coatings available to the traditional conformal coating materials. They include Parylene, fluorinated Nano-coatings, Molecular Vapour Deposition (MVD) and Atomic Layer Deposition (ALD) thin films.
There are several new and old alternative coatings available to the traditional conformal coating materials. They include Parylene, fluorinated Nano-coatings, Molecular Vapour Deposition (MVD) and Atomic Layer Deposition (ALD) thin films.

Parylene (XY) coatings

Parylene is the trade name for a variety of chemical vapor deposited poly(p-xylylene) polymers used as moisture and dielectric barriers.

Parylene is a conformal coating that is deposited as a gas in a vacuum chamber.

It is a dry process compared to the standard “wet” liquid conformal coatings.

Fluoropolymer (FC) coatings

Surface Modifiers are ultra thin coatings that are applied at less than a few microns in thickness.

Liquid conformal coatings are applied in the range of 25-75um so they are considerably thicker in nature.

There are several variations in ultra thin conformal coatings out in the market now but two of the most popular types are liquid materials and partial vacuum deposition.

Atomic layer deposition (ALD)

ALD belongs to the family of chemical vapor deposition methods (CVD).

It is a deposition process at a Nano-scale level within a vacuum chamber.

The deposition process forms ultra-thin films (atomic layers) with extremely reliable film thickness control.

This provides for highly conformal and dense films at extremely thin layers (1-100nm).

Molecular vapour deposition (MVD)

MVD belongs to both the families of chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods.

Unlike traditional CVD and ALD flow systems the MVD reaction takes place in a chamber under static pressure resulting in extremely low chemical use.

The MVD process produces highly conformal thin film coatings, typically less than 100nm in thickness.

The coating provides excellent barrier properties and surface energy control.


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