Requirements for setting up a conformal coating facility


 

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The set up of a conformal coating production line regardless of the application method has many similar characteristics.

Any coating facility will need the basic requirements put in place that would be standard for any piece of electronic manufacturing process.

These include ESD systems, facilities for the machines, the environmental requirements and the normal Health & Safety (HSE) considerations.

Also, the conformal coating production line, whether it is an operator manually brush coating printed circuit boards (PCBs) or an inline robotic spray coating process is typically made up of several stages.

These stages are shown below:

 

Not all the stages are mandatory or may be required.  However, each should be considered on an individual basis.


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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 are conformal coating masking boots and why do they save you money?


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Reusable conformal coating masking boots are a highly cost effective alternative masking material to masking tape.

Many components on printed circuit boards must remain uncoated when applying conformal coating. The purpose of masking is to prevent migration of the conformal coatings into components and keep out areas.


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The use of masking tapes, dots and liquid latex can be an effective process in protecting components from ingress of conformal coating. However, the application of the masking can be labour intensive, especially in higher volume applications.

Custom made masking boots can offer a labour saving alternative in both the masking and de-masking stages of the coating process.These completely reusable masking boots are applied over the components such as connectors,

These completely reusable masking boots are applied over the components such as connectors, plugs and sockets that require protection from the conformal coating applied. They can be simple boots that fit over the top of a connector or a more sophisticated design such as the examples below.

They provide reliable protection for many different types of components for all the conformal coating application techniques that include batch and selective robotic spraying (as an alternative to difficult technical programming), vertical and horizontal dipping and vapour deposition of coatings such as Parylene.


Three reasons to switch from tape to reusable masking boots for conformal coating masking

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These simple reasons can save you up to 80% of your costs compared to masking tape:

1. The masking application time is reduced. Masking boots can be 4-5 times quicker than masking tape.
2. The de-masking (removal) time is reduced. Again it is much quicker to remove boots compared to masking tape.
3. It is much less likely that the masking boot will leak and the component requires rework. Masking boots don’t leak and tapes can.

This means you can save a lot of money very quickly when switching to conformal coating masking boots compared to masking tapes.


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What are the different methods available for cleaning electronic circuit boards?


 

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The topic of cleaning printed circuit board assemblies (PCBAs) effectively before application of conformal coating can be daunting. This is because the process of cleaning circuit boards can be difficult especially with so many variables to consider.

When considering cleaning the circuit you need to assess many factors including:

  1. The board and component compatibility with the cleaning method
  2. The ability of the cleaning method to remove the contamination effectively from the circuit
  3. Any residues the cleaning process may leave behind that may be harmful to the circuit in the long term.
  4. The reasons for cleaning the circuit (e.g. contamination removal, adhesion promotion etc.)?

After considering these factors you can compare with the processes available.


The main methods of cleaning printed circuit boards

The main methods of cleaning used in everyday electronics processing before conformal coating application can include:

  • Aqueous washing
  • Semi-aqueous washing
  • Solvent & chemical washing
  • Plasma surface cleaning

These processes can be mixed, the method can be varied but the fundamental concepts still apply.

However, whatever method you choose you still have to consider that the key to success in cleaning circuit boards is similar to the success made with conformal coating.

You need to match the cleaning process, the cleaning materials and the circuit board together.

If you do this then this will give you the best results for cleaning the circuit board assembly.


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The science behind fluoropolymer coatings for protecting electronic circuit boards


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Nano-coatings are being used regularly to protect printed circuit board assemblies.

This is because the material they are made of fluorinated polymers and these materials have very specialized properties that produce very different results to traditional conformal coatings.

So, what are the unique properties of a Nano-coating?

To understand the properties of a Nano-coating you have to understand what a fluoropolymer coating is made of.

Typically, the coating itself is comprised of fluorocarbons and characterised by carbon-fluorine bonds.

Since the coating is made of fluorocarbons then the film surface is not susceptible to Van der Waals forces (interfacial electrostatic bonds).

Therefore, the surface energy of the fluoropolymer coating is extremely low and hydrophobic (water repellent). It acts like Teflon on a frying pan.

This non-wetting of water on the circuit board is one of the many key properties making them so popular.

What other properties do Nano-coatings have that may help protect circuit boards?

For electronic circuit board assemblies there are several key properties that are being used.

These include being:

  • Hydrophobic: Being highly water repellent
  • Ultra-thin: Protection whilst being extremely thin
  • High moisture barrier: Low water vapor transmission rate provides excellent corrosion resistance
  • Chemically resistant: Having a high chemical resistance helps protect the circuits from chemical attack.
  • Good dielectrics: fluoropolymer coatings have high dielectric properties

However, to really understand the benefits you have to look at the key difference compared to a conventional conformal coating.

That is that no masking is required when applying the Nano-coating.

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The key reason for using a Nano-coating in protecting electronics is that no masking is required whilst applying the coating to the printed circuit board.

In the image above the connector is coated completely in the Nano-coating. It was just dipped in the coating. The image shows the water is being repelled from the connector. However, electrically the connector works perfectly fine.

So, why is there no need to mask when using a Nano-coating?

First, consider a normal conformal coating is applied at typical thicknesses of 25um or more.

Also, the conformal coating is a high insulation material.

Therefore, at this thickness the conformal coating would electrically insulate components like connectors and it must not be applied to any part that needs electrical conductivity.

However, this is not a problem for a Nano-coating.

Since the Nano-coating can be applied at ultra-thin thicknesses (1-2um in thickness or less) without any protection performance reduction, then the extremely soft coating is easily removed or scratched away when the connectors are joined and the electrical circuit is easily made.

This key parameter of not requiring masking during Nano-coating application combined with the hydrophobic nature of the coating material makes the Nano-coatings highly effective in protecting electronic circuit boards at a very low cost per unit.


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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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Do you need UL qualification for your conformal coating?


UL are the Underwriters Laboratories and are a global safety certification body for consumer electronics.

They are a 3rd Party test service. There is no self-certification available.

A qualification to UL can be used as a sign of quality control and testing independently of the coatings protective ability.


When do you need UL?

Sometimes it may be required that the conformal coating needs UL approval to be applied to a circuit board. It will depend on the market the electronics goes into.

UL approved coatings can be used on electronics in many diverse sectors such as the domestic home, industrial controls, telecommunications and aerospace.

In fact, in nearly all areas conformal coatings tend to be used for protection.

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UL approved coatings can be requested in nearly all areas conformal coatings tend to be used for protection for electronics including domestic home, industrial controls, telecommunications and aerospace.


What standards are used from UL for conformal coatings?

For conformal coating, two parts of the UL standards are important.

These are:

  • UL 94: Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing.
  • UL 746E: Standard Polymeric Materials: Industrial Laminates, Filament Wound Tubing, Vulcanized Fiber and Materials Used in Printed-Wiring Boards

The two standards together control the quality of the conformal coating. They are discussed more in detail below.


UL 94

It is a plastic material flammability standard.

The standard classifies plastics according to how they burn in various orientations and thicknesses.

UL 94 measures conformal coating material’s tendency either to extinguish or to spread the flame once the test specimen has been ignited.

Classifications

For conformal coatings the test coupons are normally tested horizontally.

Tests are normally conducted on coupons of the minimum approved thickness with different types of laminate. The conformal coating thickness range also is normally specified.

The ratings are as follows:

  • V-2 burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed.
  • V-1: burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.
  • V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.

Most conformal coatings aim to achieve V-0 status (top result).


UL 746E

UL 746E is the Standard Polymeric Materials: Industrial Laminates, Filament Wound Tubing, Vulcanized Fiber and Materials Used in Printed-Wiring Boards

UL 746E measures the ignition resistance of the conformal coating to electrical ignition sources.

The conformal coating material’s resistance to ignition and surface tracking characteristics is described in UL 746E.


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How do I selective apply my conformal coating?


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Selective application of conformal coating is this case is applying the conformal coating without using masking to shield components from ingress.

Technically using masking tapes, latex and boots is a selective process. But, we want to look at coating application without masking.

This leaves a couple of different options.

The first is brushing. This is a simple selective process that can be highly effective.

Click Nexus conformal coating brushing technique to find out more.

The second, and the more obvious option, is selective robot.

This process uses a small spray valve (there are many conformal coating spray valve types) that is attached to a robot that follows a set pattern applying the conformal coating selectively to the circuit board.

Click Nexus selective conformal coating application to find out more.


Advantages and disadvantages of selective conformal coating robots

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The five advantages and disadvantages of selective application of conformal coating are:

Advantages

  • Masking can be eliminated or minimised
  • High speed coating process
  • High volume process
  • High quality finish when under control
  • Simple process to operate when set up

Disadvantages

  • High engineering skill required for set up and programming
  • High engineering skill required for troubleshooting and supervision
  • High capital cost initially
  • Process controls need to be enforced for good quality production
  • Environment can influence the process controls

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What application methods are available for applying conformal coating?


SCH UK_1Conformal coatings can be applied to printed circuit boards (PCB) in the production process environment in many alternative ways.

Listed below are the common methods of applying the conformal coating materials.

  • Brushing
  • Aerosol spray
  • Batch spray
  • Selective spray
  • Dipping
  • Vapour deposition

It is possible to split the different application methods and their suitability in many different ways including:

  • Compatibility with the conformal coating material
  • Design of the circuit board
  • Suitability to low, medium and high volume
  • Manual versus automatic process
  • Type of method of application
  • Level of control required

Each of the various methods has different advantages and disadvantages and should be carefully considered.

This thought process relates to the Nexus holistic approach to conformal coating processing that states that you should consider the conformal coating material, the application process and the circuit board together and not as three separate entities.


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What is a hydrophobic acrylic conformal coating?


Nano-coating fluoropolymers Collage 640_SCH UK

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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