How do I spray coat my printed circuit board with conformal coating?

Nexus 1Batch spraying of conformal coating on printed circuit board assemblies (PCBA) is one of the most widely used techniques in low and medium volume production processing.

Typically, using either a compressed air spray gun or an aerosol, the conformal coating is sprayed across the whole of the circuit board to provide the protection it requires.

The batch spray process can produce a high quality conformal coating finish that gives the best protection due to good tip edge coverage of components.

However, it is not a selective process and all parts are coated on the circuit board.

Therefore, masking may be required to protect components that must not be conformal coated.


What is a typical spray process for application of the conformal coating to the circuit board?

Nexus 2

Batch spraying using an aerosol or hand spray gun is normally a simple process.

A typical method of application for spraying is as follows:

  1. Dilute the conformal coating with thinners according to the manufacturers recommended instructions (typically the viscosity is close to 24cps and almost like water).
  2. Mix the blend thoroughly but without creating bubbles (if bubbles occur wait for them to dissipate) and apply a test pattern to ensure the material from the spray gun is flowing properly.
  3. If “spider webs” occur (similar to candy floss in the air) then dilute further with more thinning solvents.
  4. Position the circuit board horizontally
in front of the spray gun operator.
  5. Hold the spray gun at a 45° angle and at the recommended distance (typically 20 to 25 cm from the circuit board).
  6. Spray a thin and uniform coat onto the circuit board with an even motion using “spray and release” strokes in a raster pattern. Do not over apply too much liquid.
  7. Turn the circuit board 90°and repeat until the board has completed one complete 360° rotation.
  8. If a second coat is required, wait 2-3 minutes (may be longer with certain solvent types so check manufacturers recommended instructions) and repeat steps 5-7.

Once complete follow the cure instructions for the coating on the circuit board.


Need to find out more?

Go directly to our conformal coating batch spraying section or contact us directly and we can help you.

If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating eBook.

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Why use Parylene to protect printed circuit boards?

Parylene2

Parylene is a conformal coating film that is applied using a specialised vapour deposition application process.

This means it is very different to all of the other liquid conformal coatings available on the market.


Three reasons why Parylene is better than traditional liquid conformal coatings

The fact that the Parylene film is deposited onto circuit boards in a vacuum leads to many unique advantages.

Here are three key reasons to use Parylene:

  1. The Parylene coating is completely conformal to the surface of the Printed Circuit Board (PCB) or product. The coating has a uniform thickness and is pinhole free. Therefore, components with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids.
  2. Parylene coating provides an excellent barrier that exhibits a very low permeability to moisture and gases compared to traditional liquid conformal coatings. This means that circuit boards coated in Parylene generally are more “waterproof” than the same circuits coated in a liquid conformal coating.
  3. Parylene has excellent electrical properties compared to normal conformal coatings. These include low dielectric constant and loss with good high-frequency properties, good dielectric strength, and high bulk and surface resistance.

Need to find out more?

Go directly to our Parylene section in Nexus  or contact us directly and we can help you.

If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating eBook.

The science behind Molecular vapour deposition (MVD) in protecting circuit boards

Nexus has been examining a new, novel technique that may be able to superior protection for electronic circuit boards compared to the standard coating methods like conformal coatings and Parylene but also actually be cost-effective.

This process is a hybrid ALD (Atomic Layer Deposition)/CVD (Chemical Vapor Deposition) technique called Molecular Vapor Deposition (MVD).

This method uses multiple layers of ultra-thin coatings with differing properties to build a completely protective coating.


So, why is this new coating so good compared to Parylene and other conformal coatings?

The final MVD coating built up is much thinner than the other traditional coatings including Parylene. However, its protective performance has been found to be superior to them all in most categories of testing so far.

Key performance indicators like Water Vapor Transmission Rate (WVTR), optical clarity, temperature resistance and hydrophobicity have been found to be much better than the other coatings.

Further, the really exciting part about this technology is the cost of processing that is extremely low.

Since the coating is extremely thin then it has been found that no masking is required. This is because when components like connectors are joined together then the ultrathin coating does not prevent electrical connection.

This means that the cost of process is purely the cost of application of the material and nothing else.

Since the process is relatively low cost then this does offer a very interesting alternative to the traditional coating materials.


So what does the science of Molecular Vapor Deposition coating (MVD) look like?

The actual film is built up of alternating layers of ALD and CVD thin coating layers.

The ALD is a ceramic-based material providing the insulating properties. The CVD film provides the barrier protection.

First, an ALD layer is applied to the substrate. Then a CVD layer is applied. Then a further ALD layer is applied and so on.

This continues until the correct number of layers is built that has the right protection.

Description of Hybrid ALD_CVD Technology

Finally, once the required film thickness is achieved with the alternating layers, then a final hydrophobic thin film layer is applied, that combines with the ALD and CVD layers to provide a highly effective barrier.


So, just how good is the hybrid coating as a protective material for electronic circuit boards?

Generally, with protective coatings for electronics then Parylene is considered the gold standard in most cases.

So, Nexus compared Parylene with the MVD material.

Property Parylene MVD Coating
Hardness Soft Hard
Wear resistance/Handling Ease Poor Excellent
Water Vapor Transmission Rate Good Excellent
Temperature Resistance (extended time) 100°C 350°C
Color Gray/white Clear
Adhesion to various materials Poor Excellent
Scalable to large production Poor Excellent
Process Time 8 – 12 hrs 8 – 12 hrs
Hydrophobicity Good Good – Excellent
Cost High Low – Med

What Nexus also identified for the material were some key properties.

  • The Water Vapor Transmission Rate (WVTR) is superior to Parylene so the coating is far more waterproof.
  • Coating adhesion is superior as it covalently bonds to the substrate. So, the lifetime of the material will be better on the circuit.
  • The temperature range of the material can be up to 350C without any degradation.
  • The hybrid coating is UV stable whereas Parylene in general is not. This is an important criteria for coatings exposed to UV light.
  • The coating stayed 100% transparent during testing (no loss of lux).
  • The coating thickness of the hybrid material is x10 LESS than the Parylene. This aids light transmission and electric connectivity

So, in reality the MVD material could just be what industries like the automotive and LED sectors are looking for in protecting their circuits where cost and protection abilities are critical.


Need to find out more?

Contact us directly and we can help you with this new material.

If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating eBook.

How do you remove Parylene completely from a printed circuit board?  

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

Although Parylene is a conformal coating it is different compared to the standard “wet” liquid conformal coatings in that it is deposited as a gas in a vacuum chamber and it is a dry process.

This method and the material itself give Parylene unique material properties that give it a lot of advantages compared to other traditional conformal coatings.

However, these unique properties also make it an extremely difficult material to remove.

Why is Parylene difficult to remove?

Compared to typical liquid conformal coatings like acrylics and polyurethanes that more easily dissolve in mildly aggressive solvents then Parylene is much tougher to remove.

The reasons are many why but a key point is that the Parylene coating itself is chemically inert. It has a high chemical resistance so the solvents don’t work well.

This means any chemical attack tried with solvents or other liquid chemicals on the Parylene is as much likely to damage the circuit board than remove the actual coating.

So, chemical removal is almost impossible.

This leaves another well know method for Parylene removal that is mechanical abrasion.

Mechanical abrasion of a coating can be done crudely by scraping off the Parylene with a knife or tool. Or, removal can be done with a media blast system that gradually erodes the Parylene coating away.

However, mechanical abrasion is a time consuming process and is highly skilled. Any wrong action could result in irreparable damage.

Further, mechanical abrasion tends to be a localised repair and removal technique. The concept of completely removing all of the Parylene of a circuit by mechanical abrasion is considered almost impossible unless a ridiculous amount of time and effort is injected into the process.

Therefore, a specialist method is required to remove Parylene completely from a whole circuit board.

A new method for completely stripping Parylene from a PCBA

Due to new research two processes have been developed for completely removing Parylene coating from the surface of a circuit.

They are:

  • A method for <15um thickness of Parylene coating on the board
  • A method for >15um thickness of Parylene coating on the board

Removal with <15um thickness of Parylene coating on the circuit

When the Parylene coating is under 15um then the removal from the whole circuit is a relatively simple process.

To successfully remove the Parylene a technique has been developed involving plasma etching.

The plasma-etch process uses a proprietary blend of gasses, and a custom system to mechanically etch off the Parylene. This specially designed alchemy of gasses specifically attacks Parylene rather than the solder mask.

The technique successfully removes the Parylene from all over the board.

Also, the etching process is quick relative to the other mechanical methods. Typically, the circuit can be completely etched of Parylene in under an hour.

Further, the process is safe. It does almost no harm to the circuit and is one of the safest methods for complete removal of Parylene.

Removal with >15um thickness of Parylene coating on the circuit

When the Parylene coating is greater than 15um then the removal from the whole circuit is a little more complex. In fact, it becomes a two-stage process.

First, you can use the plasma etch treatment to loosen the Parylene from the surface of the circuit. Normally the Parylene is bonded well to the surface and this loosening allows for a second stage process.

In the second stage a media blaster like the SWARM system can be used to remove the coating. Since the coating has been loosened it does tend to come off much easier and quicker.

That said it is still a little slow and costs are higher. But, it still can be removed more easily.


Need to find out more?

For further information on Parylene removal then contact us directly and we can help you.

If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating eBook.