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Why use silicone (SR) conformal coatings to protect electronic circuit board assemblies?

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Silicone (inorganic) conformal coatings can be very different in nature to the organic chemistry coatings like the acrylics and polyurethane materials.

For example, they tend to have a much wider temperature range of operation compared to the other conformal coatings.

Why use plasma cleaning for cleaning circuit boards?

For a conformal coating to be effective in protecting a circuit board it should be applied evenly over the circuit board assembly and it should have good adhesion.

Otherwise, the circuit may suffer an early failure in the field.  

For good coverage and good adhesion the surface properties of the circuit board should be compatible with the conformal coating.

Do you need UL qualification for your conformal coating?

In conformal coating, there are many International, National and internal company standards for manufacture of coating materials, processing and inspection.

The UL standards (and qualification) are used to qualify a conformal coating material to a certain specification. Therefore, it is a material manufacture standard only.

The question of whether a conformal coating actually needs the UL qualification to the standard is down to whether the test regime is relevant to the user of the conformal coating.

Why may I have to clean my circuit board before conformal coating?

Whether to clean circuit boards before conformal coating is a well discussed problem.

However, I thought it may be interesting to summarize the reasons for considering the cleaning process before coating as it can be critical to the success of the product in the field.

First, consider that cleaning of a printed circuit board (PCB) before applying conformal coating is normally done for two key reasons:

These are:

Coating LEDs with Molecular Vapor Deposition (MVD)

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Coating LEDs to protect them from the harsh environments they are placed in is big business.

The volume of manufactured LEDs globally is growing at an exponential rate and there is no end in sight.

However, the challenges to protect the LEDs from long-term failure are not small especially in an outdoor environment.

What is a fluoropolymer coating?

A fluoropolymer coating is a material comprised of fluorocarbons and characterized by carbon-fluorine bonds.

Fluorocarbons are not susceptible to Van der Waals force. This gives the coatings their signature properties.

That is that a fluoropolymer coating is non-stick and friction reducing.

So, these coatings are naturally hydrophobic and also have some of the lowest coefficients of friction.

How thick should I apply my conformal coating?

A regular question asked in conformal coating is how thick should the coating film be?

Well, the simple answer is thick enough so the conformal coating works and protects the circuit board.

But not too thick as this can cause problems for the circuit board in the long term.

After all, the performance of the conformal coating is dependent on the thickness applied.

But, it is possible to quantify this a little more.

What is Parylene?

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

The actual material is deposited as a gas in a vacuum chamber.

Therefore it is a completely different dry process compared to the standard “wet” liquid conformal coatings.

Choosing the right conformal coating application process

Choosing the right process for your conformal coating production line is a complicated and involved process.

It normally is a case of balancing a number of factors and making compromises.

This is because of the three major parts of the complete conformal coating process that can offer different options and create difficult choices.

These parts are:

  • The conformal coating material
  • The application method
  • The circuit board design

When you consider these three points you have many factors to consider.

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