How do I measure conformal coating thickness?

 

nexus1Measuring the exact thickness of your conformal coating across the whole of a circuit board is not simple.

In fact, due to the geometry of the components and the fact that the coating is measured in microns, it makes this task almost impossible.

However, what you can do is measured the thickness of the conformal coating in a few key places and use the thickness information found to infer how the coating coverage is for the rest of the board.

This is how nearly all companies measure conformal coating thickness.


So, how is the conformal coating thickness measured on a circuit board?

There are several ways to measure the conformal coating thickness on a printed circuit board (PCB).

The methods used can be for either a dry or wet conformal coating.

These techniques include:

  • Non-destructive eddy current system
  • Micrometer screw gauge
  • Wet film gauge

These techniques are explored further below.


Non-destructive eddy current system

A fast method for measuring conformal coating thickness after drying is a system using eddy currents.

The process works by placing the test probe head flat on the surface of the conformal coating and a measurement taken.

The system provides an immediate repeatable result for thickness measurement of conformal coating.

The process is quick and accurate to ±1 um. Using a gauge and flying probe also means the measurement system is extremely easy to use.

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Using a test probe system can quickly give you conformal coating thickness measurements without damaging the circuit board. Image from SCH Technologies

There are a couple of issues using an eddy current system like this.

First, there needs to be metal in the circuit board directly below the tested point. Otherwise, the system cannot function correctly as the eddy current will pass directly through the board.

Second, there needs to be a flat area on the board large enough for the test probe. The smallest practical probe is approximately 6mm diameter so any area smaller than this is not practical.

Finally, the surface measured for the probe needs to be flat. If not then there will be errors in the measurement. So, measuring components is extremely difficult.

To overcome these problems it may be better to measure test coupons.

Apply the conformal coating to the test coupons at the same time as the circuit board allows an easy measurement process. It also provides a permanent measurement.

In fact, test coupons are the ideal method for measuring the coating thickness, whatever the conformal coating process and method of measurement.


Micrometer screw gauge

An alternative to the eddy current system for dry film measurement is a calibrated micrometer screw gauge.

It’s a low cost, low-tech method for measuring conformal coating thickness and can normally measure down to ± 10 um.

The process is relatively simple.

First measure a point on the board or test coupon before coating. Next, apply the coating. Cure the coating well and finally re-measure at the same point.

The difference in the two measurements gives you the conformal coating thickness.

gauge-88268_640_NEXUS

A couple of pitfalls to avoid with this technique are ensuring the conformal coating is cured hard enough since if it is soft it could compact and give a false reading.

Also, do not measure one point. Take an average of at least 3 or 4 points across the coupon since this will give a better result statistically.

Again, for this technique test coupons are the ideal method.


Wet film gauge

A final method that can be used is a wet coating measurement technique that is very cost effective.

The technique uses a comb with different size patterns that is placed in the wet conformal coating and the imprint left indicates the wet film thickness.

Knowing the solids content of the material means that the material thickness can be calculated.

wet
A wet film gauge is a low cost method for measuring coating thickness while the conformal coating is wet. Using the solids content in the material and the wet film thickness allows the dry film thickness to be estimated.

Need to find out more?

Click conformal coating thickness measurement for further information 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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Five important facts about silicone conformal coatings

  1. Silicone (SR) conformal coatings are inorganic materials. They are polymers in which atoms of silicon and oxygen alternate in a chain. They differ to the organic polymers like the acrylics and urethanes.
  2. The silicone coatings normally have a very wide temperature range of operation compared to the other conformal coatings. Typical range can be -55°C to +200°C (-67°F to +392°F). Like the organic coatings (acrylics and urethanes) they have good moisture protection. They also have good chemical resistance to polar solvents.
  3. SR coatings are generally applied at 2-3 times the thickness dry film compared to organic coatings. This is reflected in the IPC standards on the recommended conformal coating thickness. This increase in material use can lead to increased costs but also better water repellency properties.
  4. Curing of silicones occurs through several different mechanisms, depending on the conformal coating, including RTV (Room Temperature Vulcanisation), Heat, UV, Moisture / Condensation and Catalysed Cure.
  5. In production they can be difficult materials to use due to the cure mechanism being difficult to control. Good housekeeping can minimise these effects. SR coatings can also require different coating equipment or options compared to the organic coatings. This should be considered as part of the holistic approach to conformal coating selection.

The different conformal coating material properties

Conformal coatings can be considered in many different ways.

This includes the different families of materials, their individual properties and the chemistries of these coatings.

The different ways we can examine conformal coatings include:

  • Classification
  • Physical Properties
  • Electrical Properties
  • Chemistry

There is a vast range of options and considerations available.

Click conformal coating material properties to gain a deeper knowledge of conformal coatings.


Need to find out more?

Click silicone conformal coatings for further information 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.

Do you need mil spec qualification for your conformal coating?

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The Mil Standard (MIL-I-46058C) is a US military conformal coating standard.

It has been inactive for new designs since November 1998.

However, the standard is still widely used for independent certification of conformal coatings.

This means that an independent test house will test the conformal coating. It is not self-certified.

All companies tested to the Mil standard are listed on a Qualified Product List (QPL).

Therefore, conformal coatings listed on the QPL will have been through rigorous 3rd party testing.

MIL-I-46058C is a guideline for quality. However, other standards like IPC-CC-830 may be equally relevant.


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What is the role of a qualification or standard in conformal coating?

The role of a standard and qualifications is to give a guideline for users on the quality of a product.

For conformal coating standards and qualifications there are two areas that they cover.

These are:

  • Material manufacture
  • Production process

The conformal coating material manufacture standards are used for making the conformal coating.

They are only really relevant to suppliers so that they can measure their coating performance against a set of tests.

The conformal coating production standards are used for qualifying a process.

They are used in the production environment.


Need to find out more?

Click conformal coating standards for further information 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.

Five important facts about polyurethane (UR) conformal coatings

conformal coated PCBs Collage 640x480_NEXUS

  1. Polyurethane (UR) conformal coatings generally provide good humidity & moisture protection although not always as good as the acrylics. However, it normally is enough to protect the circuit board.
  2. Normally, urethanes are selected for their excellent chemical resistance. This is because the coatings cure rather than dry. That is they cross-link by one of many different methods including heat, UV, moisture and catalysed cure.
  3. Typically they have higher dielectric properties compared to the acrylic conformal coatings.
  4. Their chemical resistance, however, can be a limitation since rework and repair generally is more difficult than the acrylic coatings.
  5. UR coatings are normally available as either single or two-component formulations. Pot life is dependent on the cure mechanism but can be more limited than the acrylic coatings.

Need to find out more?

Click organic conformal coatings to find out about polyurethanes 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.

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

 

The cleaning of a printed circuit board (PCB) before conformal coating application is normally done for two key reasons:

These are:

  • Contamination removal
  • Process improvement

They have different effects on the lifetime of the circuit board but can be equally important.


Contamination removal

OLYMPUS DIGITAL CAMERA

The removal of contaminants from the surface of a printed circuit board before coating application could be important.

The contamination may be harmful and affect the long-term reliability of the circuit. Defects like corrosion can be devastating to a circuit performance in the field.

Also, applying the coating over the contamination will not necessarily improve the reliability.

Therefore, cleaning the PCB before coating could be advantageous.


Process improvement

OLYMPUS DIGITAL CAMERACleaning can aid the conformal coating application process.

It could avoid conformal coating defects like de-wetting and delamination. It can also help promote adhesion of the coating to the PCB.

The surface preparation can be extremely important, especially where some conformal coatings may have compatibility issues with the circuit board.

Therefore, cleaning a PCB before conformal coating may improve the coating quality and minimise defects.


What types of contamination may be present on a circuit board?

Cleaning is used to remove many different types of contaminants from the manufacturing and assembly processes.

They can affect the long-term reliability of the circuit after conformal coating.

The residues can come from many areas including:

  • Board laminate manufacture
  • Component manufacture
  • Soldering assembly processes (fluxes)
  • Glue and ruggedizing processes
  • Operator handling (finger prints, hair)
  • Machine contamination (oils and greases)
  • Environmental contamination (dust)

Removing the contamination may be a priority depending on their harmfulness.

How should I clean the printed circuit board?


Cleaning circuit boards before coating is a huge topic by itself.

Cleaning of a circuit before conformal coating at SCH 640_NexusSuccessful cleaning of circuits can be achieved by a variety of techniques.

These cleaning techniques include:

  • Aqueous
  • Semi-aqueous
  • Solvent & chemical
  • Plasma

The key to success in cleaning is similar to the success made with coating.

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

This will give the best results for application of the conformal coating.


How do I validate my cleaning process?

There are many techniques that can be used to measure cleanliness. They include many IPC test methods.

The techniques available include:

  • Ionic Contamination Testing (ROSE)
  • Ion Chromatography (IC) or High Performance Ion Chromatography (HPLC)
  • Surface Insulation Resistance (SIR) testing

Further information can be found in the IPC HDBK 001 on different cleanliness assessment methods or talk to us.


Need to find out more?

Click conformal coating cleaning for further information 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.

What are conformal coating masking boots?

Nexus Image 1Masking boots are formed shapes used to cover components before conformal coating.

They are made to replace tape and dots in the conformal coating masking process for both Parylene and liquid conformal coating processes.

Using masking boots in protecting printed circuit boards (PCBs) from conformal coating application errors can be a lower cost, more reliable solution to using tapes.

The use of masking tapes, dots and liquid latex can be an effective process in protecting components from ingress of conformal coating.

However, the masking can be labour intensive, difficult and time consuming.

Masking boots can offer a labour saving alternative in both the masking and de-masking stages of the coating process. This saves time and money.

Further if the boots are recyclable then the savings can be even higher.

Three reasons to use masking boots

  1. Masking time is normally reduced. Masking boots can be 4-5 times quicker than masking tape.
  2. De-masking time is reduced. Again it is normally much quicker to remove masking boots.
  3. Masking boots are much less likely to leak. So there is potentially less damage and rework of the circuit.

This means you could save a lot of money very quickly when switching to masking boots.

Nexus Image 2
Masking boots can offer a labour saving alternative in both the masking and de-masking stages of the coating process. This saves time and money.

 Need to find out more?

Click conformal coating masking for further information 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.

Why use plasma cleaning for cleaning circuit boards?

NEXUS 1Normally, conformal coatings must exhibit good adhesion to the PCB in order to be effective in the long term in protecting the circuit.

Therefore, the surface properties of the circuit board can be critical to the success of the coating adhesion.

Cleaning of circuit boards before conformal coating has taken place for many years.

The reasons for doing this have always remained the same:

  • Improve the surface cleanliness of the circuit to protect against corrosion and the effects of contamination on the surface from the process.
  • Improve the surface energy of the surface to improve the adhesion of conformal coating.

Traditionally, cleaning and adhesion promotion has been achieved by either using a wet chemistry treatment like washing or applying extra undercoats (priming).

Now, there are new methods and techniques appearing on the market for improving cleaning, adhesion and actually coating circuit boards differently to the traditional methods.

One of these techniques is plasma treatment.


Why use Plasma to clean circuit boards?

Here are a few key reasons to use plasma cleaning and surface treatment of printed circuit boards:

  • Plasma cleaning can clean surfaces of a product 100% to improve adhesion and surface energy of the product.
  • Many wet chemistry cleaning processes can be eliminated. Methods using cleaning chemistries, water processing and drying energy are unnecessary.
  • Activate the surface of the circuit by changing the surface energy. This allows easier bonding and better adhesion to the surface. The significant improvement in adhesion enables the use of alternative coatings that may have difficulty adhering to surfaces without the treatment.
  • The plasma process is a simple, safe and environmentally friendly technology.
  • The plasma process has both batch (offline) and inline capability.

This means the plasma process can be highly effective on printed circuit boards.


Need to find out more?

Contact us directly and we can help you with your plasma treatment requirements.

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.