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.

positector
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.

Advertisements

How to Measure the Conformal Coating Thickness on your Printed Circuit Board

Conformal coating thickness measurement can be a challenge expecially when trying to interpret standards. There are several methods to be used and advantages and trade offs for each.

The Nexus How To Guides are a range of Technical Bulletins that will be available weekly.

Each How To Guide will be initially launched through the FREE subscription service. The Guides will then be available for access online here below. There will also be a Printer Friendly version and a PDF version for easy download if required.

Dont miss out on your copy of the NEW How To Guides from Nexus as they are published

If already subscribed, Click Measure the Conformal Coating Thickness on your Printed Circuit Boardto access.

To join Nexus for Free click Nexus Conformal Coating Resource Sign Up