Measuring conformal coating thickness on a circuit board

Co-authors and contributors

  • Dr Lee Hitchens, Nexus

measuring conformal coating thickness on a circuit board is not easy
Measuring 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 techniques for measuring the conformal coating thickness on a printed circuit board (PCB).

These techniques include:

These techniques are explored further below.

Non-destructive eddy current system

Positector measurement system for measuring conformal coating thicknessA 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.

Key points in the measurement method

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

  1. There needs to be metal in the circuit board directly below the tested point. Otherwise, the system cannot work as the eddy current will pass directly through the board.
  2. 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.
  3. 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

conformal coating thickness measurement with micrometer 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.

Key points in the measurement method
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

Tricomb measuring conformal coating thicknessA 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.

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