I have been asked to apply a really thick layer of conformal coating to my printed circuit board. What are the problems with this approach?
Applying conformal coating really thick is not a good idea especially if you exceed the recommended thickness from the manufacture.
There are several reasons for this but the two key ones are:
- Applying a thick coating can cause long-term reliability issues. This is because the conformal coating can crack and craze in the long-term. It can also possibly cause damage to the PCB due to Coefficient of Thermal Expansion (CTE) mismatch between components and the coating.
- Defects like bubbles can occur when applying really thick conformal coating layers.
If in doubt, more is not better. Conformal coatings are not designed as encapsulates and potting compounds. The IPC and mil spec recommend certain thicknesses such as 30-150um for acrylics. It’s a good idea to keep within these tolerances and use conformal coating thickness measurement systems to check.
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My Customer has asked for two layers of conformal coating applied. Is this a problem?
Getting the application of the conformal coating wrong is more critical than the number of coating layers applied
Generally, the critical factor in applying conformal coating is not the number of coats applied. Factors that can influence the performance of the coating are:
- The total coating thickness. Too thick and the coating may crack in the long term. Too thin and it will have a less effective performance
- The defects created when applying the coating layer. Again, thickness is the overriding influence and applying too thick a coating can cause issues such as bubble entrapment in the film
That said it is best to avoid creating laminating layers of coating with too much time between coating layers.
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How to verify the conformal coating thickness on a printed circuit board in production for a dip coating system
When using a conformal coating dipping system it is important to know that the dry film conformal coating thickness is correct on the circuit board. Ultimately, the conformal coating will only be protective if applied at the right thickness so control of the process is crucial.
First, when we are coating PCBs we must assume that the variables for the conformal coating dip coating process are set up correctly for the PCB. That is that
- The viscosity of the conformal coating is correct
- The withdrawal speed of the dip system is correct
- The temperature of the conformal coating is stable
These three parameters ultimately control the final thickness of the conformal coating on the circuit. Change one of the these and you can change significantly the conformal coating thickness.
Once this is set up and everything is in order we now want to monitor the coating thickness.
There are two options:
Option one: Measure the circuit board directly.
This can be done with a micrometer or a conformal coating eddy current system such as a Positector. However, this assumes that there is enough room for the probe or micrometer on the circuit board and (ii) there is metal in the PCB at the point of measurement for the Positector. Using this system after the coating is cured hard enough to test gives excellent readings
If this is not the case there is an alternative solution.
Option 2: Measure a test coupon
If the conformal coating thickness is correct on a circuit board when qualifying the process then it is possible to dip a test coupon to measure the equivalent coating thickness. This then allows you to use a test coupon at the start of the day’s production process to confirm everything is in order and no parameters have changed.
The process is the same as with measuring the circuit board. Dip the coupon. Dry it sufficient to measure the coating thickness and verify that the process is stable and there is no significant variation.
This is an easy method that is reliable. It also backs up your viscosity control measurements for your conformal coating and ensures your conformal coating dip process is in control.
Click Conformal Coating Thickness Measurement for further information.
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How do I measure the thickness of a conformal coating on a Printed Circuit Board?
Positest 6000 Thickness Test System cure and measure the test coupon again at the same point. The difference gives you the coating thickness.
The low cost method is using a calibrated micrometer screw gauge that can measure down to ± 10 um. First measure a point on the board or test coupon, apply the coating,
A couple of pitfalls to avoid are ensuring the 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 since this will give a better result statistically.
Test coupons are the ideal method for measuring the coating thickness, whether is it spraying or dipping, and can be kept as a physical record of the performance.
An alternative method is using a non contact technique using eddy currents. A system like the Positest 6000 offered by SCH is extremely quick and accurate to ±1 um.
A final method is a wet film measurement technique which is very cost effective. This uses a comb with different size patterns which is placed in the wet coating and the imprint left indicates the wet film thickness. Knowing the solids content of the material means that the material
Conformal Coating 