How do I measure the coating thickness on a PCB?
There are several ways to measure the coating thickness on a conformal coated circuit board.
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, cure and measure the test coupon again at the same point. The difference gives you the coating thickness.
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 offered by SCH is extremely quick and accurate to ±1 um.
Using a gauge and flying probe for the measurement the system is extremely easy to use. Placing the test probe head flat on the surface of the conformal coating, the measurement is almost instantaneous and provides an immediate repeatable result for thickness measurement of conformal coating.
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. Apply the coating to the test coupons at the same time as the circuit boards provides a permanent measurement and an accurate guide to the coating thickness.
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 thickness can be calculated.
Eddy current systems like the Positector 6000 are perfect for measuring flat areas of coated PCBs with metal in the laminate.
How does the conformal coating wet film thickness method work?
The wet film conformal coating thickness method ensures quality control while the coating is still wet. Applying too much coating can be expensive. Wet film measurements are also useful for conformal coatings where the dry film thickness can only be measured destructively or over application of conformal coating could be problematic.
The wet film gauges are applied to the wet conformal coating and the teeth indicate the thcikness of the conformal coating. The dry film thickness can then be calculated from the measruement.
Manufactured in plastic, the disposable wet film gauge has been designed for one-off use only, saving time on cleaning the teeth after use, giving a quick and economical test of the wet film thickness. They can also be left to dry and kept as a permanent record of the conformal coating thickness. Metric values are on the front side of the gauge and the imperial values are on the back.
You can also purchase a single stainless steel comb which is reusable.
Wet film measurement of conformal coating on a printed circuit board.
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