Conformal coatings are usually applied as very thin coatings, providing the maximum protection possible using the thinnest amount of material. This minimizes heat entrapment, additional weight addition, and a variety of other concerns. Normal thickness with most conformal coatings is anywhere between 1 to 5 mils (25 to 127 microns) with some coatings applied at an even thinner level. Anything greater than this thickness is usually an encapsulate or a potting compound, which typically provide more mass and thickness to protect the boards. There are four primary ways to measure the thickness of a conformal coating:
Wet film thickness can be measured directly by using the appropriate gauge. These gauges incorporate a series of notches and teeth, each tooth has a known and calibrated length. The gauge is placed directly onto the wet film to take the film measurement. See http://www.geionline.com/wet-film-gauge. This measurement is then multiplied by the percent solids of the coating to calculate the approximate dry coating thickness.
Micrometer thickness measurements are taken on the board (or on a test panel) on several locations before and after coating takes place. The cured coating thickness is subtracted from the uncoated measurements and divided by 2, providing the thickness on one side of the board. The standard deviation of the measurements is then calculated to determine the uniformity of the coating. Micrometer measurements are best taken on harder coatings that do not deform under pressure.
Eddy current measurement of conformal coating thickness uses a test probe that directly measures the thickness of a coating by creating an oscillating electromagnetic field. The thickness measurements are non-destructive and very accurate but can be limited depending on the availability of a metal backplane or metal under the coating, and the direct contact available of the test sample. Without metal below the test area no measurements will be made, and if the probe does not fit flat on the test area, readings will inaccurate.
- Ultrasonic thickness gauge
This type of gauge measures coating thickness using ultrasonic waves. It has the advantage over eddy current probes because it does not need a metal backplane. Thickness is determined by the amount of time sound takes to travel from the transducer, through the coating, bounce off the surface of the PCB, and back. A conductive medium, like propylene glycol or water, is needed to provide good contact with the surface. This is generally considered a non-destructive test unless there is a concern with the conductive medium affecting the coating.