This is a situation you want to avoid. As a technician or engineer, you have to ensure that the coating machine program apply the conformal coating thickness evenly on the circuit card assemblies. This article will show you how to ensure the coating program give you the right thickness when applying acrylic coating to your machine, and how to test if the product is compliant to the specification.
Standards for Conformal Coat Thickness
The most popular conformal coatings used in industry today are Humiseal 1B31 and MG Chemicals 419C The material is relatively cheap, easy to use and provide excellent moisture protection. There are industry standards for conformal coat thickness for acrylics and when adhering to IPC-610, type AR conformal coatings, and the thickness should be in between . 002” and . 005. ” MIL-I-46058C indicates that the thickness should be . 002” +/- . 001. ” Most manufacturers aim between about . 002” to . 003,” and meeting that guideline will ensure that you stay within the two major thresholds of the two major specifications called out in the standard.The standard is in the company's product specification sheet. Everyone who is involved in the process should know what the standard is. When calibrating the conformal coating machine, the right thickness must be applied to the finished substrate. If the standard thickness is not achieved, then the coating fails to protect the final PCB from moisture ingress.
With the correct coating machine that match the PCB requirement, the standard can be met. Now it is up to the programmer to set the right pattern and amount of passes in the software to ensure the correct thickness is achieved. Too many or too few passes results in improper conformal coating.
All it takes is for the programmer to adjust the software setting to achieve the desired range. There are ways to measure the thickness of the conformal coating - either using a manual process or testing equipment.
Coating Thickness Measuring Techniques
It is important to achieve the standard of the desired thickness, regardless of the conformal coating equipment used. This can be achieved by measuring the coating and calibrating the process. There are two categories of measuring tools - dry and wet film measurements.Dry Coating Thickness Measurement
Conformal coating for PCB dry film measurements are made after the coating dries enough for any contact made with the coating, so that any contact made during the process won’t damage the coating. For this method, measurements are taken on unencumbered, flat and cured surface of a PCB, or on a coupon processed with the part.Test coupons are normally used to measure the coating as it is easy and affordable.. The coupons also provide a record for the quality assurance department to look over to make sure the coating characteristics and parameters are up to par with the specifications and can be sprayed the same time that the PCBs are.
One way to spray coating to test involves procuring some coupons made up of nonporous material, such as glass or metal. Glass slides that are like the ones you used in biology or chemistry work really well for this. Test coupons made out of the same material that the PCB you are spraying usually work the best. The “sticky dot” method works really well, too. Identify the areas you want to take a measurement and measure it with micrometers or an indicator that’s accurate to 12. 5+_2. 5µm on, and begin the test by spraying it with the acrylic material.
Dry the area. Then measure the area once more. The difference between the beginning and end measurements will be your record for thickness. Write these measurements down in a standardized record that shows proof that thickness was calibrated before beginning a run of production. Run other tests periodically to ensure the process remains correct.
Another dry method that is used frequently is testing equipment that utilizes eddy currents to test conformal coat thickness. It is a little more expensive, but involves a non-contact technique. The measurements involve lightly placing a test probe head on the surface of the coating to get an almost instantaneous readout. If the production amount of PCBs on conformal coating printed circuit boards is high, you might want to invest in this method. The measurement devices are extremely accurate, up to thin films of 25-50 um and ±1 um for a device such as the Positestor 6000. This is recommended for high volume because they are very quick to display measurement readings.
Wet Film Conformal Coating Measurement
This method involves using a wet film gauge to measure the coating while it’s still wet. This allows operators to test and help accomplish quality assurance before the coating dries.There are advantages to using this method. For one thing, applying to much conformal coating costs your business a lot of money, and it’s also useful for conformal coating processes where dry coating thickness can only be measured by using destructive methods.
How to do it: the wet film gauge is placed into the wet conformal coating after application until the teeth of it contacts the substrate or surface of the components measured. There are measurements on the gauge that indicate how thick the coating is. Count the measurement line that the conformal coating has risen to. You will be able to tell the dry film coating thickness by calculating it from measurement s derived from the solid resin content of the coating on the PCB.
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