Conformal Coating

Chemtronics offers the CircuitWorks® Conformal Coating Remover Pen that allows you to remove a tight area of coating around a repair area without affecting the rest of the PCB. You first saturate the tip by tapping it lightly on a surface, which opens the valve and releases solvent. Holding the tip down may oversaturate it, which could lead to solvent flowing into unintended areas.

Certifications are used to separate conformal coatings from general purpose varnishes and shellacs. There are dozens of user and industry specifications, but the two major certifications are most commonly referenced: IPC-CC-830B / MIL-I-46058C This standard originated with the military standard MIL-I-46058C, which became obsolete in 1998. The civilian version IPC-CC-830B is nearly identically, so if a board passes the IPC spec it should also pass the MIL spec., and visa versa. IPC-CC-830B is a battery of tests that includes appearance, insulation resistance, UV fluorescence, fungus resistance, flexibility, flammability, moisture and insulation resistance, thermal shock, and hydrolytic stability. All Chemtronics Konform [https://www.chemtronics.com/conformal-coatings] coatings are IPC-CC-830B certified. UL746E Underwriters Laboratories (UL) is considered a credible and reliable safety certification body worldwide, and UL certification is commonly required for consumer goods. UL746E tests for electrical safety and flammable safety of the coated electronics. For electrical safety, there is a battery of tests similar to IPC-CC-830B, but with a cycling current load to constantly measure the failure of the isolative properties of the coating. The flammability test uses the UL94 standard like IPC-CC-830B, which involves attempting to light the cured coating with an open flame and observing the sustainability of the flame. Once a coating has passed UL746E, it can be registered with UL and assigned a registration number. Products certified and registered to UL746E standards can include the UL symbol (which looks like a backward “UR”). To maintain the registration, a coating much be retested annually. Konform AR Acrylic Conformal Coating [https://www.chemtronics.com/konform-ar] and Konform SR Silicone Coating [https://www.chemtronics.com/konform-sr] are both certified and registered for UL746E.

You may be required to remove a conformal coating from the PCB to replace damaged components or other types of rework. The basic methods as cited by IPC are: Solvent Removal – While most conformal coatings can be dissolved in solvent, you should make sure the solvent won’t damage parts or components. Acrylic  is the fastest and easiest coating to remove with solvent. Silicone and urethane coatings will take more soak-time and will probably require brushing to fully remove the coating. Chemtronics offer Electro-Wash Two Step [https://www.chemtronics.com/electro-wash-two-step], which when heated, quickly removes acrylic, silicone and urethane coatings. CircuitWorks Conformal Coating Remover Pen [https://www.chemtronics.com/circuitworks-conformal-coating-remover-pen] is available to dissolve small areas of coating. Peeling – Some conformal coatings can be peeled from the circuit board. This is mainly a characteristic of some silicone conformal coatings and some flexible conformal coatings. Thermal/Burn‐through – A common technique of coating removal is to simply burn through the coating with a soldering iron as the board is reworked. This method works well with most forms of conformal coatings. Microblasting – Micro blasting removes the conformal coating by using a concentrated mix of soft abrasive and compressed air to abrade the coating. The process can be used to remove small areas of the conformal coating. It is most commonly used when removing Parylene and epoxy coatings. Grinding/Scraping – In this method, the conformal coating is removed by abrading the circuit board. This method is more effective with harder conformal coatings, such as parylene, epoxy and polyurethane. This method is only used as a method of last resort, as serious damage can be incurred.

Conformal coatings are generally very thinly applied, which provides the best protection possible, but minimizes heat entrapment, excess weight, and other issues.  Typically, conformal coating is applied between 1 to 5 mils (25 to 127 microns). There are four primary ways to measure the thickness of a conformal coating: Wet film thickness gauge - Wet film thickness can be measured directly by using gauges that use a series of notches and teeth, with each tooth having a calibrated length. The gauge is placed directly into the wet film, and the measurement is then multiplied by the percent solids of the coating to approximate dry coating thickness. Micrometer - Measurements are taken on the PCB at several locations before and after coating. 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.  This type of measurement is most accurate on hard coatings that don’t deform under pressure. Eddy current probes – A test probe is used to directly measure 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 open contact area available on the test sample. Without metal below the test area no measurements will be made. If the probe does not sit flat on the test area (i.e. a highly populated PCB), readings will be inaccurate. Ultrasonic thickness gauge – It measures coating thickness using ultrasonic waves, so does not need a metal backplane like eddy current probes. Thickness is calculated from the time sound waves take to travel from the transducer, through the coating, bounce off the surface of the board, 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.

Common application methods for conformal coatings: Manual spraying - For low volume production when capital equipment is not available, conformal coating can be applied by an aerosol can or handheld spray gun. This can be time-consuming and may need to be masked. It is also operator dependent, so variations are common from board to board. Automated spraying - Programmed spray systems can move PCBs on a conveyor under a reciprocating spray head. Selective coating – An automated spray system can actually be programmable, with a robotic spray nozzle applying the conformal coating to specific areas on the PCB. This is popular for high volume assembly because it eliminates the laborious masking process. Dipping – This is a popular conformal coating method for high volume assembly. Masking is usually necessary before PCBs are coated. Dipping is only practical when coating both sides of the board. Immersion speed, withdrawal speed, immersion time and viscosity determine the resulting film formation. Brushing - Brushing is used mostly for repair and rework. Conformal coating is brushed onto specific areas on the board, not generally the entire PCB. It is a low cost but labor intensive and highly variable method, so suited more for small production runs.

Typically, conformal coatings are not waterproof. These coatings are semi-permeable, so do not fully water-proof or seal the coated electronics. They protect from environmental exposure, improving the durability of the electronic device, while still being practical to apply and repair. The exception to this are parylene coatings, 2-part epoxy coatings, and potting compounds. Parylene coatings, epoxy coatings, and potting compounds are not offered by Chemtronics, so are outside of the scope of this article.