Conformal Coating

Measuring the thickness of conformal coating is essential to ensure proper application and adherence to the specified requirements. There are several methods used to measure the thickness, and the choice of method may depend on the type of conformal coating and the level of accuracy required. Here are some common methods:

• Calibrated Thickness Gauges: These are handheld devices that use non-destructive methods to measure the thickness of the coating. They typically use magnetic induction or eddy current principles to determine the distance between the probe and the substrate, which correlates to the coating thickness.
• Cross-Sectioning and Imaging: This method involves cutting a sample of the coated surface and examining it under a microscope. The thickness can be determined by measuring the height of the coating in the cross-sectional view.
• Ultrasonic Testing: Ultrasonic thickness gauges use sound waves to measure the thickness of the coating. The device emits ultrasonic pulses, and the time taken for the waves to bounce back from the substrate-coating interface is used to calculate the thickness.
• X-ray Fluorescence (XRF): XRF can be used to measure the thickness of conformal coatings. It involves analyzing the intensity of X-rays emitted from the coating material due to bombardment by X-rays. The thickness is determined based on the X-ray penetration depth.
• Electrical Capacitance Method: This method utilizes the change in capacitance between electrodes positioned on opposite sides of the coated surface. The coating's thickness affects the capacitance, allowing for indirect measurement.
• Weight Difference Method: For some applications, the thickness can be estimated by measuring the weight difference of a substrate before and after coating. This method is more suitable for thicker coatings.

Before choosing a specific method, it is important to consider factors like the type of coating material, the size and shape of the substrate, the required measurement accuracy, and whether destructive or non-destructive testing is acceptable for your specific application. Additionally, it is essential to calibrate the measurement equipment and follow any manufacturer's guidelines for accurate and reliable results.

IPC-CC-830 is an industry standard focused on conformal coatings for printed wiring assemblies, while MIL-I-46058 was a military specification for electrical insulating compounds used on printed circuit assemblies, which has since been replaced by more comprehensive standards like MIL-STD-883 or adopted industry standards like IPC-CC-830. 

Yes, all our coatings meet IPC-CC-830 standards. IPC-CC-830 defines the requirements for electrical insulating compounds, commonly known as conformal coatings, used to protect printed circuit boards and electronic assemblies from environmental factors such as moisture, dust, chemicals, and temperature extremes. The standard outlines the testing methods and performance criteria for these insulating compounds to meet industry requirements. It includes specifications for material properties, application procedures, and performance testing for conformal coatings. You might see IPC-CC-830B, 830C, etc. The extra letter refers to the revision. The changes between revisions general do not affect all types of coatings, so coatings are often not retested to the latest standard.

1. Prepare the PCB: Ensure the PCB is clean, dry, and free from any contaminants. Clean it using an appropriate PCB cleaning solution or isopropyl alcohol to remove any dirt, flux residues, or other impurities that could interfere with the coating adhesion.
2. Select the Conformal Coating: There are various types of conformal coatings available, such as acrylic (AR), silicone (SR), and urethane (UR). Choose a coating that suits your specific application and environmental conditions.
3. Choose the Application Method: Conformal coatings can be applied using different methods, including spraying, dipping, brushing, or dispensing. The method you choose depends on the type of coating and your equipment's capabilities.
4. Spraying: Suitable for large-scale production. It requires a specialized spray booth and spray gun to evenly apply the coating.
5. Dipping: Involves immersing the entire PCB into a reservoir of conformal coating. This method provides uniform coverage but may trap air bubbles.
6. Brushing: Manual application using a brush is suitable for small-scale production or touch-ups.
7. Masking Uncoated Areas (optional): If certain components or areas on the PCB should remain uncoated (e.g., connectors, switches, or heat sinks), use a masking material (e.g., Kapton tape or liquid latex) to protect those areas during the coating process.
8. Dispensing: Applying the coating using a controlled dispensing system, which is useful for selective coating on specific areas. Make sure to apply a thin and even layer, avoiding excessive buildup, which could lead to uneven coating thickness or encapsulation of sensitive components.
9. Curing and Drying: Allow the applied conformal coating to cure and dry as per the manufacturer's instructions. This typically involves leaving the coated PCB in a controlled environment, such as an oven, for the recommended duration.
10. Post-Coating Inspection: After the coating has cured, inspect the PCB to ensure proper coverage and to check for any defects, such as bubbles, pinholes, or insufficient coating.

The drying and curing time of conformal coatings can vary depending on the type of coating used and the environmental conditions. Conformal coatings are applied to protect electronic components from moisture, dust, chemicals, and other contaminants. Acrylic (AR) coatings typically dry quickly, and the initial tack-free time can be anywhere from a few minutes to an hour. However, the full curing time can range from 24 to 72 hours, depending on the thickness of the coating and environmental conditions like temperature and humidity. Silicone (SR) coatings usually have a longer tack-free time, which can take several hours. The complete curing time can vary from 12 hours to a few days. Urethane (UR) coatings have a relatively fast drying time, often within a few minutes. However, full curing may take 24 to 48 hours.

Conformal coatings are typically thin layers of material applied to electronic components or circuit boards to protect them from environmental factors like moisture, dust, chemicals, and mild mechanical stress. They are commonly used in various industries, including electronics, automotive, aerospace, medical electronics and more.

For more information, check out "The Ultimate Guide to Conformal Coating".

While conformal coatings can offer some level of protection against ESD by providing a barrier between the sensitive electronic components and the surrounding environment, they are not specifically designed as ESD control measures.

Full cure is when it meets all the final specifications. There might be some out-gassing, but it will be as hard as it is going to be, and adhesion is as good as it gets.  Tack-free is as the name suggests, not sticky so you can move it along the assembly process.

Every organization using hazardous chemicals within their facility has the responsibility to equip their facility and personnel to maintain exposure levels below the TLV. Personal monitoring badges can be used to measure exposure of a specific material. Then, depending on the threshold limit and the application, exposure can be controlled with PPE like masks, face shields, respirators, and even coveralls. If they don’t reduce exposure below the recommended limit, you will need to consider a special ventilation hood or even containment booth. As you can see, as the exposure limit gets down to a certain level, the equipment required to safely use the solvent can get impractical. At that point, your best option is to consider a safer alternative.

The dielectric strength is material intrinsic property and withstand voltage is surface property which depend on thickness of the material. They can be slightly different for thicker materials, but for conformal coating, the two numbers should be very close or the same. That is because we test coating at 3-5 mils thickness, calculate, then report the value per mil.

Conformal coating with UV tracer can be inspected with any typical UV lamp which has wavelength of 320-380 nm.

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 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 and Konform SR Silicone Coating 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.

Choose the coating resin based on the protective properties you need:

Acrylic Resin (AR) – AR coatings are relatively economical, provide good overall protection, and are easy to apply and repair. They have high dielectric strength, and fair moisture and abrasion resistance. Acrylic coatings are easily and quickly removed by a variety of solvents, often without the need of agitation. This makes rework and even field repair practical and economical. Acrylic coatings are not effective at protecting against solvents and solvent vapors, like jet fuel fumes common in aerospace applications. 

Silicone Resin (SR) – Silicone conformal coating provides excellent protection in a very wide temperature range. SR provides good chemical resistance, moisture and salt spray resistance, and is very flexible. Silicone conformal coating isn’t abrasion resistant because of its rubbery nature, but that property does make it resilient against vibrational stresses. Silicone coatings are commonly used in high humidity environments, like outdoor signage. Removal can be challenging, requiring specialized solvents, long soak time, and agitation like from a brush or an ultrasonic bath.