Nine Questions to Ask About Contamination, Cleaning, Failures, and Defects in Conformal Coating

To ensure a conformal coating provider has the credentials and expertise necessary for a specific application, certain criteria must be considered. The first step is to ensure the provider is knowledgeable about working with the five primary types of conformal coating: acrylics, urethanes, silicones, epoxies and Parylene. A company with this broad experience will provide the appropriate coating solution for an application not just the coating solution they prefer.

The next step is to ask the provider the following nine questions: 

What is your process for preventing ionic and non-ionic contaminants? 
Ionic contaminants cannot be seen visually, but they can turn the coating layer from non-conductive into a massive short circuit that connects all the components underneath it. They can cause corrosion or dendrites, which are small, metal outgrowths from a circuit component that can damage the conformal coating and cause short circuits.

Non-ionic contaminants sit on top of a device prior to coating and can usually be seen visually. If not removed, they can prevent the coating from adhering to the substrate.

How do you clean circuit boards and other devices prior to conformal coating?
Water will remove most ionic contaminants, but the water must be pure; otherwise, it can leave behind salt and other compounds when it evaporates. A conformal coating provider should thoroughly clean the application using a solvent or a surfactant to remove any non-ionic contaminants. 

After cleaning, do you test the device for remaining contaminants? 
The Resistivity of Solvent Extract (ROSE) method measures the solution’s conductivity, uses the solution to wash the substrate being tested and measures the solution’s conductivity post-wash. If the solution’s conductivity goes up after the wash, it is a sign the device has an ionic contaminant. A residue test with special glass slides coated with aluminum can then be performed:

• Rinse the substrate with a solution containing acetonitrile onto the aluminum-coated slides
• Rinse the substrate onto the slide upon evaporation of the solvent
• Repeat six times
• Inspect the slides for residue

If anything appears, it is a sign of non-ionic contamination. If nothing appears, the technician will inspect the slide under a microscope. If nothing appears under the microscope, the final step is to inspect the slides using infrared light to identify any remaining residue.

How do you avoid capillary flow for the conformal coatings you apply?
Capillary flow is a type of coating migration that happens when a liquid conformal coating pulls away or “flows” from one area of the substrate to another; this usually happens during the drying and curing process, resulting in a patchy, uneven finish. Some areas of the substrate may be left completely uncoated; this is often caused by the low viscosity or high-surface tension of the coating material, too much coating material being applied to the substrate, or any low-surface energy of the substrate.

Capillary flow is best prevented by making sure the substrate is clean prior to application of the coating; reducing the use of thinners to increase viscosity; reducing the coating thickness; trying a solvent-based coating instead of a water-based coating; and, heating the board prior to coating to help it dry more quickly.

How do you prevent the cracking of your conformal coating?
Cracking occurs when a smooth surface of coating fractures into sections. The cracks between the sections leave the area below exposed to potential contaminants. Common causes may include the temperature being too high during the curing process; the coating cured too quickly; the film being applied too thickly; and, the operating temperature being too high or low for coating specifications. To prevent cracking, lower the cure temperature, allow additional drying time at room temperature, apply the coating only to specified thickness levels, and choose a coating with a wider-effective temperature range as well as flexibility.

How do you prevent de-wetting of your coatings?
De-wetting occurs when a liquid conformal coating will not evenly coat the surface. It is most commonly caused by non-ionic contamination, often from the manufacturing, transport or handling process. Ensure the substrate is completely clean prior to applying the conformal coating.

How do you prevent delamination?
Delamination happens when a conformal coating lifts from the surface of the substrate, leaving the area below exposed. It is usually caused by contaminants on the surface of the substrate, lack of compatibility between the coating and the substrate material, moisture, and improper curing of the conformal coating. To prevent delamination, reduce the coating’s thickness and force drying, clean the board thoroughly prior to coating, choose a different coating material, and apply a primer material known to bond well with the substrate and the conformal coating.

How do you prevent orange peel?
Orange peel happens when the coating is uneven and textured, often appearing dull—very similar to the skin of an orange. Common causes include improper application of the coating materials as well as not enough coating applied or curing time. To prevent orange peel, reduce the viscosity of the coating, apply the coating to the recommended thickness, and increase the “flash-off time” for the solvents to evaporate before turning up the temperature to speed the curing process.

How do you prevent bubbles, pinholes and foam? 
Bubbles happen when pockets of air become trapped under a layer of conformal coating. Pinholes occur when a bubble bursts through the coating layer. Foam is a form of extreme bubbling that happens when the coating is applied too thick or viscous or is applied with incorrect equipment settings or pressure.

To prevent foam, ensure the coating is applied only to the recommended thickness; apply several thin coats, allowing bubbles to dissipate between layers; use a lower viscosity version of the conformal coating; and, if brush coating, blend the coating so it flows easily into all areas of the substrate.

If you find a provider with expertise in applying all five coatings, and the provider can confidently answer these nine questions, it is a good sign they can provide the conformal coating protection needed for your application.