Does My Product Need to Be Clean Before Conformal Coating?

Once you have decided to utilize conformal coatings to protect your device, there are two common questions that often arise:

• Does my product need to be cleaned before conformal coating?
• What do I need to know to prepare my device for conformal coating success?

Understanding some basic knowledge about conformal coatings will help to answer these questions so you successfully find the best protection for your component.

5 Essentials for Conformal Coating Success

1. Common Contaminants: Ionic vs. Non-Ionic
A device might become contaminated for a variety of reasons. The manufacturing process, transport, handling and storage can all introduce different types of contaminants. There are two broad categories of contaminants, each with a different impact on the conformal coating process.

Ionic contaminates break down into separate molecules inside the coating itself. This process can make the coating into a conductor, potentially creating a massive short-circuit between every element beneath it. Ionic contaminants can also lead to corrosion or small vertical defects in the coating known as dendrites.

Non-ionic contaminants will not short circuit boards, but they will prevent the coating from adhering to the substrate beneath.

2. Origin of Contaminants
Many ionic contaminants come from the manufacturing process, but sweat from workers can also cause ionic contamination.

Non-ionic contaminants are usually organic materials such as grease, oil or hand lotion. Rosin and silicone can also act as non-ionic contaminants.

3. Cleaning Techniques
Ionic contaminants are often removed with water. Care must be taken to use pure water during the cleaning process. Impure water will make the problem worse by depositing ionic compounds, such as salt, onto the substrate after the water evaporates.

Non-ionic contaminants can be removed using solvents or surfactants. A thorough cleaning is usually good enough to eliminate these types of contaminants.

4. Testing for Contamination
The Resistivity of Solvent Extract (ROSE) method can be used to identify. ionic contaminants The ROSE method begins by measuring how conductive a solution is. Then the solution is used to wash the substrate being tested.

After the wash, the solution is tested again for contamination. If the solution’s conductivity goes up, it is an indication that the board is contaminated with ionic substances.

To identify non-ionic contaminants, a residue test with special glass slides coated with aluminum can be used. The test begins by rinsing the substrate with a solution containing acetonitrile onto the aluminum-coated slides. When the solvent evaporates, the substrate is rinsed onto the slide again. This cycle is repeated, usually six times, and then a technician inspects the slides for residue.

If anything appears, it’s a sign of non-ionic contamination. If nothing appears, the technician will then inspect the slide under a microscope. If nothing appears under the microscope, the final step is to inspect the slides using a special spectroscopic process that uses infrared light to identify any remaining residue on the slide.

5. Consequences of Poor Preparation of a Device Before Conformal Coating
If your device isn’t clean before coating, severe degradation of insulation resistance and dielectric strength can be the result.

In addition, labor, not materials, is often the biggest factor that determines the cost of conformal coating. Rework is labor intensive and can be expensive.

What Other Preparation Work Needs to Be Done?

There are two common preparation steps in addition to cleaning to consider: masking and pre-coating.

Masking
Most conformal coating projects require at least a bit of masking. Devices often have areas that need to remain uncoated. Examples include connection points that will connect a circuit board to the rest of a device.

A-174 Silane Pre-coating
When using Parylene coating, it might be necessary to prepare the surface of the device using A-174 Silane. This will help the Parylene coating “stick” to the item being coated, helping to ensure the protection needed for the device.