Does My Product Need to Be Clean Before Conformal Coating?

Once you’ve decided to use conformal coating for your device, a question that often comes up is:

• Does my product need to be cleaned before conformal coating?

The short answer is: yes. Your device absolutely needs to be cleaned. But there’s also a broader question we recommend you ask:

• What’s everything I need to know to prepare my device for conformal coating success?

6 Essentials for Conformal Coating Success

Understanding a few fundamentals about conformal coating will help you be successful. Here are a few of the major topics you should be familiar with:

1. Common Contaminants: Ionic vs. Non-Ionic

Your 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 won’t short your circuit boards, but they will prevent your coating from adhering to the substrate beneath.

2. Where Contaminants Come From

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

To identify ionic contaminants, we use the Resistivity of Solvent Extract (ROSE) method. 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’s a sign the board is contaminated with ionic substances.

To identify non-ionic contaminants, we can use a residue test with special glass slides coated with aluminum. 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. And nothing takes more labor than rework.

6. Who “Owns” Responsibility for Conformal Coating Preparation

When choosing what company to work with for conformal coating, be sure to ask who’s responsible for preparing your device for coating.

The five types of conformal coating all have unique preparation procedures. A good conformal coating partner will help you perform much of the preparation work that needs to be done prior to coating.

If you plan to perform conformal coating in-house, we still recommend working with a conformal coating company.

You’ll need employees who are highly skilled at applying conformal coatings. A conformal coating partner with deep knowledge of the different coating types will be able to train and equip your engineering team to be successful.

Other Than Cleaning, What Other Preparation Work Needs to Be Done?

There are two common preparation steps in addition to cleaning you’ll need to think about: masking and pre-coating.

1. 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.

2. 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 you need for the device.

How Can I Be Sure My Device Are Properly Cleaned and Prepared for Conformal Coating?

A good conformal coating provider will know how to prepare your device for coating. However, some providers are more experienced than others.

Make sure you’re working with a company that knows how to prepare your device for coating. Here are a few questions to ask:

• Does the company have technicians who are experienced preparing products for the type of coating you want to use?
• Have they supplied clients from a wide variety of industries?
• Do they have a track record of satisfied clients?

These are usually good signs that a company can meet your conformal coating needs.