Parylene Coating Blog by Diamond-MT

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

Posted by Sean Horn on Fri, May 13, 2016 @ 07:30 AM

How do you ensure that a potential conformal coating provider has the professional credentials and expertise necessary to avoid costly mistakes?

The first step is to make sure the provider you’re considering has expertise working with all five of the major types of conformal coating:

  1. Acrylic
  2. Urethane
  3. Silicone
  4. Epoxy
  5. Parylene

A company with this type of broad experience will be able to help you find the right coating solution for your device, not just the coating solution they prefer.questions.png

Step two is to ask the coating provider a series of basic questions about their process.

Here are nine questions we recommend you ask:

1. What Is Your Process for Preventing Ionic and Nonionic Contaminants?

Ionic contaminants can’t 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.

Ionic contaminants can also cause corrosion or dendrites, small metal outgrowths from a circuit component that can damage the conformal coating and cause short circuits.

Nonionic contaminants sit on top of a device prior to coating and can usually be seen visually. If not removed, nonionic contaminants can prevent coatings from adhering to the substrate.

2. 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.

Your coating provider should also thoroughly clean your device using a solvent or a surfactant in order to remove nonionic contaminants.

3. After Cleaning, Do You Test the Device for Remaining Contaminants?

We use the Resistivity of Solvent Extract (ROSE) method, which works as follows:

  • Measure the solution’s conductivity.
  • Use the solution to wash the substrate being tested.
  • Measure the solution’s conductivity post-wash.

If the solution’s conductivity goes up after the wash, it’s a sign the device has an ionic contaminant.

We also use a residue test with special glass slides coated with aluminum. That test is administered as follows:

  • 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’s a sign of nonionic 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 infrared light to identify any remaining residue.

4. 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 even be left completely uncoated.

It is often caused by:

  • Low viscosity of the coating material
  • High surface tension of the coating material
  • Too much coating material applied to the substrate
  • Low surface energy of the substrate

Capillary flow is best prevented by:

  • Making sure the substrate is absolutely 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
  • Heating the board prior to coating to help it dry more quickly

5. How Do You Prevent Cracking of Your Conformal Coating?

Cracking happens when a smooth surface of coating fractures into sections. The cracks between the sections leave the area below exposed to potential contaminants.

Common causes include:

  • Temperature too high during curing process
  • Coating cured too quickly
  • Film was applied too thickly
  • Operating temperature too high or too 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
  • Choose a coating with a wider effective temperature range
  • Choose a more flexible coating

6. 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 nonionic contamination, often from the manufacturing, transport, or handling process.

To help prevent it, ensure the substrate is completely clean prior to applying conformal coating.

7. How Do You Prevent Delamination?

Delamination happens when a conformal coating lifts from the surface of the substrate, leaving the area below it exposed.

It is usually caused by:

  • Contaminants on the surface of the substrate
  • Lack of compatibility between the coating and the substrate material
  • Moisture
  • Improper curing of the conformal coating

To prevent it:

  • Reduce the coating’s thickness
  • Reduce force drying
  • Clean the board thoroughly prior to coating it
  • Choose a different coating material
  • Apply a “primer” material known to bond well with the substrate and the material you’re using for your conformal coating

8. 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
  • Not enough coating applied
  • Not enough time curing

To prevent it:

  • Reduce the viscosity of the coating
  • Apply the coating to the recommended thickness
  • Increase the “flash off time” for the solvents to evaporate before turning up the temperature to speed the curing process

9. 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.

It happens when:

  • Coating applied to thick
  • The coating is too viscous
  • The coating is applied with incorrect equipment settings or pressure

To prevent it:

  • Ensure the coating is applied only to recommended thickness
  • Apply several thin coats, allowing bubbled to dissipate between layers
  • Use a lower viscosity version of the conformal coating
  • If brush coating, blend the coating so it flows easily into all areas of the substrate

The Value of Asking These Questions

The above questions cover the most common problems that happen during the conformal coating process.

If you find a provider with expertise applying all five coatings, and the provider can confidently answer the questions we’ve listed here, it’s a good sign they can help you get the protection you need for your device.

To learn more about conformal coating defeects, why they happen and how to prevent them, download our whitepaper:

Conformal Coating Defects (And How to Avoid Them)

Tags: conformal coating, conformal coating service, conformal coating issues, conformal coating bubbles, conformal coating defects