Parylene Coating Blog by Diamond-MT

6 Conformal Coating Defects (And How to Best Avoid Them)

Posted by Sean Horn on Fri, Mar 18, 2016 @ 10:21 AM

Conformal Coatings are polymeric materials used to protect circuitry, parts, and related components. They are most commonly used to protect printed circuit boards (PCBs) and electronic devices.

However, conformal coatings can be applied to a wide variety of materials, including metal, plastic, silicone, ceramics, glass, and even paper. We use the term "substrate" to refer to an object or material that's been coated with a conformal coating.

Conformal Coating Costs

The price for conformal coating varies depending on the material being used and the preparation required. Raw parylene dimer, for example, can cost between $100 and $10,000+ per pound.

In many cases, however, the biggest cost of conformal coating is manual labor. Tasks such as cleaning, masking, and inspection take time and must be completed by a well-trained technician.

This is important for conformal coatings, because repairing or reworking a failed coating is often a costly process.

The Cost of Defects and Rework

Depending on the material used, the process of removing a conformal coating might include using chemical solvents or thermal, mechanical, or microabrasion techniques.

Coatings made of acrylic, for example, are relatively easy to remove using chemical solvents. Parylene and epoxy coatings, on the other hand, are difficult to remove. They usually require mechanical or microabrasion methods

Whatever the material, the best way to keep your costs in line is to avoid these defects in the first place.

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6 Common Causes of Conformal Coating Defects and Rework

Here are the six most common causes of conformal coating defects we see, along with recommendations for how to avoid them:

1. Capillary Flow

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.

The most common causes for capillary flow are:

  • 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

To avoid these problems:

  • Make absolutely sure the substrate is clean before applying coating
  • Reduce the use of thinners to increase viscosity
  • Reduce the coating thickness
  • Try using a solvent-based coating instead of a water-based coating
  • Heat the board prior to coating to help it dry more quickly

2. Cracking

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 for cracking are:

  • High temperature during curing
  • Coating cured too quickly
  • Coating was applied too thick
  • 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 to specified thickness levels
  • Choose a coating with a wider effective temperature range
  • Choose a more flexible coating

3. De-Wetting

De-wetting occurs when a liquid conformal coating will not evenly coat the surface. De-wetting usually happens because of a nonionic contamination, often from the manufacturing, transport, or handling process.

The best way to prevent de-wetting is to ensure the substrate material is completely clean prior to applying conformal coating.

4. Delamination

Delamination happens when a conformal coating lifts away from the substrate, leaving the area below it exposed. Delamination is most often 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 help prevent delamination:

  • 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

5. Orange Peel

Orange peel happens when the coating is uneven and textured, often appearing dull, very similar to the skin of an orange. It is caused by:

  • Improper application of the coating materials
  • Not enough coating applied
  • Not enough time curing

To mitigate the effect of orange peel:

  • 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

6. 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. Common causes of bubbles and foam are:

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

To help prevent these problems:

  • Ensure the coating is applied only to recommended thickness
  • Apply several thin coats, allowing bubbles to dissipate between layers
  • Us 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 No. 1 Thing You Can Do to Prevent Coating Defects

The most effective way to prevent conformal coating failures is to ensure you’re working with an experienced conformal coating provider.

Experienced technicians will know how to apply a wide variety of coatings without causing any of the problems we’ve discussed in this article.

If you’d like to know more about conformal coating failures (and how to avoid them), download our white paper:

Conformal Coating Defects (And How to Avoid Them)

Tags: conformal coating, conformal coating rework, conformal coating costs, conformal coating bubbles, conformal coating defects