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Cova Scientific By Cova Scientific • February 26, 2016

Conformal Coating Fundamentals

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A conformal coating is a thin film used to enhance the performance of electronic assemblies. Conformal coatings typically protect components of printed circuit boards from dust, dirt, moisture, chemicals, and temperature extremes that would damage or fail boards left exposed to these conditions.

This post is intended to give an overview of conformal coatings. Specifically we'll address the following questions:

  • What are conformal coatings and why are they important?
  • What are the different types of conformal coatings?
  • How are conformal coatings typically cured?
  • How are conformal coatings typically applied?

 

What Are Conformal Coatings And Why Are They Important?

Printed circuit boards (PCBs) are expected to perform in a huge range of environments and are no longer limited to machine rooms. As a result, PCBs must be designed to withstand a vast array of conditions including:

  • Water
  • Dust
  • Mechanical stress
  • Shock
  • Vibration

Conformal coatings have been developed to protect circuit boards from such potential threats. They are so named because they “conform” to the dimensions of their substrate to allow efficient protection. This conformity is normally achieved as a liquid and a curing process is used to solidify the protective layer.

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Many choices exist for conformal coatings, including choices of base chemistry, application method, and curing process. However, all conformal coatings should provide at a minimum the following properties:

  • A protective layer against dust, water, and other airborne contaminates
  • Electrical insulation to prevent electrical shortages
  • Structure to protect against physical and thermal stresses

However, many additionally properties may be chosen on an application basis. Depending on the intended environment and operation conditions a conformal coating may provide:

  • Extreme temprature (both high and low) durability
  • Flame resistance
  • Abrasion resistance
  • Re-workability (if underlying components may need to be later repaired)
  • And more...

 

What Are The Different Types Of Conformal Coatings?

Coatings can be divided in various types depending on their base chemistry: Acrylics, Polyurethanes, Epoxies, Silicones, and Parylenes.

Acrylics

  • Reasonable priced
  • Easy to apply and cure via a quick solvent drying process
  • Good environmental protection and moisture resistant
  • Great for applications that will likely be exposed to water
  • Able to retain clarity and can resist darkening over time
  • Have limited resistance to solvents making them a good choice for applications that might require re-work

Polyurethanes

  • More difficult to remove than acrylic alternatives, limiting re-workability
  • Excellent chemical and moisture resistance
  • Especially suitable for salt spray protection or applications where solvent resistance is required
  • Superior protection in harsh environments
  • Very good abrasion resistance
  • Flexible even at low temperatures

Epoxies

  • Good dielectric properties
  • Only to about 150 degrees Celsius
  • Excellent chemical and abrasion  resistance
  • Relatively short cure time
  • Can be difficult to remove for repairs
  • Care must also be taken since there is a possibility of shrinkage with some formulations and therefore could damage delicate parts.

Silicones

  • The most difficult to remove, abrasion is the only process that can remove them
  • These coatings are ideal for use in operating temperatures that can reach up to 200 degrees Celsius
  • Highly flexible
  • Provides protection against mechanical shock
  • Good moisture and UV resistance
  • Suited for use in the oil and gas industry because of their resistance to specific chemicals.

Parylenes (polypara-xylenes)

  • Excellent uniformity
  • Outstanding protection from a number of damaging elements
  • Moisture, chemical, and temperature resistant
  • High dielectric strength
  • Requires special application of vapor deposition polymerization. Before applying, it must be ensured that the surface is perfectly clean.

 

How Are Conformal Coatings Typically Cured?

Different curing methods can be used on many types of conformal coating, each offering different advantages and disadvantages.

  • Solvent-Evaporation - solvent-evaporation curing can be used to cure some acrylics, epoxies, and urethanes, although they may take a longer time, particularly urethanes. Although more costly, water-based and other non-VOC coatings or low-VOC coatings are also available. Meanwhile, for standard solvent-based acrylics, the normal process of curing is usually air-drying or film forming, but this is not advisable if speed is required.
  • Heat Curing - Heat curing is a faster alternative to solvent-evaporation. While this is an option for some coatings, heat curing requires a specialized setup to control temperature. This process is also more prone to shrinkage.
  • UV Curing - The quickest cure time is afforded by UV curing and this can be achieved in a matter of seconds. When production speed is of paramount importance like when there is a high volume of demand from users, this is considered as the most viable option. UV curing is also in demand because of the level of processing ease, its environmental friendliness, and thermal cycling resistance.

 

How Are Conformal Coatings Typically Applied?

A conformal coating, to be effective, must completely cover the circuit board. It must also provide good coverage of sharp edges and other contours. To ensure that the conformal coating is effective, the application method has to be taken into consideration.

There are four main ways of applying a conformal coating:

  1. Dipping - which is applied on materials that cannot be cured quickly by moisture, light, or oxidation
  2. Selective robotic coating - which is used for high volume applications
  3. Spraying - which involves hand spray using spray booth or aerosol can
  4. Brushing - which is usually done by proficient and skilled operators

When it comes to protecting printed circuit boards, the best type of conformal coating to be used, depends majoritively on the environment where the boards are placed and the kind of protection required by these boards.

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