What is Conformal Coating on a PCB?

Soldering PCB Basics 

What is conformal coating? What are the different types of conformal coatings used on a PCB?

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Editorial Team - PCB Directory

Aug 17, 2020

A conformal coating is a polymeric thin film applied over a PCB surface to protect the board and its components from corrosion and environmental contaminants. The performance of a PCB can be impacted by many external or environmental factors like moisture, extreme temperature, salt sprays, and chemicals. A conformal coating ensures the protection against PCB against these types of environmental contaminants and conditions.

By providing protection from contaminants and environmental factors, the conformal coatings prevent corrosion of conductors, solder joints and short circuits. They also provide durable insulation to help reduce thermal and mechanical stress on the components.

Conformal coatings are an important part of printed circuit boards and are used in a wide range of industries including aerospace, automotive, military, marine, lighting, consumer electronics and industrial.

The thickness of the conformal coating usually varies from 3-8 mil (0.075-0.2 mm). 

Types of PCB Conformal Coating

Based on the chemical composition, the conformal coatings are classified into five types. They are acrylic, epoxy, urethane, silicone, and paraxylylene. The selection of a particular coating is based on the application of PCB and electronics requirements. Protecting a PCB is important, but the selection of appropriate material is essential.

Acrylic Conformal Coating:

Acrylic Resins(AR) are preformed acrylic polymers that are dissolved in a solvent and used to coat a PCB surface. The acrylic conformal coating is applied by using a hand brush, sprayed, or dipping the board in an acrylic resin coating. This is the most used conformal coating for PCBs.

Polyurethane Conformal Coating:

Polyurethane (UR) coatings offer excellent protection against the effects of chemicals, humidity, and abrasion. Polyurethane (UR) conformal coating is easy to apply but difficult to remove. Spot repairability (by thermal or soldering iron means) is not recommended as it will result in the emission of toxic gasses (isocyanate).

Epoxy (Type ER):

Epoxy offers excellent performance as a conformal coating in harsh environments. It is easy to apply, but can damage the circuit when being removed. Epoxy is normally available as a two-component thermosetting mixture. Single-part compounds are cured by thermal or by UV exposure.

Silicones (Type SR):

Silicone (type SR) conformal coatings are used in high-temperature environments. This coating is easy to apply and has low toxicity and provide protection against abrasion, moisture, and humidity. Silicone resin coatings are single-component compounds.

Paraxylylene: 

A chemical vapor deposition process is used to apply the paraxylylene coating on PCBs. Paraxylylene will turn into a gas when heated. After the cooling process, it is placed into a chamber, where it polymerizes and changes into a film. This film is then applied over the surface of PCBs.

Quick Reference Guide for PCB Conformal Coatings

Characteristic 

Acrylic (AR)

Polyurethane (UR)

Epoxy (ER)

Silicone (SR)

Humidity Resistance

Excellent

Excellent

Good

Excellent

Abrasion Resistance

Fair

Good

Excellent

Good

Acid Resistance

Good

Excellent

Excellent

Good

Alkali Resistance

Fair

Excellent

Excellent

Good

Stress on Coated Components Resulting from Temperature Variation

High

High

High

Low

Temperature Resistance

Good

Fair

Fair

Excellent

Organic Solvent Resistance

Poor

Excellent

Excellent

Good

Dielectric Constant @ 23°C, 1 MHz

2.2–3.2

4.2–5.2

3.3–4.0

2.6–2.7

Repairability

Difficult

Difficult

Difficult

Easy

The type of conformal coating is decided based on the thickness of the coating needed, the area that needs to be covered and how well the coating sticks to the board and its components.

How is Conformal Coating applied to a PCB?

  • Hand coating by using a Brush
  • Hand coating by using an Aerosol
  • Hand coating by using an atomized spray gun
  • Automated Dip Coating
  • Using a Selective Coating Machine

Conformal Coating Standards:

  1. MIL-I-46058C – Common standard conformal coating materials
  2. Def Stan 59/47 - It is similar to MIL-I_46058C. The British military is using this standard for coating high-end electronics.
  3. IEC 61086 - Standard based on the supplier self-certification
  4. IPC-CC-830B - It is very similar to MIL-I-46058C. This standard was introduced to act as an alternative to the MIL-spec.   
  5. IPC-CC-830B - The standard used for conformal coating material qualification.
  6. UL94V0 - The standard is used to judge the flammability or self-extinguishing behavior of conformal coating.
  7. UL746E - This standard provides the series of dielectric breakdown evaluation and UL94 flammability testing.


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