Conformal Coating

Introduction

A thin electrically non-conductive protective coating that conforms to the contours of the PCBs or electronic assemblies. The primary purpose for conformal coating of PCBs in space applications is the protection of electronic assemblies from environmental contaminants including particles, dust, dirt, moisture, corrosive vapors, and fungus.

Why use Conformal Coating?

Many electronic assemblies are tightly packed with components, thus having small spaces between conductors (e.g., solder pads and leads). Particles, that settle onto electronic modules during production, ground testing, and launch or buoyant particles in the zero-gravity space environment on orbit that contact and span two or more closely spaced conductors may result in a short circuit, leading to potentially serious electrical failures in the satellite system. Despite efforts to keep satellite assemblies clean and free from contaminants, the possibility that stray particles, metal shavings, dust, and dirt will be present always exists.

Benefits

• Moisture and corrosive vapors over time may result in corrosion of metals in PCB assemblies. Though conformal coatings are not airtight and thus do not seal electronics from moisture and solvent vapors, they do act as barriers to slow down the diffusion of moisture and corrosive vapors through the polymer before reaching electronics.
• Biological contaminants such as fungus may also lead to degradation; conformal coatings help to protect electronics from biological contaminants.
• Structural Integrity: It is intuitive that a conformal coating applied over electronic components on PCBs provides structural integrity and protection for solder joint connections against mechanical shock and vibration.
• Tin Whisker Mitigation: Tin whiskers are elongated thread-like protrusions of tin that are observed growing on surfaces of pure tin in electronics. These thin strands can break away from the base material and cause problems such as electrical shorts and arcing in electronics. The conformal coating slow downs the formation of tin whiskers compared to uncoated specimens. Moreover they serve as a dielectric layer to protect electronics from tin whiskers that have penetrated the coating.

Risks

Some risks include:

• Mechanical damage of electrical assemblies and electrical damage due to electrostatic discharge (ESD) incurred during handling
• Electrical failures due to stresses induced by the coating on solder joints
• One well-known risk regarding conformal coatings is the potential for stresses to be induced by the coating on electronic components during thermal cycling. This is especially true if the coating is very thick (> 5 mils for many coatings) and if there is a significant coefficient of thermal expansion mismatch between the coating and the substrate materials being coated.

Precautions and Advice

• Spray coating is practical and is the most widely used method
• Generally, a 2.0 mil thickness (+/-0.5 mil) will give the best protection
• Rinse thoroughly with clean isopropyl alcohol or other cleaning solvent

While there are risks involved in conformal coating electronic assemblies, they can be mitigated.

The majority of risk is associated with general handling, transport, and processing of PCBs, factors that are present in other production processes and are not specific to conformal coating. Appropriate handling, electrical grounding, packaging, and transport of assemblies before, during, and after coating will help to prevent mechanical and electrical damage to PCB assemblies.
We can avoid undue stresses which cause damage to components and assemblies by applying conformal coating of thickness within a recommended range.

Companies providing conformal coating services in India are:

• indiamart
• mectronics
• pcbconformalcoating.net
• conformalcoatinghelp.com

References

• http://www.techni-tool.com/site/ARTICLE_LIBRARY/Chemtronics%20-%20Conformal%20Coatings%20Guide.pdf
• http://www.humiseal.com/products/acrylics/ - Acrylic conformal coating contacts and info.
• https://www.smta.org/chapters/files/Central-Texas_Coatings_and_Pottings-CTEA.pdf
• https://www.electrolube.com/pdf/conformal-coatings-brochure-electrolube.pdf

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