Introduction
According to Wikipedia, a conformal coating is "a thin polymeric film which ‘conforms’ to the contours of a printed circuit board to protect the board's components". The main purpose for conformal coating of PCBs in satellites or other space applications is the protection of electronic assemblies and components from environmental contaminants which include particles, dust, dirt, moisture, corrosive vapors, and fungus. [1]
Why use Conformal Coating?
Because there are mass and volume constraints in space, the packing of components is very tight in many electronic assemblies, which results in very small space between conductors (like solder pads, or leads). If a particle settles in this small space, it may lead to a short circuit. Particles can settle on electronic modules during manufacture, soldering, ground testing, launch or buoyant particles in the zero-gravity space environment in orbit. The short circuit can lead to potentially catastrophic electrical failures in a satellite. Inspite of all efforts to keep satellite assemblies clean and free from contaminants, the possibility of stray particles being present always exists. [1]
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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