Conformal Coating
Contents
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
- Conformal coatings, though not airtight, slow down the diffusion of moisture and corrosive vapours before reaching the electronics, slowing down the corrosion of PCB assemblies. [2]
- Conformal coatings help to protect PCBs and electronics from biological contaminants, which cause degradation. [2]
- Structural Integrity: It is easy to see that a conformal coating provides structural integrity. It also provides protection to solder joint connections against mechanical vibration and shock. [1]
- Tin Whisker Mitigation: Tin whiskers are thread-like protrusions of tin that have been observed growing on surfaces of pure tin in electronics. These can cause problems if they break away from the base surface since they then act like conductive particles which can short connections. Conformal coating slows down the formation of tin whiskers when compared to uncoated specimens. Moreover they protect electronics from tin whiskers that have penetrated the coating, by serving as a dielectric layer. [1]
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
- 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 significant difference in the thermal expansion coefficients of the coating and the substrate. [1]
Precautions and Advice
- Spray coating is easy, practical and thus, the most widely used method
- Generally, the best protection is obtained by a 2.0 mil thickness (+/-0.5 mil) [3]
While there are risks involved in conformal coating electronic assemblies, they can be mitigated.
We can avoid undue stresses which cause damage to components and assemblies by applying conformal coating of thickness within a recommended range.
Proper handling, electrical grounding, packaging, and transport of assemblies before, during, and after conformal coating will help to prevent mechanical and electrical damage to PCB assemblies.
Companies providing conformal coating services in India are:
- indiamart
- mectronics
- pcbconformalcoating.net
- conformalcoatinghelp.com
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