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Why you should select the conformal coating for PCBs with care

The components used in printed circuit boards (PCBs) are sensitive to a host of environmental conditions that can have far-reaching consequences and ruin the entire electronics assembly. The correct a

By Potshangbam July

Considering their affordability and reliability, conformal coatings are the most pragmatic choice to protect PCBs and electrically insulate the components from harsh environmental stress. They form a thin polymeric film and can be applied either by spray coating, brushing, dip coating, etc, and usually take up little space. Much like how the human skin functions, they shield the components from impairment by filtering out multiple harmful external elements like moisture, salt, dirt, fungi, etc. They significantly improve the performance and prolong the life span of PCBs. Despite their light application, conformal coatings are very effective, and prevent the premature breakdown of the components.

Benefits of conformal coatings
Conformal coatings are also a protection against extreme temperatures. The electronic parts in the PCBs are highly vulnerable to these conditions. When they are left exposed without any protection, the parts start to deteriorate at an alarming speed, and worse, the performance of the board is hindered considerably. In such cases, conformal coatings play a protective role. Besides, they are lightweight and flexible, and conform easily to the irregular contours of the board. They are applied as thin films, typically 50 µm thick, ranging from 25-75µm, and possess insulating properties that protect against shocks (thermal and mechanical), and simplify the complex process of encapsulating the circuit board. Chris Palin, EMEIA (Europe, Middle East, India, Africa) manager for HumiSeal Europe, says, “The basic purpose of a conformal coating is to extend the operating life of a PCB that functions in high moisture or corrosive chemical environments. The benefits are reduced field returns caused by failures related to the operating environment and increased product reliability. On the design side, conformal coatings allow an 80 per cent reduction in track spacing compared to an uncoated board.”


Types of coatings
There are different types of conformal coatings to choose from, such as acrylic resin, silicone resin, epoxy resin, urethane resin and parylene resin. Their functions and attributes are largely influenced by the chemistry of the conformal coating. Among these options, acrylic coatings are very popular as they do not involve a complex application process, and also possess good moisture and fungus resistant properties. In addition, they do not shrink while curing and are very affordable. However, acrylic coatings are best avoided for high temperature applications and when the curing period needs to be quick.

Epoxy conformal coatings are rigid. They have good abrasion and moisture resistance, and perform well in harsh conditions. Their drawbacks are that they are difficult to remove and require a soldering iron for rework or repair. Besides, they shrink during the curing process.
Silicone resin coatings are single-component compounds that provide excellent protection in extreme temperatures, though are not recommended for use in temperatures below 125 degrees Celsius.

Also, reworking or removing silicone coated circuits is challenging due to their strong chemical resistance.

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