Conformal Coatings

Conformal coating is a protective coating or polymer film 25-75µm thick (50µm typical) that ‘conforms’ to the circuit board topology. Its purpose is to protect electronic circuits from harsh environments that may contain high humidity, a range of airborne contaminants and varying temperatures. By being electrically insulating, it maintains long-term surface insulation resistance (SIR) levels and thus ensures the operational integrity of the assembly. It also provides a barrier to air-borne contaminants from the operating environment, such as salt-spray, thus preventing corrosion.



Conformal coatings are a breathable protective layer that will protect against the environment the PCA is exposed to by filtering out the airborne contaminants and maintaining long-term surface insulation resistance but will also allow any moisture trapped in the circuit board to escape.



The concept of a conformal coating of electronics was originally proposed and developed through research to meet the demanding requirements of the military decades earlier. HumiSeal was a part of that early research and has remained a pioneer and leader for over 60 years as the technology has expanded to an array of industries today including:



• Aerospace
• Automotive, including Hybrid and EV
• Industrial Controls
• White Goods
• Telecoms
• Energy both Traditional and Renewable

Conformal coatings come in a wide range of forms. From a physical standpoint, conformal coatings are commonly dispersed in organic solvents (solvent-based) or supplied in high or 100% solids (silicones and most UV curables).



From a chemical standpoint, conformal coatings’ standard chemistries include:



Acrylics



Acrylic conformal coatings are perhaps the most popular of all conformal coating materials as they are easy to apply and remove, with a wide process window for use. Acrylic coatings dry rapidly, are fungus resistant and provide long pot life. Additionally, acrylic materials produce little or no heat during the curing process, eliminating potential damage to heat sensitive components. The coatings possess low shrinkage, provide significant humidity resistance and exhibit low glass transition temperatures.



Urethanes



Polyurethane coatings are available in either single or two-component formulations. Both options provide excellent humidity resistance and far greater chemical resistance than acrylic coatings. Single component polyurethanes, while easy-to-apply and enjoy long pot life, sometimes require long curing regiments to achieve a full or optimum cure. In some instances, the cure cycle can be accelerated with the application of heat. Two-component formulations can reach optimum cure properties in as little as one to three hours with the assistance of heat.



Silicones



Silicone conformal coatings are most widely used in high-temperature environments due to their innate ability to withstand prolonged exposure to higher temperatures than most other conformal coating chemistries. This attribute has made them the primary choice for under hood automotive applications. They are also capable of being applied in thicker films making them useful as a vibration dampening/isolation tool if the coated assembly is to be placed in a high vibration environment.



Synthetic rubbers



HumiSeal’s Synthetic Rubber products are unique formulations that may enable the user to achieve specific protection / environmental goals not offered by “traditional” coatings. Synthetic Rubber materials are characterized by extremely low glass transition temperatures, resulting in stable modulus over a wide temperature range and extreme flexibility. Synthetic Rubbers have the least moisture permeability of all resins currently available. These products do not necessarily represent our complete Synthetic Rubber range.



UV curable acrylated urethanes.



HumiSeal’s UV curable conformal coatings represent an exciting innovation that provides superior performance. When combined with extremely fast curing, allows for short cycle times and maximum production speeds. These products can be sprayed using traditional automated spray equipment with the relatively simple addition of a small-footprint UV light curing unit.



Each chemistry is associated with certain physical properties that are suited for certain conditions and end-uses. For example, silicones tend to have the highest temperature resistance while synthetic rubbers and UV curables often provide the highest level of humidity resistance.

The advantages of conformal coatings can be summarized as follows:



1. Insulating property allow a reduction in PCB conductor spacing of over 80%
2. Can help eliminate the need for complex, sophisticated enclosures
3. Light weight
4. Completely protect the assembly against chemical and corrosive attack
5. Eliminate potential performance degradation due to environmental hazards
6. Minimize environmental stress on a PCB assembly
7. Inherent flexibility conforms to varied component profiles\



Ideally, conformal coatings should exhibit the following characteristics:



1. Simple application
2. Easy removal, repair and replacement
3. High flexibility
4. Protection against thermal and mechanical shock
5. Protection against environmental hazards