There are several options for coating technologies, and the best option for your particular application should depend primarily on your level of necessary protection. The application method and the ease of rework are also important factors, but should generally be considered secondary to the necessary protective performance.
What we call “traditional” conformal coatings are 1-part systems that have a resin base and can be diluted with either solvent or (in rare cases) water. Traditional coatings are semi-permeable, which is why they are not fully hermetic nor do they seal the coated electronics. They provide resistance to environmental exposure, which increases PCB durability while keeping application and repair processes in practice. However, they are NOT fully water-proof.
The following categories are based on the basic resin of each coating. The chemical composition of each conformal coating determines its major attributes and functions. Choosing the proper conformal coating for your application is determined by the operational requirements of your electronics.
Acrylic conformal coating provides fair elasticity and general protection. Acrylic conformal coating is recognized for its high dielectric strength, and fair moisture and abrasion resistance. What generally distinguishes acrylic coating from other resins is its facility for removal. Acrylic coatings are easily and quickly removed by a variety of solvents, often without requiring agitation. This makes rework and even field repair very practical and economical. On the other hand, acrylic coatings do not protect against solvents and solvent vapours, which could result in less than ideal performance for an application that involves something like pumping equipment. Acrylic coatings can be considered basic, entry-level protection, because they are economical and protect against a broad-level of contamination. However, they are not the best-in-class for any characteristic except possibly dielectric strength.
Silicone conformal coating provides excellent protection in a very wide temperature range. SR provides good chemical resistance, moisture, and salt spray resistance, and is very flexible. Silicone conformal coating isn't abrasion resistant because of its rubbery nature, but this property does make it resilient against vibrational stresses. Silicone coatings are commonly used in high-humidity environments. Special formulations that can coat LED lights without colour shift or reduction of intensity are available, and make SR conformal coatings a popular choice for applications such as outdoor signs. Removal can be challenging, requiring specialized solvents, long soak time, and agitation from a brush or an ultrasonic bath.
Urethane conformal coating is known for its excellent moisture and chemical resistance. It is also very abrasion resistant. Combining those factors with its solvent resistance results in a conformal coating that is very difficult to remove. Like silicone, full removal generally requires special solvents, long soak time, and agitation with a brush or an ultrasonic bath. Urethane conformal coating is commonly specified for aerospace applications where exposure to fuel vapours is a common concern.
Epoxy resins (ER) are usually available as two-part compounds and create a very hard coating. Epoxy conformal coatings provide very good humidity resistance and are not generally permeable, unlike traditional conformal coatings. They also have high abrasion and chemical resistance. Typically, they are very difficult to remove once cured and are not as flexible as the other materials. Epoxy coatings are common in potting compounds, which in contrast to conformal coatings, completely cover the electronics in a solid and level layer of material.
Parylene conformal coatings are a unique type of coating applied by vapour phase deposition. They provide excellent dielectric strength and superior resistance to moisture, solvents, and extreme temperatures. Because of the vapour deposition method, parylene coatings can be applied thinly and still provide excellent circuit board protection. However, removal for rework is very difficult, requiring abrasion techniques, and without access to vapour phase deposition equipment, recoating with parylene is impossible.
A coating is dissolved in a fluorocarbon-based carrier solvent and applied with a spray or dip method to create a very thin coat, although not at a nanometre scale as the nickname suggests. They are commonly used to provide a minimal amount of hydrophobicity, which may prevent losses from very quick exposure to water. This type of coating does not offer the level of surface protection that other coating methods do.