Co-Authors and contributors
- Dr Lee Hitchens, Nexus
The majority of conformal coatings used in the world are organic coatings.
These organic conformal coatings can be further subdivided into resin types as
- Acrylic Urethane Blend
These resin types are examined further below.
Acrylic (AR) Conformal Coating
Acrylic coatings properties include having an excellent humidity resistance.
Solvent based acrylic conformal coatings have been historically the number one choice for moisture protection of printed circuit boards.
They dry quicker than most other conformal coatings. They reach optimum properties within minutes. This is because the acrylic resin does not polymerise (does not cure) and actually just dries out.
This means the coating can actually be re-dissolved back up in the evaporated solvents and reform the coating. It also means that it is easy to repair and rework since it has very little chemical resistance. They can be soldered through easily.
Finally, they have excellent pot life compared to many other types.
Polyurethane coatings also provide good humidity & moisture protection.
They also offer excellent chemical resistance plus generally have higher dielectric properties compared to the acrylic conformal coatings.
UR coatings cure rather than dry. That is they cross-link by one of the many methods including solvent evaporation (initial stages), heat, UV, moisture and catalysed cure. This provides the chemical resistance for which they are generally selected.
Their chemical resistance, however, can be a limitation since rework and repair generally is more difficult than the acrylic coatings.
UR coatings are normally available as either single or two-component formulations.
Pot life is dependent on the cure mechanism but can be more limited than the acrylic coatings.
Epoxy conformal coatings are not normally used on circuit boards except in extreme cases. This is due to their inflexible nature when cured which can cause CTI mismatch issues.
They do provide reasonable humidity protection and excellent mechanical and chemical protection. In fact, it is extremely difficult to remove an epoxy coating safely without damaging the underlying circuit board.
Epoxy conformal coating materials are normally sold as two part compounds. This limits their effective pot life considerably.
Acrylic / Urethane Blends (ARUR)
Some conformal coatings are hybrid blends of the base resins. In this case an acrylic and polyurethane monomer combination. Other combinations exist.
This can lead to coatings with optimised properties in a certain area.
The Chemical Structure of an Organic Conformal Coating
The material used for the resin has principal ingredients derived from animal or vegetable matter or from some compound of carbon. It is made up of two principal components: the vehicle and a colourant.
The vehicle is always there. It is made up of the film-forming materials and various other ingredients, which divide into the monomer, solvents, additives and fillers.
The colourant is not always there. In 99% of cases the colourant is generally UV activated.
It is not a visible colour in daylight, like a paint in most cases, but visible only under a UV wavelength light. It is possible to have a visible colour like a paint in some products.