Co-Authors and contributors
- Dr Lee Hitchens, Nexus
A conformal coating is made of two significant parts.
These are the volatile and non-volatile materials of the conformal coating.
Volatile materials are can be further subdivided into two sections in the case of the conformal coating. These are (a) Solvents and the (b) Additives.
(a) Volatile solvents
The solvents are the carrier of the conformal coating.
The solvent section of the coating can consist of several component solvents, all with different jobs to do in the final coating application process.
They are chosen due to several reasons.
These include their solvation ability of the resins, the speed of drying required and their environmental and hazardous nature.
Water can also be considered a solvent, specifically for water based conformal coatings, although in reality most coatings based upon water are dispersions and emulsions.
They tend to be used where VOC (Volatile Organic Compounds) content is required to be low.
An organic solvent is classed as a VOC in general, although classification and exemption can apply.
(b) Volatile additives
The additives added to the Volatile Material generally aid anti-skinning (prevents or retards skinning caused by oxidation during storage ) and film-formation.
The non-volatile materials comprise of two components.
These are the (c) binders and (d) colourants.
The binder is the component of the non-volatile (dried coating), which comprises of the resins and the other additives.
The resins are the organic (AR, UR etc) and inorganic (SR) polymers, which form the film on the surface of the substrate.
These may be single component or multicomponent, depending on the complexity of the conformal coating.
The non volatile additives, which form part of the binder, can be varied and wide, depending on the binders and solvents used.
Their properties can influence many different factors including but not limited to:
Wetting: how the coating actually bonds to the surface whilst wet and can influence factoring like de-wetting.
Foaming: When a coating is applied the bubbles created need to be minimal to ensure good coverage.
Drying: Additives can speed up the drying process of the coating.
Preservation: Long term some types of coating (eg some urethanes) can be attacked and a high moisture environment can destroy a conformal coating
UV stabilisation: Exposure to natural and artificial UV light can damage conformal coatings.
For a simple acrylic based coating in a solvent system there may actually be no requirement for additives.
The coating itself functions well under most circumstances without aid.
However, in many non-VOC solvent based formulations, such as water based and 100% solvent free coatings, these additives are required.
The colourants are the collective terms for the dyes and pigments added to paints.
In conformal coatings, there are few occasions a pigment is added. Normally, it is a dye. The dye is normally fluorescent under UV / black light and this aids inspection.
Note on fillers in conformal coatings
Fillers are powder like materials that are used to improve properties of coatings.
This includes properties like adhesion and hardness. Fillers are also used to even out irregularities in substrates to be painted.
They are generally not used in conformal coatings as they change the optical properties (they make the coating opaque) and this may affect inspection criteria.
Not all the parts described here are essential for all conformal coatings.
However, the combinations of the parts possible mean that there are many types of conformal coating now available in the market.
Each conformal coating will have its own physical and electrical properties, both in the wet and dry state, that offer flexibility in selection of the conformal coating.
The key to getting it right is the selection of the right conformal coating. These are investigated further in later chapters.