Liquid

Co-Authors and contributors
  • Dr Lee Hitchens, Nexus

The liquid properties are defined for the conformal coating material before it is used in the application process.

So, it is still in its liquid state and stored in the container.

These properties include:

  • Flash point
  • Storage lifetime
  • Appearance and colour
  • Density
  • Solids content
  • Viscosity

Each are addressed below in detail.


Flash point

The flash point of a conformal coating can be defined as the temperature at which a particular organic compound (solvent) gives off sufficient vapour to ignite in the air.

This value can be significant when considering the application of the conformal coating and the machines and systems that will be using it.

If the machine is not designed correctly to use flammable coatings then it could be a high risk process.


Storage lifetime

The Storage lifetime (shelf life) of a conformal coating is dependent on many factors.

This includes but is not limited to the chemical design of the material, its cure mechanism, the ambient conditions the product is stored in and whether the container has been opened or not.

Generally, most conformal coatings do not separate with time.

They do not require stirring like paints since they have no colourants within the conformal coating.

However, there are exceptions and they should be considered.

Ultimately, to prolong the lifetime of the conformal coating, the storage parameters should be addressed.

Meeting the Manufacturers recommendations should be the guide.

Further, handling of the coating during application use should also be done as per the manufacturers instructions if the shelf life of the coating is critical.


Appearance and colour

The Colour and Appearance of a conformal coating is defined in the Material Safety Data Sheet (MSDS).

The coating will be expected to be clear in most cases.


Density

The density of a conformal coating is normally defined in the Technical Data Sheet (TDS).

Measuring the density of the conformal coating can be used as part of a Goods In Inspection process and is a relatively easy test to make.

It is normally measured with a pycnometer.

There are different methods that can be used including ASTM, Meth. D1475 (American Society for Testing and Materials International) and DIN EN ISO 21881-1 (German institute for standardization).


Solids content

In technical terms, the solids content is the non-volatile material left after application once the coating is fully dried and / or cured.

The time taken to reach this point may be hours, weeks or months depending on the cure mechanism, temperature, drying method etc.

However, it is an important parameter when considering cost of material since the solids content defines the amount of actual coating available for application over the surface of a circuit board.

When comparing price of material across a range of coating it is very important to consider just how much coating and how much solvent you are purchasing!


Viscosity

Viscosity can be defined as a quantity showing the magnitude of internal friction in a fluid, as measured by the force per unit area resisting uniform flow.

The more viscous a coating is the more it does not want to flow.

The viscosity of a conformal coating is one of the most important parameters.

It has a first order influence on many areas of coating application and many problems can be related back to viscosity.

Therefore, it is very important in most conformal coating application methods to control the viscosity.

Conformal coating viscosity check using a flow cup and stop watch