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Manual brushing is just one of many methods commonly used for applying conformal coating materials to PCBs and other components. It is a hand-applied method that requires little investment into automation or equipment. With practice, it can be one of the simplest methods available, especially for small-batch production or isolated applications.

This blog is the second installment in our two-part series on viscosity (with special consideration on adhesives and coatings).

In our previous viscosity blog, we discussed the properties of viscosity and how viscosity can be affected by temperature, the addition of dilution or thinning agents, and other similar factors.

This is the first in a short series on material viscosity, in which we will review what it is and how to measure it. We will start with the definition of viscosity. Viscosity is the internal friction of moving fluid and a measurement of the resistance of a fluid deformed by shear stress. It is measured in centiPoise. The lower the number, the less viscous the material, as can be seen by this comparison chart: starting with water at 1 cPs and ending with honey at anywhere between 2,000 – 10,000 cPs.

While conformal coatings increase the lifetime of electronic devices by providing a thin barrier that strengthens their ability to withstand harsh environmental conditions, adhesives and encapsulants provide a significantly higher level of protection on the entire assembly.

The most common properties of adhesives and encapsulants include:

• creating environmental seals that protect against moisture and corrosion,
• providing high-temperature resistance,
• increasing shock and vibration dampening,
• generating strong bonds to a variety of substrates,
• maintaining low shrinkage, and
• offering thermal conductivity.

UV curable conformal coating materials such as HumiSeal UV40 are frequently used in military, aerospace and automotive applications due to superior resistance to chemicals and mechanical stress. These properties make them difficult to remove when rework under the coating is required.

There are products available for chemical removal of conformal coatings, but many are restricted by government regulation and health and safety concerns.

An effective method for localised removal of conformal coatings, allowing the rework of the components, is the powder or micro-abrasion equipment.

IPC CC-830 revision B is a conformal coating qualification standard mostly used by board fabricators, OEM design engineers, and coatings suppliers.  

There are two fundamental uses of this IPC standard:

• Coatings’ performance capabilities
• Quality consistency

In this blog, we will go over some of the specifications along with tests requirements, passing criteria, and quality characteristics.

The application of the conformal coating in a uniform and consistent coverage is a key challenge that defines the reliability of the finished product.

A thin coating will provide inadequate protection against corrosion or insulation between conductors. Conversely, if the conformal coating is too thick, it will increase the risk of forming other types of coating defects such as bubbles, insufficient curing, and potential cracking during thermal shock cycles.

Besides the verification of the process parameters such as coating viscosity and the application method is correctly setup, the amount of conformal coating material applied on the printed circuit boards can be measured before curing. For this, wet film conformal coating thickness gauges are used. This method ensures quality control while the coating material is still wet.