Conformal coating brush application

Co-authors and contributors

  • Dr Lee Hitchens, Nexus
  • Karl Hardcasle, SCH Technologies
  • Bob Willis, Consultant

The application of a conformal coating using a brush to “paint” on the material to the circuit board is a common practice.

This can be done in main stream production or as a touch up process before final inspection.

The conformal coating is applied selectively by the operator according to the requirements of the circuit board.

Areas not be coated are avoided by the technician.

Process control and finish are completely operator dependent.

The key areas for brush coating

Is Brush Coating a Complex Process?

On first examination it is a very simple process.

However, brushing conformal coating is a manual process and the process quality relies completely on the operator.

That person is completely responsible for the quality and finish of the coating.

This can be a highly skilled process.

They are also ensuring the conformal coating is applied in the right place using the instructions supplied.

Conformal coating brushing is a simple process but can require high levels of skill to produce the right level of quality
Conformal coating brushing is a simple process but can require high levels of skill to produce the right level of quality. Photo courtesy of SCH Technologies

What equipment do you need for brushing conformal coating?

Requirements for set up are:

  • A quality brush or brushes
  • Set of containers to store the conformal coating
  • A suitable area to apply the coating

This is a very low cost set up.

Consideration should be made to the workstation. Lighting conditions can make the difference when applying coatings. Hand brushing under a UV light can help an operator to complete the task.

This will be more successful if there is a UV trace in the conformal coating.

Also, if the conformal coating is hazardous how the fumes are handled and removed is important.

conformal coating brushing in booth
The use of a dedicated UV inspection booth makes the whole process of inspecting conformal coating much easier. Photo courtesy of SCH Technologies.

What is the correct technique for applying the conformal coating with a brush?

The correct technique for applying conformal coating by brush is to flow the coating on.

The material should not be brushed on like a paint.

The coating should be loaded on to the brush and flowed onto the circuit.

Video courtesy of Bob willis

Which conformal coatings can be brush coated?

All conformal coatings can be applied by brush.

Most conformal coatings lend themselves well to the technique.

Key points to be aware of are:

Material pot life

The pot life of the material may be finite so the coating could cure.

This may make the material useless to apply after a certain amount of time.

Conformal coating viscosity

The way the conformal coating flows is critical when applied by brush coating.

The viscosity of the material is highly influential on the flow so control of the viscosity is critical to allow good coverage.

A highly viscous coating will be harder to apply correctly than a lower viscosity material.

Is brush Conformal Coating a batch or inline process?

It certainly is a batch application process as it a completely human process.

How cost effective is the brush coating process?

At low volume it can be highly cost effective depending on the cost of labour.

It is also a great technique for finishing and reworking circuits after the de-masking and touch up stage.

At medium and high volume it depends on the area to be coated, the quality of the finished required and the labour costs.

Is brushing conformal coating a low, medium or high volume technique?

Depending on the cost of labour and the quality of the coating finish required then this could be high volume technique.

However, normally brushing is considered a low volume process.

What variables control the quality of conformal coating finish with brushing?

Key variables that influence the quality of the process include:

Operator skill

The operator determines the quality and thickness of the conformal coating finish.

Using the correct brush coating technique can reduce defects like bubbles significantly.

Quality of the brush

Use a cheap, poor quality brush and you get a poor quality finish.

Use a high quality brush and the material finish quality can be much higher.

Viscosity of the coating

Setting the viscosity of the coating correctly is crucial to achieving a good finish

If the coating is too thick it will not flow, leaving brush marks and defects all over the circuit board

If the conformal coating is too thin, then it will capillary into areas that you do not want to coat.

The coating environment

If the area is designed for conformal coating then this can significantly improve on the quality of the coating process.

For example, if the area is well lit for UV light then this helps.

Complexity and size of the board

If the board is complex or large then the operator has to be more skillful in his approach and ability.

Conformal coating material type

Some materials are easy to brush coat and others are more difficult.

Some dry quickly and give a short pot life.

Some are too viscous.

How fast is the brush coating process?

It depends on how much area of the board is to be coated and the complexity of the board design.

A large, complex board will take a lot more time to process on an individual basis compared to a small board with a small area to coat.

 

What are five advantages of brushing conformal coating?

  1. Low cost to set up
  2. Simple process to learn
  3. Quick to start the process
  4. Very little process control required
  5. Perfect for low volume and rework

What are five disadvantages of brushing conformal coating?

  1. Poor Quality Control of process
  2. Operator dependence on correct coating coverage
  3. Speed of process slow for large areas
  4. Variation between operator performance is high
  5. Extremely easy for an operator to make an error

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