Article by Dr Lee Hitchens

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Selecting a conformal coating material is a complicated and involved process usually balancing a number of tradeoffs and compromises. This is because there is no such thing as a universally applicable conformal coating.

However, if no other information is taken from this article then the single most important rule is:

Choose the conformal coating material and the application process to match the design of your printed circuit board

Follow this guideline in your conformal coating production process and you will remove or minimise most of your problems immediately.

So Why Is This So Important?

Put simply if you choose the wrong material it may not work with your desired application process or be useable on your circuit board due to a difficult design.

Not all coatings work with all application processes or are easily applied to your circuit. It’s a fact and needs to be addressed as soon as possible and the best time is at the design stage.

Want To Know How To Multiply Your Costs Of Conformal Coating Production Without Knowing It?

Try and dip a circuit board with only a small area to coat and the rest to be masked / populated with a high volume of connectors

  • Try to selective spray a 3D circuit board that must have coating over all surfaces and underneath devices hidden by other components
  • Try to waterproof a circuit board using standard liquid conformal coatings
  • Try to coat close to connectors with a conformal coating that’s viscosity stability is sensitive to heat

All of these concepts and more can suddenly drive your costs up and your production engineers mad with the process.

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Figure 1: Trying to apply conformal coating close to connectors with a selective spray robot can be extremely challenging. Designing the PCB to take this into account can save a lot of time and effort.

Sounds Silly And You Wouldn’t Do it?

Examining these points it’s quickly understandable why this occurs. After all, why shouldn’t you be able to use the same production process on the next design of PCB?

However, sometimes it just does not work or is a disaster in labour costs to make it work.

The same problems occur when a company is “Going Green” and wants to move away from solvent-based coatings. They can get absorbed into the concept of the “drop in” replacement coatings.

Unfortunately, reality does not always give you the perfect replacement and this is the same in conformal coatings. There are changes in process that means the costs of manufacture can spiral quickly out of control and this is not what is wanted.

So What Factors Do You Need To Consider?

Here are a few guidelines to help you move forward in New Product Introduction or Process Improvement Programs:

1. Consider the three key areas of material, process and circuit board design as the same priority.

2. Identify the key properties of the conformal coating material that you need to consider. This can include

  • Temperature Operating Range
  • International, Customer or National Standards
  • Environmental Considerations
  • Chemical Resistance
  • Requirement to Rework the Conformal Coating
  • Curing the conformal coating
  • National, State, Regional Legislation and Insurance
  • Supplier Performance
  • Price

3. Split out the wish list into MUST HAVES and WOULD LIKES. This defines your material.

4. Repeat the task for the application process. Consider factors such as through put, capital costs, technical knowledge required to operate, ease of use and running costs to give you your preferred system of coating.

5. Now, critically examine both the material and the application process. Will your favourite coating and selected conformal coating application method work with your circuit board or is there a better option?

6. If not, then look at your material and process and adjust accordingly. What can you change that will suit your circuit board design?

7. Repeat until you have found the ideal solution or at least compromised enough not to make the product process price impossible or lead you to regularly recruit production staff, as they cannot deliver your process.

Conclusions

Selecting a Conformal Coating is a crucial stage of the design process. However, it should be considered in the context of all the factors outlined above. Select the conformal coating material and application process to fit the design of the circuit board.

If not, then it should not be a surprise to find problems later down the process stream that could have been avoided from the start. If in doubt, check with the supplier and independent experts.