Designing defect-free conformal coatings

Louise Smyth
Electrolube coating in use

Identifying potential production problems at the design stage will always be favourable and far easier than trying to fix problems or concerns following the conclusion of the engineering drawings. Here, we cover the key points to enable engineers to achieve the best outcomes for their coating processes.

How important is the reworkability characteristic of a conformal coating?

If you want to save yourself a lot of aggravation in the long term, then reworkability of a coating is a very important consideration in conformal coating selection, especially when boards are high value and have long service life expectancies. A coating may display favourable characteristics but it might also make the assembly very difficult to rework in the factory or repair in the field. A difficult-to-rework coating will not only make repairs and upgrades time consuming, but will also add cost and complexity to the product.

Does a thicker level of conformal coating coverage equal greater protection?

The simple answer here is ‘it depends’. Thicker coating coverage can achieve greater protection of your assembly but it is not unusual to push the limits on coating thickness. Coatings should be applied in thicknesses that they were designed to be applied at. Very thick application of conformal coatings can actually result in harmful levels of stress on components.

How can designers ensure a smoother production process?

When design and production work seamlessly well together, the outcome will nearly always be successful, particularly when a designer understands the intricacies of conformal coatings. One area where designers can really help their production colleagues is to specify where coatings are optional or “don’t care” in the engineering drawing. In specifying the optional conformal coating, the designer provides more flexibility to the coater in assembly operations. It is best practice to specify the areas that need to be coated and the areas that don’t as well as the ‘don’t care’ areas to help the coating process run as smoothly as possible.

Can a designer’s instructions be easily misconstrued?

We might need to duck for cover here, but the answer is yes! Engineering drawings should avoid specifying “100% coverage” as this means many different things to many different people. The only real way to get 100% coating is to have assemblies with no connectors, no uncoated components, and be either dip coated or vapour deposited.

How is conformal coating thickness specified?

Cured conformal coating thickness varies by chemistry. It’s also advisable to be aware of what these coverage requirements do to your resulting application method. Adequate coverage particularly on corners, sides or under leads can be a challenge in thin film applications, whereas thick film processes can increase the flow characteristics of a coating application and can be more difficult to manage around no-coat areas.

How is the best method of conformal coating application determined?

There isn’t necessarily a best method to apply a conformal coating. Choosing the most pertinent application method for a particular assembly will depend upon which existing equipment is available to the manufacturer, the coating processes in use, the average time interval between the start of production of one unit and the start of the next and the overall design of the assembly, which will dictate what can and can’t be achieved. The ‘best’ application method would ensure that each board to be coated receives coating coverage on all required metal surfaces at a sufficient thickness to afford protection against the environment. These requirements will change from board design to board design, and environment to environment, and invariably they need to be tested and verified ahead of the production run.

Implementing a defect-free conformal coating process is a fine balance of material selection, understanding the engineering requirements for coverage and thickness, as well as choosing a suitable application method.

Understanding the subtleties of coatings pays huge dividends in providing an engineering drawing that isn’t prone to misinterpretation.

Phil Kinner is with Electrolube