How to 'design out' production problems
Phil Kinner*, Global Business and Technical Director for Electrolube’s Conformal Coatings Division, provides some design pointers that will help you avoid some common pitfalls when applying conformal coatings.
I have quite a few years’ experience and knowledge of the coatings business, and I’m still learning, but there are certain production-related issues that cause problems time and time again — problems that could so easily have been resolved at the design stage.
I intend to highlight these and, by doing so, hope to get the design guys fully on-side with their production colleagues. So, here you are: five essential facts to consider if you intend to conformally coat your electronics products.
Fact 1: All things are not equal…
…and this is certainly the case with solder resists. Adhesion results with conformal coatings can be very varied when utilising what appears to be the ‘same’ spec of solder resist from different suppliers and this can create havoc. A quick and very effective solution to this can be to specify a surface energy of >40 dynes/cm on incoming bare boards and ensure that each batch is religiously tested and rejected if it does not meet this minimum value.
Fact 2: Leave a buffer
Conformal coatings are usually liquid when applied and will flow with a combination of gravity and the capillary forces present. Whether you are masking or relying specifically on selective conformal coating, a production team will be greatly relieved if you leave a buffer of at least 3 mm clear between the area to be coated and uncoated areas. This small buffer will make the production process easier.
Fact 3: Simplify the coating process at the design stage
By the simple mechanism of placing connectors and components that must not be coated along one edge of the assembly, the conformal coating application process will be simplified. This will also allow dip coating to be explored as a potential methodology and the net result will be quicker application times and reduced costs.
Fact 4: Discrete components have a downside
Large arrays of discrete components represent a massive coating challenge due to the high levels of capillary forces present and the result is often quite disastrous, with areas of no coverage/protection on the board and conversely areas of excessive thickness prone to stress cracking, de-lamination and other coating defects. Ultimately this will lead to premature failure of the assemblies. Try to avoid this if at all possible!
Fact 5: Bigger isn’t necessarily better
Tall components present challenges of their own by creation of shadowed or hard-to-reach areas. Splashing is another associated problem. Try to avoid siting tall components next to ‘must coat’ components to minimise this.
Remember: Thoughtful design will pay huge dividends down the line — and the designers among you will have friends for life among your production colleagues by making their jobs just that little bit easier!
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