Coatings: five ‘number one’ essentials for designers

By Phil Kinner, Technical Director For Electrolube’s Conformal Coatings Division

As I sit down to write my next column on the subject of conformal coatings, I’m reminded – as we in the UK continue to experience unseasonably cool, almost sub-arctic weather conditions this Spring – that it is always best policy to be prepared for those unexpected environmental elements. And where electronic circuit board protection is concerned, that means being prepared for all eventualities. So, as the central heating is fired up a notch or two and the barbecue stove is wheeled back into the garage, let’s consider what we’ve covered so far on the subject of conformal coatings.

True to form, I’ll be providing five design pointers each month to help you avoid some common pitfalls when applying conformal coatings. We have so far looked at tricky production related issues that could so easily have been resolved at that all-important design stage, examined some of the more common dos and don’ts; those issues that come up time and time again, despite meticulous attention to design detail, and which must be taken into consideration when coatings are finally applied. We’ve explored the implications of housing design on conformal coating performance, particularly the influence that fixtures and fittings can have in terms of thermal shock loading and the adverse effects of forced air cooling when abrasive particles are present. So, for this issue let’s cover how the physical shape of board components and the interaction with solder paste, solder masks and fluxes can affect conformal coating integrity.
The focus for this month will be my absolute ‘number one’ essential facts – my ‘never leave home without….’ list, so to speak!

Fact 1 In an ideal world, designs would not have an inherent weak point for corrosion; unfortunately, in the real world, they do. A really neat way of determining this weak point is to use stress tests such as powered condensation tests. When a weak point is revealed, you are better equipped to deal with it. Often the spacing of components, board finish and distance to ground planes can be optimised for corrosion resistance.

Once the design is optimised for corrosion resistance, the conformal coating will often be even more effective in ensuring corrosion free operation. Stress tests (thermal shock, for example) are not intended to be used on final assemblies for product validation. Failure mechanisms may well be different, and may be missed by stress tests. Product validation testing should mimic end use environments as closely as possible. Here at Electrolube, our testing is designed to push products even further than in actual usage scenarios.

Fact 2 If you assume that a conformal coating will be necessary from the very beginning of the process, you will not go far wrong. Plan for this, design for this and test for this. You can bet your life that a coating will be required and by factoring this into the design process, it will have the knock on effect of simplifying the manufacturing process and you will have a happy team.

Fact 3 We would all love a universal all-singing, all-dancing coating but life is not like that. Conformal coating is always a compromise and there is no fixed solution. Plan ahead and bear this in mind. Why not have two or three internally approved and tested materials at your fingertips? This will allow you the freedom of choice and will ensure that you can select the best one for the environment in mind, rather than the one that is used the most or the operators prefer using; after all, the most familiar or popular is not always the best for your application.

Fact 4 Technical specifications and standards give a warm fuzzy glow but the use of MIL spec conformal coatings does not in itself guarantee the coating will perform well in your application! The coatings are tested on a flat, bare FR4 substrate, without solder mask, flux chemistry or the effects of multiple thermal excursions. A key tip is to obtain the MIL document and familiarise yourself with the test conditions and properties tested and remember the standard is a guide, intended to prevent truly unsuitable coating materials from being considered by the end user. The MIL spec doesn’t really distinguish between materials or explore the areas of use for which they might be suitable.

Fact 5 Make sure you check the boilerplate language contained within your drawings to ensure it is appropriate and relevant. For example, MIL-I-46058C was declared inactive for new designs in 1999. However, it still remains ‘active’ due to the boilerplate language that requires a MIL-I approved coating that has been propagated onto new drawings produced since 1999.

I’m very keen to learn if any of the recommendations and hot tips offered in my columns over the past few months have been put into effect by readers, as it would be great to have some feedback from you and hopefully some good news about transforming your process. If you have found my design tips useful and have achieved some success through their implementation, let’s hear about it! Sharing your experiences with fellow readers – if you are at liberty to do so, of course – can only have a positive impact on our industry; so spread the word.

In the meantime, I’ll do some ‘coating’ of my own and pull on an extra layer of clothing! Roll on summer.

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