By Phil Kinner, Technical Director For Electrolube’s Conformal Coatings Division
Phil Kinner joined Electrolube in May 2014 as Technical Director for the company’s Conformal Coatings Division, which is represented in over 55 countries across the globe. In this, the second of a series of posts that take a closer look at production issues, Phil provides some design pointers that will help you avoid some common pitfalls when applying conformal coatings. In each post, he lists five essential ‘Facts’ about conformal coatings that will help you to streamline your production processes.
Well, a little time has passed since I last posted some advice offering guidance on the importance of ‘designing for manufacture’ – essentially to spot those ‘wrinkles’ that might have an adverse effect on production, ironing them out before you even think about applying conformal coatings to your products. I do hope you found it useful and that it has, at least, started the conversation about the importance of making sound early-stage design decisions. In this post, I’m going to take a look at some issues you are sure to face – and which must be taken into consideration – when, finally, you apply those conformal coatings. Here are my five essential facts:
Fact 1 – Conformal coatings are generally applied in liquid form and are subject to capillary forces and gravity during drying. They are unlikely to yield perfectly straight edges. Often coating drawing is presented as a rectangular box, and this is likely to cause issues during inspection. Better to specify areas that must be coated and areas that must not be coated. Anything else is a ‘don’t care’ area.
Fact 2 – The old adage that if ‘some is good, more is better’ doesn’t necessarily hold true with conformal coating. Conformal coatings are designed to be applied at the thickness specified on the datasheet. Exceeding the recommended thickness is unlikely to provide better protection, but may introduce a range of production issues ranging from dramatically increased cycle time to solvent-entrapment, stress-shrinkage/de-lamination/cracking. If you need a coating on the thick end of the specification, two thin coating applications are better than one thick coating. If you need more thickness than specified, use a coating that is designed to be applied thickly (Electrolube’s 2K series two-part coatings, for example) or consider a resin product.
Fact 3 – Conformal coatings are not under-fill materials. They generally contain no filler and have relatively Z-axis thermal coefficients of expansion. They have been shown to reduce the lifetime of ball grid array (BGA) and quad flat no-lead (QFN) terminations during thermal cycling conditions. If you need to under-fill a device, use one of the many under-fill formulations especially designed for this purpose, such as Electrolube’s ES501 under-fill resin, which is guaranteed not to degrade thermal cycle performance.
Fact 4 – Conformal coatings are available in many generic types (search the Electrolube website at www.electrolube.com and you’ll see what I mean!). Each has its strengths and weaknesses. Choose the right coating for the intended use and operational environment, rather than one that is used by your subcontractor or qualified on another product line for a different end-use environment. Be sure to test your design to ensure that it is suitably ruggedized for the intended application.
Fact 5 – Conformal coatings are not generally waterproof. They will allow moisture to permeate – albeit very slowly – through them, and this will eventually react with contaminants from production, such as flux, solder and adhesive residues, that could ultimately lead to corrosion beneath the coating. Cleaning is therefore highly recommended prior to the application of conformal coating.
I don’t pretend that it’s an easy task to choose the correct conformal coating for your product, let alone have certainty that you will have achieved the ultimate goal of protecting your electronics by applying it. Frankly, that’s our job; just give us a call. We’re no strangers to these problems and can help you overcome them at that all-important early design stage.
For further information, please visit http://www.electrolube.com.