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PCBD-Mar2016

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40 The PCB Design Magazine • March 2016 on the datasheet. Exceeding the recommended thickness is unlikely to provide better protec- tion, 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 speci- fied, use a coating that is designed to be applied thickly (such as Electrolube's 2K series two-part coatings) or consider a resin product. Fact 3 Conformal coatings are not under-fill mate- rials. They generally contain no filler and have relatively Z-axis thermal coefficients of expan- sion. They have been shown to reduce the life- time 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 es- pecially designed for this purpose. Fact 4 Conformal coatings are available in many generic types. Each has its strengths and weak- nesses. Choose the right coating for the intend- ed 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 de- sign to ensure that it is suitably ruggedized for the intended application. Fact 5 Conformal coatings are not generally water- proof. They will allow moisture to permeate, albe- it very slowly, through them. 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. It's critical to communicate with your conformal coating sup- plier, whose technologists can help you address your problems and overcome them at the all- important early design stage. See you next month. PCBDESIGN Phil Kinner joined Electrolube in May 2014 as technical director for the company's Conformal Coatings Division, which is represented in more than 55 countries across the globe. design and production: some essential facts A novel technique known as in-situ plasma processing is helping scientists get more neutrons and better data for their experiments at the Spallation Neutron Source at the Department of Energy's Oak Ridge National Labora- tory. "Plasma cleaning is a well-known technique in electrostatics," said ORNL's Research Accelerator Di- vision Director Kevin Jones, "but it has not been ap- plied before to superconducting cavities, and there are some interesting tricks that you have to develop and apply in order to make it all work." ORNL staff scientist Marc Doleans and postdoc- toral researcher Puneet Tyagi found a better solution. "Basically, the oxygen gobbles up all the bad atoms off the surface of the niobium, and afterwards everything gets pumped out as a gas," Jones said. After the trial procedures were complete, the team tested the cryomodule's per - formance and saw a significant improvement. That gave the go ahead for the team to carry out the in- situ procedure in the accelerator tunnel during the facility's scheduled winter maintenance outage. The work is already generating a lot of buzz, and it's being viewed very positively, Jones says. Plasma Processing Technique Takes SNS Accelerator to New Energy Highs

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