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The Importance of Conformal Coating Thickness and Edge Coverage



The Importance of Conformal Coating Thickness and Edge Coverage
The performance of new silicone and urethane materials, designed for coverage and thickness, is compared with an acrylic and ultra-thin material.
Materials Tech

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Authored By:


C.E Allen, A.Duffy, B.G Turner, P.J Kinner
Electrolube Ltd
Leics, UK

Summary


As electronics continue to become ever more densely populated, and expected to operate in ever more hostile environments, the use of conformal coating is becoming more and more essential to protect the assembly from its operating environment, and ensure acceptable reliability for the application intended.

Conformal coated assemblies are often exposed to harsh operating environments, including high humidity, high temperatures, corrosive gases, condensing environments and rapid changes in operating temperature.
It is important that the conformal coating can withstand its anticipated operating environment. In previous papers, populated SIR (Surface Insulation Resistance) test assemblies were subjected to a harsh sequential load and their ability to withstand corrosion was assessed by SIR.

During this testing it was seen that most of the coatings tested failed to provide good protection during powered salt-spray testing. Further testing, performed under condensing conditions, confirmed the importance of, and difficulty in, achieving good coverage with liquid applied conformal coatings.

In this paper, we compare the performance of new silicone and urethane materials, designed for coverage and thickness, with a popular acrylic and ultra-thin material, in a variety of experiments designed to determine, how thick is thick enough?

Conclusions


From these relatively simple experiments, it is clear that thickness and coverage are of vital importance in determining whether an assembly will survive life in the field, whether the risk of failure comes from humidity, condensation, salt-splashes, arcing or tin whisker formation.

Immersion testing has traditionally been an extremely difficult test to pass with conventional conformal coatings. The new Silicone and Urethane coatings were formulated to address the coverage and thickness issues so prevalent with liquid applied coatings.

It can be seen from the results of the first experiment that the acrylic material did not significantly improve the SIR value when saturated, even at 150 microns. The ultra-thin material, defined as being < 12.5 microns required 25 microns to show any improvement over no coating, and even at 100 microns did not match the insulation capabilities of the Silicone or Urethane materials.

In the second experiment, by omitting the current limiting resistor and measuring the actual leakage current, we were able to see again that the conventional and ultra-thin materials yielded significantly greater leakage currents than the silicone and urethane materials.

Extending this methodology to a populated B-52 assembly and looking at topography and components as opposed to flat SIR combs, we once again saw the silicone and urethane materials provide a dramatic improvement in the survival time immersed, powered-up in 3.5% saline solution. The silicone and urethane material survived these conditions for 30 hours without evidence of corrosion or excessive leakage current, whereas the acrylic and ultra-thin material survived less than 20 seconds, despite the application of multiple coats.

The Urethane board was cross-sectioned to investigate the thickness of material applied by the selective spray-process, as well as the degree of coverage, looking at the QFP, which was where the leakage current was measured, in figs 18-20, it is apparent how well the component was coated and the result is not surprising considering typical coverage seen in fig 1.

Initially Published in the SMTA Proceedings

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