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Question:

Could conformal coating affect the properties of an inductor by adhering to its windings?

Context and problem:

I have a simple boost regulator circuit based around the TPS61230 IC. It has a handful of SMD resistors and capacitors and one largish SMD inductor (http://www.mouser.com/ProductDetail/Bourns/SRU1028-1R0Y/?qs=%2fha2pyFaduga8qwkov01I2ofW5jqNjYb0PGY4sjOrrG99%2faGtQIpWg%3d%3d).

The circuit worked fine when I built it up, but I decided to conformal coat it to protect it before installing in the final product. Since I conformal coated it I'm not longer getting my 5 V on the output and instead when powered there's a -0.5 output voltage which then goes to zero. I can't see how the conformal coating could affect the rest of the circuit, but the inductor has openings on the top which my conformal coating spray probably got into.

The question I have is how might the spray have affected the properties of the inductor to cause this behavior? Is this the likely problem? I don't have any extra inductors so am trying to hone in on the issue before ordering more.

Picture of circuit boardenter image description here, and yes, I covered my connector pins during conformal coating.

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  • \$\begingroup\$ I'd be tempted to stab/scrape through the coating with test leads and see if the inductor still shows continuity. I assume you don't have access to a scope to look at it while operating? Also check all your solder joints, especially those on the ic. \$\endgroup\$ – Chris Stratton Dec 20 '15 at 2:37
  • \$\begingroup\$ I'll burn off the coating on the inductor terminals and test continuity, and I can probably hook it up to a scope too. What are you suggesting I look for on the solder joints on the other components? Note that this circuit worked perfectly before conformal coating so I don't think it's a build issue. \$\endgroup\$ – archaeopteryx Dec 20 '15 at 2:42
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    \$\begingroup\$ You could have had something which previously worked because it was in mechanical contact but was not actually soldered (solder might be present but not wetted to both surfaces), and has now either been lifted or penetrated by the coating. \$\endgroup\$ – Chris Stratton Dec 20 '15 at 2:43
  • \$\begingroup\$ The inductor still has continuity and the rest of the components look ok, but all I've done for them is a close visual inspection. I'll hook it up to a scope tomorrow when I get a chance. Thanks for the suggestions. \$\endgroup\$ – archaeopteryx Dec 20 '15 at 2:53
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    \$\begingroup\$ @archaeopteryx By itself, conformal coating can not noticeably affect a power inductor in a boost converter. There ought to be something else going on in your device. \$\endgroup\$ – Nick Alexeev Dec 20 '15 at 3:04
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Anything intended to be used as a conformal coating should have no effect.

If it was electrically conductive it may.
This is easily tested.

If it was bulk conductive and formed a screen but had very low DC conductivity across terminals it may manage to do something to a circuit but this is unlikely.

If it contained a solvent it may have dis-solved something.
I have seen IPA dissolve 'expensive stuff' during "cleaning".

If power was on during curing it may have been conductive and caused a problem.


Did you use heat in the process?
Was power on during the application process or during curing?
What DC resistance do you see per square cured and "wet" ?

Does it contain a conductive filler (eg Nickel or carbon)?

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  • \$\begingroup\$ The answer to the question seems to be that conformal coating should have no effect on the inductor. This answer and the comments have all been helpful and though I haven't solved the problem in the circuit the question I posted has been sufficiently answered. Thanks! \$\endgroup\$ – archaeopteryx Dec 21 '15 at 1:28

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