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Conformal Coat (Urethane) & bubbles

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Conformal Coat (Urethane) & bubbles | 24 January, 2006

We've got a problem here, where we're hand spraying a urethane conformal coat on over a rectangular inductor. The inductor body is a plastic tube, with potting to fill. The top is not always flat (sometimes concave). The problem is that bubbles appear in the conformal coat on the potted surface, sometimes right after bake and sometimes days after.

Any ideas on what the root cause is and how to prevent the bubbles?

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Conformal Coat (Urethane) & bubbles | 24 January, 2006

Contact Graham Naisbitt he is an expert in this field.

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Conformal Coat (Urethane) & bubbles | 24 January, 2006

Graham Naisbitt is definitely an expert on conformal coating, he has posted answers here for years.

Check your material to see what the cure mechanism is. Most urethanes need moisture from the air to cure, the higher the relative humidity the faster the cure. If the coating forms a skin before all of the solvent has had a chance to escape the solvent will form bubbles inside the coating. A little less humidity and you will end up with a matte finish, if it is really dry you'll have a slow cure that could be a shiny surface. If this is the problem, any areas that have a thicker coating would be the most likely to show bubbles.

Another possibility is the coating material is reacting in some way with the potting material. To check this out you will need to find out what the potting material is and then talk to the coating manufacturer. It may mean you have to change to a different brand of urethane.

Good luck, Mike F

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Conformal Coat (Urethane) & bubbles | 24 January, 2006

We all genuflect to Graham Naisbitt, a generous contributor to SMTnet.

Moisture-curing urethanes may form minute bubbles if the coating is too thick or the coating environment is too humid.

So, tell us more about your urethane and the method you use in controlling application thickness. Also, tell us about the potting compound and how you determine that it's fully cured.

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