Thermal Camera Varnish problems?

Question

Have you ever had bad experiences with black Varnish for thermal measuremen ts? We use this varnish for 10 years without problems, in many different pr ojects, but this time wenn it is applied, many short circuits appear over t he PCB in different positions... Strange thing is that I used the same varnish again over an old version PCB and it was ok... There were some minimal layout changes between old versio n and the new version, but the layout rules are the same. Also tried differ ent PCB manufactures, with same result. Maybe this new batch was contaminated with something in the production line , but the controls there are extremelly rigid...

And yes, I already made the thermal measurements without the varnish, were no big deviations from the old measurements, but still I need to indentify where does this weakeness come from.

Phil Hobbs · Accepted Answer

Carbon loaded paint is dangerous that way. The good news is that clear varnish works just as well out at 8-14 microns (the usual thermal IR band). It needs to be >~ 250 um thick so that it's nice and opaque but not a significant thermal insulator.

Cheers

Phil Hobbs

John Larkin · Answer

Black whiteboard marker works well and rubs off afterwards. Kapton tape has emissivity close to unity, too.   --  John Larkin         Highland Technology, Inc lunatic fringe electronics jlarkin att highlandtechnology dott com

Phil Hobbs · Answer

My roll of Kapton tape is roughly 3 mils.  How thick is yours? It doesn't look like the black smears at the bottom do anything much--is that the whiteboard pen? Cheers Phil Hobbs --  Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net

Adrian Tuddenham · Answer

Any possibility that the pot hasn't been stirred properly in the past and you are now using up the thicker dregs from the bottom?

It seems possibly that a small difference in carbon density could produce a large change in conductivity as the particles begin to come into contact with each other.

-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply)

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John Larkin · Answer

It calipers at 2 mils, adhesive and all. Given that, it's remarkably dark at thermal wavelengths. Just crud, I think.

Thermal Camera Varnish problems?

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