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Working With Transparent Conductive Coatings

2004

We are designing an electronic device, which requires a transparent-dielectric circuit board. This will be a microwave circuit, running at 2.45 GHz, and so excellent electronic qualities are important.

So far, we have narrowed the dielectric (substrate) down to Dupont ST505. The question is, what to deposit? I see ITO (indium-tin oxide) everywhere, but its resistance is not good. (50 ohms/sq. BTW, WTF is a sq? sqft?)

I've also noticed anti-EMI films that have a very acceptable resistance of 5 ohms, but there's no indication whether it could be used in an electronic circuit (etchable, parts-mountable), much less microwave. No indication of what metal it is, whether its bondable/solderable nor why it's so much better a conductor.

Gold should be a good conductor; ductile, so could tolerate a little bending; sputterable, solderable. Very conductive for microwave, which prefers to travel only on the 'skin' (outside few atoms). But no one seems to be sputtering it on PET regularly.

Our part, after electronics are mounted and checked, will be completely cast in polyurethane, so will be safe from bending/scratching/etc. The -conductive- material doesn't have to be transparent; translucent is OK. Even opaque. But low resistance and low RF parasitics is important. (non-ferrous)

If the answer is thick ITO, can you direct us to a good resource for working with ITO in electronics. Can we solder it? Epoxy? Will it corrode in contact with polyurethane? A finger? Can you etch it, and how? What are the gotchas. Any resources on working with ITO or other sputtered layer in electronics, is appreciated.

While I'm here, does anyone know what the microscopic surface of electroplated gold looks like? My brother says it's covered with whiskers, giving it the reflectivity, and so isn't good for microwave. Whereas immersion gold has large 'grains' (crystals?), and so propagates a microwave signal much better? Can anyone confirm or deny?

C.A. Cook
- Dallas, Texas, US

You asked in your question WTF is the reference to sq.? We had the same question a couple years back. We decided that the answer was square anything units wise. It could be square inches, feet, meters ... We think that is the correct answer.

Jerry Tate
- Wichita, Kansas
2004

Ohms per sq. is properly 'sheet resistivity'. And yes, it doesn't matter what units. Any changes due to the 'length' are exactly cancelled out by the same change in 'width'. That's what makes it such a handy measure. Note that sheet resistivity DOES change with thickness though! As to skin depth, in copper it's around 50 microinches. You definitely want a smooth surface. Good Luck.

Tom Gallant
- Long Beach, California, USA
2004

Mr. Cook, I came across your post, and thought that a conductive nano particle Zinc oxide might be an answer to your problem if it still exists. This is a fresh product we are currently promoting, and I'll be happy to assist you with any information you need.

BHOOMIT SHAH
- White Plains, New York
2005