Letter 0005

Gold plating of waveguide cavity

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I am working for West Virginia University. We have a microwave cavity that needs to be plated with gold (1-2 um).

Maybe it sounds simple for professionals but not for us. We have tried phosphate gold solution (1.3 g KAu(CN)2, 120 g K2HPO4, 30g KH2PO4 per/L) and current density 1mA/cm2. The gold layer looks matte and brownish. Is there any way to get shiny finish without polishing?

We would be glad if you could help us with advice. Thank you.    

Alex K [last name deleted for privacy due to age of posting]
-West Virginia

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First, what type of waveguide/cavity material? I'll assume Cu although Al is also possible as well as ferrous based (you did say you were in an edu).  

The next issue is that of the matte brown finish. Sounds like you're burning the plate. I suggest a Hull cell analysis. Essentially, you plate up on a brass shim that is placed at an angle to the anode. This allows you to compare varying current density to plated appearance.  

I am also assuming that you are using an appropriate anode w/ appropriate surface area. A Pt coated anode works best although S.S. can be used for short periods of time without damaging the bath. The surface area of the anode to cathode should be about 1:1 although 2:1 will be ok.  

Good luck, Jeff  

Jeff Albom
- El Granada, California

I would recommend you look at Plasma Enriched Deposition (aka Sputter coating) your university will have this facility I am sure. The film will have better adhesion, better structure and better uniformity.

Vijay Simha
- Bangalore, India

Alex: the overwhelming percentage of professionals buy proprietary gold plating solutions rather than trying to mix them from scratch. That's probably what you should do. Our supporting advertisers who offer small quantities include Cohler.

Vijay: thanks for the feedback. But there is a place for gold electroplating. The third advantage you listed for sputtering is true, i.e., that it's easier to get coatings of uniform thickness with sputtering than plating. The other two purported advantages aren't. In fact, in many cases electroplating yields 'perfect adhesion', which means that it is impossible to separate the materials at the boundary -- either the coating or the substrate will rip instead. Plus, it's not clear that this is a line-of-sight application where sputtering would work; in fact it sounds like it isn't.

Ted Mooney, P.E.  finishing.com Brick, NJ

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