Gold plating of waveguides

Letter 0005

Gold plating of wave guides

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 1 mA/cm2. The gold layer looks matte and brownish. Is there any way to get a shiny finish without polishing?

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

Alex K
-West Virginia

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,
Virtually all professionals buy proprietary gold plating solutions rather than trying to make them from scratch according to old formulas; that's perhaps what you might want to do. Our supporting advertisers who specialize in this include Gold Touch, and Technic.

Jeff,
As you probably know, many wave guides are electroformed because it's such a great way to build complicated hollow shapes. Suppose Alex were to polish an aluminum or wax mandrel, then electroplate it with gold, then back it up and give it integrity by electroforming with copper or nickel, then dissolve away the aluminum or wax. He'd have a copper or nickel waveguide with a highly polished gold surface. From your experience do you see any reason this wouldn't be feasible? Thanks.

Vijay,
Thanks for the feedback. But wave guides often are electroformed, and it's a good application 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, but 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, this application doesn't sound line-of-sight where sputtering would work; it sounds like Alex wants to plate the cavity (interior) of a waveguide, not the outside of it.

Readers, learn more:
Letter 49862 proposes a cleaning sequence for copper wave guides that might be applicable. Letters 1799, 39679 and 45737 address electroforming of wave guides. Letter 49813 is about a cracking problem in plating of copper electroforms. Letter 43876 addresses the general issue of making robust copper electroforms. Patient application of the search engine will uncover other related topics.

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

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