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"Sn-Co alloy coating"


It was found that it's very difficult to deposit a Sn-Co alloy coating with a thickness of more than 5 microns(without agitation). The problem is that the deposit will become dendritic with prolonged plating duration. If you have any idea about overcoming this problem, Your help and suggestions will be highly appreciated.

Keming Chen
- Loughborough, Leicestershire, UK


Well, I could not find cobalt-tin but I did find nickel-tin and they will be close. Since it is a stannous bath, you certainly do not want to use air. My guess is that you are forming some stanic tin which is precipitating out giving you shelf roughness. My first try would be to increase filtration which will also give you more fluid agitation.

James Watts
- Navarre, Florida


Over this side of the world, SnCo alloy plating has been used as a replacement for Hex Chrome. It looks similar to chrome but not exactly like chrome. The deposit is very thin (about 0.3micron). I may be missing something but why do you need such a thick deposit? Air agitation is a definite no-no as suggested by Mr.Watts.

SK Cheah
New Crown Hong Kong Co.Ltd. - Malaysia


Tin-cobalt alloy plating is quite unusual. I know the International Tin Research Institute in Uxbridge (UK) did some work some years ago with tin-nickel, but they used an alkaline bath and could only get very thin decorative deposits. If the tried to get thick ones, the deposit became very brittle and flakey. However, none of this should deter you from trying to develop a tin-cobalt system. I would suggest you look at high tin and low cobalt compositions as one of the major problems with cobalt deposits is the codeposition of the two phases. This leads to very high stress and poor adherence. It may be possible to use the ITRI basic system but replace the nickel salts with cobalt. There are numerous cobalt baths, based on the well known nickel formulations, that claim to deposit cobalt, but my experience is not as positive as those claimed! The baths tend to be inefficient and do not respond to additives as one would expect. Furthermore, there is the big problem with stress.

As far as dendritic growth is concerned, I would suggest you look at different levelers and brighteners. You may also find benefit from using low current densities and possibly temperatures about 40C. However, all this depends on your bath formulation, which you do not give.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

Ed. note: You may want to hear "The Lost Art of Tin-Nickel Plating".


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