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Cobalt electroless bath does not work with EDTA as complexant

Hi, I am doing a cobalt electroless plating process with pH 8.85, sodium hypophosphate as a reducing agent. Also, I use EDTA as my complexing agent, however, no plating is observed on my Silicon substrate. Instead, there is a distinct blackening of my substrate. Any thoughts on this? The plating solution works with the same concentration of sodium citrate and malic acid as complexing agents, but not EDTA.

Su Wen Yu
student - Singapore
2004

It would appear that your cobalt is too strongly bound to the EDTA to get it to plate out. Possible areas to look at are weaker EDTA solutions, a different pH and higher temperatures. Will it plate out on a clean piece of steel?

James Watts
- Navarre, Florida
2004

I would like to do electroless plating of a Cobalt-Phosphorus-Iron alloy. However, when I add Iron sulphate to my electroless Cobalt bath consisting of hypophosphite, sodium citrate, boric acid and ammonium sulphate as well as cobalt sulphate, no plating occurs at all. The cobalt plating bath works without the addition of iron sulphate. May I know of a suitable electroless bath which I can use? I am getting desperate, thanks a lot.

Su Wen Yu
student - Singapore
2004

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First of two simultaneous responses --

Su wen Yu, I am a bit confused. In your initial submission you say you cannot get your electroless cobalt bath to deposit when you use EDTA as the complexant; I am not surprised because EDTA is about the strongest complexing agent you can find. To get the cobalt bath to work, you need a milder complexant. The most common ones are lactates, citrates, glycolates or maleates. I would suggest trying citrate.

You then go on to ask about a cobalt-phosphorus-iron system. Is this a different problem, or is it a follow up to the first one? Alloy deposition is always difficult and electroless alloys are most unusual. The reduction potentials of cobalt and iron (II) are quite similar (-0.28 and -0.403V respectively, but the water complexes have slightly different strengths and they inhibit the reduction of cobalt in favour of iron; that is why you get reversed preferential deposition of iron in an electrolytic system. If you use Fe(III), the reduction potential is stated as -0.037V, but the reduction goes through an Fe(II) step, so there is no benefit in using ferric ions. Furthermore, Fe(III) is not a good start for any plating solution. The alloy you want is quite esoteric, so I suggest you do a good literature search before seeking the easy way by asking for help on the Internet

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2004

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Second of two simultaneous responses --

The temperature I use is 80 °C, which is the normal plating temperature. Due to the nature of my project, I am comparing the effect of different chelating agents on the plating of cobalt. I use the same functional group to metal ion ratio for each chelating agent. Will this still result in overstability of complex ion? also, I noticed that many edta solutions use hydrazine instead of hypophosphite as a reducing agent, can this also be a reason?

Yu Su Wen
- Singapore
2004

This is a followup to the first part. Part of my project is to change chelating agent and observe the different effects on plating. The other part is to incorporate iron into the plating and observe how different concentrations of iron affect the plating. Citrate and Malic acid have worked perfectly for my case, but EDTA does not. As for the iron, I have read some texts and found that this is indeed possible. Using the solutions provided in the text, I have made the plating bath only for it not to plate at all. Upon addition of exaltants like boric acid, plating does indeed occur, but at a much slower rate.

Yu Su Wen
- Singapore
2004