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Poor adhesion of Cobalt plating, copper substrate

May 6, 2009

I am working for an automotive part manufacturer. we are trying to plate Co onto Cu substrate. the process is as follows, Co strike layer (0.5 micron 4ASD), then Co plating (3-5 micron, 4ASD). We just noticed that the bonding strength between Co strike layer and Co plating was poor but the bonding strength between Co strike layer and substrate was good. The bonding strength was slightly improved when I lowered the current density of Co strike layer from 4 4ASD to 3 ASD.

I just wonder what the possible reasons are for this problem and how to improve it? many thanks.

Adam

Adam Zhang
Plating shop employee - Nagoya, Japan

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It could be any number of reasons, but the most probable is the strike is slightly passivating in the air because it is taking too long to get from the strike rinse tanks to the plating tank. It is very much like nickel.
There is also a possibility that you have some organic contamination in the strike tank. pH of the plating tank can be extremely important. What are you using? I assume that this is a cobalt sulphate tank.

James Watts
- Navarre, Florida
May 8, 2009

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First of two simultaneous responses -- May 8, 2009

Hello Adam,

lower more down the ASF for the Strike process. Also wondering me which kind of Co-Strike you are using, are you doing the strike in the same solution like the proper Co-Process?

Regards,

Dominik Michalek
- Mexico City, Mexico

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Second of two simultaneous responses -- May 11, 2009

Thanks James.

Yes, I use sulphate bath for Co plating but Chloride bath for the strike layer.

I was thinking of two possible reasons, pH, as you suggested, and internal stress. If latter is the case, probably I can do nothing about it at the time being.

As to the passivation, this is very important information. Many thanks. May I ask another question regarding to this: how to re-activate the Co coating? by acid dipping?

Adam Zhang
- Japan

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May 15, 2009

Although it is widely assumed nickel and cobalt are very similar in their metal deposition characteristics, do not be lulled into a false sense of security; they do have a lot of subtle differences.
For instance, cobalt tends to be more highly stressed than nickel when deposited from comparative solutions, so using cobalt chloride may be one of your problems. Personally I would suggest you use a cobalt sulphamate strike - since this is not a well known system I suggest you base your formulation on a nickel sulphamate strike.
I would also tend to use a sulphamate bath in place of a sulphate one for the same reasons as above.
Regarding reactivation of cobalt, I have never had any real problems with it, with the caveat that I never let it really passivate. If you are having a problem, it may be worthwhile you reactivating it in the plating bath by using an anodic "etching" with about 2ASF for about 30 seconds followed by conventional cathodic deposition. This will be dependent on the bath pH, so I would suggest you use a lower pH system in preference to a higher one.
One thing I found was that the conventional nickel brighteners are nowhere as good with cobalt.

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