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Removing chromate passivation from electroplated copper foil

June 15, 2010

I've purchased some electrodeposited copper foil which was sold with a chromate passivating layer which I intend to remove. The suppliers recommended a few seconds bath of 10% sulphuric acid or 25% hydrochloric to remove this layer.

I've made a number of attempts to remove the chromate passivation from the copper using both recommended acid baths. These processes have removed most of the powdery 'loose' looking part of the chromate layer, but leaves a matte residue on the foil that can be removed by scraping to reveal bright copper beneath - I suspect this might be more chromate that has not been removed by the acid, but can't be sure. I cannot use physical rubbing or scraping methods to remove the matte layer from the foil pieces that are to be used in the experiment, as this would leave marks and stresses in the material that would make it impossible to carry out my experiment.

Is the matte layer I see after the acid bath likely to be residual chromate that could be removed by an etch or by some other method? How can I restore the bright copper surface?

Eleanor Chalkley
Physicist - Birmingham, United Kingdom

June 18, 2010

There may be a very thin layer of electroplated brass or other metal underneath the chromate, and this may be the cause of the residue.

There are several proprietary surface roughening techniques that are applied to one side of copper foil to improve the bond strength when it is laminated to a circuit board. The foil that you purchased may have this extra coating.

I think that you will need to clean the surface more aggressively or to give it a microetch, such as a short immersion in a peroxide/sulfuric acid or sodium persulfate/sulfuric acid solution to remove 30-40 millionths of an inch of metal.

Good luck!

Lyle Kirman
consultant - Cleveland Heights, Ohio

June 21, 2010

You may need to use a microetch to remove 30-40 millionths of an inch from the surface. In circuit board fabrication, this is usually a sulfuric acid/sodium persulfate bath at ambient temperature.

It is common to have a thin elecroplated layer between the foil and the chromate conversion coating to promote some roughness and better adhesion during lamination. This is often a plated coating of brass or bronze. Your foil may have one of these.

Lyle Kirman
consultant - Cleveland Heights, Ohio

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