Plating on Aluminum Revisited

Plating on Aluminum Revisited
In my July 2007 column, I answered a question about plating of copper and tin on an aluminum substrate.

Don Baudrand, a reader, responded with an e-mail suggesting what might be going on in this case. An abbreviated version of the original question and my response are printed below. Don’s comments follow.

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Plating on Aluminum
Q. We’re having problems with plating copper and tin on a 6000 series aluminum alloy. Our process consists of using a zincate followed by copper and finally tin plate. The copper thickness is approximately 0.00007–0.00014 inch and the tin thickness is approximately 0.0002–0.0005 inch.

Finished parts are baked at 325°F for one hour. We find that some of the parts blister.
What can you suggest? C. J.

A. Your one-hour bake at 325°F is a rather harsh test for plated aluminum, and it’s possible that some of the copper layer is diffusing into the zincate layer. Copper diffusion into the zincate layer can can cause blistering due to the formation of intermetallic compounds. I suggest a lower bake temperature, no more than 275°F.
I would also investigate the zincate process step you’re using. You don’t state whether you’re using a “home brew” or a formulated zincate bath. There are a number of different formulated alloy zincate baths that perform much better than the home brews.

Are you performing a single zincate step or the preferred double zincate step? In the double zincate process, the aluminum is zincated and then the zincate is partially stripped from the aluminum using a nitric acid dip. The part is then zincated a second time. Check with your chemical supplier to determine how best to accomplish the double zincate process.
One last thing to think about is that the tin might be diffusing through the copper and causing problems. A possible solution, if this is the case, is to apply a heavier copper layer.

DON’S COMMENTS:
Regarding the plating of copper and tin on aluminum (or steel): The very thin copper under-layer will diffuse into the tin, forming a weak eutectic alloy that can fracture. Also by alloying with tin, the copper is separated from the basis metal aluminum (or steel) as the case may be. I found that for “re-flowing” plated tin over copper it was necessary to have at least 0.0002 inch of copper to prevent blistering. The lower baking temperature would suggest that a thinner copper deposit would work, but not as thin as reported.