Letter 18036

Post Cleaning of Electrodeposited Tin over Autocatalytic Nickel  

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Our company manufactures electronics test equipment that incorporates aluminum (6061-T6) enclosures. The enclosures are electroless plated with Ni (197 uin) & electrodeposited bright Sn (100 uin). Our post-plate, pre-assembly process requires the enclosures to be cleaned using isopropyl alcohol & a soft lint-free cloth. Usually one light wiping removes any surface residue. However,we have experienced that after 10+ wipings, a heavy gray/black, & sometimes amber colored residue is still present. The presence of this residue reduces the electrical sealing/shielding effectiveness of the enclosures. Prior to cleaning, the appearance of the enclosures vary: some are uniformly bright & shiny, & some reveal amber blotches. The cleaning results are inconsistent as well. Typically the amber blotched enclosures do not clean well, however the bright & shiny enclosures have yielded the same results.

1)What is the amber finish color (it does not clean off)? Is this perhaps thin areas of Sn with Ni revealed? Perhaps staining from the plating and/or rinse processes?
2)What is the gray/black residue? Perhaps oxidation? Is it corrosive? If oxidation, is there any documented change in surface resistance and/or transfer impedance?
3)Could the cleaning/wiping process be abraiding the soft Sn? If so, why is this not evident in all cases (i.e. the cloth comes up clean)?
4)Sometimes the residue is amber in color, usually after several (3-4) wipings. Perhaps Ni removal or another process residue?
5)Any suggestions to improving our cleaning process?

Thanks for your assistance.

Marty Garrett
- Wichita, Kansas

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This is a low probability item but the only idea I can suggest is that you are getting brightener residues from the tin plating. Are the parts run through a post plate dip in an alkaline solution? Are they passivated after tin? If not I think there could be some possiblity of what I suggested since the reaction products used in the brighteners are generally alcohol soluble.

Gene Packman
- Syosset, NY

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Gene,

Thanks for the response. To answer your questions, the plater is using a non-alkaline post cleaning process. I am uncertain about a passivation process; is this a post induced passivation process (as the result of a post alkaline dip solution), or is it naturally occurring via the environment? We just received a batch of reworked enclosures that had been stripped and replated. The replated enclosures are uniformly bright and shiny with no heavy gray/black residue. The plater has informed us that they are still investigating the root cause of the residue problem. They are monitoring all facets of their process very closely to eliminate as many variables as possible.

Thanks,

Marty Garrett
- Wichita, Kansas

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