I recommend looking at IEC 60950-1 Annex J. It gives the electrochemical potentials between various dissimilar metals and sets a limit of 0.6 V for the electrochemical potential. Zinc plating to gold could be a problem. The type of stainless steel used will also affect corrosion, so you will need to know if you are using austenitic stainless or chromium stainless.
Ted Eckert American Power Conversion/MGE http://www.apc.com/ The items contained in this e-mail reflect the personal opinions of the writer and are only provided for the assistance of the reader. The writer is not speaking in an official capacity for APC-MGE or Schneider Electric. The speaker does not represent APC-MGE's or Schneider Electric's official position on any matter. "Marko Radojicic" <mradojicic@strat alight.com> To Sent by: <[email protected]> [email protected] cc Subject 01/10/2008 01:57 Zinc passivation vs. Stainless PM Steel hardware selection Hello, If anyone has a materials background, your opinion would be appreciated. We are specifying hardware (screws, washers) to assemble anodized aluminum modules to gold PCB pads. These connections will also be providing grounding and I would like to ensure long-term corrosion control. The stack-up would thus look like the pancake below and hence have two Galvanic interfaces to address: Aluminum <screw, washer> Gold The only two real choices for this type of hardware are stainless steel and CRS (cold-rolled steel) with zinc passivation. I see from Wikipedia that zinc is a preferred interface material for dissimilar metals but I’m not quite sure I understand why (sacrificial anode?). Stainless is closer to gold on the galvanic scale while zinc is almost at the other end. The only rule-of-thumb that I’ve used in the past is 0.5V maximum Galvanic potential difference for non-salt humidity (and 0.25V if in a salt humidity environment). This situation should be fairly common with aluminum heat-sinks mounted on PCBAs as well. I thought I knew the best strategy but, like most things, the more one pokes around, the more you find out you don’t know. Thanks, Marko Marko Radojicic Manager, Qualifications and Regulatory StrataLight Communications, Inc. 151 Albright Way Los Gatos, CA 95032 [email protected] Desk: 408.385.3033 Cell: 650.575.3865 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
