Marko, We regularly design in this type of construction in equipment intended for adverse environments, and always use the stainless steel solution. I have never known any problems with using stainless steel. A quick catalogue search reveals that stainless steel fasteners are readily available in both A2 grade (304 S 15) and A4 grade (316 S 16). Either of these grades should be suitable for your application. Best regards
Neil R. Barker CEng MIET HonFSEE MIEEE Manager Quality Engineering e2v technologies (uk) ltd 106 Waterhouse Lane Chelmsford Essex CM1 2QU UK Tel: (+44) 1245 453616 Fax: (+44) 1245 453571 Mob: (+44) 7801 723735 P Please consider the environment before printing this email. From: Marko Radojicic [mailto:[email protected]] Sent: 10 January 2008 19:57 To: [email protected] Subject: Zinc passivation vs. Stainless 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 Sent by E2V TECHNOLOGIES PLC or a member of the E2V group of companies. A company registered in England and Wales. Company number 04439718. Registered address 106 Waterhouse Lane, Chelmsford, Essex, CM1 2QU, UK. - ---------------------------------------------------------------- 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
