Gary, Chapter 33 of my book Robust Electronic Design Reference Book, Volume 1, discusses bonding. On page 33-2, for direct metal-to-metal bonds it recommends: * 8,300 kilopascals (1,200 pounds/square inch) pressure for soft metals. * Up to 10,300 kilopascals (1,500 pounds/square inch) pressure for hard metals.
Tables 33-1 and 33-2 on the same page give recommended torques for metric machine screws/bolts from M3x0.5 through M10x1.25, and english machine screws/bolts from #5-40 through 3/8-16. These tables also give the recommended maximum ampacity (current-carrying capacity) for single and multiple wires connected by metal-to-metal compression bonds. With 58 references for that chapter, I don't remember just where I found this information originally, but I believe it was in several military standards. The most important factor is to have enough pressure (or scrubbing while making contact) to break through oxides/sulfides/crud on the metal surfaces, and to get the first few asperites to form contact dots. Once this has been achieved, the resistance drops much more slowly with increasing pressure, mainly from a reduction in the constriction resistance (distance that current must flow to travel through the nearest contact dot). And once contact has been made, we must maintain sufficient pressure to maintain the contact dots. Tin is a favorite plating material for EMC shields, because it is soft and its oxide(s) are fragile. Thus a relatively-low pressure overall will give us stable low-resistance electrical contact. Figure 2 in http://www.bita.org/datasheets/Accu-PT2008ContactResistance[1]-SE-LU.pdf illustrates this effect for beryllium copper, tungsten, and palladium. John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE dBi Corporation http://www.dbicorporation.com/ 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/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
