http://www.ieee-pses.org/symposium http://www.emc2004.org/ -------------------------------------------------- Chris I'm not sure of what you are inquiring. See comments below.
Chris Wells wrote: http://www.ieee-pses.org/symposium http://www.emc2004.org/ -------------------------------------------------- I want to bond my PCBs to my metal housing effectively for many reasons: mechanical, thermal, electrical safety and EMC etc. I know there are a lot of safety related threads we have talked about on this topic in the past - But my practical focus right now is getting a bond without tearing up the pads while still being effective for all those design issues. I am looking for your advice on the best approach - all tangents welcome but be sure to address the practical topic as well. Background I have 9 PEM nut stand offs that support the board which take # 6 size screws that come in from the top side of the PCB with standard internal star toothed electrical lock washers. 6 of these are nickel plated brass hex standoffs (with male thread on one side)and the remaining 3 are white zinc panheads. I understood that these internal toothed designs help give a bite and provide a gas tight seal on metal surfaces. My product is used in an industrial enviroment with large thermal swings and occasional condensation. The product also needs to perform for at least 10 years. * Question 1 - Does this gas tight topic apply to the plated through hole pads of a PCB if the surface is tined copper? Gas tight is always a good idea if moisture or corrosive atmosphere are involved. If you can't do gas tight you do gold. * * What about the gold treatments used when there are fine pitch BGAs? I have that gold treatment on my board. Gold will contaminate most solders and cause solder bridging. * * Question 2 - What is the optimum plating for the screw, lock washer and PEM nut? I'm not sure there is an optimal plating or why it is of concern. Other factors will drive plating thickness. * * Question 3 - I could adjust the solder paste mask to apply solder to the Plated through hole surface to give more material to connect with - good idea? Probably not a good idea. This may increase solder bridging and the solder may flake when the screw is removed. Tiny whiskers of solder can bring a nightmare to field repair. * * Question 4 - What is a practical weight for the copper on the top and bottom of a .090" 10 layer board that must support fine pitched BGAs? I would think you would stick with 1 oz cu unless there are high current problems. * * Question 5 - Are there better styles of lock washers to use? I have heard of some dimpled styles that may be more forgiving. Internal tooth or external tooth actually cut through the solder and make a pressure weld to the copper. They will do the same for the plating on the screw. This is preferable for higher currents. Dimpled washers do not cut through and may be problematic if oxides are formed. Split locks tend to open the fibers and can be problematic on reuse. In other words field service might see problems the factory never would see. * * Queston 6 - Are there any form of pastes added to the threads or other treatments to the bond in general that would help? I can't think of any pastes that would help. I can tell you 'Loctite' is corrosive and bad news. * * Note that my production group wants to keep it simple and I must justify all extra procedures. For example I had captive washers on the screws so they did not have to add it by hand. Have you checked out www.ipc.org? I have found their recommendations to be very reliable. Fred Townsend * * Thanks Chris Wells Senior Design Engineer - Eaton Electrical [email protected] ------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. IEEE PSES Main Website: http://www.ieee-pses.org/ To post a message send your e-mail to [email protected] Instructions for use of the list server: 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 ------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. IEEE PSES Main Website: http://www.ieee-pses.org/ To post a message send your e-mail to [email protected] Instructions for use of the list server: 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
