http://www.ieee-pses.org/symposium http://www.emc2004.org/ -------------------------------------------------- Rich - thanks for responding! Some comments embedded. Chris
----- Original Message ----- From: [email protected] To: [email protected] ; [email protected] Sent: Sunday, August 08, 2004 6:46 PM Subject: Re: PCB star washers - ones that won't tear up pads??? Chris, In order to meet safety requirements for connection of a protective earth, (in 950 anyway) it is necessary to use a washer that has resilience so that it can compensate for possible shrinkage in the pcb laminate. None of the star washers I've seen fit this bill and I'm not sure that a dimpled design would either, unless perhaps you used a solid washer against the pcb, then the dimpled washer, then the nut. Chris response - It has been suggested to me to use conical washers against the PCB. For example http://www.wclco.com/conical-washers.htm or in a SEMS arrangement http://www.wclco.com/pdf/thread/WCLTF22-23.pdf My PTH are reinforced with ~12 vias all around the main hole and tied to the upper and lower pad. I am not relying on the PCB connection for safety but redundancy makes sense to me. I have used two types of washer that you may like to consider, one is a split design made from steel. The other is a 'wavy' design that was made out of phosphor-bronze (though you may be able to get spring steel, and may have to with RoHS for Europe). Alternatively, you could use the type of 'hollow rivet' that goes through the pcb laminate (I've seen them used on pcbs but cannot tell you who the supplier is). Using this construction the compression associated with through-laminate screws etc is taken up by the metal washer rather than the laminate. You could then use star washers if you wished. Chris response - I understand the rivet approach but production wise it would be undesirable extra step which I will avoide if possible. Also - Dimensionaly I suspect I would have to watch the height of the rivet since I need to hold this rather tight. If you want your design to be truly gas tight and keep out moisture (problems with electrochemical corrosion with different metals in the kinds of environment you describe) then perhaps a proprietary spay-on oil mix is what you need? Perhaps someone in military could advise, or take a look on the www. Of course, your production dept may be none to keen on this proposal. I do not know if it is possible to get ready-treated parts (washers, nuts, etc). BGAs can be very sensitive and suffer from cracking of the 'solder legs' with thermal stress over time: you also mention varying moisture concentrations and that implies variation in dimensions if the pcb laminate is hydroscopic (as FR4 is). My suggestion would therefore to be to consult with all your BGA device suppliers to see what they recommend in terms of pcb substrate, copper weight etc and then fit your mechanical design around that. It may well be that the surface layers turn out to be too thin for good mechanical strength - no problem, use plane washers against the pcb or fit the type of hollow rivet mentioned previously. Chris response - We coat our boards with a conformal coating. Our coating can not cover all parts of course so one still gets some enviromental effects. While it is possible to get chemicals that improve adhesion of the manufactured part you may also want to think how you would deal with repairing failed units in the field. If the product is destined for Europe then ease of disassembly to facilitate easy(ish) compliance with the WEEE regulations may be a consideration for you (your product life is 10 years? - environmental regulations in Europe are getting more stringent, not more relaxed). Chris response - The four plug-in boards can be removed in the field. Yes the European Gauntlet does help on job security....:-) I am just now coming to grips with lead free solder :-( Just some ideas to get you started... Richard Hughes ------------------------------------------- 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
