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Rich _ I will look into the fourwave washers. Yes I am concerned about the performance of the BGAs - I have 6 of them in various sizes and pad arrangements. Already in production I have seen these be fragile. I used a .090" board and support it in 9 spots against a metal back plate but the metal will expand at a different rate and so that could get interesting. In the past I have seen some issue with the conformal coatin getting under chips and stressing the solder connect during thermal cycling. I am concerned about conformal coating under the BGAs - I will have to do some research to see if coating BGAs works or not. I agree that HALT testing makes a lot of sense. > Chris, > > The problem you may find with conical washers is that if the spring is weak enough to deflect when you screw down the pcb so that you are able to take up slack if the pcb were able to contract, then you are likely to get damage to the copper pad on the pcb - unless you use a plain washer underneath, which then defeats the idea of having an all-in-one fixture. If the spring is so stiff that it does not compress much then you may still damage the pcb but you would also not ensure a reliable contact to the pcb in case of pcb shrinkage. > > The type of washer I have used to good effect (including telecoms applications that also require long life) are those described in http://www.wclco.com/pdf/spring/fourwave.pdf. > > Good point about adding via holes around the main annulus, I use such a technique where it is necessary to obtain a high current-carrying capable connection to one or more inner layers of the pcb. > > As regards the mounting of BGAs, as I understand it one of the reliability problems you can run into with these devices stems from the fact that the connections formed by the solder balls are more likely to fracture (more brittle, I guess) than conventional lead-frames. If the semiconductor device expands at a different rate to the pcb it is mounted on for any reason (e.g. differences caused by different coefficients of thermal expansion or different rates of moisture take-up) then you will set up a force that wants to shear the electrical joints. Obviously we are not talking about huge forces, but we are also not talking about strong materials either. Thus you may find that the assembled pcb works fine for some months – or perhaps years – but it does not give you the long product life you are looking for in the aggressive operating environment you described. Conformal coating will help by excluding moisture (providing it is itself not hydroscopic) but it will not help with temperature changes. You may therefore like to consider having some Highly Accelerated Life Testing (HALT) done, or at least talk to your suppliers of laminate and chips if they can offer any advice. [I am not a purveyor of such testing services and so I have no commercial interest in whether you take-up this advice or not.] > > Regards, > > 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
