>-----Original Message----- >From: Kim Flint [mailto:[email protected]] >Sent: Monday, February 10, 2003 6:15 PM >To: [email protected] >Subject: best screw/washer choices for attaching PCB to Chassis? > > > >Hi- > >We have a fairly straightforward requirement of mounting a PCB >into a steel chassis. The chassis has PEM type standoffs and >the PCB is held to them with machine screws. These connections >provide an electrical ground path between the PCB and the chassis. > >In other words, we have what seems to be the fairly ordinary >and common set of requirements. We need to have mechanical >reliability (so the PCB is held in place), nothing should >break during assembly, the screws should not be able to work >themselves out, and electrical conductivity should be low for >a low impedance ground path. Seems simple, yet all of us here >have a different opinion about how to do this properly, we've >all done it a variety of ways in our past, and none of us >seems to have the right expertise to really claim to know the >definitive answer. > >I'm hoping to get some expert opinions from this group, or at >least some idea of what others do assuming there is some >reasonable justification for it. Can you help? Or perhaps >point me towards some good discussion on the subject? (I did >search the 2+ years of mail I have from this group, since the >archives don't seem to be online...) > >The questions are: > >Screw/Washer choice: >- should a washer be used or not? >- if a washer is used, should it be a locking washer? >- Is it possible for a locking washer to cause unwanted damage >to the PCB? >- Is an adhesive like Loctite a reasonable alternative to lock >washers in this case? >- should the screw be zinc plated? or some other plating? > > >PCB layout: >- Should we use a plated through-hole with ground planes >connected inside the hole? >- or a non-plated hole with vias in the surface layer pad >connecting to the ground plane? >- What electrical or reliability concerns relate to this choice? >- how large should the surface pad be assuming a 4-40 screw? > >anything else we should be considering? > >Thanks for any input you have! > >kim
Kim: For the board, I would use a plated-through hole. The pad on top should be a bit larger diameter than the bolt head. The pad on the bottom should be a bit larger diameter than the PEM stand-off face. For the bolt, use a machine bolt with a pan style head. Put one lockwasher under the bolt head. You don't need a flat washer (unless your board is oddly fragile); keep it simple. But if you do use a flat washer, put it between the lockwasher and the board top pad. The lockwasher should be a split-ring style. True, a toothed lockwasher will "cut" into the board pad, but you want axial compressive force, not torque withstanding ability. Besides, the toothed washer might leave some FOD in your box. Use bright nickel plated hardware. Put a dab of Locktite into the female thread on the PEM before assembly. Don't worry about conductivity loss from male to female threads; your primary ground path is from the lower pad of the board to the face of the PEM stand-off. (And I'm assuming that the PEM will mashed into the baseplate with an excellent bond.) Make sure the PEM faces are clean before dropping the board on them. Avoid getting finger oils on the mating faces of PEM and board. Torque bolt till the washer is flat. Regards, Ed This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc 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] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
