That's interesting. I can understand that the glass in the PCB is a fluid and will flow over time. But is there something more quantifiable than "gradually squashes"? Would this mean by next Thursday, next year, or 1000 years from now? By how much would it deform over time? If I wanted to calculate this effect, how would I do that? (I'm an electrical engineer - fluid dynamics is a very distant memory at this point....)
In the particular case I'm looking at right now, this connection is not serving as a DC power path so I'm not so concerned about arcing here. However, it would be interesting to understand this better for future designs. In fact I might be more concerned about the screw actually coming loose as a result of this compression, however I've never heard of such a thing. I guess that returns me to my locking washer question. thanks, kim From: Robert Johnson [mailto:[email protected]] Sent: Monday, February 10, 2003 8:09 PM To: Kim Flint; [email protected] Subject: RE: best screw/washer choices for attaching PCB to Chassis? You should not include the PWB as part of the sandwich in an electrical connection. It gradually squashes and leaves a loose connection. The answer is generally to use a soldered jumper and lug to make connections from board to chassis. The same applies to other board connections such as board to busbar or similar connections. We have experienced arcing and board destruction due to such designs on DC power output connections. Bob Johnson ITE Safety From: [email protected] [mailto:[email protected]] On Behalf Of Kim Flint Sent: Monday, February 10, 2003 9:15 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 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
