Has anyone tried square-cone sems screws for holding down PCBs? I am tempted to recommend them, as I've seen them used in this type of application, They have properties of flat and locking washers in a single, captive washer (fewer parts to manage). They come in 4-40 phillips pan head, with zinc or other platings.
Angus McGill Regulatory Engineer Cascade Engineering Services, Inc. ang...@cascade-eng.com From: Robert Johnson To: 'Kim Flint'; emc-p...@majordomo.ieee.org Sent: 2/11/03 8:28 PM Subject: RE: best screw/washer choices for attaching PCB to Chassis? The product I'm referring to used screws to fasten a two layer FR4 fiberglass/epoxy PWB to copper bars. I think it had a 5V output at about 50A. The memories are really foggy since this was about twenty years ago, but as I recall it used a couple screws with flat and split lock washers. Although properly torqued in initial manufacture, they started coming back >from the field after a year or two with failures where the hardware had loosened and the resulting loose connection would progress to heating and arcing with destruction of the board in the joint area. The smoke and charring brought it to the attention of product safety, but failures never generated further risk due to enclosure protections. We didn't do a lot of research as to how the failure progressed, just reworked the product so that etch was securely soldered to the connections and did not depend on the screws. Board compression may have been due to heat cycling or simply cold flow. However, the connection eventually loosened and that was what we had to fix. I have often seen grounding done with board mounting screws and have recommended against it. Grounding doesn't experience any heat cycling, nor does it usually experience sustained currents which burn the connection, but if it loosens, it is not usually obvious in the field. You probably can't calculate a prediction about compression. You would need to do lab experiments plotting compression against time, pressure and temperature. It's easier to just find an alternative construction. Bob Johnson ITE Safety From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of Kim Flint Sent: Tuesday, February 11, 2003 7:52 PM To: emc-p...@majordomo.ieee.org Subject: RE: best screw/washer choices for attaching PCB to Chassis? 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:john...@itesafety.com] Sent: Monday, February 10, 2003 8:09 PM To: Kim Flint; emc-p...@majordomo.ieee.org 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: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of Kim Flint Sent: Monday, February 10, 2003 9:15 PM To: emc-p...@majordomo.ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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
