Re: best screw/washer choices for attaching PCB to Chassis?
I read in !emc-pstc that Price, Ed ed.pr...@cubic.com wrote (in b78135310217d511907c0090273f5190d0b...@curly.ds.cubic.com) about 'best screw/washer choices for attaching PCB to Chassis?' on Mon, 10 Feb 2003: Put a dab of Locktite into the female thread on the PEM before assembly. - provided you will junk the *whole product* if anything fails on the board. I've had Loctited screws hold until the head is a mangled mess or the PEM separates from its mounting surface. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! 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
RE: best screw/washer choices for attaching PCB to Chassis?
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
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
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
RE: best screw/washer choices for attaching PCB to Chassis?
Kim, Screw/Washer choice: - should a washer be used or not? ONE COMPANY I WORKED FOR USED SCREWS WITH INTEGRAL STAR WASHERS. WE NEVER HAD ONE WORK LOOSE EVEN IN MOBILE APPLICATIONS. - Is an adhesive like Loctite a reasonable alternative to lock washers in this case? I WOULD THINK LOCKTITE WOULD BE GOOD BUT I WOULD DESIGN IT TO MAKE SURE THE SCREW IS NOT REQUIRED AS A FAULT CURRENT PATH. - should the screw be zinc plated? or some other plating? I FAVOR USING THE SAME PLATING THAT THE PEM NUT USES. PCB layout: - Should we use a plated through-hole with ground planes connected inside the hole? WE USED PLATED THRU BUT THE CONNECTION TO THE NTERNAL LAYERS CAN FAIL SO I WOULD NOT RELY ON THEM. - or a non-plated hole with vias in the surface layer pad connecting to the ground plane? YES VIAS, OUTSIDE THE SCREW HEAD AREA ARE PREFERRED. THE PAD SHOULD BE ON THE PEM NUT SIDE SO THAT THE SCREW IS NOT A PART OF THE FAULT CURRENT PATH. I WOULD DEVELOP A TORQUE SPEC, USE A TORQUE WRENCH, AND PUT THIS IN THE ASSEMBLY DOCUMENTATION. Dave Cuthbert Micron Technology 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
Re: best screw/washer choices for attaching PCB to Chassis?
Kim, Here's how I would do it. Note that I'm assuming that we're dealing with SELV circuits here and your concern is SI/EMC on circuit packs for a shelf system. Safety grounding is another issue entirely. First screws: I'd use split washers with flat beneath (if lock washers at all) to protect your surface. Screw/washer materials selection should be anti-corrosive and compatible with the surface finish of your PCB. Loctite or equivalent should be available applied to the screw tips from the screw supplier - this will ensure you don't get loctite on any of your contact surfaces. Now the hard to explain part - pads I would recommend flexibility on the pad end. In my earlier days at a test lab, I saw way too many products fail when either *all of the standoffs* or *one/none/few of the standoffs* (take your pick) were connected to digital ground. When we cut traces/removed mounting screws/made ground connections to nearby caps/etc..., the EMC problems would often go away. BUT then a respin was in order - and then manufacturing test - and then DVT - and then a compliance retest - and what if it made something else worse? At any rate, you're a month off schedule (minimum). Instead I would recommend surface layer pads and unplated holes for your standoffs with one or more DNI (or installed if that's your thing) components bridging the pads to digital ground. Pad size should be slightly larger than the mating surface diameter. This is a common practice these days in the telecom arena and will allow flexibility without respinning your board. If a problem is found in the lab that can be attributed to a grounding issue, it is simple to change the population of the DNI components to achieve a passing result that also has acceptable signal integrity (hopefully you're using differential signalling anyway for anything really fast). The best part about this method is that it is easy to convince your designers (who will undoubtedly be devoted to a certain grounding practice) to take this approach since the default population can reflect their current grounding ideology. For implementation of the above, I would stick with fairly wide SMT resistors - I found that 1812 zero ohm resistors and low value capacitors (220-2000pF range) are readily available. Remember to use VERY SHORT and relatively wide (3W rule) traces on the pad/component trace. For the digital ground end of the component, I would have more than one via (to dgnd) attached to the pad to minimize inductance to ground. Get creative to address card insertion ESD drain. And here is what it all comes down to: The worst thing that could happen is that manufacturing would have to change the component population for new production and retrofit the existing products. This gets particularly rewarding when a whole bunch of product is already made and you can just change components instead of scrapping the whole lot. (OK, it's not the worst thing) I think I explained this clearly (?) If you have questions, feel free to ask. Keep in mind this is just my view on a topic where opinions and practices vary wildly (and are often accompanied by good results from all sides) Best Regards, Dave Heald (currently an unemployed victim of the telecom market but looking) 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
RE: best screw/washer choices for attaching PCB to Chassis?
This is a multi-part message in MIME format. 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 Title: 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 -Original Message- 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
RE: best screw/washer choices for attaching PCB to Chassis?
-Original Message- From: Kim Flint [mailto:kfl...@inkra.com] Sent: Monday, February 10, 2003 6:15 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 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: 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
Re: best screw/washer choices for attaching PCB to Chassis?
Kim, you ask some interesting and well thought out questions. I'm sure you will get a lot of diverse opinions. You have entered into the domain of system grounding. System grounding may be governed by a number of codes, such as NEC, UL, and CSE for the North American continent. A lot more for the rest of the world. More importantly the ground system will impinge on the signal integrity of the PCB circuit. Hopefully the circuit designer has already addressed these issues although, from your question I suspect he/she hasn't fully addressed them. Let me tell you what has worked for me. Kim Flint wrote: 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? IPC (www.ipc.org) has many documents that will clarify your questions. Washers are always a good idea but may be made part of the PCB, the screw or as a separate part. Split locks may be used on top of flat washers. If split locks are used directly against the PCB it will tend to gouge or craze the PCB gel coat. Loctite, preferably blue can be used but I do not recommend it unless there is no way to use a lock washer. It is more expensive, hard to QC, and generally a pain to use. It also increases the ground resistance in some instances and may represent a latent defect. Loctite is also dependent on the type of plating used. I use stainless steel, binding head or washer head screws for normal applications. I use nickel plated brass screws with internal tooth star lock washers for high current applications. 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? Again, we are dealing with the system ground circuit design which will dictate whether the mounting holes are grounded or not. My preference is for unplated and ungrounded. Wherever possible I like to use mounting holes for tooling holes. This is easy to do if unplated. It also saves a manufacturing step since unplated holes don't have to be plugged for wave soldering. I like to use as big a pad as possible, somewhere between .150 and .180 since big pads are great for automatic screw drivers and such. In no way should the system ground be floated. By using an unplated hole it is assumed there is a system ground through a connector or other scheme. If there is no other ground, then by all means use a plated hole to connect as necessary. Finally, don't forget to an ESD (Electrostatic Discharge) analysis. You may find ESD avoidance driving your overall grounding plan. Fred Townsend 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
