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 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
