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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! kimTitle: 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 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
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