Tom Olafsson asked:
>Hi,
>
>We have been having a problem with assembled PCB's having improperly
>torqued hardware and making it out of the factory and think the
>problem is that the hardware loosens after wave solder. We have
>done some simple experiments and confirmed that this happens. The
>hardware is in common applications such as stiffener bar attachment,
>voltage regulator attachment and connector attachment.
You have my somewhat shallow sympathies.
>- Do any of you have experience with this?
Yes.
>- Do some types of hardware work better than others?
Yes.
>- Is there a good guideline for proper torque specs for certain
>types of hardware on PCB's?
Maybe, contact Mountz Tools, Inc. 800-456-1828 they can supply some specs.
You may expect that heating/recooling any assembly with a screw-nut
connection will cause some tendency to loosen.
Even if the fastener was torqued properly, if it does not have some type of
locking device, it may appear to be loose. I'd suggest you start be
ensuring that there is a lockwasher specified and installed correctly. What
this lockwasher should do is permit some movement of the assembly while
keeping a normal force on the axis of the screw throughout the expansion
and contraction of the thermal excursion (or shock, vibration, etc).
The way I envision this is that the board will expand in the z axis when
heated. If the tightened fastener constrains this, it will compress the
board material, and when the board contracts, that compression means the
fastener may be loose.
As to torque, I suggest you start by using a controlled torque tool,
tightening the fastener with the board, standoff, connector or whatever you
have designed. Torque the fasteners at progressively higher settings until
you see damage after the unit is taken apart. The extreme limit of this
would be that the fastener will break. Knowing the point at which the
assembly suffers damage allows you to back off by some percentage (say 30%)
and make that your set point.
Once you establish your tool set point, then establish a minimum "check"
force, which should not move the fastener when applied after assembly, at
some force below the set point.
But I would tell you that the accuracy of commonly available tools is not
so good for the very low torque forces you will be seeing with 4-40 or 2.5
mm fasteners, and it will vary greatly depending on the type of runup (the
way the clutch engages on your driver) and other hard to control factors
such as plated finish, lubricants, technique, etc. You can expect to see
some weirdness and arguments between inspectors and assemblers if you set
this procedure up - seen it several times. Even great assembly engineering
cannot (always) overcome a bad design.
Bottom line: look first at picking fasteners which are forgiving regarding
torque. I'd guess that you need a better lockwasher, self-locking nuts, or
that you should apply a thread locking compound.
What you may find is that the fasteners have not actually "turned" to
create the looseness you see, but that the pieces being clamped have
deformed. When this happens, the screw will seem to have loosened, even
though it was once "properly" tight.
At a still more cosmic level, remember that some types of materials just
can't be fixed together with screw-nut connections and maintain integrity.
If this is the case, consider a clamp, rivet, eyelet, or even an adhesive
joint.
YMMV, but good luck!
Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com
Data is not information is not knowledge is not wisdom (and this applies
double to specifications).
***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to: *
* [log in to unmask] with <subject: unsubscribe> and no text. *
***************************************************************************
|
