Mike Cussen asked:
> Dear Scholars,
>
> I would like to know if there is information available, or opinions that
> address this subject of torque.
> September 20, 1995
^^^^^^^^^^^^^^^^^^
Geez, Mike, you must have a slow internet feed.
>Our design engineers have requested a torque specification for a
>circuit board. This is a two layers board, FR-4 material, thickness
>0.062", and copper is 2oz. This torque requirement is due to a
>mechanical assembly operation. We are inserting a threaded SCR
>through the circuit board, where we will secure this device with a
>lock washer and nut.
Those crazy designers just never quit. eh?
The SCR has a threaded stud, or a separate screw running through the board
and the SCR? I suggest using a relatively large flatwasher on the side of
the board with the nut to reduce pressure concentration on the laminate,
and a star-type lockwasher. You will have a lot of pressure there.
>This assembly operation is very similar to
>your (IPC'S) description on page 10, fig. 2-10 of the
>"Acceptability of Electronic Assemblies," the only difference is
>there is no insulating washer.
>
>I do have the torque specification for the part being assembled
>which is 22 Lb.in (US-units), and I found no problems with this
>pressure.
Wow. That's enough torque for a UNC #10 steel screw thread fastener. Must
be a big old honker of an SCR.
>Still the design engineer is requiring the torque specification for
>the FR-4 product.
Is he an electrical type, perchance?
>Can you please assist me in qualifing this torque requirement.
You'll need some torque drivers, check with:
Mountz, Inc. (manufacturer - HIOS - Japan)
1080 N. 11th St.
San Jose, CA 95112
(800) 966-3117
or Standard Pneumatic. The offer both fixed setting clutch or adjustable
hand tools, or power drivers with good control. Mountz has a nice catalog
with some recommended starting points.
>My first concers are, could an assembler tighten this part tight enough to
>crush the barrels or feed-throughs. "I will answer that myself," if no
>constraints are given, sure with a big enough vise or lever damage would
>occur.
At the 22 in-lb, you might actually break the fasteners if you are below
#10 size. Sounds like an awful lot. I would have guessed you were using
like a #4-40 screw-nut and a spec I wrote in 1991 called for 2.8 in-lb
inspection setting, 3.5 working tool setting, and 4.7 maximum. This is for
steel fasteners for general mechanical assembly. For #6, I called out 9.9,
7.4, and 5.9 in-lb, and for #8, 19.4, 14.6, and 11.6 in-lb. The inspection
setting should not move a previously tightned screw-nut. Max is I think 80%
of failure based on elongation and yield of the threaded fastener.
Without knowing the screw size and material, I's simply tell you to use an
adjustable tool to find a setting you know will collapse the lockwasher
partially (this should keep the fasteners from loosening). Note that. Keep
moving up to where you break the screw. Make sure you have not damaged the
laminate between the fastners. Note that. Back down to a midway point, and
get some torque controlled drivers for the operators to use. Establish your
torque setting based on the actual parts you are using, not some textbook
number.
cheers,
Jerry Cupples
Interphase Corporation
Dallas, TX
http://www.iphase.com
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