Darrel Therriault <[log in to unmask]> said

>We have a backplane mounted to a chassis that has many #10 screws holding
>it in place.  Standard torque specs I use don't indicate what torque is
>appropriate for FR-4, and I am fairly certain it would not be the same as
>for a metal/metal connection.
>
>Question:
>
>Does anyone have a spec or formula for fastening FR-4 to steel/aluminum
>chassis using steel #10 screws (SEMS)??  (Or any size for that matter!!)

The torque of tightening a steel screw into a threaded fastener will
produce a compressive force on the PWB which may be greater than the yield
strength of the laminate. If this is the case, the fasteners will not
remain "tight".

Make sure that you apply no more compressive force per unit area
(distributed by washer, stiffeners, etc.) than the material can tolerate. A
#10 screw is a pretty big hunk of steel.

The force can be calculated using the formula:

F = T*pi*N/4

If you know the yield strength in compression of the materials, I'd suggest
you use a factor of 50% and  find the related torque using:

T = 1.27*F/N

in the above, F = tensile force in pounds, T = torque in in-lbs,  N =
thread TPI

>I am speculating that FR-4 with pad/pth for screws would compress more then
>metal/metal
>and am concerned with some damage/fracture to the FR-4 if not torqued
>correctly.

I'd be in agreement with your speculation and concern. Test some parts to
failure, then back off to set a torque limit. If you have a steel 10-32 SS
screw engaging tapped 1/8" Al sheetmetal (about 4 threads) then you have a
strength limit on the aluminum in failure of the fasteners, but it would be
well over
1,000 pounds tensile. Seems to me it would be easy to damage laminate with
these fasteners.

For such large holes, I would suggest you use flatwashers under the
screwheads and bolts to distribute the force, and a good locking device
such as a B-Cu or phos-bronze Belleville washer to ensure a consistent
compression without going past the yield limit on the laminate.

Note that none of this considers electrical integrity of the holes or any
grounding connection, that is another matter, then you had better think
about the exact type of washer making contact with your pads.

>Any assistance or information appreciated...


regards,



Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com

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