Michael,

Generally speaking, we do not allow via barrels to be put under any compression.
However, when clamping FR-4 the compression is localized around the bearing 
surfaces and the pwb returns to a relatively stress-free state rather quickly
depending on the geometry, composition, and loading of the clamped joint. You 
must determine the via keepout area for your paticular application but outside 
this area you should have no via reliability concerns.

Mike
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Mike McNulty
Lockheed Martin GES
Moorestown NJ
Phone:(609)722-7323
Fax:(609)722-2207
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Dear Scholars,

 I would like to know if there is information available, or opinions that
 address this subject of torque.
 September 20, 1995

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. 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. 

Still the design engineer is requiring the torque specification for
the FR-4 product.

Can you please assist me in qualifing this torque requirement.

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.

 
Sincerely your,
Medar Inc,
Michael Cussen