Also, I have used 'Ny-lock' or nylon patch treated machine screws in place
of any type of 'biting' or surface deforming device like a star washer or
split-lock washer under pan head style machine screws. These pre treated
fasteners can prevent loosening of the fastener under vibration or even from
thermal excursions.

The nice thing about them is they can be loaded into a screw feeder machine
and installed automatically because the thread locker is cured and dry and
don't fall off the screw thread during handling. This type of treated
fastener will limit the torque required to retain the screw in the treads
ultimately retaining the board to its hardware mounting points...without
galling the top surface of the board all up.

http://www.nylok.com/products_trueblue.htm


Depending on the application, 'trilobular' style screws are a potential
option too.  They grip the mating threads because they are not perfectly
round and bite into the threads as the screw fastener is engaged.

Or there are also lock-tite liquids or 'sticks' that can be very effective.
But they are hand applied and cannot be machine inserted for high volume
applications to the best of my knowledge.

http://www.taptite.net/powerlok.asp

It would be extremely advisable to avoid breaking the fibers in the board by
over applying torque to the fasteners. That sort of injury to the sealed
integrity of the board could potentially allow moisture incursion into the
interior of the epoxy laminate and eventually causing adhesion or
delamination failures. Under vibration the failure of the mechanical
fastening feature would be likely be next to follow.

http://www.boltdepot.com/fastener-information/US-Recommended-Torque.aspx

Torque values for fasteners used with printed circuit materials would be a
really great research project and white paper... anyone want to tackle it?
:)


Best regards,

Bill Brooks
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California


-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Thursday, October 12, 2006 12:02 PM
To: [log in to unmask]
Subject: Re: [TN] Compression Limit on FR4?

Hi, Jack
I can tell you that typically the torque setting used for hardware on
pwb assemblies is between 1 to 6 in/lbs. More than that tends to deform
the fiberglass over time. Of course, it all depends on the type of board
material, layer count, etc.
It also depends on the application.
Realize that a lot of relaxation occurs during any type of thermal
excursion, and the hardware will loosen up. It is always a good idea to
use a Bellville washer in conjunction with a serrated or star washer to
continue to hold the torque. It is also a good idea to use Glyptol or
some other threadlocker.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Thursday, October 12, 2006 1:54 PM
To: [log in to unmask]
Subject: [TN] Compression Limit on FR4?

Does anyone know of any guidelines or recommendations that suggest a
maximum limit on compressing board material?

Can any damage occur by over-torque'n screws, for example?

I've never heard anyone discuss that...

Jack

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