This much I can provide you for now. It is based on FR-4-4.
Outside of that, the pressures would need to be qualified through some
type of thermal cycling to determine the rate of relaxation. FR-4 and
other laminates all have different creep rates. I do not know of any
standard or guidelines that address Z-axis compression and torque
ranges, I only know this from practical experience.
{{{{{-----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 and type of laminate material.
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
Subject: Re: Compression Limit on FR4?
From: Stadem, Richard D.
Reply-To: TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date: Thu, 12 Oct 2006 16:12:38 -0500
Content-Type: text/plain
I forgot about those. thanks
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brooks,Bill
Sent: Thursday, October 12, 2006 3:58 PM
To: [log in to unmask]
Subject: Re: [TN] Compression Limit on FR4?
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: TechNet [mailto:[log in to unmask]] On Behalf Of Dharma Kemp-Bresett
Sent: Wednesday, January 24, 2007 1:24 PM
To: [log in to unmask]
Subject: [TN] maximum recommended compression force
Hi All,
I received a call from one of my customers today on a subject I do not
have the answer for. I also do not have access to any of the IPC
standards that would have the answer to this question, as I am part of a
design services bureau and not a fabricator or assembler. I was hoping
that one of the more knowledgeable (than I) members on this forum would
be able to give me some sort of guidelines, even just a general one, if
possible.
The question is as follows:
"I'm trying to find out what the maximum recommended compression force
would be for bolting a circuit board to a metal bar, for the purpose of
making a 70 amp electrical connection. My concern is not damaging the
board (an 8 layer board, in this case), and not causing the TG170 FR4
material to flow out from underneath the bolt and washer. I realize
that the answer may depend on the total thickness of FR4 material and
the total thickness of copper in the stackup....."
I have already done some poking around in the archives but have not come
across the answer, as of yet. Therefore, I defer to the expansive
knowledge of those on this forum......
Thanks in advance,
Dharma
KAW/USA Ltd.
39 Simon Street, #4
Nashua, NH 03060
603-886-8711 x212
603-881-8763 fax }}}}}}}}}}}}}}}}}}}}}}}}}}}
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