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February 2006

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Subject:
From:
Cheryl Tulkoff <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Fri, 10 Feb 2006 13:13:25 -0600
Content-Type:
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On a closely related topic, we've been working with Intel on issues related
to strain/deflection of the PCB around BGAs. This has become a critical
issue for RoHS parts and processes. Some overview information can be found
at:

http://www.smta.org/files/oregon_chapter_presentation0615-1.pdf

Frank Joyce is very willing to work with other interested parties in the
industry on this.

Industry Guidelines:

IPC/JEDEC 9704 contains the Printed Wiring Board Strain Gage Test outlines
(how to perform tests).

IPC/JEDEC 9702 contains the Monotonic Bend Characterization of Board Level
Interconnects.

Thanks,

Cheryl Tulkoff
Senior Reliability Engineer
Phone:(512) 683-8586
Fax: (512) 683-8847
National Instruments
11500 N. Mopac Expressway
Building A
Austin, TX 78759-3504





             "Brooks,Bill"
             <[log in to unmask]
             >                                                          To
             Sent by: TechNet          [log in to unmask]
             <[log in to unmask]>                                          cc

                                                                   Subject
             02/10/2006 01:00          Re: [TN] Populated PCB deflection
             PM                        spec?


             Please respond to
              TechNet E-Mail
                   Forum
             <[log in to unmask]>
             ; Please respond
                    to
               "Brooks,Bill"
             <[log in to unmask]
                     >






Alan,

I will attempt to address your question as best I can given the information
you provided.

A properly designed non-flexing enclosure with rigid mounting for the board
should restrain the rigid board from flexing. If it isn't properly
restrained, it isn't designed correctly to survive for long. How long must
it survive the potential stress it will encounter? Which particular mode of
'flexing' are you most concerned about?

A pressure from a human finger pressing a switch or button? Or, perhaps the
flexure that happens under continuous sinusoidal vibration at some given
frequency? Or perhaps instantaneous shock of some given number of g-force
in
inch/lbs? Or simply the expansion and contraction of the material over
temperature... perhaps all four?

The survival of the circuit depends on it NOT being stressed beyond its
ability to recover intact. Work hardening of solder joints and the
components mounted to the board through that solder joint can cause them to
fail. Repeated flexing can cause breakdown of the epoxy/fiberglass board,
glass fibers, or other materials, or even the copper conductors and their
adhesion to the board material can suffer from work hardening and
structural
failure can be a possibility... if the vibration is intense enough and
sustained over a long period of time.

As far as my seeing a spec that calls out an acceptable maximum single
flexure of a rigid board electrical assembly, I really have not seen
anything that specifies a restriction on that particular design feature.
Most requirements are saying that it must not fail given a certain maximum
or minimum environmental exposure. If you were using Flex circuit materials
it would really be able to survive a lot of flexing, but the components
might not... it all depends on how you protect the solder and components
from being exposed to undesirable stresses that cause failures.

Your board may survive a tremendous flex of many inches... once, or twice,
but if you repeat it over and over, eventually you will find the breaking
point and the weakest link will fail. Each board will be different
depending
on the modulus and tensile strength or even ductility of the materials
involved and what amount of stress is being applied to the assembly and
where and how often and at what temperature and humidity.

Sorry it isn't a simple yes or no question... I don't suppose that any of
that helped with your question a bit.


Best regards,



Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com

-----Original Message-----
From: Alan Young [mailto:[log in to unmask]]
Sent: Friday, February 10, 2006 9:49 AM
To: [log in to unmask]
Subject: Re: [TN] Populated PCB deflection spec?

Hi,

Here is a question that has just popped across my desk from an engineer.

A board has been populated (All SMT - 0402's, BGA's, etc.) mounted into its
housing on stand-offs.

How much deflection is acceptable if the board has not been properly
supported in the middle?

Is there a specification for this?

Thanks In Advance.

Alan.

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