Hi Bill,
In a message dated 10/23/97 14:42:41, you wrote:
>Greetings Mr. Engelmaier!
>My name is Bill Barthel.  I work for a contract assembly company called
>Plexus.  We are rather involved in area array packages and recently your
>name was mentioned as a possible source for resolution to a re-occurring
>question.
>Most PCBs will not be perfectly flat, especially after assembly processes
>and if they have high (10+) layer counts.  We have numerous examples but
>always find the boards to be well within the IPC spec. of 7ppm.
> None-the-less, many designs include stiffener bars due to their size and
>weight.
>The question I am looking for help on is:
>When stiffeners are applied to the PCB, stress is induced on all joints.
>What is the effect on fatigue life (per PCB bend radius?), particularly
>for BGA?
>I have posed this to some well connected people and no one seems to know
>of any work published in this area.  Could you direct me to a possible
>source?  Or, could you provide you opinion and any level of
>quantification possible?

I can not recall any work published in this area either.
You are looking at two damage mechanisms that could lead to failure, but
neither is related to fatigue.
(1) Solder joint overstress failures can be caused,essentially immediately
after soldering, if the PCBs assume a warp geometry after the soldering
process that produces tensile stresses in the solder joints that are large
enough and rapid enough to cause overstress failures. The strain rate has to
be high enough for the solder not to be able to accommodate the displacement
strains by creep at the prevailing temperature, thus allowing the tensile
stresses in the solder joint to build-up the fracture magnitudes, also at
this prevailing temperature.
(2) Solder joint creep rupture failure can be caused, over time, if the PCBs
assume a warp geometry after the soldering process that produces tensile
stresses in the solder joints that are large enough to cause creep rupture in
the solder joint. That could be at total displacements strains in excess of
~30%. The time to failure depends on the magnitude of the total displacement
strains as well as on the prevailing temperature.
In both these conditions, component size (smaller is better) and solder joint
height (taller is better) play important roles; as does PCB thickness
(thinner is better, i.e. single-sided PCMCIA). If failure does not occur as a
result of these conditions (which are present to some degree in all
assemblies), not significant effect on the fatigue life of the solder joints
need to be anticipated.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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