Hi Earl,
The problem with you (or any other person to whom respond) getting my
meassages much earlier than the rest of TechNet has to do with the dumb
'confirmation requirement' for TechNet. That typically delays my broadcast
responses 24 hours. My 'Reply" button puts the Technet and initial address on
the reply.
Well, on the requirement of soldering for diffusion amd formation of
intermetallic compounds—one certainly can argue whther penetration into a
surface by any amount (atomic monolayers) does or does not constitute
diffusion; however, the requirement for the formation of IMCs in my mind is a
given.
You said: >someone having said you (Werner Engelmaier) believe HASL to be
>representative of most subsequent soldering processes.
I do not even know what this statement is suposed to mean. I certainly
endeavour never to make such a vague statement—when I say something, I
typically jump into it with both feet.
You quote:
>An AMP (INC.) study says: "Reductions in solder joint strength must be
avoided
>due to the stress
>the solder joint must withstand. The cyclic forces of expansion and
contraction,
>caused by
>coefficient of thermal expansion (CTE) mismatches between the package and the
board
>material, are
>mostly absorbed by the lead and the solder joint. Lead compliance, determined
by
>the lead stiffness
>or diagonal flexural stiffness, is chosen to enable the lead and joint to
resist
>fatigue,
>especially in surface mount devices. Defects which reduce the joint strength
will
>allow fatigue to
>destroy the bond. In addition to temperature induced stresses, the solder
joint
>must withstand
>vibrational fatigue." I simply question whether the last part of the above
study
>contradicts what
>you said before it?
I do not know any of the details of this study nor in what context the quoted
statement was made. However, the statement is false (see my two feet?).
Solder joint fatigue has nothing to due with stress or insufficient strength
of the solder joints. Granted, you can fail solder joints by overstressing
them, but that is not fatigue. The premature fatigue failure of solder joints
is the result of excessive cyclic strains (displacements due to global and
local thermal expansion mismatches) for the assembly design. That is one of
the reasons solder joints with smaller cross-sections due not fail earlier
than those with thicker cross-sections, everything else being equal. Solder
joint fatigue is strain-driven not stress-driven, which for a creeping
material is a critical difference (for non-creeping metals it does not that
much of a difference). Modern fatigue technology also works much more with
Manson-Coffin (strain-life) plots rather than S-N (stress-life) plots.
If you are really interested in this stuff, you should be taking one of my
workshops.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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