Hi Bill,
There are a number of "good modeling techniques"--I prefer my model, the
Engelmaier-Wild solder fatigue equation, which is widely featured in IPC
documents. It is the only one that is NOT offered as a 'black box'--so you
know what is going on and which parameters have what impact--and it also is
the only one with stated caveats and limitations. All solder fatigue
models--in fact, all fatigue models--are based on empirical results, because
there is no known fundamental relationship between fatigue and loading
conditions. Thus, all these models, which vary significantly in complexity
(more complexity requires more assumptions, but promises more detailed
answers or answers in more complex situations) should give similar answers.
FEA is a real problem, because it requires a large number of inputs, many of
which are only superficially known, and are greatly influenced by the choice
of boundary conditions and element design--in addition, the output consists
of highly localized strains and stresses for which no relationship to fatigue
life exists.
Werner Engelmaier

In a message dated 8/1/00 12:21:05, [log in to unmask] writes
>Yes, your comments are very helpful.  Thank you!
>Evidently some companies, such as Solectron, are still using MDS, as
evidenced
>by the 1999 paper by Ladhar and Fremd: "Deployment of BGA Assembly Process
>Worldwide" (Proceedings of Pan Pacific Microelectronics Symposium).  I
>understand that MDS is much more useful for comparative studies and process
>optimization than for reliability prediction.
>Your response raises one question in my mind: by "good modeling techniques"
are
>you referring to FEA or perhaps software such as Jean-Paul Clech's "Solder
>Reliability Solutions"?
>Thank you once more.  I won't take advantage of your generosity by again
>responding with more questions!
>Bill Dorner
>Wavetek Wandel Goltermann

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