Hans,
Below are some comments on the issues you raised.
In a message dated 97-03-21 11:06:58 EST, you write:

<< I'm Investigating mechanical failures (cracking) of solder joint for   
 J-leaded surface mounted components (SOJ).  The primary casue is due to   
 the CTE mismatch between the PWB and the cermic SOJ.
 
COMMENT: Local CTE mismatch between solder and the lead (especially when the
leadframe is Alloy42 or Kovar), and too stiff of a lead may also be
contributing factors.
 Any sources of information for the following?
 
 Stress/Strain relationship of eutectic solder (63 Sn, 37 Pb)
 COMMENT: The International Tin Research Institute (UK) has publications on
the subject. You may be able to locate ITRI on the Web, or by contacting Bill
Hampshire (Hampshire Technical Services) in Columbus, Ohio.
In general, these stress/strain relationships are isothermal (+ fixed strain
rate). While they give useful information on the mechanical strength of
solder, they do no infer anything about fatigue.  Cyclic creep-fatigue is
what matters most for SMT reliability.

 Any alternatives for mounting these J-leaded devices using the same   
 footprint?
 
 Any similar experiences to this type of problem and how was it solved?
COMMENT: two possible approaches (each with its own implications) are: 1) use
a component with more compliant leads. 2) use a substrate with lower CTE. On
rare occasions, similar problems were resolved by switching from standard
Sn-Pb to an alloy with a higher fatigue resistance (a more daring approach
because material compatibility issues may have to be worked out).
  >>

Attachment reliability is application-dependent and there are many variables
involved (board, component, lead geometry + material properties).
I can't be more specific without seeing your package outline (size + lead
geometry) but I hope this helps.
Jean-Paul
________________________________________________________________
Jean-Paul Clech
EPSI Inc., P. O. Box 1522, Montclair, NJ 07042 - USA
tel: +1 (201)746-3796  fax: + 1 (201)655-0815
Home page: <A HREF="http://members.aol.com/Epsiinc1/index.html">http://members
.aol.com/Epsiinc1/index.html</A>
- click on the above or copy: http://members.aol.com/Epsiinc1/index.html
__________________________________________________________________

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