Hi,
If you are dealing with high temperature solders and high temperature
operating  and concerned about fatique stength, creep, cracking etc. you must
understand that these solders may constantly change character above 150 deg C.
The high lead solders develop large coarse grains and cracking generally
occurs across the grains at a much lower fatique life than first tests on the
solder would indicate.  Some of the high tin solders also have property
changes, if thermally aged before testing.  The Au80Sn20 seem to be more
resistant but there is a phase change at 190 deg C that could also affect its
properties.  If you are going to use high temperature solders at high
operating tempertures, it is best to age the joints for 4-8 hours at the
operating temperature before starting you life test.  See books and articles
by Frear and Yost about some of these high temperature effects.  These are
usually available a University Science Libraries or SMTA has a bookstore that
I think lists some of them.

Phil HInton

Phil Hinton

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