All,
It is best to look at what the life-environment requiremnt is for the
assembly.  In parts of this industry we qualify the solder and assembly by
aging at 200oC for 200 hours and then cycle for 100 cycles -40 to 200oC using
6 hours per cycle ( 2 hours top and 1 hour bottom).  This represents the
operating environment that the assembly will experience.  Surprisingly, some
of the soft solders do survive, but not Sn63.  I would do the pre-cycle aging
if the operating environment is maybe 100oC, but would use 125oC for a 200
hours.  The thermal aging induces many of the metallurgical changes that the
assembly may undergo in its lifetime, and shortens the test time need to
qualify a product.  (the poor ones fail early and drastically).  The thermal
aging oven time is cheap.  If operating temperatures are lower, there is no
need to have the higher temperature, I use operating temperature plus 25oC.
No scientific basis, but it has seems to work.

Phil Hinton