Hi David,
That old wife's tale that "a faster cooling rate creates a finer grain 
structure in the solder which creates a more reliable solder" is a 'red herring'. 
Indeed, a finer grain structure results, which in accelerated testing gives 
more cycles to failure, HOWEVER, solder joint grain structures are inherently 
unstable and will coarsen with time and temperature [following an Arrhenius 
equation with an activation energy of about 0.52 eV]. Thus, these increased 
cycles to failure in accelerated testing do not translate to the product, because 
after about 1 year in the field you no longer can tell whether you started with 
a fine grained structure or not.
The problem with the faster cooling rates, particularly with SAC solders, is 
that you have thermal shock right after reflow and in fact fractured solder 
joints right after reflow have been observed.
So to answer Leland's original question, NO, having the last zone run 
slightly warmer is not detrimental and likely a benefit if you are using large 
components.

Werner



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