Hi,
Faster cooling rates will--temporarily--give you finer solder grain
structures. This is important, because if you do accelerated testing with
those fine grain structures, you will get longer fatigue lives [factors of 2
to 3] that artificially inflate the projected product reliability.
Product--after about one year on the shelf or in service--will have a coarse
solder grain structure regardless of what the grain structure is after
reflow. For product poer se, it is not very important. However, you also do
not want to cool too slowely, because you get some real alloy phase
separations--cooling too fast you get thermal shock which might actually
fracture corner solder joints due to component/PCB warpage.
Werner Engelmaier

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