In a certain Electronic Physical Design book, we're advised that, after soldering:
Do not blow on the joint, as this will cause the solder to cool too rapidly, leading to crystallization and embrittlement.
On the other hand, a (if not the most) famous user of this board, advises us to
blow on the joint gently until it hardens.
Now this sounds like one of those issues that the EEVblog or even Mythbusters would tackle. So, does anyone know of experiments where the effect of blowing on the joint has been studied?
As was pointed out in a comment below, the latter advice may be impractical as it was written because a small joint may harden too fast anyhow for blowing to be helpful in that regard. Still there may be other practical incentives to do it, like cooling the board/parts faster so you can move on to making the next joint without burning yourself (by accidentally touching the traces, parts etc.) So I think it's fair to ask if the advice given in some textbooks (against blowing) is purely ex cathedra or backed up by some empirical evidence. Alas the book I mentioned doesn't cite anything in support of their stance.
After a bit more searching, I found some anecdotal evidence in an EDN blog supporting the claim from the book. Still it seems rather unsatisfactory and possibly not scientific enough since this blog said that all the joints examined at that site were cold joints ("solder was visibly cracked and crystallized in all different directions"), but that could have happened for other reasons, i.e. this anecdotal piece of evidence lacked a control.
As discussed in comments below, blowing on the joint is sometimes the poor man's fume extractor (or deflector anyway). Now, since real fume extractors are standard in most shops/labs and these have non-trivial airflow, I suspect some boffin has studied what level of airflow becomes dangerous for joint reliability.