Electronic – To blow or not to blow on the joint, that is the question

soldering

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?

Updates:

  • 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.

Best Answer

It depends on what and how you're soldering. If you are manually soldering things that come off a board, like a wire, then it's usually good to gently blow on the joint to cool it quickly. The advantages are:

  1. It hardens the joint quickly. This reduces the time you have to hold things still, which in turn reduces the chance that things are wiggling around as the solder hardens. When you're holding one or more things with your hands, this is useful.

  2. You can get a visual indication of the quality of the joint. Seeing the solder harden shows you a bit about how well the solder made thermal contact, which depends on how well it flowed around all the metal parts of the joint. It's hard to explain without showing it, but sometimes you can catch a problem by noticing that the solder just didn't look right after it cooled, or in the process of cooling.

However, when things are held in place on their own, then you get advantage from letting the solder cool slowly. This is the case, for example, with soldering a single pin of a larger component on a board. If the other pins have already been soldered, then they will hold the part in place. The part joint won't be weakend due to the part wobbling as the solder was cooling. Now letting the mechanical stresses due to uneven heating dissipate a bit by slowing the hardening process helps.

Of course when you're doing reflow soldering, follow the recommended temperature profile. In that case the equipment handles that outright, and you shouldn't be in there altering the process.

With manual hot air soldering, you generally don't want to blow on joints either. You wouldn't use hot air to solder two things you have hold still yourself. Generally you're heating a entire part. Hot air heats the board and other parts around where you are soldering. Give them a chance to cool down slowly and with the least thermal stress.

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