Phil
Welcome to Board Talk. We are the Assembly Brothers, Phil Zarrow and Jim Hall of ITM Consulting. We're here to attempt to answer your process questions to do with surface mount assembly and the like. Today's question is from G.V. It's a wave solder question.

G.V. writes, "We have seen joint voids that look explosions and after wave soldering, some members of the team believe the problem is caused by humidity brought to the process by the bare board. The problem was solved by changing the bare board lot. Have you seen this problem before? Do you agree that the cause is likely due to moisture absorption in the bare boards? Can this suspicious lot of boards be salvaged?"

Whoa, there's one of those solder joints exploding now. Son of a gun.

Jim
The answer is technically yes to everything. It's very interesting, we're talking about voids, blowholes, explosions and wave soldering joints and in the September 2009 version of Circuits Assembly magazine, there was an article by Paul Litowsky Alpha Solder on this very problem and he explained it in detail. He lists no less than 36 specific potential causes for blowholes. So, I'm sure you could find anything you want on that list.

Phil
I think blowholes in wave soldering are what solder balls are to reflow.

Jim
Certainly the most common ones are flux problems. Either the flux is not properly preheated so that there is residual solvents hitting the wave and exploding and getting trapped within the bow of the hole and then exploding when they hit the wave.

Or there is too much flux that just can't get completely activated by preheat. Certainly moisture could be a problem and Paul Lotosky lists that as one of the issues in his article, and if you did see the problem solved by board lot that might be it.

It could also be problems with the plating on the plated through holes and some of the other contamination issues that are listed in this article. So, you might try baking the boards, but you might want to do some contamination investigations on the plating, particularly the barrel of the holes to see if that could be the cause.

I said, there's 36 potential candidates for what could be causing it. Good luck.

Phil
That would be a great contest. See how many of the 36 you can name off the top of your head.

Well, thank you again for tuning into Board Talk and this is Phil Zarrow and Jim Hall of ITM Consulting. And in the meantime, wherever you go –

Jim
Don't solder like my brother.

Phil
And don't solder like my brother and watch out for those exploding solder joints.