I'm sure that an eminent co-contributor to this forum will tell you, "it
depends". If the flux is truly burnt, i.e., black, this indicates
pyrolysis, which is a fancy word meaning decomposed by heat. Pyrolysis
indicates that the residues have split apart into numerous compounds,
leaving carbon-rich stuff. Elemental carbon can be an electrical
conductor; do you want conducting particles in your assembly? They may
appear fixed in place now, but will they remain so during the life of
the equipment?
More important, WHY are they there? It may be because the operators
don't keep the bits of their irons clean. Do they wipe them on a wet
sponge before each joint is made? It may be that the time/temperature
conditions of the joint being made are far from optimal. It may be lack
of adequate training of the operators. I can't tell. Whatever,
prevention is better than cure; a lttle research into the causes may
give you the answer.
As to flux flow, maybe your solder wire simply has too much flux. Some
manufacturers allow you to choose the percentage. Yes, it is easier to
solder with an excess. It's a compromise.
What you have not told us is the essential information: what kind of
assemblies are you making. You can obviously be more tolerant of
imperfections if you are making toys than if you are making inertial
guidance or satellite systems. Probably you are somewhere between thes
extremes. "It depends"!
Brian
Sue Powers-hartman wrote:
> We fight a constant battle with operators leaving burnt flux in joints. Maybe
> only a small speck, but drives the inspectors nuts. The way I read JStd-001D,
> if they can not see it at referee inspection power, they have to accept it.
> How dangerous is this burnt flux to the PWB? If it's not seen at inspection
> power and left on the board, what happens. Also, what about no clean flux?
> Our solder training video says that if no clean flux runs out to far and is not
> heat activated, it can cause problems. Operators watch this video, but
> somehow do not get this. They say that it's no clean, they can leave it all on.
> I keep saying that this can be a problem, and then they ask me, how far out
> can the flux be away from the joint before it's unacceptable.
>
> Wow, I'm glad I found this forum, I have so many questions to ask you guys.
> Anyway, thanks for the help on this subject.
>
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