Well, no-clean burnt flux is definitely fully heated, and should be
inert...
I don't know much about burnt flux, sorry.
For no-clean flux, internally, we say that all additional flux applied
for a hand-soldering operation must be cleaned. Un-heated no-clean
residues can, like other flux types, still be corrosive and conductive,
a definite reliability concern for any product.
The definition of "flux" is "a substance that aids, induces, or
otherwise actively participates in ... flowing". When liquid flux is
applied to a surface it tends to flow away from the point of application
to cover the largest possible surface area. In a hand-soldering
operation, heat is only applied in a very localized area, and only the
flux in the immediate vicinity is fully heated and rendered inert. So
we just say if you add flux, clean it.
For the no-clean flux in solderpaste that will go through a reflow oven,
or the no-clean flux found within solder wire, that flux does get fully
heated, and does not need cleaning. I believe that a BGA rework
operation would also fully heat any flux underneath the device, so you
should not have to clean what you can't see under the device. However,
any flux residue you can see around the perimeter of the device,
definitely should be cleaned.
Hope that was of some help.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Sue Powers-hartman
Sent: May 22, 2007 2:55 PM
To: [log in to unmask]
Subject: [TN] Flux residue, burnt flux
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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