No flux can be left on for any length of time without some detrimental
effects. Here are some residual flux issues that I have recently been
involved with at clients outside of my employer here (GD).
1. A large CEM was having major issues with the hand-soldering of large
fine-pitch gate arrays. The operators complained that they could not
solder the fragile .015" pitch leads properly, that there were
solderability issues. Upon examination, it was apparent that they were
soldering to bare nickel, with poor results. Samples were removed for
SEM and solder dip testing, and they failed. However, when I examined
the components in stock that had not been soldered, and performed XRF,
it indicated that there was at least 10-35 uinches of tin/lead coating
over the nickel. When watching them perform the soldering and checking
all of the parameters, I found that many of the operators routinely
applied a standard water-soluble liquid benchtop flux to all four sides
of the components, and then began soldering the leads. It took
approximately 1 to 2 hours to solder each part, sometimes longer
depending on the particular difficulty they had. I noticed that they
also routinely applied the flux over all of the leads on all four sides
of the component, and would then get up and leave to use the bathroom,
go on break, go to lunch, go to meetings, etc. So I asked one operator
to apply the flux only to the 6 or 7 leads she could solder immediately.
She did so, and soldered them up perfectly in about 40 seconds. She
noted "this must be one of the good parts". I asked her to apply only a
drop of flux to the next 6-7 leads and solder immediately. She did so,
again perfectly. This continued until all of the leads were soldered,
and it took her just 20 minutes. Then I asked her to immediately wash
the board, inspect her soldering, touchup any that needed it, but don't
use more than a small drop of flux. She then did the next assembly, and
the next. Elapsed time: about 20 minutes per component. # of defects: 0.
So then I told another operator to use the same method. Same result.
Next we called a short meeting with all of the operators, explained what
we were doing, and asked them to do the same. Same result, all were able
to completely solder these "problem" components in less than thirty
minutes. The problem was simply leaving too much flux on for extended
periods of time, causing the solderability to progressively decrease,
causing the operators to add more flux, compounding the problem. The
flux was strong enough (it is the standard benchtop flux used by most of
the electronics industry) to eat away at the thin tin/lead finish and
expose the nickel. Once they learned what was causing the problem and
adjusted their process (and their documented process), they never had
any issues soldering these components again.
2. The second incident occured at an OEM manufacturing military
hardware. They were attempting to perform hand-soldering also, but of
small quantities of PTH parts that did not justify wave-solder. Here,
the operators were using the same exact flux as the previous example
above, but rather than dispense from the plastic bottle with a needle
attached, they were using small aluminum foil trays or cups and they
brushed the flux onto the component leads with a small paintbrush. The
longer they used the flux, the worse their soldering got, so they began
emptying the flux and refilling it every couple of hours or so. However,
they left the flux in the little aluminum foil cups overnight, and the
next day the problems would start all over again.
But there was one operator who had few or no problems soldering her
parts. Needless to say, she took great pride in her "superior soldering
skills". In studying her techniques to determine what she was doing that
the others could not, I noticed that she did not use the aluminum foil
cup or brush, but a standard needle bottle. She stated that she
preferred to use the plastic bottle over the aluminum cup, and the
reason the foil cup was used by the other operators was because one
operator accidentally "stuck her arm" on the needle, and the
Environmental Health and Safety guy then decreed no more needle bottles.
So she asked if she could continue to use the plastic bottle but with
the needle cut down to about a .100" length, and was given permission to
do so.
As it turns out, the flux would react with the aluminum foil, producing
aluminum oxides suspended in the flux. When this nice mixture was
applied to the components, it created all kinds of chemical oxidation
issues that led to poor solderability. Once the operators switched back
to plastic containers or needle bottles with the needle cut very short,
all of the soldering problems cleared up immediately. Once again they
ALL had "superior soldering skills".
To summarize, do not leave flux on component leads any longer than is
necessary. Do not substitute even little things like flux containers
from the standard tools list without some type of qualification. Do not
allow flux to remain on a circuit board for up to 7 days because the
salesman says its OK to do so. These will bring no benefit without a
decrease in quality or reliability. Typical industry practices are to
complete the soldering within a few minutes of flux application,
followed by a wash of the assembly within 30 minutes if at all possible,
even if the entire assembly/soldering operation cannot be completed in
that time. And with the exception of solder pastes with a flux formula
that allows you to do so, never allow a flux to be left on the assembly
through a subsequent bake or reflow operation (especially if RMA,
no-clean, or any alcohol-based flux is used). It will literally bake the
solids onto the PWB and make the subsequent cleaning much more difficult
to perform.
And one last tip. If you perform ROSE testing (Ionograph, Omegameter, or
similar) always choose your sample not only from assemblies that have
just completed SMT reflow or Wave solder, but after the hand-soldering
and touchup-type rework and/or hot-gas rework has been performed also,
else you have no idea as to whether any of these worst-case processes
are leaving levels of flux (or other polar contaminants) on the board
after wash.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Wednesday, October 31, 2007 10:12 AM
To: [log in to unmask]
Subject: Re: [TN] cleaning concerns
DOCUMENTATION?? In this case, I wouldn't rely on any documents, but take
samples, clean them after 1day, 7days, one month, clean and make SIR
test or other analysis. Hands on...better knowing than guessing.
My two molecules
/Inge
Can anyone tell me or direct me to............ documentation...........
that specify how long flux residue from WS609 63/37 can be left on a
board before washing?
I was informed that it could be left on for 7 days .
Linda Langley
Jabil Circuit
Training Specialist
(248) 292-6176
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