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From:
Lum Wee Mei <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Lum Wee Mei <[log in to unmask]>
Date:
Wed, 30 May 2007 20:57:34 +0800
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Hello, 

From what have discussed, I gathered that flux residue is never a good thing to leave behind on the PCBA. Regardless whether it is water soluble, no clean or RMA flux, the chemical compounds in the flux residue can react with whatever present in its surrounding over period of time and "mutate". This mutated power is so powerful that it "eats" away connector housing, give rise to dendrites and cause short, etc. 

Is that the reason why military products only use RMA flux with IPA or Ensolve or other cleaning agent and never no clean and water soluble flux?

Thanks and regards,

Wee Mei

-------------------------------------------------------------------------------------------------

I have seen many, many cases of electrical failure due to flux being
left on the board, including no-clean, water soluble, and RMA.
I have had to perform as an industry consultant for several companies
that had field returns and cancelled contracts due to flux issues
causing performance problems (crosstalk, etc.)

The worst one I ever had to deal with was many years ago (mid-'80s) when
a contract manufacturer used a no-clean flux to add (build-around) a
printer connector on a computer motherboard. They liberally spread the
flux over the SMT pads and hand-soldered the connector in place. They
did not realize that the field returns they were getting shortly
afterwards were due to the use of the no-clean flux.
The chemicals in the no-clean flux attacked the connector housing
material, turning it into a grey powder. Some of the motherboards came
back with no connector body left, only the connector leads soldered to
the pads, and a grey powder where the body used to be. This company
brought me in to find out why the connectors were deteriorating. Upon
reviewing the original process travelers, I found that the build-around
was approved without qualifying the hand-soldering of the connectors,
whereas they had been reflow soldered with water-soluble flux prior to
that.
This company was forced to take back more than 100,000 computer
motherboards, remove the connector, remove all traces of the no-clean
flux, assemble a new connector using water-soluble flux, wash the
boards, re-test the boards, and replace them into the computers, all on
their own nickel. They nearly went bankrupt.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kathy Kuhlow
Sent: Tuesday, May 29, 2007 1:52 PM
To: [log in to unmask]
Subject: Re: [TN] Flux residue, burnt flux

Inge,

When NAFTA first came into play, the company I worked for ran to the
border for cheaper products to be built. We built industrial computing
products at the time so it needed to be pretty durable. We ended up
within 9 months scrapping everything built south of the border due to
two reasons. One was a dendrite growth on all of the gold (simm sockets
mainly) due to water quality (said it was DI but apparently our
definitions of DI differed a bit). The second reason was flux under the
older 386 and 486 chip sets.
There was flux underneath that never was cleaned up. We found that the
flux was eating through the traces underneath the chips. I suspect it
was due to a solder issue and the boards were flooded for rework but
insufficiently cleaned up. About 2000 boards were scrapped because it
was impossible to determine the extent of both problems. Left south of
the border shortly after that and only used them for cables afterwards.

Kat

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Wednesday, May 23, 2007 3:11 AM
To: [log in to unmask]
Subject: Re: [TN] Flux residue, burnt flux

What I say now may cause some to think that I'm member of Ku-Flux-Klan,
which is not the case. So, we follow the general habit of removing 'all'
flux residues. However, I can't deny, that I think this flux removal
hysteria is little exaggerated. I've been in the business for so long a
time, and I have not seen many reported failures that have been caused
by flux residues. In theory, flux residues have many ingredients that
can cause corrosion, leakage current, decreased insulation etc, but it
seems as that does not happen in reality. 100% cleanliness is satifying
and beautiful, but costs a lot to obtain.
Just a thought. Would be very interesting if anyone could describe a
case with flux residues causing massive failures.
Inge

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: den 23 maj 2007 09:38
To: [log in to unmask]
Subject: Re: [TN] Flux residue, burnt flux

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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