I have seen many cases of massive failures due to flux residues. The
most spectacular was in the late 1970s. I had a client making NC machine
tools and he used boards made by a very large German company. These were
soldered with a DIN 8511 (?) F-SW32 flux, which many Germans considered
as "no-clean" as they were halogen-free (and they were hell to clean,
anyway). They were rosin fluxes, typically 15-25%, activated with alkane
carboxylic acids. The theory was that the rosin held the activators
harmless in its matrix. The practice was far different. I had a look at
some circuits that had been in service for a year or two. The copper had
all but disappeared on several of them. I was asked to find out why, as
this was the first time this problem was recorded and it was not just
one circuit but many. I visited my client's client and found the machine
in question was installed in a large workshop where there also a number
of honing machines using a water soluble cutting oil. The visibility in
the shop was about 20 m, as there was a mist of this cutting oil. I
needed no further knowledge; the circuits were operating in an aqueous
aerosol and this was reacting with the flux residues to produce a nice
acid solution. The rest is history. I recommended my client that he
clean-air-purged the NC electronics cabinet and replaced all the
existing circuits and this cured the problem.
Another spectacular case, about 10 or 12 years ago. A client, making
very high-power electronics, asked me for advice. He had a number of
failures from a recent batch of units that he had been making for a
number of years. Tracks connected to three-phase 400 V power had sinply
and suddenly volatilised, even though they were about 7 mm wide in 105
µm copper, between the input connector and the 100 A fuse holders, a
distance of about 5 or 6 cm, all three of them. To cut a long story
short, no-clean wave-soldering flux residues had their activators
sublimate as the conductors warmed up and they condensed on the top edge
of the fuse-holders. These were, unfortunately, spaced by only about 3
or 4 mm and, after some time, a short-circuit occurred sufficient to
start an arc between two phases and one of the tracks overheated to
explosively splutter molten copper around like there was no tomorrrow,
causing the other two tracks to volatilise in turn. It transpired there
had recently been a change in the formulation of the flux that was used,
which was why it had never happened before. I recommended that they used
a W/S flux with cleaning and they redesigned the PCB for more spacing
between the fuse holders. No further problems, AFAIK.
Many other cases of leeser massiveness.
Brian
Hernefjord Ingemar wrote:
> 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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