Cleaning circuit boards of solder flux residue

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This is a follow-on to the "Reverse polarity, Scotty" thread of
September 2013, where the issue was how to clean a circuit board of
soldering flux residue well enough for a 50 Gigaohm node to work.

I recently read a book on soldering ("Solders and Soldering", Howard H
Manko, 3rd edition, McGraw-Hill 1992), which has extensive treatment of
removal of flux residue.

In the Reverse-Polarity thread, use of Kester 44 cored solder was
discussed, so the flux in question is based on rosin.  

For rosin residue, the book's recommendation (page 315) is first
cleaning with a terpene solvent (to dissolve rosin and other organic
residue) followed by a hot-water wash.  This is called the
"Semi-Aqueous Cleaning Process".

On small scale, dunking in terpentine from the paint store, shaking
off, and passage through a domestic dishwasher using for instance
AlcoJet detergent, followed by a wax-dip conformal coat, would do the
job.

Joe Gwinn

Re: Cleaning circuit boards of solder flux residue
Gave us:

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  Use a brominated solvent..

"Ensolv" works very well, and they'll send you a quart sample, but the
actual product in qty ain't cheap.  So be prepared to fork out some cash
to get a really clean assembly.

http://www.envirotechint.com/industrial-solvents/npb-solvents/

Re: Cleaning circuit boards of solder flux residue
On Sat, 25 Jul 2015 11:40:06 -0400, DecadentLinuxUserNumeroUno

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  Also, you need to vacuum the assembly for a half hour, and preferably

hygroscopic.

  Also, the area where you want the leakage minimized should be a
section of PCB which is FREE of any mask layer.  i.e. BARE BOARD.

  Note too that without encapsulation, the assembly will slowly take on
water again.

Re: Cleaning circuit boards of solder flux residue
wrote:

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Bad idea. If the terpene makes the flux water soluble, you'll be
turning nice insulating rosin flux into hygroscopic conductive crud.
Water is the enemy of insulation.

"Terpene" is apparently a broad category of chemicals.

https://en.wikipedia.org/wiki/Terpene


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That's a recipe for humidity-erratic leakage.

You don't need to clean rosin flux except for cosmetics. If you do
clean them, use an organic solvent. I like acetone for small DIY
boards, as long as it doesn't damage parts and as long as you don't
inhale or absorb much of it. We have fancy expensive solvents that we
use in our non-water board cleaner. We do water wash processing, too,
but that creates leakage if not managed very carefully.

Polyurethane varnish is a good conformal coat. Bake first.


--  

John Larkin         Highland Technology, Inc
lunatic fringe electronics

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Re: Cleaning circuit boards of solder flux residue

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Yes.  The point is that it dissolves rosin without dissolving the board
or components.


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Only if you forget the second part - wash with hot water and AlcoJet
(which sponifies the terpine goo).

The hot water is also essential to get the ionic contamination off the
board.


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In my world, we always clean the boards and then conformal coat.  Rosin
undermines the conformal coat.


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Ordinarily, yes, but not at 50 Gohms.

Joe Gwinn

Re: Cleaning circuit boards of solder flux residue
wrote:

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This little board was deliberately maxi-glopped with RMA flux:

https://dl.dropboxusercontent.com/u/53724080/Gear/Keithley/Leak_Gloppy_Flux.JPG

https://dl.dropboxusercontent.com/u/53724080/Gear/Keithley/Leak_Test.JPG

and pins the meter on the 1e14 ohm range.

We have recently had some problems on some analog acquisition boards,
water soluble flux and water (saponifier and then multiple rinses in
hi-Z water) wash. Conductive stuff likes to hide under CSP/QFN
packages.


--  

John Larkin         Highland Technology, Inc
lunatic fringe electronics

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Re: Cleaning circuit boards of solder flux residue

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It sure does. Even under SC70s. When doing high-Z front ends for folks, I often specify  non-plated holes under particularly sensitive spots, so that the cleaning solutions can get under there. Makes a big difference IME.  

Cheers

Phil Hobbs  

Re: Cleaning circuit boards of solder flux residue

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JPG>
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Yeah, I remember that test.  But leakage through residue and dirt on
circuit boards is a common theme in the electrometer crowd.


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Yeah.  In the 2013 thread, this came up, and the advice was to ensure
that the mechanical design gave adequate space under the components to
allow free flow of the cleaning solutions, for precisely this reason.

Joe Gwinn

Re: Cleaning circuit boards of solder flux residue
Gave us:

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  mere "coffee breath" can cause a failure mode.

Re: Cleaning circuit boards of solder flux residue
On 7/25/2015 9:55 PM, DecadentLinuxUserNumeroUno wrote:
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That's more of a problem in semiconductor lithography.  
Chemically-amplified resist is very sensitive to amines, which prevent  
it from developing.  (My stuff was all 248-nm, not the modern stuff.)


Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Cleaning circuit boards of solder flux residue
On Sun, 26 Jul 2015 10:49:52 -0400, Phil Hobbs

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Wafers tend to be transported in big sealed plastic casettes these
days, and plugged into process machines. I don't see why anybody needs
clean rooms any more.


--  

John Larkin         Highland Technology, Inc
lunatic fringe electronics

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Re: Cleaning circuit boards of solder flux residue
On 7/26/2015 12:58 PM, John Larkin wrote:
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Belt and braces.  You have to open the mini-environments at some stage,  
and having them in a Class 1 clean room helps a lot.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Cleaning circuit boards of solder flux residue
On Sun, 26 Jul 2015 13:04:23 -0400, Phil Hobbs

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I've seen a wall where the people carrying the casettes are in one
space, and the process machines are on the other side, in a separate
space. I'm not sure which side needs to be cleaner, but they are
certainly different.

Things like this, built into a wall.

http://tinyurl.com/np4k6yz


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Cleaning circuit boards of solder flux residue
On 07/27/2015 02:02 PM, John Larkin wrote:
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Sure, fabs have been built that way forever.  The dirty side of the wall  
is called the "tool core' or just 'core'.

Back in 1991-92, I spent a certain number of hours ripping apart wafer  
steppers in IBM East Fishkill to retrofit a better alignment system.  
They were Perkin-Elmer Censors, and we managed to improve the overlay to  
the point that they could use G-line litho for another process  
generation, IBM ATX-4 ECL, before switching over to I-line (365 nm).  It  
was applied to 35 tools, and saved the company over a hundred million  
bucks in equipment.  (My colleague and I got a nice pat on the head.)

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Cleaning circuit boards of solder flux residue
On 7/26/2015 12:58 PM, John Larkin wrote:
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You claim that I pick on you unjustly.  But it is your making comments  
like this that make you stick out like the proverbial nail.  For an  
otherwise intelligent person you say the dumbest things.  Reminds me of  
the Art Linkletter show.

--  

Rick

Re: Cleaning circuit boards of solder flux residue

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CFCs were excellent.  I've heard that toluene condensation cleaners were  
the absolute best, except for the part where you die young of cancer.

I've had bad experiences with turpentine gumming in the PCB.  Automotive  
brake cleaner (benzene, acetone, isopropanol) works very well but with  
some risk to the health of components and yourself.  Overall, real rosin  
flux remover is the way to go.  Follow up with de-ionized water, gently  
bake in a vacuum, and then it's CLEAN.

I don't need 50G Ohm circuits but I need them clean and dry before I  
soak them in conformal coating to help weatherproof them.  A little damp  
flux under the coating will eventually electroplate itself a short  
circuit.

--  
I will not see posts from astraweb, theremailer, dizum, or google
because they host Usenet flooders.

Re: Cleaning circuit boards of solder flux residue

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You also need the second step, to wash in hot water with AlcoJet
detergent.


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Yep.

Joe Gwinn

Re: Cleaning circuit boards of solder flux residue
wrote:

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We just used dishwasher detergent (in a dishwasher full of PC boards,
of course).
<http://www.2spi.com/catalog/supp/Alcojet-powder-detergent.php
From the MSDS sheet:

  Sodium Carbonate          15-40%  
    (soda ash or washing soda)

  Sodium Metasilicate        5-10%  
    (strong base that reacts with fatty acids to form a soap)

  Tetrasodium Pyrophosphate  3-7%
    (removes calcium and magnesium scum)

  Sodium Phosphate          14-40%  
    (probably similar to TSP or tri-sodium phosphate to remove grease)

Looks very much like dishwashing detergent powder, which is mostly
sodium carbonate and sodium silicate.  I guess the two phosphates
might be an added bonus if your water is full of lime or your flux
uses a water soluable grease.


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Cleaning circuit boards of solder flux residue
On 7/25/2015 9:12 PM, Jeff Liebermann wrote:
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One of the problems with commercial grade chemicals is purity.  These  
are the listed ingredients but there are often contaminants that are not  
listed.  I don't know how important any of them are in this application.  
  In chemistry it was not at all unusual to refine your chemicals before  
using them for research, but that greatly depended on the research and  
the documentation detail available for the impurities.  That was also  
not trusted much and verified.

Any idea what contaminants would matter here?  Sure seems like H2O is  
the one of most concern.  Heck, even the water used would have numerous  
contaminants.

--  

Rick

Re: Cleaning circuit boards of solder flux residue

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Well, now that you mention it, there is one ingredient in consumer
dishwasher detergent that caused problems in the past.  
Sodium Dichloro-S-Triazinetrione dihydrate:
<https://en.wikipedia.org/wiki/Sodium_dichloroisocyanurate
It's a chlorine based disinfectant and bug killer.  That's quite
useful in a dishwasher full of rotting food residue, but not terribly
helpful when cleaning PCB's.  The problem was that even in such low
concentrations (about 1%), it makes a dandy bleach and was fairly good
at removing the markings off components and the rubber inspection
stamps off the PCB's.  It also seemed to cause the Chemask peelable
solder mask to prematurely fall off the PCB.  I could find no
conductivity or residue issues, just some odd irritations.  I found a
totally functional, but highly irritating method of neutralizing the
compound.  I dumped a weak acid (acetic acid or white vinegar) into
the dishwasher mixer.  That got rid of the disappearing markings and
having to fish out the Chemask from the bottom of the dishwasher, but
also filled the production area with toxic chlorine gas covered by the
smell of wet athletic socks.  Production declared my solution
unacceptable and found a supplier that did not use chlorine compounds
in their soap mix.

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I assume much of the same is done in industrial and consumer products.
However, I have no experience with producing the original compounds
and don't know how much care is taken in removing or even identifying
contaminants.  

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Well, the dishwasher runs on water, so I doubt that's considered a
problem.  Some of the junk found in the water is potentially a problem
especially since I considered Santa Clara CA tap water undrinkable and
unsuitable for decent coffee or tea.  

We had a high flow water filter before the dishwasher that was good
for removing boulder size contaminants, but did nothing for dissolved
chemicals and pollutants.  I also didn't have a proper lab for
analyzing what was in the water.  

I did manage to create a big problem when I let the dish washer drain
the hot water heater, which pumped all the calcium carbonate from the
bottom of the tank into the dishwasher.  The PCB's looked like they
were covered with white paint.  I managed to recover most of the
boards with a smelly white vinegar rinse, to produce calcium acetate,
carbon dioxide, and more water.  By itself, calcium acetate is fairly
harmless and somewhat soluble in water.  So, I tried giving the boards
a rinse in an organic solvent.  Big mistake.  Mixing alcohol and
calcium acetate produces a white, slimy, sticky, and smelly gel, that
refused to be removed by anything less than explosives.  Ok, lesson
learned.

Anyway, if you're trying to get down to megaohms per square sheet
resistivity, things like contaminants are important.  However, if your
circuit can tolerate less, the contaminants are much less of a
problem.  I've been fortunately and only had to deal with fair low
impedance designs that don't have such problems.  However, if it is a
problem, you're probably better off etching a guard ring around op amp
inputs than trying to permanently clean the PCB soldering process.


Marginally related but interesting stuff:

Design femtoampere circuits with low leakage, part one
<http://www.edn.com/design/analog/4368681/2/Design-femtoampere-circuits-with-low-leakage-part-one
INVESTIGATION OF FACTORS THAT INFLUENCE CREEP CORROSION ON PRINTED
CIRCUIT BOARDS  
<http://thor.inemi.org/webdownload/Pres/SMTA_Pan_Pacific-2012/Creep_Corr_paper.pdf
  "The present work has shown that the presence of organic  
  acid flux residue is the single biggest contributor to  
  copper creep corrosion".


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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