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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 28 Aug 2003 10:38:35 +0300
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Many questions. Unfortunately, the many answers (see below) are full of
ifs and ands and buts.

Natasha Erker wrote:

 >
 > 1) Are water soluble solders safe to use with surface mount devices?
 > As far as I can tell, they have mostly been used with through hole,
 > as the small gap of SMD does not allow water to penetrate it when
 > cleaning, and thus the highly corrosive residues remain trapped
 > beneath.

Happily, there are no water-soluble solders, just w/s chemistry :-) Yes,
they are suitable, and much used, IF you can be sure of cleaning them
correctly.
 >
 > 2) What is the difference between a saponified resin flux and an
 > organic acid flux? What other types of fluxes do WS solders use?
 > Which is preferable?

Now we are into semantics. All fluxes used for electronics are based on
organic acids, even pure rosin. It would therefore be clearer to use
"water-soluble" as being more explicit. As the term implies, what it
means is that most of the residues, after soldering, can be removed by a
suitable washing and rinsing with water. Rosin (and synthetic resin)
flux residues are not water soluble, but may be rendered so by
saponification. This is a chemical process, using very aggressive
alkalis, that react with the rosin to form a kind of soap (hence the
name). This soap then needs to be washed off and rinsed, in a similar
manner to the wash/rinse process as for w/s fluxes.

 > 3) Will ionic cleanliness testing provide an indication if the
 > cleaning process is effective?

Yes!

I believe that ROSE testing is not
 > directly aplicable to non-rosin based fluxes,

Not so, I'm afraid. It is perfectly usable with all cleaning processes
to determine general cleaning quality, but you need to determine, for
yourself, the acceptable level for your products.

You may be confusing it with "no-clean" processes, where ionic
contamination testing may be used as a general process control tool but
has no meaning in terms of absolute values.


but would ROSE, in
 > conjunction with Ion Chromatography provide useful data? How would be
 > best to monitor the cleaning process to ensure it is in control?

If you clean, ionic contamination testing, by itself, will generally be
sufficient for quality control. Ionic chromatography is a useful
qualification and troubleshooting tool.
 >
 > 4) What types of batch cleaners are available? Will an industrial
 > dish washer provide sufficient pressure to clean SMD with WS solder
 > assemblies? What price range would I be looking at for an 'effective
 > yet cheap' cleaner?

I personally am opposed to the use of cleaners employing domestic
dishwasher techniques, including lab washers, for a good number of
reasons. They usually do not give good results and I say this from
experience. My company, now liquidated, in Switzerland, pioneered w/s
techniques in the early 1970s and our initial cleaners were based on
adapted commercial lab washers. They worked well, but... In the early
1980s, we switched from this technique to one based on hotel
dishwashers, much modified for the electronics industry (you would run
into trouble if you used a standard one; effectively, other than the
chassis, housing and motorised linear spray arms, little remained of the
hotel dishwasher!). These were MUCH more effective. I've maintained some
data on our old web site at
http://www.protonique.com/plcom/files/cleaningmach.htm
which I have no hesitation in mentioning here as there are no commercial
interests left. If you surf through the pages and the video, you may
pick up some ideas.

However, drying is a bigger problem than cleaning. Especially with SMDs,
you do not want to simply allow the residual water to evaporate on the
assemblies, as any residual contaminants will dry off where the
capillaries are smallest, i.e., round the solder joints, exactly where
you want them least.

"Effective but cheap" are paradoxal terms. As a rule, you can have one
or the other, but not both (although many cleaner manufaturers producing
cheap machines will deny this vehemently, to defend their cabbage
patch). If we were selling our APL-5 range today, we would be asking
about $50-80k for a complete cleaning/drying line. Our original APL-1
(modified lab dishwasher) cost about $10k in 1975.

You have another option which would undoubtedly be the most economical
one: farm out the assembly/soldering/cleaning of this small quantity of
boards to a well-equipped company that is specialised in what you want
to do: think! no qualification, no learning curve, no capital
investment, no wasted products during start-up and, if chosen correctly,
no botched-up assemblies because of cheap equipment. Sounds good to me!

Hope this helps

Brian

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