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February 2002

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Subject:
From:
"Wenger, George M." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 28 Feb 2002 13:02:20 -0500
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Doug,
I like your answer.  One needs to know what animal they are dealing with
before one can determine that the characteristics are.  "No Clean" means
different things to different people.  There are no clean low solids
fluxes (No-Clean LSF) based on dicarboxylic acids.  There are even
No-Clean LSF's which have a rosin content.  In my years with Lucent we
used typical RMA (89% metal 11% flux vehicle)solder pastes and RMA
liquid fluxes (5-40% rosin solids) and we didn't clean them off so many
people referred to them as No-Clean but I liked to call them "leave
behind" systems.  I don't like the No-Clean name because it doesn't give
one much information.

Regards,
George

George M. Wenger (908)-546-4531 [log in to unmask]
Distinguished Member Technical Staff
Celiant Corporation, FMA Lab, 40 Technology Drive, NJ 07059



-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, February 28, 2002 11:33 AM
To: [log in to unmask]
Subject: Re: [TN] No No-clean?


Kathy Kuhlow asks:

Not to use was explained to me as such: No clean leaves a residue that
with
heat can reliquify and then start to move which will cause an air bubble
where the flux was originally at and now isn't.

So the question comes to: does and at what temperature would it take for
the no clean residue you are using to reliquify if it does at all?  Then
if
it does liquify is there any activation that is occurring?  Does the
coating withstand the proper adhesion for the application if the air
bubble
does occur?


Doug Pauls, now fully tanked up with Dew, responds.

Kathy, you are treating the low solids flux as if it were a single
animal
with tightly identified characteristics.  The residues that you have on
the
board can be generically called weak organic acids, or dicarboxylic
acids.
A paper by Al Schneider (who has forgotten more about flux than I will
ever
know) a few years back listed the menu from which a flux manufacturer
has
to choose.  If I recall, that information found its way into J-HDBK-001,
section 4.  The list is over 36 materials, each of which would give you
a
different blend of properties in the final residue.  A residue that is
50%
adipic and 50% maleic acid would give you different characteristics than
one that is 33% succinnic, 33% glutaric, and 34% fufu dust / gnats
noses.
So the question of "what point does the residue soften and creep"
depends
entirely on what flux you are using and a generic classification cannot
be
made..  Your flux manufacturer would have to answer the question.  They
would also have to address the issue of whether or not the residues had
any
kind of chemical activity in the softened condition.

Think of it this way:  At what temperature does solder reflow?  Kind of
depends on the alloy, right?

Some residues will soften under high heat and may creep.  It depends on
whether the residue acts more like a thermoplastic or more like a
thermoset.  The latter will resist heat better.  If your application has
a
high amount of heat, or point heat sources, then LSFs may not be good
for
that application, or topical cleaning may be needed for that area.  In
my
experience, you either want to leave a LSF residue totally alone, or
totally remove it.  Doing a half-ass job of cleaning results in an
amalgam
of cleaner, flux, and interactive products.  Not a pretty sight and
often
detrimental.

If you have conformal coating over a residue that will soften and creep,
I
would suspect you would have adhesion problems and blister formation.
The
same problem arises when solder mask is put over a melting metal.  The
metal melts and moves, wrinkling or blistering the mask.  Same with
conformal coat.

Doug Pauls

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