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From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 10 Feb 1999 12:19:43 EST
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In a message dated 2/10/99 4:59:12 AM Pacific Standard Time,
[log in to unmask] writes:

<< Fellow Technetters:
      In your experiences, at what point will one start to see differences in
 solubility in wave soldering.  I monitor our solder composition and am
starting
 to see my Tin deplete as time goes by.  I'm not seeing any immediate defects,
 but am wondering what is the alarm point.  None of the other contaminates are
 out of spec.  I just notice my Tin composition depleting and the balance is
the
 Pb.  Any thoughts?   I can give technical expectations from looking at the
 Eutectic, but am looking for the experience point of view.

 Jason Smith
 Lexmark Electronics
 Process Materials Engineer >>

Hi Jason!

     I asked the same question a couple of years ago when I was at another
company when I was doing the same thing you're doing, and ironically, about
the same time of year...ooooh, this is getting spooky! (GRIN)

     I got a couple of good responses, one from Aric Parr, and the other from
Dave Hillman. As far as what level contaminates will start affecting the
solder joint quality, there is a good article that was written by a Dennis
Bernier of Kester Solder that I'll paste at the end of this email. I got it at
Metcal's WEB page since Kester is re-doing their page and it ain't there no
more. It answers a lot questions about possible contributors to different
materials as well as their effects.

-Steve Gregory-

 Subject: ASSY: Low Tin Level in Wave Solder
 Author:  [log in to unmask] at internet
 Date:    1/3/97 3:41 PM


 Good Day Everyone!

      We just got our solder analysis back (it's from a lab that everyone
uses)
 for our wave solder pots, and we have one wave solder pot that's low in
Tin...
 61.4% to be exact. The recommendation is to add 4.3 lbs. of pure Tin for each
 100 lbs. of solder to return it to the nominal 63%.

      My question is; just how in tarnation did we get one wave solder pot low
 on Tin? It don't make sense...we use the same bar solder in all our machines,
we
 wave solder basically the same kinds of products in all of our machines. By
 that I mean we don't have one line waving only gold plated boards all the
time, or
 only OSP coated PCB's all the time, etc.. so what gives?

      This is the very first time that I've ever had a solder analysis come
 back and tell me to put pure Tin into the pot...this is weird. I've had
nickel get
 a little high before, but that's about it. We've got three machines and all
of
 the analysis looks like this:


                                Pot-1                           Pot-2                           Pot-3

 Tin                            62.4%                   61.4%                   62.7%
 Lead                   Balance                 Balance                 Balance
 Antimony               <0.005%                 <0.005%                 <0.005%
 Copper                 0.039%                  0.048%                  0.033%
 Gold                   0.017%          <0.003%                 <0.003%
 Silver                         <0.001%                 <0.001%                 0.002%
 Aluminum               <0.001%                 <0.001%                 <0.001%
 Arsenic                <0.010%                 <0.010%                 <0.010%
 Bismuth                0.001%                  0.001%          <0.001%
 Cadmium                <0.001%                 <0.001%                 <0.001%
 Indium                 <0.005%                 <0.005%                 <0.005%
 Iron                           <0.003%                 <0.003%                 <0.003%
 Nickel                 0.006%                  0.005%                  0.004%
 Zinc                           <0.001%                 <0.001%                 <0.001%


 The analysis was conducted according to specifications in IPC-S-815B. As you
 can see, Pot-2 is the "slacker" tin-wise. The only noticable difference that
I can
 see between the other two is that the copper content is just a tad
higher...does that
 have anything to do with anything? Looking at the tin content on the other
two pots
 they're just barely above the limit which is 62.4%-63.6%...is my bar solder
vendor
 starting to get tight with the Tin? Any light that anybody can shed on this
would be
 'preciated...
                           __\/__
                        .  / ^  _ \  .
                       |\| (o)(o) |/|
      #------.OOOo----oo----oOOO.--------#
      #          Steve Gregory                          #
      #      SMT Process Engineer               #
      #  The SMT Centre Incorporated    #
      #   [log in to unmask]     #
      #________________Oooo._______#
                           .oooO  (    )
                            (    )    )  /
                             \  (    (_/
                              \_)

Date: 03 Jan 97 16:32:11 -0500
 From: "Aric Parr" <[log in to unmask]>
 To: "[log in to unmask]" <[log in to unmask]> (Return requested)
 Subject: Re: ASSY: Low Tin Level in Wave Solder

      Tin oxidizes faster than lead. This means that dross has a higher tin
      content than the solder in the pot.

      In a previous job, with a very high dross machine, we regularly added
      tin to the bath.

      Have the dross production and dross composition from that machine
      checked and compare it with that of the other machines. The tin is
      probably coming from the dross.

      High antimony alloys were used to reduce this differential. I notice
      that you are using an antimony free alloy.

Date: Fri, 03 Jan 97 14:56:51 cst

     Hi Steve -

     One possible explanation for your one solder pot to be low in tin is
     the amount of drossing and/or drossing conditions that it has seen
     since your last analysis. You will lose tin content from your solder
     pot as part of the de-drossing operations. Since solder is more tin
     than lead (ie. 63 tin - 37 lead) and tin preferentially oxidizes
     versus lead, its almost logical that your dross will impact your tin
     content to some level. Some testing results we gathered on our dross
     content (for another reason entirely than low tin) confirmed that this
     could be one contributor. You should probably check out what method of
     solder alloy analysis was used too - what is the typical measurement
     error for the analysis method and could it impact the number you are
     getting reported as a result. Then again if you were cooking potatoes
     or getting rid of solder paste on your solder pot then lots of strange
     interactions could happen!!!! Good Luck.


     Dave Hillman
     Rockwell Collins
     [log in to unmask]

The Effect Of Metallic Impurities On The Wetting Properties Of Solder
by: Dennis Bernier Vice President, Research & Development Kester Solder
Company


SOURCE OF IMPURITIES

The solder used for this investigation was all from one batch with the
following analysis:

Element             Weight %              Element             Weight %
Sn (tin)                60.1                     Fe (iron)              0.006
Sb (antimony)           0.02                     Bi (bismuth)           0.006
Cu (copper)             0.004                    As (arsenic)          <0.01
Au (gold)              <0.002                    In (indium)            0.005
Cd (cadmium)            0.0003                   Ni (nickel)           <0.001
Zn (zinc)               0.0002                   P (phosphorous)       <0.001
Al (aluminum)          <0.001                    S (sulfur)            <0.001
Ag (silver)             0.001                    Pb (lead)
balance

Though the purpose of this testing was to determine the effect of impurities
dissolved during the soldering process, it is
important to note that national specifications for solder are not strict
enough to assure obtaining high purity metal. Secondary
or refined metal could contain excessive impurities and shorten the usable
life of the solder bath. Impurities such as copper,
antimony, zinc and aluminum have an effect on soldering quality and should be
kept to a minimum.

Assuming that high purity solder is being used, the impurities are introduce
into the solder from parts being soldered, from
holding fixtures and from the solder pot itself.

    Copper - Nearly everything on a printed circuit assembly is made of or
plated with copper which dissolves rather
    rapidly in solder. The circuit board itself, component leads and jumper
wires all introduce copper into the solder
    in a wave soldering machine.

    Gold - No longer used as an overall protective plating, gold is used on
certain component leads such as nickel-iron
    alloy used to make transistors, diodes and integrated circuits.

    Cadmium - Sheet metal chassis frames and other parts might be cadmium
plated to prevent rusting and improve
    appearance and solderability.

    Zinc - Brass is an alloy of zinc and copper; so brass terminals, lugs and
bolts are sources of impurities.

    Aluminum - Fixturing devices, bolts and fabricated metal parts might be
made of aluminum. The tough oxide film
    on the aluminum will usually prevent solder wetting; but with multiple
solder immersions or if abraded, aluminum
    can dissolve in the solder. It is doubtful that aluminum will remain in
the solder under production conditions since
    it will dross out when combining with copper, gold or antimony.

    Silver - Many parts are silver plated to preserve solderability. Like the
other coinage metals, gold and copper,
    silver will dissolve in the solder.

    Iron - Temperatures over 430 degrees C will cause the solder to dissolve
iron from the solder pot itself. An
    improperly alloyed solder using too much heat could contain excessive
iron. A new solder pot -- whether cast iron,
    cold-rolled steel or stainless steel -- will have exposed iron available
for dissolution into the solder. Excessive
    cleaning of the pot walls with a wire brush can also introduce iron into
the solder. The problem associated with
    iron contamination is excessive drossing which usually clears up as the
iron compounds are removed with the
    dross.

    Sulfur - It is very unlikely that sulfur would contaminate the solder bath
during normal production. Sulfur might be
    present in secondary metals since it is used to remove copper during the
refining process. Sulfur should be limited
    by national solder specifications to avoid its presence in solder.

    Phosphorous - The main source of phosphorous is copper that has been
deoxidized with phosphorous.

PROBLEMS ASSOCIATED WITH IMPURITIES

Phase 1 of the investigations involved a compilation of analyses performed
over the last ten years for the specific purpose of
solving soldering problems. The amount of impurities in the solder was related
to observed defects or solder conditions.
These defects, specifically cause by contaminated solder, are noted below with
some discussion about the impurities which
caused the problem. The table following this discussion shows the percentage
range of impurities which seemingly caused
the observed soldering defects.

    Icicles, Shorts, Bridges
    Cadmium, zinc and aluminum in trace amounts increase the surface tension
of the solder to cause this defect.
    Copper and gold increase the solder viscosity to cause the same problem.

    Large Solder Fillets
    Copper, gold and antimony increase the melting point of the solder and the
intermetallic compounds with tin or
    lead make the solder more sluggish. The result is larger fillets and more
solder consumed to create the solder joint.

    Unfilled Holes
    The speed of wetting is reduced by the presence of copper, gold, antimony
and cadmium. Though no instance
    occurred with zinc and aluminum, these metals are likely to also affect
wetting speed because of their ability to
    increase the surface tension of the solder.

    Dull Solder, Gritty Solder
    Cadmium and zinc in trace amounts make the solder surface dull. Gold also
dulls the surface but is quite often
    indicated by a sparkling, crystalline surface condition. Bismuth or
antimony in large amounts above 2.5% also dull
    the surface. Copper and aluminum contamination result in a gritty-looking
solder surface. Both phosphorous and
    sulfur have caused gritty solder though rarely are these two impurities
found in solder samples.

    Dross Inclusions
    Dross inclusions in the solder show up as visible particulate grit or
hidden inside a bump or pimple in the
    otherwise shiny solder surface. Quite often the source of this problem is
an unusual amount of iron in the solder.

    Cracked Joints
    Inclusions in the solder such as intermetallics of tin or lead with
copper, gold and antimony can provide the
    nucleus for crack propagation.

    Dewetting
    Zinc, antimony and phosphorous can cause solder to dewet on copper.

By looking at the real world of wave soldering and the ten years of analytical
records, we can summarize the impurity levels
which traditionally caused problems.

Impurity        % When Problems Occur
Cu (copper)             0.250 - 0.500
Au (gold)               0.005 - 0.200
Cd (cadmium)            0.005 - 0.150
Zn (zinc)               0.001 - 0.010
Al (aluminum)           0.001 - 0.006
Fe (iron)               0.010 - 0.100
Sb (antimony)           0.100 - 1.000
Ag (silver)             0.200 - 2.000
Bi (bismuth)            0.250 - 1.000
As (arsenic)            0.030 - 0.100
In (indium)             no data
Ni (nickel)             0.010 - 0.030
P (phosphorous)         0.010 - 0.100
S (sulfur)              0.002 - 0.030

Immediately obvious by an examination of this list is the fact that the
percentages established by experience are not precise
numbers. The explanation for this is that the defects cause by the impurities
may be acceptable at one company and cause for
rejection at another company. Rigid inspection requirements for aerospace or
military products might reject solder joints
which are acceptable for consumer products. Difference between fluxes,
soldering machines, circuit board density,
component layout, hole sizes, solderability and amount of heat all contribute
to the quality of soldering.


REFERENCES

  1.Soldering Manual, 1959, New York, American Welding Society
  2.C. L. Barber: Solder, 1965, Chicago, Kester Solder Company
  3.H. Manko: Solders and Soldering, 1964, New York, McGraw-Hill
  4.M. L. Ackroyd et al. : Tin Research Institute Publication No. 493, 1975,
The Metals Society.
  5.D. Mackay: Proceedings of Institute of Printed Circuits, Meeting,
September, 1972, San Francisco

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