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May 1998

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Subject:
From:
William Casey <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 27 May 1998 09:44:07 -0600
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Stephan,

This topic sounds like it would make an extremely interesting thesis.
The cleaning processes you have briefly described would more likely
induce oxidation rather than remove it from the spherical surface.  When
you have a non-uniform sphere geometry after liquidus, it is most likely
due to the oxidation concentration on the sphere or pad surface.
However, if the spheres are not melted in an inert atmosphere the oxide
skin may perpetuate your problems.

One of the techniques that I use, and it has been extremely successful,
is the use of a plasma cleaning process.   There are a number of small
reaction chambers on the market.  I currently use a March Px1000 system
for all development work.  There are also hundreds of oxide reducing
recipes already in existence however, you may just want to spend some
time developing a custom recipe which will best fit your requirements.

The following is an example recipe that I have had good results with:

Gas             02
Pressure        90 mtorr
Power           359 watts
Process time    3 min
Gas Flow rate   536 sccm

Gas             Ar
Pressure        150 mtorr
Power           250 watts
Process time    2 min
Gas Flow rate   100 sccm

Remember that argon etches organic and oxygen cleans organic.  With the
argon etch atoms are elevated to a higher energy state which with
physically displace organic molecules or atoms.  Weight loss of the
63/37 may be as high as 0.62E-12 grams per mm2.   With the oxygen atom
will undergo an oxidation-reduction reaction converting surface organics
to water and CO2.  There is basically no mass (other than the organic)
lost during this process.

The trick with your application will be limiting the sputtering effect
caused by large molecule inert gasses such as argon, while producing the
cleaning effect necessary.


William Casey
MCMS
Research & Development
Direct: (208) 898-1042
Fax:    (208) 898-2740
[log in to unmask]


>----------
>From:  Meschter, Stephan J[SMTP:[log in to unmask]]
>Sent:  Wednesday,May 27,1998 7:39 AM
>To:    [log in to unmask]
>Subject:       [TN] BGA Type Solder Sphere Deoxidation
>
>Hi,
>This is my first technet request. I have been reading the net correspondence
>with interest over the last month. I think that some of the processing
>associated with BGA spheres may be relavent to my problem.
>
>I am working on my PhD research project which is exploring the melting and
>spreading behavior of an initially solid solder sphere placed on copper
>substrates held at an elevated temperature in a gaseous formic acid/nitrogen
>environment. I have been working mostly with tin and 63Sn/37Pb solder and am
>now trying solder spheres with higher lead content.  The sphere sizes that I
>am using are between 25 and 35 mils in diameter. I found that with the tin
>and tin-lead eutectic that some pretreatment of  the spheres yield more
>consistent melting/spreading behavior. To pretreat my spheres, I place 20-30
>spheres in a 50 ml beaker with an RMA flux at 110-120C for 10 minutes
>followed by a series of rinses to get the flux off. Since the flux did not
>go up to the solder reflow temperature it is fairly easy to get off. First
>the spheres are rinsed in 3 successively cleaner isopropyl alcohol baths at
>50-60C (total time 20 minutes) then are put in a final rinse of 30-40C
>methanol for 10 minutes followed by blowdrying with nitrogen.
>
>When I pretreat the high lead content spheres using this method, I see the
>spheres darkening as I rinse them...especially when I get to the methanol.
>The resulting melting and spreading of these spheres is non-uniform. When I
>melt them, it appears that they have an oxide skin on them. I am continuing
>to tinker with the rinse process times and solvents to see if I can get a
>combination that works. I the mean time, I was wondering if anyone out there
>had any ideas.
>
>I am looking for a sphere pretreatment scheme that will eliminate the oxide
>skin and not have any residues on the sphere surface.  The process should
>not cause preferential etching of the tin or the lead on the sphere surface.
>
>PS...A while back, as I was developing the my sphere pretreatment process, I
>tried using a commercial solder brightener product followed by a DI water
>rinse on the 63Sn/37Pb spheres and it didn't really provide the consistency
>I had hoped for. I was going to try this again with the high lead content
>spheres.
>
>Thanks in advance for any help that you can offer.
>
>Steph
>
>*********************************************************
>Stephan Meschter               [log in to unmask]
>Lockheed Martin Control Systems  Phone  :(607)770-2332
>600 Main Street, MD R52F         FAX    :(607)770-2056
>Johnson City, NY 13790-1888      MARCALL: 8 * 255-2332
>*********************************************************
>
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