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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