Hi Jonathan! One possible cause for the nonreflow would be oxidation of the
solder spheres in the solderpaste. (your photo didn't make it thru the IPC
server - could you have Steve post it on his website?). If you can reflow
the paste in a forming gas environment but not in a standard N2 blanket
environment then either your solderpaste flux is defective or that the
solder spheres are so heavily oxidized that the solderpaste flux doesn't
have a chance to do its job. Good Luck.

Dave Hillman
Rockwell Collins
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Jonathan A Noquil <[log in to unmask]>@IPC.ORG> on
07/25/2001 07:07:03 AM

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Subject:  [TN] Non-Reflow


Hello Friends,
Can anybody explain why there is always a non-reflow
of solder paste as shown by the drawing below?


(Embedded image moved to file: pic07711.pcx)

The solder paste used for assembly was
88Pb10Sn2Ag and it is an Indium No clean paste.
Its liquidus temp was 290 C and solidus temp of 268 C.

Reflow Atmosphere: N2 purge as hot gas and cool gas (shown at the above
drawing)
although we tried to increase the temp to over than 300 C, it is still the
same results.
I tried the paste to other oven with a forming gas blanket, and reflow it
to a peak of 295 C
the results were good (All reflowed).

We had profiled the above oven at 3 points Left edge, Midle and. Right edge
and
temperature difference vs setting was ± 5 degree C on that three points.

Was the non-reflow due to the blanket? O2 ppm level? But why on the left
side only?
Does a localized heating have occured? But the temp profile was okay?

Do you have any insights that you can share?

God Bless You All...
Thanks


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