Hi Jim - just wanted to add a couple of items to the already good responses
you've seen posted! The mixing of old and new solder paste won't shut down
your production line but will induce more process noise. Solder paste that
has accumulated time on the stencil will have a change in flux properties
(some of the flux chemistry volitalizes the solder particles will oxidize
over time,  and  you may have solder particle shape changes (solder is a
soft material and the action of stenciling will distort the solder particle
shapes). These changes may or may not cause process defects but they
definitely contribute to overall process noise and robustness.


Dave Hillman
Rockwell Collins
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"West, Jim" <[log in to unmask]> on 08/20/99 02:05:26 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
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Subject:  [TN] Mixing used solder paste with new




Hi everyone,

TGIF

I believe this topic has come up on the technet, so I apologize if this is
a
duplicated question.  I need to know what the proper procedure for handling
solder paste.  Right now we mix solder paste that has been used all day(4-8
hours) with new solder paste.  I know it has been said that one should not
do this.  But, with our process I do not see any quality issues(I think)
that is a direct result of this mixing.  So, that leaves me with several
questions:

1. What will the mixing of old and new do to a solder joint?

2. What tells me(visually) that mixing is a bad thing?

3. What is everyone else doing with their process?

4. Does anyone measure the amount of solder they use so that they have
minimal left over at the end of the day?

5. If the old is kept separated from the new, can it be used then?

6. What does everyone do with the used solder paste?

Any response would be greatly appreciated!



Jim West
Manufacturing Engineer

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