Hi Jim,

Well...(see below, disclaimer... some of the following is my opinion and not
based on data)

-----Original Message-----
From: West, Jim [mailto:[log in to unmask]]
Sent: Tuesday, August 24, 1999 3:18 PM
To: [log in to unmask]
Subject: [TN] Mixing used solder paste with new (2nd time)


Hi everyone,

I sent the message below last week and I didn't get any response to the
questions.  I'm not even 100% sure that anyone received the e-mail, and I
hope that everyone understands that I'm not saying that I think mixing is
OK, that's why I'm asking.  I'm new to SMT and I want to understand
completely the ramifications to mixing, if any.  Thanks.


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?

A)      Probably nothing, unless the volitiles in the flux have evaporated
to the point that your  mixed paste has insufficient flux to get proper
solder joints.

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

A)      Again probably nothing...except that you paste vendor has gone
through lots of testing and     lots of quality tests of his product to
provide you with a paste that has proper slumping       characteristics and
viscosity to provide good consistent printing.  By mixing with used
paste (presumably dry from exposure) you eliminate the consistency of the
provided paste  that could lead to printing problems.

3. What is everyone else doing with their process?

A)      Going by the vendors recommended exposure time of the paste and not
mixing old with new.

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

A)      No!  We use as much paste that is required to get it to roll when
the squeegee is moving.         Very important for print quality.

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

A)      We do.   Some of the older paste that is not suitable for screen
printing 20 mil or      smaller pitch components is perfectly suitable for
boards that have a few 25 mil or just   chip components.

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

A)      Ours goes in with dross from the wave solder machine and gets
reclaimed by the bar solder     vendor.

Any response would be greatly appreciated!


Jim West
Manufacturing Engineer


......Hope this helps.

Scott
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