hi, jim,
I am sure someone more qualify than me will answer your questions...but,
here are few of the "unqualified" comments:
Personally, I do not like to mix the old paste with new for following reasons:
(a) oxidation. both flux and paste will oxidize as time going on. You may
want to see the shear rate change of the paste (viscosity at one of the rpm
is not sensitive enough for see the difference.. and shear rate is more
important for screen printing).
(b) moisture absorption. if the MFG floor is not controlled, the old paste
tend to soak up some moisture... Usually, it happened on hot summer days..
worst case, you will see splash of old paste. It is deadly for fine pitch
unless you can mix the old and new paste completely (I can't, at least not
in short time of production environment...or JIT rush).
(c) if you did your home work (the usable time of the paste v.s. humidity,
product specific printing requirement), sure you can mix them without too
much concern. except the deformation of the solder balls... I'll leave the
solder ball size, shape, distribution, slump, and other goodies to the
expert to comment.
(d) I may be too cautious. But, when you don't know what are you talking
about (like me!), better be cautious than sorry...
jk
At 04:18 PM 8/24/99 -0400, you wrote:
>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?
>
>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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