Didn't mean to ignore you but our corporate frame relay has been so
flaky lately it make me long for dial-up
1. it can do several things. mixing old "worked" paste will alter the
thixiotropic properties and change the roll and fill. this will be more
of a problem with fine pitch parts. most paste is hydroscopic and will
absorb moisture over time. this will affect slump, bridging and solder
balls. as an opposite effect, the flux solvents will dry out and affect
print quality. dry paste can also prematurely dry in the preheat stage
and form a shell over the paste which will then explode when the flux in
the wet paste volatilized thus spraying solder balls all over the place
2. often nothing. most common would be a bad print with voids or volume
variations.
3. I keep the old paste segregated and do not mix. I use only new paste
for fine pitch and other critical applications.
4. of course we try to minimize the amount leftover at the end of use
but you must have enough paste on the stencil to make a good roll or
your quality will suffer. I usually tell a new operator "bigger than
your pinkie and smaller than your thumb"
5. yes, we reuse paste on less critical assemblies without fine pitch
parts. we use clean containers and don't keep adding to them. the dry
crusty stuff will ruin you. keep an eye out for the performance of this
stuff and dump anything that gives you trouble.
6. our supplier, Alpha Metals, accepts the spoiled paste in their
re-cycling program. they want it accumulated in a separate container.
small quantities make a good tinning agent for your irons (use with
proper ventilation). don't dump it into your wave solder pot. at up to
50% flux it makes quite a mess and is a fire hazard.
7. remember this stuff and anything that comes in contact with it
(wipes, gloves, empty containers, etc.) is considered hazardous waste
and must be disposed of properly.
John Thorup
> -----Original Message-----
> From: West, Jim [SMTP:[log in to unmask]]
> Sent: Tuesday, August 24, 1999 1:18 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?
>
> 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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