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February 2001

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Subject:
From:
Bill Davis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 17 Feb 2001 01:53:32 EST
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Howard,

I believe there are many pro's and con's to re-using solder paste. One of the
biggest pro's are cost. Solder paste is expensive. Another pro is it's just
plain easier to scrape the paste off of one stencil and slap it on another
rather than taking the time to "auto dispense paste" , use a dispense gun, or
scoop it out of a fresh jar. It's just one less step in setup of the printer
you have to do. One of the biggest con's I think is the fact that no matter
how long the paste is on the screen it will dry a little every second it is
exposed to air. What would happen if an operator used 3/4 of a jar/tube of
paste to run 100 boards with 50 components per board and it took them 7.5
hours?  I think in cases like that you should definitely scrap the paste and
start with fresh paste on the next run. The biggest problem is how do you
monitor exposed paste. I strongly believe that four days is far too long to
be re-using solder paste. My previous company I worked for stated in the "AI"
that paste should only be re-used at a maximum of 72 hours. Three days was
stretching it a little I thought, Would you eat cup cakes that the batter was
exposed to air for three days before they were finally cooked? He, he, :). To
make a long story short, if you can find a reliable way to monitor paste
consistently in terms of how long it has been on the screen compared to how
many prints were done. I would follow the manufactures recommendation, if it
says eight hour stencil life, I would compare how long the paste was on the
screen compared to how much fresh was added throughout the day. If very
little fresh was added I would scrap the paste.
As far as your mid-chip solder ball problem, I believe humidity is a minor
problem that adds to a more wide spread situation. Something you should look
into is the screen design. One thing to look at is the aspect ratio. I had
the exact same problem at my previous employer, we had mid chip solder ball
problems practically every day. What was strange was the problem began
occurring just one day out of the blue. Nothing in our process or
environmental conditions changed. Eventually I did some investigating and
found that too much paste was getting deposited on the PCB. Another thing I
noticed was the settings or parameters on the various SMT equipment we used.
I found that for example, our "Quad" line, QSP-2 & IIC produced a much higher
amount of solder balls. Reason is I believe was the Z-placement. Our other
line, the MyData line, produced fewer if any solder ball problems. I found if
you could get the "Z" set right on the quad line and the part thickness
correct on the MyData line, mid-chip solder ball problems were far and few
between. I hope some of this helps. If you have any other questions feel free
to contact me offline. I am rather curious on how you determined 500 PPM post
reflow.

Bill Davis
Project Engineer
Thermo Assembly & Test
970-242-5593

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