Teo,
I have had this problem more then once in the past. In my previous encounters
with this type of failure, the issue was once the paste was on the stencil for
30-40 minutes, it started sticking to the squeegee blades and causing skips in
the prints. So cartridges would not have helped the issue for us. The culprit,
at least here in the mid-west, ended up being an older technology paste that
started "drying" as soon as the "furnace" season hit and the plant environment
was drier. We tried everything from adding thinners (not recommended, very hard
to control this process) to having the vendor mix at lower viscosities for us
(this of course, gave us other printability issues to deal with). Manually
kneading the paste every 30 minutes allowed us to at least get SOME product out.
Although our humidity in the plant was drier then in other seasons, it was not
unreasoneable, and the paste vendor was at a loss how to fix this issue. We
decided it was time for a "new paste qualification", and ended up qualifying two
new pastes which worked well in any season. Your vendor should be working hard
on this issue with you.
I know you are looking for a short term fix, but you might want to also look
further ahead. A good start would be to ask your current paste vendor if they
have a new & improved technology paste they would recommend as a replacement,
add a few of the industry leaders, grab an IPC TM-650 set of standards for paste
testing and yer on yer way! Hey, the newer pastes print fine pitch so much
better anyway! We just finished a new paste qual last month. I would be glad to
share my results with you directly. Good Luck.
Jeff Hempton
United Technologies Electronic Controls
______________________________ Forward Header __________________________________
Subject: [TN] MPM solder Paste dryness and cartridge dispenser
Author: [log in to unmask] at Internet
Date: 1/19/99 9:46 AM
Ladies & Gentlemen:
We are currently experiencing a problem in our MPM printing process.
Often, the solder paste gets dry and leads to product defects. As a
result, operators dispose the "dry" solder paste. However, the measure of
"dryness" is very subjective and varies from one operator to another.
Consequently, our facility is disposing too much solder paste. Is anybody
measuring the "dryness" of solder paste? If so, how are you doing this? I
need a scientific way to determine solder paste dryness, and a guide that
will let me know when the solder paste is no longer good. In addition, is
there any way to "recycle" dry solder paste? Are there any solvents or
chemicals that will restore the solder paste to its original condition?
The solder paste is currently being placed by hand on the stencil.
Before the stencils are washed, the solder paste is removed and placed back
into the jar. This results in cross contamination of new and used paste.
To resolve this cross contamination, we would like to use an automatic
catridge dispenser which is available with MPM printers. Is anybody using
these automatic dispensers? If so, do you have any "lessons learned" or
"recommendations" for using this new equipment?
-Thanks for your help,
Teo Tijerina
Conexant Systems, Inc.
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