Richard,
I ran into this in a previous life, and it drove me crazy for a
couple of weeks.
A couple of quick points & things to check:
Before you blame the paste, check:
- the humidity, if too dry, this could cause a problem. I had one
paste which would show problems every fall ,surprisingly it was the
same time the plant furnaces were fired-up and we had drier air! Also
check the temp/humidity inside the screen printer at the paste
location. I also had an instance where fans that normally helped keep
heat out of the printer quit working, and the area was hotter then
norm.
- If old metal blades, get new ones! they may not be releasing well.
Now I would recommend getting your supplier involved immediately, and
ask for some paste from another lot pronto...I have had unexplainably
bad lots before.
Also, obviously, dig deep to find any process changes from the
norm...operators leaving it out of the fridge 4 days, bulk storage
refrigerator at correct temp, etc....and since you mentioned you were
new at this, this is NOT the norm. Most good pastes out there have
excellent drop-off characteristics. Again, the key is to get your
supplier involved ASAP.
Oh, and are these results different then when you qualified your
vendor/paste? You DID qualify your paste for your process, using IPC
tests, didn't you? If not, now you know why we run all of those
preliminary tests to choose a paste!
(Just some friendly Friday ribbing %^)
Good Luck.
United Technology Electronic Controls
Jeff L. Hempton, Sr. SMD Engineer
3650 West 200 North
Huntington, In
46750-9002
E-mail: [log in to unmask]
______________________________ Forward Header __________________________________
Subject: [TN] Paste Printing
Author: "Hamilton; Richard -4454" <[log in to unmask]> at Internet
Date: 8/21/98 10:30 AM
Hello to all,
Well we are finally testing our SM capabilities, we are brand new at this.
We have an issue at paste printing that I would like everyone's input on.
When the squeege makes it's pass on the stencil and then lifts upward off
the stencil, the majority of the paste remains with the squeege instead of
on the stencil.
What factors are there in the paste charactistics when applied?
Thank you all in advance.
Richard Hamilton
Clemar Mfg. / Rain Bird
[log in to unmask]
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