In a message dated 8/21/98 10:43:09 AM Pacific Daylight Time,
[log in to unmask] writes:
<< Subj: [TN] Paste Printing
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]>>
Hi there Richard!!
Uh-oh, solder paste sticking to the squeegee huh? Sounds like your
squeegee's have become magnetized. Here's what you need to do to solve the
problem; go out and find a rental degaussing unit, and then you.......whaddya
mean you're not buyin' it? Geeze, you're no fun! (thought I'd try and have a
little chuckle or two with the "new guys", but they're too smart!)
Seriously, can I ask what kind of squeegee's you're using? Metal or
plastic? Plastic ones tend to have the solder paste stick a little more than
metal ones do for some reason, that's one contributor. There are also some
solder pastes that do tend to have that problem a little more than others,
especially when it's fresh out of the jar or tube.
I don't know if you've ever heard of this term before, but solder paste
is "Thixotropic" (I hope I spelled that right). What that means is that it's
rheology (basically it's viscosity) changes depending on what it's doing, or
how it's worked...and that's aside from the way it's affected by temperature.
For example; when the paste is straight out of the jar and it hasn't been
moving, it's viscosity will be thicker than the same paste, at the same temp
would be that's been on a stencil for a while moving back and forth. Then if
you let the paste set still for a while, it'll thicken back up to the same
viscosity that the fresh paste has.
I used to use a paste a couple of years ago that would do that to an extreme.
Just in the short time that the line would stop during morning breaks would be
enough for the paste to set up and stick to the squeegee when starting the
line back up again, after the paste was sheared a little (taking a spatula and
working the paste back and forth for a minute or two), everything would be
fine...just as long as the line kept running.
Another thing that can cause the paste to stick is that it isn't at room
temperature. Aside from the viscosity issues, you don't ever want to allow
cold solder paste to be exposed to ambient air until it's up at room
temperature, it'll condense the moisture in the air on to the solder paste and
you'll get solder balls from the steam it creates during reflow. One last
thing that can cause your paste to stick, is that you're not putting enough
down on your stencil in the first place. You should wind-up with enough paste
to make a roll all along the length of the squeegee about as big around as
your thumb...if you don't put enough down, the surface tension of the paste
overcomes it's own weight and takes a ride with the squeegee on it's way back
up...
Have a GREAT weekend!!
-Steve Gregory-
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