In a previous life we did this quite often. We did not dispense on paste
that was already on the pad from the screening operation. We used a
positive displacement pump on a Camalot System. I liked the driving
mechanism on the Camalot better than the Asymtek. We were turning
boards at a rate of a couple times a week at the time and to buy a stencil
for every one really saved by dispensing all the solder in the beginning.
But when the design finally settled down we used it only for the larger
pads to get that extra paste on them. We would put a number of dots, but
you will need to develop that for yourself. I would only buy the paste
in premade syringes. Eliminate the mess and the air you get in from trying
to do it yourself. Needle size is only of those things you need to work
on for your application. But my suggestion is to use about a 25 mil
needle and develop the height from the board you need to make a nice dot of
paste. I never found trying to dispense paste in a line to work real
well, we pretty much used dots for everything. If you use a Camalot or
Asymtek system the Z axix is very well controlled and you should have no
problem with that.
Good luck.
"Kane, Joseph"
<joseph.kane@BAES To: [log in to unmask]
YSTEMS.COM> cc:
Sent by: TechNet Subject: [TN] Solder Paste Dispensing
<TechNet@listserv
.ipc.org>
06/26/03 10:24 AM
Please respond to
"TechNet E-Mail
Forum."; Please
respond to "Kane,
Joseph"
We're looking at dispensing solder paste from an automated system
(we have Keihin Carolina System Technology, Inc. and Camalot). Intent is
to get some extra bulk into
solder joints on larger chip components to extend the life. Parts are
about size 1005 and up, paste is RMA. The operation is low volume,
high mix.
The idea would be to
1. Screen print paste as we do now (6-mil stencil).
2. Dispense some glue dots to build up a little standoff (higher solder
joint), and add mechanical coupling.
3. Dispense more solder paste only on those pads that need it.
4. Populate and reflow.
After looking at the archive, I get the idea that some of you are
using paste dispensing, at least for intrusive reflow. Can anyone
provide guidance on the following:
1. What kind of positive-displacement dispensing head works best?
Paste probably won't string, so do you need suck-back?
2. Does the paste dot snap off repeatably from the needle? How
critical is z-axis control?
3. What size needle?
4. Can you dispense a dot onto a solder paste brick without making
a mess, or should you dispense onto a clean pad?
5. When dispensing onto (for example) a square 1005 pad, do you
dispense a series of small dots, or one large glob, or line, or fill?
6. Can you use your normal screen-printable paste, or do you need
paste with finer mesh? Is paste available in pre-filled syringes?
Joe Kane
BAE SYSTEMS Controls
Johnson City, NY
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