We're looking at dispensing solder paste from an automated system
(we have Asymtek 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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