I have recently started to screen print surface mount adhesives.  The
process is actually quite simple.  When screen printing solder paste, it is
now conventional to use a single stroke, on-contact print cycle, ensuring
the substrate is well supported, especially under areas of fine pitch
print.  Pressures of about 1- 2 lbs per linear inch of squeegee blade,
along with 75 - 100 thou of downstop works well with metal blades printing
at 1-2 inches per second (i.p.s.).

For adhesives, we have found it best to use a print/flood cycle, i.e.
on-contact print at 1.0 i.p.s.  with similar pressure and downstop
described above, followed immediately by a flood cycle at 3.0 i.p.s. and
around 0.2 lbs pressure per linear inch.

Our process flow is somewhat as follows:-

Print adhesive
Place components on adhesive
Adhesive cure
Invert PCB
Print solder paste
Place SM components on solder paste
Reflow
Hand insertion
Wave soldering

To answer your next question regarding printing solder paste on a double
sided assembly, you will need a dedicated workholder for your solder paste
printer which has 'pockets' milled out to accommodate the passive
components on the other side of the board.  Flexible tooling could also be
used, although superior results will be obtained with a well designed
dedicated tool due to the level of support offered.  Naturally, you will
need accurate board location on your screen printer (we use MPM UP 3000's).
Further on in the process, you will also need to ensure that the pick and
place board support pins/fixtures don't interfere with the the bottom side
components.

If the process is reversed, then the tooling requirements will be similar,
although more and larger pockets will be required to accommodate top side
IC's etc while printing lower side adhesive.  However, this process will
requires an extra 'PCB flip' operation to position the glued parts on the
lower side for wave soldering.

Finally, you can print adhesive dams in addition to component dot/slot
patterns.  These may be located strategically where the risk of bridging
may be high.
Ronan Kennedy
3Com Ireland



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