Residual material on the bottom of the stencil during printing can cause
deposition of material in unwanted areas of the substrate (the wire bond leads,
for example).“Bleed” or “smearing” of silicone around the nubbins is a typical
indication of contamination on the back side of the stencil (too much squeegee
pressure could cause this as well).
Deposition of material onto the substrate will consist of the following:
silicone deposited in unwanted areas or fibers from the cleaning cloth used to
clean the stencil. Tessera has recommended Absorbent Blue Clean Room
Wipes and liberal amounts of MEK to clean the stencil during the printing
process. The wipes should be heavily soaked with solvent when cleaning. Using a
considerable amount of solvent on the towel prevents the stencil from ripping
out fibers. A final cleaning with a dry wipe should be done as a final step.
We recommend the use of Dow Corning® OS-10 Fluid to clean the stencil
instead of MEK. Wiping of the stencil should be done only enough times to
clean the stencil effectively. Repeated and circular wiping may cause more harm
than good, since it may simply spread material into unwanted areas on the
bottom of the stencil.
Post-printing cleaning
The holes of the stencil must be thoroughly cleaned to prevent
clogging/curing of material in the holes.
First a Blue Clean Room Wipe can be used to clean the top and bottom
surfaces of the stencil foil. Then a Wilshire cleanroom sponge can be used,
which can force solvent into the holes in the stencil. After the sponge is used
to clean out the holes using several passes, a Blue Clean Room Wipe can be used
to clean up any other residual silicone.
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