Recommended Lab Supplies
Wipes: Non-woven wipes that are highly durable and absorbent are
needed for cleaning the stencil. A strong, non-woven wipe is necessary so the
wipe does not snag in the stencil, creating loose fibers. The wipe also needs
to be absorbent so it will clean efficiently and not smear silicone into
Dow Corning recommends BlueWipes, TX512, from TEXWIPE1.
Sponges: A strong, non-silicone-contaminating sponge is needed for
cleaning the stencil after use. A sponge gets into the holes, cleaning areas
that cannot be reached with a wipe. A strong sponge is required so it can
withstand the metal edges on the stencil without tearing. It is important to
note that many sponges contain materials that inhibit the cure of silicones. We
highly recommend the Wilshire Ultrasov High Density Sponge, 0.375" by
3.625" by 5.375", P/N 4754HD from Wilshire Technologies2. This is the
only rugged sponge that we have found that does not inhibit silicones. If
another sponge is used, a test should be done to test its compatibility with
Gloves: Rubber and latex gloves can inhibit silicone cure. We
recommend using polyurethane gloves. A polyurethane glove is available through
Wilshire Technologies2. These are DuraClean gloves, part no: 13037 (lg.). For
other glove types, we recommend performing a compatibility test to ensure there
is no inhibition between the gloves and the silicone.
Cleaning solvent: We have found that Dow Corning® OS-10 Fluid is the
most effective solvent for removing silicones from the stencil. However,
ketones such as MEK and acetone also work well.
Stencil printers: Dow Corning recommends Speedline MPM4 as the
supplier of choice for stencil printers. The Ultraprint 2000 HiE is the model
Stencils: Stainless steel, laser-cut stencils are recommended.
Possible suppliers are Acumen5 and Southwest Stencil6.
Key factors to consider when purchasing stencils are:
Aperture size: The diameter of the aperture and thickness of the stencil
foil determine the volume of the printed nubbin.
Geometry and smoothness of aperture wall: Poor geometries and rugged
aperture walls will obstruct the paste, preventing clean transfer of the
material to the TAB tape. The technology used to create the holes is the key
factor in determining aperture wall quality.
In this process a metal foil is laminated with photoresist on both sides,
and a latent image is exposed into both sides of the photoresist. After resist
development, the apertures are etched from both sides simultaneously using a
dual-sided etching process.
jagged, “hour glass” shape
accuracy of approximately .003"
In this process, the stencil foil is held flat and tight while a laser beam
traces out and cuts the aperture.
straight, vertical, sharp cuts
accuracy of approximately .0002"
no corner ridges
natural trapezoidal wall
Information and drawings on chem-etch vs. laser-cut process courtesy of
Area ratio: The ratio between the contact area of the board (area of the
aperture opening) and the surface area of the aperture (the inner wall area).
Assuming a circular aperture; the equation:
Area ratio = πr²/2πrt = r/2t
where: r = radius of the aperture
t = stencil
Squeegee blades: Polyurethane, 90-durometer squeegees are recommended for
printing chip scale products. Blades can be bought from Speedline MPM4.