A baseline study of stencil and screen print processes for wafer backside coating
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Figure 1. Rigid squeegee for stencil print coating (patent pending).
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Figure 2. Mesh screen print process (Material B).
surface texture, and ability to replicate foil at 50 µm thick
results. Some work has already been done and also using a
to characterize coating thickness as a 198 mm diameter
function of mesh material selection for circular aperture
screen printing wafer backside coatings
1
. size.
In this study, two wafer backside coating A programmable
materials, which exhibit quite dissimilar fully automatic
rheological properties, are compared stencil printer
between mesh screen and stencil print was used to apply
processes to help establish a foundation for material on the
defining coating method capability. wafers and a special
vacuum chuck
Test design fixture secured
To simplify the study, only two screen wafers in the
printable wafer coating materials were machine during
selected for process investigation. These the print coating
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materials exhibited substantial differences process. Typical
in rheology, shown in Table 1, which polyurethane type
Figure 3. Stencil print process (Material A).
was deliberate by experiment design to squeegees of both 70
expose any unique process window effects. and 90 durometer
Material A B
Material “A” can be compared to the hardnesses were installed for coating mesh
consistency between molasses and honey, screen printed wafers, while a specialized
Stage 1 30 min at 30 min at
while Material “B” resembled more the rigid squeegee type was utilized when
125˚C 125˚C
feel of a kitchen cooking oil. Material “A” operating in stencil print mode shown in
Stage 2 60 min at —
is a non-conductive, snap curable (after Figure 1. This rigid squeegee was developed
180˚C
B-stage) wafer applied die attach adhesive. specifically for large aperture stencil print
Material “B” is a low CTE, low warpage, coating processes, delivering the following
Table 3. Cure parameters.
wafer backside applied protective coating. improvements over conventional thin/flat/
A batch of 48 bare silicon wafers (200 flexible metal squeegee blade designs.
both print adhesive materials applied at
mm dia./725 µm thick) were allocated for
print coating and cure testing as per the
• High flatness and rigidity prevents
various print process parameter settings.
schedule in Table 2. Half the wafers were
aperture scavenging, resulting in
Using mass measurements, along with
designated for printing with Material A
more planar coatings.
known print area and material specific
and the other half for Material B. Groups
• Design provides constant angle of
gravity values, it was possible to calculate
of 12 wafers for each material were printed
attack, reducing sensitivity to print
theoretical wet print thickness values.
by both mesh screen and metal stencil
pressure variants.
This data was useful to judge print process
processes. The same mesh screen and
• Strategic profiled backside
stability and helped to establish printing
metal stencil was used for both materials.
encourages cleaner material
machine parameters used in the formal
The mesh screen was designed at 120 wires
separation during squeegee travel.
print experiment. The wet print thickness
per inch using 65 µm diameter stainless
• Removal of sharp edges offers safer
target for process parameter selection (i.e.
steel wire (140 µm mesh openings/140
handling and lowers stencil wear.
Table 2, Variable 3) was loosely set at 50 µm,
based solely on matching the thickness of
µm mesh thickness) with 13 µm thickness
Test process
the metal stencil used. It was learned later,
emulsion defining a 198 mm diameter
Several bare silicon wafers were printed
different materials have different shrinkage
circular aperture. The metal stencil was
and weighed with an electronic scale for
effects in cure and it was impossible to
manufactured by electroforming a nickel
replicate the same cured
www.globalsmt.net Global SMT & Packaging – September 2009 – 15
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