A baseline study of stencil and screen print processes for wafer backside coating
or thinner foil
Thickness Trends
stencils could also be
The next several plots present thickness
methods to achieve
data grouped by location on the wafer
thinner coatings.
and arranged in specific sequences to
As long as new
illuminate potential issues associated with
screen and stencil
scooping, wedging, and material aging.
designs matched the
same aperture sizes
Scooping:
for data posted in
A concern with printing through large
Figure 9, mass based
diameter apertures, particularly with metal
analysis based on
stencils, is that the printed coating will
!
these trends is still
be thinner at the center of the pattern
valid.
compared to the circumference. While
Figure 11. Material B wafer to wafer thickness comparison.
From Figure
screen printing provides a fixed mesh
11, screen print C
layer between squeegee and wafer to
p
stubborn in its tendency to remain firmly
performance for
prevent scooping, the stencil print process
lodged in the mesh screen openings,
Material B again is shown here to be better
depends on a firm and straight squeegee
requiring extraordinary effort to unblock.
than stencil print performance, but not
blade to maintain its form across a large
It may, in fact, be the nature of this
by the margin displayed for Material A.
unsupported print area. Figure 12 displays
material to require replacement of the
The thickness differences shown between
screen print coating thickness data that
mesh screen each time a manufacturing
processes is logical in that faster print
has been grouped by process and material
run is performed. A decision scenario such
speeds tend to increase print thickness.
type, and ordered to show how thickness
as this could make the stencil print option
Print process changes within screen print
behaves as a function of radial location
look far more attractive, as this was indeed
and stencil print categories seem to have
on the wafer. The expectation is either to
much easier to clean up in comparison.
little influence on significantly changing
detect shallower deposits in the center of
The same print thickness analysis
C
the wafer or no print thickness difference
p
. The alarming data here is the poor C
p
for Material B is shown in Figure 11. The
values reported in the stencil print process,
between center and edge. It would be
coating thicknesses in general are higher
at the 1.0 level. Material B may have to
unusual to find thickness measurements
for this material compared to Material A.
be screen printed if a C
higher at the center. Typically the screen
p
level of 2.0 is
If thinner results were required for some
required. Fortunately, from a clean down
print results show uniformity in thickness
reason, further process testing could be
standpoint, there were no problems in
from center to perimeter, as expected.
explored using mass analysis to predict
cleaning the mesh screen for Material B.
The variation in thickness occurring
thickness from trends shown in Figure 9. on the stencil printed wafers is much
Mesh screens with reduced wire diameter more amplified in comparison on Figure
! !
Figure 12. Mesh screen print scoop analysis (2 plots). Figure 13. Stencil print scoop analysis (2 plots).
18 – Global SMT & Packaging – September 2009
www.globalsmt.net
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