A baseline study of stencil and screen print processes for wafer backside coating
Stylus tip size 5 µm radius/60˚
this is the process tolerance divided by six level, the mesh screen print results track
diamond
sigma, and for this testing the following quite remarkably better at an impressive
equation was used. 6.0 and above level. There is no question
Evaluation length 15 mm
that Material A is more uniformly printed
Cut-off distance 2.5 mm
with the mesh screen process, however, this
!
factor alone doesn’t necessarily define the
Table 4. Surface roughness measurement tool
The process tolerance, or specification process choice.
settings.
limit, which has customarily been accepted The stencil print process could very
for this process is ±12.5 µm, which explains well be acceptable for most applications
reported
the derivation behind the numerator at the C
p
of 2.0. It was observed in the
Cured coating thickness measurements
value. There is some debate and confusion closing minutes of the experiment during
for wafers printed with Material B were
on the use of C
pk
(process capability index) clean down that Material A was extremely
performed by a much more labor intensive
to characterize
method. Scribed wafer die were removed
the process
from measurement positions indicated on
performance
the template in Figure 5. The 21 extracted
instead of C
p
.
dies per wafer were sandwiched together
To clarify, the
and arranged in an epoxy potted stack for
use of C
p
in this
metallurgical cross-sectioning. A high-
analysis is based on
powered optical microscope was used to
characterization of
measure thickness at three positions within
print uniformity
each measurement location on the wafer.
without concern
An example measurement view is shown
to the actual
in Figure 8. A total of 21 x 3 = 63 thickness
thickness values
measurements per wafer were logged.
measured. In other
words, this testing
results is not designed
Mass analysis to measure how
!
Print coating material mass measurements accurate the print
were performed on each wafer before thickness can
Figure 8. Cross section coating thickness, Material B.
curing. The data for both Material A and be relative to a
B are organized into a single chart on specified target
Figure 9 to show how print mass translates value, which is
to cured thickness. The different color fundamental to a
symbols used to chart the data indicate C
pk
analysis.
the various process conditions tested. The Cp trends
High R-squared values suggest the best-fit for Material A on
straight line model equations can be used Figure 10 concur
with high confidence to accurately predict with the spans
thickness results. Mass analysis serves to in thickness
be useful in process monitoring and it distributions
also allows freedom to explore the effect plotted. Lower
of different process settings on thickness standard deviations
response without requiring printed wafers translate to
to be cured. greater thickness
uniformity, and
!
process control perhaps this also
Cured coating thickness measurements generates better
Figure 9. Print mass defined thickness trends.
results for Material A are shown in Figure repeatability across
10. The data has been color coded to help wafers. Higher
distinguish process sensitivity to thickness values of Cp are
and it also has been grouped to identify favorable, typically
individual wafer thickness measurement with the aim to
outcomes. The distribution of thickesses achieve a goal
appears tighter for both screen print of 2.0 in order
conditions compared to the stencil print to accomplish
processes. The screen print data also does 6-sigma level
not show a significant effect of squeegee process capability.
hardness on coating thickness. Although the
Another statistical method of stencil print process
comparing thickness control is by generally reports
!
presenting the data in the form of a Cp values at an
Figure 10. Material A wafer to wafer thickness comparison.
capability ratio, or C
p
. Mathematically acceptable 2.0
www.globalsmt.net Global SMT & Packaging – September 2009 – 17
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