A baseline study of stencil and screen print processes for wafer backside coating
coating coplanarity success for two widely such materials could be more difficult
different material rheologies that achieve to mesh screen print and may produce
±12.5 µm @ 6 sigma control down to 30 more textured surface finish results. With
µm cured thickness. Of the materials and the application of these as wafer applied
conditions tested, the process capability die attach coatings by printing already
ratio (C
p
) tends to favor mesh screen taking place, this material category will be
printing. Establishing mesh screen included in a subsequent process study.
resistance to blockage over extended runs Finally, to test more realistic substrate
as well as clean down compliance are conditions the strategy of incorporating
main concerns that warrant review for any thinner wafers into formal experiments
printing material. Demonstrated stencil are planned. Dummy 200 mm diameter
print process coating uniformity may be silicon wafers of at least to 300 µm thick
somewhat conservative based on discovery and if practical sourcing is available, to
!
of noise variables related to equipment 150 µm thick, will be incorporated into
Figure 16. Stencil print wedging trends.
setup and support tooling finish. Based forthcoming process development.
purely on viscosity differences between
occur with material thickening over time.
materials tested, results here indicate acknowledgements
Adhesive coating materials were probably
thinner materials may be more sensitive to Gratitude is extended to Stephen Ruatta
not used for a long enough period in this
printer machine component parallelism and Kevin Lindsey from the Ablestik
study to produce a distinguishable aging
and wafer chuck flatness, suggesting division of Henkel Corp. for their
effect.
such materials are more ideally suited for assistance in measuring wafer samples and
mesh screen printing. Smooth coating providing valued technical guidance.
Surface roughness
surface texture measurements obtained
Smooth print surfaces promotes better
for both print techniques using one of references
wafer level die attach adhesion to dicing
the latest generation non-conductive die 1. T. Winster, Henkel-Ablestik, A.
tape, helping to improve yield in that
attach materials available shows promising Hamstra, H. van Rijckevorsel, Philips
process. Material A was measured on a
performance consistently below 4 µm R
z
. NXP, “Wafer Backside Coating (WBC)
stylus profilometer at six positions per
of Die Attach Adhesives”, Proceedings
wafer to evaluate cured surface texture
Future work of IMAPS 2005, Brugge, Belgium, June
achieved by four printing processes. The
This study has produced some evidence 2005.
distribution of data is shown in Figure 18.
that stencil print coating thickness
Target surface roughness performance
distribution may be influenced by
is based on achieving below 10 µm
mechanical planarity attributes during
R
material deposition. This not only includes
z
. All process conditions tested were
observed to comply comfortably inside
the printer tool itself, but also the quality
this specification. The screen print results
of the wafer support tooling and squeegee
tend to produce a slightly more textured
blade. Further progress to optimize the
surface than stencil print coated wafers.
performance of the printing system for this
The “orange peel” surface characteristic
process will continue and improvements
for mesh screen-printed wafers may appear
will be reported.
visibly more obvious than what is actually
Conductive materials containing
reported by measurement.
coarse and randomly shaped filler
particle ingredients to provide electrical
conclusion
and/or thermal conductive properties
Large area thin film wafer backside
are also available
coatings can be rapidly and uniformly
in B-stageable
applied by screen and stencil print
adhesive formats.
processes. This study has demonstrated
It is speculated
! !
Figure 17. Coating thickness response to time. Figure 18. Material A surface roughness measurements.
20 – Global SMT & Packaging – September 2009 www.globalsmt.net
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