A baseline study of stencil and screen print processes for wafer backside coating
13. Although the stencil print center expressed as “wedging”. The same data parallel to one another during the print
data point tends to be low, the best fit presented previously can be ordered process. If the wafer is not presented level
straight line through all data points does differently to investigate this effect as well. to the stencil surface, this may result in
not reveal any significant slope to suggest Screen print results are shown in Figure 14. either side to side, or in this case, front
scooping is present. The good news here If wedging were present, a best-fit straight to rear print pressure discrepancy. In this
is the data confirming that the special line through the data would exhibit case, the wafer is probably more loosely
purpose rigid squeegee blade resists positive or negative slope. The screen print contacting the stencil at its bottom
deflection into the aperture, even under results do not reveal any significant slope side compared to the top. Mechanical
high print pressures used for Material A. trends to suggest the presence of wedging. adjustments to re-level the conveyer
It is interesting to point out, however, The stencil print wedging trends are rail support system should resolve this,
that there are prominent thickness shown in Figure 15. The data is formatted enabling improved stencil print coating
features occurring with Material B results. somewhat differently here as reverse and uniformity performance.
The peaks and valleys align themselves forward print directions have been plotted
consistently with one another across the independently. This print direction Material aging:
two process conditions. It is speculated filtering is actually quite important and The purpose of this test is to detect
these ups and downs may actually coincide serves to reveal a noise variable that is changes in print thickness that could
with topography occurring on the wafer causing significantly unfavorable thickness be linked with material aging. Wafer
vacuum support chuck surface. The control performance. Note the solid lines coating adhesive materials will thicken
wafer itself could in fact be mapping such tend to gain thickness from start to finish, as a function of ambient exposure time.
support surface variations, and this could while the dashed lines are losing thickness The sensitivity to open time can vary from
be showing up in the print. The screen from start to finish. This does identify a material to material. This experiment was
print process may conceal this effect due significant process fault, however, this is not intended to include material aging as
to the mesh’s ability to conform against also an issue that can be repaired. a significant variable, but nonetheless it
such surface irregularities. Compared Figure 16 provides further is useful to verify that this assumption is
to Material A, Material B may be more interpretation of the trends plotted in correct. A simple analysis is presented in
sensitive to highlight small wafer tooling Figure 15. What appears to be happening Figure 17. Time is not explicitly defined on
support surface deviations due to it’s much is that coating thickness is consistently the chart, but the data points are grouped
lower viscosity. thinner on one side of the wafer compared into common process settings. Within
to the other, which by definition is process setting groups the data is ordered
Wedging: wedging. The interesting part about the in chronological print sequence. Although
Another potential vulnerability on large data is that print direction is not found thickness data points reveal some scatter,
print area applications is for coating to have any effect on changing coating patterns of significant coating thickness
thickness to systematically grow or thickness distribution. This suggests that increase within process groups is not
shrink along the print travel axis, crudely the stencil and wafer may not be truly obvious, which would be expected to
!
!
Figure 14. Mesh screen print wedge analysis (2 plots). Figure 15. Stencil print wedge analysis (2 plots).
www.globalsmt.net Global SMT & Packaging – September 2009 – 19
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