A baseline study of stencil and screen print processes for wafer backside coating
! !
Figure 4. Measurement locations, Material A wafers. Figure 5. Measurement locations, Material B wafers.
coating thicknesses across all the process industrial convection batch oven at settings measurement is shown in Figure 7, which
designs used. listed in Table 3. Material A was initially clearly reveals the silicon base, which
During the formal print test as per B-stage cured, followed immediately by a has been exposed from scratched away
Table 2, wafers were cycled into the printer second stage to fully harden the coating. material. Major thickness axis divisions
at timed intervals approximately three Material B did not require a secondary are separated by 10 µm, finest division
minutes apart to simulate a manufacturing stage to achieve full cure. increments are 1 µm.
line process condition. Alignment fiducials The stylus profilometer was also
designed into the vacuum chuck tooling inspection procedures programmed to record surface roughness
and on the stencil were used to register the Data acquired from all 48 cured wafers measurements on Material A printed
large aperture consistently on the wafer. consisted of coating thickness values wafers. The measurement parameters are
The mesh screen print process was at several positions across the wafer. In defined in Table 4. Output data included
performed in a Flood/Print mode. The addition, surface roughness was also R
a
(average roughness), R
z
(average
rear mounted flood blade first distributes characterized for cured wafers printed maximum height), and R
t
(maximum peak
a layer of material across the large open with Material A, since this is an important to valley) values. In this report R
z
values are
mesh aperture to provide preliminary performance criteria for die attach
mesh wire lubrication. This is followed by applications. The locations measured
a second stroke in the opposite direction for thickness and roughness on
by the forward mounted polyurethane wafers printed with Material
squeegee where material is pushed down A are identified in Figure
into the open area forming the pattern 4, while Figure 5
and the surplus is removed by the edge shows the thickness
of the squeegee (Figure 2). A print gap is measurement points
quite common in mesh screen printing to on wafers printed
encourage the mesh to peel away from the with Material B.
!
surface immediately behind the squeegee, Different
Figure 6. Stylus profilometer measuring tool
leaving all the material that was in the tools were used
mesh deposited on the wafer surface. to measure the
The stencil print process was operated wafers, depending
with a different squeegee set and process on the coating material printed. Wafers
parameters. The Print/Print mode was set printed with Material A were measured for
to perform one on-contact print stroke coating thickness and surface texture by
per wafer. A set of specially designed the contact stylus profilometer instrument
straight and rigid squeegees (Figure 1) shown in Figure 6. Print thickness
were installed on both rear and forward measurements were taken at locations on
squeegee holders. Since these squeegees do the wafer labeled in Figure 4 where the
not have a sharp printing tip, a thin film stylus traversed regions of scratched away
of material is expected to trail behind the coating material.
moving squeegee and remain on the stencil Thickness measurements were
!
as shown in Figure 3. manually interpreted on printed tickets to
Figure 7. Example stylus profilometer
All 24 printed wafers were cured in an the nearest estimated 0.25 µm. A sample measurement.
16 – Global SMT & Packaging – September 2009 www.globalsmt.net
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