Establishing a precision stencil printing process for miniaturized electronics assembly
visual format. These figures are actual the holder is 15 mm, and the squeegee squeegee blades. The data in this print trial
results from the raw data collected during blade angle is 60º. Use of a 45º-angle blade proved to be erroneous for supporting the
paste product print trials. The explanation is avoided because the bead size would purpose of the printing experiment. The
of each figure is focused on the variation
need to be smaller, risking contact with the amount of variation appears to increase
present in the data. The outliers are often
blade holder, and forcing more frequent as the print trial progressed. The blades
most interesting because these data points
replenishment of the paste bead. Edge can be replaced and the print trial must be
will be the print deposits that simulate
guards on the blade holder help maintain redone. Replacing worn blades will imme-
inevitable assembly defects.
a uniform sized bead as the paste rolls diately eliminate the noticeable difference
Stencil printing for extremely small
and as the bead becomes depleted. From between forward and reverse strokes.
apertures using extremely thin stencils
experience, the typical amount of paste for The product design for miniaturized
requires minimal squeegee pressure or
the correct bead diameter (< 13.4 mm) is assembly focuses attention on the smallest
stencil damage frequency will increase.
found to weigh 130-135g. Stiffer squeegee apertures. Typically, concern about transfer
Most noticeably, there may be coining of
blades have not resulted in good perfor- efficiency of the smallest apertures is
the board pattern on the stencil foil. The
mance results, so a more flexible blade is outliers that fall below 50%, resulting with
best transfer efficiency performance results
used. insufficient paste and open joint defects.
when the paste bead is able to roll uni-
Squeegee blade wear is a difficult However, larger volume deposits will
formly across the surface of the stencil. To
tooling maintenance situation. The more remain part of the print process, complicat-
minimize variation in the uniformity of the
flexible blade does not appear to be worn ing the challenge. Outliers for rectangular
paste bead roll, each print trial begins with
as its use increases. However, the data will shaped apertures will sometimes occur.
a carefully measured amount of paste. For
indicate that there is a difference between They can be observed in the results of data
a 200 mm squeegee length, the maximum
forward and reverse squeegee strokes collection in Figure 2. This print trial con-
bead size has a diameter less than the 13.4
when the blades have reached maturity. tains rectangular aperture patterns and pad
mm. This is because the blade face below
Figure 1 is a common observation for worn sizes that are oriented both horizon
!
!
Figure 2—Rectangular Apertures: Dog Ear Outliers. The results are from eight rect-
Figure 1—Squeegees Blade Wear Variability Chart. The results are from a circular
angular aperture patterns and pad design combinations. The rectangular apertures
aperture pattern with a 12 mil diameter. There are 54 x 15-pin arrays on each test
are 2 sizes, 9 x 50 mils, and 8 x 50 mils. Both solder mask defined (SMD) and
vehicle. The minimum pitch is 0.5 mm. The stencil thickness is 5 mils. This is an
non-solder mask defined (NSMD) pad designs are available. The orientation of the
area ratio of 0.60, which typically provides an average transfer efficiency of 85-92
pads are both horizontally (H) and vertically (V) to the direction of the squeegee
%, with a standard deviation that is less than 10%. The signal for blade wear is
print stroke. The standard deviation of NSMD vertically oriented apertures ap-
(a) change in the transfer efficiency, (b) increase in variation, and (c) the difference
pears unacceptable, but is often tolerated during actual production by accepting
between forward and reverse strokes.
risk for increases in post-reflow rework activity.
!
Figure 3. Performance at different print speeds in a response-to-pause print trial.
The results are for square apertures with 11 mil sides using a 5 mil stencil. The
board pad is NSMD. The print speed can be slowed with the current paste to mini-
!
mize insufficient outliers, or print speed may be increased in with a new candidate
Figure 4. Variability chart for NSMD pad designs comparing four paste lots at a
paste product.
print speed of 100 mm/s.
www.globalsmt.net Global SMT & Packaging – August 2009 – 13
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