Establishing a precision stencil printing process for miniaturized electronics assembly
ever, maintaining a high print pressure will
damage thinner stencils. The demand for
area ratio increases to improve print trans-
fer efficiency performance of miniaturized
electronics assembly subsequently demands
thinner stencils. Consequently, controlling
minimal squeegee pressure will inevitably
become necessary as customers experience
production losses due to stencil damage
(from excessive squeegee pressure).
As production control of squeegee
!
pressure improves, production demand
!
for high speed printing processes will be
Figure 9. Pad Size (0.20 mm) x Pad Space Figure 10. Pad Size (0.15 mm) x Pad Space
challenged. Higher print speed requires a
(0.10 mm). (0.15 mm).
higher print pressure in order to get a clean
swipe across the surface of the stencil. A
mance in Figure 5. These values show in apertures did show more outliers than
clean swipe helps control uniform paste
a concise format the precise performance the square apertures. This phenomenon
roll and maximize transfer efficiency. The
differences among each paste lot. Clearly, supports a common observation trend.
precision stencil printing process could
paste lots #2 and #3 performed similarly, as For miniaturized assembly, however, the
be constrained by the challenge. New
did paste lots #1 and #4. Looking back at increased density of the smaller sized pads
squeegee designs and innovative printer
the box plots and standard deviation charts will result with predominantly NSMD pads
equipment capability could be a partial
in Figure 4, it is easier now to distinguish in the print process. Additionally, there
solution, but the demand for minimizing
the distinct differences among the lots. will be small area ratio apertures. For this
squeegee pressure is to be expected.
Additional information about the storage reason, insufficient outliers could become
Figure 4 shows some results from
history of each sample will help a decision- a significant concern with production yield
print trials on four paste lots that have
maker understand whether its storage and rates.
been done using a 4 mil stencil at a print
handling history can have an impact on A board support system for stencil
speed of 100 mm/s. The print pressure is
the product performance. printing can offer opportunity for variation
independently determined by the printer
There exists a certain confidence in the print performance. Smaller features
operator at the setup period for each print
among industry experts about the print in the print process will dictate much tight-
trial. The settings for the 200 mm squeegee
performance with respect to solder mask er tolerance for the board support system.
length using this new experimental solder
design of the pad. These insights can assist Paste print trial performance has shown
paste at 100 mm/s can range from 3.4 kg
with stencil aperture design modifica- improvement with routine housekeeping
to 4.0 kg. The operator determines the
tions, allowing for even better production maintenance following and at the start of
minimum pressure by observing during
performance. Ironically, many experts have each print trial. Dedicated support fixtures
setup whether a clean swipe and good
opposite views on the benefit of SMD are custom designed for every print test ve-
paste roll are achieved. The paste bead has
versus NSMD. Data can be presented to hicle. These fixtures provide 100% support
to be carefully measured so that it will not
support both sides of the same matter. coverage of the bottom side of the board.
risk contact with the blade holder. For the
Common attributes that filter into many of Squeegee length (overhang beyond the
setup for each lot, the trial starts with the
these SMD/NSMD situations is the board edge of the board size) is less than 10 mm,
operator using 130-135 g of paste. The
support and relative location of the pads providing maximum support while the
bead diameter will be kept uniform by edge
under surveillance in the board layout. For squeegee travels over each aperture. The
guards, and it measures just below 13.4
example, rectangular pads with a definitive- custom support box is vacuum equipped
mm.
ly large area and aspect ratio may perform to hold the test board flat and in place.
The outcome of the results from this
better using a NSMD pad design when Minimum squeegee pressure minimizes
set of print trials is expected to help distin-
positioned in the layout so that maximum potential shift in the board position during
guish performance differences among the
support is provided beneath the sites. the print cycle. No print gap is permitted
paste lots, to learn whether the transfer ef-
In another example, circular pads with in the print cycle. The printer program is
ficiency variation differs among paste lots.
an area ratio under 0.66 perform better not set to over-compensate the board thick-
Nine aperture patterns and pad design
using a SMD pad design when positioned ness so that the board is pressed into the
combinations are presented. Clearly, the
away from the start of the print stroke, stencil causing possible deformation of the
0.56 area ratio apertures characteristically
and given maximum support beneath the flatness of the stencil.
challenge the print process in all four lots.
sites. Because each expert draws on case by Since no paste debris is ever left on
It can also be observed that 0.56 area ratio
case experience, it is difficult to reach any the printer conveyor rails due to diligent
square apertures perform better than their
concrete conclusion. housekeeping, the print gap (between the
circular counterparts. It is a tough call for
Figure 6 shows results of circular and board surface and bottom side of the stencil
the best referee. Another visual presenta-
square apertures on both SMD and NSMD foil will not inadvertently occur. The regular
tion from the same data is shown in Figure
sites. The aperture distance away from the board clamps that have been traditionally
5.
start of the print stroke varies considerably. used in the conveyor system have been
The variance-to-mean ratio (VMR) is
Three squeegee speeds have been used in replaced by a new clamp system. The regular
calculated from the data for each aperture
the print trial. Other attributes of the print clamps include a stainless foil to hold the
pattern and pad design from each print
setup have been tightly controlled. The board in place, clamping down on the
trial on the four lots of an experimental
area ratio using a 5 mil stencil is 0.60. It board surface during the print cycle. Small
solder paste. It is important to note that
is not clear whether a significant differ- apertures located within 20 mm of this
a low VMR indicates better print perfor-
ence exists, except that the NSMD circular clamp are at risk of having a print gap.
www.globalsmt.net Global SMT & Packaging – August 2009 – 15
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