Establishing a precision stencil printing process for miniaturized electronics assembly
product. The results used for additional print trials to observe
show a performance performance differences among the stencils
difference between and between various paste products.
Paste A and Paste Arbitrary specification limits of
B at aperture transfer efficiency for paste printing dur-
sizes of 10 mils. ing production are 150% for the UCL
The paste handling and 50% LCL. It is commonly difficult
and storage history to standardize these values because the
indicated that paste variation in transfer efficiency is the major
may have been cause for line defects from stencil print-
improperly stored. ing, not necessarily the average amount
!
The odd results at deposited. However, using the results of
10 mil aperture sites data analysis from a print study is useful
Figure 11—Variability Chart of Pad Size and Pad Space for 0.3 mm Pitch. Using
raises concern that to characterize a stencil printing process.
square apertures proportionally sized with the pad dimension, three stencils, there
is no occurrence of insufficient outliers in transfer efficiency on the 3.0 mil thick
the paste may not Figures 12-15 are tabulated data results from
stencil.
perform as expected the initial phase of a case study aimed for
in production. A development of a 0.3 mm pitch precision
decision can be print process. As further print trials are
Any print gap during the print cycle
made to avoid using the paste for product introduced into the case study, process at-
tends to alter the transfer efficiency perfor-
designs that include the small 10 mil aper- tributes will be evaluated for improvement
mance, in part, because the stencil is mov-
ture features. opportunities of the process.
ing while paste is rolling into its aperture.
Figures 9 and 10 are images of two 0.3 It can be noted that results for the
At a print speed of 100 mm/s it has be
mm pitch pad sites on a customer test 230 micron square aperture pattern (S9_
shown to adversely affect the transfer effi-
vehicle that offers many different pad sizes NSMD) show an average transfer efficiency
ciency. Figure 7 shows two different stencil
and pad spaces. Print product test trials of 65%. The DPMO for the customer’s
and two different test vehicles with similar
of the customer paste product include specification limits ranges from 768-2376
circular 10 mil NSMD pads. The presence
three different stencils. The apertures at for the four different print speeds in these
of insufficient outliers in the data is signifi-
these sites are square shaped. The square print trials. This aperture size could show
cant when using the regular clamps.
aperture size for the 0.20 mm pad size is great promise for the customer’s new prod-
The variance-to-mean ratio line chart
0.20 mm (< 8 mils). The square aperture uct designs. In the existing process, the
shown in Figure 8 is from a print trial that
size for the 0.15 mm pad size is 0.15 mm (< data analysis shows that process capabil-
includes observation of 25 different aper-
6 mils). Each stencil has a different stencil ity (Cpk) indices of this aperture pattern
ture pattern and pad design combinations.
thickness, 4.0, 3.5, and 3.0 mils. Figure 11 range from 0.94-1.06 for the four different
Three paste product print trials were con-
shows the preliminary results of ongoing print speeds. However, the process poten-
ducted during May, June and July. Two of
print trials. It is noted that these trials in- tial (Cp) indices range from 1.59-1.70. This
the paste products used (Paste A and Paste
clude the customer’s current paste product. observation raises considerably the expecta-
C) are the same product but from different
New experimental paste products will be tions that the aperture pattern perfor-
manufacturing lots. Paste B is a different
mance has the potential for improvement.
Among the attributes to be evaluated, it is
new stencil manufacturing technology that
could show the most significant improve-
ment.
conclusion
The evaluation of solder paste print
products using the current standard print-
ing process offers a quality assessment of
!
these pastes and suggests improvement
!
techniques for the tooling and process
Figure 12. Average volume for 15 aperture patterns Figure 13. Process potential for 15 aperture patterns
setup. Using statistically based analysis to
at four print speeds. at four print speeds.
characterize the stencil printing process
demonstrates an approach for quantita-
tively benchmarking the current process.
This approach provides a measure with
which to contrast precision stencil printing
processes and materials. This approach will
be required for further miniaturization of
electronic assembly products.
This paper summarizes techniques
for establishing a precision print process
by analyzing the variation in transfer ef-
! ! ficiency. Some attribute examples shown
Figure 14. Process capability for 15 aperture pat- Figure 15. DPMO for 15 aperture patterns at four
included squeegee blade wear, rectangular
terns at four print speeds. print speeds.
aperture orientation, print speed, solder
mask design, board clamping approach,
16 – Global SMT & Packaging – August 2009 www.globalsmt.net
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