Stencil and SPI Evaluations for Miniaturized

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Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
STENCIL AND SOLDER PASTE INSPECTION EVALUATION FOR
MINIATURIZED SMT COMPONENTS
Robert Farrell
Benchmark Technologies
Nashua, NH USA
Chrys Shea
Shea Engineering Services
Burlington, NJ USA
ABSTRACT
New stencil materials and manufacturing technologies
promise improved performance over traditional options.
To determine the best technologies for a contract
electronics manufacturer assembling miniaturized SMT
components, a test was designed to assess the
performance of 11 different stencils, submitted by 3
different suppliers, using a variety of materials and
coatings. Performance metrics include print volume
repeatability and transfer efficiency for 0.5mm and
0.4mm pitch BGAs and for 0201 components with area
ratios in the challenging range from 0.5 to 0.66.
new packages, pad designs, solder paste print
performance and process evaluation tests. The devices
selected for analysis in these tests included 0.3, 0.4 and
.5mm BGAs and 0201s. Their area ratios ranged from
0.5 to 0.75. Locations and names of the specific
devices used in the stencil analysis are shown in figure
2.
The test also evaluated two methods of automated
solder paste inspection to determine the accuracy and
repeatability of the different machines and technologies
at feature sizes of 10 mil or less. Setup, execution and
results of both sets of tests are presented and discussed.
INTRODUCTION
SMT stencil printing technology continually evolves to
keep pace with device miniaturization technologies.
Printed Circuit Board (PCB) assemblers have numerous
new technology options to choose from, and need to
determine the most effective ones to produce the
highest
quality
and
most
reliable
solder
interconnections.
Figure 1. Test Vehicle
The objective of these tests was to identify the best
stencil technology for high volume production of
miniaturized SMT components. The testing grew to
include comparison of Type 4 and Type 5 solder pastes,
evaluation of SPI accuracy and assessment of aperture
wall topographies.
The SPI evaluation portion of this study used three
different SPI machines from 2 different suppliers. 2 of
the machines were from a single supplier but were
different models and ages. The other machine was
from another supplier.
EXPERIMENTAL SETUP
Test Vehicle
The test vehicle shown in figure 1 was designed inhouse for a multitude of PCB assembly tests, including
Figure 2. Features used in stencil analysis
Test Design
The stencil analysis included:
 3 different stencil suppliers
 3 different foil materials
 3 different manufacturing processes
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013

2 different nanocoatings
The experimental design was not a full factorial. Each
supplier provided stencils using technologies that were
either their top performers or developmental
technologies that they wanted to learn more about.
Three to five stencils were submitted by each supplier.
A total of 11 stencils were print tested. All were
created using the same Gerber file, and all were
specified at 0.0040” thick. The final test matrix is
shown in table 1.
Table 1. Stencil test matrix. The three different
vendors are designated A, B and C.
in-line, automatic board washer and inspected before
each test.
In addition to the print tests, an additional, abbreviated
leg was added to the DOE that compared the print
performance of Type 4 and Type 5 solder pastes.
As the experiment progressed, an evaluation of SPI
accuracy was planned and executed and an analysis of
the aperture wall topography were added. They will be
described in a later section.
MEASUREMENTS AND METRICS
The performance of the stencils was measured using
basic print measurement and evaluation techniques.
All the paste deposits were measured using a new SPI
system that promised improved accuracy over existing
technology. It is described in a later section as Machine
#2. The ten-print tests produced 1000 data points for
each of the 100 I/O device types on each stencil, and
5470 data points for the 547 I/O device. The measured
volumes for each device were used to calculate average
volumes and Coefficients of Variation (CV = std
deviation divided by mean, %) in a spreadsheet.
Whereas the average print volumes provide information
on the stencil’s paste release characteristics, the CVs
provide comparative data on volume repeatability.
Aperture sizes were measured with an Acugage model
HPL 25-3-LT4. For each device on each stencil, 5
apertures were measured on both the squeegee and PCB
side of the stencil. The five measurements from each
side were averaged and used in the aperture volume and
area ratio calculations.
Abbreviations for the materials in table 1 are as
follows:
 FG: Datum Fine Grain 301 Stainless Steel
(SS)
 PhD: Datum 304 SS
 FG, Ni Plated: Ni over FG
 Laser Ni: Electroformed Ni that has been laser
cut
 E-form: Electroformed Ni
 Nano2: DEK NanoProtek, 2-part wipe-on
coating
 Nano1: Laserjob thermally cured nanocoating
During this first phase of the experiment, all 11 stencils
were printed over a two-day span at Benchmark’s
Nashua, NH facility on DEK 265 stencil printers with
manually placed pin supports, by the same operator. A
no-clean, Type 4, SAC305 solder paste was used. The
underside of the stencil was wiped between each of ten
consecutive prints. PCBs were serialized and printed in
the same order for each run. They were cleaned in an
After completing the print tests, measurement coupons
were laser cut outside the four corners of the print area.
A micrometer was used to measure the thickness of
each coupon; the average was used in aperture volume
and area ratio calculations.
To account for any trapezoidal stencil wall geometries,
aperture volumes and area ratios were calculated using
formulas for truncated cones and pyramids.
Transfer efficiencies (TEs) are calculated as the average
solder paste deposit volume divided by the aperture
volume that was calculated from the aperture
measurements. They represent the percentage of solder
paste that was released from the aperture. They are
calculated for each of the 5 devices shown in figure 2
for each stencil tested.
Area ratios (ARs) are calculated as the area of the
stencil apertures’ circuit side opening divided by the
area of the apertures’ walls. These were also calculated
for each of the 5 devices on each stencil.
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
Plotting TE against AR provides a means of ranking
stencil release performance. Detailed descriptions and
discussion of the relationship between TE and AR can
be found in reference 1.
RESULTS & DISCUSSION
Print Volumes
The average print volumes, standard deviations and
coefficients of variation for each device type and stencil
are shown in table 2. Stencil B4 was missing the
apertures for the 0.3mm device, so no measurements
were taken.
Table 2. Solder paste deposit volumes (in cubic mils)
and CVs.
Stencil
A1
A1
A1
A1
A1
A2
A2
A2
A2
A2
A3
A3
A3
A3
A3
A4
A4
A4
A4
A4
B1
B1
B1
B1
B1
B2
B2
B2
B2
B2
B3
B3
B3
B3
B3
B4
B4
B4
B4
B4
B5
B5
B5
B5
B5
C1
C1
C1
C1
C1
C2
C2
C2
C2
C2
C3
C3
C3
C3
C3
Component Type
Avg of Vol
0201 A Horizontal
382
100BGA 3Ptich SMD
141
100BGA 4Pitch NSMD
229
100BGA 5Pitch NSMD
421
547BGA 4Pitch SMD
245
0201 A Horizontal
393
100BGA 3Ptich SMD
151
100BGA 4Pitch NSMD
243
100BGA 5Pitch NSMD
438
547BGA 4Pitch SMD
246
0201 A Horizontal
394
100BGA 3Ptich SMD
124
100BGA 4Pitch NSMD
214
100BGA 5Pitch NSMD
413
547BGA 4Pitch SMD
235
0201 A Horizontal
320
100BGA 3Ptich SMD
122
100BGA 4Pitch NSMD
183
100BGA 5Pitch NSMD
329
547BGA 4Pitch SMD
187
0201 A Horizontal
377
100BGA 3Ptich SMD
136
100BGA 4Pitch NSMD
222
100BGA 5Pitch NSMD
422
547BGA 4Pitch SMD
251
0201 A Horizontal
388
100BGA 3Ptich SMD
140
100BGA 4Pitch NSMD
228
100BGA 5Pitch NSMD
424
547BGA 4Pitch SMD
247
0201 A Horizontal
385
100BGA 3Ptich SMD
144
100BGA 4Pitch NSMD
229
100BGA 5Pitch NSMD
425
547BGA 4Pitch SMD
247
0201 A Horizontal
348
100BGA 3Ptich SMD
no data
100BGA 4Pitch NSMD
217
100BGA 5Pitch NSMD
402
547BGA 4Pitch SMD
209
0201 A Horizontal
378
100BGA 3Ptich SMD
101
100BGA 4Pitch NSMD
204
100BGA 5Pitch NSMD
395
547BGA 4Pitch SMD
205
0201 A Horizontal
399
100BGA 3Ptich SMD
159
100BGA 4Pitch NSMD
253
100BGA 5Pitch NSMD
454
547BGA 4Pitch SMD
257
0201 A Horizontal
416
100BGA 3Ptich SMD
164
100BGA 4Pitch NSMD
262
100BGA 5Pitch NSMD
470
547BGA 4Pitch SMD
269
0201 A Horizontal
407
100BGA 3Ptich SMD
161
100BGA 4Pitch NSMD
261
100BGA 5Pitch NSMD
451
547BGA 4Pitch SMD
271
StdDev of Vol
25
25
27
30
23
24
22
24
31
23
24
28
34
28
24
42
16
19
25
18
25
24
29
30
24
25
25
28
30
25
26
24
27
30
24
21
no data
22
27
20
25
23
31
25
29
24
24
24
27
26
21
23
20
26
28
21
23
18
22
21
CV%
6.4%
17.4%
11.9%
7.2%
9.3%
6.1%
14.7%
9.7%
7.1%
9.5%
6.2%
22.4%
16.1%
6.7%
10.2%
13.2%
12.8%
10.5%
7.7%
9.8%
6.6%
17.7%
13.1%
7.1%
9.7%
6.4%
18.2%
12.3%
7.0%
10.3%
6.7%
16.5%
11.8%
7.0%
9.6%
6.1%
10.0%
6.7%
9.8%
6.7%
22.7%
15.0%
6.2%
14.1%
5.9%
15.2%
9.3%
6.0%
10.1%
5.1%
14.3%
7.6%
5.5%
10.5%
5.3%
14.3%
6.9%
4.9%
7.7%
A typical guideline for print volume repeatability is a
CV of 10% or less, which was achieved in most cases,
except for the 0.3mm BGA. The 7mil aperture is too
small for a 4mil foil and a Type 4 paste (AR=0.43), but
the feature was analyzed because it pushed the limits of
both the printing and measurement processes.
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
More realistic – and production driven – evaluations are
on the 0.4 and 0.5mm BGAs, and on the 0201s. Their
area ratios were approximately:




0.4mm BGA (SMD pads): 0.53
0.5mm BGA (SMD pads): 0.60
0.5mm BGA (NSMD pads): 0.65
0201 (NSMD pads): 0.75
Actual area ratios varied from the specification due to
variation in aperture sizes and foil thicknesses.
Aperture Size Measurements
Table 3 shows the average aperture measurements and
other statistics for each device.
Table 3.
Aperture measurements (in mils) for
diameters of circular apertures and sides of square
apertures
0.3mm BGA 0.4mm BGA 0.4mm BGA 0.5mm BGA
SMD
NSMD
SMD
NSMD
SPEC
Average
Range
Min
Max
0201
NSMD
7.28
8.86
9.84
10.83
11.6
7.05
1.56
6.08
7.64
8.63
1.80
7.62
9.42
9.59
1.54
10.06
8.52
10.62
1.60
9.58
11.18
11.39
1.78
10.56
12.34
Foil Thickness Measurements
Table 4 shows the measured thicknesses of each stencil.
No thickness data was available for stencil A2, so the
average of the other PhD, No Nano stencils was
substituted in the calculations.
ratio of 0.61. If that aperture was only 8.5 mils, its area
ratio would drop to 0.53, releasing less solder paste, and
it’s volume would also drop, resulting in a much
smaller (and more inconsistent) deposit than expected.
Similarly, if the stencil is only 3 mil thick instead of 4
mil thick, the area ratio increases from 0.61 to 0.81 to
release more paste than expected, but the aperture
volume drops to 25% less than expected due to the
thinner foil.
The stencil manufacturing process has a significant
influence on the solder paste printing process; therefore,
control of the stencil manufacturing process is
absolutely critical to performance on the SMT line.
Transfer Efficiencies
The measured solder volumes were coupled with the
measured aperture sizes to estimate actual TEs, which
were plotted against actual ARs to generate print
performance curves.
Both electroformed stencils performed poorly and had
significant size and thickness inaccuracies; they were
omitted from the final plot. The 0201s (AR=0.75)
experienced some fill issues; they were also omitted
from the final plot. Figure 3 shows the performance of
the best stencils in the key 0.5 to 0.65 AR range.
Table 4. Foil thickness measurements (in mils)
Stencil Thickness - Measured on cut pieces just outside print area
STENCIL
A1
A2
A3
A4
B1
B2
B3
B4
B5
C1
C2
C3
MATERIAL
A
B
C
D
Average
FG
4.00
4.00
4.00
4.00
4.00
PhD
no data no data no data no data
4.08
FG, Ni Plated
4.35
4.40
4.25
4.35
4.34
E-form
2.70
2.75
2.50
2.40
2.59
FG
4.15
4.15
4.15
4.15
4.15
PhD, no nano
4.10
4.15
4.15
4.15
4.14
PhD, Nano
4.15
4.25
4.15
4.10
4.16
E-form
3.60
3.45
3.45
3.70
3.55
Laser Nickel
4.00
4.15
4.00
3.95
4.03
PhD, no nano
4.00
4.10
4.00
4.00
4.03
PhD, nano
4.15
4.05
4.16
4.15
4.13
FG, nano
4.15
4.15
4.20
4.20
4.18
The range of measurements and their deviation from the
specification are considerable.
While the typical
aperture size variation within a stencil was less than
2%, the stencil-to-stencil (manufacturing process-tomanufacturing process) differences were as large as
22%.
Even small deviations from specification can cause big
issues in stencil printing. For example, an aperture
specified at 9.8 mils on a 4 mil foil would have an area
Figure 3. Transfer efficiencies of different stencils
The lines in figure 3 represent 3 different variables:
 The marker styles represent the different
suppliers. Suppliers A, B and C are denoted
by squares, circles and triangles, respectively.
 The marker and line colors represent the
different foil materials. Purple lines represent
the 304 SS and orange lines represent the 301
SS.
 The line styles represent the coatings. Solid
lines indicate no nanocoating treatment,
dashed lines indicate the thermally cured
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
nanocoating; dotted lines indicate the 2-part
wipe-on nanocoating.
All of the stencils plotted in figure 3 performed well.
The results are relatively tightly grouped, with little
differentiation was noted in overall TE performance,
except for one particular stencil that stood out. The
stencil that showed the best transfer efficiencies was
C2, 304 SS with thermally cured nanocoating.
Potential problems that could cause higher print
volumes, such as positional inaccuracies or slag on the
bottom surface of the stencils were investigated, and the
stencil showed no signs of either. In fact, it had the
tightest positional accuracy and the smoothest walls
with no slag protrusions. The CVs of all prints larger
than 0.3mm were all near or less than 10%, also. It was
clearly the best performing stencil in these tests.
Type 4 vs Type 5 Solder Powders
One stencil from each supplier was selected to run Type
5 solder paste. Type 5 solder paste has smaller solder
particles in it, ranging from 5 to 15 microns diameter as
opposed to Type 4’s range of 20 to 38 microns
diameters. The smaller paste particle size should
enable denser particle packing in the aperture, better
release from the aperture, and better volume
repeatability. The prints were generated and measured
at the end of the original 11-stencil run on Day 2 of
testing, using the same techniques as the main
experiment. The transfer efficiency results are shown
in figure 4.
the 0.60 and 0.65 ARs, because the stencil
demonstrated superior release on the Type 4 paste.
Stencil C2 was also the best performer in the previous
test. Stencil A1demostrated a flat response curve with
Type 5 paste; it is assumed that experimental error,
either in the printer setup or the measurement setup,
caused the anomalous readings.
Volume repeatability is as important, and arguable
more important than transfer efficiency. Table 5
compares the average volumes and CVs of the Type 4
and Type 5 pastes for stencils B3 and C2.
Table 5. Comparison of print volumes (in cubic mils)
and repeatability for Type 4 and Type 5 solder pastes
Stencil
Component Type
B3
B3
B3
B3
B3
C2
C2
C2
C2
C2
0201 A Horizontal
100BGA 3Ptich SMD
100BGA 4Pitch NSMD
100BGA 5Pitch NSMD
547BGA 4Pitch SMD
0201 A Horizontal
100BGA 3Ptich SMD
100BGA 4Pitch NSMD
100BGA 5Pitch NSMD
547BGA 4Pitch SMD
Avg of Vol
CV%
Type 4 Type 5 Type 4 Type 5
385
406
6.7%
5.6%
144
163
16.5%
11.6%
229
252
11.8%
7.9%
425
445
7.0%
6.9%
247
258
9.6%
8.7%
416
429
5.1%
4.6%
164
168
14.3%
14.7%
262
276
7.6%
5.9%
470
476
5.5%
5.8%
269
270
10.5%
10.7%
The volumes are generally slightly higher and the CVs
slightly lower for the Type 5 solder paste, as expected.
The only exception is the performance of stencil C2,
which was equivalent (but superior to all others) for
both the Type 4 and Type 5 pastes at the higher area
ratios.
MEASUREMENT SYSTEM EVALUATION
The measured volumes and transfer efficiencies appear
higher than expected on most of the small devices. One
contributor to the difference may be the use of Type 4
solder paste, because most of the comparative data is
based on Type 3 pastes, and as previously discussed,
the smaller particle sizes enable better print quality.
Another contributor is the Solder Paste Inspection (SPI)
system itself. Most of the comparative data available is
based on phase shift, or Moire, interferometry SPI
technology. These measurements were taken with a
light and laser-based technology that promises
improved accuracy on smaller deposits by capturing
more of the actual solder paste deposit in its scan. A
photograph of the scanning system is shown in figure 5.
Figure 4. Transfer efficiency comparison of Type 4
and Type 5 solder pastes
The Type 5 powder showed better release on stencil B3
across the entire range of area ratios. The Type 5
powder showed a slight release advantage on stencil C2
in the 0.43 and 0.54 ARs, but offered no advantage on
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
Figure 7. Average area readings in initial accuracy test
Figure 5. Simultaneous light-based 2-D and laserbased 3-D solder paste inspection.
To compare the volume measurement accuracy of the
new technology against the Moire technology, a PCB
was printed and baked for 1 hour at 125°C to stabilize
the paste deposits by evaporating the liquid portion of
the paste to leave only the metal. The board was
measured 10 times with each machine. A variety of
deposit sizes and pad configurations were tested.
Figures 6 through 9 show the results for Machine #1,
the existing measurement technology, and Machine #2,
the new measurement technology.
Figure 6. Average volume readings in initial accuracy
test
Figure 8. Variation in volume readings in initial
accuracy test
Figure 9. Variation in area readings in initial accuracy
test
Machine #2 consistently measured higher volumes and
higher areas than Machine #1, with comparable
variation down to the 8.8 mill aperture size. At the 7
mil aperture size it showed far less variation in its
readings. Smaller apertures will naturally produce
more variation in deposit volumes due to their tighter
area ratios; however the divergence in CVs between the
two machines indicates considerable differences in the
measurement techniques.
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
SPI ACCURACY ASSESSMENT
The two technologies offered widely ranging
differences in measurement values. Which one was
correct?
Conversations with SPI manufactures
indicated that accuracy is typically assessed by
measuring various sized metal cylinders of known
volumes. However, the disadvantage to this test is that
the cylinders are symmetrical, shiny, and smooth,
versus solder paste deposits which are irregularly
shaped and comprised of metal spheres that are
suspended in a liquid. Solder paste deposits would
reflect light differently than metal cylinders, which
could affect the results.
The volume assessments
referenced in Figures 6 through 9 provided insight but
used dried (liquid evaporated) paste deposits which are
not representative of production conditions.
The
objective of this work was to devise a test using actual
paste deposits to asses which SPI machine was more
accurate.
Paste Weighing Experiment
This test printed the solder paste on labels, used the SPI
machines to inspect the prints, then removed the labels
and weighed them. The paste then was removed from
the labels, and they were weighed again to calculate the
mass of the paste that had been printed them. The
labels were weighed on an analytical balance with a 5digit gram scale at Custom Analytical Services in
Salem, NH. Stencil B2 was chosen for this test because
it uses common, low cost materials and manufacturing
processes, and it performed well in the previous round
of print tests.
The density of the solder paste was determined by
Benchmark’s Nashua, NH analytical laboratory by
weighing a precise volume of the paste. The density
was not available from the paste manufacturer partially
because the density can change based on allowable
variations in the metal load. The calculation was made
on the two separate lots of paste used in Run 1 and 2.
The readings were .000000071 and .000000072 grams
per cubic mil indicating the paste density between lots
was consistent and the calculations were accurate. The
average volume of each deposit was then calculated
using the mass, density and I/O count of each device.
Figure 10 shows the PCB with the labels affixed; figure
11 shows a close up of the printed labels. Again, a
variety of device sizes were tested. 8 samples were
taken for each device.
Figure 10. PCB with labels ready for print test
Figure 11. Printed labels prior to weighing.
The results of the weight tests showed consistency in
the method of determining actual print volumes.
Figures 12 through 14 depict the results from the
weight tests in the 0.5 to 0.65 AR range.
Figure 12. Weighing test results for 0.5mm BGA 100s
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013


“Better” is considered between 15% and 30%
of weighed and calculated volumes
“Bad” was higher than 30% deviation from
weighed and calculated volumes
Overall, the most consistent, fool-proof measurement
system was the weight test method. It is impractical as
a regular production control method, but can serve as an
excellent test for equipment calibrations, periodic
process control checks, or troubleshooting.
Figure 13. Weighing test results for 0.4mm BGA 620s
Figure 14. Weighing test results for 0.4mm BGA 100s
Machine Comparison to Weighing Results
In the first round of comparisons, a new, state-of-the-art
Moire technology machine (Machine #3) was installed
at the evaluation site, and Machine #1 was no longer
used in the tests. Machine #3 originally returned
readings significantly below the weight test. Machine
#2 was no longer at the evaluation site, so the readings
taken from a prior set of measurements were substituted
and were closer to the weighed results for every device.
In the second round of comparisons, Machine #3
underwent a hardware upgrade on-site and was
reprogrammed once. It then produced somewhat better
results in a second round of tests, with its best
performance after a 3rd round of programming.
Machine #2 also underwent a hardware upgrade while it
was off-site, and initially gave poorer results upon
reinstallation (run #2). It was reprogrammed and then
returned good accuracy results on run #3. Table 6
summarizes the overall results.
Table 6. Summary of paste weighing experiments

“Good” is considered within 15% of weighed
and calculated volumes
Insights gained from the accuracy tests include:
• Machine-to-machine variation influences volume
readings
• Programmer-to-programmer variation influences
volume readings
• Use extreme caution when comparing
datasets generated on different platforms,
different machines or by different
programmers
• SPI manufacturers and users tend focus more on
repeatability than accuracy, because repeatability is
easier to measure and considered more relevant in
a production environment
• None of the machines seemed to capture as much
volume as weighing; they all came in 10-65%
below the weighed amounts, depending on how
they were programmed.
• The delta between the machine reading results and
the weighing results increased as feature size
decreased
APERTURE SURFACE ANALYSIS
A final analysis of the stencils attempted to correlate
aperture wall surface topographies with transfer
efficiencies.
Experimental Method
Stencil sections were laser cut from each stencil used in
the study. They sections exposed the long side of an
1812 aperture, as shown in figure 15.
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
Figure 15. Diagram of aperture wall cut for surface
analysis.
The samples were mounted vertically (figure 16) and
scanned with a Cyber Technologies CT-300 at the
Aculogic test facility in Peabody, MA. The scanner
uses a confocal white light sensor with a measurement
range of 0.6mm and a resolution of 0.02 microns.
Figure 18. Photograph and scan of stencil wall with
peak slag height of 3 mils on a 4 mil foil. Slag
protrusions are on the PCB side of the foil.
Not all stencils demonstrated excessive slag, which is
likely related to laser cutting parameters. Table 7 lists
the findings for all the stencils in the study.
Table 7. Scan results for stencils
Figure 16. Vertically mounted stencil aperture sample
and direction of scan.
For each of the three exposed aperture walls, a
1x0.3mm area was scanned. From that data, the area
selected for analysis was 1x0.075mm to exclude bottom
side slag from the wall roughness calculation, as shown
in figure 17.
Figure 17. Scanned area, analysis area and bottom side
slag
Results
Bottom side slag was found on several stencils, and in
some cases was severe, as seen in figure 18.


Ra, or surface roughness, is measured in µmeters
Thickness and slag measurements are also
expressed in µmeters. Note that overall thickness
measurements do not include slag, which is
reported separately where applicable.
The laser cut stencils from supplier A showed the
roughest walls and greatest amount of slag. The laser
cut stencils from supplier B showed some slag, and the
laser cut stencils from supplier C showed the cleanest
walls and no slag. Note that stencil C2 was the top
performer in the print tests. Electroformed stencils do
Originally published in the Proceedings of SMTA International, Ft. Worth, TX, October, 2013
not form slag in their plating process, but did not
perform well in print tests.
test, but bottom side protrusions were noted to cause
inflated TEs on a related test.
Gage Repeatability and Reproducibility (GR&R)
An industry accepted practice to assess the repeatability
of SPI platforms is a Gage R & R study which entails a
single board with paste deposits placed in an oven at
105 to 125°C for approximately 1 hour to drive off the
liquids and solidify the paste deposits. The board is
then run through the SPI platform multiple times
including rotations of 90, and 180 degrees. The
summary of these results for Machine #2 and #3
appears below:
ACKNOWLEDGEMENTS
The authors would like to thank the numerous
participants that made these analyses possible:
 Joe Crudele, Paul Bodmer, Bruce Tostevin and Rey
Molina of Benchmark Electronics for technical
support
 Jeremy Saise for technical support and assistance
in execution of the tests
 Ray Whittier of Vicor for technical support and
assistance in execution of the tests
 Chris Tibbetts of Analogic for technical support
and Cyberscans of the aperture walls
 Bruce Guttman of Custom Analytical Services for
technical support
 Stencil providers for stencils and technical support
 SPI equipment companies for machines and
support
 Matt Holzmann of CGI Americas for funding
In running a GR&R as defined by the Automotive
Industry Action Group (AIAG) and Measurement
Systems Analysis (MSA) guidelines, findings indicated
that both Machine #2 and Machine #3 performed
acceptably over all angles, neglecting process
tolerances. When applying 50% tolerance margins to
the analysis, it was discovered that most conclusions of
repeatability were significantly different, favoring
Machine #3.
SUMMARY
In print performance tests based on transfer efficiencies
and volume repeatabilities, stencil C2, the 304 SS with
thermally cured nanocoating, was the top performer.
Characteristics that could impact paste release, such as
wall roughness, bottom side slag protrusions, and
positional accuracy were all checked; the stencils
showed the cleanest cuts and the best positional
accuracy. They are; however, more costly and have
longer lead times than typical laser-cut SS stencils, but
may provide the option for high-performance,
miniaturized print requirements.
Considerable differences were noted in the aperture
sizes and foil thicknesses, affecting area ratios and print
volumes. Electroformed stencils had the most widely
varying thicknesses and gave the poorest print
performance.
SPI systems demonstrated significant differences in
accuracy, both from platform to platform, and from
programmer to programmer. As deposit sizes get
smaller, accuracy will become increasingly important.
The SPI machines always returned values lower than
the weight test results.
No direct correlations were drawn between wall
roughness and release, partially due to the influence of
bottom side slag. The protrusions will likely create
gasketing problems that cause print quality issues in
production environments. None were noted during this
REFERENCES
[1] Shea, C. and Whittier, R., “Evaluation of Stencil
Foil Materials, Suppliers and Coatings’” Proceedings of
SMTA International, 2011
Stencil and Solder Paste Inspection
for Miniaturized SMT Components
Chrys Shea
Shea Engineering Services
Burlington, NJ
Robert Farrell
Benchmark Electronics
Nashua NH
SMTA International – Fort Worth, Texas
October 13-17, 2013
Overview & Study Development
• Original objective was to determine the best stencil
technology for printing small deposits
– Better solder paste transfer efficiencies and print volume
repeatabilities indicate a better performing stencil
• However, two Solder Paste Inspection (SPI) machines gave
very different results for paste volume
– As a result, it was decided to include an accuracy assessment of the
SPI platforms used in this study
• Platform to assess stencil wall topography was made
available and a decision was made to include this in the
experiment to determine if there was a correlation between
wall topography and print performance
Stencil Experiment
• Purpose: Choose the best stencil technology for
production
•
•
•
•
0.4 & 0.5mm pitch µBGA
0201s
Type 4 no-clean solder paste
Side leg on Type 4 vs Type 5
• Tests used:
•
•
•
•
•
Benchmark Electronics test vehicle
0.4mm µBGA
3 stencil suppliers – 12 stencils total
Same solder paste
Same 10 PCBs
10 consecutive prints off of same printer and tooling, dry wipe after each
print
• All printed same day
• SPI Machine # 2 to measure print volumes
Objective: Identify the best stencil technology for volume
production of miniaturized populated SMT components
Materials and Mfg Processes
• Materials - 3
•
•
•
Stainless steel optimized for laser cutting
Stainless steel with smaller grain size
Electroformed nickel
• Manufacturing Processes - 3
•
•
•
Laser cutting
Electroforming
Nickel plating over Stainless Steel
• Nano-coatings - 2
•
•
Can be applied only by stencil manufacturer
Can be applied by manufacturer or user
Experimental Matrix
• Three different suppliers
– A, B, C
• All used same Gerber file
• All specified 4mil thick foils
• Abbreviations for materials:
–
–
–
–
FG: Datum Fine Grain 301 SS
PhD: Datum 304 SS
FG, Ni Plated: Ni over FG
Laser Ni: Electroformed Ni that
has been laser cut
– E-form: Electroformed Ni
– Nano2: Dek NanoProtek
– Nano1: Laserjob Nanocoating
Stencil
Material(s)
A1
FG
A2
PhD
A3
FG, Ni Plated
A4
E-form
B1
FG
B2
PhD, No Nano
B3
PhD, Nano2
B4
E-form
B5
Laser Ni
C1
PhD, No Nano
C2
PhD, Nano1
C3
FG, Nano1
Test Vehicle
• Designed in-house by Joe Crudele
• Contains many tests
• SPI measured most features
• Analyzed for 0.3, 0.4,
0.5mm BGAs and 0201s
• Gave ARs from 0.50 to 0.75
547BGA 4PitchSMD
0201 A
Horizontal
100BGA
3Pitch SMD
100BGA
4Pitch NSMD
5.2”
100BGA
5Pitch NSMD
7.2”
Basic Metrics in Stencil Printing
• Aperture Area Ratio (AR)
• Paste Transfer Efficiency (TE)
• Simple statistics
– Mean
– Standard deviation
– CV% (std deviation as % of mean)
Transfer Efficiency & Area Ratio
Transfer Efficiency, TE
% TE =
Volume of paste deposited
Volume of stencil aperture
x 100
Area Ratio, AR
AR =
Area of circuit side opening
Area of aperture walls
ARs and TEs can be theoretical or actual:
 Theoretical are based on specified dimensions
- Sufficient for paste or print parameter tests that use the same stencil
 Actual are based on measured dimensions
- Needed when different stencils are used
- Shortcut AR formula: AR = D/4t
where D= circle’s dia or square’s side, t = foil thickness
This experiment used Actual Area Ratios and Transfer Efficiencies
Transfer Efficiency & Area Ratio
Paste
PCB Pad
Stencil
PWB
After the aperture is filled, the solder paste sets up and sticks to both the stencil walls and the
pads.
At separation, the forces holding the deposit to the pad must overcome the forces holding the
deposit to the stencil walls
Depending on area ratio, a portion of the paste will release to the PWB, while some will stay in
the aperture
- The smaller the AR, the lower the TE
- Higher TE’s indicate better paste release
Results
Foil Thickness (mils)
Stencil Thickness - Measured on cut pieces just outside print area
STENCIL
A1
A2
A3
A4
B1
B2
B3
B4
B5
C1
C2
C3
A
B
C
D
Average
FG
4.00
4.00
4.00
4.00
4.00
PhD
no data no data no data no data
4.08
FG, Ni Plated
4.35
4.40
4.25
4.35
4.34
E-form
2.70
2.75
2.50
2.40
2.59
FG
4.15
4.15
4.15
4.15
4.15
PhD, no nano
4.10
4.15
4.15
4.15
4.14
PhD, Nano
4.15
4.25
4.15
4.10
4.16
E-form
3.60
3.45
3.45
3.70
3.55
Laser Nickel
4.00
4.15
4.00
3.95
4.03
PhD, no nano
4.00
4.10
4.00
4.00
4.03
PhD, nano
4.15
4.05
4.16
4.15
4.13
FG, nano
4.15
4.15
4.20
4.20
4.18
MATERIAL
• Measurement coupons cut from just outside print area
• Four measurements per coupon were averaged
• Average thicknesses used in AR and TE calculations
• No data for A2; used average of other PhD, no nano thicknesses
Aperture Sizes (mils)
0.3mm BGA 0.4mm BGA 0.4mm BGA 0.5mm BGA
SMD
NSMD
SMD
NSMD
SPEC
Average
Range
Min
Max
•
0201
NSMD
7.28
8.86
9.84
10.83
11.6
7.05
1.56
6.08
7.64
8.63
1.80
7.62
9.42
9.59
1.54
10.06
8.52
10.62
1.60
9.58
11.18
11.39
1.78
10.56
12.34
5 of each aperture size were measured on each stencil
– Measured with an Acugage model HPL 25-3-LT4
– Aperture size variation within each stencil <2%
– Stencil-to-stencil variation as much as 22%!!!!
• Average measurements used in AR and TE calculations
Paste Volume Measurement
• Taken with Machine # 2 SPI system
• 10 consecutive prints
–Identical Print Set Up for Each Run
– 1000 data points for each 100 I/O BGA
& 0201s
– 5470 data points for the 547 I/O BGA
•Exported to Excel
– Pivot tables to extract average volumes
and standard deviations
– CV calculated
• Avg volumes (cu mils) used in TE
calculations
Stencil
A1
A1
A1
A1
A1
A2
A2
A2
A2
A2
A3
A3
A3
A3
A3
A4
A4
A4
A4
A4
B1
B1
B1
B1
B1
B2
B2
B2
B2
B2
B3
B3
Component Type
Avg of Vol
0201 A Horizontal
382
100BGA 3Ptich SMD
141
100BGA 4Pitch NSMD
229
100BGA 5Pitch NSMD
421
547BGA 4Pitch SMD
245
0201 A Horizontal
393
100BGA 3Ptich SMD
151
100BGA 4Pitch NSMD
243
100BGA 5Pitch NSMD
438
547BGA 4Pitch SMD
246
0201 A Horizontal
394
100BGA 3Ptich SMD
124
100BGA 4Pitch NSMD
214
100BGA 5Pitch NSMD
413
547BGA 4Pitch SMD
235
0201 A Horizontal
320
100BGA 3Ptich SMD
122
100BGA 4Pitch NSMD
183
100BGA 5Pitch NSMD
329
547BGA 4Pitch SMD
187
0201 A Horizontal
377
100BGA 3Ptich SMD
136
100BGA 4Pitch NSMD
222
100BGA 5Pitch NSMD
422
547BGA 4Pitch SMD
251
0201 A Horizontal
388
100BGA 3Ptich SMD
140
100BGA 4Pitch NSMD
228
100BGA 5Pitch NSMD
424
547BGA 4Pitch SMD
247
0201 A Horizontal
385
100BGA 3Ptich SMD
144
StdDev of Vol
25
25
27
30
23
24
22
24
31
23
24
28
34
28
24
42
16
19
25
18
25
24
29
30
24
25
25
28
30
25
26
24
CV%
6.4%
17.4%
11.9%
7.2%
9.3%
6.1%
14.7%
9.7%
7.1%
9.5%
6.2%
22.4%
16.1%
6.7%
10.2%
13.2%
12.8%
10.5%
7.7%
9.8%
6.6%
17.7%
13.1%
7.1%
9.7%
6.4%
18.2%
12.3%
7.0%
10.3%
6.7%
16.5%
Transfer Efficiencies & CVs
A1
547BGA 4Pitch SMD
100BGA 3Ptich SMD
0201 A Horizontal NSMD
100BGA 4Pitch NSMD
100BGA 5Pitch NSMD
AR
TE
AR
TE
AR
TE
AR
TE
AR
TE
0.61
84%
0.44
70%
0.76
73%
0.53
77%
0.65
94%
A1
547BGA 4Pitch SMD
100BGA 3Ptich SMD
0201 A Horizontal NSMD
100BGA 4Pitch NSMD
100BGA 5Pitch NSMD
AR
CV
AR
CV
AR
CV
AR
V
AR
CV
0.61
9.3%
0.44
17.4%
0.76
6.4%
0.53
11.9%
0.65
7.2%
A2
0.58
82%
0.43
75%
0.74
73%
0.53
80%
0.64
94%
A2
0.58
9.5%
0.43
14.7%
0.74
6.1%
0.53
9.7%
0.64
7.1%
A3
A4
0.56
74%
0.42
57%
0.76
64%
0.50
67%
0.64
82%
A3
0.56
10.2%
0.42
22.4%
0.76
6.2%
0.50
16.1%
0.64
6.7%
B1
0.58
82%
0.43
65%
0.73
70%
0.52
72%
0.63
91%
A4
B1
0.58
9.7%
0.43
17.7%
0.73
6.6%
0.52
13.1%
0.63
7.1%
B2
0.58
82%
0.43
68%
0.73
72%
0.52
74%
0.65
91%
B2
0.58
10.3%
0.43
18.2%
0.73
6.4%
0.52
12.3%
0.65
7.0%
B3
B4
0.59
0.69
80%
85%
0.44 no data
67% no data
0.74
0.87
71%
74%
0.53
0.62
72%
83%
0.65
0.77
89% 101%
B3
B4
0.59
0.69
9.6%
9.8%
0.44 no data
16.5% no data
0.74
0.87
6.7%
6.1%
0.53
0.62
11.8% 10.0%
0.65
0.77
7.0%
6.7%
B5
0.56
71%
0.40
53%
0.73
73%
0.48
72%
0.60
91%
B5
0.56
14.1%
0.40
22.7%
0.73
6.7%
0.48
15.0%
0.60
6.2%
C1
0.61
83%
0.45
72%
0.77
71%
0.54
77%
0.67
93%
C1
0.61
10.1%
0.45
15.2%
0.77
5.9%
0.54
9.3%
0.67
6.0%
C2
0.60
87%
0.43
77%
0.74
76%
0.52
83%
0.64
100%
C2
0.60
10.5%
0.43
14.3%
0.74
5.1%
0.52
7.6%
0.64
5.5%
C3
0.61
84%
0.45
71%
0.74
71%
0.54
78%
0.65
91%
C3
0.61
7.7%
0.45
14.3%
0.74
5.3%
0.54
6.9%
0.65
4.9%
MIN
0.56
MAX
71%
87%
0.40
0.45
53%
77%
0.73
0.87
64%
76%
0.48
0.62
67%
83%
0.60
0.77
82%
101%
MIN
SPEC
0.69
MAX
0.56
0.69
8%
14%
0.40
0.45
14%
23%
0.73
0.87
5%
7%
0.48
0.62
7%
16%
0.60
0.77
5%
7%
0.62
0.46
0.73
0.55
0.68
SPEC
0.62
0.46
0.73
0.55
0.68
• When comparing different stencils, TE for each stencil is the only way to normalize the data
– Comparing volumes only does not account for differences in aperture size or foil thickness
– Comparing TEs based on specified aperture sizes and foil thicknesses instead of actual does not
account for variation among stencils, either
• A4 eliminated due to varying thicknesses, B4 did not have apertures for 0.3pitch
Transfer Efficiencies (TE)
Higher TE indicates better paste release
0.5mm NSMD
(11mil)
0.4mm SMD
(10mil)
0.4mm NSMD
(9mil)
Type 4 vs Type 5 Paste
C2 shows advantages in:
• Better overall release
• No need for T5 paste at
ARs 0.6 & higher
Type 5 solder paste showed a TE advantage
•
•
•
Stencil C2: ARs below 0.6.
Stencil B3: all ARs
Stencil A1: data indicates systematic error in printing or measuring
TE Values – Seem Very High
• 75% at AR~0.52? (9mil aperture/4mil foil)
• 80% at AR~0.60? (10mil aperture/4mil foil)
• 90% at AR~0.65? (11mil aperture/4mil foil)
Measurements were not taken with traditional
structured white light SPI system that we are
accustomed to.
They were taken with a new type of color light &
laser combination measuring system (Machine # 2)
Measurement Accuracy
• Different SPI machines measured different area and volume values on
same PCBs
• Which instrument is more accurate?
Accuracy Tests
•
•
•
SPI manufacturers typically use metal cylinders of known volume for accuracy assessment
– Cylinders are a smooth, shiny, and symmetrical metal – not representative of solder
paste deposits, which have irregular shapes and are comprised of metal spheres
suspended in a liquid
– A low cost test was devised to assess accuracy using actual paste volumes
“Referee” between the machines
Weigh deposits, determine paste density and calculate volumes
Calculating Paste Volumes
•
•
•
•
Cut labels to cover component footprints on TVs
Printed PCBs with stencil B2
Measured with SPI machine
Weighed labels, scraped paste off, weighed again to
determine paste weight
• Converted to volume using density measured in lab
Weighing Test Results
Results were extremely consistent, indicating that printing was
consistent and weighing is a robust measurement method
Machine Accuracy Results
• First run:
– Machine #1 taken out of experiment and replaced with newer model
Machine #3. Reads much lower volumes than weighing test returned
– Machine #2 not available for test, reference measurements from
GR&R substituted and were much closer to weighing results
• Second run:
– Both machines get hardware upgrades
– Machine #3 reads somewhat closer to weighing results
– Machine #2 reads lower volumes than machine #3 or weighing results.
First time for machine #2 reading shiny labels, so it will get
reprogrammed and tested again.
• Third run:
– Machine #3 reprogrammed for shiny white background and provides
good readings close to weighing results
– Machine #2 also reprogrammed and also provides good reading close
to weighing results
Both machines reported volumes 10-50% less than the
method of weighing deposits
Machine Accuracy Results
Test Method
Run #1
Run #2
Run #3
SPI Machine #2
Good
Bad
Good
SPI Machine #3
Bad
Better
Good
Good
Good
Good
Weighing
Good: Within 15% of weighing, Bad = greater than 30%
Insights from results:
• Machine-to-machine variation influences volume readings
• Programmer-to-programmer variation influences volume readings
•
Use caution when comparing datasets generated on different machines or by
different programmers
• Weighing method is unique and most consistent of all; easy if 5 digit scale
is available
• Weighing paste method is more true-to-life than the metal cylinders on
equipment manufacturers’ calibration plates
Insights From Weighting Results (cont’d)
• SPI manufacturers and users focus more on repeatability than accuracy.
– Repeatability is easier to measure and considered more relevant in
production.
• None of the machines seemed to capture as much volume as weighing.
– Came in 10% to 65% below the weighed amounts depending on how they
were programmed.
• Delta between weighing and SPI results increased as the paste deposits
became smaller
• Weighing is not practical for regular production control
– Can serve as an excellent test for equipment calibration, periodic process
control checks, or troubleshooting.
• Limitations on weighing:
– White labels changed color and topography of board
– Glossy labels minimized liquid absorption into the paper.
Gage Repeatability and Reproducibility
GR&R
• An industry accepted practice to assess the repeatability of
SPI platforms is a Gage R & R study
– A single board with paste deposits placed in an oven at 105 to 125°C for
approximately 1 hour to drive off the liquids and solidify the paste deposits
– The board is then run through the SPI platform multiple times including
rotations of 0, 90 and 180 degrees.
• In running a GR&R as defined by the Automotive Industry
Action Group (AIAG) and Measurement Systems Analysis
(MSA) guidelines, findings indicated that:
– Both Machine #1 and Machine #2 performed acceptably over all angles,
neglecting process tolerances.
– When applying 50% tolerance margins to the analysis, it was discovered that
most conclusions of repeatability were significantly different, favoring
Machine #3.
Aperture Surface Analysis
Objective: Investigate correlation between aperture wall
topography and print performance
• Stencil sections were laser cut from stencils used in the
study
• Long edge of 1812 apertures were examined
• Edges were trimmed to expose stencil walls
• Stencil sections were then vertically mounted
Measurement Device
• A Cyber Technologies CT300 was used to scan the
stencil walls.
• The sensors is a confocal
white light sensor with a
measurement range of
0.6mm and a resolution of
0.02 microns.
• Good sensor for
transparent materials, like
Nanocoating
Scan and Microscope Views of Slag
A3 - Laser Cut Stainless Steel Stencil
Slag
Stencil Wall
Surface Roughness Results – Ra
(µm)
5
1
The lower the Ra (um), the smoother the stencil wall
Surface Roughness Discussion
• Laser cut stainless steel from supplier A showed excessive
slag on apertures
– Can cause gasketing issues during printing
– May produce higher paste volumes due to gasketing problems
– Did not increase average deposit height
• Laser cut SS from supplier B showed some walls with
slag, some without
• Laser cut SS from supplier C showed the smoothest SS
walls and no slag. Also, they were the top performers.
• Electroformed stencils showed no slag
• No correlation between aperture topography and stencil
performance observed. May be the result of noise
caused by slag.
Overall Discussion
• Laser cut PhD with Nanocoating 1 (Stencil C2) showed the
best overall performance
–
–
–
–
–
Checked positional accuracy: 0.01mil in X, 0.6mil in Y
Best release with T4 and T5 pastes
Release T4 just as well as T5
No slag
Longer lead times and higher cost
• Big differences in aperture sizes and foil thicknesses
– Electroform had widely varying thicknesses based on aperture density,
too hard to control
• Big differences in SPI system accuracy
– Platform-to-platform and programmer-to-programmer
– Values always less than weighing method
• Electroform had smooth walls, laser cut had rougher walls and
slag, depending on laser cutter
• No direct correlation drawn between wall roughness and
release
Acknowledgements
The investigators would like to thank:
• Joe Crudele, Paul Bodmer, Bruce Tostevin, and Rey
Molina of Benchmark Electronics for technical support
• Bruce Guttman of Custom Analytical Services for
technical support
• Chris Tibbetts of Analogic for Aperture Surface Analysis
• Stencil providers for stencils and technical support
• Jeremy Saise for technical support
• SPI equipment companies for machines and support
• Matt Holzmann of CGI Americas for funding
Thank You!
Questions?
Chrys Shea
[email protected]
Bob Farrell
[email protected]
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