01005 Chip Components In A Lead-Free System - OnBoard ...

SPECIAL Q U A L I T Y A REPORT
S S U R A N C E LEAD-FREE ELECTRONICS
ASSEMBLY
01005 Chip Components
In A Lead-Free System
A study was performed to understand
the assembly constraints of
the newest miniaturised package
chip components, 01005 (or 0402
metric). The study included all
facets of surface mount assembly:
pad design and location, printability
of the small deposits, placement
tolerances on the components,
and reflow capability of the small,
lead-free deposits in an air environment.
This study was a joint effort
between two manufacturers: a solder
paste developer and a pick and
place machine supplier.
The study tested multiple solder
deposit sizes as part of an effort to
characterise the repeatability of the
paste prints and the paste’s reflow
fusion capability in air. It also tested
several component placement
orientations and offsets, and multiple
padstacks. The result shows
strong feasibility of 01005 devices
in modern assembly systems, and
discusses the parameters that produced
the best results in the system
under consideration.
Miniaturisation in electronic products
continually drives the need for
smaller and smaller component
packages. Smaller components
bring solutions to designers seeking
smaller form factors, but they
also bring challenges to the assemblers.
Several years ago, the package size
known as 0201 (Imperial) or 0603
(metric) presented new challenges
to assemblers. Many studies were
performed to show the feasibility.
Now, the package size known as
01005 (Imperial) or 0402 (metric) is
available. The 0402 is approximately
one-fourth the size of an 0603. The
relative sizes of chip components
are shown in Figure 1.
Experimental setup
by Chrys Shea, Dr. Leszek Hozer, Hitoshi Kida, Mutsuharu Tsunoda,
Cookson Electronics Assembly Materials
Figure 1 - Relative sizes of chip
components
Four major tests were included in
the experiment. Characteristics
that were tested for included
• Printability
• Reflow capability
• Placement accuracy
• Self-alignment
A diagram of the test vehicle is
shown in Figure 2. The test vehicle
combined varying rectangular
pad dimensions, spacing between
pads, and spacing between components.
Figure 2 - 01005 test vehicle
A laser cut stainless steel stencil
was used for the print tests. It
was 80µm (3mil) thick. The rectangular
apertures were reduced
by 10% of pad area; the circular
apertures were designed with the
diameter equal to the shorter side
of the rectangular pad.
The printability test used visual
inspection, as the deposit size was
too small for accurate automatic
inspection. The major criterion
was deposit shape. A Yamaha
YVP-Xg printer used the following
parameter settings: print
speed, 50 mm/sec; squeegee type,
both metal and poly; print pressure,
50 N; separation speed, 18
mm/sec; and wipe frequency, 1
(each print). Three different Type
5 pastes were used. Two different
aperture shapes and 3 sizes were
used, for a total of six deposit
types per paste.
Reflow capability was determined
by the smallest deposit to fully
fuse in an air-environment reflow
process. The reflow equipment
was a Tamura TNR25-537PH 7-
zone oven. Two profiles were
used: one that soaked at 170°C
for 2 minutes and one that soaked
at 180°C for 2 minutes. Peak
temperature for both was 230°C.
Profiles can be seen in Figures 3
and 4.
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Figure 3 - 170°C soak profile
A chip placing test was performed
to determine the necessary accuracy
of the placer and explore
the limits of the process window
with respect to defects and selfcentring.
A Yamaha YV88Xg was
used to perform the tests. It has
a mounting accuracy of 0.05mm
(2mil). Placement pressure, location
and rotation were varied.
Placements were programmed
with each combination of following
offsets: 0 and 0.05mm in X
and Y; -0.2mm, 0, and +0.2mm in
Z, and 0 – 25 degrees theta in 5
degree increments. Tolerances of
the test boards, stencil, and components
that were used in this
test are shown in Figure 5.
Results
Three Type 5 Lead-Free pastes
were tested. They were labelled
A, B and C. Paste C provided the
best printability. Its smallest acceptable
print size was 0.16mm
(6.3mil) square or 0.18mm
(7.1mil) round with metal squeegee,
and 0.13mm (5.1mil) square
or 0.14mm (5.5 mil) round with
rubber squeegee. Photographs
of the smallest acceptable prints
with metal squeegee with Paste C
are shown in Figures 6 and 7.
Of the three pastes tested for reflow
characteristics, Paste C also
performed the best, with the finest
features to completely reflow in
air sized 0.16mm (6.3mil) square
or 0.18mm (7.1mil) round. This
coincided with the finest feature
this paste could repeatably print.
Photos of reflowed deposits are
shown in Figures 8 and 9.
Figure 4 - 180°C soak profile
Paste C exhibited the best print
and reflow characteristics, therefore
it was used for the remainder
of the tests.
A nitrogen reflow environment
was not included with this test.
It is hypothesised that smaller
features, on the order of 0.13mm
(5mil) square and 0.14mm
(5.5mil) round would reflow under
nitrogen. Recall that the smallest
acceptable print deposit size was
0.16 and 0.18mm. Therefore there
was no need to explore reflow
properties of a deposit size that
could not be repeatably printed.
Placement tests showed that the
current capability of the equipment
is sufficient for 0402/01005
placement. Even offsets of 0.05mm
(2mil) in both X and Y resulted in
Figure 6 - 0.16mm (6.2mil) square
features
Figure 8 - 0.16mm (6.3 mil) square
deposits after
Placement pressure of -0.2mm Z
was the weakest force tested, and
LEAD-FREE ELECTRONICS
ASSEMBLY
Figure 5 - Tolerance stack up of test assembly
components
Figure 7 - 0.18mm (7.1mil) round
features
Figure 9 - 0.18mm (7.1mil) round
deposits after reflow
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centred components after reflow.
Figure 10 shows the post-reflow
state of the offset components.
Theta offsets as much as 25° also
self-centred in the reflow process;
however, offsets of 10° or less are
preferred. Figures 11 and 12 show
10° rotations before and after reflow.
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SPECIAL Q U A L I T Y A REPORT
S S U R A N C E LEAD-FREE ELECTRONICS
ASSEMBLY
Figure 10 - Post-reflow alignment
of 0402/01005 devices placed with
0.05mm offsets in X and Y
Figure 11 - 0402/01005 components
placed with 10° theta offset
Figure 13 - 0402/01005 component
“floating” on solder due to
insufficient placement pressure.
The solder reflowed, but did not
wet to the component floating on
top of it
Figure 14 - 0402/01005 components
placed with 0.07mm gap between
pads. No solder bridges observed
With respect to design guidelines
for the PWB itself, this experishowed
good placement characteristics
with very little paste
spread. Placement pressures this
light require good tack force. The
paste in this test had a tack force
of 100fg, which appeared to be
sufficient to maintain the components’
positions throughout the
normal conveying process on the
assembly equipment.
This force may be a little light, as
several components “floated” on
top the solder during the reflow
process. An example is shown in
Figure 13.
The next larger force, 0mm Z,
did squeeze some paste out from
Figure 12 - 0402/01005 components
that were rotated 10° theta selfcentred
during the reflow process
under the component, but eliminated
floating issues. The largest
placement force, +0.2mm
Z, squeezed too much paste out
from under the component. This
effectively broke up the solder deposits,
making them smaller than
their reflow capability, and left
some paste unfused.
Observations made during this
test included mispicks, where the
component was left in the tape
pocket, but once a component
was successfully picked, no issues
were noted with vision alignment
or dropping of the devices prior
to placement. No tombstones or
blown off components were observed.
Figure 15 - 0402/01005 components
placed with 0.05mm gaps between
pads. Notice the solder bridges
ment showed a minimum end-toend
spacing of 0.07 mm (2.8mil).
Figures 14 and 15 show the results
of pad spacing at 0.07mm
and 0.05mm.
Conclusion
Technological feasibility of assembling
component package
sizes known as 01005 Imperial
or 0402 metric has been proven
in a lead-free alloy system. Type
5 solder pastes were used with
demanding, extended-soak reflow
profiles.
Typical assembly equipment was
used. With the exception of a
special nozzle size for the component,
no modifications were
made to the printer, chip placer,
or reflow oven. Typical print parameters
were used with a standard
laser cut stencil.
Of three pastes, the best performer
in both print and reflow
characteristics was candidate
C, which is now commercially
known as Alpha OM-325. The
finest features to print repeatably
and reflow completely in air
were 0.16mm (6.3mil) square and
0.18mm (7.1mil) round.
Initial definition of the process
window shows that with the proper
material set, the components
may be placed off-centre by as
much as 0.05mm (2mil) in both
X and Y axes. It may also be misplaced
by up to 25° in theta, but
less than 10° is preferred.
Placement pressure is critical.
Too little pressure may result in
component floating. Too much
pressure may cause solder deposits
to break up and not fully fuse
in reflow. Gross misplacement of
the components may also have
the same effect on reflow characteristics.
Component locations on the circuit
board should be spaced no
closer than 0.07mm (2.8mil) to
eliminate the defect mode of solder
bridging.
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