A new angle on printing
the effect that varying the contact angle objective measurement of the print quality.
has on aperture fill and release. Paste deposit measurements down to
Previously, the attack angle was the 0.2 mm were made with a GSI Lumonics
only blade angle that could be adjusted eas- 8200 3-D inspection system. This was the
ily in production. The primary way of ad- smallest deposit that could be robustly
justing this angle is to increase or decrease measured by this machine. Additional
the blade pressure, which in turn deflects visual inspection was done below the 0.2
the blade to a different angle. Using this mm pad size to determine the smallest ap-
method, limited adjustments can be made erture and spacing that could be effectively
without adversely affecting the solder paste printed at each of the test levels.
or the ability to wipe the top of the stencil To reduce variation across the test, we
clean. However, decoupling the attack attempted to keep the solder paste roll the
angle from the blade pressure by changing same diameter by adding a small amount
!
the contact angle significantly improves the (
~
5 g) of solder paste every six boards. Six
Figure 3: Test board design (all units in mm).
capability of the process. boards were printed for each condition,
Our study explored the effects of performing a vacuum wipe after the second
modifying the blade angle. We examined board. Data were taken on boards 5 & 6
Run Angle Pressure
two variables in our testing: applied angle of each run. A cycle time of approximately
1 45 60
and attack angle. 40 seconds was maintained throughout the
experiment. The data were replicated in
2 65 50
Test methodology random order to yield the results. We ana-
3 65 60 We undertook a systematic structured lyzed the data from only one print stroke
DoE (design of experiment) to determine direction (front to rear stroke) to eliminate
4 45 50
the effects of blade angle on print transfer another potential source of variance.
5 45 40
efficiency. The two main factors were blade Angle and pressure were varied in a full
6 55 50
contact angle (45°, 55° and 65°) and print factorial experiment with a two replicates.
pressure (40N, 50N and 60N). For this The runs were randomized and coded,
7 55 60
experiment, we used Alpha Metals OM- to minimize the effect of random error.
8 65 40
338 CSP, an IPC type 4 (22 to 38 microns) The experiment was set up as described in
lead-free solder paste with a 4 mil foil and Figure 4.
9 55 40
an Assembléon YGP printer. Blade length From our experience with a large CEM
10 45 50 was 350 mm and separation speed was customer, acceptable yields have been
11 45 60
held constant at 7 mm/sec over a distance achieved for 01005s in production using
of 2 mm. 0.170 mm square apertures with a 0.076
12 65 60
For this test, we used a board and mm stencil. This results in an area ratio
13 65 40
stencil with varying aperture sizes and of 0.56 [AR= w/4T, where w = the width].
spacing. All the test patterns used a 10 x 10 In light of this, we looked closely at the
14 55 50
matrix of square apertures. These varied apertures that were around this value. In
15 55 60
from 0.05 mm by 0.50 mm square, with this test, the 0.25 mm, 0.20 mm and 0.15
0.05 mm spacing between the apertures mm square apertures yielded area ratios of
16 65 50
(Figure 3). These patterns were printed on a 0.61, 0.49 and 0.37 respectively on a 0.101
17 45 40 bare pad with ENIG finish. The test board
mm thick stencil.
18 55 40
was designed and patented by Research in
Motion to yield both bridging and insuffi- results and discussions
Figure 4. DoE setup.
cient solder at the extremes, to allow for an The data were grouped by area ratio,
! !
Figure 5. Main effects plot all data. Figure 6. Interaction plot.
www.globalsmt.net Global SMT & Packaging – February 2009 – 37
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