1. Ultrasonic Anisotropic Conductive Films (ACFs) Bonding
of Flexible Substrates on Organic Rigid Boards at Room Temperature
Kiwon Lee, Hyoung Joon Kim, Il Kim, and Kyung Wook Paik
Nano Packaging and Interconnect Lab. (NPIL)
Department of Materials Science and Engineering
Korea Advanced Institute of Science and Technology (KAIST)
373-1, Kusong-dong, Yusong-gu, Daejeon, 305-701, Republic of Korea
phone: +82-42-869-3386, fax:+82-42-869-3310
email: kiwonlee@kaist.ac.kr
T/C bonding is often limited by high bonding temperature,
Abstract
slow thermal cure, uneven cure degree of adhesive, large
In this study, a novel Anisotropic Conductive Film (ACF)
thermal deformation of the assembly, and high bonding
bonding method for flexible substrate-organic rigid board
pressure for large number of I/O interconnections. Therefore,
bonding was investigated using an ultrasonic vibration. And
there are constant needs of lower bonding temperature and
the ultrasonic (U/S) ACF bonding was characterized in terms
faster cure ACF bonding to replace the conventional T/C
of adhesion strength, electrical continuity in comparison with
bonding at 190℃ and 15 seconds.
those of thermo-compression (T/C) bonding.
An ultrasonic bonder was used to produce ultrasonic Ultrasonic (U/S) bonding for solder and metal-to-metal
vibration in ACF joints between flexible substrates and interconnections has been widely investigated and applied in
organic rigid boards. Test boards were 25 um-thick polyimide flip chip, TAB, or surface mount technologies due to its low
based flexible substrates and 1 mm-thick FR-4 organic rigid cost, simplicity, and fast assembly time at low temperature
boards. The effect of process conditions, such as U/S powers, with reduced bonding pressure. However, most of these
bonding pressures, and bonding times on the ACF approaches are using lateral direction ultrasonic vibration
temperature were investigated. The in-situ temperature at the only. Although there have some attempt to use U/S bonding
ACF layer was measured during U/S bonding to determine as an alternative ACF bonding method, there were no
the relation between the ACF temperature and the bonding previous reports of successive ACF bonding using U/S
characteristics of ACF joints. bonding except one presentation about the demonstration of
U/S ACF flip chip bonding [1].
The optimized U/S bonding time was obtained within 5
sec at room temperature. The significant meaning of this In ACFs U/S bonding, ACFs can be rapidly heated by
result is that the ACF bonding process times can be certain ultrasonic vibration, and it can be described by
remarkably reduced by U/S bonding compared with materials’ complex young’s modulus which consists of
storage modulus and loss modulus under cyclic stress
conventional 15 sec T/C bonding at 190 ℃. The ACF joints
conditions. Storage modulus is related to elastically stored
with the optimized U/S bonding conditions showed similar
energy, and loss modulus is related to energy loss which
bonding characteristics as those with T/C bonding in terms of
converts to heat. In general, it is well known that visco-elastic
the adhesion strength and the daisy-chain contact resistance.
materials such as polymers have large loss modulus.
The Fourier Transformation Infra-Red (FTIR) spectroscopy
Therefore, it is expected that highly visco-elastic B-stage
showed that the ACF degree of cure was achieved 90 % at 3
ACFs may generate a large amount of heat by an ultrasonic
sec and 95 % at 4 sec. Reliability test results showed that the
vibration. By U/S heating in ACFs, rapid ACF curing and
U/S bonded test boards had stable daisy-chain contact
bonding can be obtained without additional chip/substrate
resistances in 85 ℃/85 % RH test and 125 ℃ high heating.
temperature storage test for 1000 hours and -55 ℃~125 ℃ In this study, a novel Anisotropic Conductive Film (ACF)
thermal cycling test for 1000 cycles. bonding method for flexible substrate-organic rigid board
1. Introduction bonding was investigated using an ultrasonic vibration. And
Anisotropic Conductive Films (ACFs) are well known the ultrasonic (U/S) ACF bonding was characterized in terms
adhesive interconnect materials which consists of conducting of adhesion strength, electrical continuity in comparison with
particles and adhesive polymer resins in a film format. And those of thermo-compression (T/C) bonding, and its reliability
they have been widely used as interconnect materials in flat was evaluated.
panel display applications such as Out Lead Bonding (OLB), 2. Experiments
chip-on-glass (COG), and chip-on-film (COF), flex to PCB 2.1 Materials preparation
bonding, and also in flip chip semiconductor packaging
Test boards were 25 um-thick polyimide based flexible
applications. ACF interconnects are simple and lead-free
substrates and 1mm-thick FR-4 organic rigid boards. Figure 1
processing as well as cost effective packaging method
shows the design of test boards which have 300 um pitch Cu
compared with solder interconnects.
patterns. The ACF was epoxy based adhesive film with 40 um
For ACF interconnection, thermo-compression (T/C) thickness, and they contained Au coated Ni particles with 8
bonding is the most common method, however it is necessary
to reduce the bonding temperature, time and pressure, because
1-4244-0985-3/07/$25.00 ©2007 IEEE 480 2007 Electronic Components and Technology Conference

2. um diameter as conductive particles. Table 1 summarizes the In-situ ACF temperature during U/S bonding was measured
specifications of test boards and the ACF. to investigate the effect of U/S bonding conditions such as
U/S power and bonding force on ACF temperatures. Figure 3
shows the in-situ ACF temperature measurement set-up with
40um-thick k-type thermocouples and a thermometer with
150ms sampling period.
Figure 1. The design of (a) organic rigid test board and Figure 3. A schematic diagram of in-situ ACF temperature
(b) flexible substrate measurement
Table 1. The specifications of test boards and the ACF 2.5 Adhesion strength vs. U/S bonding conditions
Materials Thickness To determine the relation between the ACF temperature
Flexible substrate Polyimide film 25um and adhesion strengths, adhesion strengths of ACF joints after
Organic rigid board FR-4 1mm U/S bonding were measured using a 90° peel tester as shown
Base film Epoxy based adhesive 40um in figure 4. Peel test was performed with a peel rate of 10
ACF Conductive 8um mm/min, and adhesion strengths of ACF joints were
Au coated Ni particles monitored during peel test using a load cell.
particle diameter
2.2 Equipment
Figure 2 shows the ultrasonic horn and test boards in this
experiment. The ultrasonic energy of 160 W ~ 240 W was
applied perpendicularly on the test boards
Flexible substrate
ACF
U/S horn
Organic rigid board
Figure 4. A schematic diagram of 90° peel test
Flexible substrate
ACF
2.6 Daisy-chain contact resistance vs. U/S bonding time
Organic rigid board Daisy-chain contact resistances of test boards were
measured after various U/S bonding conditions to examine
Figure 2. An ultrasonic bonder set-up showing an U/S horn the electrical continuity of ACF joints. Figure 5 shows the
and a test sample daisy-chain structure of test boards.
2.3 Decomposition temperature of materials
During U/S bonding, test boards or the ACF can be
decomposed due to rapid increase of ACF temperature.
Therefore, decomposition temperatures of flexible substrates,
organic rigid boards and the ACF were measured to prevent
decomposition of materials during U/S bonding. I
V
Decomposition temperatures were measured by TGA Figure 5. The daisy-chain structure of test boards showing a flexible
(Thermo-Gravimetric Analysis) with 10℃/min heating rate. substrate bonded on a organic rigid board using ACFs
2.4 ACF temperature vs. U/S bonding conditions
481 2007 Electronic Components and Technology Conference

3. 2.7 Reliability evaluation of U/S bonding
Reliability tests were performed with the optimized U/S 600
bonding conditions in terms of the adhesion strength and the 160W
daisy-chain contact resistance. Reliability requirements were 500 200W
85 ℃/85 % RH test and 125 ℃ high temperature storage test 240W
ACF temperature ( C)
o
for 1000 hours, and -55 ℃~125 ℃ thermal cycling test for 400
1000 cycles. Using 10 test boards for each test, total daisy-
300
chain contact resistances were measured for every 100 hours
and 100 cycles. 200
100
3. Results and discussion
3.1 Decomposition temperature of materials U/S on U/S off
0
0 2 4 6 8 10 12
Bonding time (sec)
1.1
1.0
Figure 7. ACF temperatures vs. U/S powers at 5.6 MPa bonding
pressure
Relative weight
0.9
Figure 7 shows the ACF temperature during U/S bonding
0.8
with 160 W, 200 W and 240 W powers at constant 5.6 MPa
0.7
bonding pressure. As shown in the graph, the ACF
temperature increased as the U/S power increased.
0.6 Flexible substrates The increase of ACF temperature can be explained with
Organic rigid boards U/S vibration amplitudes. U/S vibration amplitudes increase
0.5
0 100 200 300 400 500 600 with larger U/S powers at constant pressures, because the
o work by U/S vibration increases as the U/S power increases.
Temperature ( C)
According to the well-known equation 1,
f (∆ε )2E’’
dQ =
1.1
1.0 2
Equation 1. Heat generation by cyclic deformation [2]
Relative weight
0.9
0.8 which explains heat generation under cyclic deformation, heat
generation (dQ) is proportional to cyclic strain (∆ε).[2] And
0.7 cyclic strain increases as U/S vibration amplitude increases.
Therefore, the increase of U/S power causes the increase of
0.6
U/S vibration amplitude, and cyclic strain of the ACFs
0.5
resulting in more heat generation.
0 100 200 300 400 500 600
o
Temperature ( C)
500
Figure 6. TGA results of flexible substrates, organic rigid boards, 4.6MPa
6.7MPa
ACF temperature ( C)
400
and ACF
8.6MPa
o
Figure 6 shows TGA results of flexible substrates, organic 300
rigid boards and the ACF. As shown in the graph, the weight
of FR-4 organic rigid boards rapidly decreased at above 300 200
℃. However, flexible substrates and the ACF showed
negligible amount of decomposition up to 300 ℃. Therefore, 100
ACF heating temperature during U/S bonding was maintained
below 300 ℃ to prevent FR-4 decomposition. U/S on U/S off
0
0 2 4 6 8 10 12
Bonding time (sec)
3.2 ACF temperature vs. U/S bonding conditions
Figure 8. ACF temperatures vs. bonding pressures at 180W power
482 2007 Electronic Components and Technology Conference

4. Figure 8 shows the ACF temperature during U/S bonding
at 4.6 MPa, 6.7 MPa and 8.6 MPa bonding pressures at
500
constant 180 W power. As shown in the graph, the ACF
temperatures decrease as bonding pressures increase. 8.6MPa
200W
ACF temperature ( C)
400
The decrease of ACF temperatures can be explained with
o
U/S vibration amplitudes. U/S vibration amplitudes decrease 180W
as bonding pressures increase at constant U/S powers. 300
Because the work by U/S vibration is constant at a certain U/S
power, it means that a smaller vibration amplitude can be 200
obtained at larger pressures. Therefore, the increase of U/S
powers causes the decreases of U/S vibration amplitude, and 100
cyclic strain of the ACF resulting in a less amount of heat
generation.
0
As explained above, ACF temperatures were dependent 0 2 4 6 8 10 12
on both U/S powers and bonding pressures. Therefore, in Time (sec)
order to maintain the ACF temperature below 300 ℃, various
U/S powers were selected at 4.6 MPa, 6.7 MPa and 8.6 MPa Figure 9. ACF temperatures during U/S bonding with various
bonding pressures. Figure 9 shows the ACF temperatures bonding conditions
during U/S bonding with various bonding conditions.
At 4.6 MPa and 180 W condition, the ACF temperature
increased up to 300 ℃ . However, test boards were
decomposed at 6.7 MPa and 200 W condition due to over-
500
heating above 400℃. At other U/S bonding conditions, ACF
4.6MPa
180W temperatures were maintained about 200 ℃ which was
ACF temperature ( C)
400
relatively lower than that of 4.6 MPa and 180W condition.
160W
o
300
3.3 Adhesion strength vs. U/S bonding conditions
Figure 10 shows adhesion strengths of U/S bonded ACF
200 joints at various U/S bonding conditions. As shown in the
graph, the maximum adhesion strength was above 600 gf/cm
100 at 3 sec bonding time at 4.6 MPa and 180 W condition.
However, at other U/S bonding conditions, adhesion strength
0
showed relatively low 400 gf/cm adhesion strengths. The
0 2 4 6 8 10 12 adhesion strength behavior is well matched with the previous
Time (sec) ACF temperature behavior at figure 9. The ACF temperature
during U/S bonding at 4.6 MPa and 180 W condition was
about 300 ℃, and those with other U/S bonding conditions
were about 200 ℃. Lower adhesion strengths was mainly be
due to lower degree of cure of ACF at lower temperatures.
500
6.7MPa
200W
ACF temperature ( C)
400
o
180W
4.6MPa, 160W 3sec
800
300
5sec
7sec
peel strength (gf/cm)
200 600 9sec
100
400
0
0 2 4 6 8 10 12 200
Time (sec)
0
0 1 2 3 4 5 6 7
Displacement (mm)
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5. 800
4.6MPa, 180W 3sec 800
8.6MPa, 200W 3sec
5sec 5sec
Peel strength (gf/cm)
7sec 7sec
Peel strength (gf/cm)
600 9sec 600 9sec
400 400
200 200
0 0
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Displacement (mm) Displacement (mm)
Figure 10. Adhesion strengths of U/S bonded ACF joints at various
U/S bonding conditions.
6.7MPa, 180W 3sec
800
5sec
Figure 11 shows adhesion strengths of U/S bonded ACF
joints at 4.6 MPa and 180W condition for 2 sec, 3 sec and 4
Peel strength (gf/cm)
7sec
600 9sec sec bonding times. As shown in the graph, the adhesion
strength rapidly increased by ACF curing, and reached the
maximum value of 630gf/cm at 3 sec bonding time. And
400 630gf/cm maximum adhesion strength obtained by the
optimized U/S bonding was similar to the typical 620gf/cm of
200
T/C bonding with 15 sec bonding time and 190 ℃ bonding
temperature.
0
0 1 2 3 4 5 6 7
Displacement (mm)
600
Peel strength (gf/cm)
8.6MPa, 180W 3sec 400
800
5sec
7sec
Peel strength (gf/cm)
600 9sec 200
400
0
2 sec 3 sec 4 sec
200 Bonding time (sec)
Figure 11. Adhesion strengths of U/S bonded ACF joints for 2 sec, 3
0
0 1 2 3 4 5 6 7 sec and 4 sec bonding times at 4.6 MPa and 180 W condition
Displacement (mm)
As shown in figure 12, the degree of ACF cure measured
by FTIR shows more than 90% after 3 sec at 4.6 MPa and
180W condition.
484 2007 Electronic Components and Technology Conference

6. 125℃ high temperature storage test
Total daisy-chain resistance (Ohm)
100 10
90 8
ACF degree of cure (%)
80
6
70
4
60
2
50
0
40 0 200 400 600 800 1000
2 3 4
Test time (hrs)
U/S bonding time (sec)
Figure 14. Daisy-chain contact resistance during 125℃ high
Figure 12. ACF degree of cure vs. U/S bonding time at 4.6 MPa and
temperature storage test
180 W condition
3.4 Daisy-chain contact resistance vs. U/S bonding time Figure 14 shows daisy-chain contact resistances vs. aging
Daisy-chain contact resistances of ACF joints were times during 125℃ high temperature storage test. U/S bonded
measured after U/S bonding at the optimized conditions 4.6 test vehicles showed no significant change of daisy-chain
MPa bonding pressure and 180 W U/S power. As shown in contact resistance during the 125℃ storage test. Figure 15
figure 13, stable daisy-chain contact resistances were obtained and figure 16 show daisy-chain contact resistance at 85℃
regardless of bonding times after 3 sec. The average daisy- /85% RH test conditions and -55℃~125℃ thermal cycling
chain contact resistance was 1.13 Ohm at 3 sec U/S bonding test conditions. No significant changes of daisy-chain contact
time. And it was similar to 1.08 Ohm which was obtained by resistance were observed during both tests.
the typical 15 sec T/C bonding at 190℃. These results show
that not only similar adhesion strengths but also similar daisy-
chain contact resistances as the typical T/C bonding were 85℃/85%RH test
obtained using optimized U/S bonding at room temperature
Total daisy-chain resistance (Ohm)
10
and less than 5 seconds bonding time.
8
2.5 6
Total daisy-chain resistance (Ohm)
4.6MPa, 180W
4
2.0
2
0
1.5 0 200 400 600 800 1000
Test time (hrs)
1.0 Figure 15. Daisy-chain contact resistance during 85 /85%RH test
3 4 5 6 7 8 9
U/S bonding time (sec)
As the result, U/S bonded ACF joints showed similar
Figure 13. Total daisy-chain resistance with various U/S bonding adhesion strengths and daisy-chain contact resistances, and
time at 4.6 MPa and 180 W condition showed stable daisy-chain contact resistance during 125℃
high temperature storage test, 85 °C/85 % RH test and -55°
3.5 Reliability evaluation of U/S bonding C~125°C thermal cycling test compared with the typical T/C
For reliability evaluations of U/S bonded ACF joints, bonded ACF joints.
three kinds of tests were performed with 10 test boards at
each conditions. U/S bonding was performed for 3 sec
bonding time at room temperature with 4.6MPa bonding
pressure and 180W U/S power.
485 2007 Electronic Components and Technology Conference

7. -55℃/125℃ thermal cycling test
reduced from typical 15 seconds to less than 5 seconds by
Total daisy-chain resistance (Ohm) using the U/S bonding.
10
Therefore, conventional T/C ACF bonding processes can
be replaced by the novel U/S ACF bonding process
8
demonstrated in this study.
6
5. References
1. Kiwon Lee, Hyoung Joon Kim, Myung Jin Yim, and
4
Kyung Wook Paik, “Curing and Bonding Behaviors of
Anisotropic Conductive Films (ACFs) by Ultrasonic
2
Vibration for Flip Chip Interconnection”, 56th Electronic
Components and Technology Conference, San Diego,
0
0 200 400 600 800 1000
California, USA, May 30 – June 2, 2006
2. M. N. Tolunay, P. R. Dawson, and K. K. Wang, “Heating
Test cycle (cycles)
and bonding mechanisms in ultrasonic welding of
thermoplastics”, Polymer engineering and science, Vol.
Figure 16. Daisy-chain contact resistance during -55°C~125°C
23, No. 13, 1983
thermal cycling test
4. Conclusion
In this study, a novel ACF bonding method using
ultrasonic vibration was investigated, and its process
conditions were optimized for flexible substrate-organic rigid
board bonding applications.
The optimized U/S bonding time was 3 sec at room
temperature at 4.6 MPa bonding pressure and 180 W U/S
power. It was demonstrated that the ACF bonding process can
be significantly enhanced by U/S bonding method compared
with conventional 15 sec T/C bonding at 190 ℃. Using the
optimized U/S bonding conditions, the ACF joints showed
similar bonding characteristics as T/C bonding in terms of the
adhesion strength, the daisy-chain contact resistance, and
stable electrical resistances during 125 ℃ high temperature
storage test, 85 ℃/85 % RH test, and -55 ℃~125 ℃ thermal
cycling test.
Table 2 summarizes results of the optimized U/S ACF
bonding in comparison with those of typical T/C bonding.
Table 2. The optimized U/S ACF bonding condition and their results
in comparison with those of typical T/C bonding.
T/C ACF bonding U/S ACF bonding
at 190℃ at room temperature
for 15 sec for 3 sec
Peel strength
622.47 (±28.59) 633.41 (±52.14)
(gf/cm)
Daisy-chain
contact
1.08 (±0.02) 1.13 (±0.04)
resistance
(Ohm)
These results indicate that ACF bonding temperature can
be significantly reduced from the typical 190 ℃ to room
temperature, and bonding time can be also significantly
486 2007 Electronic Components and Technology Conference