Microelectronics Reliability 42 (2002) 1945–1951
www.elsevier.com/locate/microrel
Study of short-circuiting between adjacent joints under
electric field effects in fine pitch anisotropic
conductive adhesive interconnects
Y.W. Chiu
a
a,*
, Y.C. Chan a, S.M. Lui
b
Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong
b
Varitronix Limited, 22 Chun Cheong Street, TKO Industrial Estate, Tseung Kwan O, Hong Kong
Received 2 April 2002; received in revised form 14 May 2002
Abstract
Anisotropic conductive adhesive film (ACF) has been extensively used in the liquid crystal display (LCD) industry
for decades on chip-on-glass (COG) applications. It offers the advantages in terms of fine-pitch capability and more
environment compatibility. One of the very important performance requirements of using ACF in fine pitch interconnection is not to create leakage of electric current between adjacent joints as it may lead to abnormal display
segments/pixels of the LCD. In this work, the possibility of short-circuiting between adjacent joints in fine pitch ACF
interconnections under the effects of electric field was investigated. Insulation resistance measurements of the selected
adjacent joints against electric field strength 1 V/lm for a 24 h testing duration were discussed and analyzed. The results
showed a strong dependence of curing degree of the ACFs on the chance of short-circuiting between adjacent joints
under field effects.
Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
Direct mounting of chip-on-glass (COG) substrate
using anisotropic conductive adhesive film (ACF) is
being widely used to satisfy the increasing demand for
finer pitch interconnections in the liquid crystal display
(LCD) industry. Although the extensively use of ACF
for electrical conduction in LCD applications has been
industrialized for many years, it is still under development and have stimulated significant interest.
ACF interconnection is a rapid developing technology having the advantages of being lead-free, offers finer
pitch capability, and lower cost with less processing
steps at lower bonding temperatures. ACF is composite
in nature, containing conductive particles in an insulating adhesive matrix. The principle of ACF joints in
*
Corresponding author. Fax: +852-2788-7579.
E-mail addresses: eeywchiu@cityu.edu.hk (Y.W. Chiu),
eeycchan@cityu.edu.hk (Y.C. Chan).
COG assemblies is by interposing the adhesive so as to
trap conductive particles between flip chip bumps and
substrate tracks. Pressure and heat (thermo-compression) are applied simultaneously so that the conductive
particles can make contact with both bumps and substrate tracks and to be electrically connected. A sufficient
number of the conducting particles must be trapped
during the bonding process to guarantee the electric
contact with both conductor surfaces, but on the other
hand, the particles loading should be kept low enough
(ranging between 0.5 and 5 vol.% of conducting particles) [1], so that the material is not conductive between
particles.
Theoretically, the conductivity of ACF material is
restricted to the z-direction only and with electrical
isolation provided in the x–y plane. However, for very
fine pitch applications, such as chips with less than 15
lm bump gap, it becomes more challenging to maintain
in-plane electrical insulation. One of the important reliability issues concerning the use of ACF for fine pitch
interconnections is the possibility of short-circuiting
0026-2714/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 6 - 2 7 1 4 ( 0 2 ) 0 0 0 9 7 - 5
1946
Y.W. Chiu et al. / Microelectronics Reliability 42 (2002) 1945–1951
between adjacent joints under applied electric field. Instability of joints is frequently found over time when
voltage is applied, particularly at situations where cluster of conductive particles are trapped between bump
gaps. A proposed failure mechanism––movement of
conductive particles at a microscopic level in the presence of applied electric field––will be discussed. Previous
studies have shown the movement of charged conducting particles under electric field in other applications
such as viscous insulating liquid [2,3]. Modelling fluid
flow for ACF during assembly [1,4] and study on the
probability of bridging between bumps has also been
studied earlier [1].
In this work, COG test vehicles using two different
types of ACFs were prepared under various extent of
curing reaction. The focus is on comparing the dependence of short-circuiting on the curing degree of the
ACFs under the effects of electric field strength. Fourier transform infrared spectrophotometer (FT-IR) was
employed to investigate the degree of cure as a function
of time. The use of voltage source for insulation resistance measurement was performed to investigate the
possibility of short-circuiting between joints.
Table 1
Specifications of ACF 1 and ACF 2
Description
ACF 1
ACF 2
Thickness (lm)
Structure
Conductive particle
30
Single layer
Ni/Au plated
resin
Yes
30
Double layer
Ni/Au plated
resin
No
51
125
51
111
Insulating coating on
particle surface
Particle size (lm)
Tg (°C)
glass substrate is 1.1 mm thick and the conductive trace
pattern is sputter-deposited indium tin oxide, ITO (less
than 1 lm thick). Two different types of thermosetting
ACFs (ACF 1 and ACF 2), employing a random dispersion of conductive particles, were used in this study.
They are commercially available and believed to have
the capability for fine pitch interconnection. The detailed specifications given by the manufacturers are
summarised in Table 1.
2.2. Bonding process
2. Experimental
2.1. Test chips, glass substrates and ACF materials
In the design of test chip and glass substrate used in
this study, five bumps with 10 or 15 lm bump gap are
considered as a set of measuring unit. Two individual
bumps are designed for insulation resistance test and
three connecting bumps are intended for contact resistance test, as shown in Fig. 1. The test chips have a size
of 3 11 mm2 , and there are 300 bumps around the
periphery, with bump area of 50 70 lm2 . The bumps
are electroplated with gold and the bump height is approximately 17 lm. A total of 24 sets of bumps with a
choice of 10 or 15 lm bump gaps are available for insulation resistance measurement from a single chip. The
All glass substrates were carefully cleaned using organic solvent, such as acetone, prior to use in order to
remove any contaminants. ACFs were cut to the correct
size to cover the bonding area automatically by the
Toray SA2000 flip chip bonder and were preliminarily
bonded to the glass substrate using low pressure (0.07
MPa) and low temperature (95 °C) for 2 s. After preliminary bonding, the separator was peeled off from the
adhesive film immediately. Then the substrate pattern
and the position of the chip bumps were aligned. Finally, the chip was bonded to the glass substrate by applying heat and pressure simultaneously for a specific
duration. The bonding conditions are shown in Table 2.
2.3. Curing process
Curing degree of the two ACFs was examined by
using FT-IR spectroscopy. A Perkin–Elmer Spectrum
One FT-IR Spectrometer, with a resolution of 4 cm1 ,
was used to get spectra from samples prepared at
T ¼ 180 °C for five different reaction times (t ¼ 0, 5, 10,
15 and 20 s), see Fig. 2 for the typical spectra of ACF 1
and ACF 2 at t ¼ 0 s.
Table 2
Bonding conditions for the two types of ACFs
Description
Fig. 1. Layout of a set of measuring unit on test chip.
Bonding temperature (°C)
Bonding pressure (MPa)
Bonding time (s)
ACF 1
180
0.18
5, 10, 20
ACF 2
Y.W. Chiu et al. / Microelectronics Reliability 42 (2002) 1945–1951
1947
Fig. 2. FT-IR spectra of the two different ACFs, (a) ACF 1 and (b) ACF 2, at bonding time t ¼ 0 s.
The extent of curing reaction (a) at time t was calculated from the initial area of epoxy and reference
peak, Aepoxy;0 and Aref;0 respectively, and their corresponding values at time t, Aepoxy;t and Aref;t , according to
the following equation:
a¼1
Aepoxy;t =Aref;t
Aepoxy;0 =Aref;0
ð1Þ
The peak at 915 cm1 was used to measure the
amount of remaining epoxy group. The aromatic ring
band at 1507 cm1 , which did not vary during cure, was
considered as the internal reference [5–7]. Two enlarged
views of the epoxy group absorption for ACF 1 and
ACF 2, which comprises of peaks at 915 cm1 , are
shown in Figs. 3 and 4 respectively.
2.4. Insulation resistance test
Open-short test using a digital multi-meter was performed prior to insulation resistance test. Test joints
were inspected and selected under an optical microscope. Only pairs of open-circuited adjacent joints with
cluster of particles trapped in between were chosen to
undergo the insulation resistance test (see Fig. 5). As the
particles are expected to move at a microscopic level
only, the said joints may give a higher chance of obtaining the predicted results.
As measurement of insulation resistance is an indicator of short-circuiting, high resistance value (>108 X)
indicates no current flow between adjacent joints while
low resistance value (<104 X) indicates shorting is pre-
sent. In this study, insulation resistance was measured
between adjacent termini (bump gap, 10 or 15 lm) with
the use of a Keithley 617 digital electrometer. A constant
voltage was applied and resistance value was continuously measured and recorded during the aging process.
Pre-selected samples cured for three different curing
durations (t ¼ 5, 10 and 20 s) were tested under electric
field strength of 1 V/lm for 24 h.
3. Results and discussion
As thermo-compression is applied during bonding
process, the ACF first softens and begins to flow and
finally becomes hardened when the curing reaction has
completed. The chemistry behind this is that the curing
of ACF begins with the formation and linear growth of
polymer chain in the epoxy resin that soon begins to
branch, and then to cross-link. Finally, finishes as a
three-dimensional, highly cross-linked chemical network
[8]. As the reaction proceeds, transformation of epoxy
resin from a rubbery state to a glassy state and becomes
hardened [1]. Properties of an ACF depend on the degree of cross-linking from the curing reaction. In general, the cross-linking in the epoxy resin means the
degree of curing [8] and its completeness greatly affects
the rigidity of the ACF. From the results of FT-IR
analysis, the continual decreasing of the epoxy band
area at 915 cm1 over time shows that there is progressively consumption of epoxy group in cross-linking
during the curing reaction. The lesser the epoxy band
area remained, the more complete the cross-linking and
1948
Y.W. Chiu et al. / Microelectronics Reliability 42 (2002) 1945–1951
Fig. 3. Enlarged view of changes in epoxy absorption peaks for ACF 1, curing temperature at 180 °C for (a) t ¼ 0 s, (b) t ¼ 5 s,
(c) t ¼ 10 s, (d) t ¼ 15 s and (e) t ¼ 20 s.
Fig. 4. Enlarged view of changes in epoxy absorption peaks for ACF 2, curing temperature at 180 °C for (a) t ¼ 0 s, (b) t ¼ 5 s,
(c) t ¼ 10 s, (d) t ¼ 15 s and (e) t ¼ 20 s.
the stronger the chemical bonding, hence, the higher the
rigidity of ACF. In other words, undercured ACF such
as sample bonded at t ¼ 5 s (<50% cured) was observed
to have a relatively large epoxy band area remained
at 915 cm1 after thermo-compression, hence, a lower
cross-link density and a weaker bonding force. Therefore, higher mobility of conductive particles may then be
allowed, especially if an external electric field is applied.
Conductive particles are randomly distributed over
the adhesive film and the final positions reached by the
particles are determined by the flow of the adhesive
during the bonding process [4]. In the case when a
cluster of particles are trapped between two adjacent
bumps and exposed to electric field, the particles may be
charged. If the applied electric field is high enough, the
forces acting on the charged conductive particles can
Y.W. Chiu et al. / Microelectronics Reliability 42 (2002) 1945–1951
1949
Fig. 5. Photograph of cluster of conductive particles trapped between a 15 lm bump gap: (a) top view and (b) cross-sectional view.
become strong enough to overwhelm the holding forces
of particles by the adhesive (such as viscous drag), the
particles may start to move and pull adjacent particles
closer to form conductive chains. Hence, causing undesirable shorting. A schematic representation of the particle movements under field effect is shown in Fig. 6.
Based on the FT-IR spectra obtained, higher degree
of curing will result in more reacted epoxy. The extent of
curing reaction of the two ACFs can be calculated from
Eq. (1) and is plotted as a function of cure time in Fig. 7
with the same curing temperature at 180 °C. The overall
reaction rate (%) of ACF 1 is higher than that of ACF 2
under the same curing times (Fig. 7). When processed
under the same bonding temperature, the extent of reaction increases with time. The rate of reaction is rapid
at the beginning and slows down after t ¼ 10 s. A reaction rate of 80% or above is reached for both ACFs
after curing for 10 s.
From the results of the insulation resistance test,
short-circuiting failures were found only from the samples cured at t ¼ 5 s. Examples of the changes in insulation resistance during the 24 h measurement for the
failed (i.e. short-circuit) and passed (i.e. open-circuit)
test samples were shown in Figs. 8–10. A dramatic drop
of resistance from a high magnitude (ranged from GX to
TX) to a low resistance (kX) was observed in the failed
samples (see Fig. 8), whilst samples cured at both t ¼ 10
and 20 s (see Figs. 9 and 10 respectively) survived the
test rather well. Although some of them might have
experienced slight fluctuations of resistance over the test
period, they still stayed at a high resistance level and no
short-circuiting was found after the 24 h test.
Fig. 11 shows the statistics of short-circuiting failures
found after the insulation resistance test. At electric field
of 1 V/lm, higher chance of shorting (25% failure) was
found in both types of ACF at cure time t ¼ 5 s, where
Fig. 6. Schematic representation of particle movements under electric field effects.
1950
Y.W. Chiu et al. / Microelectronics Reliability 42 (2002) 1945–1951
Fig. 10. Examples of failed samples cured at t ¼ 20 s after the
insulation resistance test.
Fig. 7. Plots of reaction rate (%) versus cure time (s) at cure
temperature 180 °C.
Fig. 11. Percentage failures of ACF 1 and ACF 2 against different degree of cure after applying electric field strength at 1 V/
lm for 24 h.
Fig. 8. Examples of failed samples cured at t ¼ 5 s after the
insulation resistance test.
Fig. 9. Examples of failed samples cured at t ¼ 10 s after the
insulation resistance test.
their degrees of cure were below 50%. None was found
from samples with cure time at both 10 and 20 s (above
80% cured) after 24 h. These results showed that the
degree of curing of ACF plays an important role in
determining the possibility of short-circuiting between
adjacent joints under the field effects. Chances of failure
were found higher in the undercured samples and a
certain degree of curing (in this case, >80% cured) was
needed for obtaining stable conductive adhesive joints
without short-circuiting under the effects of electric field.
One could see that the results of the two ACFs are
very similar although the structure of conductive particles between the two is different, as particles in ACF 1
containing insulation layer at their outermost. There
should be no electron flow between a perfect insulator
and Au bump, even if they are in contact. Particles with
insulation, therefore, should not be charged up and
would not be affected by the field. However, failures
were found and this may be explained by the wear-out
Y.W. Chiu et al. / Microelectronics Reliability 42 (2002) 1945–1951
1951
under the field effects at a microscopic level and are
seemed to be connected as conductive chains. Hence, the
adjacent joints become short-circuited. All failures
found in this study were long-term shorting; these results
were well matched with our hypothesis, as the moved
particles will remain in their final positions after withdrawal from the electric field and causes permanent
failure. It can be concluded that curing degree of the
ACFs will greatly influence the chance of short-circuiting between adjacent joints under electric field effects
and there is no doubt that proper curing is very important for joint reliability.
Acknowledgements
Fig. 12. Schematic diagrams of different mode of frictions
during bonding process: (a) particle-to-particle, (b) particleto-bump and (c) particle-to-substrate.
mechanism of insulation layer during the bonding process. The insulation layer of the particles is very thin. It
is only around 0.2–0.4 lm thick. When heat and pressure are applied during bonding, the adhesive resin is
pressed out of the clearance between the Au bumps and
the glass substrate with an accompanying flow of particles. Some of the particles are trapped in between the
bonding electrodes while some of them are squeezed out
of these narrow gaps to the bump gap. The frictions
between particle-to-particle (Fig. 12a) or particle-tobump (Fig. 12b) or even particle-to-substrate (Fig. 12c)
can easily damage or break the insulation layers, especially in the area where cluster of particles is present.
Hence, the Au layer underneath is being exposed and the
particles become conductive. Under applied electric
field, short-circuiting problem may then happen.
4. Conclusions
In this study, a higher chance (25%) of short-circuiting, under an applied electric field was found in the
undercured ACF samples (<50% cured) after the insulation resistance test. However, none of the samples were
found short-circuited in ACF samples with more than
80% cured. We believe that in the presence of electric
field, the bonding force of the undercured ACFs may
not be strong enough to hold the conductive particles in
place. Particles may be allowed to move closer together
The authors would like to acknowledge the financial
support provided by the Hong Kong Research Grant
Council fund for Co-operative Research Center on
Conductive Adhesive Technology for High Density
Electronic Packaging (project no. 8720003) and the
Hong Kong SAR Government––Innovation and Technology Fund (ITF), University-Industry Collaboration
Programme (project no. UIT/15). The authors also wish
to thank Ms. Edwina Luk of City University of Hong
Kong for proof reading.
References
[1] Liu J. Conductive adhesives for electronics packaging. UK:
Electrochemical Publications Ltd; 1998. p. 99–116.
[2] Choi C, Yatsuzuka K, Asano K. Dynamic motion of a
conductive particle in viscous fluid under DC electric field.
IEEE Trans Ind Appl 2001;37(3):785–91.
[3] Atten P, Boissy C, Foulc JN. Attraction force between
slightly conducting spheres immersed in a dielectric liquid.
In: Conference Record of the ICDL Õ96, Italy, 15–19 July
1996.
[4] Dudek R, Meinel S, Schubert A, Michel B, Dorfmuller L,
Knoll PM, et al. Flow characterization and thermomechanical response of anisotropic conductive films. IEEE
Trans Compon Pack Technol 1999;22(2):177–85.
[5] Cherdoud-chihani A, Mouzali M, Abadie MJM. Study of
crosslinking AMS/DGEBA system by FTIR. J Appl Polym
Sci 1998;69:1167–78.
[6] Mijovic J, Andjelic S. Monitoring of reactive processing by
remote mid infra-red spectroscopy. Polymer 1996;37(8):
1295–303.
[7] Meyer F, Sanz G, Eceiza A, Mondragon I. The effect of
stoichiometry and thermal history during cure on structure
and properties of epoxy networks. Polymer 1995;36(7):
1407–14.
[8] May CA. Epoxy resins. New York: Marcel Dekker Inc;
1988.