Microelectronics Reliability 42 (2002) 1195–1204
www.elsevier.com/locate/microrel
Effects of bonding parameters on the reliability performance
of anisotropic conductive adhesive interconnects
for flip-chip-on-flex packages assembly
II. Different bonding pressure
Y.C. Chan *, D.Y. Luk
Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong
Received 2 April 2002
Abstract
The effects of different bonding pressure during flip-chip-on-flex (FCOF) assembly in relation to the performance of
anisotropic conductive film (ACF) interconnect were investigated. Two types of ACF were used in this study. ACF 1 is
designed to create good interconnection when the connecting bumps and pads are in close contact while ACF 2 can give
good connections when the conductive particles are in contact with the bumps and pads, hence the deformation of
conductive particles within ACF 2 FCOF packages were less than that within ACF 1 packages. ACF 2 gave much
better interconnection performance when compared to ACF 1 indicating that ACF 2 is more flexible and can tolerate a
wider range of bonding pressure.
Ó 2002 Published by Elsevier Science Ltd.
1. Introduction
Electronic packages nowadays are becoming smaller,
lighter with higher in/out (I/O) count and better performance that are more cost competitive. The trend in
electronic packaging runs from bulky plastic ball grid
array (PBGA) with solder joints to miniature flip chip
with anisotropic conductive adhesive interconnects. The
new fabrication technology in the electronic industry is
flip-chip-on-flex (FCOF) assembly, in which anisotropic
conductive films (ACFs) are used as the adhesive to bind
the desired interconnecting chip and the polyimide (PI)
flexible substrate. The electrical path is formed by connecting the bumps on the chip and the electrode on the
PI film via the conductive particles in the ACFs [1]. ACF
offers several advantages for electronics assembly including extreme fine pitch capability, low temperatures
*
Corresponding author. Tel.: +852-2788-7130; fax: +8522788-7579.
E-mail address: eeycchan@cityu.edu.hk (Y.C. Chan).
and simple processes. It is also a lead-free and fluxless
bonding process meaning that the need for post-assembly cleaning is eliminated.
ACFs consist of mixtures of conducting fillers in an
insulating matrix. This arrangement allows the material
to conduct in the z-direction while remaining insulators
in the x–y plane [2]. The aim of these adhesives is to trap
at least one conductive particle between the conductive
bumps on the flip chip and the corresponding pads on
the substrate. This has to be achieved without the occurrence of bridging between the pads. The particles are
randomly distributed in the matrix in most anisotropic
materials, which can cause problems especially in ultrafine pitch applications. This is because the concentration
of particles within the material varies at different locations, and hence may result in open or short circuit
therefore the size of the conductive particles has to be
small (about 5 lm) [3].
In previous study, it was found that the conductive
particles were trapped tightly between the Ni bumps
and Cu pads in the Ni bump FCOF packages, however,
the conductive particles within the bumpless FCOF
0026-2714/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd.
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Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1195–1204
packages were not fully compressed between the Al and
Cu pads and hence leaving small gaps [4]. This finding
was thought to be another factor that caused the ACF
interconnects in bumpless FCOF packages to be less
effective and may be influenced by the bonding pressure.
The degree of spread of the conductive particles, determined by the amount of pressure applied during the
bonding process, has a great influence on the contact
resistance of the ACF joints. This is because if the particles are too spread out between adjacent bumps or
pads, caused by too much pressure applied, they may
end up contacting each other creating the same effect as
short-circuiting; whereas if the bonding pressure is too
low, the particles may not be able to make contact between the connecting bumps and pads. It is believed that
the bonding pressure needed to compress the ACF before the conductive particles within will come into contact between the Al pads in bumpless chips and the pads
on the flexible substrate is larger than that for chips with
Ni bumps. With regard to bumpless chips, an ideal
bonding force is not easy to obtain. Flip chips with Ni
bumps create a surface with lands and grooves. This
arrangement reduces the chance of adjacent conductive
particles being bridged since they can be spread into the
groove areas. The average diameter of a conductive
particle is 3.50 lm and the bump height is about 4 lm
hence, the conductive particles can be buried in the
grooves escaping from causing bridges. However, bumpless chips have Al pads with bump height of only 1 lm
meaning that the groove areas are too shallow to cater
even a single conductive particle. This phenomenon is
shown in Fig. 1.
Since pressure is applied to force the conductive
particles to make contact between the flip chip and
flexible substrate, the degree of deformation of the
conductive particles can affect the performance of ACF
interconnects. Ideally, the conductive particles should
be squashed until just before the metallic layers begin
to break. At this point, the contact area between the
bonding surfaces is the largest. However, if the pressure
applied is too great, the metallic layers of the particles
will burst open, exposing the polymer to the bonding surfaces, and hence would not be conductive. This
phenomenon is shown in Fig. 2.
This series of studies concentrate on the effects of
different bonding parameters during the assembly of
FCOF packages in relation to the reliability of the ACF
joints. The aim of this study was to investigate the effects
of different bonding pressure on the contact resistance of
ACF joints. The results of this study would allow development of ACF joints using fine pitch flip chips on
flexible substrates with better reliability and performance.
2. Experimental procedure
The FCOF packages are made up of three different
materials, namely silicon (Si) chip, ACF and flexible
substrate.
2.1. Silicon chips
Fig. 1. Schematic showing the possibility of creating bridging
in (a) Ni bump and (b) bumpless FCOF packages (the shape of
the conductive particles is also different depending of their locations within the ACF matrix; oval between connection bumps
and pads but spherical between adjacent bumps or pads).
The dimensions of the silicon (Si) chips are 10:87 3:14 mm2 , with rectangular bumps (70 50 lm). The
bumps are arranged in sets of five as a group; with two
adjacent bumps for measuring insulation resistance and
three for contact resistance. There are a total of twelve
sets of these daisy-chained bumps within the chip. The
layout of the chip is shown in Fig. 3.
Fig. 2. Conductive particle with (a) minimum contact and (b) maximum contact with the bump and pad and (c) too much pressure
causing the particle to burst open.
Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1195–1204
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Fig. 3. Schematic showing a corner of a Si chip with daisy-chained bumps.
Electroless nickel (Ni) bumping process involves aluminium (Al) cleaning, Al activation, electroless Ni deposition and immersion gold (Au) coating [2]. The bump
height of the Ni bump and Al pad are 4 and 1 lm, respectively. The last two steps in the bumping process of
bumpless chips were omitted.
to prevent electrical conduction of adjacent conductive
particles in the x- and y-direction, and it is designed to
become soft when subjected to thermal compression
during bonding. Fig. 4 shows the structure of the two
types of ACF and their specifications are summarized
in Table 1.
2.2. ACFs
2.3. Flexible substrate
Two types of ACF were used in this study in order to
compare their flexibility with different bonding parameters. ACF 1 is a double layer ACF that consists of an
epoxy layer and another one filled with conductive and
insulation particles. The conductive particles of ACF 1
are polymer spheres plated with a thin layer of nickel
followed by a thin layer of gold. ACF 2 is a single layer
ACF that consists of an epoxy layer filled with conductive particles. The conductive particles of ACF 2 are
polymer spheres plated with a thin layer of nickel followed by a thin layer of gold with an additional insulation layer. The purpose of the insulation layer is
The flex substrates used in this study were about 40
lm thick and the electrode is gold/electroless nickel
coated copper (Au/Ni/Cu). 12 lm thick of copper (Cu)
traces was electro-deposited onto a 25 lm thick PI,
followed by 4–5 lm thick of electroless nickel (Ni) and
finally sputtered with a 0.4 lm thick gold (Au) layer.
Since the flex substrate is of ultra-fine pitch (the smallest
gap between the traces was 10 lm), Ni was plated onto
the Cu traces to prevent Cu migration. Au sputtering
was necessary to prevent the Ni layer from oxidation.
During the pre-bonding process, the ACF was laminated onto the flexible substrates, by using the Karl Suss
Fig. 4. Schematic showing the structure of (a) ACF 1 and (b) ACF 2 and their conductive particles.
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Table 1
Specifications of the ACF
Description
Film thickness (lm)
Conductive particle
Insulation coated
Particle size (lm)
Pre-bonding temperature
(°C)
Pre-bonding time (s)
Pre-bonding force (MPa)
per unit area of bump
Bonding temperature (°C)
Bonding time (s)
Bonding force (N)
a
Specification
ACF 1
ACF 2
30
Au/Ni coated
polymer
No
3.5
90
30
Au/Ni coated
polymer
Yes
3.5
90
5
10
5
10
200a
10
60
190a
10
60
Selected according to our other research [4].
manual flip chip bonder. The final bonding of flip chip
onto the ACF/flex was carried out using the Toray semiautomatic flip chip bonder. The alignment accuracy is
2 lm. Different bonding pressures were used in this
study, as shown in Table 2 and the schematics of the
bonding process is shown in Fig. 5.
The contact resistance of the ACF joints of the
FCOF packages was measured by using the four-point
probe method as shown in Fig. 6.
In the four-point probe test, 1 mA was applied to the
circuit constantly and the voltage was measured for each
set of bumps using the Hewlett Packard 3478A Multimeter. The contact resistance was calculated by using
R ¼ V =I.
The samples were moisture-soaked under 85 °C/
85%RH conditions for 336 h and the contact resistance
was measured again. The samples were then mounted in
epoxy resin and cross-sectioned. The Philips XL40 FEG
scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) was used to inspect and
analyse the microstructure and microjoints of the FCOF
packages, especially the chip/conductive particle metallization interface. Dimensions of the conductive particles
were also measured.
3. Results and discussion
Table 2
Bonding pressure used
Bonding pressure (N)
Standard
Tests
Fig. 6. Contact resistance measurement of ACF joints using the
four-point probe method (I ¼ 1 mA).
60
70, 80, 90, 100, 110, 120, 130
From Fig. 7, one can see that the FCOF packages
with different types of ACF behaved differently. Packages with ACF 2 showed very steady interconnection
regardless of the bonding pressure whereas those with
Fig. 5. Schematic showing the formation of flip chip interconnections with (a) bumped chip and (b) bumpless chip using ACFs.
Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1195–1204
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Fig. 7. Average contact resistance of Ni bumps and bumpless packages with ACF 1 and ACF 2 assembled at various pressures
(a) before and (b) after ageing.
ACF 1 gave fluctuating joints’ performance. In general,
the contact resistance of the FCOF packages with ACF
1 decreased dramatically then increased steadily whereas
those with ACF 2, more or less, stayed the same. For
FCOF packages with Ni bump/ACF 1 bonded at 60–70
N, the contact resistance decreased from 28.8 to 13.3 mX
but then increased systematically when the bonding
pressure was above 70 N. The contact resistance of the
bumpless/ACF 1 packages decreased from 38.5 to 20.4
mX when bonded at 60–80 N but then increased with
diligence when the bonding pressure used was above 80
N. By looking at the contact resistance obtained when
ACF 1 was used as the connecting material, FCOF
packages with Ni bump assembled at 70 N and bumpless
packages assembled at 80 N gave the best contact, with
initial average contact resistance 13.3 and 20.0 mX, respectively. For ACF 2, packages with Ni bumps and
bumpless packages gave best interconnections at 80 and
100 N respectively, with initial average contact resistance of 1.07 and 4.11 mX, respectively.
The contact resistance of the packages increased
slightly after being moisture soaked under 85=85 con-
ditions, with the Ni bump/ACF 2 FCOF packages increased the least. Humid environments favour oxidation
of the Al pads and hence the bumpless packages gave
higher contact resistance after the moisture soak test.
The contact resistance of the packages with Ni bumps
did not increase much due to the thin layer of gold
sputtered on top therefore the Ni bumps are less prone
to oxidation. The addition of oxide layer is thought to
impede the flow of electrons through the connecting
bump-conductive particle-pad path [4].
When pressure is applied, the conductive particles
within the ACF would experience certain amount of
compressive force. As a result, the conductive particles
would change from being spherical to oval, with the
length in the x-direction being greater than that in the
y-direction. This phenomenon is shown in Fig. 8. Conductive particles that are caught in between the connecting bumps and pads tend to be oval while the ones
not being trapped remains spherical, as shown in Fig. 1.
During the bonding process, the ACF is being compressed thermally and the conductive particles are sandwiched between the connecting bumps and pads. As a
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Fig. 8. Schematic showing the change in shape and dimensions of a conductive particle after pressure was applied during bonding (x1
and y1 represents length in x- and y-direction before bonding respectively; x2 and y2 represents length in x- and y-direction after
bonding, respectively).
result, the particles ‘‘shrunk’’ in the y-direction and
‘‘grown’’ in the x-direction. According to the specifications given by the manufacturer, the average diameter of
the conductive particles within ACF 1 and ACF 2 is
3.50 lm. The optimal bonding pressure needed to create
best interconnections was different when using different
types of flip chip and ACF, as summarized in Table 3.
The average dimensions of the conductive particles
within the different combinations of FCOF packages,
assembled at their ideal bonding pressure, are shown in
Table 4. The conductive particles within the Ni bump/
ACF 1 and bumpless/ACF 1 packages after the bonding
process changed to (4.56, 1.53 lm) and (4.31, 2.06 lm),
respectively. The average dimensions of the conductive
particles changed to (4.52, 1.42 lm) and (4.37, 1.92 lm)
for Ni bump/ACF 2 and bumpless/ACF 2 packages,
respectively.
The bonding pressure applied during assembly of
FCOF packages affects the deformation of the conductive particles within the ACF, which in turn affects the
performance of the interconnects. The optimal pressure
needed to compress the ACF during bonding should
squash the conductive particles until just before the
Table 3
Ideal bonding pressure needed to create the best interconnections with different types of ACF and flip chip
Fig. 9. SEM micrograph showing the ideal shape of a conductive particle with largest surface area in contact with the
connection bump and pad.
outermost layer of coating breaks, as shown in Fig. 9.
This amount of pressure should create a maximum
surface area of conductive particles in contact with the
connecting bumps and pads therefore a larger volume
for electrons to flow through the bump-conductive
particle-pad path. However, if the pressure applied is
greater than the outermost layer of the conductive
Ideal bonding pressure (N)
Ni bump
Bumpless
ACF 1
ACF 2
70
80
80
100
Table 4
Dimensions of the conductive particles within different FCOF
packages after bonding with their ideal bonding pressure (initial
diameter of a conductive particle ¼ 3:50 lm)
Dimensions (lm)
ACF 1
Ni bump
Bumpless
ACF 2
x
y
x
y
4.56
4.31
1.53
2.06
4.52
4.37
1.42
1.92
Fig. 10. SEM micrograph showing the broken outermost layer
of the conductive particle exposing part of the polymer sphere.
Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1195–1204
particle can withstand (metallic coating in ACF 1 and
insulation layer in ACF 2 particles), the conductive
particle may burst open, exposing the polymer sphere,
which is an insulator, to the connecting bump and pad.
Hence, the conductive property of the particle is lost,
leaving it to be an insulator that acts as an obstacle
impeding the electrons from trying to flow through the
connecting bump and pad.
During the bonding process, the ACF is being cured
and flows. The flow of ACF provided the conductive
particles with sufficient mobility to distribute themselves
evenly between the bumps and pads [4]. During their
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travel through the ACF matrix, friction is generated [5]
and the outermost layer of the conductive particle might
be rubbed off, as shown in Fig. 10. As a result, the
performance of the interconnects may be different even if
the same number and size of conductive particles were
present. The function of the insulation layer of ACF 2
conductive particles is to prevent electrical conduction in
the x- and y-direction between adjacent particles, and it
is designed to become soft and breaks when subjected
to thermal compression during bonding. However, this
additional layer may serve another purpose. Theoretically, having three layers covering the polymer core
Fig. 11. SEM micrographs showing the deformation of conductive particles within the Ni bump/ACF 2 packages assembled at (a) 70
N (b) 80 N (c) 90 N (d) 100 N and (e) 110 N, with a combination of trapped conductive particles with intact and incomplete outermost
layers.
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should be better than two layers. This is because the
insulation layer of the conductive particles in ACF 2 can
act as a friction-proof layer making it difficult to rub off
the two inner metallic layers. Without the third layer for
protection, the metallic coatings on the polymer spheres
in ACF 1 are easily removed during the travel through
the ACF matrix when compared to ACF 2 and hence
unable to maintain their conductive properties when
reached between the connecting bumps and pads.
Referring to Figs. 11 and 12, at least one conductive
particle was able to distribute itself in between the
connecting bumps and pads. However, the degree of
their deformation and the completeness of their outer-
most layer were different. This could be one of the reasons why certain amount of pressure applied can result
in interconnects with lower contact resistance. In Fig.
11(a), the conductive particles were not compressed
enough leaving the connecting bumps and pads in contact with only part of their surface, which is not enough
to give the best contact. In contrast, the conductive
particles in Fig. 11(c) and (d) were compressed too much
and their outermost layer was broken. The conductive
particle in Fig. 11(e) was deformed too much and its
conductive property might be lost. In Fig. 11(b), when
80 N was applied during the assembly of the FCOF
package, the conductive particles were being compressed
Fig. 12. SEM micrographs showing the deformation of conductive particles within the bumpless/ACF 2 packages assembled at (a) 70
N (b) 80 N (c) 90 N (d) 100 N (e) 110 N and (f) 120 N, with a combination of trapped conductive particles with intact and incomplete
outermost layers.
Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1195–1204
Fig. 13. SEM micrograph showing a conductive particle pushed into the aluminium oxide layer on the Al pad.
and deformed in the desirable manner, with their surfaces in contact with the bump and pad at maximum,
hence resulting in lowest contact resistance and therefore
gave the best interconnection.
In general, one would expect the more pressure applied the more deform the particles would be as the gap
between the chip bump and substrate pad becomes
smaller. This assumption is seen with the FCOF packages with Ni bumps (Fig. 11), however, the dimensions
of the conductive particles and the gap between the
connecting bumps and pads seem to be unaffected by the
bonding pressure in bumpless FCOF packages (Fig. 12).
Previous study showed that the Al pad is made up of
two layers, namely the Al pad itself and a layer of aluminium oxide [4]. Aluminium oxide is a softer material
with respect to nickel. When the conductive particles
were subjected to compression during bonding, the layer
of aluminium oxide was acting like a ‘‘soft’’ sponge allowing the conductive particles to embed in it. In this
way, part of the pressure applied to the interconnects
was absorbed by the oxide layer, which in turn compressing less onto the conductive particles. However,
the distance the conductive particles can travel into the
oxide layer is abruptly stopped when they reach the
oxide/Al pad interface, and at this point, deformation of
the conductive particles begins due to the absence of
‘‘shock absorbent’’, as shown in Fig. 13. This observation was not found in Ni bump packages since the
conductive particles were unable to push into nickel and
also absent in ACF 1 FCOF packages due to the narrower working window of ACF 1 and its sensitivity to
different pressure.
4. Conclusions
By correlating the results obtained from contact resistance measurement and change in dimensions of the
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conductive particles, the most suitable bonding pressure
for different combinations of the types of ACF and flip
chip used can be determined. For ACF 1, Ni bump and
bumpless packages performed best when 70 and 80 N
pressure was applied, respectively; whereas for ACF 2,
packages gave the best contact when 80 and 100 N was
applied to Ni bump and bumpless packages, respectively. ACF 2 gave much better interconnection performance when compared to ACF 1 indicating that ACF 2
is more flexible and can tolerate a wider range of bonding
pressure. However, the pressure required to assemble
FCOF packages with ACF 2 is higher than that for ACF
1. This means that expenses of the companies may increase but in return with higher yield of products that
would perform better and are more reliable. In addition,
the presence of the extra insulation coating on the ACF
2 conductive particles may act as a friction-proof layer,
protecting the two inner metallic layers from being
rubbed off during their travel through the ACF matrix, hence retaining their conductive properties when
reached in between the connecting bumps and pads.
The performance of the ACF interconnects greatly
depends on the deformation of the conductive particles.
It is difficult to establish how much the conductive
particles should deform in order to create the best interconnections. It is equally hard to judge to what dimensions the conductive particles should reach during
the bonding process since their initial size varies slightly.
However, by looking at the measurement of the conductive particles, we conclude that the change in dimensions should reach about 30% in the x-direction and
about 50% in the y-direction when ACF 1 is used. Since
ACF 1 is designed to create good interconnection when
the connecting bumps and pads are in close contact
while ACF 2 can give good connections when the conductive particles are in contact with the bumps and pads,
the change in dimensions of the conductive particles for
ACF 2 is smaller when compared to that of ACF 1.
Hence, for best performance of the interconnects when
ACF 2 is used, 25% change in the x-direction and 40%
change in the y-direction should be achieved.
Although the presence of aluminium oxide in bumpless FCOF packages may impede the flow of electrons
through the interconnecting bumps and pads and hence
creating poorer interconnections, it may also act as a
‘‘shock absorbent’’ preventing the conductive particles
from experiencing too much pressure during bonding.
This explains why the dimensions of the conductive
particles within bumpless/ACF 2 FCOF packages did
not alter much when different pressure was applied.
Simply by looking into changing the bonding pressure alone cannot produce the highest yield of interconnects, all the parameters should be coupled together
in order to create best performance. This series of study
that concentrates on investigating the affects of bonding
temperature and pressure have provided fundamental
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Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1195–1204
understanding of how ACF behaves under different
bonding conditions.
Acknowledgements
The authors would like to acknowledge the Strategic
Research Grants (Project no. 7001080) of the City University of Hong Kong.
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