Download Effect of microwave preheating on the bonding performance of flip

Microelectronics Reliability 44 (2004) 815–821
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Effect of microwave preheating on the bonding performance
of flip chip on flex joint
R.A. Islam, Y.C. Chan
*
Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon tong, Kowloon, Hong Kong
Received 7 July 2003; received in revised form 17 September 2003
Abstract
A microwave (MW) preheating mechanism of anisotropic conductive adhesive film (ACF) has been introduced in
order to reduce the bonding temperature for flip chip technology. Thermal curing of epoxy shows a very sluggish and
non-uniform curing kinetics at the beginning of the curing reaction, but the rate increases with time and hence requires
higher temperature. On the other hand MW radiation has the advantage of uniform heating rate during the cycle. In
view of this, MW preheating (for 2/3 s) of ACF prior to final bonding has been applied to examine the electrical and
mechanical performance of the bond. Low MW power has been used (80 and 240 W) to study the effect of the MW
preheating. It has been found that 170 C can be used for flip chip bonding instead of 180 C (standard temperature for
flip chip bonding) for MW preheating time and power used in this study. The contact resistance (0.015–0.025 X) is low
in these samples where the standard resistance is 0.017 X (bonded at 180 C without prior MW preheating). The shear
forces at breakage were satisfactory (152–176 N) for the samples bonded at 170 C with MW preheating, which is very
close and even higher than the standard sample (173.3 N). For MW preheating time of 2 s, final bonding at 160 C can
also be used because of its low contact resistance (0.022–0.032 X), but the bond strength (137.3–145 N) is somewhat
inferior to the standard one.
2003 Elsevier Ltd. All rights reserved.
1. Introduction
Anisotropic conductive film (ACF) consisting of
conducting particles and adhesives provides both
attachment and electrical interconnections between
electrodes. They are widely used for high-density interconnections between liquid crystal display (LCD) panels
and tape automated bonding (TAB) as a replacement of
traditional soldering or rubber connectors. ACF has the
advantage of low temperature assembly, high-density
interconnections, fluxless bonding and low fabrication
cost [1–4]. Therefore, their use is also expected to increase as new interconnect materials in semiconductor
packaging which requires smaller and thinner dimen-
*
Corresponding author. Tel.: +852-2788-7130; fax: +8522788-7579.
E-mail addresses: [email protected] (R.A. Islam),
[email protected] (Y.C. Chan).
sions. The application on flexible substrate such as smart
card, disc drives and driver chips for LCDs have attracted much interests and widespread use [5].
ACFs are adhesive films with anisotropic properties
induced by dispersing 0.5–5 vol% of conducting particles
into polymer matrices. By controlling the volume fractions of conducting particles, anisotropic conductivity
can be imparted to the adhesive film. In a thermomechanical system the adhesive resin cured with the
application of heat and pressure and conducting particles dispersed in the matrix is trapped between the
conducting surfaces. The mechanical contacts between
conducting particles and bonding electrodes are retained
by strong adhesion strength of the polymer matrix.
Therefore, the electrical and mechanical properties of
the polymer matrix have a significant influence on the
interconnection reliability of ACFs [6,7].
Polymerization, as the key processing step for generating polymer, is directly related to the polymers
performance and is therefore of great importance.
0026-2714/$ - see front matter 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.microrel.2003.08.014
816
R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815–821
Conventionally, thermal heating is used to initiate and
carry out the polymerization. But in thermal curing, a
non-uniform heating at the early stage of the process
produce localized curing and needs higher curing temperature or higher curing time to complete the polymerization. Furthermore thermal curing (during die
bonding) is costly and therefore it is always desirable to
find an alternative curing process in the ACF technology. Other than thermal, MW radiation can be a possible candidate for performing the curing operation [8].
Microwave (MW) as a heating source for curing polymer and polymer-based composites has been of interest
for many years. The proposed curing mechanism is that,
the alternating electric field of MW causes re-orientation
of the long-chain molecules of polymers, which results in
the friction among molecules; this friction converts the
MW energy into the thermal heat [9,10]. For example, a
high heating rate and a more uniform temperature distribution can be easily obtained because all molecules
are heated simultaneously without the requirement of
thermal conduction.
Most literature reported the same kinetics for curing
reaction between MW and thermal cure. Discrepancy
was merely centered in the issue of curing extent. Some
authors reported the partial completion of cross-linking
by MW, which is believed that MW radiation accelerated the curing rate at the early stages of reaction and
the induced rapid cross-linking created a molecular
network [11,12]. However, partial polymer reaction was
only observed at the early stages of cross-linking, which
implies that further curing by thermal energy would
enable the reaction to complete uniformly [13,14]. Fig. 1
illustrates the molecular state of epoxy before curing,
after MW preheating and after the final curing. Con-
Fig. 1. Epoxy molecule re-orientation and conductive particle
distribution by curing process: (a) uncured state, (b) after preheated by MW radiation and (c) cured state.
tinued heating with MW can be the cause of conductive
filler metal arcing. So an identical or even much higher
curing degree can be obtained with MW preheating
followed by thermo-mechanical bonding even at lower
temperature than that used in conventional thermal
bonding.
The objective of this study is to see the effect of MW
preheating of ACF on the bonding performance of flip
chip on flex circuit. Here the contact resistance and the
shear force at breakage of the bond will characterize the
electrical and the mechanical performance of the bonding.
2. Experimental
The flex substrates are 50 lm thick and the height of
gold/electroless nickel coated copper (Au/Ni/Cu) electrodes is about 14 lm (the pitch between two Au/Ni/Cu
electrode is 35 lm in average). Sony ACF that has the
capability for the fine pitch on flex is used in this study.
The thickness of ACFs is about 35 lm. Conductive
particles with diameter of 3.5 lm (resin + Ni/Au plating + insulating coating) are distributed in the adhesive
matrix, with a concentration of 3.5 million/mm3 . A
schematic view is shown in Fig. 2. The test chips have a
size of 11 · 3 mm2 with 300 Au/Ni bump having a height
of 4 lm and a size of 70 · 50 lm2 .
The MW preheating was done in a electronically
controlled Sharp R652B microwave oven used for
domestic purpose (having a fixed frequency of 2.45 GHz
and a maximum power of 800 W) with the power of 80
and 240 W and the exposure time of 2 or 3 s. The built in
controller of the oven, having a very high resolution,
was set to desired time and output power and the oven
was turned on for preheating. Then the prebonding
process is carried out using the Karl Suss FCM manual
flip chip bonder at a temperature of 90 C with 0.3 MPa
pressure for 5 s. Final bonding was done by using the
Toray FC 2000 semi-automatic flip chip bonder at 150,
160 and 170 C and 60 N load for 10 s. For every
condition, five samples were prepared using the same
experimental setup. The control/standard sample was
prepared by using no MW preheating and the final
bonding temperature of 180 C with other variables
remaining the same [15,16]. Again the five samples were
prepared to compare with the newly bonded samples.
The contact resistance was measured by the 4-point
probe method.
Then the chip package is mounted on the FR4 board
as shown in Fig. 2. The die shear test was carried out by
using INSTRON Mini 44 Tester with a cross-head
speed of 10 mm/min. The experimental setup is shown in
Fig. 3. The shear force was measured at breakage when
chip is detached from the substrate. The tests were repeated five times for each condition using the same
R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815–821
817
Flex Substrate
FR4 Board
Polymer
ACF
Thin Au Layer
Chip
Thin Ni Layer
Insulating layer
Fig. 2. Schematic view of a conductive particle and the side view of a chip package prepared for die shear test.
Crosshead
Chip Package
Specimen Holder
BASE
at 80 W and bonded at 170 C. The samples preheated at
240 W show some higher contact resistance for both the
case. Here some factors have to be considered. They are
the initial cross-linking of molecules by MW heating,
final cross-linking by thermal heating, distribution of
conductive particles etc.
Thermal curing, during the flip chip bonding, is a
localized heating process. Here the cross-linking starts at
some molecules getting thermally excited. Then the
number of excited molecules increases with increasing
temperature and finally the curing process completes.
Fig. 3. Experimental setup for die shear test.
0.16
Contact Resistance (ohm)
experimental setup and the average results with data bar
have been reported (Figs. 8 and 9). The samples were
then cross-sectioned and mounted on epoxy resin. The
Philips XL40 FEG scanning electron microscope (SEM)
equipped with energy dispersive X-ray (EDX) was used
to inspect and analyze the micro-joints of the FCOF
packages, especially the chip/conductive particle metallization interface. Optical microscopy was used for
examining the presence of air bubbles in the fracture
surface.
80 watt
0.14
240 watt
0.12
0.1
0.08
0.06
0.04
0.02
0
145
0.017 ohm
150
155
160
165
170
175
Bonding temperature in degree celsius
3. Results and discussion
Fig. 4. Variation of contact resistance with bonding temperature (MW preheating time: 2 s).
3.1. Contact resistance
0.16
Contact Resistance (ohm)
Figs. 4 and 5 represent the contact resistance vs.
bonding temperature at 2 and 3 s MW preheating respectively. It is quite clearly observed that with increasing
MW power and MW exposure time the contact resistance increases except at 170 C. With the same MW
power and the MW preheating time, the higher the
bonding temperature, lower the contact resistance. In
both the figures the black line represents the contact
resistance of the standard sample using no MW preheating and bonded at 180 C. It is clearly observed that
the samples preheated in MW oven with 80 W and 2/3 s
can reach the contact resistance very close to the standard one and even smaller, especially for 3 s preheating
80 watt
0.14
240 watt
0.12
0.1
0.08
0.06
0.04
0.02
0
145
0.017 ohm
150
155
160
165
170
175
Bonding temperature in degree celsius
Fig. 5. Variation of contact resistance with bonding temperature (MW preheating time: 3 s).
818
R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815–821
Fig. 6. SEM images showing uneven distribution of conductive particle bonded at 150 C (bond line thickness: 0.592 lm).
The reason behind this is the initial reaction rate is much
slower because of the poor thermal conduction. On the
other hand, MW radiation is a volumetric heat source,
which accelerates the curing process at the early stage of
the curing operation. All the molecules get excited
simultaneously from the beginning. As the nucleation or
polymerization starts everywhere due to the replacement
of thermal heating by MW radiation at the early period
of the curing, set a stage for continued growth and crosslinking of epoxy molecules by the thermal heat. If the
MW preheating time is much longer then the internal
temperature achieved could be higher then it will be
difficult for thermal heating at 170 C to allow the
reaction to proceed and additional shrinkage to occur,
which is a typical irreversible behavior of thermosetting
polymers. But if the preheating time in MW oven is just
enough to initiate polymerization volumetrically then
additional thermal heat is required to reach the desired
cure point. This can be observed in Fig. 4. With MW
preheating time of 2 s at 80 W, even the final bonding at
160 C produces a very low contact resistance (0.022 X),
which is very close to the standard value (0.017 X). The
final bonding at 170 C at this condition produces even
lower contact resistance (0.018 X), which can be considered as almost the same with the standard one.
Thermosetting polymers are irreversible polymers.
Once set, they cannot be reshaped. They are formed by a
large amount of cross-linking of linear prepolymers.
Once the cross-linking has been established and it is
cooled down, it cannot be altered and it will be very
difficult to enhance the curing [17]. In Fig. 5, same feature can be seen that is the bonding at 150 and 160 C
produces very high contact resistance even in 80 W. This
can be explained as the preheating time of 3 s reaching a
level of the curing process where the final bonding at
150/160 C is not very effective to overcome the already
cured state. But a higher temperature of 170 C could
complete the curing process within the bonding time.
Thus it produces a very low contact resistance (0.015 X),
even lower than the standard value.
Another factor that affects the contact resistance is
the distribution of conductive particles. During curing,
the epoxy molecules interconnect by cross-linking and
the conductive particles distribute evenly during the
adhesive flow. In the heating and polymerization (curing) process, there will be a good adhesive flow at an
appropriate temperature, which results in an even distribution of the conductive particles. When this situation
prevails, there is a very good chance that between every
contacting electrode, there will be a considerable
amount of conductive particles. The outcome will be a
very low contact resistance. This feature has been found
in the case of samples bonded at 170 C with MW
preheating and even some of the samples of 160 (MW
preheating time of 2 s). However, for higher MW power
exposures the viscous flow stage is bypassed and there is
potentially an uneven distribution of conducting particles. According to this situation there is a very little
chance of trapping a good amount of well-distributed
conductive particles between every contacting electrodes. This can be clearly shown in Fig. 6. Here uneven
distribution of particles is shown. In one joint there is
only one particle within the bonding electrode and in
other joint there are several number of conducting particles found very close to each other in the samples
bonded at 150 C. In Fig. 7 an even distribution of
particles are shown in the samples bonded at 170 C.
Fig. 7. SEM image showing the even distribution of conductive
particle between the joints bonded at 170 C (bond line thickness: 0.296 lm).
3.2. Die shear test
The important characteristics governing bonding
outcome is the viscosity of the adhesive and hence its
flow property. If the viscosity of the adhesive is low only
for a short time, wetting will be far from complete and
air bubbles will easily be entrapped in the joint, which
will act as a stress raiser [18].
In the figures (Figs. 8 and 9) for shear force vs.
bonding temperature, the curves show a similar trend.
For 2/3 s MW preheating time, a smaller die shear force
is required for the specimens bonded at 150 C. The
force increases with the bonding temperature, indicates
the increase in the formation of molecular network. If
there is air bubble present, proper bonding cannot take
place due to reduction of cohesion force between the
adhesive and substrate and also between the adhesive
and die. The viscosity of an adhesive can be lowered by
heat energy (MW radiation and temperature) leading to
an increase in the degree of wetting. Again bonding at
temperature 150 and 160 C produces a thick bond line,
which generally is a weakening feature as the mechanical
strength of the unsupported resin film is likely to be less
than that of the substrates. However, this type of bond
line can, in general, be more flexible than the substrates
and is also more susceptible to failure when a pressure is
applied [18]. In Fig. 6, a thick bond line of 0.592 lm is
observed for the samples bonded at 150 C, whereas
much thinner bond thickness (0.296 lm) is observed for
819
180
173. 3 N
170
160
150
140
130
120
80 Watt
110
100
145
240 Watt
150
155
160
165
170
175
Bonding Temperature in degree celsius
Fig. 8. Variation of force required to shear the die with
bonding temperature (MW preheating time: 2 s).
Shear force (at breakage) in N
This feature is another explanation of why there is a
high contact resistance in the samples bonded at 150 C
(all samples) and 160 C (for 3 s preheating time).
Shear force (at breakage) in N
R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815–821
180
173. 3 N
170
160
150
140
130
120
80 Watt
110
100
145
240 Watt
150
155
160
165
170
175
Bonding Temperature in degree celsius
Fig. 9. Variation of force required to shear the die with
bonding temperature (MW preheating time: 3 s).
the sample bonded at 170 C which is due to the complete curing at the curing temperature. So this is another
point why the cohesive force is much lesser in the samples bonded at 150/160 C.
Fig. 10. Optical images showing various amount of air bubble affecting the bond strength.
820
R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815–821
MW radiation is a volumetric heat source, which
starts curing (or polymerization) the epoxy adhesive at
all nuclei of the material. So it takes very short time for
the curing process. That is why for high power and increased time, there is a chance of reaching a level of
polymerization beyond which further heat at identical
temperature cause little effect to the cured adhesive. In
this case the adhesive would be less viscous for a very
short time and results in improper flow of adhesive.
High MW power can heat very fast and generate voids
or trap air bubbles in the adhesive, which is very detrimental for strong adhesive joint [18]. These features are
observed with 240 W samples––the shear force at
breakage is less than that of the samples preheated at 80
W. MW preheating time of 3 s at 240 W can easily reach
such a situation and there is a significant reduction of
cohesion energy, therefore, a smaller force is required to
shear the die. Fig. 10 shows the air bubble trapped between the ACF and the substrate. At 150 C a huge
amount of air bubbles are trapped. The amount decreases with increasing bonding temperature, decreasing
preheating power and time.
4. Conclusions
MW activated bonding process has been studied in
order to reduce the final bonding temperature. It has
been successful to reduce the maximum curing temperature by 10–170 C for curing if MW preheating of
ACF is used for 2–3 s prior to final bonding. When high
bond strength is not required, 160 C curing can also be
used as final bonding temperature after the MW preheating of ACF for 2 s at very low power (80 W). The
benefits of reducing the final bonding temperature up to
10–20 C are the reduction in the operation cost and
processing time. Above 100 C, it is quite expensive to
affect a small increment of temperature and also time
consuming to reach at high temperature. So by introducing the MW preheating, the processing time and
operation cost can be minimized. Here the contact
resistance for samples bonded at 170 C and a MW
preheating of ACF is used prior to that has shown a low
contact resistance of 0.015 X (MW: 3 s at 80 W), which
is even lower than the samples bonded at 180 C without
any MW preheating. The shear force measured at
breakage in the die shear test was found 176.8 N maximum, which is higher than the standard one (173.3 N).
Optical microscopy of this joint shows a low concentration of air bubble which is the basic criteria for the
high bonding strength. The samples bonded at 160 C
for 2 s preheating time also show low contact resistance
(0.022–0.032 X), which is very close to the standard
value (0.017 X) but the bond strength is not up to the
mark due to considerable amount of air bubble
entrapment. So it can be concluded that, 80 W is the
ideal power for MW preheating and the time depends
upon the bonding temperature. For 170 C, 2–3 s would
be ideal but for 160 C, not more than 2 s preheating
time must be provided.
Acknowledgements
The authors would like to acknowledge the financial
support from Strategic Research Grant of City University of Hong Kong––Comparative Study of Thermal
and Microwave Cured Anisotropic Conductive Joints
for Liquid Crystal Display Application (project no:
7001419). Special thanks must be given to Mr. Ahmed
Sharif and Ms. Samia Islam for their endless support.
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