Materials Science and Engineering B98 (2003) 255 /264
www.elsevier.com/locate/mseb
Corrosion study of anisotropic conductive joints on polyimide flexible
circuits
C.W. Tan, Y.W. Chiu, Y.C. Chan *
Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong
Received 11 October 2002; accepted 30 January 2003
Abstract
In application, ACF joints often undergo thermal foray and since under most circumstances moisture absorption exists at the
same time, the combined effects are, therefore, important. Autoclave test was used in this study as a stimulator for stress-corrosion
cracking in the strong presence of hygroscopic swelling-induced stress. SEM/EDX analysis showed evidences of metal oxidation and
stress-corrosion cracking at ACF joints. Apart from the physical damages, results of FTIR spectroscopy showed chemical
modification to aged ACF joints under the effect of autoclave test condition. ACF joints lost their epoxy functionality after 288 h
and hydrolysis was becoming obvious after 384 h of the autoclave test.
# 2003 Published by Elsevier Science B.V.
Keywords: Anisotropic conductive joints; Autoclave test; Hydrolysis; Stress-corrosion cracking; Swelling
1. Introduction
Polymer-based conductive adhesive materials have
been vastly used in a number of interconnect applications, including direct chip attachment, i.e. chip on
glass, chip on ceramics, flip chip on board, etc. It offers
advantages in terms of reduced package size and finer
pitch, improved environmental compatibility, and lower
assembly temperature [1,2]. Anisotropic conductive
adhesive film (ACF) is by nature a composite consisting
of conducting particles that are dispersed in the thermosetting epoxy matrix. The epoxy resin is being cured to
provide mechanical and thermal interconnections and
particles are utilized for electrical connections.
Long-term performance of ACF joints can be evaluated using accelerated environmental tests. Elevated
temperature and relative humidity (RH) environments
can accelerate many failure mechanisms of ACF joints
[3 /6]. Typical tests conducted at 85 8C/85% RH can
cause chemical changes in the adhesive matrix, or at
* Corresponding author. Tel.: /852-2788-7130; fax: /852-27887579.
E-mail address: eeycchan@cityu.edu.hk (Y.C. Chan).
joining interfaces [3 /5]. This is especially true for ACF
joints with unprotected aluminum metallization.
Nowadays, many flat panel displays use aluminum
metallization as their bonding pad materials. In our
study, bumpless chips with 1 mm aluminum bump have
been used since they are cheaper compared with conventional Au bumped or Au/Ni bumped chips. Despite
this advantage, the hard oxide layer on the aluminum
surfaces becomes a challenge to the formation of stable
mechanical contacts that are electrical conductive. Besides, galvanic corrosion process is the dominant failure
mechanism in the presence of moisture and non-noble
metal interfaces. Since elevated temperature and humidity are seemed to be a threat to ACF joints, autoclave
test was chosen to simulate corrosion condition in this
study.
In general, corrosion can be defined as electrochemical reaction in a metallic field. It will occur when two
metals with different electrochemical potential are in
contact under wet condition. The less noble metal
(electrochemical potential less than 0.4 V) will act as
an anode while the metal with higher electrochemical
potential will act as a cathode. For most of the cases,
metal hydroxide will be formed and it finally becomes a
more stable form of metal oxide.
0921-5107/03/$ - see front matter # 2003 Published by Elsevier Science B.V.
doi:10.1016/S0921-5107(03)00047-3
256
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
Autoclave test is widely recognized as an accelerated
test for humidity resistance and stress corrosion cracking of non-hermetic packaged solid-state devices. It
employs severe conditions of vapor pressure, humidity
and temperature that accelerate the penetration of
moisture through the external protective material (encapsulant or seal) or along the interface between the
external protective material and the metallic conductors
that pass through it. In some studies, it was also used as
a stimulator for stress-corrosion cracking in the strong
presence of hygroscopic swelling-induced stress [6].
This paper reports the corrosion resistance of anisotropic conductive joints under autoclave test conditions
121 8C, 100%RH and 1 atm, referring to JEDEC
standard JESD22-A102-B. A better understanding of
these degradation mechanisms under corrosive environment will allow manufacturers to develop highly reliable
and low cost electronic products using ACFs flip chip
on flex (FCOF) applications.
and 1 atm. The test readout points were selected at 0, 48,
96, 144, 192, 240, 288, 336 and 384 h.
Micro-structural and chemical analyses of the crosssectioned samples were obtained by using Philips XL 40
FEG Scanning Electron Microscope (SEM) equipped
with Energy Dispersive X-ray analysis (EDX). This
equipment calculates the atomic percentage (relatively)
from each spectrum.
The structure of epoxy resins is generally determined
by means of infrared spectrometers. These instruments
measure the absorption of infrared energy as a function
of wavelength and plot an absorption spectrum for the
resin. The instrument used in this study was a Perkin /
Elmer Spectrum ONE Fourier Transform Infrared (FTIR) Spectrometer equipped with an Attenuated Total
Reflectance (ATR), and its resolution is of 4 cm 1.
Chemical and structural changes of the cured and aged
ACF samples were identified by comparing their spectra.
3. Results and discussion
2. Experimental method
The flex substrates that are used in this study are of 50
mm thick and the height of gold/electroless nickel coated
copper (Au/Ni/Cu) electrodes is about 14 mm. A
commercially available ACF that has the capability for
fine pitch on flex is used in this study. Conductive
particles with diameter of 3.5 mm (Resin/Ni/Au
plating/Insulating coating) are distributed in the adhesive matrix. Fig. 1 shows the schematic diagram of
test specimen in cross-section.
The pre-bonding process was carried out by using
Karl Suss FCM manual flip chip bonder at a temperature of 90 8C and a pressure of 0.3 MPa. Then, followed
by final bonding using Toray FC 2000 semi automatic
flip chip bonder at 200 8C and 160 N for 10 s. All
samples were submitted to the autoclave test that was
designed according to JEDEC Standard No. 22 Method
A102-B, with test conditions set at 121 8C, 100% RH
ACF joints were shear-separated at the Flex/Cu-trace
interface (refer to Fig. 1) prior to SEM examination.
The enlarged views at ACF/Cu-trace interface of these
separated surfaces are shown in Fig. 2. Initially, there
was no crack at ACF (cohesive) or ACF/Cu-trace
interface (adhesive). After samples underwent the autoclave test for 336 and 384 h, spalls were found in these
samples as shown in Fig. 2(b) and (c). Obviously, bondline cracks (adhesive) and cohesive cracks were observed
at the ACF/Cu-trace interface after 384 h of the
autoclave test. Most probably, these cracks are due to
the hygroscopic swelling-induced stress, and residue
stress induced by thermo-mechanical bonding process.
Oxide layer growth was detected at the Ni/ACF
interface from samples underwent 336 and 384 h of
the autoclave test. These oxides grew from the Cu-trace/
flex interface, which is potentially not covered by Au
coating. More oxide dendrites were observed in samples
Fig. 1. Schematic diagram of ACF joint in cross-section.
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
257
Fig. 2. SEM micrographs taken from the bottom view at the ACF/Cu
trace interface of the shear-separated Cu trace from flex after (a) 0 h,
(b) 336 h (c) 384 h in the autoclave test.
underwent autoclave test for 336 h than those underwent for 384 h. One of the possible reasons is that an
accelerated life test is basically a study on the probability for a product to fail and the failure rate is never a
linear function to the test conditions. In this study, the
probability of having oxide growth is dependent on the
water penetration rate, Cu-trace/flex interface break up
readiness, the electrochemical potential differences between Au and Ni, and etc. These oxide growths have
broken up the binding between ACF and Cu-trace, and
reduced its adhesion area, thus weakening its mechanical strength enormously.
Top surfaces of the Cu-traces on the bond pads that
had been separated after shear test are shown in Fig. 3.
The dark regions observed at these micrographs are the
compression marks induced by the conducting particles
while bonding. These exposed surfaces were examined
by using EDX as integration of all parts of the field and
Fig. 3. SEM micrographs of the top surfaces of bond pad after shear
separation for samples underwent (a) 0 h, (b) 192 h and (c) 384 h in the
autoclave test.
the results are presented in Fig. 4. The amount of
oxygen content prior to autoclave test was about 0.80
at.% only. An increase in the oxygen content (9.08 at.%)
was observed after 192 h of the autoclave test and then a
further increment in the oxygen content (25.72 at.%) was
found in samples after 384 h. These are the evidences of
oxidation at bond pads and Cu-trace in the autoclave
test.
Fig. 5 shows some examples of cracking after the
autoclave test at different interfaces in the ACF joints.
In Fig. 6, cracks or separations at Cu-trace/flex interface
are observed from samples that had undergone the
autoclave test for 0, 96, 192 h as well as 336 h and the
most severe separations were found in samples after
testing for 336 h.
258
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
Fig. 4. EDX spectra of the exposed bond pads as shown in Fig. 3.
Fig. 5. Examples of cracks observed from autoclave tested ACF joints
at (a) Die/ACF and (b) Pad/ACF interfaces (before Al bump).
Oxidation and cracks were observed in our study,
which agrees with the failure mechanism proposed by
Liu [5]. The paper concluded that oxidation in hot and
humid conditions (85 8C/85% RH) causes an increase in
electrical resistance but does not cause catastrophic
failure. The presence of crack formation must also be
involved as one of the failure mechanisms, which
explains the resistance change during the humidity test.
EDX analysis was performed on the areas located at
the ACF/Cu-trace interface of samples before (0 h) and
after underwent the autoclave test for 384 h, as shown at
location 1 and 2 in Fig. 2(a). The interaction volume
produced by a 15 kV electron beam in polymer is about
2.4 mm and that in metal is about 0.6 mm. At location 1,
the contents of carbon and oxygen were found to be
64.59 and 19.96 at.%, respectively, for samples at 384 h
and they were relatively high when comparing with
samples at 0 h of the autoclave test (C /19.09 at.% and
O /12.86 at.%). The presence of carbon and oxygen
detected at this area could be from the ACFs itself.
Similar observation was found at location 2. At this
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
259
Fig. 6. Examples of cracks observed from autoclave tested ACF joints at bond pad/flex for (a) 0 h, (b) 96 h, (c) 192 h and (d) 336 h.
location, carbon and oxygen contents for samples at 0 h
of the autoclave test were 17.76 and 10.32 at.%
correspondingly and increased to a content of 57.47
and 19.43 at.% after 384 h. Carbon and oxygen detected
at these locations reveal that the ACFs epoxy after 384 h
in autoclave test might have experienced changes in its
chemical structure.
SEM micrograph of a selected ACF joint that had
undergone 336 h of the autoclave test is shown in Fig. 7.
ACF matrix at the bond pad/ACF interface had been
degraded and was spalling like fragile materials. This
observation indicates that ACF matrix has lost its
ductility and adhesion after long hour’s exposure to
the test condition. Combined effects of hygroscopic
swelling and metal oxidation lead to reduction in
contact area of conducting particles with metallizations
and bond pads, as reported by Liu [5]. These spalling
and fragile like cracks, see Figs. 2 and 7, lead to a
possibility of degradation in the property of ACFs.
Fig. 7. Example of spalling and fragile like cracks observed at the
surrounding of bond pads from one of the autoclave tested ACF
joints.
A major problem with the use of structural adhesives
concerns the adverse effects on the mechanical properties of the joint. Notable amongst these includes
temperature extremes, stress and radiation. However,
the most severe problem is associated with the influence
of atmospheric moisture. Moisture absorption is generally concentration independent. Therefore, the diffusion of moisture into cured epoxy resin is generally
considered to obey Fick’s Law. Absorbed water acts as
an efficient plasticizer for cured epoxy resins and as a
consequence, the glass transition temperature of the
resin is reduced [7,8].
Since absorbed moisture is accommodated into the
molecular structure of the resin through dipole /dipole
and hydrogen bond interactions, this leads to swelling.
Many debates have centered on the extent of this effect
and the accommodation of water in the free or
unoccupied volume. However, matrix swelling negates
the restrained contractions, which has already occurred
during cooling process through the strain-free temperature from the post-cure temperature after fabrication.
The IR spectra of cured ACF joints at 0 h of
autoclave test, as well as that of the aged ones were
shown in Fig. 8. Changes were observed at wavenumbers 3450, 2930, 1728, 1655, 1176, and 916 cm 1, which
are corresponded to the following groups and molecules:
hydroxyl (/OH), alkyl groups ( /CH3, CH2), ester link
( /(C /O) /O), the carbonyl group (C /O), the ether links
(C /O /C), and epoxy functional group ( ) [8 /12].
After 384 h of the autoclave test, an increase in
hydroxyl group (3450 cm 1, see Fig. 9), alkyl group
(2930 cm1, see Fig. 10), and carbonyl group (1655
cm 1, see Fig. 11) together with a corresponding drop
in ester peak (1728 cm 1, see Fig. 12) supports our
contention that the material is undergoing chemical
260
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
Fig. 8. FTIR spectra of autoclave tested ACFs at different test time.
hydrolysis with increased aging of the ACF under
autoclave test condition [8]. The hydroxyl bonds could
be formed at the same time as uncrosslinking of the ester
groups occurred, which would increase the total hydroxyl group proportion in the network and, therefore,
cause the increase of the peak at 3450 cm1 [1,8,9]. On
Fig. 9. Enlarged view of changes in water absorption peak.
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
261
Fig. 10. Enlarged view of changes in alkyl group.
the exposure of cured adhesive to temperature and
moisture, both moisture absorption and further curing
can be observed. After a certain time, further curing will
not be observed, however, degradation effects may be
seen instead. Moisture degradation is believed to occur
by hydrolysis of the ester linkages (‘R /(C /O) /OR’).
Fig. 11. Enlarged view of changes in carbonyl group.
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
262
Fig. 12. Enlarged view of changes in ester peak.
Such hydrolytic attack breaks the polymer chain and
results in creating two new end groups, a hydroxyl and a
carbonyl [1], as shown in Figs. 9 and 11, respectively.
Moisture sorption effects may be reversible or irreversible, and are usually small enough to detect the
molecular changes during absorption. The decreasing of
epoxy functionalities (see Fig. 13) is the most obvious
and thus further progress of the cure reaction. Attributed to a newly formed hydroxyl groups, which are free
groups, which could be formed by further curing, or as
an oxidation product resulting from thermo-oxidative/
degradative processes [1,8,9].
In quantitative analysis, relative absorbance intensities of the molecular bonds and groups were determined.
The corresponding relative intensities of the cured
sample and the aged sample were compared. The base
line method was used to calculate the absorbance
intensities of different groups and molecules. The
relative absorbance intensities of any two peaks were
calculated as the ratio of their intensities. The relative
intensity between the reactive peak and reference peak
would be calculated as the following equation,
Er Ereactive
Eref
(1)
Since benzene ring (1508 cm 1) is the most stable
structure in the bisphenol-A network and is not likely to
react with water or oxygen, it is valid to assume that it
would not be lost or changed during aging. Therefore, it
was used as the reference peak in this quantitative
analysis. The absorbance intensities of peaks at the
above mentioned wavenumbers were calculated and the
results are listed in Table 1, which quantifies and reflects
the results shown in Figs. 9/13. The major observation
from Table 1 is the loss of epoxy functionality (916
cm 1) in the ACF joints after 288 h in the autoclave
test.
4. Conclusions
Water can exert an effect on the ACFs by chemical
modification (hydrolysis) or by physical damage (microcracking), both of which can be regarded as irreversible.
Since absorbed moisture is accommodated in the
molecular structure of the resin through dipole /dipole
and hydrogen bond interactions, this leads to hygroscopic swelling. In the presence of elevated temperature
and swelling-induced stress under corrosive environment, stress corrosion cracking was observed at ACF/
metal interface. The uncrosslinking of bisphenol-A
network could be well attributed to the breaking of
C /O bonds of the ester groups under the attack of water
and the results of FTIR analysis can be the evidences.
These chemical degradation observed in autoclave tested
ACF joints are potentially causing degradation in
mechanical strength of these ACF joints which has
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
263
Fig. 13. Enlarged view of changes in ether link, carbonyl, and epoxy functional group.
Table 1
Relative absorbance intensity of different peaks
Time (h)
0
48
96
192
288
384
Relative absorbance intensity
Hydroxyl
Alkyl
Ester
Ether
Carbonyl
Epoxy
0.629
1.377
1.544
2.392
4.135
15.909
0.912
0.759
0.726
1.363
1.478
4.222
0.122
0.056
0.061
0.059
0.039
0.016
0.623
0.695
0.716
0.597
0.641
0.571
0.595
0.855
0.871
0.856
1.151
3.253
0.212
0.021
0.020
0.016
0.000
0.000
been reported in our previous studies [13,14]. Contact
resistance of an ACF joint is dependent on the contact
area between conducting particles and metal electrodes.
Therefore, weak adhesion, and bond-line microcracking
at ACF/metal interfaces will indirectly attribute to the
undesired relatively high contact resistance.
Acknowledgements
The authors would like to acknowledge the financial
supports from Research Grants Council of Hong Kong,
Cooperative Research Center on Conductive Adhesive
Technology for High Density Electronic Packaging
(Project no. 8720003) and grant from Innovation
Technology Commission, The Government of the
Hong Kong Special Administrative Region (ITS/182/
00), Conductive Adhesive Technology Programme for
Fine Pitch
9440006).
Electronic
Interconnect
(Project
no.
References
[1] J. Liu, P. Lundstrom, in: J. Liu (Ed.), Conductive Adhesive For
Electronics Packaging, 1999, pp. 212 /253.
[2] C.P. Yeh, K.W. Wyatt, K.X. Hu, IEEE Trans. Component
Packaging Manufact. Technol. Part A 20 (4) (1997) 470 /477.
[3] P. Palm, J. Maattanen, A. Tuominen, E. Ristolainen, Microelectr.
Reliab. 41 (2001) 633 /638.
[4] R.D. Burg, J.H. Constable, J.J. Decker, J.W. Weidler, Electrical
characteristics of an ACF bond as a function of temperature and
humidity aging. Proceedings of IEEE Electronic Component and
Technology Conference, 2000, pp. 906 /913.
[5] J. Liu, Int. J. Adhesion Adhesives 16 (4) (1996) 285 /287.
[6] E.H. Wong, K.C. Chan, R. Rajoo, T.B. Lim, The mechanics and
impact of hygroscopic swelling of polymeric materials in electronic packaging. Proceedings of IEEE Electronic Component and
Technology Conference, 2000, pp. 576 /580.
264
C.W. Tan et al. / Materials Science and Engineering B98 (2003) 255 /264
[7] B. Ellis, Chemistry and Technology of Epoxy Resins, Chapman &
Hall, UK, 1993.
[8] C.E. Park, B.J. Han, B.E. Bair, Polymer 38 (15) (1997) 3811 /
3818.
[9] B. Gao, J. Unsworth, Y. Li, Degradation of epoxy insulation
under combined high humidity and elevated temperature. Proceedings of the Fourth IEEE International Conference on
Properties and Applications of Dielectric Materials, 1994, pp.
752 /755.
[10] M. Sun, Microelectr. Reliab. 32 (2001) 197 /203.
[11] F. Fraga, S. Burgo, E.R. Nunez, J. Appl. Polym. Sci. 82 (2001)
3366 /3372.
[12] R.M. Silverstein, F.X. Webster, Spectrometric Identification of
Organic Compounds, sixth ed, Wiley, Canada, 1998.
[13] C.W. Tan, Y.C. Chan, N.H. Yeung, Effect of autoclave test on
anisotropic conductive joints, Microelectr. Reliab. 43 (2003) 279 /
285.
[14] C.W. Tan, Y.C. Chan, N.H. Yeung, Behaviour of anisotropic
conductive joints under mechanical loading’, Microelectr. Reliab.
43 (3) (2002) 481 /486.