Materials Transactions, Vol. 51, No. 12 (2010) pp. 2329 to 2331
#2010 The Japan Institute of Metals
RAPID PUBLICATION
Characteristics of Isotropically Conductive Adhesive (ICA) Filled
with Carbon Nanotubes (CNTs) and Low-Melting-Point Alloy Fillers
Byung-Seung Yim and Jong-Min Kim*
School of Mechanical Engineering, Chung-Ang University, Seoul 156-756, Korea
A new class of isotropic conductive adhesive (ICA) using a carbon nanotubes (CNTs) and a low-melting-point alloy (LMPA) fillers has
been developed. We investigated the fundamental materials characteristics including curing behavior and temperature-dependant viscous
property of ICA. In addition, the morphology of conduction paths in each ICA was investigated using X-ray inspection systems and an optical
microscope. Mechanical and electrical characteristics of formulated ICAs were determined and compared with those of three kinds of
conventional ICAs filled with Ag flakes. The CNT-filled solderable ICA formed good metallurgical interconnection between upper and
corresponding lower electrode. In addition, the results indicated that the CNT-filled ICA exhibit lower electrical resistance and higher
mechanical strength, as compared with those of conventional ICAs. [doi:10.2320/matertrans.M2010301]
(Received September 6, 2010; Accepted October 6, 2010; Published November 17, 2010)
Keywords: carbon nanotuubes (CNTs), isotropically conductive adhesive (ICA), low-melting-poing alloy, polymeric composites
1.
Introduction
In recent years, conductive filler-filled polymeric composites have been used for a wide range of applications due
to their versatile properties including thermal stability,
mechanical strength, electrical resistance, and adhesive
characteristics.1,2) Among these composites, isotropically
conductive adhesives (ICAs) filled with organic or inorganic
fillers have been investigated as a lead-free alternative in
microelectronic packaging.3,4) In addition to the environmental issue, they have attracted attention due to advantages
including lower temperature processing, more flexible and
simpler processing, and compatibility with nonsolderable
materials.5,6) An ICA is mainly composed of a polymer
matrix and electrically conductive fillers. ICAs offer alldirectional conductivity by incorporating a higher filler
concentration (typically 25 to 30%).4) Silver (Ag) flake is the
most common conductive filler for ICAs due to its high
conductivity and easy processing. However, a conventional
ICAs has several critical limitations such as lower conductivity, unstable contact resistance with non-noble metal
finished components, Ag migration, and poor mechanical
performance compared to traditional solder materials.7–9)
To improve the mechanical and electrical properties of ICA
assembly, a new class of ICAs using carbon nanotubes
(CNTs) and low melting point alloys (LMPAs) has been
developed. As shown in Fig. 1, an electrical conduction path
is mainly established by the rheology-coalescence-wetting
behaviors of molten LMPA fillers in an ICA during the
curing process, and the shear strength of the assembly is
reinforced by the CNTs.
In this study, a reinforced ICA using CNTs and a LMPA is
considered. The influence of CNTs on the mechanical and
electrical properties of the assembly was investigated and
compared to the mechanical and electrical properties of
commercial ICAs.
*Corresponding
author, E-mail: 0326kjm@cau.ac.kr
(a)
(b)
(c)
(d)
Fig. 1
2.
Schematic of CNT-filled solderable ICA bonding.
Experimental
The proposed ICA consists of CNTs, a LMPA, a thermoset
polymer resin, and other minor organic additives. Multiwalled CNTs (MWCNTs) with diameters ranging from 15
to 40 nm and lengths ranging from 30 to 50 mm (Hanwha
Nanotech Co. Ltd., Korea) were used. Sn-58Bi LMPA filler
with a diameter of 45 mm (Senju Metal Co., Japan) was used.
The polymer matrix was formulated with diglycidyl ether
of bisphenol-A (DGEBA) for the binder, diaminodiphenyl
sulfone (DDS) for the curing agent, and an amine-type
catalyst. Carboxylic acid was used to remove the oxide layer
from the surface of the LMPA fillers and the electrodes. The
CNTs were functionalized using carboxylic acid.10) Functionalized MWCNTs and LMPA particles were added into
the polymer matrix and sonicated to produce a homogeneous
CNT-filled solderable ICA. To compare mechanical and
electrical properties, two kinds of ICAs were formulated:
ICA 1 (without CNT and reductant), and ICA 2 (with CNT
and reductant). Also, three types of conventional ICAs with
Ag flakes were used to compare the mechanical and electrical
2330
B.-S. Yim and J.-M. Kim
Table 1 Components of ICA formulations.
ECA
Polymer
Filler
ICA 1
Epoxy-type
(W/O reduction Capa.)
Sn-58Bi
ICA 2
Epoxy-type
(With reduction Capa.)
Sn-58Bi (40 vol%) +
MWCNTs (0.05 wt%)
ICA 3
Epoxy-type
ICA 4
ICA 5
Ag flake
Polyester-type
properties of assemblies. The ICA formulations are shown in
Table 1. To test adhesive bonding, a Quad Flat Package
(QFP) with 44-pin I/O terminals plated with Sn (14 14 2:7 mm3 in size, and 1.0 mm in lead pitch) and an
18 mm-thick Cu-plated printed circuit board (PCB, 32 32 1:0 mm3 ) were used. The curing behavior of the
polymer and the melting behavior of the LMPA filler were
monitored using differential scanning calorimetry (DSC).
The temperature-dependant viscosity of the formulated
polymer matrix was investigated using a torsional parallel
rheometer. The substrate was cleaned with acetone for 1 min,
rinsed with deionized (DI) water, and dried with an air jet.
ICAs were selectively applied on the QFP leads. The QFP
was then aligned on the PCB, and adhesive bonding was
performed using a chip bonder (Lambda, Finetech Co.,
German). After the adhesive bonding, the electrical properties were investigated through a daisy chain-connected probe
terminal using a multimeter (34410A, Agilent Tech., USA).
The bonding characteristics of the LMPA fillers in each ICA
assembly were observed using an X-ray inspection system
(SMX-160, Shimadzu Co., Japan). In addition, the morphology of the conduction path was observed using an optical
microscope (VHX-100, Keyence Co., Japan). To observe the
mechanical strength of each ICA joint, a 45 degree pull test
(JIS Z 3198-6) on the QFP lead was performed. The QFP
lead was pulled upward at 6 mm/min, and a total of 22 leads
were pull-tested for each ICA assembly.
3.
Fig. 2
Curing behavior of polymer matrix and melting behavior of LMPA.
Results and Discussion
Figure 2 shows the DSC results of the curing behavior of
the polymer matrix and the melting behavior of the LMPA.
The peak curing peak temperature of the polymer matrix
was about 427 K, and the endothermic peak at around 414 K
indicates the melting point of the LMPA. The viscosity of
the polymer matrix was about 100 cPs around the melting
point of the LMPA. From our previous work, we knew that
the ICA should not cure too much around the melting
point of the LMPA in order to achieve a good metallurgical
interconnection between the upper and corresponding
electrode. Otherwise, the flow-coalescence-wetting behavior
of LMPA fillers in the ICA during the adhesive bonding
process is hindered by excessive curing of the ICA.11,12)
As shown in the results, the polymer matrix does not
excessively. In addition, the polymer matrix was maintained
at a sufficiently low viscosity for the molten LMPA fillers to
flow, coalesce with each other, and wet the upper and lower
electrodes.
Fig. 3 Morphology of the conduction path formed between metallizations
of QFP and PCB substrate by (a) ICA 1 and (b) ICA 2.
Figure 3 shows the morphology of the conduction path of
each formulated ICA. The interconnected parts were divided
into two distinct regions. One is a conductive path made
by melted fillers, and the other is a polymer wrapped around
the outside of the conduction path. The white dotted line
in Fig. 3 shows the cured polymer region.
Characteristics of Isotropically Conductive Adhesive (ICA) Filled with Carbon Nanotubes (CNTs) and Low-Melting-Point Alloy Fillers
25
Solderable ECA without reductant & CNT (ICA 1)
Solderable ECA with reductant & CNT (ICA 2)
Conventional ICAs (ICA 3, ICA 4, ICA 5)
1000
100
42.45
40.00
43.63
10
0.99
1
0.1
ICA 1
ICA 2
ICA 3
ICA 4
ICA 5
Pull Strength, S / N
Contact Resistance, R / mΩ
10000
Solderable ECA without reductant & CNT (ICA 1)
Solderable ECA with reductant & CNT (ICA 2)
Conventional ICAs (ICA 3, ICA 4, ICA 5)
20
Pull Speed: 6mm/min
14.93
15
10
6.91
5.35
5
Electrical resistances of QFP assemblies.
As for non-reductive ICA 1, the wetting of the LMPA
fillers on the surface of the metallization of the substrate was
not remarkable, and a large quantity of LMPA fillers
remained. As for CNT-incorporated ICA 2 with reduction
capability, the LMPA fillers were mostly wet and metallurgically interconnected between the metallizations of the
QFP lead and substrate, as shown in Fig. 3(b). The spherical
LMPA fillers remained outside the interconnection region.
Since the remaining LMPA fillers were covered with the
polymer matrix, the QFP assembly maintained its insulation
properties between adjacent leads.
Figure 4 shows the electrical resistance per joint of the
QFP assemblies in which three commercial Ag-filled
ICAs were incorporated. Although ICA 1 without reduction
capability did not form any electrical connection, the CNTincorporated ICA 2 with reduction capability showed much
lower electrical resistance (0.99 m) than those of other
ICAs (42.45 m for ICA 3, 40.00 m for ICA 4, and
43.63 m for ICA 5). This was due to a stable electrical
conduction path caused by sufficient wetting, and the
coalescence behavior of the LMPA fillers in ICA 2 during
the curing process.
Figure 5 shows the results of the pull test of the QFP
assemblies with each ICA. The QFP assembled with CNTincorporated ICA 2 exhibited excellent pull strength properties. Its pull strength was 14.93 N, which was more than three
times greater than the QFP assemblies with conventional
Ag-filled ICAs (ICA 3, ICA 4, and ICA 5). This was due to
the robust interconnection achieved through metallurgical
bonding between the metallizations of the chip and substrate.
Conclusion
A novel ICA filled with CNT and LMPA fillers was
developed. The morphology of the achieved conduction path,
which was formed by the flow-coalescence-wetting behavior
of LMPA fillers in the ICA, was investigated experimentally.
4.04
4.59
ICA 4
ICA 5
0
ICA 1
Fig. 4
4.
2331
ICA 2
ICA 3
Fig. 5 Pull strengths of QFP assemblies.
When the reduction capability was introduced into the ICA,
a stable metallurgical interconnection was formed between
metallizations of the chip and substrate by totally wetted and
coalesced LMPA fillers in the ICA.
Due to the stable and robust metallurgical interconnection
in the ICA, the CNT-incorporated ICA 2 with reduction
capability showed much lower electrical resistance and
higher pull strength than conventional ICAs.
Acknowledgement
This research was supported through Chung-Ang University Research Grants in 2009.
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