Materials Transactions, Vol. 46, No. 3 (2005) pp. 704 to 708
#2005 The Japan Institute of Metals
Effect of Solvent Evaporation and Shrink on Conductivity
of Conductive Adhesive
Woo-Ju Jeong1 , Hiroshi Nishikawa2 , Hideyuki Gotoh3 and Tadashi Takemoto4
1
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Center for Advanced Science and Innovation, Osaka University, Suita 565-0871, Japan
3
Harima chemical, Inc., Tsukuba 300-2635, Japan
4
Joining and Welding Research Institute, Osaka University, Ibaraki 567-0047, Japan
2
This paper describes the effect of solvent evaporation and shrink in conductive adhesive. The adhesion mechanism of conductive adhesive
strongly depends on the curing of the polymer matrix. The curing is preceded by polymer matrix chemical reactions, such as cross-linking,
solvent evaporation and shrink. Accordingly, it is important to understand the effect of solvent evaporation and conductive adhesive shrink in
curing. The curing behaviors and solvent evaporation of conductive adhesive were investigated using a differential scanning calorimeter (DSC)
and thermo gravimetric analysis (TGA). As curing time increases, the silver particles in the polymer are concentrated due to the incremental
solvent evaporation rate and the shrink rate. As a result, the silver particles in the polymer form an electric path. These results reveal that the
shrink rate and solvent evaporation rate increase in conductive adhesive during the curing process improved their conductivity.
(Received July 20, 2004; Accepted January 17, 2005)
Keywords: conductive adhesive, curing, shrink, solvent evaporation, electric path
1.
Introduction
The advent of environmental protection laws has lead to an
increasing interest in lead-free chip technologies. From this
standpoint, conductive adhesives have been studied actively
as a solder substitute due to their advantages, such as
miniaturization, environmental compatibility (no lead, no
flux) and lower bonding temperatures.1–5) Initially, conductive adhesives were used to bond bare silicon die and the lead
frames as part of the packaging process. Now, they are
widely applied as an interconnecting material for the
connection between a chip or die and the substrate.6–11)
Conductive adhesives are also composed of conductive metal
particles and a polymer matrix.12,13) Conductive metal
particles and the polymer matrix provide electrical and
mechanical interconnections between the chip and the
substrate.14) The polymer used in conductive adhesives is
classified as thermosets and thermoplastics. Thermosets are
cross-linked polymers and typically have an extensive threedimensional molecular structure. Cross-links are chemical
bonds occurring between polymer chains that prevent
substantial movement even at elevated temperatures. Additionally, thermoplastics are a class of polymers that can be
heated to a specific melting point or melting range without
significantly altering their intrinsic properties. The conductive metal particles used in conductive adhesives are silver,
copper or nickel. Silver is the most commonly used
conductive filler. Its most important feature is the high
conductivity of the oxide, meaning that there is almost no
change in conductivity as silver particles oxidize. Copper,
Table 1
Conductive
adhesives
A
B
Metal filler
Sphere silver
(3 mm)
which appears to be the logical choice, produces adhesives
that become non-conductive after exposure to heat and
humidity.15) Thermoset-based conductive adhesive is cured
as to a specific crosslink density, and the electric path is
formed by polymer shrink during the curing process.
Furthermore, their electrical properties depend on the class
of fillers, content of fillers, and curing conditions but also on
shape, size of fillers and packing structure of the metal
particles.16–19) However, the effects of solvent evaporation
and shrink rate should be considered due to their influence on
the conductivity. That is to say, it can be thought that they are
one of the important factors in bonding processing. In this
paper, the effect of solvent evaporation and shrink rate on
conductivity of conductive adhesives in curing is investigated.
2.
Experiment Procedure
Conductive adhesives A and B were composed of 3 mm
silver particles (Spherical type) and epoxy matrix (from
Harima Chemical Inc., Ltd.). The types of conductive
adhesive are shown in Table 1. Differential scanning calorimeter (DSC) analysis was performed with a DSC-7000M at
a heating rate of 10 K/min in order to investigate curing
profile. The solvent evaporation of conductive adhesive was
measured by thermo gravimetric analysis (TGA). An uncured
conductive adhesive was scanned at a heating rate of 10 K/
min at room temperature 573 K in an air environment. The
solvent evaporation rate for curing time was obtained with
the formula (1).
Component of conductive adhesives, in mass%.
Epoxy
Solvent
(Butyl-carbitol-acetate)
Coupling
agent
Curing
agent
93.5
1.6
4.3
0.2
0.4
89.6
4.0
5.1
0.4
0.9
Effect of Solvent Evaporation and Shrink on Conductivity of Conductive Adhesive
0.3
A
V
A
B
24mm
-2.0
Cu terminal
(5 5mm)
5mm
0.2mm
451.5K
-6.0
Schematic diagram of a test piece for four point probe method.
Wi represents weight of conductive adhesive before curing,
Wf is weight of conductive adhesive after curing, and Ws is
solvent weight in conductive adhesive before curing. The
variation of shrink rate was investigated with a Hyper-XYZ
ver.1.02. First, the height of uncured conductive adhesive
was measured and then the height of cured conductive
adhesive was measured. The shrink rate was determined from
the following eq. (2).
Shrink rate (%) ¼ ðHo Hf Þ=Ho 100
-8.0
273
ð1Þ
ð2Þ
where Ho is average height of conductive adhesive before
curing, and Hf is average height of conductive adhesive after
curing. In order to investigate the electrical resistance of
conductive adhesive, the four-point probe method was used
as shown in Fig. 1. A metal mask was placed on the FR-4
substrate, conductive adhesive (24 mm 5 mm 0:2 mm)
was pasted onto the metal mask, and curing was performed.
Micro-structures of conductive adhesive versus curing time
were examined through scanning-electron microscopy
(SEM).
373
573
473
Temperature, T / K
Fig. 3
Solvent evaporation rate, (%)
Solvent evaporation rate (%) ¼ ðWi Wf Þ=Ws 100
TG analysis of conductive adhesive (heating rate: 10 K/min).
100
A
B
98
stable
96
94
92
90
88
86
0
0.9
1.8
2.7
3.6
Curing time, t / ks
Fig. 4 Variation of solvent evaporation in conductive adhesive for curing
time.
Results and Discussion
DSC and TG analysis were applied to determine the curing
temperature of conductive adhesive. Figures 2 and 3 show
the DSC and TGA results of conductive adhesives A and B.
6.0
DSC, mV
A
B
447.8K
10
9
Shrink rate, (%)
3.
441.4K
-4.0
H0
FR-4
Fig. 1
TG, (%)
Conductive
adhesive
705
4.0
434.8K
A
B
8
7
stable
6
5
4
3
2
1
0
0
2.0
1.8
2.7
3.6
Curing time, t / ks
Fig. 5
0
273
0.9
323
373
423
473
Variation of shrink rate of conductive adhesive for curing time.
523
Temperature, T / K
Fig. 2 DSC analysis of conductive adhesive (heating rate: 10 K/min).
For conductive adhesive A, curing concluded at 435 K, and
solvent evaporation concluded at 441 K; for conductive
adhesive B, curing was completed at 448 K and the solvent
W.-J. Jeong, H. Nishikawa, H. Gotoh and T. Takemoto
(a)
Solvent and polymer
Silver particle
Electrical resistivity, r / 10 -7 Ω m
706
7
A
B
6
5
4
3
2
stable
1
0
0
0.9
1.8
2.7
3.6
Curing time, t / ks
(b)
(c)
Fig. 6 Surface micro-structure of conductive adhesive B (a) 0 ks, (b)
0.9 ks, (c) 3.6 ks.
evaporation end temperature was 452 K. Consequently, the
curing temperature was set up at 473 K, that is, higher than
the curing reaction end point and solvent evaporation end
point to improve electrical conductivity for conductive
adhesives. The relationship between solvent evaporation
and curing time is shown in Fig. 4. As curing time increased,
solvent evaporation in conductive adhesive increased. Both
Fig. 7
Relation of electrical resistance and curing time.
conductive adhesives A and B were almost entirely evaporated at a curing time of 0.9 ks. There is a remarkable
difference between A and B’s solvent evaporation of the
conductive adhesive when curing time is 0.3 ks. There was a
difference in the speed of solvent evaporation between
conductive adhesives A and B because 1) B has more solvent
than A and 2) the curing time is too short for the solvent to
completely evaporate. Figure 5 depicts the shrink rate
variation of A and B according to curing time. During the
curing process, conductive adhesive was cured through the
crosslink effect and shrunk simultaneously. Finally, the
shrink rate was shown to have a parabolic relation to the
curing time. In this study, the shrink rate of B exceeded that
of A because conductive adhesive B contains more polymer
that influence on the shrink as shown in Table 1. Figure 6
illustrates the surface micro-structure of conductive adhesive
B. The silver particles, which are enclosed in solvent and
polymer, were randomly distributed in the initial conductive
adhesive. The distribution of silver particles was constrained
by shrink and solvent evaporation of conductive adhesive
proportionally to curing time increase. Figure 7 represents
the electrical resistance variation versus curing time. As
curing time increased, electrical resistance decreased. Perhaps this occurred because the polymer could be cured by the
increase of cross-linking density, and the silver particles in
the polymer were closed due to a shrink rate increase of the
polymer and an increase of solvent evaporation (Figs. 4 and
5). The electric path in the conductive adhesive was formed
during the curing process. Accordingly, the electrical
resistance decreased. Moreover, conductive adhesive A
exhibited lower electrical resistance than conductive adhesive B. At low volume fraction of metal particles, the
possibility of generating continuous contacts is relatively
small because the metal particles are distributed randomly
throughout the polymer matrix. While at high volume
fraction of metal particles, the conductivity becomes high
due to the larger continuous contacts produced between the
particles.3,18) That is, as the number of metal particles in
conductive adhesive increases, the possibility of forming
electric paths increases. Figure 8 illustrates a magnified
Effect of Solvent Evaporation and Shrink on Conductivity of Conductive Adhesive
Solvent and Polymer
(a)
707
Silver Particle
Rd
Silver particle
Rs
Rt
Rs
Rd
Rt
a) Curing time : 0ks
Solvent and polymer
Rd
Rs
(b)
Rt
b) Curing time : 3.6ks
Fig. 9 Simplified schematic of SEM image of conductive adhesive B in
Fig. 8.
Rt ¼ Rs þ Rd þ R c
(c)
Fig. 8 Silver particle morphology in conductive adhesive B for curing time
(a) 0 ks, (b) 0.9 ks, (c) 3.6 ks.
surface micro-structure of the conductive adhesive in order to
examine the relation among particles. As the curing time
increased, the solvent that enclosed the particles in the
conductive adhesive evaporated and the solvent volume
decreased (Fig. 4). Typically, the total electrical resistance
for conductive adhesive consists of volume resistance and
contact resistance. Both resistances are functions of volume
loading of metal particles.18–21) However, the solvent and
polymer resistances could not be disregarded as factors
because they influence electrical resistance. For this reason,
total electrical resistance for conductive adhesive was
determined with the following formula:
where Rt is total electrical resistance, Rs is volume resistance,
Rd is solvent and polymer resistance, and Rc is contact
resistance. The magnification of Fig. 8 was simplified and
shown in Fig. 9 to more easily understand the effect of
solvent and polymer resistance. For initial curing time (0 ks),
the solvent remained at the interface among the particles,
preventing electrical current and producing high electrical
resistance. As the curing time increased, the solvent in the
conductive adhesive evaporated completely, and the solvent
resistance decreased. However, the polymer matrix did not
evaporate. Hence, solvent evaporation in conductive adhesive is one of the primary factors that affect electrical
resistance. Moreover, contact resistance decreased because
the distance between particles decreased as the polymer
shrunk with increasing curing time.
Total electrical resistance decreased so that:
Rt ¼ Rs þ Rd ð#Þ þ Rc ð#Þ
As shown in Figs. 8 and 9, the reduction of solvent volume in
the curing process decreased the total electrical resistance for
conductive adhesive. The rate of change of the electrical
resistance for conductive adhesives A and B stabilized after a
curing time 0.9 ks. This result is in a good agreement with
Figs. 4 and 5. The rate of change of solvent evaporation and
shrink stabilized after a curing time 0.9 ks as shown in Figs. 4
and 5. Therefore, the solvent evaporation and shrink of
conductive adhesive in the curing process are considered
important factors that affect electrical resistance.
4.
Conclusion
Conductive adhesives A and B comprised of 3 mm silver
particle were used in order to study the effects of solvent
708
W.-J. Jeong, H. Nishikawa, H. Gotoh and T. Takemoto
evaporation and shrink on characteristics of conductive
adhesive. At longer curing times, the distribution of the silver
particles was closed due to an increment of solvent
evaporation rate and shrink rate, resulting in the electrical
resistance decrease. Both solvent evaporation and shrink
were confirmed to affect electrical resistance for conductive
adhesives in this paper.
Acknowledgements
The authors would like to thank SHORAI Foundation for
Science and Technology and HARIMA CHEMICALS, Inc.
for their financial support.
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