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Effect of Solvent Evaporation and Shrink on Conductivity

of Conductive Adhesive

Woo-Ju Jeong

1

, Hiroshi Nishikawa

2

, Hideyuki Gotoh

3

and Tadashi Takemoto

4 1

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 2Center for Advanced Science and Innovation, Osaka University, Suita 565-0871, Japan

3Harima chemical, Inc., Tsukuba 300-2635, Japan

4Joining and Welding Research Institute, Osaka University, Ibaraki 567-0047, Japan

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,

conduc-tive 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 three-dimensional molecular structure. Cross-links are chemical bonds occurring between polymer chains that prevent substantial movement even at elevated temperatures. Addi-tionally, 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 conduc-tive metal particles used in conducconduc-tive 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,

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 investi-gated.

2. Experiment Procedure

[image:1.595.48.551.733.785.2]

Conductive adhesives A and B were composed of 3mm silver particles (Spherical type) and epoxy matrix (from Harima Chemical Inc., Ltd.). The types of conductive adhesive are shown in Table 1. Differential scanning calo-rimeter (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 temperature573K in an air environment. The solvent evaporation rate for curing time was obtained with the formula (1).

Table 1 Component of conductive adhesives, in mass%.

Conductive

adhesives Metal filler Epoxy

Solvent (Butyl-carbitol-acetate)

Coupling agent

Curing agent

A Sphere silver 93.5 1.6 4.3 0.2 0.4

B (3mm) 89.6 4.0 5.1 0.4 0.9

(2)

Solvent evaporation rate (%)¼ ðWiWfÞ=Ws100 ð1Þ

Wi represents weight of conductive adhesive before curing, Wf is weight of conductive adhesive after curing, andWs 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 ðHoHfÞ=Ho100 ð2Þ

where Ho is average height of conductive adhesive before

curing, andHf 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 (24mm5mm0:2mm) 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).

3. 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.

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

A

V

5mm 24mm

0.2mm Cu terminal

(5 5mm)

FR-4 Conductive

H0

adhesive

Fig. 1 Schematic diagram of a test piece for four point probe method.

A B

0 2.0 4.0 6.0

DSC

,

m

V

434.8K 447.8K

273 323 373 423 473 523

Temperature,

T

/ K

Fig. 2 DSC analysis of conductive adhesive (heating rate: 10 K/min).

573 -8.0

-6.0 -4.0 -2.0 0.3

TG, (%)

273 373 473

Temperature,

T

/ K

441.4K

451.5K

A B

Fig. 3 TG analysis of conductive adhesive (heating rate: 10 K/min).

86 90 94 98 100

96

92

88

Solv

ent e

v

apor

ation r

ate

, (%)

0.9 1.8 2.7 3.6

Curing time,

t

/ ks

stable

A B

0

Fig. 4 Variation of solvent evaporation in conductive adhesive for curing time.

0 1 2 3 4 5 6 7 8 9 10

Shr

ink r

ate

, (%)

0.9 1.8 3.6

Curing time,

t

/ ks

A B

stable

2.7 0

[image:2.595.319.532.75.254.2] [image:2.595.55.282.75.185.2] [image:2.595.317.535.287.471.2] [image:2.595.316.537.519.708.2] [image:2.595.63.278.590.767.2]
(3)

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

conductive adhesives A and B were almost entirely evapo-rated 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. Per-haps 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 adhe-sive 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

(a)

Solvent and polymer

Silver particle

(b)

(c)

Fig. 6 Surface micro-structure of conductive adhesive B (a) 0 ks, (b) 0.9 ks, (c) 3.6 ks.

Electr

ical resistivity

,

r

/ 10

-7

m

0 0.9 1.8 2.7 3.6

Curing time,

t

/ ks

stable

A B

0 1 2 3 4 5 6 7

[image:3.595.58.280.65.650.2] [image:3.595.322.534.75.262.2]
(4)

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:

Rt¼RsþRdþRc

whereRtis total electrical resistance,Rsis 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 adhe-sive 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:

Rt¼RsþRdð#Þ þRcð#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 3mmsilver particle were used in order to study the effects of solvent

Solvent and polymer

Silver particle

(b)

(a)

(c)

Fig. 8 Silver particle morphology in conductive adhesive B for curing time (a) 0 ks, (b) 0.9 ks, (c) 3.6 ks.

Rd

Rs Rt

Silver Particle

Solvent and Polymer Rd

Rs Rt

Rt Rs

Rd

a) Curing time : 0ks

b) Curing time : 3.6ks

[image:4.595.69.271.66.585.2] [image:4.595.323.532.71.320.2]
(5)

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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Figure

Table 1Component of conductive adhesives, in mass%.
Fig. 1Schematic diagram of a test piece for four point probe method.
Fig. 6Surface micro-structure of conductive adhesive B (a) 0 ks, (b)0.9 ks, (c) 3.6 ks.
Fig. 8Silver particle morphology in conductive adhesive B for curing time(a) 0 ks, (b) 0.9 ks, (c) 3.6 ks.

References

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