Three Bond Technical News
Issued July 1, 2004

47

Anisotropical Conductive Adhesives
Introduction ________________________________________________
Anisotropical conductive adhesive is a high insulation adhesive with uniformly dispersed conductive
particles that is used to establish electrical connections between relative electrodes of electronic
components, isolate neighboring electrodes, and fix chips.
This functional material was first introduced onto the LCD market approximately 15 years ago.
Since then it has been improved to meet the needs for higher performance, keeping pace with thedevelopment of LCDs. This material is now expected to find new applications in other devices.
This issue provides related information and introduces recent products and technologies developedby Three Bond.

Table of contents
Introduction
1. What is anisotropical conductive adhesive? .................................................. 2
2. Applications and types of anisotropical conductive adhesives ...................... 3
3. Print-type anisotropical conductive adhesive--ThreeBond 3373.................... 4
4. Film-type anisotropical conductive adhesive--ThreeBond 3370K.................. 7
5. Future development ..................................................................................... 10

1

1.

What is anisotropical conductive
adhesive?

Anisotropical conductive adhesive is used toestablish an electrical connection between relativeelectrodes and fix them. It is also useful formaterials that cannot be fixed by soldering andthose that cannot be exposed to high temperaturesduring soldering.
Anisotropical conductive adhesive is made ofglue (binder) and will hold electrodes andconductive particles that are uniformly dispersed in
the binder. The binder is required to have adhesivity
to serve as glue, and to be sufficiently electricallyinsulative to isolate neighboring electrodes withhigh reliability. If the binder meets those basic
requirements, it can be any resin, such as synthetic
rubber, thermoplastic, or thermocure resin.
The conductive particles lie between electrodes asshown in Fig. 1. Thus, they are required to havestable conductivity between relative electrodes, anda shape and dispersed population that will not cause
them to contact with neighboring electrodes. They
can be metals (nickel or composites that are
gold-coated nickel), metal-coated plastic or resin, or
those that have conductive cores covered byinsulative skin that is broken due to pressure1) orEquivalentcircuit

Schematic description of
the resulting contact
Electrode A (R1)
Contact resistance (R4)high temperatures.2) The materials that have shapeof approximately round, and diameter from severalµm to a few tens µm are selected in accordancewith the requirements, particularly those concerning
the electrode gap.
As the anisotropical conductive adhesive, a film
type, a paste type that can be printed on electrodes,
and a liquid type that can be applied to electrodes
are available to meet different types ofmounting-process needs.
The bonding process uses heat and pressure to
spread the binder, and thereby enables the
conductive particles to contact the electrodes. With
some adhesives, ultraviolet light is used instead ofheat to cure the binder.
The resulting electrical connection has fiveresistances: resistance of electrode-A, contact
resistance between electrode A and the conductive
particle, intrinsic resistance of the particle, contact
resistance between electrode B and the particle, andresistance of electrode-B. Accordingly, resistance
between electrodes must be uniform among any
pairs of electrodes.
Thus, the materials with anisotropical conductive
adhesives are fully adjusted in order to function
according to the applications.

Assuming r1 = R21 + R31 + R41
r2 = R22 + R32 + R42
r3 = R23 + R33 + R43

Conductive filler (R3)Electrode l B (R5)

Contact resistance (R2)

BinderFigure 1. Schematic description of the electrical connection established with the anisotropicalconductive adhesive
Table 1. Anisotropical conductive adhesive application areas
Area
Compact LCD devices

Application
LCD/Printed circuit
Printed circuit/PCB

Large LCD, Plasma displays

Types of anisotropical conductive adhesives
Print type

LCD/TCP, FPC
TCP, FPC/PCB

Thermocure film type

LCD/IC (COG)
Bear-chip mounting

2

Bear chip IC/PCB

Thermocure paste type

2.

Applications and types ofanisotropical conductive adhesives

As shown in Table 1, the current anisotropicalconductive adhesive market can be roughly divided
into three categories.
The first category includes compact LCD devices,which are often used in low-cost mass-produced
products such as electronic calculators and PDAs
that have small displays for showing relatively
simple numerals and characters. Such products
normally use low-cost polyester in their printed
circuit boards. Print-type anisotropical conductive
adhesives are particularly useful for such products,
as they can be used as they are in the printing
process (Fig. 2-1).
In similar applications, they are used to makeelectrical connections in EL devices and plastic
LCD devices for making LCD components lightand durable.
The second category consists of large LCD
devices, typically notebook PCs, word processors,
LCD TV sets, and car navigation systems, all ofwhich display colorful images and a diverse range
of information. To ensure reliable, efficientelectrical connection between those LCD device

electrodes, film-type anisotropical conductive
adhesives are employed.
A few types of electrode bonding are available forsuch use. The bonding most commonly used is thejunction (Fig. 2-2, upper) between LCD display
panel and chips that are called as Tape Carrier
Package (TCP). Recently, the Chip On Glass (COG)
technology used to fix ICs directly on the frame of
the LCD display for higher portability and wider
display areas has been employed in 5" to 6"
navigation systems and TV sets. This technology isused for bonding (referred to as "Outer Lead
Bonding") between LCD chips and control chips. In
the meantime, anisotropical conductive film usedinstead of solder is also seen for bonding between
control devices and keyboard circuits (Fig. 2-2,
lower).
Also, the usage on the junctions that need highconnection current such as for PDP (Plasma Display
Panel) is accelerated. PDP is remarked as successor
display device of CRT and LCD. As for the third
category, anisotropic paste is studied as one of flip
chip mounting techniques at the inside of chips
called as BGA, PGA, and MCM (Fig. 3).

(1) [Bonding with print-type anisotropical conductive
adhesive]
LCD

PET board (FPC)
Flip chip

Anisotropical conductive
Anisotropicaladhesive
conductive
adhesive(2) [Bonding with film-type anisotropical conductive
adhesive]

Bump
BinderConductive filler

Electrode on the PCB side

LCD

Figure 3. Application of anisotropicalconductive adhesive (2)Anisotropicalconductive adhesive
LCD

Anisotropical conductive
adhesive

Driver IC

The next section introduces the anisotropicalconductive adhesive newly developed by Three
Bond for the applications described above.

Anisotropicalconductive adhesive

Figure 2. Application of anisotropicalconductive adhesive (1)33.

Print-type anisotropical conductive
adhesive--ThreeBond 3373

This type of adhesives is used in the bonding ofcompact LCD electrodes, as previously described.
This section introduces the usage for ThreeBond
3373, along with its basic properties.
3-1. Details of bonding applications
Reducing cost of basic materials and shortening
production time are among the key requirements for
the LCD-related components of electronic
calculators and similar devices. They are common
requirements for the components used for electronic
connection in LCDs. These points are similar withthe materials of the electrode connections for LCD.
Thus, low cost materials are used for the conductive
material of the printed circuit board used to drive
LCDs.
Specifically, it is not a copper etched circuit board,but a flexible printed circuit board that is made byprinting circuits with silver or carbon paste onpolyester film or similar materials. On one hand,such material is low cost, but on the other hand,heat resistance is poor. Therefore, the anisotropicalconductive materials that are able to bond at low
temperatures in a short time must be used for thisconnection. Table 2 shows the major characteristicsof the polyester film3), base board material used forthis application, and the polyimide film4) used on
the connections for large LCD display pannels.

CompanyA

Company
B

CompanyC

Conductive
paste forcircuit-forming

PET basematerial

Sub-materials
Printing processcompanyHeat-seal connector
Assemble
Device manufacturer

Figure 4. ThreeBond 3373 supply system

Table 2. Typical characteristics of polyester films and polyimide films
Item
Mechanical properties
Tensile strength (MD)Elongation (MD)Electric properties
Specific volume resistance
Surface resistance
Thermal properties
Melting point
Linear expansion coefficient
Flame resistancePrice ratio

Unit

Polyethylene terephthalate

Polyimide

MPa
%

20.6
160

39.2
30

Ω • cm
Ω

10
16
10

17

10
17
>10

°C
/°C
-

263
-5
1.5 × 10None

17500 or higher (glass transition point)-5
2.0 × 20
UL94VTM-0
1:20 or higher* Film thickness: 25 µm

Table 3. Characteristics of ThreeBond 3373 and recommended film-forming conditions
Properties

Basic properties

Recommended
film-forming
conditions

4

Product nameType
Appearance
Main composition
Conductive filler
Viscosity
Specific weightContact resistanceInter-line insulation resistancePossible minimum conductor
width
Bonding force
Printing conditions
Mesh type
Emulsion thickness
Squeegee type
Squeegee angle
Drying conditions

ThreeBond 3373
One-liquid solvent, volatile type
Pale yellow-green paste
Synthetic rubber type adhesive
Special compound powder
100Pa•s(100, 000cp)
0.98
12 Ω
9
10 Ω
0.2mm
490 N/m(500 gf/cm)
Stainless steel screen, 80 mesh
10-20 µm
Urethane rubber
30-60°
100°C × 10-20 min
120°C × 5-10 min

RemarkVisual
BH-type viscometer
No. 7 × 10 rpm
JIS6835 specific-weight cup
ITO/printed circuit
0.4-mm pitch
ITO/PET

Use a fan driven heat-dryer
furnace3-2. Heat-seal connector

the thickness of completed films is very important

As illustrated in Fig. 4, ThreeBond 3373 is apoints. Therefore, precise printing process control

paste-type anisotropical conductive adhesive and it

must be required. Final properties of completed

is formed into a film by the printing company.

products change depending on drying temperature,

The printing company buys a base film (such as

drying duration, and drying facility. If the solvent ispolyester film) from Company A as the base of theleft in the film, the heat resistance may be affected.

printed circuit board, prints conductive resin for

Also, if the drying temperature and its duration are

forming printed circuit (silver or carbon paste)

extremely high and long, its adhesivity willpurchased from Company B on the base, and prints

decrease. Table 3 shows the characteristics ofThreeBond 3373 thereon to make a complex

ThreeBondmaterial. This complex material is referred to as afilm-forming conditions.

"heat-seal connector." Then, process to connectheat-seal connector to LCD panel follows.

3373

and

the

recommended

Figure 5 is a photo of the surface of completed
film. It shows some conductive particles jutting

Because ThreeBond 3373 is used in such afrom the binder layer. The binder layer should be atcomplex material, its properties must match theleast 15-25 µm in thickness, depending on thetechnology for forming circuit. For this reason,

bonded parts, i.e. LCD panel or PCB for driving.

Three Bond needs close technological collaboration

Table 4.

with the printing company. Three Bond will

Recommended bonding conditions
for ThreeBond 3373

continue to perform sufficient follow-up activities

Pressure-bonding

for such technological collaborations.

temperature (°C)

120-160
2.9

Applied pressure (MPa)

3-3.

2

(kgf/cm )Processing key points

Following the screen-printing of ThreeBond 3373,
the solvent is dried to leave a film. In this process,Pressure bonding duration
(sec)

(30)
5-10

Conductive particle

Adhesive binder
Polyester base film

Figure 5. Cross-section of a ThreeBond 3373 print53-4. Basic properties of ThreeBond 3373

takes the method that fixes to LCD with utilizing
heat adhesiveness, but not heat melting. The
conductive particles are sufficiently large to jut
beyond the binder surface, so that they easily
make connections with the target electrodes whencollapsed with applying heat and pressure. Highreliability is thereby attained over a broadbonding conditions. Table 4 shows therecommended bonding conditions.
Figure 6 demonstrates the adhesivity ofThreeBond 3373 as proven in a reliability test, andFig. 7 shows the resulting contact resistance.

Peeling strength (N/m)

As previously discussed, ThreeBond 3373 is used
as a heat-seal connector material. Thus, it must beable to complete bonding as short as possible
because of the restrictions such that it can beconnected below the degradation temperature of thebase PET film and that it is incorporated into low
cost products. Reliability must be maintained withinthose conditions. ThreeBond 3373 has theformation that is shown in Fig. 5 previously, inorder to achieve high reliability.
Synthetic rubber that has low fluidity during heatapplied bonding is used for the binder resin. It

Test time (h)Circuit resistance (Ω)

Figure 6. ThreeBond 3373 reliability test at 60°C × 95% RH (adhesivity change)

Pressure-bonding time (sec)Test time (h)Figure 7. ThreeBond 3373 reliability test at 60°C × 95% RH (circuit resistance change)

64.

Film type anisotropical conductive
adhesive--ThreeBond 3370K

adhesive.

This section describes the fine pitch compatible,
film type thermocure anisotropical conductive
adhesive that has been developed for electricalconnections between a large LCD panel and driver
ICs on TCP. The related peripheral technology
trends are also discussed.

114-1. Requirement functions in the market(1) Change of bonding peripheral materials
The resolution is increased in large LCDs, and the
LCD glass panel is made thinner, from theconventional 1.1 mm to 0.7 mm, which also makes
it lighter in weight. As the transparent electrodematerials, not only antimony doped ITO (indium tinoxide), but aluminum or chromium coatedelectrodes are getting popular.
Some TCPs with an outer electrode pitch as
narrow as 70 µm have been introduced onto themarket. Even pitches of 20-30 µm have beenrealized in prototypes. In accordance with this trend
toward finer patterns, thickness of the copper foilthat forms TCP electrode has had a wide variety
such as 1, 3/4, and 1/2 ounces for 18µm2. Adhesives
used to bond TCP polyimide and copper foil has
been improved to satisfy the characteristics. Figure
8 shows actual cross section of the bonding of LCD
and TCP with film-type anisotropical conductive

Polyimide

Adhesive for Cu foil

Electrode (Cu/Sn plate)
Conductive filler
Transparent electrode (ITO)

Glass

Figure 8. Cross- section of the LCD/TCP
bonding and its microscopic view
Table 5 summarizes the basic characteristics ofThreeBond 3370K. The following is a further
explanation of those characteristics.

Table 5. Basic characteristics of ThreeBond 3370K
1. Film characteristics
Item
BinderConductive particle

Separator

Characteristics item
Appearances
Thickness (µm)
Width (mm)Composition
Mean particle size
Conductive plating
Insulation coating
Particle density
Base material
Peeling, separation

Characteristics
Gray (semi-transparent)
17, 23
1.5, 2.0, 2.5
Thermocure resin
50 µm
Ni/Au plating
None
2
Approx. 6400/mm
White PET
Semi-transparent OPP

Remarks
Visual
Measurement using a micrometer
30m, 50m
Epoxy resin

50 µm in thickness
20 µm in thickness

2. Bonding conditions
Item
Temporary bonding

Separation/peeling strength
Tack ability
Permanent bonding

Temperature (°C)
Pressure (MPa)Time (sec)
Peeling strength (N/m)
Peeling strength (N/m)
Temperature (°C)
Pressure (MPa)Time (sec)Condition
100 or less1 or less3 or less
2-6
3-5 (25 °C)
160-180
2-3
20-30

7

(2) Specifications of ThreeBond 3370K
1) Width, thickness, reel size
In accordance with the trend toward thinner LCD
panel frames, tapes 1.5 mm, 2.0 mm, and 2.5 mm inwidth are available. In addition, tapes 17 µm and 23µm in thickness are available to match the thicknessof the TCP chip electrodes (copper foil). ThreeBond
3370K is normally 50 m in length, and is available
in the form of a φ130 outer reel with a 1 inch inner
hook.
2) Tack ability, transparency
Tack ability and transparency are the keys to theanisotropical conductive adhesive for use in suchapplications. The adhesive, which is manufactured
as shown in Fig. 9, is required to have atransparency that allows the electrode gaps to bechecked during the manufacturing process and atack ability that allows easy temporary bonding.
ThreeBond 3370K can be set with one second at
70-80°C.
3) Curing ability
ThreeBond 3370K is made of epoxy resin and iscapable of completing curing with high reliability
under normal pressure bonding conditions of 20 sec
at 160-170°C. Figures 10-12 give an example of its
performance. ThreeBond 3370K is a highly reliablematerial, maintaining stable elasticity up to hightemperatures as shown in Fig. 13.

4) Repairability
Despite such high reliability, it must be peeled off
for repair if mal positioned electrodes are foundafter bonding. ThreeBond 3370K can be gently
peeled off if heated to approximately 180°C. It
restores thermal adhesivity to the surface of the
LCD panel by cleaning its surface withketone-based solvents.
ThreeBond 3370K setting

Printed onto the LCD glass

Tack ability isneededPeel-off separator

TCP/LCD-panel electrode matchingTransparency isneededTemporal TCP bonding

Tack ability isneededTCP/LCD-panel permanent bonding

Figure 9. ThreeBond 3370K bonding process
flow chart

Peak temperature of lost
elasticity: 132.8°C

Loss coefficient2

Elasticity (dyn/cm )Cross point of dynamic-elasticity tangent lines: 122.6°C

Dynamic elasticity
Lost elasticity
Loss coefficient

Temperature (°C)

Figure 13. Temperature dependency of dynamic elasticity, lost elasticity, and loss coefficient

8

Contact resistance (Ω)

Max.
Min.
Mean

Test time (h)Peeling strength (N/m)

Figure 10.ThreeBond 3370K reliability test results at 85°C × 90% RH (resistance change: 170°C, 20-sec
pressure bonding)

Test time (h)Pressure-bonding
temperature (20 sec)

Early stage

Figure 11. ThreeBond 3370K reliability test at 85°C × 90% RH (peeling-strength change)

Resistance (Ω)

85 °C×90%RH
100 °C-40 °C
Heat shock

Immediately afterpressure bonding

Test time (h)Figure 12. ThreeBond 3370K reliability test (interline resistance change)

9

5.

Future development

ThreeBond 3370K was developed for connections between LCD panel and TCP chips. As described earlier,
however, development for the applications of the connection between COG5), PDP, TCP chips, and input-side PCB
is going to further.

Electric Business Development Section, Development Department
Manager Akihiko Miyauchi
Eiichi Tomioka
Hiroshi Kimura
Masayuki Nagata

1456 Hazama-cho, Hachioji-shi, Tokyo 193-8533, Japan
Tel: 81-426-61-1333

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