Issued January 1, 1999

Recent Technical Trends in Conductive Adhesives
Introduction
Three Bond has been supplying conductive adhesives for about 15 years. Most of these products take the form of paste containing silver powder. They are used for mounting quartz oscillators and semiconductors. Today, new technologies are developing very rapidly in the electricand electronic industries, creating faster and lighter products, especially in the field of information. Accordingly, electric connectivity, which is not its original purpose, is required of theadhesives.
This issue of Recent Technical Trends in Conductive Adhesives discusses conductive adhesives. The first half describes the relation between volume resistance and connection resistance.
The second half introduces two adhesives: silicone-based conductive adhesive, which is still
under development, for use in the SMD quartz oscillator, and two-part low-outgas conductive
adhesive (ThreeBond is abbreviated to TB hereafter).
Contents
Introduction
1. Conductivity theory ·················································································· 2
2. Composition of the conductive adhesives ··············································· 2
3. Conductivity······························································································ 3
4. Conductive adhesives for quartz oscillator ·············································· 4
5. Conductive adhesives for grounding························································ 6
6. Conclusion ······························································································· 8

11. Conductivity theory
There are two theories about why a conductive
adhesive is conductive. One is that conductive fillers
become connected to one another as the adhesive
contracts as it cures. The other is that an insulation
between conductive fillers breaks down because conductive fillers are scattered in the binder and are surrounded by nonconductive organic (or inorganic)
binder even after the adhesive is cured.
2. Composition of the conductive adhesives
2-1. Conductive filler and binder
A conductive filler can be composed of powder ofgold, silver, copper, nickel, aluminum, plated particles, carbon, graphite, and so on. The sizes andshapes of these differ; the typical shapes are flakes,
balls (granular), and arboroid. For most conductive
adhesives, conductive fillers of different size andshapes are blended for dense packing. Flake fillers
provide lower resistance after the adhesive is cured,
because they provide facial contacts, compared withball-shaped fillers that provide point contacts, as
shown in Fig. 1. When only flake fillers are used,however, the thixotropy ratio rises, and might reduceease of handling. To achieve an appropriate leveling,
it is better to use flaked and ball-shaped fillers
together.
The binder binds the conductive fillers, and it isused to connect the conductive fillers to one anotherwithin the curing adhesive, as well as to the adherend
when it cures and shrinks. Thus the binder has a very
important influence on conductivity.
Organic binders are the most popular binders.
Epoxy resin is common, but depending on therequired property or use, urethane, silicone, acrylic,or polyimide might be used, as well as other heatcuring resin or thermoplastic resin. As with inorganic materials, glass of low melting point is used as asecondary binder for a conductive adhesive that is
manufactured by high-temperature baking.
2-2. Conductive filler and binder
The resistance of the cured adhesive varies according to the type of binder used (see Fig. 2). There arethree factors:(1) Each binder shrinks at a different rate while curing.

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If that rate is high, conductive fillers stay closer toeach other. This makes it easier to break the insulation and thus to reduce the resistance.
(2) Each binder (resin) has different wettability forthe conductive fillers.
Wettability is affected by the polarity(hydrophilicity and hydrophobicity) of the surface of the conductive filler and the binder, andby the difference of activeness. If wettability ispoor, the binder (resin) rejects the conductive
fillers during the curing process. The conductive
fillers cannot stay close to each other, and therefore the resistance of the cured adhesive increases.
(3) Each binder has different resistance.
The conductive fillers are surrounded by insulating binders. The lower the resistance of the cured
binder, the more easily the insulation can break
down.
Flake-shapeBall-shape

Low resistancebecause of surface contact

High resistancebecause of point contact

Fig. 1. Difference of conductivity caused by thedifference of filler shapes

High resistanceLow resistance(1) Binder with little shrinkage ratio(2) Bad binder wettability with
conductive filler(3) Binder with high resistance(1) Binder with big shrinkage ratio(2) Good binder wettability with
conductive filler(3) Binder with low resistanceFig. 2. Difference of conductivity caused bydifference between binders

3. Conductivity
3-1. Volume resistanceThe conductivity of conductive adhesive is usuallydescribed by the volume resistance, and as was
explained above, the resistance of the cured adhesive
varies according to the blend. Figure 3 shows themethod to measure the volume resistance of a test
piece. In general, such an adhesive has a volume
resistance between 100 and 10-5 Ω·cm. When it isused for bonding electrodes, however, the volume isbetween 10-3 and 10-5 Ω·cm.

R = ρ · L/SLRS
Copper film
electrodeFig. 3. Method of measuring volume resistivity

3-2. Electrode and connection resistance
When electrodes are bonded with a conductive
adhesive, the resistance measured is the sum of theresistance of the conductive adhesive itself and theconnection resistance (contact resistance) between
the adherend (electrode) and the conductive adhesive. Therefore, not only the lower volume resistanceof the conductive adhesive, but also its lower connection resistance to the adherend are importantwhen it is used as the electrode adhesive.
Table 1 shows the volume resistance of conductive
adhesives for different conductive fillers. As can beseen from Table 1, silver powder is used for the conductive adhesive because it reduces the volume resistance the most. Table 2 shows the connection resistance between various boards and adhesives withvarious conductive fillers. To measure the connection resistance, a test piece was prepared (Fig. 4).
For these measurements, prototypes of conductive
adhesive were prepared with the same epoxy-based
binder, blended with the appropriate amount of pow-

Bonded area

Various substrates

Fig. 4. Method of measuring bonding resistanceder of silver, nickel, gold plating, palladium, carbon,
or other material. The results showed that theadherend and the conductive adhesives caused large
differences. Clearly, then, low volume resistancedoes not always mean low connection resistance.
For a nickel board, for example, the connection resistance with nickel paste (2.7 × 10-1 Ω·cm) was lower
than with silver paste (1.1 × 10-4 Ω·cm). The reason
for this is not yet known, but it suggests that one
main factor is the energy activating the dispersion
between metallic materials (base materials)--thecombination that is easier to make an alloy--in addition to the surface conditions of metallic boards.
On the basis of this conductivity theory, Three
Bond is developing new conductive adhesives. Twoof these new adhesives are introduced here.

Table 1. Volume resistivity (ρ) of various conductive adhesives

Volume resistivity (Ω·cm)

Silver paste

Nickel paste

Gold paste Palladium paste Carbon paste

1.1 × 10

2.7 × 10

2.1 × 10-2

-4-1

8.2 × 10-2

1.3 × 10-1

Table 2. Bonding resistance (mΩ) between various conductive adhesives and various substrates

Type of substrate

Silver paste

Nickel paste

NickelSilverGold
Aluminum
CopperTin
SolderPhosphor bronze

700
1.0
0.60
6000
0.33
4.0
34
2.9

140
2000
6.5
200
8.3
22
1800
23

Gold paste Palladium paste Carbon paste
61
1.4
0.83
1200
18
4900
1800
520

27
1.7
1.9
10000
34
1400
4200
1700

12000
900
170
0.80
3900
26000
10000
60000
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4. Conductive adhesives for quartz oscillators
4-1. Recent trends in quartz oscillators
Most Three Bond's conductive adhesives are used
to assemble quartz oscillators. Figure 5 shows the
typical structure of a quarts oscillator with a support.
Recently, as electric units have become smaller andthinner, the production of quartz oscillators of thesurface mount device (SMD) type (Fig. 6), whichhave no lead, has been increasing. One big reason
for this increase is the rapid popularization of mobile
communication equipment such as mobile phones.
4-2. Characteristics required for the adhesives
The conductive adhesives for the SMD quartz
oscillator present the following problems notencountered with the one for the support type:(1) The adhesive must not only support the quartz,
but also bond it to the base, providing the electrical connection.
(2) Because the electrode is gold-plated, and gold
does not adhere well, the adhesive has to provide
a special kind of adhesion,(3) The adhesive must be resistant to heat and disintegration. To be made airtight, it has to be
exposed to high temperature, though only for ashort time.
The adhesive, therefore, plays an important rolewith SMDs. Silicone-based and polyimide-based
conductive adhesives are the potential candidates.
Both have advantages and disadvantages. Recently,
higher frequencies are required for quartz oscillators,
and the drop test has become more severe. But thesealing condition has been moderated in terms ofheat. For this and other reasons, it is likely that thesilicone-based conductive adhesive will become themainstream adhesive for SMDs. Three Bond has
developed such an adhesive.

4-3. Silicone-based conductive adhesive underdevelopment (TB3303E)
The one-part solvent-type, silicone-based conductive adhesive that Three Bond is developing for the
SMDs has the following improvements:(i) To improve ease of handling, a solvent withlow volatility is used.
Usually, the solvent used to dissolve silicone resin
has low affinity and so is more volatile. If it is left
too long after it is applied, its wettability deteriorates.
TB3303E maintains wettability. Figure 7 shows theresults of measurements of the strength of a bond
between a ceramic chip and a sheet of glass. Chips
were coated with the adhesive, but attached only after
a period of time shown on the horizontal scale. As
the chart shows, the bonding strength after a wait of
up to 10 minutes between coating the chip andattaching it showed almost no change. Even after
longer intervals, the bonding strength did not change
substantially.
(ii) Improving adhesion to gold plating
Table 3 shows the strength of a bond between aceramic chip and a gold-plated panel. The adhesion
strength is higher than that of products of other manufacturers.
(iii)Preventing deterioration at high temperature
The effects of temperatures from 300 to 400˚C areshown in Fig. 8 for volume resistance, in Fig. 9 forbonding strength, and in Fig. 10 for outgas (weight
reduction by heating). From these results, it can beassumed that the adhesive can hold for 20 minutes at
300˚C, 10 minutes at 350˚C, and 5 minutes at 400˚C.

Table 3. Strength of chip bonding to a gold-plated board obtained with the Three Bond product andwith the vendor productDeveloped product (TB3303E)
Other vendor product

4

Unit

On a glass board

On a gold-plated board

MPa
MPa

3.6
2.0

3.4
2.0

Electrode
Blank (crystal piece)
Blank (crystal piece)
SupportElectrode
Conductive
adhesive
Conductive adhesive
Base (metal)

Base (ceramic)

3.0

Fig. 6. Structure of crystal oscillator of SMD type

Volume resistivity (×10-4Ω·cm)

Chip adhesive strength (MPa)Fig. 5. Structure of crystal oscillator of support
type

2.8
2.6
2.4
2.2
2.0
0

5

3.0
2.5
2.0
1.5
1.0
300°C
350°C
400°C

0.5
0.0

10 15 20 25 30 35 40 45

0

5

15

20

Fig. 8. Variation of volume resistivity duringhigh-temperature aging

5.0

3.0

Weight reduction by heating (%)

Fig. 7. Relation of chip adhesive strength toleaving time

4.0
3.0
2.0
300°C
350°C
400°C

1.0
0.0
0

5

10

15

25

Aging time (minutes)

Setting time (minutes)

Chip adhesive strength (MPa)

10

20

25

Aging time (minute)Fig. 9. Variation of chip adhesive strength duringhigh-temperature aging

300°C
350°C
400°C

2.8
2.6
2.4
2.2
2.0
0

5

10

15

20

25

30

35

Aging time (minute)Fig. 10. Variation of weight reduction duringhigh-temperature aging

5

5. Conductive adhesives for grounding
5-1. Recent trends of grounding
Electric products often need to be grounded.
Grounding is especially important during the assembly of small motors, HDD magnetic heads for PCs,
and VTR magnetic heads for video players. In suchelectric products, conductive adhesives can be used
not only to make electrical connections, but also to
simplify the manufacturing process. For example, anadhesive can be used for grounding as well as forsealing.

5-3. Two-part conductive adhesive under development (TB3380B)(i) Viscosity after mixing
After a two-part conductive adhesive is mixed, it
continues its reaction at room temperature. With
such an adhesive, therefore, workability is an important factor. Figure 12 shows how the viscosity
changes after the two-fluids are mixed and left at
25˚C. As chart shows, the adhesive has low initialviscosity, and even after one hour, it has viscosity
low enough to be coated with a dispenser.

5-2. Properties required of adhesives
To counteract the effects of the heating process andthe working conditions, adhesives are required tohave the following properties:(1) Improved ease of handling(2) Low curing temperature, 60 - 80˚C(3) Low outgas(4) No solvent(ii) Curing conditions and properties
Figure 13 shows the volume resistance for different
curing conditions. Figure 14 shows the bonding
strength for the chips. Table 4 shows the weight
reduction after aging at 120˚C for 30 minutes, andTable 5 shows the weight reduction after aging at
120˚C for 60 minutes.
To summarize the results, the curing time must be
2 hours or more when the curing temperature is
between 60 and 70˚C, or 1 hour if at 80˚C. The property seems to stabilize fastest at 80˚C.
Also, the amount of outgas tends to increase whenan adhesive containing solvent is cured at low temperature. That is because solvent cannot vaporize
completely when the curing temperature is not highenough, and this causes more outgassing. The twopart conductive adhesive offered by Three Bond uses
no solvent, and so is suitable for curing at low temperature.

Figure 11 shows the organization of the TB3300
series of conductive adhesives. Among these, theepoxy-based two-part conductive adhesive meets therequirements. TB3380, however, does not meet all
the requirements, and so a new epoxy-based two-part
conductive adhesive is under development.

Nonsolvent type
Reaction type

Epoxy adhesives
TB3301, TB3301B
Epoxy adhesives
Other TB3301 series

Solvent type
One-part type

Urethane adhesives
TB3302 series
Silicone adhesives
TB3303 series

Volatile solvent typeTwo-part type

Reaction type

Solvent type

Nonsolvent type

Thermoplastic adhesives
TB3350 series
Epoxy adhesives
TB3380Acrylic adhesives
TB3381

Fig. 11. Various products of silver paste

6Viscosity (Pa·s)

100
80
60
40
20
0
0

1

2

Elapsed time after mixing (hours)

Volume resistivity (×10-4Ω·cm)

Fig. 12. Viscosity variation after mixing two fluids

50
60°C
70°C
80°C

40
30
20
10
0
0

1

2

3

4

Hardening time (hours)

Chip adhesive strength (MPa)Fig. 13. Relation of volume resistivity to hardening conditions

20
15
10
60°C
70°C
80°C

5
0
0

1

2

3

4

Hardening time (hours)

Fig. 14. Relation of chip adhesive strength to hardening conditions
Table 4. Weight reduction after aging at 120˚C for
30 minutes

60˚C
70˚C
80˚C

1-hourhardening

2-hourhardening

0.41
0.36
0.22

0.25
0.24
0.13

Table 5. Weight reduction after aging at 120˚C for
60 minutes

60˚C
70˚C
80˚C

1-hourhardening

2-hourhardening

0.53
0.47
0.29

0.34
0.33
0.19
7

6. Conclusion
The requirements for conductive adhesives are
becoming more and more sophisticated nowadays,
and the adhesive is becoming an integral part of thefunctional device. Thus, each electric or electronicpart needs its own specific conductive adhesive.
Future conductive adhesives need to be developed
specifically for each application as well as for theirconventional general properties.

Three Bond Co. Ltd.
Research and Development Division

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

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