Electrically Conductive Adhesive Filled with Mixture of Silver Nano and Microparticles

Pavel MachI, Radoslav RadevI, Alena Pietrikova2

ICzech Technical University in Prague, Faculty of Electrical EngineeringTechnicka 2, 16627 Prague 6, Czech Republic

Email:mach@fel.cvut.cz, Phone: ++42022435 2214, Fax: ++4202 2436 39492Technical University ofKosice, Faculty ofElectrical Engineering and Informatics

Letna 9, 04200 Kosice, Slovak RepublicEmail: Alena.Pietrikova@tuke.sk, Phone: ++421 55 6023194, Fax: ++421 55 602 3195

AbstractElectrically conductive adhesive with isotropical

electrical conductivity modified with addition of silvernanoparticles has been investigated. The electricalresistance, nonlinearity of a current vs. voltagecharacteristic and the tensile strength of adhesive jointsformed of this adhesive have been measured. Thespecimens have also been aged at the temperature of125°C and at the combined climate 80 °C/80 % relativehumidity for 700 hours. It has been found that silvernanoparticles added into the electrically conductiveadhesive cause decrease of its conductivity, increase of itsnonlinearity and increase of the tensile strength.

IntroductionThe resistance of adhesive joints formed of

electrically conductive adhesives with isotropicalelectrical conductivity is higher in comparison with theresistance of soldered joints. The ratio of conductivity ofadhesive vs. soldered joints is mostly higher than 1:5 orhigher. The reason is that electrical conductivity of solderis dominantly influenced with a phonon-electroninteraction, whereas electrical conductivity of adhesive isdominantly influenced with tunneling of electrons acrossbarriers between conductive particles [1]. Electricalconductivity of soldered joints is also improved usingdifferent types of fluxes; electrically conductiveadhesives are applied directly between a pad and a lead ofa component without some additive substance improvingcontact quality.

There are different ways, which have been tested toimprove electrical and mechanical properties of adhesivejoints. The use of nanoparticles is not a new idea.Analysis of conductivity of electrically conductiveadhesives filled with lead free solder nanoparticles ispresented in [2]. Properties of electrically conductiveadhesives filled with silver coated carbon nanotubes havebeen investigated in [3]. Effect of silver nanowires onelectrical conductance of system composed of silverparticles has been described in [4]. The resistivity ofpolymeric conductive adhesives filled with nano-sizedsilver particles has been investigated in [5].

Basic parameters used for description of quality ofadhesive joints are the joint resistance and the tensilestrength, usually. We have added these measurementswith the measurement of nonlinearity of a current vs.voltage characteristic.

With respect to the structure of electrically conductiveadhesives with isotropical electrical conductivity thedominant conductive mechanism is tunneling betweenneighboring filler particles and tunneling between theseparticles and a pad on PCB or a lead of a component.Tunneling mechanism is nonlinear and therefore themeasurement of the adhesive joint current vs. voltagecharacteristic nonlinearity can be useful tool forevaluation of joint quality. Influence of addition of silvernanoparticles into the standard type adhesive on the jointresistance has been already investigated. Knowledge ofsuch information can be interesting with respect to thepossible application of adhesives in a high frequencyrange, where joint nonlinearity can cause problemsfollowing of generation of intermodulatoin signals onsuch nonlinearity.

1. Theoretical BackgroundThe total electrical resistance of an adhesive joint

consists of following components:• Electrical resistance of filler particles.• Electrical resistance of contacts between filler

particles.• Electrical resistance of a contact between filler

particles and a pad on a PCB.• Electrical resistance of a contact between filler

particles and a lead of a component.The value of the electrical resistance of filler particles

is substantially lower in comparison with the value of thecontact resistance between filler particles and betweenfiller particles and a pad or a lead of a component if Agfiller particles are used. If the filler particles are plasticballs covered with a thin conductive metal film, the fillerparticle resistance can be comparable with the contactresistance. However, such the particles are used foradhesives with anisotropical electrical conductivity onlyand we have tested adhesive with isotropical conductivityfilled with Ag flakes.

The contact resistance between two particles of thefiller consists of following parts:

• The constriction resistance.• The tunnel resistance.The total contact resistance can be calculated as a sum

of these two components. The reason of the constrictionresistance is as follows: let us assume that twosemiinfmity conductive segments are mutually connectedby only a small circle contact with the diameter of 2a. Inthe area of a contact the current lines taper and the result

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The tunnel resistivity can be calculated using aformula:

of this effect is an increase of the resistance. If theconductive segments have the same area and if they arefabricated of the same material or of materials with thesame resistivity p, the constriction resistance Rc can bewritten as:

Rc = l!- (1)2a

This formula has been derived for a small circlecontact with the diameter 2a joining two infmity bodies;however, it is also possible to use it for calculation of theconstriction resistance between two flakes with sufficientaccuracy [1].

The second component of the contact resistance is thetunnel resistance. This resistance depends strongly on thethickness and quality of an insulating barrier. The tunnelresistance Rr is calculated using the tunnel resistivityu(nm2

). The resistance of a tunnel contact with thediameter of2a can be calculated as follows:

Where: A ... contact area between flakes.The equations (1) to (7) describe properties of

contacts between conductive balls and betweenconductive flakes used as filler of adhesive. If the totaladhesive joint resistance should be calculated, theconductive net formed of filler particles has to besimulated, and the contacts in contact points are describedusing equations (6) or (7). The resistance of fillerparticles is small in comparison with the cont~ct

resistance and therefore it can be neglected for calculatIonof the total resistance.

If conductive nanoparticles (grains or balls) are addedinto electrically conductive adhesive with isotropicalelectrical conductivity, they can be distributed amongfiller particles of adhesive as follows:

• The nanoparticles are not aggregated and arerandomly located between flakes.

• The nanoparticles are partially aggregated andcreate additional conductive bridges betweenflakes.

The total electrical resistance of the tunnel jointformed of electrically conductive adhesive withisotropical conductivity, where flakes are used as thefiller, is dominantly influenced by the tunnel resistancebetween flakes. Obviously, the tunnel resistance betweenflakes and pad and between flakes and component leadare also significant. When nanoparticles are added intothe adhesive and are not aggregated, they are randomlydistributed between flakes. The total number of contactsin conductive net increases and the total resistance of theconductive net too.

On the other hand, if the nanoparticles, after additioninto electrically conductive adhesive, are able toaggregate and to create additional conductive bridges,parallel with contacts between flakes, the joint resistancedecreases.

Different theories have been developed for descriptionof nonlinearity of a current vs. voltage characteristic.Theory of nonlinearity, based on an assumption thatnonlinearity of current vs. voltage characteristic isdominantly influenced by thermal movement of vacanciesinside the material has been presented Zhigalsky [6].

Other theory of nonlinearity is based on anassumption that nonlinearity of current vs. voltagecharacteristic is dominantly caused by potential barriersinside the material. According to this theory the currentflowing through the potential barrier can be calculatedusing the equation:

and the total contact resistance can be described with thetunnel resistance only.

R= RT=~ (7)

A

(2)

(4)

(5)

(3)

-6RfOB = 1, 265 .10 <I> - -s£r

5 ( 7,2)A=7,32·10 s-~

10-22 A 2AB

a =a(s, <l>,cr ) =-2-1+ AB e

Where: s ... thickness of a tunnel barrier, (/J ... outputwork of an electron from a metal, Gr ••• dielectric constantof the tunnel barrier material.

The total contact resistance between filler particles isa sum of the constriction and the tunnel resistance.

R= Rc +RT =.£!....+~ (6)2a 1l.a

The next part of the contact resistance is the resistancebetween the filler particles and the pad or lead of acomponent. Quality of these contacts depends on thematerials of the filler particles and the pad or lead. If thepad or lead are fabricated, or if their surface finish is, .ofsuch metals as eu, Sn, Ni or Pb or of other metals wIthlow electrochemical potential, the contact between thesemetals and metals with the high electrochemical potentialsuch as Ag, Au or Pt can create a galvanic a voltaic cell ifwater is present. This cell can cause electrochemicalcorrosion, which decreases quality of the contact.Therefore it is recommended to use for surface finish ofpads or leads of components a metal with similarelectrochemical potential like Ag.

For contact between flakes it is hard to calculate acontact area. However, it is assumed that the contact areasare larger than those between balls. This assumption canbe confIrmed using SEM analyzis. Therefore theconstriction resistance between flakes can be neglected

11422nd Electronics Systemintegration Technology Conference

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Formulation

Fig. 1 Measurement of the tensile strength of adhesive joints

Movingplug

Adhesive

Jumper

82 83 84 85 86 87

FR4

Cu pad

100

90

80

70

a- 60

.§, 50

~ 40

30

20

10

0801 81

5. Results and discussionThe starting values of the resistance, nonlinearity of

the current vs. voltage characteristic and tensile strengthof adhesive joints are shown in Fig. 2, Fig. 3 and Fig. 4.

Fig. 2 Resistance of adhesive joints formed of modifiedadhesives, SO 1 is the basic adhesive used for modification

Changes of the resistance, nonlinearity and tensilestrength of the adhesive joints after thermal ageing at thetemperature of 125°C for 700 hours are shown in Fig. 5,Fig. 6 and Fig. 7.

Nonlinearity of the current vs. voltage characteristic ofadhesive joints has been measured using equipmentdesigned and realized at the Department ofElectrotechnology. The principle of the measurement isthe evaluation of a level of intermodulation productsgenerated by nonlinearity of the adhesive joint, which ispowered with two sinusoidal signals with differentfrequencies.

The tensile strength has been measured using a pull­off test. KERN MHI0KIO digital tester has been used forthis measurement. The arrangement for the measurementof the tensile strength is shown in Fig. I.

Following climatic load has been applied on adhesivejoints formed of the samples of modified adhesivemarked S1 to S3:

• Temperature 125°C for 700 hours.• Combined climatic load 80 °C/80% RH for 700

hours.

Nanoparticles Concent-Sample dimensions ration Processing

#(nm) 0/0 (wt.)

81 (3 - 55) 10 U)"C CD:.! £

(3 - 55) 20~:::s.c't:

82 .!!~~ns'I- '1i) ~ c.en .c 0c(:::s c:

83 (3 - 55) 30 U) nsc:

S4 (80 - 100) 3,8 "CCD"C"C

3,8ns

85 (6- 8) U)CDU

86 (80 - 100) 7,4 :ensc.0

(6 - 8) 7,4c:

87 nsz

Tab. 2 Modifications of the basic adhesive undertest. Basic adhesive: leA, bisphenoi-epoxy resin,

silver flakes 6 to 8 J.lm, 75 % wt.

4. Measurement of electrical and mechanicalparameters

The electrical resistance has been measured using aMCP TH 2818 Automatic Component Analyzer.

Where NT ... total number of barriers, A ... constantdependent on the material, e ... charge of an electron,<I>i ... high of the potential barrier, k ... Boltzmannconstant, T ... temperature, U ... voltage across the joint.

3. ExperimentalAdhesive bonds have been formed by adhesive

assembly of resistors with zero resistance Gumpers, 1206ORO for adhesive assembly) on test boards (FR4, Cu40 Jlm).

One-component isotropically electrically conductiveadhesive composed of bisphenol-epoxy resin and silverflakes (dimensions 6 to 8 ~m) has been used as the basicadhesive for experiments. Concentration of silver flakesin this adhesive has been 75 % (wt.). This adhesive hasbeen modified by addition of silver nanoparticles.

Nanoparticles have been grains of 3 dimensions: 3 to55 nm, 6 to 8 nm and 80 to 100 nm.

Addition of nanoparticles into the basic adhesive hasbeen carried out by following ways:

• A part of silver flakes has been substituted withnanoparticles to preserve the constant contentsof silver in adhesive.

• The total amount of silver in adhesive has beenincreased by addition of nanoparticles,

Following modifications of the basic adhesive havebeen used for experiments:

Sets of 42 adhesive joints have been formed of everysample of modified adhesive and the same number ofadhesive joints has been formed of the basic adhesive tohave a standard for investigation of properties changescaused with addition of nanoparticles.

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801 81 82 83 S4 85 86 87

Formulation

15

10~0

CD 50)c::ca 0.c(,)

~ -5'CcaCD.5 -10'E0z -15

-20801 81 82 83

Formulation

120

~0CD 100C)c::ca

80.t:.(,)

.t:.'& 60c::CD'-1;) 40

J!"iii 20t:CDt-

o801 81 82 83

the relative humidity of 80 % for 700 hours are shown inFig. 8, Fig. 9 and Fig. 10.

The increase of the joints resistance after addition ofsilver nanoparticles into the adhesive (Fig. 2) is caused byadditional contacts, which are created by nanoparticles inconductive net of adhesive. When the same weight ofnanoparticles is added, the frequency of these contacts isthe higher, the lower are dimensions ofnanoparticles.

Therefore the increase of the resistance is higher fornanoparticles of lower dimensions (see formulations 85and 87 in Fig. 2).

These "new" contacts have, as for the resistance, aconstriction part and a tunnelling part, because they arecreated by particles of small dimensions. Both these partsare nonlinear. Therefore addition of nanoparticles causesmostly increase of the joint nonlinearity (see Fig. 3).

Exceptions (modifications 81 and 84) are sampleswith a small amount of nanoparticles added and thenonlinearity decrease can be joined with the decrease ofthe resistivity in comparison with the basic adhesive (see

Fig. 6 Nonlinearity change of adhesive joints formed ofmodified adhesives after thermal ageing (] 25 °C~ 700 hours).SO1 is the basic adhesive used for modification

Formulation

Fig. 7 Tensile strength change of modified adhesives afterthermal ageing (] 25 °C~ 700 hours)~ SO] is the basic adhesive

used for modification

53

5382

Formulation

51 52

Formulation

81

501

801o

12

o

10~

0.05

T(MPa)

18

16

I14

12

I10

8 I6

4

2

0

8» 8c::ca.c(,) 6CD(,)c::~ 4.;

&. 2

0.3

0.25

0.2

>"..- 0.15::;)

0.1

Fig. 3 Nonlinearity of adhesive joints formed of modifiedadhesives~ SO 1 is the basic adhesive used for modification

Fig. 4 Tensile strength of modified adhesives. SO] is the basicadhesive used for modification

Fig. 5 Resistance change of adhesive joints formed ofmodified adhesives after thermal ageing (] 25 °C~ 700 hours).SO] is the basic adhesive used for modification

Changes of the resistance, nonlinearity and tensilestrength of the adhesive joints after combinedthermal/humidity ageing at the temperature of 80°C and

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Formulation

Fig. 9 Nonlinearity change of adhesive joints formed ofmodified adhesives after combined thermal/humidity ageing(80 °C~ 80 % RH~ 700 hours). SO1 is the basic adhesive usedfor modification

(I)80

0»c: 60co

.s::.u 40.s::.0,_

!~ 20U; 0J!·iii

-20c:(I)...

-40801 81 82 83

Formulation

This damage has been caused by oxidation or otherchemical process. It has been found (see Fig. 5) that thehigher is the contents of nanoparticles, the higher isincrease of nonlinearity after thermal ageing. The reasonis the increase of the total number of contacts in theconductive net after adhesive modification.

Nonlinearity change of the adhesive joints afterthermal ageing is negative for the basic adhesive andnegative and positive for different formulations with thenanoparticles (Fig. 6). Such the result can be explained bydifferent conductivity mechanisms, which dominate indifferent formulations after the thermal ageing.

Increase of the tensile strength for all measuredformulations (Fig. 7) is caused with additional hardeningof the adhesive during thermal ageing.

Positive influence of nanoparticles, added into theelectrically conductive adhesive, on the resistance andnonlinearity of adhesive, has been found for combinedclimatic ageing. It has been detected lower increase of theresistance (Fig. 8) and significantly lower increase ofnonlinearity (Fig. 9) after combined thermal/humidityageing for the samples with nanoparticles in comparisonwith the samples without them. The reason is that thecontacts between nanoparticles have lower dimensions incomparison with the contacts between flakes. Thereforethe contact pressure is higher for these contacts and theyare more resistive against damage with chemical layercreated as a consequence of combined climatic ageing.

Tensile strength of the formulation S1 has decreasedafter combined climatic ageing (but this decrease hasbeen lower in comparison with the decrease of the tensilestrength of the basic adhesive), whereas the tensilestrength of formulations S2 and S3 has increased (see Fig.10). The reason is as follows: the adhesive have beenadditionally hardened during this type of ageing and thebonds between the nanoparticles and adhesive arestronger in comparison with the bonds between flakes andthe adhesive. Therefore the penetration of watermolecules into the adhesive is more limited informulations with nanoparticles than in the standardadhesive.

Fig. 10 Tensile strength change of modified adhesives aftercombined thermal/humidity ageing (80 °C~ 80 % RH~ 700hours). SO1 is the basic adhesive used for modification

83Formulation

Aones Ive sam pie

801o

60

100

90

~ 800

G) 70enc 60co.c

50uG)u 40ccoU) 30'enG) 200:::

10

0

801 81 82 83

180

160

~ 140G)

~ 120co"5 100

:E 80coG)

.=~ 40z

20

Fig. 8 Resistance change of adhesive joints formed ofmodified adhesives after combined thermal/humidity ageing(80 °C~ 80 % RH~ 700 hours). SO1 is the basic adhesive usedfor modification

Such the increase could be assumed when theadhesives would be filled with carbon nanotubes or silvernanowires. However, it has been found that alsonanoparticles can improve mechanical properties ofadhesives. The reason is that the chemical bonds betweennanoparticles and the resin are stronger than bonds in thereSIn.

The resistance of adhesive joints has increased afterthermal aging due to partial damage of contacts betweenthe conductive particles.

Fig. 2). The significant information is that the decrease ofthe resistivity has been found for "big" nanoparticles andlow concentrations only (see Tab. 1).

It has been found that the tensile strength of adhesivesmodified with the nanoparticles is higher in comparisonwith the basic adhesive (see Fig. 4).

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ConclusionsThe resistance, nonlinearity and tensile strength of the

adhesive joints formed of ICA modified with silvernanoparticles of different dimensions have beenexamined. It has been found that the nanoparticles do notimprove the electrical resistance and nonlinearity of thejoints. The reason is that the total number of contactsbetween the conductive particles increases after additionof the nanoparticles into the basic adhesive. It has alsobeen found that addition of nanoparticles improves thetensile strength and the resistance against combinedclimatic ageing of the adhesive. The reason is that thecontact areas between nanoparticles are substantiallysmaller in comparison with the contact areas betweenflakes. Therefore the contact pressure betweennanoparticles is higher than between flakes and thecontacts are more resistant to the extemalload.

AcknowledgmentsThe work has been carried out as a part of a project

"Diagnostics ofMaterials", number MSM6840770021

References1. Su, B.: Electrical, Thermomechanical and Reliability

Modeling of Electrically Conductive Adhesives.Dissertation. Georgia Institute of Technology, May2006, pp. 64-65.

2. Verma, S.CH., Guan, W., Liu, J.: "Flip-ChipInterconnection Using Anisotropic Conductive withLead Free Nano-Solder Particles", Proc. 1st

Electronics Systemintegration TechnologyConference, 2006, vol.l, pp. 282-286.

3. Wu, H.P., Wu, X.J., Ge, M.Y., Zhang, G.Q., Wang,Y.W., Jiang, J.: "Properties investigation onisotropical conductive adhesives filled with silvercoated carbon nanotubes." Composites Science andTechnology: Vol. 67, Issue 6 (2007), pp. 1182-1186.

4. Chang, Ch., Li, W., Ruilin, L., Guohua, J., Haojie,Y., Tao, Ch.: "Effect of silver nanowires on electricalconductance of system composed of silver particles."Journal of Materials Science. Vol. 42, No 9 (2007).pp.3172-3176

5. Lee, H. H., Chou, K. S., Shih Z. W.: "Effect ofnano­sized silver particles on the resistivity of polymericconductive adhesives." International Journal ofAdhesion and Adhesives, Vol. 25, Issue 5 (2005), pp.437-441

6. Zhigalski, G.P.: "Noise l/f and nonlinear effects inthin metal films (in Russian)." Uspechy fyziceskichnauk. Vol. 6 (1997), pp. 623-647.

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