Electrically Loaded Adhesive Bonds Formed on Different Surfaces

Pavel Mach, Radoslav RadevDepartment of Electrotechnology, Faculty of Electrical Engineering, CTU Prague,

Technicka 2, 166 27 Prague 6, Czech Republicmach@fel.cvut.cz

Abstract: Bonds of three types of electrically conductive adhesives with isotropic electricalconductivity have been formed on three types of surfaces - Cu, immerse Au and Sn (HAL). Epoxyresin types ofadhesives with Agflakes have been used. The bonds have been fabricated by assemblyofjumpers. Two types of electrical load have been applied. DC current JA and rectangular currentpulses with the frequency of 500 Hz, mark-space ratio 1.] and amplitude of JA. The resistances,nonlinearity and noise of the bonds have been measured. It has been found that quality of the bondson Sn surface finish is unacceptable through instability and level of measured parameters. Theresistances and nonlinearity of the bonds formed on Au surface finish have beenhigher incomparison with those measured on Cu pads, noise have been comparable. Both types ofthe currentload have caused low changes ofall parameters tested.

1. INTRODUCTION

Electrically conductive adhesives are materialswith growing significance in electronics packaging.Adhesives are used for bonding of different materialsand types of surface finishes. It can influenceelectrical as well as mechanical properties of thebonds [1].

Surface finish and surface oxide films on padsinfluence quality of adhesive bonds more than qualityof the soldered bonds. Fluxes used in a solderingprocess cleanse oxides from pads and make betterjoining between solder and a pad possible.

Electrically conductive adhesives are filled withelectrically conductive particles. The particles of fillerare usually Ag balls, Ag flakes, or a combination ofboth these types of particles [2], [3], [4]. Other typesof conductive particles are plastic balls covered withdifferent types of conductive films; with Ag or Aufilm the most often. Some types of these particleshave the conductive film covered with a very thininsulating film, which intercepts to contacts amongthe particles in x-y plane. The adhesive bondingprocess, contrary to the soldering process, uses nofluxes, which would improve quality of the adhesive

bonds. Therefore material and surface finish ofjoinedsurfaces has significantly higher influence on finalelectrical and mechanical quality of adhesive bondsthan on quality of soldered ones.

However, on the other hand, adhesive bondinghas one advantage in comparison with soldering [5].Adhesives shrink after curing. Therefore the volumeof adhesive bond decreases, but the volume of thefiller does not change. The contacts among the fillerparticles improve and also the contacts among thefiller particles and the surface of the pad improve. Thefiller particles are pressed to the pad surface and apossible oxide film covering the pad or the contactsurface is broken. Therefore quality of the bondimproves by curing.

It is very good known that mobility of Ag ionsis high and that their migration can cause bridgingbetween near contacts if there is a DC voltageconnected between them. Such the bridge can causethe breakdown of equipment [6]. Electricallyconductive adhesives are mostly filled with Ag ballsor flakes. By this reason it can be assumed that theproperties of electrically conductive adhesive bondscan also be sensitive to the DC current or currentpulses flowing through the adhesive bonds [7]. Such

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the property of the adhesive bonds would limit theiruse in some applications.

The goal of the work has been as follows: tofind out how material and surface finish of surfacesjoined by the use of electrically conductive adhesivecan influence the electrical properties of adhesivebonds and to find out whether the DC current of ahigh level and rectangular current pulses influenceelectrical properties of the adhesive bondssubstantially.

2. EXPERIMENTAL

2.1 Specimens under Test

Isotropic electrically conductive adhesives (two ofa one-component type, one of a two-component type)based on epoxy resin matrix with Ag filler have beenused for fabrication of the adhesive bonds. The typesof adhesives used and their basic properties are shownin Tab. 1

Tab. 1 Types of adhesives used for experiment

Formulation Phenolic-epoxy rezin

Curing temperature 200 °C

Curing time 6 min

Resistivity 3,0 to 3,5 10 -6 QmNumber of components 1

Formulation Phenolic-epoxy rezin

Curing temperature 180 °C

Curing time 6 min

Resistivity 4,0 to 7,5 1 0 -5 Qm

Formulation Phenolic-epoxy rezin

Curing temperature 180 °C

Curing time 9 min

Resistivity 7,0 to 1i0,0 10 7 Qm

Number of components 2

The bonds have been formed by adhesive assemblyof jumpers on pads of a test board. The surfacefinishes of the pads have been Cu; immerse Au (2 ptm)and Sn (HAL).

2.2 Measurement ofthe Electrical Resistance, Noiseand Nonlinearity ofthe Bonds

The bonds have been loaded with the DCcurrent 1A or with rectangular current pulses havingthe amplitude of 1A. The mark-space ratio of thepulses has been 1:1, the frequency 500 Hz. The loadhas been applied for 50 hours. It is necessary toremark that the DC current load has caused increase ofthe temperature of the bonds by 55 °C, the pulse loadhas increased their temperature by 37 'C. Theresistance has been measured in a four-pointarrangement using an impedance-meter HP 4284A,nonlinearity using a spectral analyzer HP 8560 andnoise using a lock-in amplifier PAR 124 A [8].

Fig. 1 Test board and the four-point measurement of thebond resistances

The schematic diagram for the measurement ofnoise is shown in Fig. 2.

R TRt

Bat C~~~~~~RC

Fig. 2 Schematic diagram of noise measurement, V is alock-in amplifier, R is a measured bond

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The schematic diagram of nonlinearity of the currentvs. voltage characteristic measurement is shown inFig. 3.

Fig. 3 Schematic diagram of the nonlinearitymeasurement

By the reason that the temperature of the bonds hasrisen during their current load with the DC current orwith the current pulses, the results of allmeasurements have been calculated as differences.The process of calculation of the bond resistances,noise and nonlinearity has been carried out infollowing steps:

1. The jumper assembled with adhesive has beenloaded with the DC current (current pulses) andthe resistance, noise and nonlinearity of acombinations adhesive bond - jumper -

adhesive bond have been measured. Thetemperature of the adhesive bonds has also beenmeasured. An infrared thermometer and a Cu-Constantan thermocouple have been used forthis measurement.

2. The resistance, noise and nonlinearity of theindividual jumpers at the normal temperatureand at the temperatures, which have beenmeasured on the adhesive bonds loaded withthe current and current pulses, have beenmeasured.

3. The specimens with the jumpers assembledwith adhesive have been heated in an oven andthe resistance, noise and nonlinearity of thecombination adhesive bond - jumper -

adhesive bond have been measured at the sametemperatures, which have been measured on theadhesive bonds loaded with the current andcurrent pulses.

4. The values of the bond resistances at the normalas well as elevated temperature have beencalculated by subtraction of the jumperresistance measured at the normal or elevatedtemperature from the resistance of thecombination adhesive bond - jumper -

adhesive bond measured at the sametemperature.

5. The values of the bond noise at the normal aswell as elevated temperature have beencalculated by subtraction of the jumper noisevoltage measured at the normal or elevatedtemperature from the noise voltage of thecombination adhesive bond - jumper -

adhesive bond measured at the sametemperature.

6. The values of the bond nonlinearity at thenormal as well as elevated temperature havebeen calculated by subtraction of the jumpernonlinearity voltage measured at the normal orelevated temperature from the nonlinearityvoltage of the combination adhesive bond -

jumper - adhesive bond measured at the sametemperature.

7. Then the change of the bond resistance, noiseand nonlinearity caused by the current or thecurrent pulses has been calculated as adifference between the value of every parametermeasured under the load of the bond and thevalue of this parameter measured at the sametemperature without the electrical load.

3. RESULTS OF THE MEASUREMENTS

The results of the measurements are shown in Fig.4 to Fig. 9. It has been found that the changes of thebonds resistances caused by the current load andcurrent pulses are low for both types of surfacefinishes. The bonds resistance is lower for the Cufinish.

Noise depends on the electrical load of the bondsslightly. It has been found that the noise voltageincreases for adhesive AX20 if the bonds are loadedwith the DC current and current pulses, whereas thisvoltage decreases for adhesive 656 S in dependenceon the electrical load. The bonds on Cu pads havelower noise than the bonds on Au pads.

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60

50

40

rAX20ER55MN

10-

No load DCCu

current pus

Type of the load

irr.ses

Fig. 4 Changes of the bonds resistances caused with theDC current and current pulses. Finish of the pads: Au

Fig. 6 Noise of the bonds. Finish of the pads: Au

25 65: 20tX 0r2:!~~ ~ ~ ~~A2

No loadD

C~~~~~~~~ur

current pulses

Type of the load

Fig. 5 Changes of the bonds resistances caused with theDC current and current pulses. Finish of the pads: Cu

Fig. 7 Noise of the bonds. Finish of the pads: Cu

0.1

0.77ll l

| _ ~~~~~~ER55MN

cuurrncretpulses| ~~~Type ofthe loadl

Fig. 8 Nonlinearity of the current vs. voltage characteristic Fig. 9 Nonlinearity of the current vs. voltage characteristicof the bonds. Finish of the pads: Au of the bonds. Finish of the pads: Cu

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40

350:ibEw 15 JEiAX62MN

10 , , _

No load DNioaDCCurr.

current pulses

Type of the load

1.6

0.2 65

IV1v0

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The noise level is very low. Therefore the noisevoltage of more bonds (of 14 usually) connected inseries has been measured and the noise voltage of onebond has been calculated. Different combinations ofindividual bonds have been measured in series to findout whether the final noise voltage of the bonds joinedin series is not caused by one or two bondsdominantly.

It has been found that nonlinearity of the bonds isinfluenced with the DC current and current pulsesslightly only. The bonds formed on the pads with theAu finish have had lower level of nonlinearity incomparison with the bonds formed on the Cu pads.

Quality of the bonds formed on the pads with theSn surface finis has been very low and the results havenot been repeatable.

CONCLUSIONS

The adhesive bonds formed of three types ofelectrically conductive adhesives with isotropicelectrical conductivity on three different types of thesurface finishes have been investigated. The bondshave been loaded with the DC current and currentpulses.

It has been found that the Cu finish is better fromthe viewpoint of all measured electrical parameters.Nonlinearity of the bonds formed on the Cu pads hasbeen substantially lower than nonlinearity of thebonds prepared on the Au pads.

The changes caused with the electrical load havebeen low. It means that both the phenolic-epoxy resinand epoxy resin do not make migration of Ag ionspossible. Therefore almost no changes of electricalparameters have been observed after the electricalload although the DC current density and the densityof the pulse current have been so high that they havecaused heating of the bonds.

The results of the measurements of the bondsformed on the pads with the Sn finish have been notrepeatable. The values of all measured parametershave been sizes higher in comparison with them,which have been measured on the bonds formed onthe Cu or Ag finishes. Therefore the Sn surface finishis not usable for adhesive assembly.

REFERENCES

[1] MACH, P., DURAJ, A.: Adhesive Joining or Lead FreeSoldering? Proc. MIDEM, 41< International Conference onMicroelectronics, Devices and Materials. Bled, Slovenia.2005, pp. 73-86

[2] Blackwell, G. R. The electronic packaging handbook. CRCPress and IEEE Press. 2000

[3] Brown, W. D. Advanced electronic packaging. IEEE Press.N. Y. 1999

[4] Das, J. H., Morris, J. E. IEEE Trans. on CPMT. Vol. 17. 1994,pp 620-625

[5] SUZUKI, K. et al. Conductive adhesives materials for leadsolder replacement. IEEE Transactions on components,packaging, and manufacturing technology - part A. Vol.21.No.2, pp.252-258. June 1998

[6] Tummala, R. R. et al.. Microelectronics PackagingHandbook. Chapnan and Hall. N. Y. 1997

[7] Kotthaus, S. et al. Current-Induced Degradation ofIsotropically Conductive adhesives, IEEE Trans. on CPMT,Vol. 21, No.2, June 1998, pp.259-265

[8] M\ACH, P. et all: Diagnostics of Adhesive Bonds. Proc. 3rdEuropean Microelectronics and Packaging Conference withTable Top Exhibition. IMAPS CZ&Slovak Chapter. Prague.2004, pp 83-88

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