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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 9, NO. 7, JULY 2019 1235

A Study on the Magnetic Dispersion of theConductive Particles of Anchoring-Polymer-

Layer Anisotropic Conductive Films andIts Fine-Pitch Interconnection Properties

Jun-Ho Byeon, Dal-Jin Yoon , and Kyung-Wook Paik

Abstract— As the display resolution has been rapidly increased,the pitch between two electrodes has been continuously decreasedto less than 20 µm pitch. Therefore, fine-pitch interconnectiontechnology has become very important in display technology.In our previous research results, anchoring polymer layer (APL)structure was successfully introduced into anisotropic conductivefilm (ACF) system to form fine pitch interconnection by suppress-ing the conductive particles movement during the ACF assembly.In general, the agglomerated conductive particles between twoelectrodes can cause short-circuit problems at ACF assembly.In this paper, the magnetic field was applied to the Ni-coatedpolymer conductive particles in the polyvinyl fluoride (PVDF)APL structure to disperse the conductive particles uniformlyin the xy plane. Then the effects of the magnetic fields on thedispersion of the Ni-coated conductive particles in the PVDFAPL structure and the characterization of fine-pitch chip-on-glass (COG) assembly using PVDF APL ACFs with magneticallydispersed conductive particles were investigated. By optimizingthe magnetic fields on the PVDF APL structure, 80% dispersedparticle rate was successfully obtained. After the ACF bondingprocess, the conductive particles capture rates and contactresistance properties of magnetically dispersed PVDF APL ACFs,and the PVDF APL ACFs with no magnetic field applied wereinvestigated at 20-µm-pitch COG applications. Both PVDF APLACFs showed a similar capture rate of conductive particlesand electrical insulation property at 20 µm pitch. The PVDFAPL ACFs with no magnetic field applied showed 100% ofinsulation property at 20 µm, but short circuits at less thanpitch. However, for less than 20-µm pitch, only the PVDF APLACFs with magnetically dispersed conductive particles showed100% insulation circuit rate down to the 11-µm pitch becausethe conductive particles were not agglomerated but existed assingle particles, resulting in no electrical short between fine-pitch adjacent bumps. As a result, magnetically dispersed PVDFAPL ACFs can be used as new ACF materials for ultrafine-pitchinterconnection applications without any electrical short.

Manuscript received January 17, 2019; revised April 30, 2019; acceptedMay 23, 2019. Date of publication June 5, 2019; date of current versionJuly 18, 2019. This work was supported by the Wearable Platform MaterialsTechnology Center (WMC) funded by the National Research Foundation ofKorea (NRF) under Grant 2016R1A5A1009926 of the Korean Government(MSIT). Recommended for publication by Associate Editor D. Lu upon eval-uation of reviewers’ comments. (Jun-Ho Byeon and Dal-Jin Yoon contributedequally to this work.) (Corresponding author: Kyung-Wook Paik.)

The authors are with the Material Science and Engineering Department,Korea Advanced Institute of Science and Technology, Daejeon 34141, SouthKorea (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TCPMT.2019.2921055

Index Terms— Anchoring polymer layer (APL), anisotropicconductive films (ACFs), conductive particle dispersion, chip onglass (COG), fine pitch assembly, magnetic field.

I. INTRODUCTION

TO REALIZE higher resolution displays, the number ofpixels per unit area should be increased and more num-

bers of interconnection between display panels such as glassesand organic light-emitting diode (OLED) plastics and driverICs should be realized [1].

In our laboratory, the new concept of anchoring polymerlayer (APL) anisotropic conductive films (ACFs) was previ-ously introduced using the high-tensile-strength thermoplasticpolymer APL structure into ACFs to suppress the conductiveparticles movement during the ACF assembly. Basically, APLACFs can solve the electrical short problem due to the aggre-gated conductive particles between two electrodes during ACFresin flow by suppressing the conductive particle movement byhigh-tensile-strength APL structure [2], [3].

Even though the APL structure can provide excellentconductive particles suppression effect during the APL ACFassembly, the initial conductive particles of as-coated APLstructure can be agglomerated at the APL coating stage,resulting in the potential possibility of electrical short afterACF assembly. Therefore, it is necessary to uniformlydisperse these conductive particles and reduce agglomeratedconductive particles in the as-coated APL structure to furtherenhance the electrical short problems at ultrafine-pitchinterconnection applications.

The idea of dispersing the Ni-coated conductiveparticles using magnetic fields comes from the principleof magnetorheological fluids (MRFs). MRFs are solutionsconsisting of nonconductive solvents and micron-sizedparticles that can be magnetized. If a magnetic field is notapplied to this solution, particles are randomly distributedas shown in Fig. 1(a). The solution does not have greatresistances to flow. However, when the magnetic field isapplied, particles can be magnetized and arranged in adirection parallel to the applied magnetic field resulting inhigher resistance solutions to flow as shown in Fig. 1(b) [4].

In contrast, if all the magnetic particles in a monolayerstructure are in the same phase and dispersed in the xy plane,

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https://orcid.org/0000-0002-9581-4300

https://orcid.org/0000-0003-1743-0376

1236 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 9, NO. 7, JULY 2019

Fig. 1. Schematic images of MRF. (a) Before applying a magnetic field. (b) After applying a magnetic field.

Fig. 2. Schematic images of the APL structures with a monolayer of conductive particles (a) before and (b) after applying vertical-direction magnetic field.

TABLE I

SPECIFICATION AND SEM IMAGES OF TEST SI CHIPS AND GLASS SUBSTRATES

only the repulsive force pushing other particles will act, result-ing in the dispersion behavior of magnetic particles. In addi-tion, material properties and characteristics are improved bythe particles dispersed by the magnetic field [5], [6].

As the polyvinyl fluoride (PVDF) APL used in this papercontains a monolayer of conductive particles coated with aferromagnetic nickel material, they can be magnetized in thexy plane by applying a magnetic field [7]. Monolayer ofNi-coated conductive particles in the APL layer can be mag-netized in the same phase, and then agglomerated conductiveparticles can be separated and randomly distributed as shownin Fig. 2.

II. TEST VEHICLES

Si chips with Au bumps and glass substrates with thin-filmAu electrodes were prepared to measure electrical prop-erties as shown in Table I. Si chip had a dimension of

16 mm × 1.5 mm, and the Au bump width and spacewere 12 and 8 μm, respectively. The glass substrate had adimension of 26 mm × 23 mm, and thin-film Au electrodewidth and space were the same as the Au bump dimensionof Si chips. The Si chip and glass substrate had a 20-μmpitch dimension [3]. When the pitch decreased, the bump andelectrode width and space decreased at the same rate. If the20-μm pitch decreases to 15-μm pitch, the decrease rate is0.75. Then, the width decreases from 12 to 9 μm and thespace decreases from 8 to 6 μm. Applying the same principle,for 13- and 11-μm pitch, the bump and electrode width andspace are shown in Table II. However, misalignment methodwas used to reduce the pitch because of the difficulty andlimitations of the process of manufacturing test vehicles below20 μm. Table II shows the actual implemented specificationsat the possible level of laboratory equipment to maximize thecalculated width and space specifications.


BYEON et al.: STUDY ON THE MAGNETIC DISPERSION OF THE CONDUCTIVE PARTICLES 1237

TABLE II

SPECIFICATION OF WIDTH AND SPACE AT THE PITCH LESS THAN 20 μm

Fig. 3. Schematic images of (a) 20-, (b) 15-, (c) 13-, and (d) 11-μm pitch COG assembly.

Fig. 4. Schematic image of the magnetic dispersion during a comma-roll coating and drying processes of the PVDF APL.

To achieve less than 20-μm pitch size, intentional mis-alignments between chips and glass substrates were used.For example, if 2.5 μm of bump-to-bump misalignment wasintentionally applied, 15-μm pitch composed of 9.5 μmof bump size plus 5.5 μm of space could be realized tomeasure ultrafine-pitch insulation characteristics. In this way,pitch sizes down to 11 μm were achieved as shown inFig. 3.

III. EXPERIMENTS

A. Materials

For preparing PVDF APL structures, PVDF was dissolvedin the mixture of dimethylacetamide (DMAC) solvent, and3.25-μm-diameter Au–Ni-coated conductive particles wereadded [8]. Nonconductive films (NCFs) composed of ther-mosetting epoxy and curing agents were also prepared to



Fig. 5. Schematic images of (a) magnetic structure consisted of two magnets, (b) magnetic field in repulsive interaction, and (c) magnetic field in attractiveinteraction.

Fig. 6. Schematic image of the magnetic structure consisted of magnets andmagnetic shielding materials.

laminate on the top and bottom of the APL structure as adhe-sive layers to PVDF APL structure. Ferrite (Fe), Neodymium35 (Nd35), and Nd 52 magnets were prepared to apply mag-netic fields on conductive particles in PVDF APL structures[9].

B. Magnetization of the Conductive Particles

In order to confirm whether the conductive particles weremagnetized by magnetic fields, hysteresis loops of the con-ductive particles, PVDF APL solution, and PVDF APL weremeasured by the vibrating sample magnetometer (VSM) [10].

The magnetic dispersion of the conductive particles inthe APL structure was conducted using a comma-roll filmcoater with the magnetic structures as shown in Fig. 4. First,PVDF APL films were fabricated by a comma-roll film coater.When PVDF APL films entered in the magnetic structure,the receiving roll was stopped to apply the magnetic fieldto the PVDF APL for 30 s. After magnetization, the PVDFAPL entered into the 100 °C drying zone to completelyevaporate solvents. Then, the PVDF APL was observed by anoptical microscope and scanning electron microscope (SEM)to evaluate the distribution of conductive particles.

The magnetic structure made of two permanent magnets wasused to examine the effect of the magnetic field directionsas shown in Fig. 5(a). In order to apply the horizontal(y-axis) or vertical (z-axis) magnetic fields to the PVDF APL,repulsive and attractive interaction were realized by changingthe poles of the magnets in the magnetic structure as shownin Fig. 5(b) and (c).

C. Dispersed Particle Rates Depending on VerticalMagnetic Field Strength

In order to concentrate the magnetic field in the verticaldirection, the magnetic structure composed of magnets and

high-permeability magnetic shielding materials [11] was fabri-cated as shown in Fig. 6, and installed in the comma-roll filmcoater to examine the effect of the magnetic field strengthsduring the coating process.

Various magnetic field strengths were realized using per-manent magnets that have various magnetic field strengthsand adjusting the gap sizes between the magnet and magneticshielding material.

Magnetic dispersion of the conductive particles in PVDFAPL was conducted in the same way as the above experiment.Then, PVDF APL films were observed by an optical micro-scope to evaluate the dispersed particle rate. The dispersedparticle rate was defined as the ratio of the number of dispersedconductive particles to the number of conductive particles ina specific area of the PVDF APL structure.

D. Fabrication and Characterization of the PVDF APL ACFsWith Magnetically Dispersed Conductive Particles

The PVDF APL with magnetically dispersed conductiveparticles was fabricated by using a comma-roll film coaterequipped with the magnetic structure with optimized magneticfield strengths. After PVDF APL films were treated by themagnetic structure for the magnetic dispersion of the conduc-tive particles, PVDF APL films were dried in a 100 °C dryzone to completely evaporate solvents, and then the PVDFAPLs with magnetically dispersed conductive particles wereobserved by SEM to evaluate dispersed particle rates andmorphology.

The PVDF APL films with magnetically dispersed conduc-tive particles were laminated by two high-viscosity NCFs tomake ACFs for chip-on-glass (COG) applications. Final PVDFAPL ACFs consisted of three-layers structure by laminating12- and 2-μm thickness of high-viscosity NCFs on the PVDFAPL film.

In order to characterize the PVDF APL ACFs with magneti-cally dispersed conductive particles, the as-coated PVDF APLACFs with no magnetic field applied were used as a reference.

First, the capture rates of the PVDF APL ACFs wereanalyzed by the optical microscope on Au bumps beforeand after the ACF bonding process. The applied bondingconditions were 170 °C, 70 MPa for 10 s. The particle capturerates were defined as the ratio of the number of the conductiveparticles per gold bump area before and after ACF bonding.



Fig. 7. Test fixtures of measuring (a) insulation resistance and (b) bump joint contact resistance.

Fig. 8. Magnetization hysteresis loops of (a) Ni-coated conductive particles and (b) PVDF APL solution and PVDF APL films with added Ni-coatedconductive particles.

Fig. 9. SEM images of the conductive particles distribution under (a) N–N repulsive interaction structure and (b) N–S attractive interaction structure.

Second, the insulation properties were also evaluated bymeasuring the insulation resistance of 26 joints as shownin Fig. 7(a) because the agglomerated conductive particlesbetween neighboring bumps after ACF bonding sometimescaused short-circuit failures.

Last, the single-Au-bump ACF contact resistances weremeasured by using the four-point probe method as shownin Fig. 7(b) to characterize the electrical joint properties ofPVDF APL ACFs with dispersed conductive particles and nomagnetic field applied.

IV. RESULTS AND DISCUSSION

A. Behavior of Conductive Particles Under Magnetic Fields

The conductive particles were magnetized by magneticfields as shown in the hysteresis loop of Fig. 8(a), and the

magnetizations of conductive particles themselves, PVDF APLsolution, and PVDF APL were almost the same as shownin Fig. 8(b). These results showed that the conductive particlescan be magnetized identically under all three conditions.

In the magnetic structure having a repulsive interactionwith 250 G y-axis magnetic field strength and 10 G z-axismagnetic field strength, the agglomerates of the conductiveparticles were formed as shown in Fig. 9(a). However, in themagnetic structure having attractive interaction with 140 G y-axis magnetic field strength and 370 G z-axis magnetic fieldstrength, the conductive particles were dispersed as shownin Fig. 9(b).

From these results, it was found that the y-axis magneticfield caused the attractive force between the conductive parti-cles resulting in many lined agglomerates. The z-axis magneticfield caused the repulsive force between conductive particles,



Fig. 10. Simulations of the magnetic fields. (a) When the magnet and the magnetic shielding material are far and (b) when the magnet and the magneticshielding material are closer.

Fig. 11. z-axis magnetic field strengths adjusted by various permanentmagnets and gap sizes of the magnetic structure.

resulting in the better dispersion of conductive particles inxy planes. Therefore, the minimization of the y-axis magneticfield strength and the maximization of the z-axis magnetic fieldstrength were needed to maximize the magnetic dispersion ofconductive particles.

B. Dispersed Particle Rates Depending onVertical Magnetic Field Strengths

In order to realize magnetic fields that had minimized y-axismagnetic field strength and maximized z-axis magnetic fieldstrength, the principle of the magnetic shielding was usedas shown in Fig. 10. The magnetic fields were attracted andconcentrated in perpendicular to a magnetic shielding materialas a shielding material closer to a permanent magnet [12].Based on the simulation, the magnetic structure made of themagnet on the top and the magnetic shielding material on the

bottom was made. As a result of measuring the z-axis magneticfield strength in the magnetic structure, it was confirmed thatthe z-axis magnetic field was highly concentrated as shownin Fig. 11.

As the z-axis magnetic field strengths were stronger, themagnetization of the conductive particles was increased andthen saturated at the specific magnetic field strength as shownin Fig. 8, hysteresis loops of the conductive particles. The dis-persed particle rates were measured by various magnetic fieldstrengths because the magnetization determined the repulsiveforces between the conductive particles.

As shown in Fig. 12, the agglomerates of the conductiveparticles were not dispersed at 100 G. However, as thez-axis magnetic field strengths increased, the agglomeratesbecame more dispersed and the numbers of agglomerates alsodecreased. Over 1600 G in which magnetization was saturated,with most of the conductive particles well dispersed.

In terms of dispersed particle rates as shown in Fig. 13,50% dispersed particle rate was obtained at 100 G. However,as the magnetic field strengths increased, dispersed particlerates increased, and at above 1600 G, dispersed particle rateswere saturated at 80% due to the saturation of magnetizationof Ni-coated conductive particles.

From the result, 1600 G of the z-axis magnetic field strengthwas sufficient enough to achieve 80% dispersed particle rate.Therefore, when the magnetic field was 1600 G or more,it was found that the dispersed particle rate of the PVDFwith no magnetic field applied increased about 2.6 timesfrom 30%. The magnetic field strength (1600 G) conditionwith maximum dispersed particle rate was optimized. In thiscondition, PVDF APL was fabricated and ACF structure wasfabricated after roll laminating with NCF.

C. Characterization of the PVDF APL ACFs WithMagnetically Dispersed Conductive Particles

As shown in Fig. 14, it was confirmed that PVDF APL thathas 80% dispersed particle rate and the stable film morphologywas obtained.

The thickness of PVDF APL films with dispersed conduc-tive particles and no magnetic field applied was almost thesame as 800 nm, and there was no difference in the APLstructure as shown in Fig. 15.



Fig. 12. Optical microscopic images of the conductive particles distribution under z-axis magnetic field strengths of (a) 0, (b) 100, (c) 220, (d) 500, (e) 1600,(f) 1850, and (g) 1960 G.

Fig. 13. Dispersed particle rates depending on z-axis magnetic field strengths.

In both PVDF APL ACFs with and without dispersion,nine conductive particles were placed on a gold bump beforeACF bonding, and seven conductive particles were capturedby a gold bump after ACF bonding due to NCFs resinflow. Both PVDF APL ACFs showed the same 74% capturerate.

In addition, both PVDF APL ACFs showed 100% insulationcircuit rate and also similar 360-m� contact resistance for20-μm-pitch COG assembly because of the same capturedconductive particles after ACF bonding.

However, when the COG pitches were reduced, conductiveparticles started agglomerating from 15-μm pitch, resulting inan electrical short circuit using the PVDF APL ACFs with nomagnetic field applied. As shown in Fig. 16(a), the electricalshort circuits are formed due to the agglomerated conductiveparticles 3 to 4 in the space, resulting in electrical short circuitsfrom below the 15-μm pitch. As a result, 100% insulation

circuit rate could not be achieved at below 15-μm pitchesusing the PVDF APL ACFs with no magnetic field applied asshown in Fig. 16(b).

On the other hand, for the PVDF APL ACFs with magnet-ically dispersed conductive particles, the conductive particlesare individually present in the space as shown in Fig. 17(a) andhave no contribution to the formation of electrical short circuitsup to 11-μm pitch. Therefore, 100% insulation circuit rateswere achieved using the PVDF APL ACFs with magneticallydispersed conductive particles down to 11-μm pitch as shownin Fig. 17(b).

V. CONCLUSION

It was found that the Ni-coated conductive particles canbe magnetized and can be moved by the magnetic fields.In monolayer dispersed Ni-coated conductive particles inthe PVDF APL structure, the y-axis magnetic field induced thelined agglomerates of the conductive particle because of theattractive force between them. However, the z-axis magneticfield induced the magnetic dispersion by providing repulsiveforces between magnetized conductive particles. By usingthe principle of the magnetic shielding method, the magneticstructure with maximized z-axis magnetic strength with littley-axis magnetic component was made.

As the z-axis magnetic field strengths increased, dispersedparticle rates were increased. Over 1600 G in which the mag-netization of the conductive particles was saturated, dispersedparticle rates were saturated at 80%.

It was found out that PVDF APL films with 80% dispersedparticle rate and stable film morphology were successfullyfabricated by the optimized magnetic structure at 1600 G.The PVDF APL ACFs with magnetically dispersed conductiveparticles showed similar electrical properties with PVDF APLACFs with no magnetic field applied in terms of capture rates,insulation circuit rates, and contact resistances for 20-μm fine-pitch COG assembly. However, for less than 20-μm pitches,



Fig. 14. (a) Top optical, (b) top SEM, and (c) tilted SEM images of PVDF APL with magnetically dispersed conductive particles.

Fig. 15. Cross-sectional SEM images of PVDF APL (a) with magnetically dispersed conductive particles and (b) with no magnetic field applied.

Fig. 16. (a) Top view of optical images of the agglomerated conductive particles in electrode spaces and (b) insulation circuit rates at various pitches ofPVDF APL ACFs with no magnetic field applied.

Fig. 17. (a) Top view of optical images of the single conductive particles in electrode spaces and (b) insulation circuit rates at various pitched of PVDFAPL ACFs with magnetically dispersed conductive particles.

only PVDF APL ACFs with magnetically dispersed conductiveparticles showed 100% insulation circuit rate down to 11-μmpitch because the conductive particles were not agglomeratedbut existed single particle, resulting in no electrical shortbetween very fine-pitch adjacent bumps.

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Jun-Ho Byeon received the B.S. degree in mate-rial science and engineering from Sejong Univer-sity, Seoul, South Korea, in 2016, and the M.Sdegree from the Korea Advanced Institute of Scienceand Technology (KAIST), Daejeon, South Korea,in 2019.

His research interests include the magnetic dis-persion of the conductive particles in anchoring-polymer-layer anisotropic conductive films forfine-pitch interconnection in electronic devices.

Dal-Jin Yoon received the B.S. degree in materialsscience and engineering from the Sejong University,Seoul, South Korea, in 2014, and the M.S. degree inthe Korea Advanced Institute of Science and Tech-nology (KAIST), Daejeon, South Korea, in 2016,where he is currently pursuing the Ph.D. degree withthe Nano-Packaging and Interconnect Laboratory.

His current research interests include ultrafine-pitch interconnection technologies using anchoring-polymer-layer anisotropic conductive films forPelectronics application.

Kyung-Wook Paik received the B.S. degree inmetallurgical engineering from Seoul National Uni-versity, Seoul, South Korea, in 1979, the M.S.degree from the Korea Advanced Institute of Scienceand Technology (KAIST), Daejeon, South Korea,in 1981, and the Ph.D. degree in materials scienceand engineering from Cornell University, Ithaca, NY,USA, in 1989.

He was a Research Scientist with KAIST from1982 to 1985 and was involved in the developmentof gold bonding wires. He was a Senior Technical

Staff Member with the Interconnect Multichip Module Technology andPower IC Packaging, General Electric Corporate Research and Development,Brookline, MA, USA, from 1989 to 1995. He rejoined the Departmentof Materials Science and Engineering, KAIST, as a Professor in 1995,where he is currently with the Nano-Packaging and Interconnect Laboratory(http://npil.kaist.ac.kr) and is involved in flip-chip bumping and assembly,adhesive flip-chips, embedded capacitors, and display packaging technologies.


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