A Study on the Nanofiber-Sheet Anisotropic Conductive Films(NS-ACFs) for Ultra-Fine-Pitch Interconnection Applications

SANG-HOON LEE1 and KYUNG-WOOK PAIK1,2

1.—Department of Materials Science and Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu,Daejeon 305-701, Republic of Korea. 2.—e-mail: kwpaik@kaist.ac.kr

Nanofiber-sheet anisotropic conductive films (NS-ACFs) were invented toovercome the limitations of high joint resistance and short-circuit issues ofultra-fine-pitch interconnections. The NS-ACFs have great advantages interms of suppressing conductive particle movement during the flip-chipbonding process. In a 20-lm ultra-fine-pitch with 7-lm bump spacing ultra-fine-pitch chip-on-glass assembly, suppression effects of conductive particlemovement were significantly improved by using the NS-ACFs because anunmelted NS inside the ACFs suppressed the mobility of conductive particlesso that they would not flow out during the bonding process. The NS-ACFscould significantly increase the capture rate of conductive particles from 31%up to 81% compared to conventional ACFs. Moreover, excellent electricalcontact properties were obtained without melting the nanofiber materialwhich was essential for the conventional nanofiber ACFs. The NS-ACFs arepromising interconnection materials for ultra-fine-pitch packaging applica-tions.

Key words: Nanofiber-sheet (NS), nanofiber, electrospinning, anisotropicconductive films (ACFs), ultra-fine-pitch interconnection,plasma etching

INTRODUCTION

Fine-pitch interconnection technology has becomea very important field in packaging industry sincethe demands of smart electronic devices such astablet personal computers, televisions, smartphoneshave risen dramatically. The packaging industryrequires more advanced technology to fulfill cus-tomer needs in electronic devices such as high-performance, multi-functionality, and compact-ness.1–3 As a consequence, more input/output pinsare required within the limited space resulting inthe narrower pitch and bump space for ultra-fine-pitch interconnection technology of electronicdevices.4–6

In the flip-chip packaging area, anisotropic con-ductive films (ACFs), well-known interconnectingadhesive materials which consist of thermosettingresin and conductive particles in a film format, have

been widely used to provide anisotropic electricalconduction and adhesion between bumps and elec-trodes.7–9 However, in ultra-fine-pitch interconnec-tions, electrical problems have become seriousissues with the conventional ACFs as the pitchand space between bumps and electrodes havebecome finer and finer.10 Bumps with longer lengthand narrower width have caused less number ofconductive particles captured between the bumpand the electrode resulting in a high contact resis-tance problem. Moreover, narrow space between thebumps has become a serious problem, becauseconductive particles that have flown out are agglom-erated between bumps causing short-circuit fail-ures.11,12 To solve these fine-pitch interconnectionproblems, nanofiber ACFs were introduced.13,14 Itwas reported that the nanofiber ACFs could solvethe electrical interconnection problems in ultra-fine-pitch interconnection by suppressing the movementof conductive particles during the ACF bondingprocess. However, there was a limitation of(Received May 3, 2016; accepted August 19, 2016;

published online September 6, 2016)

Journal of ELECTRONIC MATERIALS, Vol. 46, No. 1, 2017

DOI: 10.1007/s11664-016-4895-5� 2016 The Minerals, Metals & Materials Society

167

nanofiber polymer selection for nanofiber ACFs inwhich the melting temperature of the nanofiberpolymer should be lower than the bonding temper-ature for stable electrical conduction. Therefore,only limited polymer materials with low meltingtemperature and low tensile strength could beselected and they have shown insufficient suppres-sion effects. Moreover, nanofiber ACFs required atwo-step bonding process including a resin flowingprocess and a main bonding process which eventu-ally causes low productivity.14,15

In this study, nanofiber ACFs are further devel-oped and a modified concept of nanofiber ACFs called‘‘nanofiber-sheet (NS) ACFs’’ is introduced whereconductive particles are located in a very thin layerof NS with top and bottom surfaces exposed by aplasma etching process. For the NS-ACFs, an elec-trical path is created through captured conductiveparticles without melting the nanofibers. As a result,polymers with higher tensile strength and highersuppression effects can be selected for the nanofibermaterials. In this study, polyvinylidene fluoride(PVDF) is used for the NS polymer material. Poly-mer particles coated by Ni/Au are incorporated intoPVDF nanofiber by an electrospinning process and avery thin NS is obtained by a compression processfollowed by a plasma etching process. After that, NS-ACFs are fabricated by a non-conductive film (NCF)lamination process. Finally, the effects of NS-ACFsare investigated in terms of conductive particlemovement and electrical contact properties.

EXPERIMENTS

Materials and Test Vehicles

For the NS-ACFs, PVDF which has a meltingpoint of 168�C was dissolved in the mixture ofdimethylacetamide (DMAC) and acetone. And then,conductive particles were added to a nanofibersolution for electrospinning to fabricate PVDFnanofiber with incorporated conductive particles.For the conductive particles, polymer core–shellstructures with total diameters of 3.5 lm with a0.25 lm Ni/Au coating were used. For chip-on-glass(COG) assemblies, silicon chips with a 12-lm-highAu bumps having a 20-lm pitch and a 7-lm bumpgap were used. And for glass substrates, 1300 A Ti/Au thin-film electrodes with the same pitch and gapas the bumps of the silicon chip were used.15 Testvehicles were designed to measure the contactresistance at single bump joints using the Kelvinresistance measurement method and also to inspectany short-circuit failure between 24 neighboringbumps using insulation resistance patterns.

Fabrication of the NS Structure UsingThermal Compression and Plasma EtchingProcesses

To fabricate the NS-ACFs, four fabrication pro-cesses: electrospinning, compression, plasma

etching, and lamination were used. In the electro-spinning process, PVDF nanofibers containing con-ductive particles were electrospun at an optimizedcondition as shown in Table I. Then, in the com-pression process, the electrospun PVDF nanofiberswere pressed with heat so that the conductiveparticles could be located in a very thin compressednanofiber. Finally, both the top and bottom of thecompressed nanofiber were plasma etched to fabri-cate NSs. The plasma etching process was con-ducted under the O2 plasma condition at2.67 9 10�3 kPa pressure with 100 W of radio fre-quency power to remove polymer layers around theconductive particles to expose the Ni/Au metalsurface of the conductive particles for excellentelectrical contact as shown in Fig. 1.

After the NS containing conductive particles wasfabricated, the NCFs lamination process was fol-lowed, where two high-viscosity epoxy NCFs werelaminated on the top and bottom of the NS by a rolllaminator. And then, low-viscosity epoxy NCF waslaminated on the top once again to fabricate a four-layer ACF structure as shown in Fig. 2. The min-imum viscosity of high-viscosity NCF was78,580 Pa s and that of low-viscosity NCF was3668 Pa s as shown in Fig. 3. Fabrication of ACFsby using NCFs with different viscosities couldcapture more conduction in COG assembly, wherethe thickness of Ti/Au glass electrodes was very thin(1300 A) as shown in Fig. 4.

Effects of NS on the Conductive ParticleMovement

Nanofiber ACFs required the two-step bondingprocess which was divided into a resin flowingprocess and a main bonding process. During theresin flowing process, a bonding temperature belowa resin curing temperature and below a meltingtemperature of nanofiber polymer was applied toflow out the resin without curing and withoutmelting the nanofibers at the same time, so thatthe nanofibers could suppress the movement ofconductive particles. However, for the main bonding

Table I. Optimized electrospinning conditions ofPVDF nanofibers incorporating conductiveparticles

PolymerPolyvinylidenefluoride (PVDF)

Concentration (wt.%) 18Conductive particle content PVDF 1 g : particle 0.6 gSolvents Dimethylacetamide

(DMAC)/acetoneConductive particlediameter (lm)

3.5

Applied voltage (kV) 8Spinning rate (lL/min) 20

Lee and Paik168

process, temperature above the resin curing tem-perature and the melting temperature of thenanofiber polymer was applied to cure the resinand melt the insulating nanofibers at the same timefor stable electrical conduction between bumps andelectrodes.

However, in this study, only the main bondingprocess was performed for the NS-ACFs. For theNS-ACF bonding, 70 MPa was applied at 160�C for8 s, where the bonding temperature was above theresin curing temperature, but below the meltingtemperature of PVDF nanofiber, 168�C. Since theNi/Au metal surface of the conductive particles werealready exposed after the plasma etching process, itwas not necessary to melt the insulating nanofiberpolymer for electrical conduction. As a result, thetwo-step bonding process of nanofiber ACFs could bereduced to a one-step bonding process by using theNS-ACFs.

Once the ultra-fine-pitch COG package intercon-nection was assembled using the NS-ACFs, conduc-tive particle movement was analyzed and comparedwith the conventional ACFs by an optical micro-scope to see how the NS contributed to suppressionof the movement of conductive particles. It wasexpected that the NS-ACFs could suppress themovement of the conductive particles very effec-tively because the NS polymer remained unmeltedafter the bonding process, retaining the NS.

Fig. 1. Fabrication process of NS with incorporated conductiveparticles.

Fig. 2. Fabrication processes of NS-ACFs by the NCF lamination process.

Fig. 3. Viscosities of NCFs.

Fig. 4. A scheme of a COG assembly.

A Study on the Nanofiber-Sheet Anisotropic Conductive Films (NS-ACFs) for Ultra-Fine-PitchInterconnection Applications

169

Characterization of the NS-ACF Joint Prop-erties

After the ultra-fine-pitch COG interconnectionpackage was assembled using the NS-ACFs, scan-ning electron microscopy (SEM) was used to analyzecross-sectional morphologies of the NS-ACF joints.They were analyzed to verify the existence of the NSbetween the bumps and electrodes after the bondingprocess, and to observe any insulating polymerlayers captured between the conductive particlesand bumps or electrodes.

The electrical properties of the NS-ACF jointswere analyzed by using the Kelvin resistance mea-surement method to measure the contact resistanceof a single bump joint and by using insulationresistance patterns to inspect any short-circuitfailure between 24 neighboring joints. The contactresistance was measured to detect any high contactresistance problem caused by remaining NS layersaround the conductive particles after the plasmaetching process. The insulation resistance was alsoinspected to detect any short-circuit failures atultra-fine-pitch interconnections caused by agglom-erated conductive particles between fine bump gaps.Finally, 85�C 85% relative humidity testing wasconducted for 1000 h, and the change in contactresistance was compared with the conventionalACFs to investigate if there was any defect relatedto the NS.

RESULTS AND DISCUSSION

Fabrication of NS with Incorporated Conduc-tive Particles

Electrospun PVDF nanofibers containing the con-ductive particles are shown in Fig. 5. The PVDFnanofibers were ejected uniformly without formingany beads, and the average diameter of PVDFnanofibers was 400 nm.

To fabricate PVDF NS, the electrospun PVDFnanofibers were compressed with heat. The

thickness of the compressed nanofiber became thin-ner as the applied temperature increased, as shownin Figs. 6 and 7. When the applied temperaturereached to 180�C, the thickness of the compressednanofiber became 3.5 lm which was almost thesame as the diameter of the conductive particles,and the conductive particles were located in a singlelevel.

After the nanofibers were compressed, the plasmaetching process was conducted by using O2 plasmato completely remove the insulating polymer layeraround the conductive particles, as shown in Fig. 8.The compressed nanofibers were plasma etched for120 s, 240 s, and 360 s, and 450-nm, 900-nm, and1400-nm thicknesses of the top compressed nanofi-ber were etched, respectively. The etching rate ofPVDF NS was 233 nm/60 s and the optimizedetching time was 240 s because the etching targetwas 1 lm.

Since the top and bottom Ni/Au metal surface ofthe conductive particle should be exposed forstable electrical conduction, the bottom side of theNS was once again plasma etched as shown inFigs. 9 and 10. It was clear to see that both top andbottom surfaces of the conductive particles wereexposed after plasma etching which would eventu-ally create an electrical path between bumps andelectrode in COG interconnections without meltingthe nanofiber polymers.

Conductive Particle Movement of the NS-ACFs

Conductive particle movement of the NS-ACFswas analyzed by counting the number of conductiveparticles captured between bumps and electrodesbefore and after ACF bonding process and compar-ing them with the conventional ACFs as shown in

Fig. 5. Electrospun PVDF nanofibers with incorporated conductiveparticles.

Fig. 6. Thickness profile of compressed PVDF nanofibers atincreasing temperatures.

Lee and Paik170

Fig. 11. For the conventional ACFs, there was anaverage of 42 conductive particles on a single bumpat the initial state, but only 13 of them werecaptured after the bonding process. However, forthe PVDF NS-ACFs, the same 13 conductive parti-cles were captured after the bonding process usingNS-ACFs with only 16 conductive particles at theinitial state. In other words, the NS-ACFs showedan 81% conductive particle capture rate, while theconventional ACFs showed only 31%. For the con-ventional ACFs, the conductive particles were flownout easily with polymer resin during the bonding

process. But for the NS-ACFs, the NS was unmeltedand remained during the bonding process, and itprevented the conductive particles from flowing out.

Fig. 7. Compressed PVDF nanofibers at (a) room temperature, (b) 50�C, (c) 160�C, and (d) 180�C at 3 MPa for 10 s.

Fig. 8. PVDF NS after plasma etching for (a) 0 s, (b) 120 s, (c) 240 s, and (d) 360 s.

Fig. 9. SEM images and cross-sectional focused ion beam (FIB)images of PVDF NS (a) before plasma etching and (b) after plasmaetching of both the top and bottom of the NS for 240 s.

Fig. 10. SEM images and cross-sectional FIB images of PVDF NS(a) before plasma etching and (b) after plasma etching of both thetop and bottom of the NS for 240 s.

A Study on the Nanofiber-Sheet Anisotropic Conductive Films (NS-ACFs) for Ultra-Fine-PitchInterconnection Applications

171

Therefore, NS-ACFs could suppress the movementof the conductive particles significantly, resulting inthe great improvement of the conductive particlecapture rate. By using the NS-ACFs, the capturerate was even more improved compared with thenanofiber ACFs which showed 65%.15

It was very important to improve the conductiveparticle capture rate in ultra-fine-pitch interconnec-tions because there was more chance of having short-circuit failure by agglomerated conductive particlesbetween fine bump gaps. The short-circuit rates of a 20-lm ultra-fine-pitch COG assembly using the NS-ACFsand the conventional ACFs were measured as shown inFig. 12. The conventional ACFs showed an 8.3% short-circuit rate, while the NS-ACFs showed 100% insula-tion. NS-ACFs required only 38% of conductive parti-cles at the initial state to capture the same number ofconductive particles compared with the conventionalACFs. By using NS-ACFs, it was possible to signifi-cantly reduce the amount of conductive particlescaptured between fine bump spaces resulting in noshort-circuit failure. Moreover, they could reduce thecost of ACFs since Ni/Au-coated conductive particles

represent a major portion of the ACF’s cost. Insummary, cost-effective NS-ACFs could perfectly pre-vent short-circuits in ultra-fine-pitch COG assembly.

Electrical Contact Property of the NS-ACFJoint

The contact resistances of ultra-fine-pitch COGassemblies with the conventional ACFs and the NS-ACFs were measured as shown in Fig. 13. Theaverage contact resistance of the conventional ACFswas 254 mX, while that of the NS-ACFs was244 mX. They showed similar results since NS-ACFs were designed to capture the same number ofconductive particles after the bonding process forthe same contact area.

The bonding was conducted at 160�C which waslower than the melting temperature of the PVDFNS. Although the insulating NS was unmelted andremained after the bonding process, there was nodifference in contact resistance of PVDF NS-ACFscompared to the conventional ACFs, because the topand bottom polymers around the conductive

Fig. 11. (a) Average number of captured conductive particles per (a) bump and (b) capture rates of conventional and the NS-ACFs.

Fig. 12. Short-circuit rates of conventional and PVDF NS-ACFs. Fig. 13. Single bump contact resistances of conventional ACFs andPVDF NS-ACFs.

Lee and Paik172

particles were plasma etched. As a result, the Ni/Aucoating layer of the conductive particle in the NS-ACF was directly in contact with the bump and theelectrode similar to the conventional ACFs, asshown in Fig. 14. Therefore, by using the NS-ACFs,stable contact resistance could be obtained.

Reliability Test of the NS-ACFs

For the reliability test, COG test samples assem-bled by using conventional ACFs and PVDF NS-ACFs were placed in a high-temperature humidity

chamber under an 85�C/85% relative humidity (RH)condition for 1000 h. Then, changes in contactresistances were measured and plotted as accumu-lated graphs as shown in Fig. 15. According to thereliability test results, no open failure was observedfor either conventional ACFs or NS-ACFs. Thecontact resistance slightly increased up to 500 hdue to the hygroscopic expansion of polymer resinsthen stabilized afterward. By comparing the relia-bility test results, it could be concluded that therewere no significant defects caused by NS, which wasunmelted and remained after the bonding process.

Fig. 14. COG joint morphologies by using (a) conventional ACFs and (b) PVDF NS-ACFs.

Fig. 15. 85�C/85% relative humidity test of (a) conventional ACFs and (b) PVDF NS-ACFs.

A Study on the Nanofiber-Sheet Anisotropic Conductive Films (NS-ACFs) for Ultra-Fine-PitchInterconnection Applications

173

CONCLUSION

A 20-lm ultra-fine-pitch with a 7-lm bumpspacing COG interconnection was successfullydemonstrated using the PVDF NS-ACFs. TheNS-ACFs significantly suppressed the movementof the conductive particles and remarkablyincreased the capture rate up to 81% comparedwith 31% of conventional ACFs and 65% ofnanofiber ACFs. Since the surfaces of the conduc-tive particles were exposed after the plasmaetching process, excellent electrical paths werecreated between bumps and electrodes, andstable contact resistance was obtained withoutmelting the NS. As a result, the two-step bondingprocess of the nanofiber ACFs could be reduced toa one-step bonding process using the NS-ACFs.Moreover, the NS-ACFs could overcome the limi-tation of nanofiber polymer selectivity by usingthe plasma etching technique. Therefore, by usingNS polymers with higher tensile strength, wecould suppress the movement of conductive parti-cles more effectively and prevent the short-circuitfailures in ultra-fine-pitch assemblies. This studyhas shown that the NS-ACFs are one of thepromising interconnection materials for ultra-fine-pitch packaging applications.

REFERENCES

1. J.U. Knickerbocker, P.S. Andry, L.P. Buchwalter, and A.Deutsch, IBM J. Res. Dev. 49, 725 (2005).

2. M.J. Yim, C.K. Chung, and K.W. Paik, IEEE Trans. Elec-tron. Packag. 30, 306 (2007).

3. C.K. Chung, G.D. Sim, S.B. Lee, and K.W. Paik, IEEETrans. Compon. Packag. Manuf. Technol. 2, 359 (2012).

4. K. Takahashi, M. Umemoto, N. Tanaka, K. Tanida, Y.Nemoto, Y. Tomita, M. Tago, and M. Bonkohara, Micro-electron. Reliab. 43, 1267 (2003).

5. M.J. Yim and K.W. Paik, Int. J. Adhes. Adhes. 26, 304(2006).

6. M.J. Yim, C.K. Chung, and K.W. Paik, IEEE Trans. Elec-tron. Packag. 30, 306 (2007).

7. C.K. Chung, G.D. Sim, S.B. Lee, and K.W. Paik, IEEETrans. Compon. Packag. Manuf. Technol. 2, 359 (2012).

8. G. Taylor, Math. and Phys. Sci. 313, 453 (1969).9. W.S. Kwon and K.W. Paik, IEEE Trans. Compon. Packag.

Technol. 29, 528 (2006).10. C.T. Peng, C.M. Liu, J.C. Lin, H.C. Cheng, and K.N. Chiang,

IEEE Trans. Compon. Packag. Technol. 27, 684 (2004).11. Y.W. Chiu, Y.C. Chan, and S.M. Lui, Microelectron. Reliab.

2002, 42 (1945).12. C. Jang, S. Han, J. Ryu, S. Cho, and H. Kim, IEEE Trans.

Adv. Packag. 30, 2 (2007).13. S.H. Lee, K.L. Suk, K. Lee, and K.Y. Paik, IEEE Trans.

Compon. Packag. Manuf. Technol. 2, 2108 (2012).14. K.L. Suk, C.K. Chung, and K.W. Paik, J. Nanosci. Nan-

otechnol. 13, 351 (2013).15. S.H. Lee, T.W. Kim, K.L. Suk, and K.W. Paik, J. Electron.

Mater. 44, 4628 (2015).

Lee and Paik174