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Research ArticleReceived: 1 June 2009 Revised: 29 August 2009 Accepted: 14 October 2009 Published online in Wiley Interscience: 1 December 2009

(www.interscience.wiley.com) DOI 10.1002/sia.3134

Multi-metal shell coated with surfacemodification of polystyrene bead coresto improve its adhesion metal shellJun-Ho Lee,a Youngkwan Leeb and Jae-Do Namc∗

We investigated the delamination problem at the metal-polymer interface and the mechanical buckling of the metal layerat a localized area of the metallic shell under compression between two parallel plates. First, polystyrene (PS) beads weresynthesized by dispersion polymerization and then their sulfonation process. After sequential electroless deposition, theaverage size of multi-metal coated sulfonated polystyrene (SPS) bead was ca 4.95 µm. Using the electromechanical indentation,the electrical resistance of a single metal-coated SPS bead decreased with increasing compressive strain without delaminationat the metal-polymer interface, and its electrical resistance showed 5.65 �. Copyright c© 2009 John Wiley & Sons, Ltd.

Keywords: adhesion; metal–polymer; conductive filler; electroless

Introduction

As electronics packaging continues to improve in terms ofperformance miniaturization and weight, the flip-chip package isgaining popularity in the electronics packaging industry becauseof its smaller size and light weight.[1 – 3] The flip-chip technologyhas focused on the use of an anisotropic conductive adhesive(ACA), which is composed of an insulating adhesive polymermatrix and conductive fillers in place of solder, and underfillfor the attachment of electronic chips to the electrodes on theglass or organic substrate.[3,4] In general, the particles are oftenspherical and range 3–30 µm in size for ACA applications.[2] Thesimplest fillers are metal particles such as gold, silver, copper,lead-free solders. Although, the tin/lead alloys are widely used asinterconnects in flip-chip packages, there are increasing concernsabout the use of tin/lead alloy solders, because lead, a majorcomponent in the solder, has long been recognized as a healththreat to human beings. Therefore, some ACA systems employnonconductive particles with a thin metal coat.[3 – 7] Polystyrene(PS) is often selected as the core material because the coefficientof thermal expansion of metal-coated PS beads is close thermosetadhesive as well as PS beads deform when compressed betweenopposing contact surface, and thus provide a large contact area.

Recently, several approaches have been proposed to formmetallic or inorganic layers on a polymer bead, including directmetallization, electroless deposition and layer-by-layer (LbL) self-assembly method to produce conductive filler. One of them,electroless deposition of a metallic layer onto a polymer surface,which is widely used in electronics system, is a simple andconvenient method for metallizing a polymer surface.[8 – 12]

For example, we previously reported the fabrication ofnickel/gold multilayered shells on PS bead cores by sequen-tial electroless deposition processes.[11] However, metal-coatedconductive fillers without surface modification often have thedelamination problem at the metal–polymer interface and themechanical buckling of the metal layer at a localized area of themetallic shell under the large deformation.

In particular, the metal coating on polymer beads in the flip-chipbonding technique should be strong enough to sustain severalphysicochemical downstream processes, which include bindermixing film forming, thermal bonding, etc. Surface modificationof polymer including chemical treatment, plasma treatmentand graft-copolymerization technique provides various functionalgroups on the surface of the polymer substrates in order to induceenhanced physicochemical bonding.[13 – 15]

In this study, monodispersed PS beads were synthesized bydispersion polymerization. In order to induce the strong bondbetween metal layer and polymer beads surface, PS beads weresulfonated by chlorosulfonic acid. Finally, Sn, Pd, Ni–P andAu were sequentially deposited on them using the electrolessdeposition. Furthermore, the effect of metal–polymer interfacewas investigated with micro-indentation tester.

Experimental

Materials

2,2-Azobisisobutyroitrile of analytical grade (AIBN, Junse Chemical)was used as an initiator without further purification. Nickel-platingsolution (ICP Nicoro GIB) and gold-plating solution (ICP Nicoro OCP)were obtained from Okuno Chemical Co. Ltd. All the following

∗ Correspondence to: Jae-Do Nam, Department of Polymer Science and Engi-neering and Department of Energy, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea. E-mail: jdnam@skku.edu

a Gyeonggi Regional Research Center, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea

b Department of Chemical Engineering, Sungkyunkwan University, 300Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea

c Department of Polymer Science and Engineering and Department of Energy,Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746,South Korea

Surf. Interface Anal. 2010, 42, 36–39 Copyright c© 2009 John Wiley & Sons, Ltd.

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Polymer–metal adhsersion

Figure 1. Schematic representation of the procedure used to prepare the polymer beads coated with a metallic shell: (I) PS beads were synthesized bydispersion polymerization, (II) PS beads were sulfonated by chlorosulfonic acid, (III) SPS beads were sensitized by SnCl2 solution, (IV) the sensitized SPSbeads were activated by PdCl2 solution, (V) nickel electroless plating, and (VI) gold electroless plating on the SPS beads.

Figure 2. SEM image of PS beads synthesized by dispersion polymerization(a) and the particle size distribution of PS beads using the laser diffractionparticle size analyzer (b).

chemicals were obtained from Aldrich and used as received:poly(vinylpyrrolidone) (PVP, Mn: 40 000), palladium(II) chloride(99%), hydrochloric acid, chlorosulfonic acid, sodium borohydride(NaBH4), styrene monomer, ethanol and tin chloride.

Preparation of PS beads

PS beads were synthesized by using a dispersion poly-merization.[16 – 17] Styrene monomer (30 ml) and PVP (3.0 g) weredissolved in a mixture of ethanol and water. The AIBN concentra-tion was 1.5 wt% relative to the total amount of monomer. Thereaction mixture was heated at 60 ◦C and the polymerization wasconducted under a nitrogen atmosphere at a constant agitationspeed of 200 rpm for 12 h. After the completion of the polymeriza-tion, the resultant product was repeatedly washed with methanoland water in order to remove the remaining PVP and styrenemonomer. Finally, PS beads were dried by freeze dryer.

Sulfonation of PS beads

The synthesized PS beads were sulfonated by chlorosulfonic acid.PS beads were (1 g) immersed in chlorosulfonic acid for 300 min at0 ◦C. Finally, reaction was then terminated by adding ethanol.In order to measure degree of sulfonation of PS beads, theion exchange capacity (IEC) was determined by an acid–basetitration method.[18] Briefly, sulfonated polystyrene (SPS) beadswere immersed in saturated sodium chloride (NaCl) solution andthe mixture was stirred for 24 h. The released H+ ions weretitrated with 0.1 M sodium hydroxide (NaOH) solution. The IEC wascalculated from the consumed NaOH via the following equation:

IEC = consumed NaOH(mL) × molarity of NaOH

dried polymer weight (g/mol)(1)

Preparation of conductive fillers

SPS beads (2 g) were dipped in a SnCl2 solution for 1 h to sensitizethe SPS beads. Sn2+ ions are adsorbed onto the negatively chargedSPS beads due to the electrostatic interaction. Sensitized SPS beadsimmersed in a PdCl2 solution (30 mM) for 1 h at 60 ◦C to give apalladium ion-absorbed SPS surface through an ion exchangereaction, which was then reduced using the sodium borohydride(NaBH4, 10 mM) for 20 min at 60 ◦C. These beads were sequentiallydeposited by Ni–P and Au, respectively.

Characterization

The size, morphology and structure of the resulting productswere examined by scanning electron microscopy (SEM, JEOL,

Surf. Interface Anal. 2010, 42, 36–39 Copyright c© 2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia

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J.-H. Lee, Y. Lee and J.-D. Nam

Figure 3. SEM images of the beads for (a) SPS beads, (b)SPS beads activated by Pd, (c) Pd-modified SPS beads coated with nickel, and (d) Ni-plated SPSbeads coated with Au.

JSM 890) and transmission electron microscope (TEM, JEM-2000EXIT). A micro-compression tester (Shimadzu, MCT-W) anddigital multimeter (Advantest, R6552) were used to measure theelectrical resistance of the individual metallic shell coated SPSbead.

Results and Discussion

Figure 1 shows a schematic diagram of the general procedureused to prepare the conductive fillers. Figure 2(a) shows a SEMimage of the pristine PS beads. The morphology of the pristine PSbeads was perfectly spherical, uniform size and smooth withoutany aggregation. Moreover, Fig. 2(b) shows the particle size distri-bution of the pristine PS beads determined using a laser diffractionparticle size analyzer. The major peak is centered at 4.60 µm andthe particles sizes range around 4.55–4.65 µm. After the sulfona-tion process, IEC value obtained is 2.1 meq/g. Consequently, itconfirmed the formation of sulfonic acid groups on the surface ofPS beads.

Figure 3(a–d) shows a representative SEM image of the SPSbeads, activated SPS beads, nickel and gold deposited SPS beads,respectively. SPS beads retained their spherical shape and theirsurface remained smooth without any visible deformation andaggregation slits being formed on them (Fig. 3(b)). As can beseen in Fig. 3(c) and (d), the Ni- and Au-deposited beads havea slightly rough surface. This suggests that nickel particlescan form selectively on the PS surface when SPS beads areimmersed in a solution containing Pd2+. The anionic site canthen interact with a palladium ion due to an electrostaticinteraction. After the sequential nickel and gold depositions, thePd/Ni–P/Au particles form and grow in the vicinity of the SPSsurface.

Figure 4. Cross section TEM image of the conductive filler.

Figure 4 shows cross section TEM image of the conductive filler.It can be seen that the metal layer formed a continuous phaseat a thickness of ca 210 nm, and significant void or interlaminardelamination could not be observed, highlighting the robustadhesive bonding between the metal and PS bead. In order tomake binding between polymer surface and metal layer clearer,we measured the displacement–resistance relation of a preparedsingle conductive filler by using the micro-indentation tester.

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Figure 5. The resistance and force of single resulting bead plotted as afunction of the compressive strain.

Figure 5 shows the electrical resistance and compressive forceas a function of the compressive strain. It can be seen that electricalresistance of the conductive filler decreased with increasingcompressive strain without a noisy signal. The electrical resistanceshowed 5.65 �. This indicates that sulfonic acid groups induced astrong bond between the PS beads surface and the metal layer.

Conclusions

In this paper, we presented a conductive filler using electrolessdeposition with functional PS beads for the improvement ofinterfacial adhesion between polymer beads and metal layer.Monodisperse PS beads ca 4.6 µm in size have been synthesizedby dispersion polymerization and then they are sulfonated byusing chlorosulfonic acid. According to SEM and TEM, the metalliclayers with thickness of ca 210 nm were successfully formed acontinuous phase on the surface of the SPS without significant voidor interlaminar delamination. Using a micro-indentation tester, theresistance of a single conductive filler decreased substantially to

5.65 �. There were no cracks or void found at the PS beads tometal layer interface from our results. Hence the formed sulfonicacid groups on PS beads induced a strong bond between thepolymer surface and metal layer.

Acknowledgements

This work was supported by the Postdoctoral Research Program ofSungkyunkwan University (2009). We also appreciate the projectand equipment support from Gyeonggi Province through theGRRC program in Sungkyunkwan University.

References

[1] M. Datta, T. Osaka, J. W. Schultze, Microelectronic Packaging, CRCpress: London, 2005.

[2] R. R. Tummala, Fundamentals of Microsystmes Packaging, McGRAW-Hill: New York, 2001.

[3] F. Coombs Jr, Printed Circuits Handbook II, McGRAW-Hill: New York,2001.

[4] J. Y. Myong, J.-S. Hwang, J. G. Kim, J. Y. Ahn, H. J. Kim, W. Kwon,K.-W. Paik, J. Electron. Mater. 2004, 33, 76.

[5] T. Laurila, V. Vuorinen, T. Mattila, J. K. Kivilahti, J. Electron. Mater.2005, 34, 103.

[6] L. Shiau, C. Ho, C. Kao, Solder. Surf. Mt. Tech. 2002, 14, 25.[7] J. Liu, R. Rogren, J. Electro. Manuf. 1993, 3, 205.[8] S.-H. Cho, W.-Y. Kim, G.-K. Jeong, Y.-S. Lee, Colloid Surf. A. 2005, 255,

79.[9] T. Lei, K. Aoki, J. Electroanal. Chem. 2000, 482, 149.

[10] J.-H. Lee, Y. Lee, J.-D. Nam, Macromol. Rapid Commun. 2009, 30, 52.[11] J.-H. Lee, J. S. Oh, P.-C. Lee, D.-O. Kim, Y. Lee, J.-D. Nam, J. Electron.

Mater. 2008, 37, 1648.[12] J.-H. Lee, H. Choi, J.-D. Nam, J. Mater. Res. 2009, 24, 253.[13] H. W. Giobson, F. C. Bailey, Macromolecules. 1980, 13, 34.[14] J. Fredrich, G. Kuhn, R. Mix, W. Unger, Plasma Processes Polym. 2004,

1, 28.[15] B. Wang, Z. Ji, F. T. Zimone, G. M. Janowski, J. M. Rigsbee, Surf. Coat.

Technol. 1996, 19, 64.[16] B. Thomson, A. Rudin, G. Lajoie, J. Appl. Polym. Sci. 1996, 59, 2009.[17] M. A. Awan, V. L. Dimonie, M. S. El-Aasser, J. Polym. Sci., PArt A: Polym.

Chem. 1996, 34, 2651.[18] S. Fisher, R. Kunin, Anal. Chem. 1955, 27, 1191.

Surf. Interface Anal. 2010, 42, 36–39 Copyright c© 2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia