Nanofiber Anisotropic Conductive Films (ACF) - Springer Link

Journal of ELECTRONIC MATERIALS, Vol. 44, No. 11, 2015

DOI: 10.1007/s11664-015-4021-0 Ó 2015 The Minerals, Metals & Materials Society

Nanofiber Anisotropic Conductive Films (ACF) for Ultra-FinePitch Chip-on-Glass (COG) Interconnections SANG-HOON LEE,1 TAE-WAN KIM,1 KYUNG-LIM SUK,1 and KYUNG-WOOK PAIK1,2 1.—Department of Materials Science and Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea. 2.—e-mail: [email protected]

Nanofiber anisotropic conductive films (ACF) were invented, by adapting nanofiber technology to ACF materials, to overcome the limitations of ultra-finepitch interconnection packaging, i.e. shorts and open circuits as a result of the narrow space between bumps and electrodes. For nanofiber ACF, poly(vinylidene fluoride) (PVDF) and poly(butylene succinate) (PBS) polymers were used as nanofiber polymer materials. For PVDF and PBS nanofiber ACF, conductive particles of diameter 3.5 lm were incorporated into nanofibers by electrospinning. In ultra-fine-pitch chip-on-glass assembly, insulation was significantly improved by using nanofiber ACF, because nanofibers inside the ACF suppressed the mobility of conductive particles, preventing them from flowing out during the bonding process. Capture of conductive particles was increased from 31% (conventional ACF) to 65%, and stable electrical properties and reliability were achieved by use of nanofiber ACF. Key words: Anisotropic conductive films (ACF), ultra-fine-pitch, chip-onglass (COG) interconnection, nanofiber, electrospinning

INTRODUCTION The demand for smart electronic devices, for example telephones and tablet personal computers, has risen dramatically in recent decades. The display industry requires more demanding technology, for example ultra-high-definition, to fulfil the needs of customers who require high performance, multifunctionalization, and miniaturization. As a consequence, more input/output pins are placed within the same space and the space between bumps has decreased, resulting in fine-pitch interconnection. Fine-pitch packaging technology has therefore become very important in assembly processes.1 In flip-chip packaging, anisotropic conductive films (ACF), well known interconnecting adhesives which consist of thermosetting resin and conductive particles in a film format, have been widely used to provide interconnections and attachment between bumps and electrodes.2 However in ultra-fine-pitch interconnections, the pitch and the space between (Received March 2, 2015; accepted August 27, 2015; published online September 11, 2015)

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bumps and electrodes have become extremely small, resulting in such interconnection problems as short circuits and open circuits, which did not occur with conventional ACF bonding. As a result, a novel ACF concept, called ‘‘nanofiber ACF’’, was introduced to solve these issues.3–5 It was proposed that nanofiber ACF would be a promising interconnection material for ultra-fine-pitch assembly, because they could suppress the mobility of conductive particles and eliminate electrical interconnection problems at ultra-fine-pitch. In this study, nanofiber ACF were investigated for ultra-fine-pitch chip-on-glass (COG) interconnections in which the assembly structure was established by a flip-chip bonding, by use of nanofiber ACF. In this process, an electrical path was formed via conductive particles captured between a bump of the silicon chip and a thin-film electrode of the glass substrate. Nanofiber ACF incorporating conductive particles were investigated to determine conductive particle capture and electrical properties for 20 lm ultra-fine-pitch COG interconnections for which the bump gap was only 7 lm. Poly(vinylidene fluoride( (PVDF) and poly(butylene succinate) (PBS)

Nanofiber Anisotropic Conductive Films (ACF) for Ultra-Fine-Pitch Chip-on-Glass (COG) Interconnections

were selected as nanofiber polymer materials.6 Conductive particles coated with insulating polymer beads were incorporated into PVDF and PBS nanofibers by electrospinning. Finally, conductive particle movement and electrical resistance were compared for PVDF and PBS nanofiber ACF on the basis of their thermal properties. EXPERIMENTAL Materials and Test Vehicles For nanofiber ACF, PVDF was dissolved in dimethylacetamide (DMAC) and acetone, and PBS was dissolved in chloroform and 3-chloro-1-propanol. Conductive particles of diameter 3.5 lm coated with Ni–Au and an insulating polymer layer were then added to each nanofiber solution. Silicon chips with 12 lm thick Au bumps with 20 lm pitch and ˚ 7 lm bump space and glass substrates with 1300-A Ti–Au thin-film electrodes with 20 lm pitch and 7 lm electrode space were used for COG interconnections, as shown in Fig. 1. Each COG test sample contained four-point contact resistance patterns to enable measurement of the contact resistance of a COG joint and insulation resistance patterns to enable measurement of electrical shorts between 24 nearby joints. Fabrication of Double-Layer Nanofiber ACF Nanofiber ACF with conductive particles incorporated into nanofibers were fabricated by ‘‘electrospinning’’, as shown in Fig. 2. When a high voltage was applied to a droplet of a polymer solution on a syringe needle and a grounded target (the collector), the body of the polymer solution became charged, and electrostatic repulsion counteracted the surface tension and stretched the droplet,

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resulting in a Taylor cone shape. Above a critical voltage a charged liquid jet was formed. As the jet dried in flight, it became elongated, forming a nanofiber.7,8 To fabricate nanofibers incorporating conductive particles, the conductive particles were added to the PVDF and PBS nanofiber solutions before electrospinning. For PBS nanofiber solution, 0.05 wt.% tetrabutylammonium bromide was added to increase the conductivity of the solvents and reduce the diameters of PBS nanofibers, to achieve uniform nanofiber diameter and particle distribution.9,10 For the conductive particles, polymer particles with core–shell structure with total diameters of 3.5 lm were used. Polymer beads of diameter 3.25 lm with a 0.25-lm nickel–gold coating were used as core polymer particles; the particles were coated with an insulated polymer layer 50–250 nm thick (Fig. 3). These conductive particles were also used for conventional ACF. Finally, two high-viscosity nonconductive films (NCF) were laminated on the top and bottom of electrospun nanofiber fabrics containing conductive particles by use of a roll-laminator. To fabricate double-layer ACF the top of the NCF-laminated nanofiber was also laminated with low-viscosity NCF, as shown in Fig. 4.1 The minimum viscosity of the high-viscosity NCF was 10 times higher than that of the low-viscosity NCF. Finally, the laminated nanofiber ACF were pressed once again by use of a vacuum laminator at 50°C under 80 psi for 1 min, to remove captured voids. Double-layer ACF have been used for conventional COG ACF and are regarded as suitable for capturing more conductive particles because of the COG structure, in which the electrodes of glass substrates were thin-film electrodes. Low-viscosity NCF were designed to fill the empty space between bumps whereas high-viscosity

Fig. 1. Images of ultra fine pitch COG (a) test chip and (b) glass substrate.

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NCF suppress conductive particle movement. In this research, the same concept of the double-layer ACF format was used to fabricate double-layer nanofiber ACF.

Fig. 2. Electrospinning method.

Lee, Kim, Suk, and Paik

Effects of Nanofibers on the Movement of Conductive Particles and Insulation Resistance In this experiment, two-step bonding was performed. During the first bonding process, called the ‘‘resin flowing process’’, 70 MPa was applied at 80°C for 10 s. In this process, unmelted nanofibers suppressed the mobility of conductive particles so that the particles were captured between bumps and thin-film electrodes while flowing out of the resin. In the second bonding process, called the ‘‘main bonding process’’, 30 MPa was applied at 190°C for 5 s for PVDF nanofiber ACF and 30 MPa was applied at 160°C for 5 s for PBS nanofiber ACF, to melt the nanofiber polymers and cure the resins. The main bonding temperature conditions were decided on the basis of the melting points of each nanofiber polymer. Finally, movement of the conductive particles when using PVDF and PBS nanofiber ACF was analyzed and compared with that for conventional COG ACF, to investigate the effects of the nanofibers on conductive particle movement. The particle density of conventional ACF was 50 k/mm2 whereas that of the optimized nanofiber ACF was 20 k/mm2, only 40% that of the conventional ACF. It was expected that nanofiber ACF can perform the same as conventional ACF with lower particle density because capture by nanofiber ACF is expected to be higher. Characterization of ACF Joint Properties

Fig. 3. SEM image of Ni/Au and insulating polymer layer coated polymer.

After the 20 lm ultra-fine COG interconnection package was assembled by using the nanofiber ACF, the cross-sectional morphology of the ACF joints was analyzed to investigate whether the nanofiber layers melted and electrical conduction occurred. Scanning electron microscopy was used to analyze the morphology of the COG nanofiber ACF joints. The electrical properties of COG joints assembled by use of nanofiber ACF were analyzed by using Kelvin structures for single-bump contact resistance measurement and insulation patterns to measure the insulation resistance of 24 nearby joints, as shown in Fig. 5. The contact resistance

Fig. 4. Fabrication procedures of double layer nanofiber ACF by NCF lamination.


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Fig. 5. (a) Single contact resistance pattern using the Kelvin 4 point probe pattern and (b) an insulation resistance pattern.

Effects of Nanofibers on Movement of Conductive Particles and on Insulation Resistance

Fig. 6. Die shear test for adhesion strength measurement with 100 lm/s shear speed.

was measured to reveal any open circuit or higher contact resistance problems caused by missing or an insufficient number of trapped conductive particles or unmelted nanofiber layers between the bumps and electrodes. The insulation resistance was also measured to reveal any short circuits caused by agglomerated conductive particles between the neighboring bumps at ultra-fine-pitch interconnections. Finally, a die shear test and reliability test were conducted to detect any nanofiber related defects caused by nanofibers inside ACF resins. The die shear test with nanofiber ACF was performed to measure the adhesion strength of the COG assembly, as shown in Fig. 6. For reliability testing a 85°C, 85% RH test was performed for 1000 h, and the change in contact resistance was measured. The adhesion strength and reliability results for nanofiber ACF were then compared with those for conventional ACF. RESULTS AND DISCUSSION Fabrication of Nanofiber ACF PVDF and PBS nanofibers were electrospun under the optimized electrospinning conditions listed in Table I. Conductive particles were successfully incorporated into uniform nanofibers (Fig. 7).

Particle movement in conventional ACF and nanofiber ACF was analyzed by counting the number of conductive particles captured between bumps and electrodes before and after main bonding (Fig. 8). For conventional ACF, there was an average of 42 conductive particles on a single bump before bonding; only 13 of these were captured after the main bonding. However, for the PVDF and PBS nanofiber ACF, only an average of 21 conductive particles was required for capture of 13 conductive particles after the main bonding. In other words, only 31% of conductive particles were captured by use of conventional ACF whereas nanofiber ACF captured 65% of conductive particles, as shown in Fig. 9. In conventional ACF the conductive particles were free to move as a result of polymer resin flow during the main bonding process. However, the nanofiber layers in nanofiber ACF suppressed free movement of conductive particles and significantly increased the number of captured conductive particles after the main bonding. It is important to increase capture of conductive particles per bump in ultra-fine-pitch interconnections, because open circuit and/or high contact resistance and short circuit problems were caused by low capture of conductive particles. Therefore, by use of nanofiber ACF capture of conductive particles was significantly increased compared with conventional ACF. Insulation resistance of 20 lm ultra-fine-pitch COG assemblies with PVDF and PBS nanofiber ACF and with conventional ACF were measured and compared. Fir conventional ACF the incidence of short circuits was 8.3% whereas no short circuits were observed for both nanofiber ACF as shown in Fig. 10. Since the nanofiber ACF contained only a half of conductive particles in the initial state compared with the conventional ACF, there was significantly less chance of having short circuit caused by agglomerated conductive particles between bumps. As a summary, nanofiber ACF increased the conductive particle capture rate after the main bonding resulting in a lower amount of conductive particles needed at the initial formulation of

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Table I. Optimized electrospinning conditions for PVDF and PBS nanofibers with incorporated conductive particles

Concentration (wt.%) Particle content (w/w) (polymer:particle) Solvent (w/w) Applied voltage (kV) Spinning rate (lL/min) Working distance (cm)

PVDF

PBS

18 1:0.3 DMAC–acetone (3:2) 8 10 10

12 1:0.3 Chloroform–3-chloro-1-propanol (9:1) 12.5 20 12.5

Fig. 7. Conductive particles incorporated (a) PVDF nanofiber and (b) PBS nanofiber.

Fig. 8. Conductive particles captured on bumps before and after main bonding.


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Fig. 9. (a) Number of captured conductive particles per a single bump and (b) the capture rates of various ACFs.

Fig. 11. Contact resistance using various ACFs bonded at 160°C. Fig. 10. Short circuit rate of various ACFs joints with 20 lm pitch and 7 lm bump spacing.

nanofiber ACF, and this eventually improved the insulation property of ultra-fine-pitch COG interconnection. Characterization of ACF Joint Properties Contact Resistance The contact resistances of ultra-fine-pitch COG assemblies with PVDF and PBS nanofiber ACF were measured and compared with those for conventional ACF, as shown in Fig. 11. At 160°C, contact resistance was stable at approximately 300 mX, for conventional ACF and the PBS nanofiber ACF whereas for the PVDF nanofiber ACF the contact resistance was >2000 mX, regarded as open circuit. The reason for this high resistance was the melting temperature of PVDF polymer. Differential scanning calorimetry (DSC) was conducted to measure the melting temperatures of

PVDF and PBS polymers. According to the DSC results shown in Fig. 12, the melting temperatures of PVDF and PBS nanofibers were 165°C and 118°C, respectively. At 160°C, the main bonding temperature, the PVDF nanofiber did not melt, because the melting temperature of the nanofiber polymer was higher than 160°C. The unmelted PVDF nanofiber between a conductive particle and an electrode was observed by cross-sectional analysis as shown in Fig. 13. For the conventional ACF and the PBS nanofiber ACF, the bump and the electrode made full metal contact via captured conductive particles, because the PBS melting temperature was 118°C. However, for the PVDF nanofiber ACF, a thin PVDF nanofiber layer remained between a conductive particle and an electrode. As a result, good electrical conduction was not possible with the ACF joints, because the remaining PVDF nanofiber layers resulting in an open circuit or high contact resistance.

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Fig. 12. DSC melting temperature analyses of (a) PVDF and (b) PBS nanofibers.

Fig. 13. SEM images of COG joint using (a) conventional ACFs, (b) PVDF nanofiber ACF, and (c) PBS nanofiber ACF bonded at 160°C.

had the same stable contact resistance of approximately 300 mX, as shown in Fig. 14. Both PVDF and PBS nanofiber ACF had the same contact resistance as conventional ACF, because both nanofibers were completely melted where the bonding temperature was 190°C. As a result, no remaining nanofiber layers were found at 190°C for PVDF nanofiber ACF, as shown in Fig. 15. Therefore, it is clear that nanofibers inside the ACF resins must be melted during the main bonding process achieve stable contact resistance. Adhesion Strength and Reliability Testing of Nanofiber ACF Fig. 14. Contact resistance using various ACFs bonded at 190°C.

Because the unmelted PVDF nanofiber layers disturbed electrical conduction, the main bonding temperature was increased to 190°C to completely melt the PVDF nanofiber. At 190°C, all three ACF

According to the die shear test, the adhesion strengths of all three ACF were almost the same, as shown in Fig. 16. The adhesion strength of the conventional ACF was 34 MPa whereas that of the PVDF and PBS nanofiber ACF was 33 MPa. For the reliability tests, COG samples were bonded at 160°C for conventional ACF and the PBS


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Fig. 15. SEM images of COG joint using (a) conventional ACF, (b) PVDF nanofiber ACF, and (c) PBS nanofiber ACF bonded at 190°C.

increased slightly because of hygroscopic expansion of the polymer resins, but never exceeded 1000 mX, and, again, no open-circuit failure was observed for any of the three ACF. It is obvious that use of PVDF and PBS nanofibers had very little or almost no effect on adhesion strength or on the reliability test. CONCLUSION

Fig. 16. Adhesion strength test results using various ACFs.

nanofiber ACF. For the PVDF nanofiber ACF the samples were bonded at 190°C to melt the PVDF nanofiber layers and achieve stable contact resistance. According to the 85°C 85% RH reliability test results, no open-circuit failure was occurred at any ACF, as shown in Fig. 17. Contact resistance increased continuously but started to stabilize after 500 h. After 1000 h, the contact resistance

A COG assembly with 20 lm ultra-fine-pitch and 7 lm bump spacing was successfully achieved by use of PVDF and PBS nanofiber ACF. Nanofibers contributed to suppressing the mobility of the conductive particles during the main ACF bonding process, and 65% of the initial conductive particles were captured after the main bonding process. This was a remarkable result, because conventional ACF captured only 31% of the initial conductive particles after the main ACF bonding process. Use of nanofiber ACF resulted in no short circuits in the 20 lm pitch and 7 lm bump spacing COG assembly. Stable bump contact resistance and no open circuits were also observed for nanofiber ACF. Finally, it was also shown that the nanofibers inside the ACF resins had no effect on the adhesion strength or reliability of the COG package. This study showed that

Fig. 17. 85°C/85% RH reliability test of (a) conventional ACF, (b) PVDF nanofiber ACF, and (c) PBS nanofiber ACF up to 1000 h.

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nanofiber ACF are promising interconnection materials for ultra-fine-pitch packaging applications. REFERENCES 1. M.J. Yim, J. Hwang, and K.Y. Paik, Int. J. Adhes. Adhes. 27, 77 (2007). 2. M.J. Yim and K.Y. Paik, in Proceedings of the 1st IEEE Symposium Polymeric Electronics Packaging Conference (PEP’97), p. 233 (1997). 3. K.L. Suk, C.K. Chung, and K.Y. Paik, in Proceedings of the 61st Electronic Components and Technology Conference, p. 656 (2011).

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