OVER the last several decades, the size of electronic

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1 2108 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 12, DECEMBER 2012 Study on Fine Pitch Flex-on-Flex Assembly Using Nanofiber/Solder Anisotropic Conductive Film and Ultrasonic Bonding Method Sang-Hoon Lee, Kyung-Lim Suk, Kiwon Lee, and Kyung-Wook Paik Abstract A new concept of anisotropic conductive film (ACF) called nanofiber/solder ACF combined with an ultrasonic bonding method can overcome limitations of fine pitch flex-on-flex (FOF) assembly using conventional ACF, such as short circuit issues, low current-handling capability, and poor reliability. To fabricate the nanofiber/solder ACF, SAC305 (96.5% Sn, 0.5% Cu, and 3% Ag) conductive solder balls are added to polyacrylonitrile polymer solution and ejected as nanofibers containing SAC305 conductive solder balls by an electrospinning method. This new concept of nanofiber/solder ACF successfully prevents short circuits between neighboring conducting electrodes by limiting the free movement of conducting balls and insulating individual conductive solder balls with coated nanofiber layers. Moreover, SAC305 conductive solder balls in nanofiber/solder ACF are completely melted and made metallurgical alloy contact between FOF electrodes during the ultrasonic bonding, resulting in 41% more electrical power being carried, and showing significant improvement in reliability performance compared with conventional nickel ball ACF. Index Terms Anisotropic conductive film (ACF), flex-on-flex (FOF), nanofiber, solder, ultrasonic bonding. I. INTRODUCTION OVER the last several decades, the size of electronic devices has been continuously miniaturized. Therefore, more components with various functions have been able to be integrated as modules with the same size of devices. In this modularization trend, flex-on-flex (FOF) assembly structure has been introduced for its great advantages in flexibility, space efficiency, miniaturization, and fine pitch capability. However, as the size of electronic devices has gradually decreased through modularization, packaging and interconnection technology have become very important [1] [3]. In many electronic assembly processes, the most promising method is to use conductive adhesives for obtaining strong adhesion strength as well as reliable electrical conductivity in order for electronic devices to function properly. Anisotropic conductive film (ACF), well-known adhesive materials, which consist of conducting balls and adhesive polymer resin in a film format, have been widely used in the electronic packaging industry. However, conducting balls in conventional ACF like metal coated polymer balls or nickel balls show insufficient performance for certain applications requiring good current handling capability and reliability. For viable alternatives of metal coated polymer or nickel balls, SAC305 solder balls with better electrical property and reliability have been introduced. Despite solder ACFs showing improved performances [4], [5], electrical short circuit issue between neighboring lines has been raised for fine pitch flex circuit when bonding with solder ACF due to the bridging phenomenon, whereby electrodes are electrically shorted by containing conductive balls at fine pitch cases. Therefore, it is imperative to prepare ACF that suit for the fine pitch assembly and perform not only good electrical conductivity but also high reliability. In order to satisfy these requirements, a new type of ACF called nanofiber/solder ACF where conductive balls were coupled and insulated by nanofiber layers was introduced using an electrospinning method [6], [7]. The objective of this paper is not only to assure the advantages of the nanofiber/solder ACF in terms of fine pitch capability, power carrying capability, and reliability, but also to ascertain the benefits of the nanofiber/solder ACF in fine pitch FOF assembly compared with the conventional ACF. II. EXPERIMENTS A. Preparation of FOF Substrates Flexible printed circuits (FPCs) having dimensions of 24-mm width 30-mm height 0.05-mm thickness with 100-μm pitch, as shown in Fig. 1, were made of polyimide material. For the metal surface finish, 12-μm-thick copper electrodes were coated by organic solderability preservative material. The bottom FPCs contained six four-point contact resistance patterns to measure the contact resistance of an FOF joint and four insulation resistance patterns to measure electrical short between lines. Manuscript received January 13, 2012; revised August 23, 2012; accepted August 24, Date of publication November 16, 2012; date of current version December 3, Recommended for publication by Associate Editor D. Shangguan upon evaluation of reviewers comments. The authors are with the Department of Materials Science and Engineering, KAIST, Daejeon , Korea ( svl5083@kaist.ac.kr; stones01@kaist.ac.kr; kiwonlee@kaist.ac.kr; kwpaik@kaist.ac.kr). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TCPMT /$ IEEE B. Fabrication of Nanofiber/Solder ACF ACF containing solder balls that were electrically insulated by nanofibers called nanofiber/solder ACF was fabricated by an electrospinning method. For the electrospinning optimization process, applied voltage varied from 10, 12.5, and 15 kv while working distance, pumping rate, and needle diameter were fixed as 15 cm, 20 μl/min, and 250 μm.

2 LEE et al.: STUDY ON FINE PITCH FOF ASSEMBLY 2109 temperature at ACF was adjusted at 240 C. During the bonding process, silicone interposers were used to uniformly apply the vibration to the assembly. Fig. 1. FOF images of (a) top and bottom flexes, (b) pitch, (c) insulation resistance patterns, and (d) contact resistance patterns. Two types of nanofiber/solder ACF having different size of solder balls were fabricated. For the solder balls, SAC305 (96.5% Sn, 3% Ag, and 0.5% Cu with 217 C melting point) with diameters of 15 and 25 μm were used. The nanofiber solutions consisted of 8 wt% polyacrylonitrile polymer, 72 wt% dimethylformamide solvent, and 20 wt% SAC305 solder balls. Once the nanofiber/solder solutions were well dispersed, they were electropun by an electrospinnning process. However, the solder balls often sank due to their weights and gravity while electrospinning nanofibers. As a solution to this problem, the electrospinning machine was modified to rotate the syringe so that it could prevent the solder ball sinking problem. Finally to fabricate the nanofiber/solder ACF, two acrylic nonconductive films were laminated on top and bottom of the electrospun nanofiber by a roll-laminator at 65 C. Then, the laminated ACF was laminated once again by a vacuum laminator to remove voids at increasing temperature up to 75 C for 5 min. C. Demonstration of Fine Pitch FOF Assembly Using Nanofiber/Solder ACF and Ultrasonic Bonding Method In this paper, ultrasonic bonding method was used to bond FOF assembly. [2] For the bonding parameters, 3 MPa of bonding pressure was applied with 65% of amplitude which vibrated 13 μm in vertical direction for 5 s. The local D. Characterization of ACF Joint Properties and Reliability Once the fine pitch FOF assembly was bonded using the nanofiber/solder ACF and ultrasonic bonding method, joint morphologies of the assembly were analyzed to see whether the nanofiber layers were removed and metallurgical solder alloy joints were formed between solder balls and top and bottom electrodes. Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to analyze the morphology and metal alloy composition of the nanofiber/solder ACF joints. Then, degree of cure of the nanofiber/solder ACF after ultrasonic bonding process was analyzed by Fourier transform infrared spectroscopy (FT-IR) analysis. The FOF assembly was peeled and the reflected IR from the remaining acrylic resin on the ACF joint was detected. Finally to test the fine pitch capability of FOF assembly using the nanofiber/solder ACF, insulation resistance of 56 patterns was measured under the constant voltage condition and the resistance below 10 7 was regarded as short circuits as showninfig.1(c). Contact resistance of a single FOF joint was measured to characterize the electrical properties of the nanofiber/solder ACF joints and compared with conventional Ni ACF. The contact resistance of a 1-mm length and 50-μm width joint was measured by four point probe by applying current and voltage to the electrodes as shown in Fig. 1(d). Power carrying capability was conducted to measure the maximum amount of electrical power that the FOF assembly could carry before failure occurred. A power supply was used to apply voltage bias (V) in every 0.2 V to the FOF assembly and increasing current were recorded until it reached the maximum point and dropped to zero. The recorded current values were converted to the power to show the power carrying capability. Finally, the reliability tests were carried out to evaluate the reliability of fine pitch FOF assembly using nanofiber/solder ACF and ultrasonic bonding method. For the reliability tests, the high-temperature storage test (HTST) was carried out at 150 C for 500 h, and the pressure-cooker test (PCT) was performed at 121 C, 2 atm, and 100% relative humidity for 48 h. The results of two reliability tests were compared with FOF assemblies using conventional Ni ACF. III. RESULTS AND DISCUSSION A. Fabrication of Nanofiber/Solder ACF Nanofibers containing SAC305 solder balls electrospun at various applied voltages are shown in Fig. 2. When 10 and 12.5 kv of voltage were applied, nanofibers were ejected nonuniformly with many beads among them. However, when the voltage was increased up to 15 kv, the nanofibers were ejected uniformly without forming any beads. It can be explained that 10 and 12.5 kv were not strong enough to eject SAC305 solder balls from the nanofiber solution.

3 2110 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 12, DECEMBER 2012 (a) (b) Fig. 3. (a) Cross-sectional image and (b) magnified image of 50-nm thin nanofiber layer on SAC305 solder balls. Fig. 2. Nanofibers containing 15-μm diameter SAC305 solder balls electrospun at (a) 10 kv, (b) 12.5 kv, and (c) 15 kv, and 25-μm diameter SAC305 solder balls electrospun at (d)10 kv, (e) 12.5 kv, and (f) 15 kv. More applied voltage was necessary to pull heavy SAC305 solder balls, and the electrospinning process required at least 15 kv to form uniform nanofibers containing SAC305 solder balls. In the cross-sectional image of electrospun nanofiber containing SAC305 solder balls shown in Fig. 3, about 50-nm thick nanofiber layers were observed at the solder balls, and they were expected to electrically insulate between solder balls during fine pitch FOF assembly. Differential scanning calorimeter (DSC) analysis of fabricated nanofiber/solder ACF was carried out as shown in Fig. 4. Dynamic scan mode was applied which temperature was increased from 30 C to 240 C at a rate of 10 C/min. Since acrylic resin was used for this type of ACF, it had curing onset temperature of 90 C. Unlike common acrylic ACF which only one curing exothermic peak appears at 125 C peak, another exothermic peak was appeared beyond 200 C. This peak represented the temperature where SAC305 solder balls in the nanofiber/solder ACF started to melt down at its melting point. B. Demonstration of Fine Pitch FOF Assembly Using Nanofiber/Solder ACF and Ultrasonic Bonding Method 1) Analysis of FOF Joint Morphology: The cross-sectional SEM images and EDS analyses of a solder ball after the FOF assembly using nanofiber/solder ACF and the ultrasonic bonding method are shown in Fig. 5. Intermetallic compounds were grown in the dark area while solder bulk was remained in the Fig. 4. DSC analysis of nanofiber/solder ACF representing ACF resin curing and solder melting. Fig. 5. Cross-sectional SEM images and EDS analyses of SAC305 nanofiber/solder ACF joints. (a) 25-μm SAC305 solder ball. (b) 15-μm SAC305 solder ball. bright area. According to the EDS analysis, significantly more amount of copper ranging from 17 to 44 At% was observed at the joint compared with the solder composition of SAC305 (96.5% Sn, 3% Ag, and 0.5% Cu) itself. It seemed that

4 LEE et al.: STUDY ON FINE PITCH FOF ASSEMBLY 2111 Fig. 6. Insulated circuit rate of 100 μm using various solder ball size nanofiber/solder ACF and conventional Ni ACF. Fig. 8. Contact resistances and contact areas per electrode at various ACFs. Fig. 9. Number of captured balls per electrode for nanofiber/solder ACF with (a) 25-μm diameter solder balls and (b) 15-μm diameter solder balls. Fig. 7. Potential electrical short due to the misalignment effect in FOF assembly. about 30-mm thick insulation nanofiber layers were removed during the bonding process while temperature and pressure were applied, and the copper was diffused from the copper electrodes to the SAC305 solder balls when they were melted. Similar result was obtained for 15-μm diameter solder ball. As a conclusion, excellent metallurgical alloy joints were achieved for SAC305 nanofiber/solder ACF during ultrasonic bonding. 2) Analysis on the Degree of ACF Cure: FT-IR analysis was carried out to analyze the degree of ACF cure after ultrasonic bonding method. The acrylic type of ACF generally contains a carbon double bond peak between 990 and 1000 wavenumber (cm 1 ). The intensity of this spectrum peak will be reduced if the resin becomes fully cured. The FT-IR result of the nanofiber/solder ACF after ultrasonic bonding shows that the nanofiber/solder ACF after assembly was completely cured. 3) Insulation Resistance: The insulation resistances for FOF assembly using 25- and 15-μm diameter solder balls in nanofiber/solder ACF as well as 8-μm diameter Ni ACF are shown in Fig. 6. The nanofiber/solder ACF using 25-μm diameter solder balls shows 97% insulation result. It was a great improvement compared with the conventional solder ACF with 25-μm diameter solder balls since it showed only Fig. 10. Power carrying capability of nanofiber/solder ACF and conventional Ni ACF. 67% insulation result in 100-μm pitch assembly. The reason for 3% short circuit is presumably a misalignment tolerance showninfig.7.inthisfig.7,μm of misalignment can easily cause the short between neighboring lines especially when the ball size was 25-μm diameter. Moreover, the nanofiber/solder ACF with 15-μm diameter solder balls showed 100% insulation result. Therefore, it is necessary to use smaller diameter solder balls to prevent the electrical short due to the alignment tolerance.

5 2112 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 12, DECEMBER 2012 Fig. 11. HTST result of (a) conventional Ni ACF, (b) nanofiber/solder ACF with 25-μm diameter solder balls, and (c) nanofiber/solder ACF with 15-μm diameter solder balls. Fig. 12. PCT result of (a) conventional Ni ACF, (b) nanofiber/solder ACF with 25-μm diameter solder balls, and (c) nanofiber/solder ACF with 15-μm diameter solder balls. C. Characterization of ACF Joint Properties and Reliability 1) Contact Resistance: The contact resistances of fine pitch FOF assembly using the nanofiber/solder ACF with 25- and 15-μm diameter SAC305 balls were measured and compared with those of conventional Ni ACF as shown in Fig. 8. Nanofiber/solder ACF with 25-μm diameter SAC305 balls showed the highest contact resistance among the three types of ACF. The contact resistance of the nanofiber/solder ACF with 25-μm diameter solder balls was 25 ± 5m. And both the nanofiber/solder ACF with 15-μm diameter solder balls and the conventional Ni ACF_8 μmwere20± 1.5 m,whichwas 20% lower than nanofiber/solder ACF with 25-μm diameter solder balls. It is because the nanofiber/solder ACF with 25-μm diameter solder balls showed the lower contact area than other two ACF as shown in Fig. 9. The reason why the nanofiber/solder ACF with 25-μm diameter solder balls shows the highest contact resistance and smallest contact area per electrode was because of the size of SAC305 balls. Although each ACF had the same

6 LEE et al.: STUDY ON FINE PITCH FOF ASSEMBLY 2113 weight percent of conducting balls, the volume percent was different. More number of solder balls was found for the nanofiber/solder ACF with 15-μm diameter solder balls and this affected the number of captured balls shown in Fig. 9. In conclusion, 15-μm SAC305 solder balls were more favorable for 100-μm pitch FOF assembly with respect to the contact resistance. 2) Power Carrying Capability: Power carrying capability results of fine pitch FOF assembly using the nanofiber/solder ACF with 15- and 25-μm diameter SAC305 balls and the conventional Ni ACF are shown in Fig. 10. Nanofiber/solder ACF with SAC305 balls showed similar results regardless of solder ball sizes. They could carry 30% more current and 0.2 more voltage bias resulting in carrying higher electrical power by 41% than conventional Ni ACF before failure. Unlike power carrying capability of FOB assembly [5], ACF joints with 15- and 25-μm solder balls were both burnt at FOF metal lines. It seemed that the finer electrodes of 50 μm are weaker than 250 μm of [5]. Nevertheless, both could undergo until the metal lines burn out which caused the similar tendency of 15- and 25-μm solder balls. 3) Reliability Tests: For reliability tests, HTST and PCT are shown as Figs. 11 and 12. According to the HTST results, no open failure was occurred at both nanofiber/solder ACF joints with 25- and 15-μm diameter solder balls, and the contact resistances were stably maintained for 500 h. However, 3% of open failure was observed at the conventional Ni ACF joints for first 100 h and the failure rate increased to 17% for 500 h, because physical contact-based Ni ACF joints can be degraded by the s of ACF polymer resin by humidity and temperature. However, the solder joint was not affected by the humidity and temperature. More remarkable results were observed during the PCT test. No open failure was observed at nanofiber/solder ACF joints. However, 30% and 57% of conventional Ni ACF joints were failed after 24 and 48 h, respectively. The fine pitch FOF assembly using nanofiber/solder ACF showed excellent reliability than conventional Ni ACF in both HTST and PCT reliability tests. It is because SAC305 solder balls were completely molten and formed metallurgical joints between top and bottom copper electrodes. IV. CONCLUSION 100-μm fine pitch FOF assembly using nanofiber/solder ACF and ultrasonic bonding method was successfully demonstrated with improved power carrying capability and excellent HTST and PCT reliabilities. During the ultrasonic bonding, rapid resin curing and melting of solder balls occurred, and metallurgical alloy joints were made using nanofiber/solder ACF. During the bonding, copper diffused into the solder joints and formed intermetallic compounds at the solder and copper electrodes. In addition, excellent insulation was achieved for nanofiber/solder ACF with 15-μm diameter solder balls, because nanofiber coating prevented electrical short. In terms of ACF joint properties and reliability of fine pitch FOF assembly, nanofiber/solder ACFs showed significantly better results compared with conventional Ni ACF. Nanofiber/solder ACF carried 30% more current carrying and 41% more electrical power handling. Nanofiber/solder ACF also showed excellent reliabilities in HTST and PCT tests without any open failure. This paper has shown the feasibility of demonstrating 100-μm fine pitch FOF assembly by incorporating solder balls into nanofiber and ACF. It is expected that fine pitch FOF assembly will be feasible using new nanofiber solder ACF. And this technology can provide new solution for high performance, multifunctionalization, and miniaturization in electronic packaging industry. REFERENCES [1] C. Jang, Issues in assembly process of next-generation fine-pitch chipon-flex packages for LCD applications, IEEE Trans. Adv. Packag., vol. 30, no. 1, pp. 2 10, Feb [2] K. Lee, Ultrasonic anisotropic conductive films (ACF) bonding of flexible substrates on organic rigid boards at room temperature, in Proc. 57th Electron. Compon. Technol. Conf., Reno, NV, 2007, pp [3] J. Kiilunen, The effect of bonding temperature and curing time on peel strength of anisotropically conductive film flex-on-board samples, IEEE Trans. Device Mater. Rel., vol. 12, no. 2, pp , Jun [4] W. C. Kim, A study on the characteristics of solder anisotropic conductive film (ACF) flex-on-board (FOB) joints using an ultrasonic bonding method, M.S. thesis, Dept. Mater. Sci. Eng., KAIST, Daejeon, Korea, [5] K. Lee, High power and high reliability flex-on-board assembly using solder anisotropic conductive films combined with ultrasonic bonding technique, IEEE Trans. Compon. Packag. Technol., vol. 1, no. 12, pp , Dec [6] K. L. Suk, Nanofiber anisotropic conductive adhesives (ACAs) for ultrafine pitch chip-on-film (COF) packaging, in Proc. 61st Electron. Compon. Technol. Conf., Lake Beuna Vista, FL, 2011, pp [7] S. Y. Gu, Process optimization and empirical modeling for electrospun polyacrylonitrile (PAN) nanofiber precursor of carbon nanofibers, Eur. Polymer J., vol. 41, pp , Nov Sang-Hoon Lee received the B.Sc. degree in mechanical engineering from Pennsylvania State University, University Park, in 2010, and the M.Sc. degree from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2012, where he is currently pursuing the Ph.D. degree with the Department of Materials Science and Engineering. His current research interests include nanofiber solder anisotropic conductive films and fine pitch flex-on-flex module interconnection in electronic devices. Kyung-Lim Suk received the B.S. degree in nanotechnology and advanced materials engineering from Sejong University, Seoul, Korea, in 2007, and the M.S. degree in materials science and engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2009, where she is currently pursuing the Ph.D. degree with the Department of Materials Science and Engineering. Her current research interests include flip chip assembly, adhesive materials for advanced packaging, and nanomaterials and their applications in highly reliable electronic packaging.

7 2114 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 12, DECEMBER 2012 Kiwon Lee received the B.Sc., M.Sc., and Ph.D. degrees in materials science and engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea in 2005, 2007, and 2012, respectively. He is currently a Senior Engineer with the Advanced Circuit Interconnection R&D Center, Samsung Electro-Mechanics, Suwon, Korea. He is a Primary Researcher in an ultrasonic bonding process joint research project by NOKIA and KAIST. His current research interests include the development of room temperature ultrasonic bonding processes using anisotropic conductive films and solder and adhesive hybrid joints for high-density flex-on-board module interconnection in electronic devices. Dr. Lee was the recipient of the Motorola-IEEE Fellowship Award as the author of the best student paper at the Electronic Components and Technology Conference, Reno, NV, in 2007, the Best Student Paper Award at the International Conference on Electronic Materials and Packaging in Daejeon, Korea, in 2007, Second Prize in the Samsung Inside Edge International Thesis Competition in 2009, and the Travel Award and the Best-In-Session Paper Award at the Electronic Components and Technology Conference. Kyung-Wook Paik received the B.Sc. degree in metallurgical engineering from Seoul National University, Seoul, Korea, the M.Sc. degree from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, and the Ph.D. degree in materials science and engineering from Cornell University, Ithaca, NY, in 1979, 1981, and 1989, respectively. He was a Research Scientist with KAIST from 1982 to 1985 and was involved in the development of gold bonding wires. He was with General Electric Corporate Research and Development, Brookline, MA, as a Senior Technical Staff Member, and was engaged in research on interconnect multichip module technology and power IC packaging, from 1989 to He rejoined the Department of Materials Science and Engineering, KAIST, as a Professor in 1995, where he is currently with the Nano-Packaging and Interconnect Laboratory and involved in research on the flip-chip bumping and assembly, adhesive flip-chips, embedded capacitors, and display packaging technologies. He was a Visiting Professor with the Packaging Research Center, Georgia Institute of Technology, Atlanta, from 1999 to 2000, and was involved in packaging education and integrated passives research programs. He was visiting Portland State University, Portland, OR, from 2005 to 2005, where he was engaged in flip-chip polymer materials evaluation. He has authored or co-authored more than 80 technical papers in journals and conferences, and holds 14 U.S. patents. Dr. Paik has been the Chairman of the Korean IEEE Components Packaging and Manufacturing Technology Chapter since 1995, and is a member of the International Microelectronics and Packaging Society, SEMI, and MRS.