ANISOTROPIC conductive film (ACF) is a film-type

Transcription

1 1350 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 5, NO. 9, SEPTEMBER 2015 Effects of Bonding Pressures and Bonding Temperatures on Solder Joint Morphology and Reliability of Solder ACF Bonding Yoo-Sun Kim, Seung-Ho Kim, Ji-Won Shin, and Kyung-Wook Paik Abstract In this paper, in order to improve the reliability of anisotropic conductive film (ACF) interconnections, solder ACF joints were investigated in terms of solder joint morphology. ACFs are film-type interconnection adhesive materials that consist of polymer adhesive resins and randomly dispersed conductive particles. Recently, in order to obtain high reliability of ACF joints, solder ACFs that use solder particles as conductive particles of ACFs have been introduced combined with an ultrasonic bonding method. However, further researches are needed in the area of crack initiation and the propagation of solder ACF joints at high temperature and high humidity conditions due to the stress generated by hygroscopic expansion of ACF resin. Therefore, in order to solve the crack initiation of solder ACF joints, solder ACF joint morphology should be controlled and optimized to minimize the solder crack-related reliability problems. In this paper, solder ACF joint reliability was investigated depending on the solder morphologies of ACF joints bonded with various bonding pressures and temperatures. According to the results, as bonding pressure increased from 2 to 6 MPa, aspect ratio (joint diameter/joint gap) increased due to the increased joint area and decreased joint gap. In the pressure cooker test (PCT) reliability tests, as solder aspect ratio increased, electrical resistances were more stable after 60 h of the tests due to higher joint strength. In the target bonding temperature profile of 250 C, solder joints showed a concave shape. However, joints bonded at in the target bonding temperature profile of 200 C showed a convex shape. It is mainly due to the lower degree of cure of resin when ACF temperature reached to solder Melting Point (MP) in the target bonding temperature profile of 250 C compared with that in the target bonding temperature profile of 200, because ACF temperature reached to solder MP in shorter time in the target bonding temperature profile of 250 C. Concave-shaped solder joints showed higher PCT reliability than the convex-shaped solder joints due to the lower stress concentration. These results indicate that solder ACF joint morphology was a significantly important factor for highly reliable solder ACF joints in high temperature and high humidity reliability conditions. Index Terms Anisotropic conductive film (ACF), bonding parameters, electronic packaging reliability, solder joint morphology. Manuscript received October 10, 2014; revised May 14, 2015; accepted May 21, Date of publication July 12, 2015; date of current version September 18, Recommended for publication by Associate Editor J. E. Morris upon evaluation of reviewers comments. The authors are with the Nano Packaging and Interconnect Laboratory, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon , Korea ( kys870505@kaist.ac.kr; s-lovely-h@kaist.ac.kr; jwon08@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 I. INTRODUCTION ANISOTROPIC conductive film (ACF) is a film-type interconnection adhesive material that consists of polymer adhesive resins and randomly dispersed conductive particles. Electrical interconnection is made by the mechanical trapping of conductive balls between two metal electrodes. ACF interconnection has several advantages such as fast bonding time, low bonding temperature, fine-pitch capability, and cost effectiveness [1]. Therefore, ACFs have been widely used as an interconnection method in Liquid Crystal Display (LCD) drive ICs, touch screen panel, and various flexible assembly applications [2]. However, hygroscopic expansion of ACF polymer resin in high temperature and humidity environment may cause expansion of polymer resin, resulting in unstable electrical problems due to the loose contact of trapped conductive balls [3]. In order to solve such reliability problems of conventional ACFs, solder ACFs that consist of solder balls instead of conductive metal particles have been introduced. Solder balls in solder ACFs can form solder metallurgical joints at the metal electrodes and show better electrical properties compared with conventional metal conductive balls at a harsh environment [4]. However, unlike conventional ACFs, where its bonding temperature are purely determined by the curing properties of polymer resin, the bonding temperature for solder ACFs should be determined by considering both curing properties of polymer resin and the melting point of solder, because the wetting of solder on the metal electrodes needs to be achieved [4], [5]. In the applications of low temperature solder ACF bonding, Sn58Bi solder balls have been used to bond at less than 200 C. However, the Sn58Bi solder balls have relatively poor mechanical properties compared with conventional SnAgCu-based solder balls, and solder joint crack-induced failures may occur during high-stress applying reliability tests condition such as an autoclave test. The solder crack-induced failures have close relationship with morphology of solder ACF joints, and therefore it is important to control the solder joint morphology and optimize the solder ACF materials and bonding conditions. In this paper, the effects of bonding pressures and bonding temperatures on solder joint morphologies and reliabilities of solder ACFs were investigated at flex-on-board (FOB) applications. In addition, reliabilities depending on the joint morphologies were also evaluated IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 KIM et al.: EFFECTS OF BONDING PRESSURES AND BONDING TEMPERATURES 1351 Fig. 1. FOB vehicles used in this paper. Fig. 2. Schematic of the experimental setup of the US bonding. Fig. 3. Schematic of a solder ball joint after ACF bonding. II. EXPERIMENTS A. Materials Acrylic-based polymer resin and 20 wt% Sn58Bi solder balls with a diameter of 25 μm wereusedtopreparesolder ACFs. The thickness of ACF films was 35 μm and5wt% Ni balls with a diameter of 8 μm were also used as spacers of solder ACFs to maintain uniform gaps between metal electrodes. 1-mm-thick rigid printed circuit boards (PCBs) was used as PCB substrates and 25-μm-thick flexible PCBs (FPCBs) was used to fabricated FOB test vehicle. Fig. 1 shows the FOB test vehicle used in the experiments. Both substrates have 500-μm pitch Cu patterns with electroplated Ni/Au surface finish. Electrode width and length were 250 μm and 2 mm. The number of the electrodes on a PCB and an FPCB was 32. B. Bonding Conditions In this paper, an ultrasonic (US) bonding method was used to obtain stable metallurgical joint formation. By this bonding method, solder oxide layers can be broken by US vibration demanding solder melting and good wetting of solders on electrodes. Fig. 2 shows the experimental setup of the vertical US bonding method. In the US bonding process, the bonding pressures were applied simultaneously with 40-kHz US vibration. The ACF temperature was controlled by controlling a US amplitude up to 12 μm [6]. In order to investigate the effects of bonding pressures on the solder ACF joint morphology and reliability, the various bonding pressures of 2, 4, and 6 Mpa were used at the bonding temperature of 200 C. In addition, two ACF target temperatures of 200 C and 250 C were also used at the fixed bonding pressure of 2 MPa to investigate the effects of bonding temperatures on the solder joint morphology and reliability. 9-s bonding time was used to guarantee full cure of ACF resins. Table I shows the bonding parameters of this paper. C. Observation of Solder ACF Joints 1) Aspect Ratio of Solder Joints: Aspect ratio of solder ACF joints was calculated by measuring of solder joint gaps and diameters of solder joint areas [7]. Fig. 3 shows the schematic of a solder ball of ACF joints. The joint gaps between electrodes were measured by cross-sectional analysis and scanning electron microscope (SEM). The diameters of solder joint areas were measured by observing the remaining solders on PCB and Flexible Printed Circuit (FPC) substrates after peeling off FPCBs from PCBs. The joint gaps and diameters at eight joints in two samples were observed in each bonding conditions. In the calculations, solder ball joints between electrodes were assumed as a cylindrical shape. 2) Solder Ball Joint Shape: In order to investigate the solder ball joint shape, degree of cure of resin was measured when the ACF temperature reached to solder MP during bonding process using Fourier Transform Infrared (FT-IR) spectroscopy. In this method, IR-absorbance peaks were obtained on the surfaces of the ACF joints after peeling off FPCBs from PCBs.

3 1352 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 5, NO. 9, SEPTEMBER 2015 TABLE I US BONDING CONDITIONS USED IN THIS PAPER Fig. 4. Schematic of a solder ball joint morphology and geometry factors of a stress concentration region. At high temperature and high humidity conditions, solder joints were under the tensile stress, which generated by hygroscopic expansion of ACF resin. Stress concentration in solder ACF joints is related to lengths and radii of curvature at the stress concentration region of solder joints, as shown in Fig. 4. In the equation of stress concentration commonly used, when solders are under tensile stress, maximum stress surrounding a stress concentration region is proportional to the stress concentration region (a) and inversely proportional to the curvature of the dented region in solder joints (ρt) as shownin[7] where σ m σ 0 a ρt σ m = σ 0 ( a ρt ) 1/2 (1) maximum stress surrounding a stress concentration region; nominal applied tensile stress; length of stress concentration region; radius of curvature of stress concentration region. In order to investigate the effects of solder ball joint shape on reliability, lengths and radii of curvature at the stress concentration regions were measured at the various bonding temperatures at eight joints in two samples by cross-sectional analysis using SEM. D. Electrical Test and Pressure Cooker Test A joint contact resistance was measured using the four-point kelvin method, as shown in Fig. 5. A joint contact resistance Fig. 5. Four-point kelvin method used for measuring joint contact resistance. was measured and averaged from 15 ACF joints in three samples for each condition. For the reliability evaluation of the solder ACF joints, pressure cooker test (PCT) tests were performed at 121 C, 2 atm, and 100%RH and joint contact resistances were observed during the test. III. RESULTS AND DISCUSSION A. Effects of Bonding Pressures on Solder ACF Joint Morphology and Reliability Fig. 6 shows the solder ACF joints depending on the bonding pressures, and Fig. 7 shows the measured joint diameters, gaps, and calculated aspect ratio (joint diameter/gap). As bonding pressure increased from 2 to 6 MPa at the bonding temperature of 200 C and the aspect ratio increased from 5.75 to because higher bonding

4 KIM et al.: EFFECTS OF BONDING PRESSURES AND BONDING TEMPERATURES 1353 Fig. 6. Solder ACF joint morphologies bonded at bonding pressures of (a) 2, (b) 4, and (c) 6 MPa at 200 C bonding temperatures. Fig. 7. (a) Diameters of joint diameters, (b) joint gaps, and (c) aspect ratios of solder ACF joints at various bonding pressures. Fig. 8. Joint contact resistances of solder ACF joints bonded at various bonding pressures of (a) 2, (b) 4, and (c) 6 MPa during PCT test at 121 C, 2 atm, and 100%RH. pressures caused larger amount of resin flow and larger deformation of solder. Joint strength may increase as bonding pressures increase due to higher aspect ratio. Fig. 8 shows the joint contact resistances during the PCT test bonded with various bonding pressures. In the solder ACF joints with 2-MPa bonding pressure, initial joint contact

5 1354 Fig. 9. IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 5, NO. 9, SEPTEMBER 2015 Solder ACF joint morphologies after 48-h PCT test bonded at (a) 2, (b) 4, and (c) 6 MPa, and 60-h PCT test bonded at (d) 2, (e) 4, and (f) 6 MPa. Fig. 10. Solder ACF joint morphologies bonded at (a) 200 C and (b) 250 C bonding temperatures. Fig. 11. Actual ACF temperatures during bonding process. resistances were lower than 25 m. However, after 48 h in PCT tests joint contact resistances increased up to 93 m, as shown in Fig. 8(a), presumably because solder joint crack was propagated during PCT test, as shown in Fig. 9(a). Fig. 12. FT-IR absorbance curves of the solder ACF acrylic resins after peeling off bonded FPCB. In addition, at 2-MPa bonding pressure, some solder joints showed an open failure after 60 h, as shown in Figs. 8(a) and 9(d). In the case of solder ACF joints with 4-MPa bonding pressure, 10% open failure rate was observed

6 KIM et al.: EFFECTS OF BONDING PRESSURES AND BONDING TEMPERATURES 1355 TABLE II CALCULATED GEOMETRY FACTOR [(a/ρt) 1/2 ] AT THE STRESS CONCENTRATION REGIONS DEPENDING ON THE BONDING TEMPERATURES after 60 h due to large crack propagation, as shown in Fig. 9(e). However, solder ACF joints with 6-MPa bonding pressure showed no open failures up to 60 h due to stable ACF joints, as shown in Figs. 8(c) and 9(f). The results indicate that the ACF joints with higher aspect ratio have advantages of maintaining the interconnection stability during PCT test.

7 1356 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 5, NO. 9, SEPTEMBER 2015 Fig. 13. Joint resistances of solder ACF joints bonded at (a) 200 C and (b) 250 C bonding temperatures during PCT test at 121 C, 2 atm, and 100%RH. Fig. 14. Solder ACF joint morphologies after 48-h PCT test depending on the (a) 200 C and (b) 250 C bonding temperatures. It was presumably due to higher joint strength in higher aspect ratio [7], because solder joints with high aspect ratio have longer time to fail by crack propagation. B. Effects of Bonding Temperatures on Solder ACF Joint Morphology and Reliability Fig. 10 shows the solder ACF joints bonded at the bonding temperatures of 200 C and 250 C. The solder joints bonded at 200 C showed a convex shape, but the joints bonded at 250 C showed a concave shape. In order to explain the solder joint shape results, degree of ACF resin cure was investigated at each bonding conditions when ACF temperature reached to Sn58Bi solder melting temperature of 138 C as shown in figure 11. For the investigation, in the case of 200 C target bonding temperature, degree of resin cure was measured on the PCBs bonded with a bonding time of 1.5 s because ACF temperature reached to 138 C within 1.5 s. On the other hand, in the case of 250 C target bonding temperature, degree of resin cure on PCBs was measured after bonding with a bonding time of only 0.5 s because of higher initial heating rate compared with that of 200 C target bonding temperature. In the FT-IR analysis of Fig. 12, peak area represents the not-reacted acrylic resin during the bonding process. Acrylic resin around solder joint at the bonding temperature of 250 C showed 13% degree of resin cure at solder melting temperature, which was significantly lower 45% degree of resin cure with the bonding temperature of 200 C. It is mainly due to that ACF temperature with higher target bonding temperature reached to solder melting temperature faster due to the higher heating rate. Therefore, solder ACF joints bonded at the bonding temperature of 250 C showed a concave shape with large amount of solder spreading, because lower degree of resin cure allowed molten solder to be spread more. In contrast, solder joints formed at the bonding temperature of 200 C showed a convex shape because of higher degree of resin cure when ACF temperature reached to solder MP. The stress concentration regions in solder ACF joints are represented with arrows in Fig. 10. Table II shows the calculated geometry factors [(a/ρt) 1/2 ] in stress concentration regions, which is proportional to the degree of concentrated stress. The solder joints bonded at the bonding temperature of 250 C showed about 6.5 times larger curvature (ρt) of stress concentration regions by a concave shape compared with that of solder joints bonded at the bonding temperature of 200 C. The average value of geometry factor of joints bonded at 250 C was 1.63, which was about three times

8 KIM et al.: EFFECTS OF BONDING PRESSURES AND BONDING TEMPERATURES 1357 smaller than that of solder joints bonded at 200 C. As a result, three times lower stress can be applied to the concave solder joints bonded at 250 C compared with the convex solder joints bonded at 200 C. 50% of solder ACF joints bonded at 200 C showed joint resistances of higher than 50 m after48hinpctasshown in Fig. 13(a), because crack was initiated and propagated from the dented region in solder joints during the PCT tests, as shown in Fig. 14(a). On the other hand, the joint resistances of solder ACF joints bonded at 250 C did not change after 48 h in Fig. 13(b). Furthermore, solder joints were stable after 48 h of PCT test as shown in Fig. 14(b), although tensile stress was applied during PCT test by polymer resin expansion. The results can be explained by the difference of maximum stress at the stress concentration region depending on geometry factors as explained by (1). Lower stress was concentrated in solder joints bonded at 250 C compared with solder joints bonded at 200 C, resulting in higher reliability. IV. CONCLUSION In this paper, the morphologies and reliabilities of solder ACF joints depending on the bonding pressures and temperatures were investigated. As bonding pressure increased from 2 to 6 MPa, the aspect ratio of solder ACF joints was also increased from 5.8 to Therefore, solder ACF joints bonded at higher bonding pressure provided a better PCT reliability due to higher aspect ratio of solder joints. In terms of bonding temperature, solder joints bonded with ACF temperature profile of 250 C target bonding temperature showed a concave shape. On the other hand, solder joints bonded with that of 200 C target bonding temperature showed a convex shape. It is mainly due to the lower degree of cure of resin when ACF temperature reached to solder MP in the target bonding temperature profile of 250 C compared with that of 200 C, because ACF temperature reached to solder MP in shorter time in the target bonding temperature profile of 250 C. Furthermore, joints bonded at 250 C showed a higher PCT reliability because of lower stress concentration compared with that of joints bonded at 200 C. In conclusion, in the solder ACF bonding, solder ACF joints with high aspect ratio and concave solder shape significantly showed a better performance in PCT tests. REFERENCES [1] M.-J. Yim and K.-W. Paik, Design and understanding of anisotropic conductive films (ACF s) for LCD packaging, IEEE Trans. Compon., Packag., Manuf. Technol. A, vol. 21, no. 2, pp , Jun [2] S.-Y. Jang et al., FCOB (flip chip on board) reliability study for mobile applications, in Proc. IEEE 54th Electron. Compon. Technol. Conf., Jun. 2004, pp [3] L. L. Mercado, J. White, V. Sarihan, and T.-Y. T. Lee, Failure mechanism study of anisotropic conductive film (ACF) packages, IEEE Trans. Compon. Packag. Technol., vol. 26, no. 3, pp , Sep [4] K. Lee and K. W. Paik, High power and fine pitch assembly using solder anisotropic conductive films (ACFs) combined with ultrasonic bonding technique, in Proc. IEEE 60th Electron. Compon. Technol. Conf., San Diego, CA, USA, Jun. 2010, pp [5] M.J.Rizvi,H.Lu,C.Bailey,Y.C.Chan,M.Y.Lee,andC.H.Pang, Role of bonding time and temperature on the physical properties of coupled anisotropic conductive nonconductive adhesive film for flip chip on glass technology, Microelectron. Eng., vol. 85, no. 1, pp , [6] K. Lee, S. Oh, I. J. Saarinen, L. Pykari, and K.-W. Paik, Highspeed flex-on-board assembly method using anisotropic conductive films (ACFs) combined with room temperature ultrasonic (US) bonding for high-density module interconnection in mobile phones, in Proc. IEEE 61st Electron. Compon. Technol. Conf., May/Jun. 2011, pp [7] W. D. Callister, Materials Science and Engineering: An Introduction, 3rd ed. New York, NY, USA: Wiley, [8] J. P. Ranieri, F. S. Lauten, and D. H. Avery, Plastic constraint of large aspect ratio solder joints, J. Electron. Mater., vol. 24, no. 10, pp , Yoo-Sun Kim received the B.Sc. degree in materials science and engineering from Chonnam National University, Gwangju, Korea, in 2010, and the M.Sc. degree in materials science and engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2012, where she is currently pursuing the Ph.D. degree with the Department of Materials Science and Engineering. Her current research interests include anisotropic conductive films for electronic devices. Seung-Ho Kim received the B.Sc. and M.Sc. degrees in materials science and engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2008 and 2010, respectively, and the Ph.D. degree from the Department of Materials Science and Engineering, KAIST, in He was a Primary Researcher in touch screen panel assemblies using the ACF ultrasonic bonding process joint research project being carried out by Samsung Electro-Mechanics, Suwon, Korea, and KAIST. His current research interests include the development of self-fluxing solder anisotropic conductive films for flex-on-board assemblies. Ji-Won Shin received the B.Sc. and M.Sc. degrees in materials science and engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2008 and 2010, respectively, and the Ph.D. degree from the Department of Materials Science and Engineering, KAIST, in He was a Primary Researcher in nonconductive film for fine-pitch Cu-pillar/Sn Ag bump interconnection. Kyung-Wook Paik received the B.Sc. degree in metallurgical engineering from Seoul National University, Seoul, Korea, in 1979, the M.Sc. degree from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 1981, and the Ph.D. degree in materials science and engineering from Cornell University, Ithaca, NY, USA, in He was a Research Scientist with KAIST from 1982 to 1985, where he was involved in the development of gold bonding wires. He was a Senior Technical Staff Member in Interconnect Multichip Module Technology and Power IC Packaging with General Electric Corporate Research and Development, Brookline, MA, USA, from 1989 to He was a Professor with the Department of Materials Science and Engineering, KAIST, in 1995, where he is currently with the Nanopackaging and Interconnect Laboratory, and is involved in 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, GA, USA, from 1999 to 2000, where he was involved in packaging education and integrated passives research programs. He was with Portland State University, Portland, OR, USA, in 2005, where he was involved in flip-chip polymer materials evaluation. He has authored or co-authored over 80 technical papers, and holds 16 U.S. patents and four U.S. patents pending. Dr. Paik is a member of the International Microelectronics and Packaging Society, the Society for Emergency Medicine India, and the Materials Research Society. He has been the Chairman of the Korean IEEE Components Packaging and Manufacturing Technology Chapter since 1995.