1 884 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO., MAY 212 Ultrasonic Bonding of Anisotropic Conductive Films Containing Ultrafine Solder Balls for High-Power and High-Reliability Flex-On-Board Assembly Won-Chul Kim, Kiwon Lee, Ilkka J. Saarinen, Lasse Pykari, and Kyung-Wook Paik Abstract New solder anisotropic conductive films (ACFs) consist of a thermosetting polymer resin and fine solder balls instead of the conventional metal particles or metal-coated polymer particles. These solder balls have lower melting temperature than conventional metal conductive particles, which enable them to be melted during ultrasonic (US) bonding, and form intermetallic alloy joints with metal pads of flex on board. In this paper, excellent solder ACF joints are demonstrated using a US-bonding method for high-power and high-reliability flexon-board (FOB) assemblies. Ultrasonically bonded solder ACF joints were characterized in terms of their electrical properties and reliability. Solder alloy bonding was achieved using two kinds of solder balls, i.e., Sn 8Bi and SAC (96.Sn 3.Ag.Cu). At the same time, the acrylic ACF resin was completely cured after s of US bonding. Solder alloy ACF joints show about 2% decrease in daisy-chain electrical resistance and 1% increase in the current-carrying capability compared with conventional physical-contact-based Ni ACF joints. Solder alloy ACF joints also show significantly improved reliability with stable electrical resistances up to 24 h in an unbiased autoclave test, whereas conventional Ni ACF joints show severe electrical open failures within 4 h. Index Terms Adhesives, anisotropic conductive film (ACF), flex-on-board (FOB), reliability, solder, ultrasonic. I. INTRODUCTION ANISOTROPIC conductive films (ACFs) are well known adhesive interconnect materials which consist of thermosetting polymer resins and conductive particles. ACFs have been widely used in semiconductor and display applications for flex-on-glass, flex-on-board (FOB), flex-on-flex, chip-on-glass, chip-on-flex, and chip-on-board interconnections due to their fine-pitch capability, simple process, and cost effectiveness. Recently, the application of ACF bonding Manuscript received April 6, 211; revised November 29, 211; accepted December 2, 211. Date of publication January 18, 212; date of current version May 3, 212. Recommended for publication by Associate Editor J. E. Morris upon evaluation of reviewers comments. W.-C. Kim, K. Lee, and K.-W. Paik are with the Nano Packaging and Interconnect Laboratory, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon -71, Korea ( I. J. Saarinen is with STOD Assembly Technology FI, Nokia Corporation, Salo FI-2411, Finland ( L. Pykari is with Manufacturing Technology Research FI, Nokia Corporation, Salo FI-2411, Finland ( Color versions of one or more of the figures in this paper are available online at Digital Object Identifier 1.119/TCPMT /$ 212 IEEE has been extended to high-density FOB assemblies for module interconnection to replace conventional socket-type connectors in mobile phone applications . The most common method for ACF interconnection is thermocompression (T/C) bonding, which applies heat and pressure with a high-temperature bonding tool. However, the T/C bonding method requires high process temperatures (over 2 C) and long bonding times from 6 to 2 s. These process conditions have limitations such as thermal damages and warpage problems to components due to the high process temperatures and longer bonding times. Therefore, there has been a strong demand for an alternative bonding method to replace conventional T/C bonding. Recently, a newly developed room-temperature ultrasonic (US) bonding method was suggested and successfully demonstrated as an alternative to T/C bonding. By utilizing spontaneous heat generation at the ACF layer by a vertical US vibration, US bonding can be achieved in less than 3 s with reduced thermal damage to the assembly. At the same time, it was reported that ultrasonically bonded ACF joints showed the same degree of ACF curing, resulting in excellent adhesion strength, contact resistances, and reliability as the T/C bonded conventional ACF joints , . In spite of the improvement of the US bonding process mentioned above, conventional ACF joints that utilize physical contacts of conductive particles to metal pads have the limitations of reliability problems and low current-handling properties for high-power applications. Applying the idea of solder-filled adhesives that have been studied from the mid- 199s , the electrical properties and reliabilities of the ACF joints can be significantly improved by using solders as conductive particles. The ACF joints that contain solder conductive particles can achieve intermetallic alloy joint bonding between the electrode pad metal finish using US bonding. Therefore, the solder ACF bonded by the US bonding method can completely solve the limitations of conventional physicalcontact-based ACF joints. In this paper, new solder ACFs combined with US bonding technique are investigated for the FOB assembly. The solder ACF materials and US bonding parameters were optimized, and US-bonded solder ACF joints were characterized in terms of ACF joint characteristics such as electrical joint resistance, current-carrying capability, and autoclave reliability.
2 KIM et al.: ULTRASONIC BONDING OF ANISOTROPIC CONDUCTIVE FILMS 88 TABLE I SPECIFICATIONS OF SOLDER ACFS Resin Acrylic Solder ball composition Sn 8Bi, SAC Solder ball size 2 µm Solder ball content 1 wt% Spacer Ni ball size 8 µm Spacer Ni ball content wt% FPCB PCB A II. MATERIALS AND EXPERIMENTS A. Test Vehicle Preparation For solder ACFs, Sn 8Bi and SAC (96 Sn-3. Ag.Cu) solder balls with 2-µm diameter were used as conductive particles replacing conventional Ni metal particles. The adhesive matrix of the solder ACFs was the same thermosetting acrylic resin as conventional fast-curing acrylic ACFs. The major difference between the two solder compositions was their melting temperatures. Sn 8Bi has a lower melting temperature of 138 C, whereas SAC, the most widely used solder composition in the industry for solder assembly  , has a higher melting temperature of 217 C. Unlike conventional Ni metal conductive particles, solder balls can be melted and completely squeezed by applied pressures at below the C bonding temperature. Thus, for controlling the gap between electrodes of solder ACF joints, 8-µm diameter spacer Ni balls were added in the solder ACFs. The details of the solder ACF materials are listed in Table I. Test boards were 7-µm thickness rigid FR4 printed circuit boards (PCBs) and -µm thickness flexible PCBs (FPCs) for mobile device applications. The size of the PCB was 11 mm 48 mm and had 3-µm thick electroplated Ni/immersion gold (ENIG) finished electrodes. The FPC was made of a -µm thick polyimide based flexible substrate and had 22-µm thick ENIG finished electrodes. The size of the ACF bonding area was 19 mm 2 mm and the electrode pattern pitch was 2 µm. B. Ultrasonic Bonding Process For the FOB assembly, US bonding was performed by applying pressure and vertical vibration on the test specimen for s. A bonding tool made of steel alloy with an area of 22 mm 2 mm was used, which was larger than the ACF area. A US bonder with 4 khz of vertical vibration was used to generate US vibration, resulting in localized heating in the ACF layer. A maximum of 2 µm amplitude of vertical vibration was generated by the US bonder and the bonding temperatures were controlled by adjusting the US vibration amplitudes . The bonding temperature was 1 and 2 C for Sn 8Bi bonding and 2 and 2 C for SAC bonding. The in situ temperature at the ACF layer was measured by K-type thermocouples during US bonding. C. Observation of Solder ACF joints After US bonding, the solder ACF joints were observed and their composition analysis was made. Fourier transform Partial daisy-chain resistance Fig. 1. Schematic diagram and measuring points of daisy-chain resistance of solder ACF bonded FOB test vehicle. infrared (FT-IR) spectroscopy was conducted to analyze the degree of ACF curing, and a 9 peel test with mm/min peeling rate was conducted to measure the adhesion strengths at the ACF joints. D. Measurement of Electrical Properties To investigate the electrical properties of the solder ACF joints, daisy-chain resistances of ultrasonically bonded solder ACF joints were measured in comparison with conventional T/C bonded Ni ACF joints. As shown in Fig. 1, a daisy-chain circuit was designed to enable the measurement of the total daisy-chain resistance and 14 partial daisy-chain resistances. For the daisy-chain resistance measurement, 14 partial daisychain resistances were measured and the average was obtained. In addition, the current-handling capability of the USbonded solder ACF joints was also measured to investigate the effects of solder alloy joints. For measuring the maximum current limit of solder ACF joints from the I V curve, the voltage bias (V ) was raised by.1 V using a power supply. E. Reliability Test An unbiased autoclave test, also called as a pressure-cooker test, was performed for reliability assessment. The test conditions for the autoclave test were 121 C, 1% relative humidity, and 2 atm. pressure. During the test, the changes in the daisy-chain resistances were measured every 4 h for a total of 24 h of test time. The test results were also compared with those of conventional T/C-bonded Ni ACF joints. III. RESULTS AND DISCUSSION A. Demonstration of Solder ACF Joints Using the Vertical Ultrasonic Bonding Method 1) Morphologies of Solder ACF Joints: Fig. 2 shows the cross-sectional scanning electron microscopy (SEM) images and energy dispersive spectrum (EDS) of the solder B
3 886 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO., MAY [kV] SP = 16 WD = 1. kx 6. [µm] 2[kV] SP = 16 WD = 1. kx 6. [µm] 1. Bright area 2. Dark area Au(at.%) Sn(at.%) Bi(at.%) Ni(at.%) 1. Bright area 2. Dark area Au(at.%) Sn(at.%) Bi(at.%) Ni(at.%) [µm] 2[kV] SP = 14 WD = 1. kx 6. [µm] 1. Bright area Au(at.%) 9.9 Ag(at.%) 1 Sn(at.%) 7.99 Ni(at.%) 8.97 Cu(at.%) 4.7 Fig. 2. Cross-sectional SEM images and EDS analyses of Sn 8Bi ACF joint bonded at 1 C, Sn 8Bi ACF joint bonded at 2 C, SAC ACF joint bonded at 2 C, and SAC ACF joint bonded at 2 C. All bondings were made at. Absorbance (arb. unit) Non-cured 2 C 2 C Full-cured Sufficient curing Peel adhesion strength (gf/cm) Ref. 1 Wave number (cm 1 ) 99 2 C 2 C U/S bonding temperature ( C) Fig. 3. FT-IR spectrum of the C=C bond of methacrylate at various ultrasonic bonding temperatures. Fig. 4. Peel adhesion strengths of US-bonded FOB using solder ACFs at 2 and 2 C bonding temperatures. ACF joints. In the case of Sn 8Bi composition, which has 138 C melting temperature, they showed different joint morphologies at 1 and 2 C bonding conditions. As shown in Fig. 2, a Sn 8Bi joint bonded at 1 C shows well-captured Sn 8Bi solder balls between the electrodes. The Sn 8Bi solder ball consists of a bright area and a dark area in the SEM image. According to the EDS analysis, it was found that the bright area was a Bi-rich area and the dark area was a Sn-rich area. However, the solder joint composition was the same as its original composition. It means that there was no alloy bonding at the interface. This result indicates that the Sn 8bi joints bonded at 1 C are formed by physical contact of solder balls rather than alloy bonding, because this temperature is not high enough to completely melt the Sn 8Bi solder.
4 KIM et al.: ULTRASONIC BONDING OF ANISOTROPIC CONDUCTIVE FILMS Conventional T/C bonded Conventional T/C bonded. Fig.. Daisy-chain resistance of Sn 8Bi ACF bonded at 1 C and 2 C. Daisy-chain resistance of SAC ACF joints bonded at 2 C and 2 C [µm] Solder melting 2 [kv] SP = 12 WD = 1. kx 6. [µm] 2 [kv] SP = 16 WD = 1. kx 6. [µm] Fig. 6. Comparison of conventional contact-based Ni ACF joint and solder-alloy bonding based solder ACF joint. Fig. 2 shows the 2 C bonded Sn 8Bi ACF joint. Unlike the 1 C bonded joints, Sn 8Bi solder ACF joints contained 24 at% Au diffused from the Au layer of both ENIG finish on PCB and FPCB electrodes. The significance of this result is that solder alloy bonding can be rapidly formed even within a few seconds in US bonding with a higher ACF temperature than the solder melting temperature. Fig. 2 shows the 2 C bonded SAC ACF joints. Since 2 C is lower than the melting temperature of SAC, only physical contacts were made by the SAC solder balls. However, 2 C bonded SAC ACF joints in Fig. 2 showed 1 at% of Au diffused from the Au layer of both ENIG finish on PCB and FPCB electrodes, which indicated solder alloy bonding due to sufficiently high bonding temperatures. 2) ACF Curing Behavior and Adhesion Strength: Fig. 3 shows the FT-IR spectrum of US-bonded ACF joints. The curing of acrylic ACF resin can be analyzed by measuring the polymerization of methacrylate. Methacrylate is polymerized by breaking the C=C bond, which results in an IR peak reduction near 99 cm 1. The FT-IR spectrum showed that the C=C bond peak completely disappeared after US bonding at both 2 and 2 C. The significance of this result is that US-bonded solder ACF joints not only formed solder alloy joints but also made fully cured acrylic adhesive joints within s. In addition, fully cured solder ACF joints showed sufficient peel adhesion strengths of higher than 8 gf/cm as shown in Fig. 4. B. Characteristics of Ultrasonically Bonded Solder ACF Joints 1) Electrical Properties: As mentioned above, Sn 8Bi solder ACF joints were formed by physical contacts of solder balls at 1 C but by alloy joint bonding at 2 C. In the case of SAC, solder ACF joints were formed by physical
5 888 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO., MAY C 1 C 1 C 2 C 2 C 2 C.8.6 Conventional T/C bonded #1 #2.8.6 Conventional T/C bonded Fig. 7. Current-handling capability of Sn Daisy-chain resistance of Sn 8Bi ACF joints bonded at 1 C and 2 C. Current-handling capability of SAC ACF joints bonded at 2 C and 2 C Ni T/C ref , Ni T/C ref. Ni T/C ref., Fig. 8. Daisy-chain resistance of Sn 8Bi ACF joints bonded at 1 C and 2 C. The sane for SAC ACF joints bonded at 2 C and 2 C during an unbiased autoclave test (121 C, 2 atm. aging). contacts at 2 C but by alloy joint bonding at 2 C. As shown in Fig., stable daisy-chain resistances were achieved regardless of the bonding conditions. In comparison with T/C-bonded conventional Ni ball ACF joints, both Sn 8Bi ACF joints and SAC ACF joints had lower resistances, due to broader electrical paths because of the 2-µm size
6 KIM et al.: ULTRASONIC BONDING OF ANISOTROPIC CONDUCTIVE FILMS 889 solder balls, than conventional Ni ACF joints as shown in Fig. 6. Current-handling capability of solder ACF joints was measured from the I V curve by raising voltage bias (V ) by.1 V. At lower voltages, the current at the ACF joints linearly increased as the applied bias increased, according to the Ohm s law. However, the rate of current increase became saturated as the bias increased due to resistance increase at high temperature resulting from joule heating. At high voltages, the ACF joints began to be unstable and failed electrically. Fig. 7 shows the current-handling capability of Sn 8Bi and SAC solder ACF joints in comparison with conventional Ni ACF joints. As shown in the graph, conventional Ni ACF joints became unstable even below.6 A. However, solder ACF joints showed excellent current-handling capability compared with conventional Ni ACF joints regardless of the bonding conditions. In particular, solder ACF joints bonded at high temperatures, 2 C in Sn 8Bi and 2 C in SAC, showed excellent current-handling capabilities: up to 1% improvement compared with those of conventional Ni ACF joints was achieved. The significance of this result is that solder alloy bonding not only reduced the electrical resistances at the ACF joint but also dramatically improved the currenthandling capabilities of the ACF joints. According to these results, the solder ACF joints showed 2% decrease in daisy-chain electrical resistances and 1% increase in current-handling capability compared with conventional Ni ACF joints. 2) Reliability Test Results: AsshowninFig.8,solderACF joints bonded at lower temperatures, i.e., 1 C for Sn 8Bi and 2 C for SAC, showed early electrical failures (within 8 h) due to physical contacts of the solder balls. These results were similar to those of conventional ACF joints, which showed electrical failures within 4 h. However, solder ACF joints bonded at high temperatures, i.e., 2 C for Sn 8Bi and 2 C for SAC, showed significantly improved results due to solder alloy bonding at the ACF joints. With solder alloy bonding, a solder ball can be mechanically bonded to the metal electrodes, resulting in stable solder joint morphologies under a hygroscopic expansion of polymer resins at high temperature and high humidity conditions, whereas physically contacted conductive balls easily lose their contacts from the metal electrode surface. Furthermore, reliability improvements at higher bonding pressures are presumably due to broader alloy joints achieved by narrower gaps between metal electrodes at higher bonding pressures. daisy-chain resistance and 1% increase in current-handling capability compared with conventional Ni ACF joints bonded by T/C bonding. Solder ACF joints also showed significantly improved reliability with stable electrical resistances up to 24 h in an unbiased autoclave test, whereas conventional Ni ACF joints bonded by T/C bonding showed severe electrical failures within 4 h. Therefore, conventional Ni ACF joints bonded by T/C bonding can be replaced by solder ACF US bonding technology, which results in significantly improved electrical properties and reliability in FOB assembly. This solder ACF US bonding technology is expected to be used not only in mobile device applications but also in other ACF applications as well for better electrical properties and reliability. REFERENCES  P. Savolainen, I. Saarinen, and O. Rusanen, High-density interconnections in mobile phones using ACF, in Proc. 4th IEEE Int. Conf. Polym. Adhes. Microelectron. Photon., Apr. 24, no. AP22, pp  K. Lee, H.-J. Kim, M.-J. Yim, and K.-W. Paik, Ultrasonic bonding using anisotropic conductive films (ACFs) for flip chip interconnection, IEEE Trans. Electron. Packag. Manuf., vol. 32, no. 4, pp , Oct. 29.  K. Lee, H. J. Kim, I. Kim, and K. W. Paik, Ultrasonic anisotropic conductive films (ACFs) bonding of flexible substrates on organic rigid boards at room temperature, in Proc. Electron. Comp. Technol. Conf., Reno, NV, May Jun. 27, pp  J. Kivilahti and P. Savolainen, Anisotropic adhesives for flip-chip bonding, J. Electron. Manuf., vol., no. 4, pp , 199.  A. Z. Miric and A. Grusd, Lead-free alloys, Solder. Surf. Mount Technol., vol. 28, no. 1, pp. 19 2,  H. Ma and J. C. Suhling, A review of mechanical properties of leadfree solders for electronic packaging, J. Mater. Sci., vol. 44, no., pp , 29.  A. Choubey, H. Yu, M. Osterman, M. Pecht, F. Yun, L. Yonghong, and X. Ming, Intermetallics characterization of lead-free solder joints under isothermal aging, J. Electron. Mater., vol. 37, no. 8, pp , 28.  M. N. Toulunay, P. R. Dawson, and K. K. Wang, Heating and bonding mechanisms in ultrasonic welding of thermoplastics, Polym. Eng. Sci., vol. 23, no. 13, pp , Sep Won-Chul Kim author photograph and biography not available at the time of publication. Kiwon Lee author photograph and biography not available at the time of publication. IV. CONCLUSION In this paper, solder ACF joints were successfully demonstrated using a US bonding method for high-power and high-reliability FOB assembly. Fine solder balls 2-µm diameter were used as conductive particles to form solder alloy bonding between metal electrodes. It was found that solder alloy bonding was achieved at both Sn 8Bi and SAC solder balls. At the same time, it was also seen that acrylic ACF resin was completely cured within s of US bonding. Solder ACF joints showed about 2% decrease in Ilkka J. Saarinen author photograph and biography not available at the time of publication. Lasse Pykari author photograph and biography not available at the time of publication. Kyung-Wook Paik author photograph and biography not available at the time of publication.