Fabrication of Smart Card using UV Curable Anisotropic Conductive Adhesive (ACA) Part I: Optimization of the curing conditions

Transcription

1 Fabrication of Smart Card using UV Curable Anisotropic Conductive Adhesive (ACA) Part I: Optimization of the curing conditions K. K. Lee, K T Ng, C. W. Tan, *Y. C. Chan & L. M. Cheng Department of Electronic Engineering City University of Hong Kong 83 Tat Chee Avenue, Kowloon, Hong Kong * EEYCCHAN@cityu.edu.hk Abstract Highly demand of green electronics products has driven the use of conductive adhesive as the substitution of using conventional solders. Recently developed UV curable Anisotropic Conductive Adhesive (ACA) is suggested to be used in temperature sensitive electronic packages. The aim of this study is to optimize the bonding conditions for fabrication of smart card using UV curable ACA. A simple designed copper coil plated on a PET was used as the micro strip antenna of the smart card. The entire fabrication process is divided into three parts, high power UV curing, chip-on-flex (COF) bonding & post curing. By varying the UV curing & post-curing parameters, a number of contactless smart cards were made. Chemical analysis using FTIR (Fourier Transform Infra Red Spectroscopy) was carried out to determine the curing degree of the UV curable ACA under different curing conditions. In order to quantify the performance of the smart card, the read range between the card reader and the sample was measured. Besides, shear test was performed; the shear force that is required to break the ACA joint and its failure mode was recorded and discussed. ACA joints of the smart cards under different curing conditions were also crosssectioned and examined using SEM (Scanning Electron Microscope). By comparing the results in curing degree, shear strength of the ACA joint and the reading distance of the smart card samples, a set of parameters which gave better performance of the COF bonding were determined. {keywords} chip-on-flex (COF), contactless smart card, UV curable anisotropic conductive adhesive (ACA) Introduction Generally, shorter time is required to cure the materials at higher cure temperature as higher curing temperature usually leads to an increase in crosslink density and a homologous increase in heat resistance. However, problems can occur at high curing temperature. The higher the curing temperature, the greater the inclination for the material to shrinkage, cracks and voids and probably a lowering of its dielectric properties. Newly developed UV curable adhesive offers several advantages over the conventional epoxy resin, including rapid cure and little to no emissions of volatile organic compounds without affecting other components in the assembly [1]. Based on the aforementioned advantages, it is worth investigating the bonding properties at different curing conditions. UV curable adhesive is now used successfully in the manufacture of anisotropic conductive adshesive (ACA). Photoinitiators are the key organic compounds that are contained in the UV cure polymers [1]. Under ultraviolet light exposure, the photoinitiators can release free-radicals which react with the base monomer in the resin. Thus the material is engendered to polymerize or strengthen. In order to determine the curing degree of the UV curable adhesives, there are two major methods which have been proposed previously by several researchers. One is using Differential Scanning Photocalorimetry (Photo DSC) [2] while the other one is using Fourier Transform Infrared Spectroscopy (FTIR) [2, 3]. In Photo DSC measurements, the exothermic peaks are considered to be attributed to the thermal polymerization of the UV curable adhesives. Nevertheless, in FTIR measurements, the epoxy and reference peaks can numerically represent curing degree of the adhesives. Experimental Procedure Materials In this work, the UV curable anisotropic conductive adhesive (ACA) used is made of epoxy resin in which plastic hollow bead with nickel-gold coated particles are dispersed. The density of the conducting particles in the epoxy is about 1200 per mm 2. Each conducting particle is about 6.5µm in diameter. The dimensions of the test chip are 1.11 x 1.06 mm 2 with 4 Au bumps on the active side of the chip as shown in Figure 1. Bumps B1 and B2 are at the top right corner while bumps B3 and B4 are at the bottom left and right corners. The dimensions of B1, B3 and B4 are 104 x 104 x 18 µm 3 while that of B2 is 89 x 104 x 18 µm 3. The flexible smart card substrate used in this work is 75 µm thick PET and the area of the substrate is 70 x 76 mm 2. A custom designed copper coil is screen-printed on the PET as the antenna of the contactless smart card. The thickness of the copper coil is 17µm. Only B1 & B4 were bonded on the pads of the designed antenna. EFOS Novacure Model N2001-A1 was used as the light source of this work and its operating range of the UV light is about nm. Curing Degree FTIR-ATR measurement was carried out to find out the curing degree of the UV curable ACA. The spectrometer used was equipped with a universal ATR attachment. Two epoxy peaks, 789 and 914 cm -1 and two reference peaks, 1455 and 2922 cm -1 are suggested [2, 4] for determining the curing degree of light activated epoxy resin. For the UV curable anisotropic conductive adhesive used in this work, the most suitable epoxy and reference peaks were found to be 789 and /04/$ IEEE 134

2 1455 cm -1. The reference and specimen spectra represented the average of 16 scans recorded at 4cm -1 resolution. Figure 4. Schematic diagram of a fabricated contactless smart card sample under shear test Figure 1. Schematic diagram of the active side of the test chip (1.11 x 1.06 mm 2 ) One of the key functions of the epoxy matrix in ACA is used to adhere the chip and the substrate. The curing degree of epoxy can affect the mechanical property as well as the electrical property of the ACA [5-7]. In current study, FTIR spectroscopy is used to check the curing degree of epoxy in ACA with the following equation (1). α = 1 [(A epoxy,t / A ref,t ) / (A epoxy,0 / A ref,0 )] (1) Where A epoxy,0 and A ref,0 are the initial area of epoxy and reference peak respectively, and their corresponding values at time t are A epoxy,t and A ref,t. Fabrication of Contactless Smart Cards For the fabrication of the contactless smart card samples, 60 test chips were fabricated on the flexible substrate. For UV light activation of the reaction in the epoxy, the light intensity and exposure time were 400mW/cm 2 and 1.5s. By using a KarlSuss FCM505 Manual Flip Chip Bonder, after the alignment between the bumps and the pads was finished, the test chip was bonded on the flexible substrate for 10 seconds under the pressure of ~1N at room temperature. The fabricated samples were then post-cured at 140 o C for 4, 6, 8, 10 minutes. Shear Test After the post-curing treatment, the read range of all the fabricated contactless smart card samples were measured so as to find out the electrical perfornance of the ACA joint. For investigating the mechanical performance of the samples, they were sheared by using a Dage Series 4000 Bondtester. In this test, the shear blade was equipped with a shear hook and then fixed to the crosshead. Each sample was placed on the fixture and then clamped on the bond tester as shown in Figure 4. The shear blade was placed just 20µm right above the copper coil, which applied a tensile load with 100.0µm/s loading rate moving horizontally towards the test chip. Result & Discussion FTIR-ATR Results For choosing the bonding and curing parameters of the chip-on-flex contactless smart card application, the samples of bare UV curable ACAs were examined by using FTIR-ATR measurement. First of all, the curing degree of the bare UV curable ACAs specimens with only UV light activation was measured. Each specimen was prepared by dropping a spot of UV curable ACAs on the flexible substrate. Each spot of UV curablle ACA weighted about 0.1mg was dispensed on the flexible substrate. Three sets of specimens were exposed under the UV light of intensities of 200, 400 & 600 mw/cm 2 for 1.5 seconds with 4 specimens in each set. The result is shown in Figure 2. Curing Degree (%) Set B 50 Set C UV Intensity (mw/cm2) Set A Figure 2. Curing degree of UV curable ACA after UV light activation under different intensities (Set A 200mW/cm 2 ; Set B 400mW/cm 2 ; Set C 600mW/cm 2 ) It was shown that the curing degree of the UV curable ACA spots under higher UV light exposure became higher. This parameter was recorded to find out the most suitable UV light intensity for the fabrication of contactless smart card samples. The curing degree of set A specimens is relatively much lower than that of sets B & C. Therefore, we had to consider the time required for the alignment so as to choose which UV light intensity is more suitable. For set C /04/$ IEEE 135

3 specimens, the surfaces of the UV curable ACA spots started to be harden 2 minutes after UV light activation and were unable to bond the test chip. Based on the above reasons, another four sets of 16 specimens were prepared to measure the curing degree. They were exposed under the UV light of intensity of 400mW/cm 2 for 1.5 seconds and then was put into an oven at 80, 100, 120 & 140 o C for 4 minutes. The result is shown in Figure 3. part, the R J is contributed to the number of conductive particles in between the bump and the pad while the C J is contributed to the adhesive which affects the dielectric change in the connection area. Therefore, the curing of the adhesive and the number of conductive particles may affect the read range of the contactless smart card sample. 95 Curing Degree (%) Post-curing Temperature (Degree C) Figure 3. Curing degree of UV curable ACA after UV light activation under 400mW/cm 2 for 1.5s and then post-cured at different temperature for 4 mins Curing Degree (%) Post-curing Time (mins) Figure 5. Curing degree of UV curable ACA under 400mW/cm 2 exposure for 1.5s and post-cured at 140 o C for different time. UV From the result, the UV curable ACA spots post-cured at 140 o C for 4 minutes can be cured more than 80%. Therefore, it was decided to fabricate the contactless smart cards by using 140 o C post-curing temperature. To compare the mechanical and electrical performance of the ACA joints using UV curable ACA, the contactless smart cards samples were post-cured for different time and they were tested by determining the shear force and the read range of the samples. Moreover, in order to estimate the correspondng curing degree, bare UV curable ACA sample were measured which were prepared under the same bonding and curing paramenters. According to the above FTIR-ATR measurement, each sample spot of ~ 0.1mg UV curable ACA was exposed under 400mW/cm 2 UV light for 1.5s activation and then post-cured at 140 o C for 4, 6, 8, 10 minutes. The curing degree measurement result was also measured as shown in Figure 5. From the graph, the longer the post-curing time, the higher the curing percentage of the UV curable ACA. The curing degree can reach about 90% when post-cured for 10 minutes. However, to proof whether 90% curing of the UV curable ACA can provide a good electrical and physical join between the bump and the pad, more tests were carried out in this work. Read Range Measurement Figure 6 shows the equivalent circuit of the whole fabricated contactless smart card. C C & R C represent the capacitance and resistance of the test chip, C J and R J are the resistance and the capacitance of the ACA joint of the test chip and the copper coil. C A, R A and L A are the capacitance, resistance and inductance of the antenna. For the connection Figure 6. Schematic diagram of the equivalent circuit of the whole fabricated contactless smart card There were 40 successfully fabricated contactless smart card samples for the 4 sets of parameters. Each sample was placed horizontally right above the card reader, the longest distance that the sample could be recognized by the card reader was measured as the read range of the sample. The read range results is shown in Figure 7. Generally, the read range of contactless smart card is mainly due to the antenna design and the flexible substrate materials used. When focusing on the comparison between the conventional wire bonded contactless smart card and the flip-chip bonded smart card using UV curable ACA. By using the same test chip and flexible substrate, the average read range of aluminum wire bonded smart card samples were more than 11 mm /04/$ IEEE 136

4 Read Range (mm) Read Range Sheat Load (g) Sheat Result Post-curing Time (mins) Post-curing Time (min) Figure 7. Read range of the contactless smart card samples post-cured for different time Figure 8. Shear strength of the contactless smart card samples post-cured for different time. From the result, the longer post-curing time, the longer the read range of the samples. However, the overall read range is around 7 to 8 mm of all the samples. Therefore, the samples in this work can reach about 65% to 70% perforrnance of the general wire bonded contactless smart card. The read range difference of the samples that post-cured for 4 and 10 minutes was about 0.7mm. There was just slightly change in read range value of the samples post-cured for different time. The increment of the read range may be due to the higher curing degree of the adhesive for longer post-curing time. Shear Result After read range meansurement, the four sets of contactless smart card samples were used for shear test. The result is shown in Figure 8. The longer the post-curing time, the larger the shear load was required to totally separate the test chip and the flexible substrate. Similarly, the higher the curing degree, the larger the shear load required. There was a great increase in shear load of samples post-cured for 10 minutes. At this condition, the corresponding curing degree of the adhesive was about 90%. SEM Observation In this work, samples were observed, compared and analyzed by using Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectrometer (EDX) (Philips Model XL40). Three sheared test chips of the contactless smart card samples that post-cured at 140 o C for 4, 6 & 10 minutes were shown as Figures 9, 10 and 11 respectively. Most of the adhesive remained on the active sides of the test chips rather than on the flexible substrates. By comparing the figures, less adhesives were left on the active sides of the test chips when the post-curing time was longer. Therefore, for longer postcuring treatment, the adhesion of the UV curable ACA is greater at the flexible substrate side while it is smaller when at the test chip side. Figure 9. Active side of sheared test chip with UV curable ACA post-cured for 4 mins Figure 10. Active side of sheared test chip with UV curable ACA post-cured for 4 mins /04/$ IEEE 137

5 Figure 12. SEM photo of Au bump (B4) of a good contactless smart card sample after shear test Figure 11. Active side of sheared test chip with UV curable ACA post-cured for 4 mins From the read range measurement, there were few contactless smart card samples which could not be recognized by the smart card reader. Therefore, after shear test, the samples were observed and are shown in Figure 12 and 13. By considering the number of the conductive particles, it seemed that there were more particles remained on the bump surface in Figure 12, thus providing a better electronic conduction of the ACA joint. However, number of conductive particles may not be the major factor affecting the electrical connectivity of the ACA joint, then cross-section observation were also analyzed. From Figure 14, it is the cross-section of a good sample also post-cured at 140 o C for 10 minutes. Conductive particles were trapped in between and bump and the pad, and they also provided a good electrical contact of the ACA joint. For the failed contactless smart card samples which could not be recognized by the card reader, a cross-section picture of one of the ACA joints is shown in Figure 15. It is a sample that post-cured at 140 o C for 10 minutes. From the figure, it can be observed that the failure was due to the spacing was so large that no conductive particles could be trapped in between the bump and the pad. The alignment of the ACA joint in Figure 14 was much better than that of sample in Figure 15. Figure 13. SEM photo of Au bump (B4) of a failed contactless smart card sample after shear test Figure 14. Cross-section of the ACA joint of a good sample Figure 15. Cross-section of the ACA joint of a failed sample Conclusion /04/$ IEEE 138

6 The major objective of this work is to optimize the UV curing and post-curing conditions to fabricate the contactless smart card samples. FTIR-ATR measurement was used to firstly find out the required UV light intensity, and then the post-curing temperature. It was found that the higher the intensity, the higher the curing degree of the UV curable ACA. However, if the intensity is too high or the time for exposure is too long, the surface of the adhesive will be cured and harden that cannot be used for bonding. Therefore, the UV curing conditions was chosen to be 400mW/cm 2 UV light intensity and 1.5s light activation. For determining the required post-curing temperature, the measurement result showed that the curing degree of the adhesive can reach more than 80% when the specimens postcured at 140 o C for only 4 minutes. The curing degree could be up to about 90% when the adhesive post-cured for 10 minutes. Therefore, contactless smart card samples were fabricated under the above chosen conditions with different post-curing time. To determine the time required for postcuring procedure, not only the FTIR-ATR measurement, but also the read range measurement and shear test were also be done. This two experiments were carried out to compare the mechanical and electrical performance of the ACA joint. By using the UV curable ACA, the distribution of the conductive particles in between the bump and the pad can not be controlled. Therefore, the capacitance of the ACA joint, C J, was varied due to the dielectric change of the adhesive used. It was found that the higher the curing degree of the adhesive, the longer the read range. In order to finish optimizing the curing conditions, it was also needed to consider the mechanical performance of the ACA joint. From the shear test result, the longer the post-curing time, the larger the shear load was required. Also, the higher the curing degree, the larger the shear load was required. On the other hand, the adhesion became larger between the adhesive and the flexible substrate. Based on all the test results, the post-curing time was optimized to 10 minutes as both the mechanical and electrical performance of the samples fabricated under this condition were relatively better than the others. In addition to optimizing the curing conditions, SEM observation of the ACA joint was also done to found out the failure mechanism. The main problem of the failed samples was the large spacing between the bump and the pad that conductive particles could not be trapped and led to open circuit. The spacing problem maybe due to the unevenly distribution of adhesive and conductive particles and also the applied bonding pressure was not large enough. In conclusion, both the UV curing and post-curing conditions for the contactless smart card application was optimized. However, the read range result showed that the samples fabricated under optimized curing conditions could achieve up to 70% performance of the general wire bonded contactless smart card. Therefore, it is worth to further fabricate the contactless smart cards under the optimized conditions, and then to determine the reliability of the ACA joint of using UV curable ACA. In addition to reliability tests, microscopic observation of the failure would be another key method to enhance the fabrication procedures and curing conditions. Acknowledgments The authors would like to acknowledge the financial support from the City University of Hong Kong (ITF Project Conductive Adhesive Technology Programme for Fine Pitch Electronic Packaging, Project No.: ITS/182/00). The authors would like to thank Ms S C Tan for her consultation on chemical background of the UV curable ACA and also would like to thank MaCaPs International Limited for the technical advice on the read range measurement of contactless smart card. References 1. William S. Pataki, Optimization of Free-radical Initiation Reactions in the Electrical Industry, Electrical Insulation Conference and Electrical Manufacturing & Coil Winding Conference Proceedings, 22 25, (1997), pp J. D. Cho, H. K. Kim, Y. S. Kim and J. W. Hong, Dual curing of cationic UV-curable clear and pigmented coating systems photosensitized by thioxanthone and anthracene, Polymer Testing, Vol. 22, (2003), pp H. Nagata, M. Shiroishi, Y. Miyama, N. Mitsugi and N. Miyamoto, Evaluation of New UV-Curable Adhesive Material for Stable Bonding between Optical Fibers and Waveguide Devices: Problems in Device Packaging, Optical Fiber Technology 1, (1995), pp K. Motoki, M. Oyama, T. Imai, T. Ishii, M. Kimata, H. Horita, N. Sasaki, I. Kobayashi, T. Yokoyama, A. Ono, s. Kodate, S. Suzuki, Y. Ono, M. Kurosawa, K. Yoshinuma and H. Goto, Connecting Technology of Anisotropic Conductive Materials, Fujikura Technical Review, (2002), ( 5. Liu, J., An Overview of Advances of Conductive Adhesive Joining Technology in Electronics Applications Materials Technology, Vol. 10, pp Watanabe, I. and Takemura, K., pp [A book reference ] 7. Lai, Z. and Liu, J., Anisotropically Conductive Adhesive Flip-Chip Bonding on Rigid and Flexible Printed Circuit Substrates, IEEE Transactions on Components, Packaging and Manufacturing Technology Part B, Vol. 19, No. 3, pp /04/$ IEEE 139