Ultrasonic Bonding Acf

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  • 1. Ultrasonic Anisotropic Conductive Films (ACFs) Bonding of Flexible Substrates on Organic Rigid Boards at Room Temperature Kiwon Lee, Hyoung Joon Kim, Il Kim, and Kyung Wook Paik Nano Packaging and Interconnect Lab. (NPIL) Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 373-1, Kusong-dong, Yusong-gu, Daejeon, 305-701, Republic of Korea phone: +82-42-869-3386, fax:+82-42-869-3310 email: kiwonlee@kaist.ac.kr T/C bonding is often limited by high bonding temperature,Abstract slow thermal cure, uneven cure degree of adhesive, large In this study, a novel Anisotropic Conductive Film (ACF) thermal deformation of the assembly, and high bondingbonding method for flexible substrate-organic rigid board pressure for large number of I/O interconnections. Therefore,bonding was investigated using an ultrasonic vibration. And there are constant needs of lower bonding temperature andthe ultrasonic (U/S) ACF bonding was characterized in terms faster cure ACF bonding to replace the conventional T/Cof adhesion strength, electrical continuity in comparison with bonding at 190℃ and 15 seconds.those of thermo-compression (T/C) bonding. An ultrasonic bonder was used to produce ultrasonic Ultrasonic (U/S) bonding for solder and metal-to-metalvibration in ACF joints between flexible substrates and interconnections has been widely investigated and applied inorganic rigid boards. Test boards were 25 um-thick polyimide flip chip, TAB, or surface mount technologies due to its lowbased flexible substrates and 1 mm-thick FR-4 organic rigid cost, simplicity, and fast assembly time at low temperatureboards. The effect of process conditions, such as U/S powers, with reduced bonding pressure. However, most of thesebonding pressures, and bonding times on the ACF approaches are using lateral direction ultrasonic vibrationtemperature were investigated. The in-situ temperature at the only. Although there have some attempt to use U/S bondingACF layer was measured during U/S bonding to determine as an alternative ACF bonding method, there were nothe relation between the ACF temperature and the bonding previous reports of successive ACF bonding using U/Scharacteristics of ACF joints. bonding except one presentation about the demonstration of U/S ACF flip chip bonding [1]. The optimized U/S bonding time was obtained within 5sec at room temperature. The significant meaning of this In ACFs U/S bonding, ACFs can be rapidly heated byresult is that the ACF bonding process times can be certain ultrasonic vibration, and it can be described byremarkably reduced by U/S bonding compared with materials’ complex young’s modulus which consists of storage modulus and loss modulus under cyclic stressconventional 15 sec T/C bonding at 190 ℃. The ACF joints conditions. Storage modulus is related to elastically storedwith the optimized U/S bonding conditions showed similar energy, and loss modulus is related to energy loss whichbonding characteristics as those with T/C bonding in terms of converts to heat. In general, it is well known that visco-elasticthe adhesion strength and the daisy-chain contact resistance. materials such as polymers have large loss modulus.The Fourier Transformation Infra-Red (FTIR) spectroscopy Therefore, it is expected that highly visco-elastic B-stageshowed that the ACF degree of cure was achieved 90 % at 3 ACFs may generate a large amount of heat by an ultrasonicsec and 95 % at 4 sec. Reliability test results showed that the vibration. By U/S heating in ACFs, rapid ACF curing andU/S bonded test boards had stable daisy-chain contact bonding can be obtained without additional chip/substrateresistances in 85 ℃/85 % RH test and 125 ℃ high heating.temperature storage test for 1000 hours and -55 ℃~125 ℃ In this study, a novel Anisotropic Conductive Film (ACF)thermal cycling test for 1000 cycles. bonding method for flexible substrate-organic rigid board1. Introduction bonding was investigated using an ultrasonic vibration. And Anisotropic Conductive Films (ACFs) are well known the ultrasonic (U/S) ACF bonding was characterized in termsadhesive interconnect materials which consists of conducting of adhesion strength, electrical continuity in comparison withparticles and adhesive polymer resins in a film format. And those of thermo-compression (T/C) bonding, and its reliabilitythey have been widely used as interconnect materials in flat was evaluated.panel display applications such as Out Lead Bonding (OLB), 2. Experimentschip-on-glass (COG), and chip-on-film (COF), flex to PCB 2.1 Materials preparationbonding, and also in flip chip semiconductor packaging Test boards were 25 um-thick polyimide based flexibleapplications. ACF interconnects are simple and lead-free substrates and 1mm-thick FR-4 organic rigid boards. Figure 1processing as well as cost effective packaging method shows the design of test boards which have 300 um pitch Cucompared with solder interconnects. patterns. The ACF was epoxy based adhesive film with 40 um For ACF interconnection, thermo-compression (T/C) thickness, and they contained Au coated Ni particles with 8bonding is the most common method, however it is necessaryto reduce the bonding temperature, time and pressure, because1-4244-0985-3/07/$25.00 ©2007 IEEE 480 2007 Electronic Components and Technology Conference
  • 2. um diameter as conductive particles. Table 1 summarizes the In-situ ACF temperature during U/S bonding was measuredspecifications of test boards and the ACF. to investigate the effect of U/S bonding conditions such as U/S power and bonding force on ACF temperatures. Figure 3 shows the in-situ ACF temperature measurement set-up with 40um-thick k-type thermocouples and a thermometer with 150ms sampling period. Figure 1. The design of (a) organic rigid test board and Figure 3. A schematic diagram of in-situ ACF temperature (b) flexible substrate measurement Table 1. The specifications of test boards and the ACF 2.5 Adhesion strength vs. U/S bonding conditions Materials Thickness To determine the relation between the ACF temperature Flexible substrate Polyimide film 25um and adhesion strengths, adhesion strengths of ACF joints after Organic rigid board FR-4 1mm U/S bonding were measured using a 90° peel tester as shown Base film Epoxy based adhesive 40um in figure 4. Peel test was performed with a peel rate of 10 ACF Conductive 8um mm/min, and adhesion strengths of ACF joints were Au coated Ni particles monitored during peel test using a load cell. particle diameter2.2 Equipment Figure 2 shows the ultrasonic horn and test boards in thisexperiment. The ultrasonic energy of 160 W ~ 240 W wasapplied perpendicularly on the test boards Flexible substrate ACF U/S horn Organic rigid board Figure 4. A schematic diagram of 90° peel test Flexible substrate ACF 2.6 Daisy-chain contact resistance vs. U/S bonding time Organic rigid board Daisy-chain contact resistances of test boards were measured after various U/S bonding conditions to examine Figure 2. An ultrasonic bonder set-up showing an U/S horn the electrical continuity of ACF joints. Figure 5 shows the and a test sample daisy-chain structure of test boards.2.3 Decomposition temperature of materials During U/S bonding, test boards or the ACF can bedecomposed due to rapid increase of ACF temperature.Therefore, decomposition temperatures of flexible substrates,organic rigid boards and the ACF were measured to preventdecomposition of materials during U/S bonding. I VDecomposition temperatures were measured by TGA Figure 5. The daisy-chain structure of test boards showing a flexible(Thermo-Gravimetric Analysis) with 10℃/min heating rate. substrate bonded on a organic rigid board using ACFs2.4 ACF temperature vs. U/S bonding conditions 481 2007 Electronic Components and Technology Conference
  • 3. 2.7 Reliability evaluation of U/S bonding Reliability tests were performed with the optimized U/S 600bonding conditions in terms of the adhesion strength and the 160Wdaisy-chain contact resistance. Reliability requirements were 500 200W85 ℃/85 % RH test and 125 ℃ high temperature storage test 240W ACF temperature ( C) ofor 1000 hours, and -55 ℃~125 ℃ thermal cycling test for 4001000 cycles. Using 10 test boards for each test, total daisy- 300chain contact resistances were measured for every 100 hoursand 100 cycles. 200 1003. Results and discussion3.1 Decomposition temperature of materials U/S on U/S off 0 0 2 4 6 8 10 12 Bonding time (sec) 1.1 1.0 Figure 7. ACF temperatures vs. U/S powers at 5.6 MPa bonding pressure Relative weight 0.9 Figure 7 shows the ACF temperature during U/S bonding 0.8 with 160 W, 200 W and 240 W powers at constant 5.6 MPa 0.7 bonding pressure. As shown in the graph, the ACF temperature increased as the U/S power increased. 0.6 Flexible substrates The increase of ACF temperature can be explained with Organic rigid boards U/S vibration amplitudes. U/S vibration amplitudes increase 0.5 0 100 200 300 400 500 600 with larger U/S powers at constant pressures, because the o work by U/S vibration increases as the U/S power increases. Temperature ( C) According to the well-known equation 1, f (∆ε )2E’’ dQ = 1.1 1.0 2 Equation 1. Heat generation by cyclic deformation [2] Relative weight 0.9 0.8 which explains heat generation under cyclic deformation, heat generation (dQ) is proportional to cyclic strain (∆ε).[2] And 0.7 cyclic strain increases as U/S vibration amplitude increases. Therefore, the increase of U/S power causes the increase of 0.6 U/S vibration amplitude, and cyclic strain of the ACFs 0.5 resulting in more heat generation. 0 100 200 300 400 500 600 o Temperature ( C) 500Figure 6. TGA results of flexible substrates, organic rigid boards, 4.6MPa 6.7MPa ACF temperature ( C) 400 and ACF 8.6MPa o Figure 6 shows TGA results of flexible substrates, organic 300rigid boards and the ACF. As shown in the graph, the weightof FR-4 organic rigid boards rapidly decreased at above 300 200℃. However, flexible substrates and the ACF showednegligible amount of decomposition up to 300 ℃. Therefore, 100ACF heating temperature during U/S bonding was maintainedbelow 300 ℃ to prevent FR-4 decomposition. U/S on U/S off 0 0 2 4 6 8 10 12 Bonding time (sec)3.2 ACF temperature vs. U/S bonding conditions Figure 8. ACF temperatures vs. bonding pressures at 180W power 482 2007 Electronic Components and Technology Conference
  • 4. Figure 8 shows the ACF temperature during U/S bondingat 4.6 MPa, 6.7 MPa and 8.6 MPa bonding pressures at 500constant 180 W power. As shown in the graph, the ACFtemperatures decrease as bonding pressures increase. 8.6MPa 200W ACF temperature ( C) 400 The decrease of ACF temperatures can be explained with oU/S vibration amplitudes. U/S vibration amplitudes decrease 180Was bonding pressures increase at constant U/S powers. 300Because the work by U/S vibration is constant at a certain U/Spower, it means that a smaller vibration amplitude can be 200obtained at larger pressures. Therefore, the increase of U/Spowers causes the decreases of U/S vibration amplitude, and 100cyclic strain of the ACF resulting in a less amount of heatgeneration. 0 As explained above, ACF temperatures were dependent 0 2 4 6 8 10 12on both U/S powers and bonding pressures. Therefore, in Time (sec)order to maintain the ACF temperature below 300 ℃, variousU/S powers were selected at 4.6 MPa, 6.7 MPa and 8.6 MPa Figure 9. ACF temperatures during U/S bonding with variousbonding pressures. Figure 9 shows the ACF temperatures bonding conditionsduring U/S bonding with various bonding conditions. At 4.6 MPa and 180 W condition, the ACF temperature increased up to 300 ℃ . However, test boards were decomposed at 6.7 MPa and 200 W condition due to over- 500 heating above 400℃. At other U/S bonding conditions, ACF 4.6MPa 180W temperatures were maintained about 200 ℃ which was ACF temperature ( C) 400 relatively lower than that of 4.6 MPa and 180W condition. 160W o 300 3.3 Adhesion strength vs. U/S bonding conditions Figure 10 shows adhesion strengths of U/S bonded ACF 200 joints at various U/S bonding conditions. As shown in the graph, the maximum adhesion strength was above 600 gf/cm 100 at 3 sec bonding time at 4.6 MPa and 180 W condition. However, at other U/S bonding conditions, adhesion strength 0 showed relatively low 400 gf/cm adhesion strengths. The 0 2 4 6 8 10 12 adhesion strength behavior is well matched with the previous Time (sec) ACF temperature behavior at figure 9. The ACF temperature during U/S bonding at 4.6 MPa and 180 W condition was about 300 ℃, and those with other U/S bonding conditions were about 200 ℃. Lower adhesion strengths was mainly be due to lower degree of cure of ACF at lower temperatures. 500 6.7MPa 200W ACF temperature ( C) 400 o 180W 4.6MPa, 160W 3sec 800 300 5sec 7sec peel strength (gf/cm) 200 600 9sec 100 400 0 0 2 4 6 8 10 12 200 Time (sec) 0 0 1 2 3 4 5 6 7 Displacement (mm) 483 2007 Electronic Components and Technology Conference
  • 5. 800 4.6MPa, 180W 3sec 800 8.6MPa, 200W 3sec 5sec 5secPeel strength (gf/cm) 7sec 7sec Peel strength (gf/cm) 600 9sec 600 9sec 400 400 200 200 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Displacement (mm) Displacement (mm) Figure 10. Adhesion strengths of U/S bonded ACF joints at various U/S bonding conditions. 6.7MPa, 180W 3sec 800 5sec Figure 11 shows adhesion strengths of U/S bonded ACF joints at 4.6 MPa and 180W condition for 2 sec, 3 sec and 4Peel strength (gf/cm) 7sec 600 9sec sec bonding times. As shown in the graph, the adhesion strength rapidly increased by ACF curing, and reached the maximum value of 630gf/cm at 3 sec bonding time. And 400 630gf/cm maximum adhesion strength obtained by the optimized U/S bonding was similar to the typical 620gf/cm of 200 T/C bonding with 15 sec bonding time and 190 ℃ bonding temperature. 0 0 1 2 3 4 5 6 7 Displacement (mm) 600 Peel strength (gf/cm) 8.6MPa, 180W 3sec 400 800 5sec 7secPeel strength (gf/cm) 600 9sec 200 400 0 2 sec 3 sec 4 sec 200 Bonding time (sec) Figure 11. Adhesion strengths of U/S bonded ACF joints for 2 sec, 3 0 0 1 2 3 4 5 6 7 sec and 4 sec bonding times at 4.6 MPa and 180 W condition Displacement (mm) As shown in figure 12, the degree of ACF cure measured by FTIR shows more than 90% after 3 sec at 4.6 MPa and 180W condition. 484 2007 Electronic Components and Technology Conference
  • 6. 125℃ high temperature storage test Total daisy-chain resistance (Ohm) 100 10 90 8 ACF degree of cure (%) 80 6 70 4 60 2 50 0 40 0 200 400 600 800 1000 2 3 4 Test time (hrs) U/S bonding time (sec) Figure 14. Daisy-chain contact resistance during 125℃ highFigure 12. ACF degree of cure vs. U/S bonding time at 4.6 MPa and temperature storage test 180 W condition3.4 Daisy-chain contact resistance vs. U/S bonding time Figure 14 shows daisy-chain contact resistances vs. aging Daisy-chain contact resistances of ACF joints were times during 125℃ high temperature storage test. U/S bondedmeasured after U/S bonding at the optimized conditions 4.6 test vehicles showed no significant change of daisy-chainMPa bonding pressure and 180 W U/S power. As shown in contact resistance during the 125℃ storage test. Figure 15figure 13, stable daisy-chain contact resistances were obtained and figure 16 show daisy-chain contact resistance at 85℃regardless of bonding times after 3 sec. The average daisy- /85% RH test conditions and -55℃~125℃ thermal cyclingchain contact resistance was 1.13 Ohm at 3 sec U/S bonding test conditions. No significant changes of daisy-chain contacttime. And it was similar to 1.08 Ohm which was obtained by resistance were observed during both tests.the typical 15 sec T/C bonding at 190℃. These results showthat not only similar adhesion strengths but also similar daisy-chain contact resistances as the typical T/C bonding were 85℃/85%RH testobtained using optimized U/S bonding at room temperature Total daisy-chain resistance (Ohm) 10and less than 5 seconds bonding time. 8 2.5 6 Total daisy-chain resistance (Ohm) 4.6MPa, 180W 4 2.0 2 0 1.5 0 200 400 600 800 1000 Test time (hrs) 1.0 Figure 15. Daisy-chain contact resistance during 85 /85%RH test 3 4 5 6 7 8 9 U/S bonding time (sec) As the result, U/S bonded ACF joints showed similarFigure 13. Total daisy-chain resistance with various U/S bonding adhesion strengths and daisy-chain contact resistances, and time at 4.6 MPa and 180 W condition showed stable daisy-chain contact resistance during 125℃ high temperature storage test, 85 °C/85 % RH test and -55°3.5 Reliability evaluation of U/S bonding C~125°C thermal cycling test compared with the typical T/C For reliability evaluations of U/S bonded ACF joints, bonded ACF joints.three kinds of tests were performed with 10 test boards ateach conditions. U/S bonding was performed for 3 secbonding time at room temperature with 4.6MPa bondingpressure and 180W U/S power. 485 2007 Electronic Components and Technology Conference
  • 7. -55℃/125℃ thermal cycling test reduced from typical 15 seconds to less than 5 seconds by Total daisy-chain resistance (Ohm) using the U/S bonding. 10 Therefore, conventional T/C ACF bonding processes can be replaced by the novel U/S ACF bonding process 8 demonstrated in this study. 6 5. References 1. Kiwon Lee, Hyoung Joon Kim, Myung Jin Yim, and 4 Kyung Wook Paik, “Curing and Bonding Behaviors of Anisotropic Conductive Films (ACFs) by Ultrasonic 2 Vibration for Flip Chip Interconnection”, 56th Electronic Components and Technology Conference, San Diego, 0 0 200 400 600 800 1000 California, USA, May 30 – June 2, 2006 2. M. N. Tolunay, P. R. Dawson, and K. K. Wang, “Heating Test cycle (cycles) and bonding mechanisms in ultrasonic welding of thermoplastics”, Polymer engineering and science, Vol.Figure 16. Daisy-chain contact resistance during -55°C~125°C 23, No. 13, 1983 thermal cycling test4. Conclusion In this study, a novel ACF bonding method usingultrasonic vibration was investigated, and its processconditions were optimized for flexible substrate-organic rigidboard bonding applications. The optimized U/S bonding time was 3 sec at roomtemperature at 4.6 MPa bonding pressure and 180 W U/Spower. It was demonstrated that the ACF bonding process canbe significantly enhanced by U/S bonding method comparedwith conventional 15 sec T/C bonding at 190 ℃. Using theoptimized U/S bonding conditions, the ACF joints showedsimilar bonding characteristics as T/C bonding in terms of theadhesion strength, the daisy-chain contact resistance, andstable electrical resistances during 125 ℃ high temperaturestorage test, 85 ℃/85 % RH test, and -55 ℃~125 ℃ thermalcycling test. Table 2 summarizes results of the optimized U/S ACFbonding in comparison with those of typical T/C bonding.Table 2. The optimized U/S ACF bonding condition and their results in comparison with those of typical T/C bonding. T/C ACF bonding U/S ACF bonding at 190℃ at room temperature for 15 sec for 3 sec Peel strength 622.47 (±28.59) 633.41 (±52.14) (gf/cm) Daisy-chain contact 1.08 (±0.02) 1.13 (±0.04) resistance (Ohm) These results indicate that ACF bonding temperature canbe significantly reduced from the typical 190 ℃ to roomtemperature, and bonding time can be also significantly 486 2007 Electronic Components and Technology Conference