Anisotropic Conductive Film Joints Research Articles

A Study on Transmission properties of AuNi coated polymer ball joint and Sn58Bi solder joint for flex-on-board interconnection

Flex-on-board (FOB) interconnection plays an important role in display applications. Due to the thinness of the interconnect, the contact resistance of anisotropic conductive film (ACF) joints is very small; however, the transmission property of ACF joints are still unknown. In this study, we designed a novel coplanar waveguide structure to characterize the transmission properties of ACF joints in FOB interconnections. Specifically, we compared the differences between the Au/Ni coated polymer ball joint and low melting point Sn58Bi solder ball joint in terms of their high frequency performance. Both were bonded under the same conditions of 200 °C for 10 s at 2 MPa. A high frequency test was then carried out in the range of 300 MHz to 20 GHz. The results showed that 14 GHz was the resonant frequency of FOB package that contained the Au/Ni coated polymer ball joints, while 1 GHz increased by using Sn58Bi solder joints in FOB package. To our knowledge, there is the first study to determine the high frequency performance of flex-on-board interconnections using ACFs joints and the coplanar waveguide method.

A Study on the Dynamic Bending Property of Chip-on-Flex Assembly Using Anchoring Polymer Layer Anisotropic Conductive Films

Anisotropic conductive films (ACFs) composed of polymer adhesive resin and conductive particles have been used as interconnection materials for ultrafine pitch chip-on-flex (COF) packages due to their flexibility, stable electrical properties, and bending reliability. To realize an efficient ultrafine pitch COF package, a new anchoring polymer layer (APL) was recently introduced. However, the chip flexibility in terms of ACFs materials after APL layer integration needs lot of improvement. Herein, we study the order to obtain more flexible and reliable COF packages using ACFs interconnection materials, and the effect of the ACFs material properties on the COF bending reliability was investigated. It was found that when the APL ACFs were used for the COF packages, 100% electric insulation rates were obtained due to the suppression of conductive particle movement by the APL structure compared with the conventional ACFs. Furthermore, the polyacrylonitrile (PAN) APL ACFs showed better ACFs joint properties and dynamic bending properties compared with the polyvinylidene fluoride (PVDF) APL ACFs due to its higher adhesion property caused by the chemical reaction between the PAN and the curing agent inside the epoxy polymer adhesive resin confirmed by the Fourier transform-infrared spectroscopy (FT-IR) analysis. In addition, the strain applied to the PAN APL ACFs joints during the bending test was 1.35% which was quite smaller than that of the PVDF APL ACFs joints, 2.01% resulting in better bending reliability too. For the bending reliability, there was no failure at the PAN APL ACFs joints up to 150 000 cycles, not only because of the excellent mechanical properties of PAN APL ACFs but also because of the lower strain applied to the APL ACFs joints during the dynamic bending test in COF packages.

A Study on the Bonding Conditions and Nonconductive Filler Contents on Cationic Epoxy-Based Sn–58Bi Solder ACFs Joints for Reliable Flex-on-Board Applications

Although cationic epoxy is reported as the best adhesive candidate for Sn–58Bi solder anisotropic conductive films (ACFs) reliability, ACFs bonding conditions have not been optimized yet. In this paper, ACFs bonding parameters, such as pressure and time at 200 °C, have been optimized to produce flex-on-board (FOB) applications with a higher reliability, using the cationic epoxy-based Sn–58Bi solder ACFs joints with 10- $\mu \text{m}$ average sizes Al2O3 spacers. It was difficult to achieve above 90% cure degree by less than 20 s at 200 °C, and Sn elements started to diffuse from micrometer-sized Sn–58Bi joints after 10 s. Both 80% and 90% cure degree can provide 520-kgf/cm peel adhesion strength. To obtain a larger amount of Sn phase remaining at solder joints for lower contact resistance and higher reliability and adhesion strength, 10 s at 200 °C bonding condition was selected. As large amounts of Al2O3 spacers were added into ACFs resin, the reliability of joints became lower. Direct contacts of two electrodes were obtained at 1.5 wt% Al2O3 spacers, however, no direct electrode contact was observed at 3 wt% Al2O3 spacers. As a summary, optimized bonding conditions at 200 °C for 10 s at 1-MPa pressure and solder ACFs with 3 wt% 10- $\mu \text{m}$ diameter Al2O3 spacers were suggested for nondirect contacts of electrodes interconnection and higher reliability of cationic epoxy-based Sn–58Bi solder ACFs joints for FOB applications.

Effects of Polymer Conductive Particle Contents on the Electrical Performance and Reliability of 50- $\mu \text{m}$ Pitch Flex-on-Flex Assemblies Using Anisotropic Conductive Films

Anisotropic conductive films (ACFs) composed of an adhesive polymer resin and fine conductive particles such as metal-coated polymer conductive particles are interconnect materials for fine-pitch flex-on-flex (FOF) assembly technologies due to their fine-pitch handling capability, low-temperature bonding, and stable electrical properties due to the compliance of polymer conductive particles. For the ultrafine pitch FOF assembly, the polymer conductive particle content becomes more important due to electrical storage problem between electrodes. To design better quality ACFs materials for 50-μm FOF assemblies, the effects of polymer conductive particle contents on the electrical stability and reliability of FOF ACFs joints were investigated. It was found that the bonding temperature should be less than the deformation temperature of polymer conductive particles, and proper conductive particles content should be applied to obtain the stable joint contact resistance and ACF joint morphology. When the conductive particle contents increased more than 4wt%, the gap width between the top and bottom electrodes increased and the deformation of conductive particles became less, resulting in higher contact resistance. Therefore, 4wt% of conductive particle ACFs bonded at 175 °C, 2 MPa for 10 s showed the lowest contact resistance of 36.2 mQ and excellent thermal cycle (T/C) reliability up to 1000 cycles. As a result, the optimum polymer conductive particles contents and the bonding conditions such as temperature, pressure, and times were achieved to obtain the best performance and T/C reliability of 50-μm pitch FOF assembly using ACFs.

Joint Morphologies and Failure Mechanisms of Anisotropic Conductive Films (ACFs) During a Power Handling Capability Test for Flex-On-Board Applications

In this paper, joint morphologies and failure mechanisms of anisotropic conductive films (ACFs) during a power handling capability test for flex-on-board applications were investigated in terms of four conductive particles: gold-coated polymer ball, Ni metal ball, SnBi58, and Sn–3Ag–0.5Cu particles. It was found that all the polymer ball and Ni ball joints could not handle the ACFs’ joint temperature of 200 °C presumably due to polymer resin expansion and physical contacts loss, and their corresponding contact resistance also became open circuit after the power test. In contrast, the failure mechanism of SnBi58 solder joint was that when temperature was over 138 °C and Sn element was quickly consumed by Sn-Ni intermetallic compounds resulting in brittle Bi-rich phase remaining, more severe crack occurred along the Bi-rich phase of SnBi58 joints. Sn–3Ag–0.5Cu solder ACF joints due to a higher solder melting point were the best among the four conductive particles in terms of the little crack formation in its joint morphology and the morphology and the remaining stable electrical resistance even after the highest power test to 300 °C, when FR4 printed circuit board burned. Beyond the FR-4 decomposition at 300 °C, ACF joints were burned due to epoxy resin decomposition from 330 °C.

Effect of Nanofiber Orientation on Nanofiber Solder Anisotropic Conductive Films Joint Properties and Bending Reliability of Flex-on-Flex Assembly

Anisotropic conductive films (ACFs) have been widely used as an interconnection adhesive material due to its light weight and simple and low-temperature assembly processes. However, because of the higher demands for further miniaturization, short-circuit problem of ACFs interconnection has been a major issue, when it comes to very fine pitch assembly. Also, due to the fast development of wearable devices, demands for flexible packaging as well as flexible interconnection methods such as flex-on-flex (FOF) are growing more, where the bending characteristics are important. Nanofiber incorporated ACFs were previously introduced by our research group. The nanofiber incorporated ACFs showed excellent conductive particle movement suppression capability that not only prevents short circuit but also improves the conductive ball capture rate, which eventually improves the joint reliability of fine pitch FOF assembly. In order to maximize the conductive particle movement suppression capability of the nanofiber, nanofiber was oriented using a drum-type receiver. In addition, the bending reliability of FOF assembly using nanofiber/solder ACFs with different nanofiber orientations has been also investigated. As a result, parallel nanofiber/solder ACFs showed excellent joint properties as well as bending reliability due to the stable metallurgical solder joint formation and high conductive particle movement suppression capability, and this new type of ACF technology will provide a promising solution for future flexible electronic packaging.

Optimization and reliability evaluation of COG bonding process

In order to assemble a driver IC onto a LCD glass panel, Chip on glass (COG) bonding technology using Anisotropic conductive film (ACF) has been widely implemented. The key process parameters in the technology are the temperature, pressure and time during the thermo-compression bonding. These factors mainly affect the quality of the assembled ACF joint being represented by the contact resistance and adhesion strength. This study purposes to obtain the optimal process conditions for high reliability of the ACF joint. In order to determine the optimal condition, the experiments based on the response surface methodology were designed. Based on the experiments and statistical analysis, the empirical model of second-order polynomials approximating the relationships between the process conditions and the response variables of contact resistance and adhesion strength were obtained. The optimal process condition to lead lower contact resistance and higher adhesion strength of the ACF joint was also calculated from the relationships. The optimization was verified through the bonding experiments conducted under the derived optimal process condition. In addition, the reliability of the ACF joints according to the variation of the process conditions was evaluated in high temperature and humidity environment (85°C/85%RH). Consequently, it was shown that the reliability of ACF joints assembled under the optimal process conditions was superior to others after the 85°C/85%RH tests.

A Study on the Failure Mechanism and Enhanced Reliability of Sn58Bi Solder Anisotropic Conductive Film Joints in a Pressure Cooker Test Due to Polymer Viscoelastic Properties and Hydroswelling

A study of the failure mechanism and the enhanced reliability performance of SnBi58 solder ACF joints were investigated by considering the total influence of polymer viscoelastic and hygroscopic properties on the cross-sectional morphologies and the electrical properties of Sn58Bi solder joints in a pressure cooker test (PCT) for 120 h. The Sn58Bi solder failure mechanism is demonstrated that, when polymer resin absorbed moisture, the Tg values of polymer resins decreased, and the coefficient of thermal expansion (CTE) values of polymer resins increased, due to a hydroswelling effect. Considering the total influence of polymer viscoelastic and hygroscopic properties on the cross-sectional morphologies and the reliability of Sn58Bi solder joints in a PCT for 120 h, four typical polymer resins were evaluated. As a result, cationic epoxy (128 ppm/°C) with the smallest CTE mismatch (112 ppm/°C) with SnBi58 solder joints (16 ppm/°C) in a PCT aging test (121°C, 100% humidity, and 2 atm) showed a stable electrical property in terms of joint contact resistance and remained a complete cross-sectional Sn58Bi solder joint morphology after PCT reliability for 120 h.

Bending Properties of Anisotropic Conductive Films Assembled Chip-in-Flex Packages for Wearable Electronics Applications

In this paper, a chip-in-flex (CIF) assembly that has an excellent bending performance, including a minimum bending radius without a chip fracture and the capacity to withstand dynamic bending, is developed. Chip-on-flex (COF) and CIF assemblies are fabricated using anisotropic conductive films (ACFs) as interconnection materials. The COF package is composed of 40-μm-thin silicon chips, ACF, and flexible substrates. The CIF package is fabricated by attaching a cover adhesive film and a polyimide film on the COF package to encapsulate the silicon chip. Through static bending tests, the optimal thickness of the cover adhesive film is established. The optimized CIF assembly allows a minimum bending radius of 4 mm without a chip fracture, while the chip in the COF assembly fractures at a bending radius of 10 mm. A finite-element analysis of the static bending test is performed to understand the internal stress state of the assemblies. A bending reliability test of the CIF package is also conducted at a bending radius of 7.5 mm for 160k cycles, by measuring the daisy-chain resistance during the test. The effect of the elastic modulus of the ACF resin on the fatigue endurance is investigated through the bending fatigue test. The higher modulus of the ACF resin resulted in excellent fatigue reliability with stable ACF joints showing neither delamination nor resin crazing after 160k cycles of bending.

Effects of Bonding Pressures and Bonding Temperatures on Solder Joint Morphology and Reliability of Solder ACF Bonding

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.

A Study on the Solder Ball Size and Content Effects of Solder ACFs for Flex-on-Board Assembly Applications Using Ultrasonic Bonding

In this paper, the electrical and reliability properties of various solder anisotropic conductive film (ACF) joints for flex-on-board (FOB) assembly using ultrasonic bonding were assessed in terms of short circuits, contact resistance, current handling capability, wetted solder areas, and reliability evaluation. It was found that the wetted solder areas increased and reliability was enhanced as the size and content of the solder ball increased. Regardless of solder ball size and content, the contact resistances of solder ACF joints were almost the same, because the current went through the electrode/electrode contact, which is like a short circuit to the solder joint. In addition, the exact value of current handling capability of all the ACF joints could not be evaluated, because metal lines of printed circuit board were burnt before ACF joint failure. After 120 h of pressure cooker test (PCT), the open-circuit failures were mainly observed at Ni ACFs, and 5-15-μm solder ball size from 10 to 30 wt% solder content and 25-32-μm size and 10 wt% solder content of solder ACFs, presumably due to lower wetted solder areas on metal electrodes. In addition, the crack was noted after 120 h of PCT in the solder ACF joints who has lower wetted solder areas. As a result of this paper, the solder ball size and content of solder ACFs joints were optimized at 25-32-μm sizes and above 30 wt% content for 300-μm pitch FOB assembly.

Effect of Flux Activators on the Solder Wettability of Solder Anisotropic Conductive Films

In this paper, a thermal acid generator was added as a flux activator in solder anisotropic conductive films (ACFs) to eliminate solder oxide resulting in stable solder wetting on metal pads. Since the native solder oxide causes poor wettability of the solder particles, solder oxide was chemically removed by the flux activator in solder ACFs. The test vehicles consisted of polyimide-based flexible printed circuits and FR-4 organic rigid printed circuit boards, which have 300- $\mu $ m pitch Cu pads with electroless nickel immersion gold metal surface finish. The thickness of the solder incorporated ACFs were $50~\mu $ m and it consists of epoxy-based adhesive resin, 20 wt% of 5–15- $\mu $ m diameter SAC305 (96.5Sn-3.0Ag-0.5Cu) solder particle, flux activator, and 5 wt% of 5- $\mu $ m diameter Au-coated Ni particles, which act as a spacers to maintain a stable gap between metal pads. According to the experimental results, solder wettability improved significantly, as the amount of the flux activator increased in the solder ACFs due to solder oxide removal. As a result, the flux activator caused significant improvement of electrical properties of solder ACF joints, such as contact resistance, power handling capability, and reliability. Therefore, the flux activator added solder ACFs can provide an excellent interconnection method for high-power flex-on-board and flex-on-flex assemblies.

Film-type connection system toward flexible electronics

A thin electric connector has been required for use in mobile electronics. Although an anisotropic conducting adhesive technology, e.g., the use of an anisotropic conductive film (ACF), has been developed to meet the demand, the technology is not suitable for flexible electronic devices based on plastic materials, since the plastic films used will change their mechanical properties by pressure and heat. To address these limitations, we have developed a novel film-type connector composed of a base material such as polyimide, an adhesive layer that deforms elastically against pressure, and electrodes arranged on the adhesive layer. In this study, the contact resistance of a film-type connector is investigated and compared with that of an ACF joint through four tests related to reliability. Results of the tests show that the film-type connector is more useful than the ACF joint in areas such as future flexible fine-pitch interconnections.

Ultrasonic-Assisted Thermocompression Bonding Method of Solder Anisotropic Conductive Film Joints for Reliable Camera Module Packaging

In this paper, to improve the reliability of anisotropic conductive film (ACF) joints in the camera module packaging without any mechanical damages, ultrasonic-assisted thermocompression (TC) bonding was investigated using solder ACFs. This technology uses small lateral-direction ultrasonic vibration during the TC ACF bonding to break solder oxide layers for sufficient solder wetting on the electrodes at ACF joints. Bonded solder ACF joints were investigated in terms of solder ACF joint morphology, resistance, adhesion strengths, and reliabilities. In addition, the stability of camera module components after bonding process was also observed. Ultrasonic-assisted TC bonded solder ACF joints showed excellent solder wetting compared with conventional TC bonded solder ACF joints due to broken solder oxide layers by applied ultrasonic vibration. The Au contents diffused from electroless nickel immersion gold-finished Cu pads to solders was about 12 atomic% in the ultrasonic-assisted TC bonding. The Au contents were three times higher than that of the solders in the conventional TC bonding. In the ultrasonic-assisted TC bonding, ultrasonic amplitude was optimized. At the optimized amplitude, solders were well wetted and showed no mechanical damages of camera module components. Adhesion strength of ultrasonic-assisted TC bonded solder ACF joints was almost the same as that of TC bonded ACF joints due to fully cured resins in these samples. During the unbiased autoclave test (121 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , 2 atm, 100%RH), ultrasonic-assisted TC bonded ACF joints showed no open-circuit failure compared with conventional TC bonded Ni ACF joints due to metallurgical alloy joints between solder balls and electrodes by sufficient solder wetting. These results prove that ultrasonic-assisted TC bonding using solder ACFs can be applied for highly reliable ACF assembly in the camera module packaging without any mechanical damages.

Sliding Mechanism of Lateral Thermosonic Process With Anisotropic Conductive Film for High Productivity and High Reliability

In this paper, a thermosonic flip-chip bonding process using lateral ultrasonic vibration is proposed. To enhance the reliability of the specimen after the lateral thermosonic process, a sliding mechanism is adopted with investigation of equivalent stiffness of the anisotropic conductive film (ACF) joint. By a tensile test, it is shown that the equivalent stiffness of the ACF joint gradually increases as curing proceeds. Based on these results, the sliding point where the vibration amplitude of the chip specimen begins to decrease can be adjusted by the applied pressure. Thanks to the sliding mechanism, forced excitation to the sufficiently cured chip specimen can be naturally avoided. In addition, the robustness of the degree of cure against the bonding time variation can be improved in spite of the short bonding time. To demonstrate the feasibility of the proposed sliding mechanism in practice, experiments are conducted with a commercialized driver chip assembly of a liquid crystal display with an ACF.

Effects of ACF Bonding Parameters on ACF Joint Characteristics for High-Speed Bonding Using Ultrasonic Bonding Method

As the use of anisotropic conductive films (ACFs) in flex-on-board bonding has increased recently, the need for high-speed bonding for higher productivity has developed. For high-speed bonding, a fast-curable acrylic resin has been used instead of an epoxy resin, and ultrasonic (US) bonding is introduced to allow for short bonding time at high bonding temperatures. However, it is possible for fast curable acrylic ACFs at 250°C to be fully cured before the resins flow out sufficiently between the electrodes. In such cases, the ACF joints could show poor electrical characteristics due to there being no conductive particle capture. Therefore, in this paper, in order to understand important factors for high-speed bonding, the effects of the bonding parameters, substrate geometry, and ACF material property on the ACF joint characteristics, such as resin flow and joint resistance, are investigated using acrylic-based ACFs and US bonding. The fast-curable acrylic resin is fully cured at a bonding temperature of 250°C within 1 s. However, the ACF joints bonded at 250°C show joint gaps larger than the diameter of conductive particles, i.e., 8 μm, due to insufficient resin flow and open failure due to no particle capture. As the bonding pressure increases from 2 to 4 MPa, the ACF joint gap at 250°C is smaller than the conductive particle size, and the joint resistance reaches about 20 mΩ within 7 s when using 3-mm-long electrodes and a high-viscosity resin. However, in order to use a shorter bonding time and the common bonding pressure of 3 MPa, the effects of substrate geometry and resin property on ACF joint characteristics are investigated. As the electrode length of the substrates are decreased from 3 to 1 mm, the ACF joints show stable ACF joint gaps and joint resistance at 3 MPa despite the bonding temperature of 250°C applied within 1 s. In terms of the ACF resin properties, when a resin with low minimum viscosity is used, the ACF joint gaps are smaller than the diameter of the conductive particles. Furthermore, the joint resistance of low-minimum-viscosity ACF is more stable than that of the ACF joints bonded with the high-minimum-viscosity resin at 250°C within 1 s, even though a 3-mm-long electrode is used. These results indicate that the substrate geometry of the printed circuit board and the ACF resin viscosity are significant factors for high-speed bonding to obtain stable ACF joints at a high bonding temperature of 250°C within 1 s.

Conductivities and curing properties of electron-beam-irradiated anisotropic conductive films

Radiation-curable acrylated epoxy oligomer was irradiated by using an electron beam (E-beam) with dosages of 5, 10, 20, 40, 80, 200, 400, and 550 kGy to investigate the electrical and the physical properties of anisotropic conductive films (ACFs) and to evaluate the potential application of radiation technology to flip-chip package processing. An ACF is an insulating epoxy matrix containing conducting particles that keep the electrical conductivity along the out-of-plane direction and the insulation property along the in-plane direction. The contact resistance between ACF joints cured by using an E-beam irradiation of 80 kGy was measured under a constant bonding pressure of 2 kgf/cm2 to demonstrate the effects of pad pitch size and the number of added conductive particles in the epoxy resin. Three types of PCBs, 1000-, 500-, and 100-µm pad pitches, were employed while the E-beam curable epoxy resin was mixed with conductive particles in a weight ratio of 10:1. The measured average contact resistance was 0.24 Ω with a minimum of 0.06 Ω for the samples prepared with a 100-µm pad pitch size, which is compatible with or lower than the values obtained from thermally-cured commercial ACFs. Our results demonstrate that an E-beam is an effective radiation method for curing epoxy resins at low temperatures in a short time and can be employed as a new technique for bonding circuits in high-density electric devices.

Ultrasonic Bonding of Anisotropic Conductive Films Containing Ultrafine Solder Balls for High-Power and High-Reliability Flex-On-Board Assembly

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 flex-on-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-58Bi and SAC305 (96.5Sn-3.0Ag-0.5Cu). At the same time, the acrylic ACF resin was completely cured after 5 s of US bonding. Solder alloy ACF joints show about 20% decrease in daisy-chain electrical resistance and 100% 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.

Effects of Conductive Particles on the Electrical Stability and Reliability of Anisotropic Conductive Film Chip-on-Board Interconnections

The effects of conductive particles on the electrical stability and reliability of anisotropic conductive film (ACF) joints for chip-on-board applications were investigated. In this paper, two types of conductive particles were prepared. One was a conventional Ni/Au-coated polymer ball, and the other was a Ni/Au-coated polymer ball with Ni/Au-projections. According to the results of a nano-indentation experiment of a conductive particle, the elastic recovery of a single conductive particle decreased as the applied load and the deformation of the conductive particle increased. The evaluated conductive particles which had the same polymer core showed the similar load-deformation behaviors regardless of the existence of Ni/Au-projections. The contact resistances of ACF joints using each conductive particle as a function of bonding pressure were measured, and the results showed that the contact resistances of ACF joints with the metal-projection-type conductive particles were lower and more stable than those with the conventional conductive particles. Especially at lower bonding pressure, ACF joints with the metal-projection-type conductive particles were much more stable and had lower contact resistances than the conventional ACF joints. According to the results of thermal cycling (T/C) reliability test, the metal-projection-type conductive particles also enhanced T/C reliability of ACF joints when compared with the conventional conductive particles.

Studies on various chip-on-film (COF) packages using ultra fine pitch two-metal layer flexible printed circuits (two-metal layer FPCs)

Various fine pitch chip-on-film (COF) packages assembled by (1) anisotropic conductive film (ACF), (2) nonconductive film (NCF), and (3) AuSn metallurgical bonding methods using fine pitch flexible printed circuits (FPCs) with two-metal layers were investigated in terms of electrical characteristics, flip chip joint properties, peel adhesion strength, heat dissipation capability, and reliability. Two-metal layer FPCs and display driver IC (DDI) chips with 35μm, 25μm, and 20μm pitch were prepared. All the COF packages using two-metal layer FPCs assembled by three bonding methods showed stable flip chip joint shapes, stable bump contact resistances below 5mΩ, good adhesion strength of more than 600gf/cm, and enhanced heat dissipation capability compared to a conventional COF package using one-metal layer FPCs. A high temperature/humidity test (85°C/85% RH, 1000h) and thermal cycling test (T/C test, −40°C to +125°C, 1000cycles) were conducted to verify the reliability of the various COF packages using two-metal layer FPCs. All the COF packages showed excellent high temperature/humidity and T/C reliability, however, electrically shorted joints were observed during reliability tests only at the ACF joints with 20μm pitch. Therefore, for less than 20μm pitch COF packages, NCF adhesive bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for over 25μm pitch COF applications. Furthermore, we were also able to demonstrate double-side COF using two-metal layer FPCs.