Anisotropic Conductive Film Interconnection Research Articles

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 Conductive Particle Movements in Polyvinylidene Fluoride Anchoring Polymer Layer Anisotropic Conductive Films for 20-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>m Fine-Pitch Interconnection

Fine-pitch interconnection technology in electronic packaging has become very important because of electrical interconnection issues such as high contact resistance and open-/short-circuit failure. Nanofiber anisotropic conductive films (ACFs) and nanofiber sheet ACFs (NS ACFs) have been developed and reported by our research group to address the challenges for fine-pitch interconnection problems. However, they require complicated fabrication processes such as electrospinning, thermal compression, and plasma etching. In this paper, anchoring polymer layer (APL) ACFs and their fabrication process are introduced. The goal in employing the APL ACFs was to simplify the complex fabrication steps of nanofibers and NS ACFs by coating methods without compromising electrical performance. With the APL coating method, the electrospinning, thermal compression, and plasma etching steps can be reduced to a single process. Nonetheless, APL ACFs exhibit excellent electrical conduction at 20- $\mu \text{m}$ fine-pitch interconnections by successfully suppressing the movement of conductive particles in the ACF. The capture rate of the APL ACFs was 78%, which was comparable to the value 81% of NS ACFs. APL ACFs open the possibility of cost-effective production of ACFs interconnection materials for fine-pitch applications. Therefore, we use the word “anchoring” to suppress the movement of conductive particles during resin flow. In this paper, we discuss the fabrication process of APL structure and electrical properties of APL ACFs.

The Effect of Anisotropic Conductive Films Adhesion on the Bending Reliability of Chip-in-Flex Packages for Wearable Electronics Applications

In this paper, the effects of adhesion properties of anisotropic conductive films (ACFs) interconnection on the chip-in-flex (CIF) bending reliability were investigated. Oxygen plasma treatment was conducted to increase the adhesion strength between ACFs and Si chip or ACFs and flexible printed circuit (FPC) substrates. In order to characterize the enhanced adhesion properties of the CIF packages, surface energy, surface roughness, elemental composition, and peel strength were measured. A digital image correlation method was used with cross-sectional scanning electron microscopy images to visualize the stress development at the ACFs interconnection. It was found that the interface of ACFs resin and FPC substrate showed the weakest adhesion, where the delamination was initiated. As a result of the improved adhesion at the ACFs resin and FPC substrate, the location of stress concentration was changed to the interface of Si chip and ACFs resin, leading to better dynamic bending reliability. When the oxygen plasma was treated both on the Si chip and FPC substrate, the stress concentration was observed not at the ACFs interfaces, but inside of the ACFs resin, resulting in further improved dynamic bending reliability. With the optimized plasma treatment condition and the ACFs materials, the dynamic bending reliability of the CIF packages was successfully demonstrated up to 160 000 bending cycles at a 7.5-mm bending radius without any electrical failures.

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.

Effects of moisture absorption on anisotropic conductive films interconnection for flip chip on flex applications

This paper reports the investigations into the moisture induced failures in Flip-Chip-on-Flex interconnections using Anisotropic Conductive Films (ACFs). Both experimental and modelling methods were applied. In the experiments, the joint resistance was used as a quality indicator and was measured continuously during the autoclave test. The test condition was set as 121^oC, 100%RH and 2atm. The results showed that the joint resistance of the ACF flip chip increased during the tests and nearly 25% of the joints were found open after 168h' testing. Visible conduction gaps between the adhesive and substrate pads were observed. Cracks at the adhesive/flex interface were also found. In order to have a better understanding of the experimental results, 3D Finite Element (FE) models of the ACF assembly were constructed and a macro-micro modelling technique was used to overcome the difficulty caused by the multi-length scale in the ACF assembly. The moisture diffusion and moisture-induced stresses in the ACF flip chip during the autoclave test were predicted. Modelling results are consistent with the findings in the experimental work.

Optimization of Heating Temperature History for Anisotropic Conductive Film Interconnection

Anisotropic conductive film (ACF) interconnection is used for mounting electronic components, because this method can decrease the mounting area and electric connection length, as well as the thermal stress in the connecting area. An ACF comprises thermosetting resin and conductive particles. The resin is heated and its curing rate and viscosity changes complexly with the heating temperature during the process. There are several requirements for the heating temperature history from the industrial viewpoint such as the reliability of adhesion and energy efficiency. These requirements are related to the curing rate and the viscosity of the resin. A global optimization method proposed for nonlinear programming problems is adopted to optimize the values of the curing reaction parameters and the temperature history. First, the values of parameters in the functions determining the curing rate and viscosity are identified, and the curing rate and viscosity calculated using the values of the parameters agree well with the experimental data. Then, several optimization examples clarify features of the optimum heating temperature history. It is possible to increase the final curing rate to ensure adhesion and to control the viscosity in the bubble-removing process. The period in which bubbles are removed can be changed by the setting of the optimization parameters. It is also possible to minimize the heat input and ensure the required final curing rate. These results clarify that the temperature history for ACF interconnection can be determined accurately by the presented global optimization approach.

Enhancement of electrical stability of anisotropic conductive film (ACF) interconnections with viscosity-controlled and high T g ACFs in fine-pitch chip-on-glass applications

This paper describes how anisotropic conductive film (ACF) properties including viscosity affect the electrical stability of ACF interconnections for fine pitch chip-on-glass (COG) applications. In this study, new ACFs for COG applications were designed by combining a high viscosity ACF layer and a low viscosity NCF layer to prevent the electrical shortage between bumps. As expected, the viscosity-controlled ACF showed better electrical insulation stability than a conventional ACF in fine pitch COG assemblies. According to the results of thermo-mechanical analysis (TMA) and dynamic-mechanical analysis (DMA), the viscosity-controlled ACF showed the improved thermo-mechanical properties such as lower coefficient of thermal expansion (CTE), higher storage modulus ( E′) at higher temperature region, and higher glass transition temperature ( T g ) than the conventional ACF. Furthermore, hot air reliability test and pressure cooker test (PCT) results showed that the viscosity-controlled ACF with higher T g had better hot air test and PCT reliabilities than the conventional ACF.

Effect of Bonding Conditions on Conduction Behavior of Anisotropic Conductive Film Interconnection

This paper presents an investigation on the conduction behaviors of the anisotropic conductive film (ACF) interconnections bonded at various bonding forces. The connection resistance of the ACF joints decreased with increasing bonding force up to 70 N, but subsequently converged to a value of 10 mΩ at bonding forces above 70N. This convergence of the connection resistance in the ACF joints was due to two opposing factors: decreased resistance due to increased contact area and increased resistance due to decreased currentflow path that may have been affected by the delamination of the metal film from the polymeric cores at high bonding forces. During thermal shock testing of the adhesive joints, two different conduction behaviors were observed: increasing connection resistance and the termination of Ohmic behavior. The former was due to decreased contact areas caused by warpage of the package, whereas the latter was caused by delamination at the interface.

Electrical characterization of adhesive flip chip interconnects for microwave application

Microwave performance of the anisotropic conductive film (ACF) and nonconductive film (NCF) interconnects was investigated by measuring the scattering parameters (S-parameters) of the flip chip modules employing the films. To compare the accurate intrinsic microwave performance of the ACF and NCF interconnects without lossy effect of chip and substrate, a de-embedding technique was employed. The effects of two chip materials, Si and quartz (SiO2), and of the metal pattern gap between the signal line and ground plane in the coplanar waveguide (CPW) on the microwave performance of the flip chip module were also investigated. The transmission properties of the quartz were markedly improved over those of the Si chip, which was not suitable for the measurement of the S-parameters of the flip chip interconnect. Extracted impedance parameters showed that the microwave performance of the flip chip interconnect with NCF was slightly better than that of the interconnect with ACF, mainly due to the inductive effect of the conductive particle surface and capacitance of the epoxy matrix in the ACF interconnect.

Effect of Conductive Particle Properties on the Reliability of Anisotropic Conductive Film for Chip-on-Glass Applications

This paper describes how the material properties of conductive particles in anisotropic conductive films (ACFs) affect the electrical conductivity and the reliability of ACF interconnections for chip-on-glass (COG) applications. For the conductive particles, Au/Ni-coated polymer particles with a 5-diameter were used. Two different types of conductive particles were characterized with respect to their mechanical and electrical properties, such as ball hardness, recovery behavior, and electrical resistance. In addition, two ACFs were fabricated in the form of a double-layered structure, in which the thickness of the ACF and a nonconductive film (NCF) layer were optimized to have as many conductive particles as possible on the bump after COG bonding. The electrical contact resistance of an ACF interconnection in a COG structure depends mainly on the electrical properties of conductive particles in the ACF. The electrical reliability of an ACF interconnection in a COG structure also depends more on the electrical properties than the mechanical properties of conductive particles under a high-temperature and humid condition. Conductive particles with a lower electrical resistance, higher mechanical hardness, and lower recovery rate show better reliability than conductive particles with a higher electrical resistance, lower mechanical hardness, and higher recovery rate. Cross-sectional scanning electron microscopic (SEM) pictures of a COG interconnection show the deformation of two different conductive particles after the reliability tests. The ACF interconnections in the edge or corner of a driver IC show less reliable joints due to high absorption of moisture.

Effects of bonding pressure on the thermo-mechanical reliability of ACF interconnection

Effects of reflow process on the thermo-mechanical reliability of the anisotropic conductive film (ACF) joints were investigated. Cross-sectional studies were carried out to investigate the effects using a scanning electron microscope (SEM). Swelling of ACF matrix in z-direction and formation of a conduction gap were observed in the reflowed ACF joints which were bonded in lower flip chip bonding pressures than 60 N. However, a delamination as well as the formation of a conduction gap was occurred between the ACF and Cu pad on flexible printed circuits (FPC) when ACF joints bonded in higher bonding pressures than 70 N were reflowed. This difference of failure mode was mainly due to the divergence of adhesive matrix thickness between the Au bump on Si chip and Cu pad on FPC. Finite element analyses were also conducted to interpret the failure behaviors of the ACF joints.

Flip chip interconnection with anisotropic conductive adhesives for RF and high-frequency applications

Anisotropic conductive adhesives (ACAs) are as promising interconnect materials for flip chip assembly in low cost, high-density, high-speed interconnection packages. We evaluated and compared several flip chip interconnects that use ACAs at the radio frequency (RF) and high-frequency range. The performance of high-speed circuits is limited by the package interconnect discontinuity which is due to large inductance and resistance in the high-frequency range. This discontinuity is determined by the interconnection geometry and materials used. For bumps on the integrated circuit (IC), we used Au studs, Au electroplated and electroless Ni/Au bumps, for the interconnection adhesives, we used two kinds of anisotropic conductive film (ACF) with a different dielectric constant. To evaluate the high frequency model parameters, which we based on an ACF flip chip model and network analysis, we took the high-frequency measurements of test flip chip vehicles that used different bonding materials. Furthermore, to demonstrate real applications for an ACF interconnection at the RF and high-frequency range, we applied ACF flip chip technologies to assemble a passive device that uses an RF integrated passive device. We also applied these technologies to an active device that uses a highly integrated monolithic microwave (IC) device on an RF module. Moreover, we compared the high-frequency characteristics of these devices with those of flip chip assemblies fabricated with conventional methods such as solder ball interconnection.

Effect of filler content on the dielectric properties of anisotropic conductive adhesives materials for high-frequency flip–chip interconnection

The dielectric property of anisotropic conductive film (ACF) as an interconnect materials in the flip–chip joints is becoming important concern for device packaging solution at high-frequency due to low parasitic effect on the signal transfer. The effects of non-conductive, dielectric filler content on dielectric properties of ACA materials, like dielectric constant, loss factor and loss tangent, and conductivity at high-frequency were investigated. Frequency is dominating factor in determining dielectric constant, loss factor, and conductivity. However, the filler content is dominant only on dielectric constant, not on the loss factor, and conductivity at low-frequency range. The effect of low dielectric constant (low- k) filler addition on high-frequency behavior of ACF interconnection in flip–chip assembly was also investigated. Impedance parameters of low- k ACF with Ni filler and low- k SiO 2 filler extracted from measurement were compared with that of conventional ACF with only Ni filler. The resonant frequency of conventional ACF flip–chip interconnect was 13 GHz, while the resonant frequency of low- k ACF including low- k SiO 2 filler was found at 15 GHz. This difference is originated from capacitance decrease of polymer matrix between bump and substrate pad due to change in dielectric constant of polymer matrix, which was verified by measurement-based modeling. The high-frequency property of the conductive adhesive flip–chip joint, such as resonant frequency can be enhanced by low- k polymer matrix.

Reliability of anisotropic conductive joints for bumpless flip-chip on flex packages under thermal stresses

The reliability of ACF (Anisotropic conductive film) interconnection is a serious concern especially under thermal loading condition. This paper focuses on the online contact resistance behavior of the ACF joint for bumpless flip-chip on flex packages during different thermal cycling conditions. In this work, flip chips of 11×3 mm2 having bare aluminum pad were used. Real time contact resistance (i.e. live measurement contact resistance variation with temperature) was measured by four points probe method when the packages were inside thermal shock chamber. Tests for three different thermal cycling profiles (125°C to −55°C, 140°C to −40°C and 150°C to −65°C) were carried out. The samples bonded at temperature 180°C and pressure of 2.42Mpa was used. The initial contact resistance of the bumpless samples was 0.4Ω. Contact resistance increased with the number of thermal cycles, however the effect was severe when the temperature variation was above the glass transition temperature (Tg) of the ACF matrix (131°C). Differences in co-efficient of thermal expansion (CTE) between the chip and the substrate generated thermal stresses during temperature fluctuation, which caused the pad of the substrate to slide over the Al pad of the chip. Thus variation of the contact resistance was also observed along the interconnection position in the package, i.e. corner joint showed higher value of increase in contact resistance than the middle position. Even though flex substrate was used in this study; the sliding effect was severe at the corner Al pads of the chip, where cumulative forces generated due to the thermal stress. Results show that for thermal cycling profile 140°C to −40°C, online contact resistance increased to 1.2 Ω in corner joint, whereas for the middle joints the contact resistance just increased to 0.5 Ω. Glass transition temperature (Tg) of the ACF material plays an important role on the high temperature contact resistance. For every thermal cycling profile, there is an incubation period that would have significant impact in the application of ACF. After the incubation period the contact resistance increases rapidly and the joints are no longer reliable.

Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection

The effects of different bonding parameters, such as temperature, pressure, curing time, bonding temperature ramp and post-processing, on the electrical performance and the adhesive strengths of anisotropic conductive film (ACF) interconnection are investigated. The test results show that the contact resistances change slightly, but the adhesive strengths increase with the bonding temperature increased. The curing time has great influence on the adhesive strength of ACF joints. The contact resistance and adhesive strength both are improved with the bonding pressure increased, but the adhesive strengths decrease if the bonding pressure is over 0.25 MPa. The optimum temperature, pressure, and curing time ranges for ACF bonding are concluded to be at 180–200 °C, 0.15–0.2 MPa, and 18–25 s, respectively. The effects of different Teflon thickness and post-processing on the contact resistance and adhesive strength of anisotropic conductive film (ACF) joints are studied. It is shown that the contact resistance and the adhesive strength both become deteriorated with the Teflon thickness increased. The tests of different post-processing conditions show that the specimens kept in 120 °C chamber for 30 min present the best performance of the ACF joints. The thermal cycling (−40 to 125 °C) and the high temperature/humidity (85 °C, 85% RH) aging test are conducted to evaluate the reliability of the specimens with different bonding parameters. It is shown that the high temperature/humidity is the worst condition to the ACF interconnection.

The viability of anisotropic conductive film as a flip chip interconnect technology for MEMS devices

The use of anisotropic conductive film (ACF) and stud bumping to form interconnects between die and substrates is one variation of current flip chip technologies with potential applications to MEMS devices. The key concerns associated with ACF are its long-term mechanical stability and consistent electrical performance. During this study a process methodology was developed for using ACF interconnections on MEMS devices by investigating key electrical parameters, and a practical example was investigated by packaging a pressure sensor using ACF flip chip techniques. Results of process development and contact resistance measurements using glass substrates and the anisotropic conductive film Sony FP1526 are discussed and analyzed. All required processing steps, such as stud bumping, application of ACF and thermocompression bonding of the die and substrate, were carried out using a digital wire bonder and a bench-top flip chip system. FP1526 yielded an average contact resistance of 10.23 mΩ, a stray capacitance measurement of 10 femtofarad and less than 0.1% change in contact resistance after being encapsulated in parylene C and being immersed in DI water for 72 h. Finally, the performance of an ACF-packaged MEMS piezoresistive pressure sensor was compared to that of a conventional wire-bonded device and showed a significant improvement in temperature stability (<0.34% deviation in offset voltage) with essentially no change in sensitivity.

Effects of Solder Reflow on the Reliability of Flip-Chip on Flex Interconnections Using Anisotropic Conductive Adhesives

This work describes the work of an investigation of the effects of solder reflow process on the reliability of anisotropic conductive film (ACF) interconnection for flip-chip on flex (FCOF) applications. Experiments as well as computer modeling methods have been used. The results show that the contact resistance of ACF interconnections increases after the reflow and the magnitude of the increase is strongly correlated to the peak reflow temperature. In fact, nearly 40 percent of the joints are open when the peak reflow temperature is 260/spl deg/C, while there is no opening when the peak temperature is 210/spl deg/C. It is believed that the coefficient of thermal expansion (CTE) mismatch between the polymer particle and the adhesive matrix is the main cause of this contact degradation. To understand this phenomenon better, a three-dimensional (3-D) finite element (FE) model of an ACF joint has been analyzed in order to predict the stress distribution in the conductive particles, adhesive matrix and metal pads during the reflow process. The stress level at the interface between the particle and its surrounding materials is significant and it is the highest at the interface between the particle and the adhesive matrix.

ACFを用いた電子デバイス実装における導電粒子の弾性回復と応力分布(J01-2 微細接続信頼性,J01 エレクトロニクス実装における熱制御および信頼性評価)

An anisotropic conductive film (ACF) is used for the flip chip assembly as one type of underfill. Our analysis is focused on elastic recovery arid stress distribution along the interfaces in particle-pads and underfillpads due to a thermal loading. The process for ACF interconnection is simulated by FEM. The viscoelastic properties are used in underfill. To analyze the failures by thermal shock during cool down process, three patterns of cool down rate are used. The results of stress distribution in each patterns will be presented.

Failure mechanism study of anisotropic conductive film (ACF) packages

Anisotropic conductive film (ACF) consists of an adhesive polymer matrix with dispersed conductive particles. In flip-chip technology, ACF has been used in place of solder and underfill for chip attachment to glass or organic substrates. The filler particles establish the electrical contacts between the interconnecting areas. ACF flip-chip bonding provides finer pitch, higher package density, reduced package size and improved lead-free compatibility. Nevertheless, the interconnection is different from traditional solder joints, the integrity and durability of the ACF interconnects have major concerns. Failures in anisotropic conductive film (ACF) parts have been reported after temperature cycling, moisture preconditioning and autoclave. The failures have not been well understood and have been attributed to a wide variety of causes. This paper investigates the failure mechanism of ACF using finite element simulation. From a failure-initiation point of view, the response of ACF packages to environmental (temperature and humidity) exposure is very different from standard underfilled packages. These differences cause the ACF package to fail in different ways from an underfilled package. Simulation results have shown that moisture-induced ACF swelling and delamination is the major cause of ACF failure. With moisture absorption, the loading condition at the interface is tensile-dominant, which corresponds to lower interface toughness (or fracture resistance). This condition is more prone to interface delamination. Therefore, the reliability of ACF packages is highly dependent on the ACF materials. The paper suggests a new approach toward material selection for reliable ACF packages. This approach has very good correlation with experimental results and reliability testing of various ACF materials.

The effect of reflow process on the contact resistance and reliability of anisotropic conductive film interconnection for flip chip on flex applications

The work presented in this paper focuses on the effect of reflow process on the contact resistance and reliability of anisotropic conductive film (ACF) interconnection. The contact resistance of ACF interconnection increases after reflow process due to the decrease in contact area of the conducting particles between the mating I/O pads. However, the relationship between the contact resistance and bonding parameters of the ACF interconnection with reflow treatment follows the similar trend to that of the as-bonded (i.e. without reflow) ACF interconnection. The contact resistance increases as the peak temperature of reflow profile increases. Nearly 40% of the joints were found to be open after reflow with 260 °C peak temperature. During the reflow process, the entrapped (between the chip and substrate) adhesive matrix tries to expand much more than the tiny conductive particles because of the higher coefficient of thermal expansion, the induced thermal stress will try to lift the bump from the pad and decrease the contact area of the conductive path and eventually, leading to a complete loss of electrical contact. In addition, the environmental effect on contact resistance such as high temperature/humidity aging test was also investigated. Compared with the ACF interconnections with Ni/Au bump, higher thermal stress in the Z-direction is accumulated in the ACF interconnections with Au bump during the reflow process owing to the higher bump height, thus greater loss of contact area between the particles and I/O pads leads to an increase of contact resistance and poorer reliability after reflow.