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We developed a low-cost process for assembling versatile sensors without expensive, thick metal finish on rigid and flexible PCB using anisotropic conductive films (ACF) flip-chip (FC) process. This allows a lower temperature budget than conventional FC assembly. The ACF FC process requires no expensive set up, is quick to implement and suits perfectly for sensor prototyping and low-scale manufacturing. The process was directly applied to assemble the bare die of a CMOS strain gauge sensor on flexible PCB without compromising its integrity.

Technology development of blue, green and red light micro LED display (uD) was developed in this study. Here, 64 x 32 pixels on the display, with size of 50 μm x 50 μm and pixel pitch of 100 μm were demonstrated. ITO as transparent conducting layer on the P-epilayer is used to be contact layer and contributes current spreading. Flip chip LED process as basic of process design is apply to this study, light is emitted from the back side of LED that it can prevent light emitting that is shaded from metal. Ti/Al/Ti/Au as electrode was deposited on the anode and cathode. Display was boned to IC by anisotropic conductive film, the process of display was finished eventually. Single pixel on matrix display can be driven by mix multi-electrodes addressable controlling circuit with passive IC, the image is demonstrated on the display successfully. The 430, 590 and 600 nit for blue, green and red uDs were obtained as the uDs were driven by PWM with voltage source at 5 V.

Integration technologies involving flexible substrates are receiving significant attention owing the appearance of new products regarding wearable and Internet of Things technologies. There has been a continuous demand from the industry for a reliable bonding method applicable to a low-temperature process and flexible substrates. Up to now, however, an anisotropic conductive film (ACF) has been predominantly used in applications involving flexible substrates; we therefore suggest low-temperature lead-free soldering and bonding processes as a possible alternative for flex-on-flex applications. Test vehicles were designed on polyimide flexible substrates (FPCBs) to measure the contact resistances. Solder bumping was carried out using a solder-on-pad process with Solder Bump Maker based on Sn58Bi for low-temperature applications. In addition, thermocompression bonding of FPCBs was successfully demonstrated within the temperature of 150 °C using a newly developed fluxing underfill material with fluxing and curing capabilities at low temperature. The same FPCBs were bonded using commercially available ACFs in order to compare the joint properties with those of a joint formed using solder and an underfill. Both of the interconnections formed with Sn58Bi and ACF were examined through a contact resistance measurement, an 85 °C and 85% reliability test, and an SEM cross-sectional analysis.

Demonstrated are visible GaN-based light-emitting diodes (LEDs) on economical large-area Si substrates using an advanced device and packaging architecture to improve optical output power, while reducing manufacturing costs. The process employs thin-film-flip-chip devices and wafer-level chip-scale packages and uses through-Si-via substrate and anisotropic conductive film for bonding. The improved curvature control region is applied in the epitaxial growth of the LED structure on a Si substrate to achieve flat wafers for epitaxial structures at room temperature, which is critical for wafer-level bonding. External quantum efficiency and light-output power at 350 mA increase by $\sim 12$ % compared with those of conventional flip-chip LEDs grown on a sapphire substrate. The devices also show a reverse-bias leakage current failure rate of

Advanced microelectronic packaging, driven by the multiple benefits of system performance, power, size and cost, has moved into a three-dimensional (3D) era. One of the bottlenecks to 3D IC integration is the high-density interconnections to be formed between stacked dies. Prevailing solder-based interconnect encounters constraints including high process temperature, increasing process cost and intermetallic compound (IMC) reliability issues related to the ever-decreasing pad/pitch size. Anisotropic conductive film (ACF) and anisotropic conductive adhesive (ACA) technology are two of the fine-pitch, low-temperature flip-chip material bonding options, which have been considered as potential replacements for solder interconnections. However, conventional ACF or Anisotropic conductive paste (ACP) with spherical conductive particles have limitations in terms of unpredictable numbers of trapped particles in the interconnections and the occurrence of particle agglomeration in the small gaps between interconnections. To tackle these limitations, a nanowire ACF (NW-ACF) with vertically configured, high aspect ratio metallic nanowires has been fabricated utilizing a nanoporous template and an electrodeposition method to create a novel nano-ACF material. The NW-ACF has been structurally and electrically characterized to demonstrate its feasibility as a fine-pitch and low-temperature interconnection solution for future 3D die-/wafer-level interconnection applications.

Thermosonic bonding (TSB) and thermocompression bonding (TCB) are common interconnections technique in flip chip (FC). Both techniques are used in chip to chip interconnection and both have been studied in this paper. Coining and anisotropic conductive film are used in TCB assembly process. Whereas, TSB process flow are solder free and simpler compared to TCB. It was found that TSB has lower 5.5 % electrical resistance compared to TCB. In term of physical analysis, TCB sample has an open joint after temperature cycling (TC) 500 whereas TSB still in a good joint. Therefore, TSB method is suitable for system in package.

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.

Advanced microelectronics packaging, driven by the multiple benefits of system performance, power, size, and cost, has now entered the three-dimensional (3-D) era. According to the International Technology Roadmap for Semiconductors 2012 [1], the interconnect pitch size is predicted to be 4?16 ?m at the global interconnect level by the year 2018. Silicon die incorporated with through-silicon-via (TSV) technology [2] can be stacked using solder microbumps as high-density interconnects [3]. However, solder microbump technology faces many challenges because of the intermetallic compound growth and an underfill requirement as the bump size reduces [4]. Meanwhile, the temperature of the soldering process is more than 260 ?C, which can result in high thermal stress in the devices and impact the thermal budget of the processing, particularly for the multitechnology node-stacking processes [5]. Therefore, developing interconnect methods, which can provide ultrafine-pitch capability and low-temperature process for 3-D systems, attracts continuous attention from industry [6].

In this paper, inclined conductive bumps (ICBs) are proposed as a substitute for the anisotropic conductive film (ACF). The new interconnection method ICBs can provide controlled bump deformation, uniform electrical conductivity, and fine pitch interconnection without short circuit. The ACF is widely used in the chip-substrate bonding process in today's display and semiconductor industries. This is due to the fact that the ACF has various advantages, such as low bonding temperature, low cost, and small packaging sizes, when compared with those of wire bonding. However, as the bump pitch decreases, the short-circuit problem can occur by lumped conductive particles of the ACF between adjacent bumps. In addition, the electrical conductivity of package varies due to the number of the trapped ACF particles between bonded bumps. Various alternatives, such as coating a thin insulating layer at the outside of the particle and utilizing an array of metal pillars instead of the random distributed particles have been attempted to overcome these shortcomings of the conventional ACF. As another alternative, the ICBs, which are the inclined and hollow copper bumps directly fabricated on electrodes, are suggested. In addition, in this paper is the finite element analysis, which has been conducted to predict the elastic-plastic deformations of the ICBs and their reliability issues. Then, the fabrication processes of the ICB is explained. In such process, the ICBs are fabricated on a test wafer with the inclined angles of 70° and 80°. The ICBs are selectively formed on a target pad at a pitch of 30 μm. A singulated chip with ICBs is assembled on an organic substrate using thermo-compressive bonding. After bonding, the ICBs show reasonable contact resistances of 12-27 mΩ.

Demonstrated is advanced device and packaging architecture of visible GaN-based light-emitting diodes (LEDs) combining thin-film flip-chip devices and wafer-level chip-scale package with through-silicon-via (TSV) and wafer-to-wafer alignment bonding. In addition, a new interconnect technique for LEDs is introduced using an anisotropic conductive film with metal balls. Thermal rollover in light output versus current characteristics is not observed up to 700 mA. A forward voltage at 350 mA is 3.06 V. The architecture can facilitate excellent heat removal through a TSV-formed Si wafer in addition to expected benefits of easy integration of Si-based devices in lighting modules. Light-output power at 350 mA increases by 11.1% compared with that of conventional flip-chip LEDs. A Lambertian-like emission pattern is also achieved.

Recently, the flip-chip bonding technology using adhesives has been widely used in the packaging industry because of environmental friendliness (elimination of lead material and flux cleaning), low temperature process (no soldering process), fewer processing steps (no underfill process), and the fine pitch capability. In flip-chip assembly using adhesives, the electrical interconnection is established by mechanical contact between the bumps on the chip and the corresponding pads on the substrate after the adhesive is cured. The adhesive can be categorized into two types with respect to the presence of the conductive particles: anisotropic conductive film (ACF) and nonconductive adhesive (NCA). ACF is the adhesive polymer film with dispersed conductive particles, and NCA is just adhesive which contains no conductive particles. In this review, the bonding technologies with ACF and NCA are introduced, and the principle and characteristics of each bonding method are discussed.

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.

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 35 μm, 25 μm, and 20 μm 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 5 mΩ, good adhesion strength of more than 600 gf/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, 1000 h) and thermal cycling test (T/C test, −40 °C to + 125 °C, 1000 cycles) 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 20 μm pitch. Therefore, for less than 20 μm 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 25 μm pitch COF applications. Furthermore, we were also able to demonstrate double-side COF using two-metal layer FPCs.

In this work, thermal cycling (T/C) reliability of anisotropic conductive film (ACF) flip chip assemblies having various chip and substrate thicknesses for thin chip-on-board (COB) packages were investigated. In order to analyze T/C reliability, shear strains of six flip chip assemblies were calculated using Suhir’s model. In addition, correlation of shear strain with die warpage was attempted. The thicknesses of the chips used were 180 μm and 480 μm. The thicknesses of the substrates were 120, 550, and 980 μm. Thus, six combinations of flip chip assemblies were prepared for the T/C reliability test. During the T/C reliability test, the 180 μm thick chip assemblies showed more stable contact resistance changes than the 480 μm thick chip assemblies did for all three substrates. The 550 μm thick substrate assemblies, which had the lowest CTE among three substrates, showed the best T/C reliability performance for a given chip thickness. In order to investigate what the T/C reliability performance results from, die warpages of six assemblies were measured using Twyman–Green interferometry. In addition, shear strains of the flip chip assemblies were calculated using measured material properties of ACF and substrates through Suhir’s 2-D model. T/C reliability of the flip chip assemblies was independent of die warpages; it was, however, in proportion to calculated shear strain. The result was closely related with material properties of the substrates. The T/C reliability of the ACF flip chip assemblies was concluded to be dominatingly dependent on the induced shear strains of ACF layers.

Polymer-based anisotropic conductive film has become widely used in many electronic packaging interconnect applications. The ultrasonic flip chip bond technology for anisotropic conductive film (ACF) joints in Chip-on-Glass (COG) assemblies has been investigated in part 1 of this paper [1]. In Part 2, the ultrasonic flip chip bond process for polymer-based anisotropic conductive film joints in chip-on-FR4 organic rigid board assemblies is discussed. The effects of ultrasonic bond process parameters, including the bonding time, bonding force, substrate temperature and ultrasonic power on the degree of cure and bond strength of ACF joints between chip and FR4 organic rigid board are investigated in detail. The optimized ultrasonic bonding process conditions for the Chip-on-FR4 organic rigid board assemblies are obtained as: (1) the bonding time can be from 2500 ms to 3000 ms; (2) the bonding force should be over 10 N; (3) the substrate temperature should be higher than 70 °C; (4) the optimum ultrasonic power is about 2.80 W.

Polymer-based anisotropic conductive film has become widely used in many electronic packaging interconnect applications. In this study, thermosonic flip chip bonding technology for anisotropic conductive film (ACF) joints in Chip-on-Glass (COG) assemblies is investigated. The effects of ultrasonic bonding process parameters on the degree of cure and bond strength of polymer-based anisotropic conductive film joints are discussed. Results show that (1) the bond strength and degree of cure both increase with increase of the bonding time and the optimum bonding time is 3 s; (2) the effects of the bonding force on the bond strength are not significant; (3) the bond strength of the ACF joints firstly increases and then rapidly decreases when the ultrasonic power is continuously increased; (4) the degree of cure of ACF material increases with increase of the ultrasonic power. When the ultrasonic power is 3.52 W, the degree of cure of ACF material can reach 94%. Hence, the optimum value of ultrasonic power is about 3.5 W for the studied COG assembly.

Polymer-based anisotropic conductive film has become widely used in many electronic packaging interconnect applications. In this study, thermosonic flip chip bonding technology for anisotropic conductive film (ACF) joints of chip-on-glass (COG) assembly is investigated. The effects of ultrasonic power on the curing degree and bonding strength of anisotropic conductive film joints are discussed. The results show that (1) The bonding strength of the ACF joints firstly increases and then fast decreases when the ultrasonic power is continuously increased; (2) The curing degree of ACF material increases with the increase of the ultrasonic power. When the ultrasonic power is 3.52W, the curing degree of ACF material can reach 94.1%; (3) The optimized value of ultrasonic power is 3.5W for the studied assembly.

Flip chip assembly on FR4 board using anisotropic conductive films (ACFs) is gained more attention because of its many advantages. But to obtain more reliable flip chip assembly, it is necessary to control the parameters involved in the process. In this work, the effect of bonding force of ACF, as well as the kind of ACF and the parameters evolved in the reflow process on the ACF flip chip joints were investigate. It was observed a formation of the high conduction gap for ACF joints bonded with lower bonding forces after reflow. This high conduction gap can result in electrical disconnection in the worst case.

A primary factor of anisotropic conductive film (ACF) package failure is delamination between the chip and the adhesive at the edge of the chip. This delamination is mainly affected by the thermal shear strain at the edge of the chip. This shear strain was measured on various electronic ACF package specimens by micro-Moire interferometry with a phase shifting method. In order to find the effect of moisture, the reliability performance of an adhesive flip-chip in the moisture environment was investigated. The failure modes were found to be interfacial delamination and bump/pad opening which may eventually lead to total loss of electrical contact. Different geometric size specimens in terms of interconnections were discussed in the context of the significance of mismatch in coefficient of moisture expansion (CME) between the adhesive and other components in the package, which induces hygroscopic swelling stress. The effect of moisture diffusion in the package and the CME mismatch were also evaluated by using the Moire interferometry. From Moire measurement results, we could also obtain the stress intensity factor K. Through an analysis of deformations induced by thermal and moisture environments, a damage model for an adhesive flip-chip package is proposed.

In this paper, a novel anisotropic conductive film (ACF) flip chip bonding method using ultrasonic vibration for flip chip interconnection is demonstrated. The curing and bonding behaviors of ACFs by ultrasonic vibration were investigated using a 40-kHz ultrasonic bonder with longitudinal vibration. In situ temperature of the ACF layer during ultrasonic (U/S) bonding was measured to investigate the effects of substrate materials and substrate temperature. Curing of the ACFs by ultrasonic vibration was investigated by dynamic scanning calorimetry (DSC) analysis in comparison with isothermal curing. Die adhesion strength of U/S-bonded specimens was compared with that of thermo-compression (T/C) bonded specimens. The temperature of the ACF layer during U/S bonding was significantly affected by the type of substrate materials rather than by the substrate heating temperature. With room the temperature U/S bonding process, the temperature of the ACF layer increased up to 300degC within 2 s on FR-4 substrates and 250degC within 4 s on glass substrates. ACFs were fully cured within 3 s by ultrasonic vibration, because the ACF temperature exceeded 300degC within 3 s. Die adhesion strengths of U/S-bonded specimens were as high as those of T/C bonded specimens both on FR-4 and glass substrates. In summary, U/S bonding of ACF significantly reduces the ACF bonding times to several seconds, and also makes bonding possible at room temperature compared with T/C bonding which requires tens of seconds for bonding time and a bonding temperature of more than 180degC.

In this paper, the effects of heating rate during anisotropic conductive film (ACF) curing processes on ACF material properties such as thermomechanical and rheological properties were investigated. It was found that as the heating rate increased, the coefficient of thermal expansion (CTE) of the ACF increased, and the storage modulus and glass transition temperature (T g) of the ACF decreased. Variation of the ACF material properties are attributed to cross-linking density, which is thought to be related with the ACF density. In addition, as the heating rate increased, the minimum viscosity of the ACF decreased and the curing onset temperature increased during the curing process. The similar phenomenon was also found in in-situ contact resistance measurement. As the heating rate increased, contact resistance establishing temperature increased and the contact resistances of the ACF flip chip assemblies decreased. The decrease in contact resistance was due to larger conductive particle deformation which leads to larger electrical contact area. The effect of the heating rate of ACFs on thermal cycling (T/C) reliability of flip chip assemblies was also investigated. As the heating rate increased, the contact resistances of the ACF flip chip assembly rapidly increased during the T/C test. The T/C reliability test result was analyzed by two terms of shear strain and conductive particle deformation. Reduced gap of joints due to reduced ACF viscosity resulted in larger shear strain. Moreover, many cracks were observed at metal-coated layers of conductive particles due to larger deformation.

Extensive understanding and management of the thermal-mechanical characteristics of novel packaging designs during the bonding process are indispensable to the realization of the technologies. Thus, this paper attempts to explore the bonding process-induced thermal-mechanical behaviors of an advanced flip chip (FC) electronic packaging. FC packaging employs a novel anisotropic conductive film, which is a thin composite film composed of polymer matrix and thousands of millions of highly oriented, 1-D silver (Ag) nanowires on the scale of 200 nanometers in diameter. For carrying out the process simulation, a process-dependent finite element (FE) simulation methodology that integrates both thermal and nonlinear contact FE analyses and a special meshing scheme is applied. The material properties of the nanoscale Ag wires are first explored using molecular dynamics (MD) simulations. By the characterized material properties of the Ag nanowires, the effective material properties of the composite film are derived through two theoretical approaches: 1) the rule-of-mixture (ROM) technique and 2) the proposed FE method-based approach. The predicted results by these two approaches are extensively compared with each other to examine the feasibility of using the widely used ROM technique for such cases. In addition, the validity of the proposed process-dependent FE simulation methodology is also confirmed through three experiments: 1) micro-thermocouple measurement of temperature; 2) Twyman-Green Moire interferometry measurement of out-of-plane deformations; and 3) portable engineering Moire interferometry measurement of in-plane deformations. Throughout the investigation, the effectiveness of the novel interconnect technology is demonstrated. Good agreement with the experiments is also obtained. It is found that the technology may ensure good electrical performance and structural integrity, not only at room temperature but even at elevated temperature, based on its substantial contact stresses but minor peeling stresses on the bonding line, together with a moderate, process-induced warpage on the substrate.

In this paper, the effects of anisotropic conductive film (ACF) viscosity on ACF fillet formation and, ultimately, on the pressure cooker test (PCT) reliability of ACF flip chip assemblies were investigated. The ACF viscosity was controlled by varying the molecular weight of the epoxy materials. It was found that the ACF viscosity increased as the increase of molecular weight of the epoxy materials. However, there was little variation of the thermomechanical properties among the evaluated ACFs with different viscosites. Also, the results showed that the ACFs have no differences in moisture absorption rate, die adhesion strength, and degree-of-cure. In scanning electron microscopy images, the lower ACF viscosity resulted in the smoother ACF fillet shape and the higher fillet height. From the results of PCT, the ACF flip chip assembly with the smoother fillet shape showed better reliability in terms of contact resistance changes. After 130 h of PCT, the flip chip assembly with lower ACF viscosity also showed a lesser degree of delamination at the ACF/chip interface.

The radio frequency (RF) and high frequency performance of the flip chip interconnects with anisotropic conductive film (ACF) and non-conductive film (NCF) was investigated and compared by measuring the scattering parameters (S-parameters) of the flip chip modules. Low cost electroless-Ni immersion-Au (ENIG) plating was employed to form the bumps for the adhesive bonding. To compare the accurate intrinsic RF performance of the ACF and NCF interconnect without lossy effect of chip and substrate, a de-embedding modeling algorithm was employed. The effects of two chip materials (Si and GaAs), the height of ENIG bumps, and the metal pattern gap between the signal line and ground plane in the coplanar waveguide (CPW) on the RF performance of the flip chip module were also investigated. The transmission properties of the GaAs were markedly improved on 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 RF performance of the flip chip interconnect with NCF was slightly better than that of the interconnect with ACF, mainly due to the capacitive component between the bump and substrate and self inductance of the conductive particle surface in the ACF interconnect.

Thermo-mechanical reliability of the anisotropic conductive film (ACF) joints in relation with flip chip bonding forces was evaluated by thermal shock testing. Two kinds of main failure modes were detected after thermal shock testing: formation of a conduction gap between conductive particles and Au bump or Ni/Au-plated Cu pad, and delamination of the adhesive matrix from the plated Cu pad on the flexible substrate. The determination of the failure mode was mainly affected by the variation of the bonding force. The main failure mode of the thermally shocked ACF joints was conduction gap for the joints with low bonding forces and adhesive matrix delamination for the joints with high bonding forces. Finite element analysis was also performed to determine the stress distributions within the two kinds of flip chip packages having different bonding gaps when it was stressed by the temperature rising.

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.

Failure behaviors of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects were investigated by measuring the connection resistance. The four-point probe method was used to measure the connection resistance of the adhesive joints constructed with Au bump on Si chip and Cu pad on flexible printed circuit. The interconnection reliability was evaluated by multiple reflow process. The connection resistance of the ACF joints was markedly higher than that of NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mOmega at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the reflow process, two different conduction behaviors were observed: increased connection resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.

A conductive adhesive is a promising interconnection material for microsystem packaging. The interconnect features are of great importance to system responses under various loading conditions. The flip-chip packaging system with anisotropic conductive film (ACF) joint under thermal loadings has been investigated both experimentally and theoretically. The displacement distributions have been measured by an interferometer, which could provide the in-plane whole-field deformation observation. The interconnection is of much smaller scales compared with the neighbouring components such as the chip and substrate, and there are even finer internal structures involved in the joint. The wide scale range makes both experimental observation and conventional simulation difficult. A micropolar model is thus developed. Utilizing the homogenization, this model requires low computation resource. Combination of this model with a second-order model was able to produce a highly efficient and valid prediction of the packaging system response under thermal and mechanical loadings. Comparison of the micropolar model simulation and experimental data shows good agreement.

Anisotropic conductive films (ACFs) and non-conductive films (NCFs) are film-type, adhesive polymer resins that have been used in flat panel display modules to provide high-resolution, light weight, thin profile and environmental friendliness. We investigated the radiofrequency (RF) and high frequency performance of the flip chip interconnects with ACF and NCF by measuring the scattering parameters (S-parameters) of the flip chip modules. The effects of three chip materials, Si, quartz and gallium arsenide (GaAs), and the metal pattern gap between the signal line and ground plane in the coplanar waveguide (CPW) on the RF performance of the flip chip were also investigated. The transmission properties of the GaAs and quartz chips 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. The RF performance of the flip chip module with NCF was slightly better than that of the module with ACF, mainly due to the capacitance o...

The temperature-humidity reliability of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects is investigated by measuring the interconnect resistance during temperature-humidity testing (THT) at 85°C and 85% relative humidity. The four-point probe method was used to measure the interconnect resistance of the adhesive joints constructed with Au bumps on Si chips and Cu pads on flexible printed circuits (FPCs). The interconnect resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The interconnect resistances of both interconnects decreased with increasing bonding force, and subsequently converged to about 10 mΩ and 1 mΩ at a bonding force of 70 N and 80 N, for the ACF and NCF joints, respectively. During the THT, two different conduction behaviors were observed: increased interconnect resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.

Recently, wafer-level packaging (WLP) has become one of the promising packaging technologies due to its advantages, such as fewer processing steps, lower cost, and enhanced device performance compared to conventional single-chip packaging. Many developments on new WLP design, material, and process have been accomplished according to performance and reliability requirement of the devices to be packaged [1], [2]. For a lower cost, higher performance, and environmentally green packaging process, anisotropic conductive film (ACF) flip chip assembly has been widely used, such as in ultrafine-pitch flat panel display (FPD) and general semiconductor packaging applications, too. However, there has been no previous attempt on the wafer-level flip chip assembly using ACFs. In this paper, wafer-level flip chip packages using preapplied ACFs were investigated. After ACF prelamination on an electroplated Au bumped wafer, and subsequent singulation, singulated chips were flip-chip assembled on an organic substrate using a thermocompression bonding method. Au-plated bumps were well assembled on Ni/Au pads of organic substrates. The electrical, mechanical properties and the reliabilities of wafer-level flip chip assemblies (WL-FC As) were evaluated and compared with conventional ACF flip chip assemblies using the thermocompression method. Contact resistance measurement was performed after thermal cycling, high temperature/humidity, and pressure cooker test. ACF joints between electroplated Au bumps and substrate metal pads showed stable contact resistance of 5 mOmega per a bump, strong bump adhesion, and similar reliability behaviors compared with conventional ACF flip chip joints using a thermocompression bonding. As a summary, new wafer-level packages using preapplied ACFs were successfully demonstrated for flip chip assembly. The new wafer-level packages using preapplied ACFs can be widely used for many nonsolder flip chip assembly applications such as chip-on-board (COB), chip-on-flex (COF), and chip-on-glass (COG).

Wafer bumping technology using an electroless Ni/Au bump on a Cu patterned wafer is studied for the flip chip type CMOS image sensor (CIS) package for the camera module in mobile phones. The effect of different pretreatment steps on surface roughness and etching of Cu pads is investigated to improve the adherence between the Cu pad and the Ni/Au bump. This study measures the shear forces on Ni/Au bumps prepared in different ways, showing that the suitable pretreatment protocol for electroless Ni plating on Cu pads is "acid dip followed by Pd activation" rather than the conventional progression of "acid-dip, microetching, and Pd activation." The interface between the Cu pad and the Ni/Au bump is studied using various surface analysis methods. The homogeneous distribution of catalytic Pd on the Cu pad is first validated. The flip chip package structure is designed, assembled, and tested for reliability. The successful flip chip bonding in the CIS package is characterized in terms of the cross-sectional structure in which the anisotropic conductive film (ACF) particles are deformed to about 1.5 μm in diameter. The experimental results suggest that electroless Ni/Au can be applied to the flip chip type CIS package using Cu patterned wafers for high mega pixel applications.

Thermo-mechanical reliability of flip chip packages employing anisotropic conductive film (ACF) was investigated in terms of the effect of bonding forces on the failure mode of the packages. Conventional reflow process was conducted to evaluate the reliability of the package. Two kinds of failure modes were detected in this study. The first one is formation of a conduction gap between conductive particle and Ni/Au plated Cu pad, while the second one is delamination of the adhesive matrix from the plated Cu pad on flexible substrate. The determination of the failure mode was mainly affected by the variation of the bonding force. In case of the ACF joints with lower bonding forces, a conduction gap was the main failure mode of the reflowed joints, while the delamination of the adhesive matrix was frequently observed in case of the joints with higher bonding forces. The difference in failure mode was discussed in the main text.

The effects of bonding forces and reflow process on the failure behaviors of anisotropic conductive film (ACF) interconnections were analyzed. Conventional reflow process was employed with peak temperatures of 220 � C for soldering of Sn-37Pb and 260 � C for soldering of Pbfree Sn-Ag or Sn-Ag-Cu. Two kinds of main failure mode were detected after double reflows at 220 � C: formation of a conduction gap between conductive particles and Ni/Au-plated Cu pad, and delamination of the adhesive matrix from the plated Cu pad on the flexible substrate. The determination of the failure mode was mainly affected by the variation of the bonding force. The main failure mode of the reflowed ACF joints was conduction gap for the joints with lower bonding forces and adhesive matrix delamination for the joints with higher bonding forces. However, only adhesive matrix delamination was observed after reflows at 260� C. A theoretical calculation was also conducted to predict the connection resistance of the ACF joint before and after reflows. The calculation showed that the optimum bonding forces are between 65 and 70 N. [doi:10.2320/matertrans.48.1070]

Recently, RFIC flip-chip mounting technique is very important to make small size and high frequency transceivers like mobile phones. The conventional flip-chip mounting uses Au stud bump bonding (SBB) and requires complicated process which includes forming and leveling of the bumps. In order to simplify the RFIC flip-chip process, we introduce a fiber type anisotropic conductive film (ACF) adhesive. This flip-chip mounting process does not require the SBB process and is suitable for transceiver system RFIC's having large number I/O pins. The interconnection between the RFIC and the substrate is confirmed by the 3D-CT images. The RF performance of the interconnection is measured and analyzed. The measured return loss of this interconnection is more than 10dB below 4.2GHz.

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 60N. 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 70N 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.

A novel method (the V-shaped curve) is presented to predict the failure probability of anisotropic conductive film (ACF) in IC/substrate assemblies. The Poisson function is used to calculate the probability of opening failure in the vertical gap between the pads, while the box and modified box models are used to estimate the probability of bridging failure between the pads in the pitch direction. The opening and bridging probabilities are combined using probability theory to establish four different failure prediction models. The results reveal that the model combining the Poisson function for fewer than six particles per pad with the modified box model provides the most accurate predictions of the failure probability of ACF in IC/substrate assemblies.

PurposeThis paper discusses the use of modelling techniques to predict the reliability of an anisotropic conductive film (ACF) flip chip in a humid environment. The purpose of this modelling work is to understand the role that moisture plays in the failure of ACF flip chips.Design/methodology/approachA 3D macro‐micro finite element modelling technique was used to determine the moisture diffusion and moisture‐induced stresses inside the ACF flip chip.FindingsThe results show that the ACF layer in the flip chip can be expected to be fully saturated with moisture after 3 h at 121°C, 100%RH, 2 atm test conditions. The swelling effect of the adhesive due to this moisture absorption causes predominately tensile stress at the interface between the adhesive and the metallization, which could cause a decrease in the contact area, and therefore an increase in the contact resistance.Originality/valueThis paper introduces a macro‐micro modelling technique which enables more detailed 3D modelling analysis of an ACF flip chip than previously.

The use of flip-chip technology has many advantages over other approaches for high-density electronic packaging. ACF (anisotropic conductive film) is one of the major flip-chip technologies, which has short chip-to-chip interconnection length, high productivity, and miniaturization of package. In this study, thermal fatigue life of ACF bonding flip-chip package has been predicted. Elastic and thermal properties of ACF were measured by using DMA and TMA. Temperature dependent nonlinear bi-thermal analysis was conducted and the result was compared with Moire interferometer result to verify FEM results. Calculated displacement field was well matched with experimental result. Thermal fatigue analysis was also conducted. The maximum shear strain occurs at the outmost located bump. Shear stress-strain curve was obtained to calculate fatigue life. Fatigue model for electronic adhesives was used to predict thermal fatigue life of ACF bonding flip-chip packaging. Approximately, 700 cycles have been expected under current conditions (JEDEC standard No. 22-A104)

The polymer SU-8 is becoming widely used for all kinds of micromechanical and microfluidic devices, not only as a photoresist but also as the constitutional material of the devices. Many of these polymeric devices need to include a microfluidic system as well as electrical connection from the electrodes on the SU-8 chip to a printed circuit board. Here, we present two different methods of electrically connecting an SU-8 chip, which contains a microfluidic network and free-hanging mechanical parts. The tested electrical interconnection techniques are flip chip bonding using underfill or flip chip bonding using an anisotropic conductive film (ACF). These are both widely used in the Si industry and might also be used for the large scale interconnection of SU-8 chips. The SU-8 chip, to which the interconnections are made, has a microfluidic channel with integrated micrometer-sized cantilevers that can be used for label-free biochemical detection. All the bonding tests are compared with results obtained using similar Si chips. It is found that it is significantly more complicated to interconnect SU-8 than Si cantilever chips primarily due to the softness of SU-8.

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 SiO2 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 SiO2 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.

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.