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Chip-on-film (COF) technology has been developed for liquid crystal displays (LCD) due to its high yield capability in fine pitch products. In this study, a new COF thermosonic (TS) bonding process using anisotropic conductive adhesive and ultrasonic vibrations will be introduced. Si chips with 16 cylindrical Cu bumps (¤100µm) and polyimide (PI) film substrate with a thickness of 70µm were prepared. For the bonding condition, the bonding temperature and ultrasonic time were varied from 413 to 453K and from 0.5 to 1s, respectively. Also, thermocompression (TC) bonding was conducted with a bonding temperature of 453K and bonding time of 8s to compare it to the bonding characteristics of TS bonding. The shear strength and electrical resistance of COF assemblies were measured to verify the feasibility of COF TS bonding using ACFs. The cross-sectional inspection and fracture surface analysis of COF joints were also conducted using field-emission scanning electron microscopy (FE-SEM). As a result, the mechanical and electrical properties of the COF assembly were improved by increasing the bonding temperature and ultrasonic dwell time. It was also determined that ultrasonic energy has a significant influence in improving the mechanical and electrical properties of the TS COF assembly. [doi:10.2320/matertrans.MB201210]

A new class of functionalized graphene-filled solderable isotropically conductive adhesive (ICA) has been developed using a low-meltingpoint alloy (LMPA) fillers. The mechanical and electrical characteristics of formulated ICAs were investigated and compared with those of three kinds of conventional ICAs filled with Ag particles. The functionalized graphene-filled solderable ICA formed good metallurgical interconnection between upper and corresponding lower electrode. The developed ICA exhibit lower electrical resistance and higher mechanical strength compared with those of conventional ICAs. In addition, the thermal conductivity was improved about 20% by adding functionalized graphene compared with that of solderable ICA without graphene. [doi:10.2320/matertrans.M2011365]

This study investigates the dynamic flow characteristics of capillary-driven underfill flows in a flip chip package. In the present study, we used two different bump arrays using Sn-2.5Ag solder balls with 80 mm and 100 mm diameters on commercially available flip chips, which have different pitches of 150 mm and 180 mm. First, we measured surface tension and viscosity with a rheometer and a tensiometer, respectively, and conducted an experimental visualization of the dynamic filling behavior of the underfill flows. From the captured images, we estimated the filling times, which can be affected by two important factors: bump arrangements and resin viscosities. In addition, we conducted a FVM (finite volume method)-based numerical simulation using commercial CFD code (Fluent v. 6.3.26), and compared its numerical results to both the experimental data and the analytical solutions given by the previous model described by Wan et al. (2005). The numerical predictions and analytical solutions estimating filling time were in good agreement with the experimental data, and the increase in spatial density of solder bumps allowed the flow to fill more slowly due to the increase in flow resistance. We conclude that the non-Newtonian characteristics and bump arrangement are very important factors in the design of flip-chip packaging. [doi:10.2320/matertrans.M2011151]

The preheating effect of SCM415 steel substrates on the flattening behavior of Ni20Cr flame-sprayed particles was investigated in a temperature range from 278 K to 523 K. In the present study, we calculated the particle temperature and velocity distributions of the continuous and discrete phases before collision with the substrate by using a computational fluid dynamics (CFD) code of Fluent (ver. 6.3.26). Particle velocity and gas temperature decreased rapidly in the radial direction while the particles traveled toward the substrate, suggesting that substrate size and distance from the nozzle should be carefully controlled to improve coating quality. The conventional flame spray gun was used to accelerate molten particles and K-type thermocouples were used to monitor the substrate temperature during a preheating process. Commercially available nickel-based Ni20Cr particles with a mean diameter of 45 mm and 20 mm were used for experiments to examine the particle size effect on the coating characteristics. Herein we present FE-SEM images of coated layers on the substrates. As the substrate preheating temperature increased, the flatter surface morphology was seen at the interface between substrate and coating layer because of better wetting and corresponding higher shear adhesion strength. Moreover, the splat morphology was significantly dependent on the particle size. [doi:10.2320/matertrans.M2011068]

In recent years, Anisotropic Conductive Films (ACFs) have been widely used in microelectronic packaging applications due to their potential advantages, which include low processing temperatures, high-resolution capabilities, and compatibility for nonsolderable materials. In this study, we investigated a novel, thermosonic ACF flip-chip bonding process using ultrasonic vibration. The thermosonic ACF flip-chip bonding was characterized in terms of adhesion strength and electrical property in comparison with conventional thermo-compression ACF bonding. ACF has a nickel particle with a 4-μm diameter, and its recommended thermo-compression bonding conditions were 453 K for bonding temperature and 10 s for bonding time. The investigated thermosonic flip-chip bonding conditions were 433 K and 453 K for bonding temperature and 0.5 s and 1 s for bonding time at each temperature, respectively. In addition, a cross-section of the ACF joint was observed using scanning electron microscopy. As a result, the developed thermosonic flip-chip bonding process showed good electrical and mechanical characteristics, as compared with those of the conventional thermo-compression bonding process. In the future, we expect that a thermosonic flip-chip bonding method with ACFs can be effectively applied to reduce the bonding temperature and time.

In order to overcome several crucial limitations to the traditional electrically conductive adhesive bonding used in the electronics field, a novel low-melting-point alloy (LMPA)-filled ICA (Isotropic Conductive Adhesive) was synthesized. Also, a hybrid interconnection process using it was developed, which can be achieved by melting-coalescence-wetting behavior of LMPA fillers in ICA. In order to ensure excellent coalescence and wetting characteristics of the LMPA filler particles, the fluxing capability of the resin should have been sufficient to remove the oxide layer on the surfaces of the filler particles and the electrode materials. The effect of reductant on the hybrid interconnection formation was examined. ICAs with and without reductant were formulated with a 40% volume fraction of filler. The QFP chip had a size of 14 � 14 � 2:7 mm and a 1 mm lead pitch. The test board had a 18 mm thick Cu daisy-chained pattern. Thermal characteristic of the ICA was observed by differential scanning calorimetry (DSC), and the temperature-dependant viscosity characteristic of the polymer matrix was observed by torsional parallel rheometer. Based on the results, the reflow profile of the interconnection process was determined. The dipping interconnection method was applied in the QFP bonding process. After the QFP bonding process, the electrical characteristic of ICA was measured with a multimeter. The wetting, bondability, coalescence characteristics of the LMPA filler particles and the morphology of the conduction path were observed by microfocus X-ray inspection systems and optical microscopy. As a result, the LMPA-filled ICAs with reductant had stable contact resistance, forming the metallurgical interconnection. A good electrical conduction path was achieved with the coalescence and wetting characteristics of the molten solder particles in the ICA. [doi:10.2320/matertrans.M2009109]

A non-equilibrium phase field model (NPFM) is proposed to examine the three-dimensional coalescence characteristics of low melting point alloy (LMPA) fillers, widely used for self-organized interconnection applications. This model sufficiently considers the non-equilibrium process during phase transitions, resulting in accurate prediction of interfaces between two different phases with a high density ratio. The preliminary simulation is carried out for validation of the present model and numerical predictions are in good agreement with analytical solutions for a one-dimensional solidification problem. The proposed NPFM successfully predicts the phase transition, the heat transfer from solid to liquid, and the coalescence behaviors of LMPA fillers, whereas the conventional enthalpy method fails to describe the non-equilibrium phase transition. Moreover, the results show that there exists grid dependency in the mush zone, suggesting that special care should be taken of the mush zone for better prediction of interfaces between different phases.

In this paper, we discuss our design of a new class of low-melting-point alloy (LMPA)-filled anisotropically conductive adhesives (ACA) and a self-organized interconnection process for using these adhesives, and we demonstrate a good electrical conduction for the process. Flow, melting, coalescence, and wetting characteristics of LMPA fillers in the ACA facilitate this process. In order to exploit good coalescence and wetting characteristics of LMPA fillers, the polymer matrix has a sufficient fluxing capability against oxide films of both LMPA fillers and electrode materials. Furthermore, it is essential that the polymer have a low viscosity level around the melting point of the incorporated LMPA to achieve a good electrical conduction path. In order to study coalescence and wetting characteristics, we formulated two types of ACA with different volume fractions of LMPA filler (10 vol% and 40 vol%). Our test board had an 18-mm thick Cu line-type pattern (10 mm � 0:1 mm) and an area array-type pattern (square type: 0:1 mm � 0:1 mm, circle type: ’, 0.1 mm) with five different pitches (50 mm to 250 mm). We measured thermal properties of the ACA by differential scanning calorimetry (DSC), and we determined a temperature profile for the interconnection process. We then monitored coalescence and wetting characteristics of LMPA fillers and morphology of conduction path in ACA by using a microfocusing X-ray inspection system and an optical microscope. We determined that the developed LMPA-filled ACA have good coalescence and wetting characteristics regardless of pattern types. In addition, we were able to achieve a good electrical conduction path because of the coalescence and wetting characteristics of the LMPA fillers in the ACA. [doi:10.2320/matertrans.MF200918]

In the electronic components and device packaging process, the Cu conductor surface is generally coated with Au and Ni. The Au coating is applied to prevent the oxidation of the Cu surface and enhance the solderability, while the Ni coating is applied as a diffusion barrier between the solder alloy and the Cu substrate, in order to restrict the formation and growth of intermetallic compounds. The dynamic reactive wetting characteristics of Sn-Ag-Cu alloys are related to the properties of the coating materials. In the present study, the dynamic reactive wetting behavior of Sn-Ag-Cu alloys on Cu substrates coated with Ni and Au was firstly investigated on millisecond scale. On the bare Cu surface, the metal droplet rebounded several times due to the poor wettability and started to spread from 1 s and became equilibrated by 20 s, whereas on the Ni/Au coated surface, the metal droplet settled down without rebounding because of the good wettability, which caused it to react instantaneously, and became equilibrated by 1 s.

The present study is devoted to develop a new simulation code for the numerical investigation on coalescence characteristics in selforganized interconnection using anisotropic conductive adhesive (ACA) with low melting point alloy (LMPA) fillers. In particular, this article examines the coalescence mechanism during ACA process for different resin viscosity, particle volume fractions, and provides estimated results of conduction path growth fraction for different conditions. Starting from the motion equation of filler particles, moreover, a theoretical approach is presented to find out dominant factors affecting the formation of interconnection. The new simulation code is based on the finite volume method with the use of the continuum surface force (CSF) model and the cubic interpolated propagation (CIP) method. It is confirmed that conduction path formation depends on volume fraction, viscosity, and surface tension during the process. It is also found that the pressure-driven force effect becomes not dominant in the path formation at high resin viscosity and high filler density, the particle drift velocity becomes slow due to the increase in resin viscosity, conduction path growth fraction increases with the volume fraction, and the time for path formation is shorter with the increase of the volume fraction. [doi:10.2320/matertrans.MB200819]

This article investigates numerically carrier-phonon interaction and nonequilibrium energy transfer in direct and indirect bandgap semiconductors during sub-picosecond pulse laser irradiation and also examines the recombination effects on energy transport from the microscopic viewpoint. In addition, the influence of laser fluence and pulse duration is studied by using the self-consistent three-temperature model, which involves carriers, longitudinal optical phonons, and acoustic phonons. It is found that a substantial non-equilibrium state exists between carriers and phonons during short pulse laser irradiation because of time scale difference between the relaxation time and the pulse duration. It is clear that the two-peak structure in carrier temperature exists and it depends mainly on laser pulses, fluences, and recombination processes. During laser irradiation, in particular, the Auger recombination for Si becomes dominant due to the increase in the carrier number density, whereas for GaAs, the Auger recombination process can be ignored due to an abrupt increase in SRH recombination rates at the initial stages of laser exposure.

A new class of electrically conductive adhesives (ECAs) has been developed using two different resin materials and a low-melting-point alloy filler powder. The curing or cross-linking behaviors of base resin materials and the melting behavior of filler metal were examined using a differential scanning calorimetry (DSC) instrument. The effects of the reduction capability of base resin material, metal filler volume fraction, and joint height on the coalescence of filler, and morphology of conductive path were examined using a cross-sectional optical laser microscope. The two different types of conductive paths (necking-type and bump-type) were achieved by the coalescence and wetting characteristics of melted fillers into new formulations. The results show a good metallurgical connection and a low contact resistance even at a low filler metal volume fraction of 30%.

The self-interconnection process is one of the promising methodologies for joining novel materials and assembling micro-electronic devices. Basic experiments fabricating micro joints using conductive adhesives with low melting point solders, such as Sn-In eutectic alloy, were demonstrated as an alternative method to conventional soldering or using the usual adhesive joining used in electronic assembly. The joint morphology, the formation of electrical conduction interconnections, and the self-interconnection characteristics were examined by optical and X-ray transmission microscopy. The behaviors of melting fillers such as aggregation, coalescence, and wetting were found to occur during joint formation. Especially on the copper line patterns of a glass-epoxy substrate, selective adhesional wetting of the melting alloy, enhanced by the oxygen-reduction capability of resin and capillary phenomena, was the main driving forces of the self-interconnection of joints. The alignment, joint height, and volume fraction of the filler need to be set correctly for a successful adhesion to be achieved.

Electrically conductive adhesives (ECAs) have been developed as an alternative to traditional Sn-Pb solders for electronic and optoelectronic applications. However, there are critical limitations such as the low conductivity, and unstable contact resistance. These limitations have seriously hindered the broad applications of ECAs. In order to overcome these limitations, a new formulation using Pb-free conducting filler particles was proposed. Our previous study proved that the metallurgical interconnections among conducting filler particles and between particles and the conducting pads were established by process control. In particular, it was found that the wetting behavior of conducting fillers in ECAs is one of the main mechanisms for the establishment of conduction paths. In this study, we propose the fundamental concept of an assembly process using fusible Pb-free conducting filler particles. Also, the effect of the reduction capability of base resin material on the solder wetting property was investigated. As candidates for ECA compositions, two types of resin materials and two lead-free solders were investigated. The reflow temperature profiles for each ECA formulation were determined using a differential scanning calorimeter (DSC) dynamic scan. The wettabilities of the lead-free solders were investigated using an optical microscope with a CCD camera and a microfocus Xray television system. It was found that the developed resin material with an intrinsic reduction capability shows good wettabilities in both leadfree candidates.

A new class of isotropic conductive adhesive (ICA) using a carbon nanotubes (CNTs) and a low-melting-point alloy (LMPA) fillers has been developed. We investigated the fundamental materials characteristics including curing behavior and temperature-dependant viscous property of ICA. In addition, the morphology of conduction paths in each ICA was investigated using X-ray inspection systems and an optical microscope. Mechanical and electrical characteristics of formulated ICAs were determined and compared with those of three kinds of conventional ICAs filled with Ag flakes. The CNT-filled solderable ICA formed good metallurgical interconnection between upper and corresponding lower electrode. In addition, the results indicated that the CNT-filled ICA exhibit lower electrical resistance and higher mechanical strength, as compared with those of conventional ICAs.