Material properties of anisotropic conductive films (ACFs) and their flip chip assembly reliability in NAND flash memory applications

In this paper, the material properties of anisotropic conductive films (ACFs) and ACF flip chip assembly reliability for a NAND flash memory application were investigated. Measurements were taken on the curing behaviors, the coefficient of thermal expansion (CTE), the modulus, the glass transition temperature (T_g), and the die adhesion strength of six types of ACF. Furthermore, the bonding processes of the ACFs were optimized. After the ACF flip chip assemblies were fabricated with optimized bonding processes, reliability tests were then carried out. In the pressure cooker test, the ACF with the highest adhesion strength showed the best reliability and the ACF flip chip assembly revealed no delamination at the chip-ACF interface, even after 96 h. In the high temperature storage test and the thermal cycling test, the reliability of the ACF flip chip assembly strongly depends on the T_g value of the ACF. In the thermal cycling test, in particular, which gives ACF flip chip assemblies repetitive shear stress, high value of CTE above T_g accelerates the failure rate of the ACF flip chip assembly. From the reliability test results, ACFs with a high T_g and a low CTE are preferable for enhancing the thermal and thermo-mechanical reliability. In addition, a new double-sided chip package with a thickness of 570 μm was demonstrated for NAND flash memory application. In conclusion, this study verifies the ACF feasibility, and recommends the optimum ACF material properties, for NAND flash memory application.     

Filler size and content effects on the composite properties of anisotropic conductive films (ACFs) and reliability of flip chip assembly using ACFs

Flip chip assembly on organic 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 have low coefficient of thermal expansion (CTE) value of ACFs. To control the CTE of ACF materials, non-conductive silica fillers were incorporated into ACFs. The effect of non-conductive silica filler content and size on cure kinetics and thermo-mechanical properties of ACFs was studied. Furthermore, filler content and size effects on reliability of flip chip assembly using ACFs were also investigated. In accordance with increasing filler content, curing peak temperature and storage modulus (E′) increased. But CTE decreased as the filler content increased. The effect of filler size on composite properties and assembly reliability showed similar tendency with the filler content effect. The smaller filler size was applied, the better composite properties and reliability were obtained. Conclusively, incorporation of non-conductive fillers, particularly in case of smaller size and higher content, in ACFs improves the material properties significantly, and as a result, flip chip assembly using ACFs is resulted in better reliability.     

Theoretical Prediction and Experimental Measurement of the Degree of Cure of Anisotropic Conductive Films (ACFs) for Chip-On-Flex (COF) Applications

The degree of cure of anisotropic conductive films (ACFs) was theoretically predicted and experimentally measured to investigate the effect of the degree of cure of ACFs on the electrical and mechanical stability of ACF joints and the␣reliability of chip-on-flex (COF) assemblies. The cure reaction of ACFs, observed by an isothermal differential scanning calorimetry (DSC) analysis, followed an autocatalytic cure mechanism, and the degree of cure of ACFs as a function of time and temperature was mathematically derived from an autocatalytic cure kinetics model. To simulate the ACF temperature field accurately during the COF bonding process, the thermal properties of the ACF such as the thermal diffusivity (α), specific heat capacity (C _p), and thermal conductivity (λ) were characterized experimentally. The degrees of cure of ACFs as functions of the bonding time during the COF bonding process were theoretically predicted by the incorporation of autocatalytic kinetics modeling and ACF temperature simulation. The predicted degrees of cure of ACFs were well matched with the experimental data measured by attenuated total reflectance/Fourier-transform infrared (ATR/FT-IR) analysis. The contact resistances of the ACF joints and the peel adhesion strengths of the COF assemblies were evaluated for electrical and mechanical interconnection stability. According to these results, the ACF contact resistances decreased and the ACF peel adhesion strengths increased as the degree of cure of ACFs increased. In addition, to investigate the effect of the degree of cure of ACFs on the reliability of COF assemblies, an 85°C/85% relative humidity (85°C/85% RH) test was performed. These results showed that the reliability of COF assemblies also strongly depends on the degree of cure of the ACFs.     

A study on thermal cycling (T/C) reliability of anisotropic conductive film (ACF) flip chip assembly for thin chip-on-board (COB) packages

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.     

Effects of Anisotropic Conductive Film Viscosity on ACF Fillet Formation and Chip-On-Board Packages

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.     

Ultrasonic Bonding Using Anisotropic Conductive Films (ACFs) for Flip Chip Interconnection

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.     

Thermal stability performance of anisotropic conductive film at different bonding temperatures

In this work the effect of different bonding temperatures on the thermal stability of anisotropic conductive films (ACFs) was investigated. A thermogravimetric analyzer (TGA) was utilized to determine the thermal decomposition temperature of ACF. The experimental results showed that the temperature for maximum decomposition rate of ACF, T_m decreased with increasing bonding temperature. The results obtained from Fourier transform infrared spectroscopy (FTIR), which was used to examine the curing degree and also chemical changes in the ACF, showed some networks scissoring happened on C–N bond during the bonding process. This was the main reason why the ACF bonded at high temperature 225.0 °C, gave relatively low thermal stability. Four point probe was used to measure the contact resistance performance before and after thermal aging at 260 and 300 °C. The contact resistance results suggested that ACF bonded with 10 s at 205.0 °C, yielding a curing degree of 85.0% was the best bonding parameter to obtain a low contact resistance after thermal aging. FTIR results showed there was a significant increase in the absorbance peak of carbonyl group after thermal aging. Thermal oxidation reactions which were taken place at high bonding and aging temperature had broken the polymer networks in the ACF and caused electrical failure.     

Effect of microwave preheating on the bonding performance of flip chip on flex joint

A microwave (MW) preheating mechanism of anisotropic conductive adhesive film (ACF) has been introduced in order to reduce the bonding temperature for flip chip technology. Thermal curing of epoxy shows a very sluggish and non-uniform curing kinetics at the beginning of the curing reaction, but the rate increases with time and hence requires higher temperature. On the other hand MW radiation has the advantage of uniform heating rate during the cycle. In view of this, MW preheating (for 2/3 s) of ACF prior to final bonding has been applied to examine the electrical and mechanical performance of the bond. Low MW power has been used (80 and 240 W) to study the effect of the MW preheating. It has been found that 170 °C can be used for flip chip bonding instead of 180 °C (standard temperature for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (0.015–0.025 Ω) is low in these samples where the standard resistance is 0.017 Ω (bonded at 180 °C without prior MW preheating). The shear forces at breakage were satisfactory (152–176 N) for the samples bonded at 170 °C with MW preheating, which is very close and even higher than the standard sample (173.3 N). For MW preheating time of 2 s, final bonding at 160 °C can also be used because of its low contact resistance (0.022–0.032 Ω), but the bond strength (137.3–145 N) is somewhat inferior to the standard one.     

Deformation mechanism and its effect on electrical conductivity of ACF flip chip package under thermal cycling condition: An experimental study

In this study, experimental works are performed to investigate the deformation mechanism and electrical reliability of the anisotropic conductive adhesive film (ACF) joint subjected to temperature cycling for flip chip on organic board (FCOB) assemblies. This paper presents some dominant deformation parameters governing the electrical degradation in an ACF joint between a chip and a substrate when flip chip assembly is heated and cooled. The deformation mechanism of ACF flip chip assemblies during the temperature cycling are investigated using in situ high sensitivity moiré interferometry. A four-point probe method is conducted to measure the real-time contact resistance of ACF joint subjected to the cyclic temperature variation. As the temperature increases below T_g of ACF, the bending displacement of assembly decreases linearly. At the temperature higher than T_g of ACF, there is no further change in bending behavior and in-plane deformations of a chip and a substrate become approximately free thermal expansion. It is because that soft-rubbery ACF at the temperature above T_g cannot provide the mechanical coupling between a chip and a substrate. The effect of bump location on the temperature dependent contact resistance is evident. A characteristic hysteresis in bending curves is observed and discussed. The contact resistance of the corner bumps increases with increasing temperature at a higher rate when compared to that of the middle. Failure analysis is performed to examine the ACF interconnections before and after thermal cycling test. The results indicate that during the thermal loading, the shear deformation is more detrimental to the electrical degradation of ACF joints than normal strain.     

Effects of Heating Rate on Material Properties of Anisotropic Conductive Film (ACF) and Thermal Cycling Reliability of ACF Flip Chip Assembly

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.      

Effects of epoxy functionality on the properties and reliability of the anisotropic conductive films for flip chips on organic substrates

The effects of the functionality of an epoxy monomer on the composite properties and reliability of anisotropic conductive films (ACFs) in a flip-chip package were investigated. Three epoxy monomers with different functionalities (f=2–4) were considered. The ACFs prepared using epoxy monomers with higher functionality resulted in lower molecular weight between crosslinks (M_c). As the M_c decreased, the elastic modulus (E′) and coefficient of thermal expansion (CTE) were improved. These results were highly consistent with the rubber elasticity theory. The reliability performance of the flip chip on organic substrate assemblies using ACFs were also investigated as a function of epoxy functionality. The ACFs prepared by using higher functional epoxy monomers showed improved reliability performance.     

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.     

Flip chip attachment on flexible LCP substrate using an ACF

In this study the reliability of a flip chip bonding process using anisotropic conductive adhesives (ACA) was evaluated. The flexible substrates used were made of liquid crystal polymer (LCP), which is an interesting new material having excellent properties for flexible printed circuit boards. The test samples were prepared using two different anisotropic conductive films (ACF) having the same fast-cure resin matrix, but different conductive particles. The reliability of the test samples was studied by accelerated environmental tests. In the constant humidity test the temperature was 85 °C and the relative humidity was 85%. The temperature cycling test was carried out between temperatures of −40 °C and 85 °C. To determine the exact time of a failure the resistance of each test sample was measured using continuous real-time measurement. A clear difference between the behaviour of the conductive particles was seen in the test. While the adhesive having polymer particles had only one failure during testing, the adhesive having nickel particles had a considerable number of failures in both tests. Cross sections of the samples were made to analyze the failure mechanisms.     

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

Various fine pitch chip-on-film (COF) packages assembled by (1) anisotropic conductive film (ACF), (2) nonconductive film (NCF), and (3) AuSn metallurgical bonding methods using fine pitch flexible printed circuits (FPCs) with two-metal layers were investigated in terms of electrical characteristics, flip chip joint properties, peel adhesion strength, heat dissipation capability, and reliability. Two-metal layer FPCs and display driver IC (DDI) chips with 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.     

Reliability study of underfill/chip interface under accelerated temperature cycling (ATC) loading

Interface reliability issue has become a major concern in developing flip chip assembly. The CTE mismatch between different material layers may induce severe interface delamination reliability problem. In this study, multifunctional micro-moiré interferometry (M³I) system was utilized to study the interfacial response of flip chip assembly under accelerated thermal cycling (ATC) in the temperature range of −40 °C to 125 °C. This in-situ measurement provided good interpretation of interfacial behavior of delaminated flip chip assembly. Finite element analysis (FEA) was carried out by introducing viscoelastic properties of underfill material. The simulation results were found to be in good agreement with the experimental results. Interfacial fracture mechanics was used to quantify interfacial fracture toughness and mode mixity of the underfill/chip interface under the ATC loading. It was found that the interfacial toughness is not only relative to CTE mismatch but also a function of stiffness mismatch between chip/underfill.     

Effects of thermoplastic resin content of anisotropic conductive films on the pressure cooker test reliability of anisotropic conductive film flip-chip assembly

The flip-chip technology using anisotropic conductive films (ACFs) is gaining growing interest due to its technical advantages such as environmentally friendly, simpler, and lower cost processes. Electrical performances and reliability of ACF flip-chip assembly depend on thermomechanical properties of ACF polymer resins. In this paper, the changes in ACF resin morphology due to the phase separation of thermoplastics, and subsequent changes of physical and mechanical properties were investigated as a function of thermoplastic contents of ACF formulation. Furthermore, the pressure cooker test (PCT) reliability of ACF flip-chip assemblies with various thermoplastic contents was also investigated. As thermoplastic contents increased, coefficient of thermal expansion (CTE) of ACFs increased, and elastic modulus (E′) of ACFs decreased. In contrast, water absorption rate decreased as thermoplastic content increased. As a result, PCT reliability of ACF flip-chip assembly was improved adding up to 50 wt.% content of thermoplastic. An erratum to this article is available at .     

Dynamic strength of anisotropic conductive joints in flip chip on glass and flip chip on flex packages

The work presented in this paper focuses on the behavior of anisotropically conductive film (ACF) joint under the dynamic loading of flip chip on glass (COG) and flip chip on flexible (COF) substrate packages. Impact tests were performed to investigate the key factors that affect the adhesion strength. Scanning electron microscopy (SEM) was used to evaluate the fractography characteristics of the fracture. Impact strength increased with the bonding temperature, but after a certain temperature, it decreased. Good absorption and higher degree of curing at higher bonding temperature accounts for the increase of the adhesion strength, while too high temperature causes overcuring of ACF and degradation at ACF/substrate interface––thus decreases the adhesion strength. Higher extent of air bubbles was found at the ACF/substrate interface of the sample bonded at the higher temperature. These air bubbles reduce the actual contact area and hence reduce the impact strength. Although bonding pressure was not found to influence the impact strength significantly, it is still important for a reliable electrical interconnect. The behaviors of the conductive particles during impact loading were also studied. From the fracture mode study, it was found that impact load caused fracture to propagate in the ACF/substrate interface (for COG packages), and in the ACF matrix (for COF packages). Because of weak interaction of the ACF with the glass, COG showed poor impact adhesion.     

Design of a novel chip on glass package solution for CMOS image sensor device

This paper presents a chip-on-glass (COG) package solution for CMOS image sensors based on highly precise and reliable bumping and flip-chip bonding techniques. The package is fabricated using three core techniques, namely the damage-free image sensor bumping technique, the wafer form glass substrate patterned technique, and the damage-free flip-chip bonding technique. Since the proposed package concept is new, the effects of the package geometry and material properties on the package reliability are uncertain in the initial design stage. A three-dimensional nonlinear finite element model of the proposed CMOS image sensor package is created. In the simulations, the applied thermal load is cooled from 200 °C to ambient temperature (25 °C) to model the thermal deformation and warpage of the package during the practical ACF assembly cooling process. The design parameters influencing the reliability of the package, i.e. the material properties of the ACF, the thickness of the image chip and the thickness of the optical glass are investigated. Two control levels are specified for the chip, glass, and ACF factors and a 2³ factorial design is created to determine the appropriate combination of material properties and geometric size. It is found that the glass thickness and the ACF properties significantly affect the thermal deformation of the package, while the chip and glass factors, and the interaction between them, significantly affect the warpage. Regression models are developed to perform a series of surface response simulations. Using the developed statistical tests and regression models, suitable material selection criteria and geometric sizes can be specified to satisfy various reliability considerations in the initial design stage.     

Reduced thermal strain in flip chip assembly on organic substrateusing low CTE anisotropic conductive film

Flip chip assembly directly on organic boards offers miniaturization of package size as well as reduction in interconnection distances, resulting in a high performance and cost-competitive packaging method. This paper describes the usefulness of low cost flip-chip assembly using electroless Ni/Au bump and anisotropic conductive films on organic boards such as FR-4. As bumps for flip chip, electroless Ni/Au plating was performed as a low cost bumping method. Effect of annealing on Ni bump characteristics informed that the formation of crystalline nickel with Ni₃P precipitation above 300°C causes an increase of hardness and an increase of the intrinsic stress. As interconnection material, modified ACFs composed of nickel conductive fillers for conductive fillers, and nonconductive fillers for modification of film properties, such as coefficient of thermal expansion (CTE), were formulated for improved electrical and mechanical properties of ACF interconnection. Three ACF materials with different CTE values were prepared and bonded between Si chips and FR-4 boards for the thermal strain measurement using moire interferometry. The thermal strain of the ACF interconnection layer, induced by temperature excursion of 80°C, was decreased according to the decreasing CTEs of ACF materials. This result indicates that the thermal fatigue life of ACF flip chip assembly on organic boards, limited by the thermal expansion mismatch between the chip and the board, could be increased by low CTE ACF     

Evaluation of the Delamination in a Flip Chip Using Anisotropic Conductive Adhesive Films Under Moisture/Reflow Sensitivity Test

Anisotropic conductive adhesive films (ACFs) have been used for electronic assemblies such as the connection between a liquid crystal display panel and a flexible printed circuit board. ACF interconnection is expected to be a key technology for flip chip packaging, system-in-packaging, and chip size packaging. This paper presents a methodology for quantitative evaluation of the delamination in a flip chip interconnected by an ACF under moisture/reflow sensitivity tests. Moisture concentration after moisture absorption was obtained by the finite element method. Then, the vapor pressure in the flip chip during solder reflow process was estimated. Finally the delamination was predicted by comparing the stress intensity factor of an interface crack due to vapor pressure with the delamination toughness. It is found that the delamination is well predicted by the present methodology.