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.     

Effects of bonding temperature on the properties and reliabilities of anisotropic conductive films (ACFs) for flip chip on organic substrate application

The effects of bonding temperatures on the composite properties and reliability performances of anisotropic conductive films (ACFs) for flip chip on organic substrates assemblies were studied. As the bonding temperature decreased, the composite properties of ACF, such as water absorption, glass transition temperature (T_g), elastic modulus (E′) and coefficient of thermal expansion (α), were improved. These results were due to the difference in network structures of cured ACFs which were fully cured at different temperatures. From small angle X-ray scattering (SAXS) test result, ACFs cured at lower temperature, had denser network structures. The reliability performances of flip chip on organic substrate assemblies using ACFs were also investigated as a function of bonding temperatures. The results in thermal cycling test (−55 °C/+150 °C, 1000 cycles) and PCT (121 °C, 100% RH, 96 h) showed that the lower bonding temperature resulted in better reliability of the flip chip interconnects using ACFs. Therefore, the composite properties of cured ACF and reliability of flip chip on organic substrate assemblies using ACFs were strongly affected by the bonding temperature.     

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 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 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 .     

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

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

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     

Highly reliable non-conductive adhesives for flip chip CSP applications

Non-conductive adhesives (NCA), widely used in display packaging and fine pitch flip chip packaging technology, have been recommended as one of the most suitable interconnection materials for flip-chip chip size packages (CSPs) due to the advantages such as easier processing, good electrical performance, lower cost, and low temperature processing. Flip chip assembly using modified NCA materials with material property optimization such as CTEs and modulus by loading optimized content of nonconductive fillers for the good electrical, mechanical and reliability characteristics, can enable wide application of NCA materials for fine pitch first level interconnection in the flip chip CSP applications. In this paper, we have developed film type NCA materials for flip chip assembly on organic substrates. NCAs are generally mixture of epoxy polymer resin without any fillers, and have high CTE values un-like conventional underfill materials used to enhance thermal cycling reliability of solder flip chip assembly on organic boards. In order to reduce thermal and mechanical stress and strain induced by CTE mismatch between a chip and organic substrate, the CTE of NCAs was optimized by filler content. The flip chip CSP assembly using modified NCA showed high reliability in various environmental tests, such as thermal cycling test (-55/spl deg/C/+160/spl deg/C, 1000 cycle), high temperature humidity test (85/spl deg/C/85%RH, 1000 h) and high temperature storage test (125/spl deg/C, dry condition). The material properties of NCA such as the curing profile, the thermal expansion, the storage modulus and adhesion were also investigated as a function of filler content.     

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.     

Study of anisotropic conductive adhesive joint behavior under 3-point bending

Flip chip interconnections using anisotropic conductive film (ACF) are now a very attractive technique for electronic packaging assembly. Although ACF is environmentally friendly, many factors may influence the reliability of the final ACF joint. External mechanical loading is one of these factors. Finite element analysis (FEA) was carried out to understand the effect of mechanical loading on the ACF joint. A 3-dimensional model of adhesively bonded flip chip assembly was built and simulations were performed for the 3-point bending test. The results show that the stress at its highest value at the corners, where the chip and ACF were connected together. The ACF thickness was increased at these corner regions. It was found that higher mechanical loading results in higher stress that causes a greater gap between the chip and the substrate at the corner position. Experimental work was also carried out to study the electrical reliability of the ACF joint with the applied bending load. As per the prediction from FEA, it was found that at first the corner joint failed. Successive open joints from the corner towards the middle were also noticed with the increase of the applied load.     

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.     

An optimization study of underfill dispensing volume

The underfill dispensing volume has been modeled and verified through both experiments and application of statistical technique. The model established is capable of estimating the operating range of the dispensing volume of a defined flip chip assembly and is targeted to reduce wastage as well whilst fulfilling the reliability requirement. The model has taken into consideration the reliability factor, e.g., presence of fillet; manufacturing tolerance of bumps size and standoff variation. In this study, the actual volume for flip chip assemblies prepared in a controlled manner, so that fillets were seen on all sides, was compared with the recommended underfill volume. It was found that the model tended to yield a higher volume and it is concluded that this variation is related to the over estimation of the fillet element in the formulation. For reliability assessment, these flip chip assemblies were examined under C-mode SAM and no voids were found. These flip chip assemblies also passed electrical testing after 500 cycles of air to air thermal cycles and therefore are proven to meet the reliability requirement.     

Effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue lifetime significantly. The reliability of solder joint in flip chip assembly for both rigid and compliant substrates was evaluated by accelerated temperature cycling test. Experimental results strongly showed that the thermal fatigue lifetime of solder joints in flip chip on flex assembly was much improved over that in flip chip on rigid substrate assembly. Debonding area of solder joints in flip chip on rigid board and flip chip on flex assemblies were investigated, and it was found that flex substrate could slow down solder joint crack propagation rate. The mechanism of substrate flexibility on improving solder joint thermal fatigue was investigated by thermal mechanical analysis (TMA) technique. TMA results showed that flex substrate buckles or bends during temperature cycling and this phenomenon was discussed from the point of view of mechanics of the flip chip assembly during temperature cycling process. It was indicated that the thermal strain and stress in solder joints could be reduced by flex buckling or bending and flex substrates could dissipate energy that otherwise would be absorbed by solder joints. It was concluded that substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during temperature cycling.     

Corrosion testing of anisotropic conductive adhesive interconnections on FR4, liquid crystal polymer and polyimide substrates

Anisotropic conductive adhesive films (ACF) have been widely studied for numerous applications. However, their resistance to corrosion in highly corrosive environments has been studied only very little. This study investigated the reliability and behaviour of ACFs in corrosive salt spray environment. ACF was used to attach flip chip (FC) components on FR4, liquid crystal polymer (LCP) and polyimide (PI) substrates and the FC packages were subjected to a salt spray test lasting 3000 h. The FC packages had daisy chain structures which were measured continuously in real time during testing. After testing cross sections of the tested packages were examined using an optical microscope and a scanning electron microscope (SEM). Most components failed during the test and the results showed significant differences between the various substrate materials. The LCP substrate performed considerably better than the other substrates and the PI substrate proved to have the poorest reliability. Corrosion of the pads on the substrates as well as open joints was seen in all substrate materials. The corrosion behaviour as well as the differences between the substrates showed that the substrate structure and material are critical factors in corrosive environments and should be carefully considered. The reliability of the ACF FC package with the LCP substrate was found to be good, as the test was very severe and no failures occurred during the first 625 h of testing and only 20% failed during the first 1000 h.     

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.     

Study of the Formation of Bubbles in Rigid Substrate-Flexible Substrate Bonding Using Anisotropic Conductive Films and the Bubble Effects on Anisotropic Conductive Film Joint Reliability

The formation of process-related bubbles that become entrapped inside the anisotropic conductive film (ACF) layer during bonding processes remains an issue. The formation of these bubbles is strongly influenced by the process variables, such as bonding pressure and bonding temperature. Therefore, bonding process variables of bonding temperature, bonding pressure, and type of flexible substrate (FS) were changed in order to investigate the effects of the changes as they concern the formation of bubbles. According to the results, the tendency toward bubble formation was closely related to these three factors. The bubble area increased as the bonding temperature increased. Moreover, the shape and tendency of bubbles coincided with temperature distribution in␣the ACF layer. Two different types of FS, each with different surface roughnesses and energies, were used. The bubbles formed only on the FS with the larger roughness and lower surface energy. According to the results from a surface energy measurement of FS types using goniometry, a FS with a higher surface energy is favorable for a bubble-free assembly, as the higher surface energy provides better wettability. In addition, in order to investigate the effect of bubbles on the reliability of ACF joints, the pressure cooker test (PCT) was performed, and all samples with bubbles electrically failed after 72 h of a PCT, as the process-related bubbles provided a moisture penetration path and entrapment site for moisture. However, all type 1 test vehicles (TVs) survived even after 120 h of a PCT. Therefore, Ar and O₂ plasma treatments were performed on the FS with the lower surface energy in order to improve the surface energies and wettability. Following this, the bubbles were successfully removed at rigid substrate (RS)–FS bonding joints using ACFs.     

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.     

Adhesion and Reliability of Anisotropic Conductive Films (ACFs) Joints on Organic Solderability Preservatives (OSPs) Metal Surface Finish

The effect of final metal finishes of Cu electrodes on the adhesion and reliability of anisotropic conductive film (ACF) joints was investigated. Two different metal surface finishes, electroless Ni/immersion Au (ENIG) and organic solderability preservatives (OSPs) coated on Cu, were selected in this study for ACF bonding. The adhesion strength of ACF/OSP joints was higher than that of ACF/bare Cu and ACF/ENIG joints. The fracture sites of the ACF/bare Cu and ACF/ENIG joints were ACF/metal interfaces, while those of ACF/OSP joints were inside the ACF. Transmission electron microscope (TEM) and Fourier-transform infrared (FT-IR) analyses showed that the OSP coating layer on the Cu electrodes reacted with the epoxy resin of the ACFs but still remained at the bonding interface. According to the in-depth X-ray photoelectron spectroscopy (XPS) analysis, additional C-N bonds formed after the OSP-epoxy reaction and the outermost nitrogen of the OSP layer participated in curing of the epoxy resin of the ACF. Therefore, the OSP layer acted as an adhesion promoter to ACFs. Furthermore, this role of the OSP layer enhanced the reliability of the ACF/OSP joints under high temperature and humid environments, as compared to the ACF/ENIG joints.     

Effects of silica filler and diluent on material properties and reliability of nonconductive pastes (NCPs) for flip-chip applications

In this paper, thermomechanical and rheological properties of nonconductive pastes (NCPs) depending on silica filler contents and diluent contents were investigated. And then, thermal cycling (T/C) reliability of flip chip assembly using selected NCPs was verified. As the silica filler content increased, thermomechanical properties of NCPs were changed. The higher the silica filler content was added, glass transition temperature (T/sub g/) and storage modulus at room temperature became higher while coefficient of thermal expansion (CTE) decreased. On the other hand, rheological properties of NCPs were significantly affected by diluent content. As the diluent content increased, viscosity of NCP decreased and thixotropic index increased. However, the addition of diluent deteriorated thermomechanical properties such as modulus, CTE, and T/sub g/. Based on these results, three candidates of NCPs with various silica filler and diluent contents were selected and used as adhesives for reliability test of flip chip assemblies. T/C reliability test was performed by measuring changes of NCP bump connection resistance. Results showed that flip chip assembly using NCP with lower CTE and higher modulus exhibited better T/C reliability behavior because of reduced shear strain in NCP adhesive layer.