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.     

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.     

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.     

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.     

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 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.     

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 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 .     

A continuous contact resistance monitoring during the temperature ramp of anisotropic conductive adhesive film joint

In this study, a systematic experimental work was performed to evaluate the reliability of the anisotropic conductive adhesive film (ACF) joint at high temperature for flip-chip-on-flex (FCOF) assemblies. A four-point probe method was developed to measure the contact resistance at high temperature. Measurement was also conducted along the length of the chip. The correlation between the increased resistance and the failure mechanism was investigated using scanning electron microscopy (SEM). Initially, the contact resistance increased linearly with rising temperature, but later, it increased abruptly. This changeover was related to the glass-transition temperature (T_g) of the ACF matrix. The coefficient of thermal expansion (CTE) is very high at temperatures above T_g; thus, the ACF swells too much, reducing the mechanical contact of the particles with the bump and/or pad. Again, as the adhesive strength becomes weaker at temperatures above the glass transition, it is unable to resist the thermal stress of the flex. The cumulative thermal stress at the edges dislodges the particles from the interconnection. Even below T_g, the thermal stress at the edges is higher than the middle point. Thus, the contact resistance varied from the middle joint of the chip to that of the corner at the same high temperature. To reduce the contact resistance at the corner joint of the FCOF packages bonded by ACF, a square-shaped chip instead of a chip with a higher aspect ratio should be used. It was also suggested to use an adhesive with a higher glass-transition temperature and lower CTE.     

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     

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.     

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.     

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.     

In process stress analysis of flip-chip assemblies during underfill cure

Low cost flip chip on board assemblies are analyzed during the underfill cure process to determine residual stress generation. In situ stress measurements are performed over the active face of the die during processing and relative in-plane stresses are measured. Experimental measurements are made using flip-chip test vehicles, based on the Sandia National Laboratories’ ATC04 assembly test chip. Four different commercial underfill materials have been evaluated and a relative comparison is presented with respect to the residual stresses produced by each underfill on the flip-chip assemblies. Significant stress variations are observed between the four underfills studied. Correlation between the glass transition temperature (T_g) and storage modulus (G^′) are made relative to residual stresses produced during underfill cure. Stress relaxation characteristics are also evaluated for the low cost flip-chip assemblies.     

Low-Temperature Curable Photo-Active Anisotropic Conductive Films (PA-ACFs)

Photo-active anisotropic conductive films (PA-ACFs) with curing temperatures below 120°C were introduced using photo-active curing agents. The PA-ACFs showed no curing before UV activation, and the crosslinking systems of the PA-ACFs were not activated under fluorescent light exposure. However, after UV activation, the PA-ACFs were completely cured at 120°C within 10 s. Flex-on-board (FOB) assembly using PA-ACFs had adhesion strength and joint resistances similar to those of the FOB assemblies using conventional epoxy-based ACFs. This study demonstrates that PA-ACFs provide reliable interconnection and minimal thermal deformation among all the commercially available ACFs, especially for low T _g substrate applications.     

Correlation of flip chip underfill process parameters and materialproperties with in-process stress generation

Electronic packaging designs are moving toward fewer levels of packaging to enable miniaturization and to increase performance of electronic products. One such package design is flip chip on board (FCOB). In this method, the chip is attached face down directly to a printed wiring board (PWB). Since the package is comprised of dissimilar materials, the mechanical integrity of the flip chip during assembly and operation becomes an issue due to the coefficient of thermal expansion (CTE) mismatch between the chip, PWB, and interconnect materials. To overcome this problem, a rigid encapsulant (underfill) is introduced between the chip and the substrate. This reduces the effective CTE mismatch and reduces the effective stresses experienced by the solder interconnects. The presence of the underfill significantly improves long term reliability. The underfill material, however, does introduce a high level of mechanical stress in the silicon die. The stress in the assembly is a function of the assembly process, the underfill material, and the underfill cure process. Therefore, selection and processing of underfill material is critical to achieving the desired performance and reliability. The effect of underfill material on the mechanical stress induced in a flip chip assembly during cure was presented in previous publications. This paper studies the effect of the cure parameters on a selected commercial underfill and correlates these properties with the stress induced in flip chip assemblies during processing     

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.     

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.     

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.     

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.