Anisotropic Conductive Film Material Research Articles

A Study on the Dynamic Bending Property of Chip-on-Flex Assembly Using Anchoring Polymer Layer Anisotropic Conductive Films

Anisotropic conductive films (ACFs) composed of polymer adhesive resin and conductive particles have been used as interconnection materials for ultrafine pitch chip-on-flex (COF) packages due to their flexibility, stable electrical properties, and bending reliability. To realize an efficient ultrafine pitch COF package, a new anchoring polymer layer (APL) was recently introduced. However, the chip flexibility in terms of ACFs materials after APL layer integration needs lot of improvement. Herein, we study the order to obtain more flexible and reliable COF packages using ACFs interconnection materials, and the effect of the ACFs material properties on the COF bending reliability was investigated. It was found that when the APL ACFs were used for the COF packages, 100% electric insulation rates were obtained due to the suppression of conductive particle movement by the APL structure compared with the conventional ACFs. Furthermore, the polyacrylonitrile (PAN) APL ACFs showed better ACFs joint properties and dynamic bending properties compared with the polyvinylidene fluoride (PVDF) APL ACFs due to its higher adhesion property caused by the chemical reaction between the PAN and the curing agent inside the epoxy polymer adhesive resin confirmed by the Fourier transform-infrared spectroscopy (FT-IR) analysis. In addition, the strain applied to the PAN APL ACFs joints during the bending test was 1.35% which was quite smaller than that of the PVDF APL ACFs joints, 2.01% resulting in better bending reliability too. For the bending reliability, there was no failure at the PAN APL ACFs joints up to 150 000 cycles, not only because of the excellent mechanical properties of PAN APL ACFs but also because of the lower strain applied to the APL ACFs joints during the dynamic bending test in COF packages.

A Study on the Magnetic Dispersion of the Conductive Particles of Anchoring-Polymer-Layer Anisotropic Conductive Films and Its Fine-Pitch Interconnection Properties

As the display resolution has been rapidly increased, the pitch between two electrodes has been continuously decreased to less than $20~\mu \text{m}$ pitch. Therefore, fine-pitch interconnection technology has become very important in display technology. In our previous research results, anchoring polymer layer (APL) structure was successfully introduced into anisotropic conductive film (ACF) system to form fine pitch interconnection by suppressing the conductive particles movement during the ACF assembly. In general, the agglomerated conductive particles between two electrodes can cause short-circuit problems at ACF assembly. In this paper, the magnetic field was applied to the Ni-coated polymer conductive particles in the polyvinyl fluoride (PVDF) APL structure to disperse the conductive particles uniformly in the xy plane. Then the effects of the magnetic fields on the dispersion of the Ni-coated conductive particles in the PVDF APL structure and the characterization of fine-pitch chip-on-glass (COG) assembly using PVDF APL ACFs with magnetically dispersed conductive particles were investigated. By optimizing the magnetic fields on the PVDF APL structure, 80% dispersed particle rate was successfully obtained. After the ACF bonding process, the conductive particles capture rates and contact resistance properties of magnetically dispersed PVDF APL ACFs, and the PVDF APL ACFs with no magnetic field applied were investigated at 20- $\mu \text{m}$ -pitch COG applications. Both PVDF APL ACFs showed a similar capture rate of conductive particles and electrical insulation property at $20~\mu \text{m}$ pitch. The PVDF APL ACFs with no magnetic field applied showed 100% of insulation property at $20~\mu \text{m}$ , but short circuits at less than pitch. However, for less than 20- $\mu \text{m}$ pitch, only the PVDF APL ACFs with magnetically dispersed conductive particles showed 100% insulation circuit rate down to the 11- $\mu \text{m}$ pitch because the conductive particles were not agglomerated but existed as single particles, resulting in no electrical short between fine-pitch adjacent bumps. As a result, magnetically dispersed PVDF APL ACFs can be used as new ACF materials for ultrafine-pitch interconnection applications without any electrical short.

Effects of Polymer Conductive Particle Contents on the Electrical Performance and Reliability of 50- $\mu \text{m}$ Pitch Flex-on-Flex Assemblies Using Anisotropic Conductive Films

Anisotropic conductive films (ACFs) composed of an adhesive polymer resin and fine conductive particles such as metal-coated polymer conductive particles are interconnect materials for fine-pitch flex-on-flex (FOF) assembly technologies due to their fine-pitch handling capability, low-temperature bonding, and stable electrical properties due to the compliance of polymer conductive particles. For the ultrafine pitch FOF assembly, the polymer conductive particle content becomes more important due to electrical storage problem between electrodes. To design better quality ACFs materials for 50-μm FOF assemblies, the effects of polymer conductive particle contents on the electrical stability and reliability of FOF ACFs joints were investigated. It was found that the bonding temperature should be less than the deformation temperature of polymer conductive particles, and proper conductive particles content should be applied to obtain the stable joint contact resistance and ACF joint morphology. When the conductive particle contents increased more than 4wt%, the gap width between the top and bottom electrodes increased and the deformation of conductive particles became less, resulting in higher contact resistance. Therefore, 4wt% of conductive particle ACFs bonded at 175 °C, 2 MPa for 10 s showed the lowest contact resistance of 36.2 mQ and excellent thermal cycle (T/C) reliability up to 1000 cycles. As a result, the optimum polymer conductive particles contents and the bonding conditions such as temperature, pressure, and times were achieved to obtain the best performance and T/C reliability of 50-μm pitch FOF assembly using ACFs.

The Effect of Anisotropic Conductive Films Adhesion on the Bending Reliability of Chip-in-Flex Packages for Wearable Electronics Applications

In this paper, the effects of adhesion properties of anisotropic conductive films (ACFs) interconnection on the chip-in-flex (CIF) bending reliability were investigated. Oxygen plasma treatment was conducted to increase the adhesion strength between ACFs and Si chip or ACFs and flexible printed circuit (FPC) substrates. In order to characterize the enhanced adhesion properties of the CIF packages, surface energy, surface roughness, elemental composition, and peel strength were measured. A digital image correlation method was used with cross-sectional scanning electron microscopy images to visualize the stress development at the ACFs interconnection. It was found that the interface of ACFs resin and FPC substrate showed the weakest adhesion, where the delamination was initiated. As a result of the improved adhesion at the ACFs resin and FPC substrate, the location of stress concentration was changed to the interface of Si chip and ACFs resin, leading to better dynamic bending reliability. When the oxygen plasma was treated both on the Si chip and FPC substrate, the stress concentration was observed not at the ACFs interfaces, but inside of the ACFs resin, resulting in further improved dynamic bending reliability. With the optimized plasma treatment condition and the ACFs materials, the dynamic bending reliability of the CIF packages was successfully demonstrated up to 160 000 bending cycles at a 7.5-mm bending radius without any electrical failures.

A Study on Flex-on-Fabric Interconnection Using Anisotropic Conductive Films and Ultrasonic Bonding Method

Flex-on-fabric (FOF) interconnections using anisotropic conductive films (ACFs) and ultrasonic bonding method were investigated for wearable devices applications. In this paper, the conduction mechanism of FOF and the effects of the conductive fabrics porosity and ACFs materials on the FOF interconnection were investigated. Two types of conductive fabrics with different porosities such as knitted and woven fabrics, and different viscosities of ACFs resin were used. For the conductive particle of ACFs, metal-coated polymer ball was used. According to the results, the conduction of FOF consists of both conductive balls and fabrics to metal electrode direct contacts. Among these two conduction paths, fabric direct contact was the major conduction path because the contact resistance of the ACFs conductive balls to metal contact is about ten times larger than that of the fabric to metal direct contact. In addition, it was found that laminated nonconductive films cover layer and the increased ACFs minimum viscosity (from 940 to 27000 Pa·s) substantially increased the peel adhesion strength twice due to the decreased resin flow through the pore of fabrics and increased FOF joint gap size. Finally, both conductive fabrics showed no electrical failure after 500 h 85°/85% relative humidity reliability test. However, among two types of fabric, knitted fabric had better humidity reliability performance than the woven fabric presumably due to its better compliance.

Nanofiber Anisotropic Conductive Films (ACF) for Ultra-Fine-Pitch Chip-on-Glass (COG) Interconnections

Nanofiber anisotropic conductive films (ACF) were invented, by adapting nanofiber technology to ACF materials, to overcome the limitations of ultra-fine-pitch interconnection packaging, i.e. shorts and open circuits as a result of the narrow space between bumps and electrodes. For nanofiber ACF, poly(vinylidene fluoride) (PVDF) and poly(butylene succinate) (PBS) polymers were used as nanofiber polymer materials. For PVDF and PBS nanofiber ACF, conductive particles of diameter 3.5 μm were incorporated into nanofibers by electrospinning. In ultra-fine-pitch chip-on-glass assembly, insulation was significantly improved by using nanofiber ACF, because nanofibers inside the ACF suppressed the mobility of conductive particles, preventing them from flowing out during the bonding process. Capture of conductive particles was increased from 31% (conventional ACF) to 65%, and stable electrical properties and reliability were achieved by use of nanofiber ACF.

Optimal Yield Rate in ACF Cutting Process of TFT-LCD Module Using Orthogonal Particle Swarm Optimization Based on Response Surface Design

Anisotropic Conductive Film (ACF) is essential material in LCM (Liquid Crystal Module) process. It is used in bondingprocess to make the driving circuit conductive. Because the price of TFT-LCD is getting lower than before in recentyears, the ACF has relatively higher cost ratio. The conventional long bar ACF cutting unit is changed into short barACF cutting unit in new bonding technology. However, the new type machine was not optimized in process controland mechanical design. Therefore, the failure rate of new ACF cutting process is much higher than the one of theconventional process. This wastes the ACF material and rework cost is considerably large. How to make themanufacturing cost down effectively and promote the product quality is the main issue to maintain competitioncapability for the product. Therefore, the orthogonal particle swarm optimization (OPSO) is used to analyze theoptimal design problem. The ACF cutting yield rate is selected to be objective function for optimization. The qualitycharacteristic function for yield rate is used in orthogonal particle swarm optimization. Three control factors such asplasma clean speed, ACF peeling speed and ACF cutter spring setting are selected to study the effect of the yieldrate. Results show that the proposed method can provide good optimal solution to improve the ACF cutting processfor TFTLCD manufacturing process.

Effects of ACF Bonding Parameters on ACF Joint Characteristics for High-Speed Bonding Using Ultrasonic Bonding Method

As the use of anisotropic conductive films (ACFs) in flex-on-board bonding has increased recently, the need for high-speed bonding for higher productivity has developed. For high-speed bonding, a fast-curable acrylic resin has been used instead of an epoxy resin, and ultrasonic (US) bonding is introduced to allow for short bonding time at high bonding temperatures. However, it is possible for fast curable acrylic ACFs at 250°C to be fully cured before the resins flow out sufficiently between the electrodes. In such cases, the ACF joints could show poor electrical characteristics due to there being no conductive particle capture. Therefore, in this paper, in order to understand important factors for high-speed bonding, the effects of the bonding parameters, substrate geometry, and ACF material property on the ACF joint characteristics, such as resin flow and joint resistance, are investigated using acrylic-based ACFs and US bonding. The fast-curable acrylic resin is fully cured at a bonding temperature of 250°C within 1 s. However, the ACF joints bonded at 250°C show joint gaps larger than the diameter of conductive particles, i.e., 8 μm, due to insufficient resin flow and open failure due to no particle capture. As the bonding pressure increases from 2 to 4 MPa, the ACF joint gap at 250°C is smaller than the conductive particle size, and the joint resistance reaches about 20 mΩ within 7 s when using 3-mm-long electrodes and a high-viscosity resin. However, in order to use a shorter bonding time and the common bonding pressure of 3 MPa, the effects of substrate geometry and resin property on ACF joint characteristics are investigated. As the electrode length of the substrates are decreased from 3 to 1 mm, the ACF joints show stable ACF joint gaps and joint resistance at 3 MPa despite the bonding temperature of 250°C applied within 1 s. In terms of the ACF resin properties, when a resin with low minimum viscosity is used, the ACF joint gaps are smaller than the diameter of the conductive particles. Furthermore, the joint resistance of low-minimum-viscosity ACF is more stable than that of the ACF joints bonded with the high-minimum-viscosity resin at 250°C within 1 s, even though a 3-mm-long electrode is used. These results indicate that the substrate geometry of the printed circuit board and the ACF resin viscosity are significant factors for high-speed bonding to obtain stable ACF joints at a high bonding temperature of 250°C within 1 s.

Material Properties and Reliability of Anisotropic Conductive Films (ACFs) Modified by POSS for Chip-on-Glass Assembly

This paper describes the effect of polyhedral oligomeric silsesquioxane (POSS) in anisotropic conductive films (ACFs) composition on the material properties and reliability of ACF interconnections for chip-on-glass (COG) applications. Series of POSS modified ACFs containing 0wt%, 2wt%, 4wt%, 6wt% and 8wt% contents of octa-methacrylated POSS were formulated and characterized. The POSS added in the formulations favorably affected on the materials properties of ACFs by lowering coefficient of thermal expansion (CTE) and increasing modulus (E′). In addition, an increase in the POSS contents led to higher curing density and, as a result, thermo-mechanical properties of ACFs were improved. But, the adhesion strength of ACF materials was decreased with the POSS content, due to higher modulus originated from octa-functional POSS. The reliability performances of COG assemblies using POSS modified ACFs were also investigated. In conclusion, the materials properties and module reliability of COG assemblies were improved with increasing POSS content in ACFs.

Multi-objective optimization of yield rate and tact time of ACF in the TFT-LCD short-bar attach process using the fuzzy dominant directed graph method

This paper proposes a fuzzy-DDG method to solve the multi-objective optimization problem in regard to the anisotropic conductive film (ACF) attach process for thin-film transistor liquid crystal display (TFT-LCD). As such, this paper has been motivated by a desire to decrease the usage of ACF materials in the ACF attach process. Herein, the experimental Taguchi method will be used to find the optimal solution for this problem. This paper will study the optimization problem of yield rate and tact time with respect to control factors of TFT-LCD. In using the Taguchi method, five test cases are discussed: the first involves the optimal yield rate problem; the second, the optimal tact time problem; and the third, solving multiple problems by simultaneously considering yield rate and tact time. Additionally, the conventional multiple performance characteristics index (MPCI) method is used, although it exhibits a less than optimal fuzzy inference structure, and fuzzification as well as defuzzification processes are required; in the fourth and the fifth cases, a new fuzzy DDG method is used to determine the multiple objective problems. Notably, the conventional fuzzy inference structure is not required in this algorithm and only graph-based matrix operations are required. Testing results show that the fourth and the fifth cases are more convenient in regard to arithmetical operations, compared to the third case. The fourth case illustrates the problem with the vertex order included only; only crisp potential values are derived. The fifth case illustrates the problem with the vertex S/N ratio values included and fuzzy potential values derived. Experimental results are provided to verify the validity of the proposed fuzzy DDG method.

Flexible Chip-on-Flex (COF) and embedded Chip-in-Flex (CIF) packages by applying wafer level package (WLP) technology using anisotropic conductive films (ACFs)

Due to increasing demand for higher performance, greater flexibility, smaller size, and lighter weight in electronic devices, extensive studies on flexible electronic packages have been carried out. However, there has been little research on flexible packages by wafer level package (WLP) technology using anisotropic conductive films (ACFs) and flex substrates, an innovative packaging technology that requires fewer process steps and lower process temperature, and also provides flexible packages. This study demonstrated and evaluated the reliability of flexible packages that consisted of a flexible Chip-on-Flex (COF) assembly and embedded Chip-in-Flex (CIF) packages by applying a WLP process. The WLP process was successfully performed for the cases of void-free ACF lamination on a 50 μm thin wafer, wafer dicing without ACF delamination, and a flip-chip assembly which showed stable bump contact resistances. The fabricated COF assembly was more flexible than the conventional COF whose chip thickness is about 700 μm. To evaluate the flexibility of the COF assembly, a static bending test was performed under different bending radiuses: 35 mm, 30 mm, 25 mm, and 20 mm. Adopting optimized bonding processes of COF assembly and Flex-on-Flex (FOF) assembly, CIF packages were then successfully fabricated. The reliability of the CIF packages was evaluated via a high temperature/humidity test (85 °C/85% RH) and high temperature storage test (HTST). From the reliability test results, the CIF packages showed excellent 85 °C/85% RH reliability. Furthermore, guideline of ACF material property was suggested by Finite Element Analysis (FEA) for better HTST reliability.

Effects of Ultrasonic Power on Bonding Strength of Anisotropic Conductive Film Joint in Chip-on-Glass Assembly

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

Effects of ultrasonic bonding process on polymer-based anisotropic conductive film joints in chip-on-glass assemblies

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

ACF particle distribution in COG process

The Chip on Glass (COG) process, which bonds the driver IC onto a glass substrate via anisotropic conductive film (ACF), is applied in producing a liquid crystal display (LCD) module package. Both the stability of the ACF conductive particle conductive property and the prevention of short connections are important directions for the development of ACF material and fine pitch COG process. Better connection reliability can be achieved if more conductive particles remain on the bump with particles rarely clustered in the space between the bumps. Several types of driver ICs with different bump area ratios (total input bump area/total output bump area, I/ O ratio) and length/width ( L/ W) ratios are designed in this study to investigate the correlation between IC structure and these characteristics. The results show that the bump design influences the ACF adhesive flow causing varied capture rate effects on the bump and particle density in the space. The results provide guidance in bump design for driver ICs in the COG process.

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 ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</i> ) 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 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> 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.

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.

Contraction Stresses Development of Anisotropic Conductive Films (ACFs) Flip Chip Interconnection: Prediction and Measurement

In this study, the buildup of contraction stresses and the effect of anisotropic conductive film (ACF) properties on contraction stresses were investigated. Both ACF thickness shrinkage and modulus change of four kinds of ACFs with different thermomechanical properties were experimentally investigated using thermomechanical and dynamic mechanical analysis. Based on an incremental approach to linear elasticity, the contraction stresses of ACFs developed along the thickness direction were numerically predicted. In addition, the contraction stresses of ACFs were developed during the cooling process from the glass transition temperature of ACFs to room temperature. The buildup of contraction stresses below T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> was strongly dependent on both the coefficient of thermal expansion (CTE) and elastic modulus (E) of ACFs. A nanoscale deformation field of thin ACF layers was obtained to measure the contraction stresses experimentally using a phase shifting moire technique. Good agreement between the contraction stresses predicted from the incremental approach and the actual vertical stresses measured from a phase shifting moire analysis was obtained. Therefore, the full temperature-evolution of contraction stresses based on the incremental approach to linear elasticity is reliable and thereby can be used to predict the contraction stress behavior of polymeric ACF materials

Study of Some Parameter Effects on Warpage and Bump-Joint Stresses of COG Packages

The purpose of this paper is to investigate the parameters such as thermal loads, adhesive material properties and fillets, bump materials, and moisture, affecting the warpage and bump-joint stresses of the chip-on-glass (COG) packages with anisotropic conductive film (ACF). Two- and three-dimensional finite-element models (FEMs) are employed for calculating the warpage and bump- joint stresses in terms of such parameters. In addition, the real-time full-field Twyman-Green interferometry is used for measuring the out-of-plane deformations of the specimens and providing validation of the warpage calculated from finite-element models. Also, Suhir's close-form solution of a die-substrate assembly is applied for looking into insight of thermomechanics, especially for adhesive stresses and correlated with finite-element results. The results show that the fillets strongly affect the warpage of the COG specimens by changing the convex shape of warpage for those without fillets to the saddle shape for those with fillets. However, unlike those with fillets, the warpage of the COG is insensitive to those with or without (gold or polymer) bumps. It is also found that the two-dimensional (2-D) FEM in consistence with Suhir's solution can be used for approximately addressing both warpage and bump-joint stress issues. The COG warpage results for thermal loads, from 2-D FEM, indicate that the maximum warpage is very sensitive to the modulus of the ACFs (especially for low modulus) rather than the thermal expansion coefficients of the ACFs, which are consistent with Suhir's solution. The stress results show that the COG with gold bumps provides higher bump-joint stresses (especially for compressive transverse normal stress, which is important for maintaining good electrical contact between bumps and tracks through the conductive particles in the ACF) than that with polymer bumps for the isothermal loaded case. Note that negative isothermal load here applied represents the cooling temperature change from an uniform elevated temperature of the ACF curing in the COG packaging. Furthermore, the transverse normal bump-joint stress increases three fold, when the fillets are present for the COG either with polymer bumps or with gold bumps. The similar mechanism is found in the case of the COGs under thermal-gradient loads. It is also seen that the higher the elastic modulus (E) and thermal expansion coefficient (CTE) of the ACF, the larger the compressive transverse normal bump-joint stress. It is also interestingly found that the volume swelling of the ACF due to moisture absorption provides very relatively large tensile transverse normal bump-joint stresses for the COGs with bumps. These tensile stresses could be one order of magnitude larger than the compressive values resulted from other parameters discussed here. As a result, the approaches to well controlling ACF moisture absorption and selecting low-moisture-expansion ACF materials are very critical to improving the reliability of the COG packages

RLC effects in fine pitch anisotropic conductive film connections

PurposeThe resistance, capacitance and inductance of anisotropic conductive film (ACF) connections determine their high frequency electrical characteristics. The presence of capacitance and inductance in the ACF joint contributes to time delays and cross‐talk noise as well as simultaneous switching noise within the circuit. The purpose of this paper is to establish an experimental method for estimating the capacitance and inductance of a typical ACF connection. This can help to provide a more detailed understanding of the high frequency performance of ACF assemblies.Design/methodology/approachExperiments on the transient response of an ACF joint were performed using a digital oscilloscope capable of achieving the required ns resolution. An equivalent circuit model is proposed in order to quantify the capacitance (C) and inductance (L) of a typical ACF connection and this model is fitted to the experimental data. The full model consisted of two resistors, an inductor, and a capacitor.FindingsThe capacitance and inductance of a typical ACF connection were estimated from the measured transient response using Kirchhoff's voltage law. The method for estimation of R, L, and C from the transient response is discussed, as are the RLC effects on the high frequency electrical characteristics of the ACF connection.Research limitations/implicationsThere was decay time deviation between the calculation and the experiment. It may have resulted from the skin effect in the high frequency response and the adhesive surrounding joint as well. The main reason may be the capacitance zctric lost. Further research work will be done to determine more accurately the dielectric losses in anisotropic conductive adhesive (ACA) joint.Originality/valueThis paper presents a new method to characterise the high frequency properties of ACA interconnections and will be of use to engineers evaluating the performance of ACF materials in high frequency applications.