Anisotropic Conductive Adhesive Film Research Articles

Size Effects of Au/Ni-Coated Polymer Particles on the Electrical Performance of Anisotropic Conductive Adhesive Films under Flexible Mechanical Conditions.

In soft electronics, anisotropic conductive adhesive films (ACFs) are the trending interconnecting approach due to their substantial softness and superior bondability to flexible substrates. However, low bonding pressure (≤1 MPa) and fine-pitch interconnections of ACFs become challenging while being extended in advanced device developments such as wafer-level packaging and three-dimensional multi-layer integrated circuit board assembly. To overcome these difficulties, we studied two types of ACFs with distinct conductive filler sizes (ACF-1: ~20 μm and ACF-2: ~5 μm). We demonstrated a low-pressure thermo-compression bonding technique and investigated the size effect of conductive particles on ACF's mechanical properties in a customized testing device, which consists of flexible printing circuits and Flex on Flex assemblies. A consistency of low interconnection resistance (<1 Ω) after mechanical stress (cycling bending test up to 600 cycles) verifies the assembly's outstanding electrical reliability and mechanical stability and thus validates the great effectiveness of the ACF bonding technique. Additionally, in numerical studies using the finite element method, we developed a generic model to disclose the size effect of Au/Ni-coated polymer fillers in ACF on device reliability under mechanical stress. For the first time, we confirmed that ACFs with smaller filler particles are more prone to coating fracture, leading to deteriorated electrical interconnections, and are more likely to peel off from substrate electrode pads resulting in electrical faults. This study provides guides for ACF design and manufacturing and would facilitate the advancement of soft wearable electronic devices.

Development of a Stretchable and Removable Electrical Interconnection Solution for Ultra-Thin Electronic Components

Currently, the solutions for interconnecting electronic components with their active face facing on substrates are based on metallic soldering. The mechanical contacts are therefore rigid. In order to enhance the reliability of the bonding, underfill is usually used to redistribute the thermo-mechanical stress created by the Coefficient of Thermal Expansion (CTE) mismatch between the silicon chip and substrate. Underfill is done with epoxy resins containing silica fillers (SiO2). As a result, the removal of components is no longer possible. Moreover, this solution is not suitable for devices integrating ultra-thin silicon components (&lt;100 μm) hybridized on flexible substrates that may be subject to deformation. This is the case, for example, of medical “patches” worn on the person and continuously solicited. Indeed, the rigid contact points are likely to break. To address this issue, we are developing a thin anisotropic conductive and stretchable adhesive film inspired by the adhesion of the gecko. Thanks to the microstructuration of its toes involving about 1 million setae, the gecko can develop a large contact surface and thus a large force of attraction by the multiplication of van der Waals interactions. In this work, this “dry adhesion” based on the principle of “contact splitting”, was implemented in order to improve the adhesion of a flexible interconnection. For this purpose, the surface of a polydimethylsiloxane (PDMS) film was structured with micrometric mushroom-shaped patterns known to be the most efficient form of contact. To this end, silicon molds with varying mushroom geometries were used to shape the PDMS (with different cap and pillar diameters) and the adhesion force microstructured films were assessed (shear and pull-off experiments). To make these films locally conductive through the thickness, a conductive composite was prepared and locally deposited in the mold. One approach we investigated, was using a screen-printing mask. This approach has been implemented and characterized using electrical tests (I-V measurements) in order to select the most suitable films to make a flexible interconnection.

Research on the related properties of ACF anisotropic conductive adhesive

Bonding strength is an important indicator to measure the life of the package in the process of Electronic Packaging. Anisotropic conductive adhesive film is a kind of package bonding. In this paper, scanning electron microscopy is used to compare the microstructure and element distribution of two different types of conductive adhesives CP6920F and HA5125PL both before and after flip-chip curing. At the same time, the 280SJ four-point probe sheet resistance tester and Bruker Icon atomic force microscope are used to measure the electrical properties of the cured conductive adhesive, and we use INSTRON5985 electronic universal material testing machine to test the shear strength and tensile strength of the cured conductive adhesive film.

Facile Synthesis of Conductive Flexible Composite Sn@Ag Microspheres and their Application in Anisotropic Conductive Films

AbstractComposite microspheres with noble metal nanoshells are advantageous in flexible circuit connections, flip chips, and chemical sensors. It is difficult for Polystyrene microspheres to cover a complete Ag layer, and hard metal microspheres have no deformability. These two reasons have contributed to this research. This report demonstrates a facile synthesis route that chemically deposits an Ag layer on the surface of 30 µm Sn microspheres to form a composite Sn@Ag double‐layer structure, which is suitable for anisotropic conductive films (ACFs) with linewidths greater than 100 µm. Scanning electron microscope analysis reveals that the surface Ag coating thickness can be controlled within the range of 737 nm–5.62 µm by varying the amount of Ag source applied, and trace free Ag particles are produced during the preparation process. An anisotropic conductive adhesive film is successfully fabricated and hot‐pressed after the film‐on‐film flexible circuit board. The microspheres form an ACF single‐layer structure after hot pressing. Hot pressing in the range of 10–20 MPa can ensure deformation and avoid Ag shell cracking, and the deformation is determined when Sn@Ag is in contact with the circuit surfaces at both ends. This approach provides a potential method of preparing low‐cost and high‐performance ACFs.

Next-Generation Self-Assembled Anisotropic Conductive Adhesive Developed Using Solder Powder for Electrical Micro-Bonding in the Emerging IT Industry

The importance of the IT industry is becoming more and more prominent as it gradually changes to a non-face-to-face society after the COVID-19, and the next-generation smart devices of 5G mobile communication, micro LEDs, and wearable devices are emerging as the leading of the IT industry. Along with the development of these devices, limitations and problems of the Anisotropic Conductive Adhesive Film (ACF) used as a connection material for Film on Film (FOF), Chip on Glass (COG), Chip on Film (COF), and Film on Glass (FOG) are emerging. [1-5]

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 3000h. 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 625h of testing and only 20% failed during the first 1000h.

Recent advances in mechanical properties of anisotropic conductive adhesive film for microelectronic packaging

Purpose– This review paper aims to provide a better understanding of formulation and processing of anisotropic conductive adhesive film (ACF) material and to summarize the significant research and development work for the mechanical properties of ACF material and joints, which helps to the development and application of ACF joints with better reliability in microelectronic packaging systems.Design/methodology/approach– The ACF material was cured at high temperature of 190°C, and the cured ACF was tested by conducting the tensile experiments with uniaxial and cyclic loads. The ACF joint was obtained with process of pre-bonding and final bonding. The impact tests and shear tests of ACF joints were completed with different aging conditions such as high temperature, thermal cycling and hygrothermal aging.Findings– The cured ACF exhibited unique time-, temperature- and loading rate-dependent behaviors and a strong memory of loading history. Prior stress cycling with higher mean stress or stress amplitude restrained the ratcheting strain in subsequent cycling with lower mean stress or stress amplitude. The impact strength and adhesive strength of ACF joints increased with increase of bonding temperature, but they decreased with increase of environment temperature. The adhesive strength and life of ACF joints decreased with hygrothermal aging, whereas increased firstly and then decreased with thermal cycling.Originality/value– This study is to review the recent investigations on the mechanical properties of ACF material and joints in microelectronic packaging applications.

Thermal cycling aging effects on the ratcheting behavior of anisotropic conductive film

Purpose – The purpose of this paper is to determine: how the thermal cycling aging affects the ratcheting behavior of anisotropic conductive adhesive film (ACF); how the loading conditions and loading history affect the ratcheting strain and strain rate of ACF with different thermal cycling aging histories. Design/methodology/approach – The ACF of CP6920F was cured at 190°C in an electro-thermal vacuum drying apparatus for 30 s. The cured specimens were put into the thermal cycling chamber (−40-150°C) for aging to 25, 50, 100, 200 and 500 cycles. A series of uniaxial ratcheting tests of aged ACF after different thermal cycles was carried out under stress control at 80°C. Findings – The ACF subjected to larger number of thermal aging cycles exhibits less ratcheting strain under the same loading conditions. The ACF with the same thermal cycling aging history shows more ratcheting strain and a higher ratcheting strain rate when loaded under a larger mean stress or stress amplitude or a lower loading rate. The ratcheting behavior of aged ACF is found to be more sensitive to the lower loading rate. The higher mean stress (or stress amplitude) enhances the deformation resistance and consequently restrains the ratcheting strain of subsequent cycling with a lower mean stress (or stress amplitude). The prior lower loading rate accelerates the plastic deformation more significantly than the higher one. Originality/value – The influencing trends of thermal cycling aging, loading condition and loading history on ratcheting behavior of ACF are obtained, which is important for the design and safety assessment of ACF joints.

Interfacial thermal stresses in ACF bonding assembly

The interfacial stresses and chip cracking stress produced because of thermal and mechanical mismatch in layered electronic assembly are one of main reasons for the failure of electronic packages. The analytical model considering the nonlocal deformation of assembly was developed and applied to predict the interfacial stresses produced due to temperature variation for the short and long anisotropic conductive adhesive film (ACF) bonding assembly. The conditions of zero shear stress at the free ends and self-equilibrated peeling stresses were satisfied. Simultaneously the interfacial stresses of ACF assembly were also predicted by the corrected Suhir’s model, Wang’s model, Ghorbani’s model, local model and finite element model (FEM), which were compared with the results by the present model. In addition, the analytical expression of chip cracking stress was also obtained for layered electronic assembly. The approach is mathematically straightforward and can be extended to include the inelastic creep behavior.

A Study on the Fatigue Behavior of Anisotropic Conductive Adhesive Film

In this paper, the low cycle fatigue (LCF) tests were performed for anisotropic conductive adhesive films (ACFs). The effect of temperature on the tensile mechanical properties of ACFs was investigated. It was found that, with the increase in temperature, the tensile strength of ACFs decreases and its fracture strain increases resulting in lower Young's modulus. The stress-control LCF experiments were carried out under tension-tension fatigue loading with a stress ratio of 0 <; R <; 1, which can be also called ratcheting fatigue experiments. The fatigue life decreased with an increase in stress amplitude, but it did not show significant change with an increase in mean stress. In addition, the final ratcheting strain showed an increase with respect to mean stress or stress amplitude. The effect of hygrothermal aging on the LCF of ACFs was also studied. In this case, the LCF of ACFs decreased initially and then showed an increase. In order to predict the LCF life, several methods (the modified Goodman, Gerber, and Soderberg) were considered to investigate which method is the most reasonable when mean stress is nonzero.

Uniaxial ratcheting behavior of hygrothermal aging anisotropic conductive adhesive film

A series of uniaxial ratcheting experiments on anisotropic conductive adhesive film (ACF) were conducted under stress-control at elevated temperature using a DMA-Q800. The ratcheting behavior of ACF specimens with different hygrothermal aging times was investigated at room temperature and 120 °C. The effects of loading rate, mean stress and stress amplitude on the ratcheting behavior of unaged and aged specimens were compared. The results show that the ratcheting strains of aged specimens are smaller than those of unaged specimens under the same experimental conditions. The cycling stability of aged specimens is increased by hygrothermal aging. At room temperature, with the increase of aging time, the ratcheting strains of aged specimens increase with hygrothermal aging time when it is less than or equal to 96 h but, however, decrease when it exceeds 96 h. At 120 °C the ratcheting strains of ACF only decrease with the increase of hygrothermal aging time. Additionally, the effects of loading rate, mean stress and stress amplitude on the ratcheting behavior of unaged and aged ACF are different and their effects are weakened by hygrothermal aging.

Reliability Testing and Modeling of Anisotropic Conductive Adhesive Joints Under Temperature Cycling Test

The use of anisotropic conductive adhesives (ACAs) in flip-chip interconnection technology has several advantages compared to solders with underfills. However, when ACAs are used with organic substrates, their reliability may cause problems. Temperature cycling testing can be used to study the effect of fluctuating temperatures on the reliability of ACA joints. However, this is fairly time consuming. If modeling can be utilized in the evaluation of reliability, significant time savings can be achieved. In this paper, the prospects of using shear stress modeling in predicting the reliability of two different anisotropic conductive adhesive films (ACFs) with an organic FR-4 substrate in temperature cycling testing are examined. Finite element models were made for the test structures. Stresses and strains during the cycling tests ae calculated. To estimate the validity of the models, experimental testing is also done. Test samples with two different ACFs are tested in a temperature cycling test where the temperatures vary from -40°C to 125 °C. Testing is carried out for 10000 cycles. In addition, failure analysis is conducted on the test samples. To model the test samples accurately, the material properties of the ACFs are tested using a dynamic mechanical analyzer and a thermomechanical analyzer. The results of the modeling are compared with those of the experimental tests by comparing typical failure locations, failure modes, and the failure order of the test samples with different ACFs. According to the modeling, the highest shear stress seems to indicate the failure location. In addition, it seems that shear stress models can be used to rank ACF flip-chip joints in the order of the anticipated failure times. On the other hand, normal stress and strain models seem to describe failure mechanisms different from that observed in the experimental tests, and they are observed to be impractical for our purposes. Delamination, which may be one manifestation of failure mechanisms related to shear stress, is observed in all samples with ACF1. However, as no delamination is found in the samples with ACF2, it cannot be positively stated whether the failure mechanism of ACF2 joints is related to shear stress or not. If the samples with different ACFs have different failure factors, simple stress or strain models seem to be impractical for reliability studies, as the results of different models cannot be easily compared.

A study of degradation of indium tin oxide thin films on glass for display applications

Indium tin oxide (ITO) has been widely used in liquid crystal displays (LCD). Contamination and moisture have proved to have the adverse effect of causing ITO corrosion/degradation. The purpose of this paper is to determine if scratching the surface of ITO is a cause of ITO degradation. Additionally, the influence of the width of ITO tracks on degradation was observed under accelerated testing. Simultaneously, anisotropic conductive adhesive film (ACF) was added to the ITO surface, to determine any adverse degradation effect. It was determined that the time to initiate corrosion was faster if the ITO was scratched, and that the degradation was directly related to the surface area of the ITO electrodes.

Creep-Recovery Behaviors of Anisotropic Conductive Adhesive Film with Temperature and Hygrothermal Aging

The effects of temperature, stress level and hygrothermal aging on the creep-recovery behaviors of anisotropic conductive adhesive film (ACF) were investigated experimentally using a dynamic mechanical analyzer (DMA). It is found that the initial strains, instantaneous strains, and creep or recovery rates increase with increasing temperature，however decrease with increasing hygrothermal aging time. The change of creep or recovery rates at low temperature is apparent, however the creep or recovery rates increase obviously at temperatures above 25 oC with increasing stress level. For the hygrothermal aged ACF, the time to reach steady creep stage or steady recovery stage is reduced significantly compared with the unaged sample. The strain jumps at instantaneous loading decrease visibly and the strain jumps at instantaneous unloading decrease slightly with increasing aging time. And the strain jumps at instantaneous loading and unloading increase with increasing stress level for the unaged and aged ACFs.

Mechanical Properties of Anisotropic Conductive Adhesive Film Under Hygrothermal Aging and Thermal Cycling

Mechanical properties of anisotropic conductive adhesive film (ACF) were investigated experimentally under various environmental conditions. The temperature sweep test was conducted to investigate the effects of temperature on dynamical mechanical properties of the ACF. The ACF exhibited transitions to the glass state, viscoelastic state, and rubber state with increasing temperature, and its glass-transition temperature (Tg) was determined to be 149°C. The creep-recovery behaviors of the ACF were investigated, and it was found that the initial strains, instantaneous strains, and creep or recovery rates increased with increasing temperature. No obvious creep phenomenon was observed at low temperatures (≤0°C). The creep strain and creep rates at any time decreased with increasing hygrothermal aging time. The uniaxial tensile behaviors of the ACF were also investigated under hygrothermal aging and thermal cycling. The results show that the Young’s modulus and tensile strength of the ACF decrease with increasing hygrothermal aging time; however, they increase at first and then decrease with increasing thermal cycling time. Tg decreases slightly for the ACF after hygrothermal aging; however, it increases after thermal cycling.

Shear strength of anisotropic conductive adhesive joints under hygrothermal aging and thermal cycling

The shear strength behaviors of Anisotropic Conductive Adhesive Film (ACF) joints were investigated experimentally with various environments. The shear tests of ACF joints were performed at different constant temperature conditions. It is found that the shear behaviors of the ACF joints are strongly dependent on testing temperature, and the shear strength decrease and the maximum displacement increases with increasing temperature. The shear strength of ACF joints was also investigated under hygrothermal aging and thermal cycling. The results show that the shear strength of ACF joints gradually decreases at first, then quickly decreases and finally the rate of decrease slows down again with increasing hygrothermal aging time. However the shear strength of ACF joints after thermal cycling increases firstly and then decreases with increasing thermal cycling time.

Uniaxial ratcheting behavior of anisotropic conductive adhesive film at elevated temperature

A series of uniaxial ratcheting experiments on anisotropic conductive adhesive films (ACFs) were conducted under stress-control at elevated temperature using DMA (DMA-Q800). The effects of mean stress, stress amplitude, applied temperature and loading history on the uniaxial ratcheting behavior of ACF were investigated. The results show that Young’s modulus of the ACF declines rapidly with increasing temperature. The ratcheting strain increases as the mean stress, stress amplitude and temperature increased. Especially, when the temperature was over 80 °C, the ratcheting strain accumulated rapidly. There are significant differences in the uniaxial ratcheting behavior of ACF at 80 °C and 120 °C. The ratcheting strain rate at 120 °C is nearly twenty times that at 80 °C. The ratcheting strain decreases with increasing stress rate. Furthermore, the loading history also plays an important role in the progress of ratcheting. Previous cycling with higher stress amplitude greatly reduces ratcheting strain of subsequent cycling at lower stresses.

Fatigue life evaluation of anisotropic conductive adhesive film joints under mechanical and hygrothermal loads

The shear fatigue lives of Anisotropic Conductive Adhesive Film (ACF) joints were evaluated experimentally and theoretically under different testing conditions. The shear fatigue tests of ACF joints were performed with different loading amplitudes. It is found that the fatigue lives of ACF joints decrease with increasing loading amplitudes and Basquin’s equation is fit to predict the fatigue lives of ACF joints. Hygrothermal aging and thermal cycling tests were conducted to investigate the shear strength and lives of ACF joints. The results show that the shear strength and lives of ACF joints decrease with increasing hygrothermal aging time, however increase firstly and then decrease with increasing thermal cycling time. The fatigue life model considering aging damage is proposed and the predictions of the fatigue life agree with the experimental results at different aging time for ACF joints.

Preparation of Composite Conductive Microspheres Used in Anisotropic Conductive Adhesive Films

Polymer-based conductive adhesive materials have become widely used in many electronic packaging interconnect applications. In this paper, a new kind of polymer-based conductive particle used in anisotropic conductive adhesive films (ACFs) is prepared. The preparation of the single scattered polystyrene spheres with the particle size of 3 microns around and the less than 5% dispersion coefficient is done successfully, and a layer of nickel is coated on the surface of polystyrene spheres using chemical plating method well.

Uniaxial ratchetting behavior of anisotropic conductive adhesive film under cyclic tension

Polymer-based conductive adhesive materials have become widely used in many electronic packaging interconnect applications. Uniaxial ratchetting behavior of anisotropic conductive adhesive film (ACF) was studied by uniaxial cyclic tension experiments. The effects of mean stress, stress amplitude and loading history on the ratchetting response of ACF material were analyzed. Results show that (1) the ratchetting strain amplitude and ratchetting strain rate of the ACF material increase with increasing stress amplitude or mean stress; (2) the ACF material has a strong memory of loading history. Prior stress cycling with higher mean stress or stress amplitude restrains the ratchetting strain in subsequent cycling with lower mean stress or stress amplitude. Prior stress cycling with higher stress rate will accelerate the plastic deformation of subsequent cycling with lower stress rate for the ACF material.