Anisotropic Conductive Film Research Articles

Rapid fabrication of gold microsphere arrays with stable deep-pressing anisotropic conductivity for advanced packaging

Smooth metal microspheres with uniform sizes are ideal for constructing particle-arrayed anisotropic conductive films (ACF), but synthesis is hindered by challenges in controlling anisotropic metal growth. Here, we present a positioned transient-emulsion self-assembly and laser-irradiation strategy to fabricate pure gold microsphere arrays with smooth surfaces and uniform sizes. The fabrication involves assembling gold nanoparticles into uniform colloidosomes within a pre-designed microhole array, followed by rapid transformation into well-defined microspheres through laser heating. The gold nanoparticles melt and merge in a layer-by-layer manner due to the finite skin depth of the laser, leading to a localized photothermal effect. This strategy circumvents anisotropic growth, enables tunable control of microsphere size and positioning, and is compatible with conventional lithography. Importantly, these pure gold microspheres exhibit stable conductivity under deep compression, offering promising applications in soldering micro-sized chips onto integrated circuits.

Stretchable Anisotropic Conductive Film with Position‐Registered Conductive Microparticles Used for Strain‐Insensitive Ionic Interfacing in Stretchable Ionic Sensors

AbstractNumerous approaches are explored to achieve precise position registry of microparticles (MPs) with minimal defects; however, MP assembly in a periodic pattern or an arbitrary manner has been a challenging issue over the past several decades. Utilizing the position‐registered conductive MPs, polymer composites of the MPs are used as anisotropic conductive film (ACF) and soft interfacing. One of the remaining challenges is maintaining the MP positions while producing or utilizing the ACF. This study proposes a simple strategy to produce a stretchable ACF (S‐ACF) by mechanical rubbing, without disturbing the MP positions during the production and use. A practical means of precise MP positioning on a ultraviolet (UV)‐patternable soft template is investigated first. This study investigates, through both experiment and finite element method calculation, the relationship between local adhesion of the template and mechanical rubbing variables (pressure, rubbing velocity, and MP size). Based on this exploration, a fast and simple method to fabricate large‐area S‐ACF is presented. This study demonstrates that the S‐ACF can be used for electronic interfacing in stretchable devices and for ionic interfacing to remove the effect of external mechanical force in ionic sensors.

Anisotropic conductive film (ACF) step-by-step line ablation using NIR, green, and DUV wavelengths femtosecond laser for Micro-LED display repair

Anisotropic conductive film (ACF) step-by-step line ablation using NIR, green, and DUV wavelengths femtosecond laser for Micro-LED display repair

Effect of the solder conductive particles and substrate widths on the current carrying capability for flex-on-board (FOB) assembly

PurposeThe study aims to ascertain the influence of solder conductive particle types and substrate widths on the current carrying capability of flex-on-board (FOB) assemblies. By comparing Sn58Bi and SAC305 particles and varying substrate widths, the research sought to provide insights into the stability and performance of solder joints under different scenarios, particularly in high-power applications.Design/methodology/approachThe study used a comprehensive design/methodology, encompassing the investigation of solder conductive particle types (Sn58Bi and SAC305) and substrate widths on the current carrying capability of FOB assembly. Stable solder joints were obtained by manipulating the curing speed of anisotropic conductive films for both particle types. Various tests were conducted, including current carrying capability assessments under differing conditions.FindingsThe study revealed that larger substrate widths yielded higher current carrying capability due to increased contact area and reduced contact resistance. Notably, solder joints remained stable beyond the solder melting temperature due to encapsulation by cured epoxy resin. SAC305 solder joints exhibited superior current carrying capability over Sn58Bi in continuous high-voltage conditions. The results emphasized the stability of SAC305 solder joints and their suitability for robust interconnections in high-power FOB assemblies.Originality/valueThis study contributes by offering a comprehensive assessment of the impact of solder particle types and substrate widths on solder joint performance in FOB assemblies. The finding that SAC305 joints outperform Sn58Bi under continuous high-voltage conditions adds significant value. Moreover, the observation of stable solder joints beyond solder melting temperature due to resin encapsulation introduces a novel aspect to solder joint reliability. These insights provide valuable guidance for designing robust and high-performance interconnections in demanding applications.

An Optimal Design Method for Lightweight Heating Film of Anisotropic Heat Conduction Substrate Based on Surrogate Model.

In space missions, heating films are crucial for uniformly heating onboard equipment for precise temperature control. This study develops an optimization method using surrogate models for lightweight anisotropic substrate thermal conductive heating films, meeting the requirements of uniform heating in thermal control for space applications. A feedforward neural network optimized by particle swarm optimization (PSO) was employed to create a surrogate model, mapping design parameters to the temperature uniformity of the heating film. This model served as the basis for applying the NSGA-II algorithm to quickly optimize both temperature uniformity and lightweight characteristics. In this study, the PSO-BP surrogate model was trained using heating film thermal simulation data, and the surrogate model demonstrated an accurate prediction of the mean square error (MSE) of the predicted temperature difference within 0.0168 s. The maximum temperature difference in the optimal model is 1.188 ℃, which is 30.5 times lower than before optimization, and the equivalent density is only increased by 3.9%. In summary, this optimization design method effectively captures the relationships among various parameters and optimization objectives. Its superior computational accuracy and design efficiency offer significant advantages in the design of devices such as heating films.

The preparation of polystyrene/nickel core-shell particles for anisotropic conductive films (ACFs)

The use of core-shell conductive microspheres as conductive fillers for anisotropic conductive films (ACFs) has broad application prospects for high-precision circuit connections. However, existing preparation methods suffer from environmental pollution and a relatively long processing procedure. Therefore, we propose a bio-inspired, simple, and environmentally friendly method for preparing PS/Ni core-shell conductive particles suitable for ACFs. Specifically, this study utilized the oxidative self-polymerization of dopamine (DA) to modify the surface of polystyrene (PS) microspheres. The catechol groups in polydopamine (PDA) act as both reducing agents and anchor groups for palladium (Pd) atoms to catalyze the electroless nickel plating. Results indicated that the amount of (PDA) on the PS surface and the morphology of the formed nickel-phosphor (NiP) layer were regulated by adjusting the DA polymerization time and concentrations. The prepared PS-PDA/Ni conducting particles with an average diameter of 2 ± 0.2 μm has a low density (1.5 g/cm3) and can be applied to ACFs. The application of the prepared ACFs was demonstrated by bonding two flexible print circuits (FPC) with a line spacing of 200 μm. Good adhesion, conductivity, and anisotropic properties were observed in the bonded FPC-200s. This study provides useful information for producing suitable conductive particles with easy synthesis for ACFs applications.

Photoresponsive Janus microfibers array film with tunable electrical anisotropy

A new kind of photoresponsive film is produced by integrating photoconductive material into the type-II anisotropic conductive film. This film consists of orderly arranged Janus microfibers, with each Janus microfiber is comprised of a conductive region and an insulating region. Copper phthalocyanine, as the photoconductive material, is introduced into the conductive regions of these Janus microfibers, and thus the electrical conduction of the conductive regions can be enhanced by applying appropriate external light irradiation, while the insulating regions are basically unaffected. Based on this property, the electrical conduction along with the length direction of the Janus microfibers in the film is adjustable by light, the perpendicular direction along the film surface remains constantly insulating, and thus the electrical anisotropy of the resulting Janus microfibers array film can be tuned under light irradiation. The products have potential applications in flexible electronic devices and control systems.

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.

Experiments of Failure and Damage in ITO-Coated PC/FPC with ACF Bonding due to Bending Fatigue

Anisotropic conductive film (ACF) is frequently used in the packaging manufacture for fine-pitch conductivity and interconnection, maintaining the electrical and mechanical connections between micro-electrodes. A key determinant of good conductivity is the deformation, fatigue, and breakage of conductive particles within the ACF packaging. This study aims to measure the resistance changes of specific conductive channels and observe the microscopic fatigue damage of compressed ACF conductive particles through the fabrication of Flex Printed Circuits (FPC) / Indium Tin Oxide-coated Polycarbonate (ITO-coated PC) specimens and the setup of bending experiments. The results show that the deformation, fatigue, and breakage of conductive particles will quantitatively affect electrical conductivity performance. By microscopically observing the breakage morphology of conductive particles before and after bending, it can be found that bending in the ACF packaging area further exacerbates the previously compressed and broken conductive particles, with cracks continuing to grow and shatter.

Assembly of Ultra-thin MEMS Device on Driver Chip Using Anisotropic Conductive Film

Assembly solution for large-area, ultra-thin, MEMS-based transducer array dies on driver chips has been investigated using dummy Si dies and substrates. The dummy dies have a thickness of 50 μm, a bonding area on the order of 100 mm2 populated with more than a thousand through-silicon vias connecting corresponding metal pads and redistribution layer (RDL) on both sides. The dummy substrates have conventional thickness as well as necessary RDL and pads for bonding and monitoring electrical resistance and stray capacitance of interconnects. Flip-chip interconnection technology based on anisotropic conductive film (ACF) was selected for bonding and characterization. Handling and ACF bonding of ultra-thin dies to rigid substrates were eased by means of glass carriers attached to the inactive side of the dies. Proper bonding parameters were identified, providing sufficiently low interconnect resistance with satisfactory yield. Stray capacitance of interconnects was found in acceptable range, though lower stray capacitance should be further pursued. The results from this work have demonstrated the feasibility of using ACF for assembling large-area, ultra-thin dies on rigid substrates.

Understanding the Contact Resistance in an ACF Bonding

Anisotropic Conductive Film (ACF) bonding, the technique of choice for display interconnects, allows very fine pitch interconnect at a moderate process temperature, typically at the expense of a non-negligible interconnect resistance. Modern ACFs have conducting particles in a thin layer of highviscosity adhesive. This allows even finer pitch but comes at the risk of high interconnect resistance due to trapped adhesive between pad and conductive particle. Conductive particles with spikes are designed to penetrate such a trapped adhesive layer. In this paper, we compare spiky conductive particles with the traditional, spherical, ones. We measure interconnect resistance of individual particles in a nano-indentation test setup, as well as the interconnect resistance of ACF bonding using the two different particles, in daisy-chain measurements. Interconnect resistance of individual spiky particles is more than an order of magnitude higher than the one for spherical particles. However, when mixed in an adhesive and used for ACF bonding, the interconnect resistance of ACF with spiky particles is somewhat lower than that of the ACF with spherical particles. The interconnect resistance of ACF with spherical particles is two orders of magnitude higher than that of the individual particles, whereas the interconnect resistance of ACF with spiky particles is comparable to the individual-particle resistance. We conclude that trapped adhesive is a major concern for fine-pitch ACF bonding, and that the use of spiky conductive particles has the potential to overcome these challenges.

Anisotropic Solder Paste (ASP) Material Solution for Laser Assisted Bonding (LAB) Process

Anisotropic conductive paste (ACP) or anisotropic conductive film (ACF) consisting of a polymer matrix and a metal-coated polymer bead are introduced as interconnection materials in mini-LED display panel packaging through a thermal compression bonding process. However, this packaging solution has been leading to important problems such as alignment difficulties due to high bonding temperature and pressure, and its repair issues for bad LED devices after the bonding process. Recently, laser-assisted bonding (LAB) process for mini-LED display packaging has been introduced to solve these problems[Ref. 1-2]. This study introduces, an anisotropic solder paste (ASP) using conductive solder particles and applies it to the mini-LED packaging through the LAB process.

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.

Ablation morphology and characteristic analysis of anisotropic conductive film (ACF) using femtosecond lasers with NIR, Green, and DUV wavelengths for micro-LED display repair

Ablation morphology and characteristic analysis of anisotropic conductive film (ACF) using femtosecond lasers with NIR, Green, and DUV wavelengths for micro-LED display repair

Response of an anisotropic conductive film to shear forces and temperature stress in a miniature package

Response of an anisotropic conductive film to shear forces and temperature stress in a miniature package

Capillary Self‐Assembly Register Microspheres to Fabricate Anisotropic Conductive Film Used for Ultra‐Fine Pitch Stable Electrical Interfacing Interconnection

AbstractThe trend to integrate more electronic components in a limited space range motivates the development of advanced electronic packaging. Conventional electronic packaging is difficult to perform in stable interfacing interconnection pitch below 7 µm due to the possibility of short‐circuit problems. Herein, a kind of anisotropic conductive film (μ‐ACF) containing periodically arranged conductive microspheres for microscale pitch electrical interfacing interconnection is developed. The periodic arrangement can avert the contact of conductive microspheres, thus enabling stable interconnection with a pitch as small as 5 µm. By coating conductive microspheres with a layer of silicon insulation, the pitch can be further reduced to 3 µm, which can prevent the formation of conductive pathways between electrodes even if they come into contact with each other. Such high‐quality arrangement is achieved by lithography and capillary self‐assembly method, which is expected to be used in high‐throughput production. Thanks to the delicate design, the μ‐ACF can achieve a low contact resistance of 4.62 mΩ mm−2 and a spatial resolution of 3 µm without short‐circuit failure. The spatial resolution can be further improved by adjusting the size of conductive microspheres, which is conducive to the development of highly integrated devices.

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.

Mechanical stability and insulation reliability of polymeric layers constructed by thermal fusion of polymer shell particles heterocoagulated on conductive nickel‐plated core particles

AbstractIn this study, the mechanical stability and insulating reliability of polymeric layers constructed on the surface of nickel‐plated particles for anisotropic conductive films were measured by means of a microcompression test. The polymeric layers were constructed by heterocoagulation between the nickel‐plated core particles and the polymeric shell particles and successive thermal fusion. Uniform core‐shell particles with a coverage of approximately 60%–70% were prepared at NaCl concentrations near the critical coagulation concentrations of the shell particles. Continuous polymeric layers were successfully formed by heating the heterocoagulates at a temperature slightly higher than the glass transition temperature of the shell particles. The microcompression test was applied to measure the compression displacement until the heterocoagulates were electrically connected. The larger the shell particles and the higher the coverage of heterocoagualtes, the greater the displacement, reaching a maximum displacement of 0.72 μm for heterocoagulates with shell particles of 217 nm before heating. Heating the heterocoagulates and thermally melting adjacent shell particles could further improve the insulating properties of polymeric layer. Thermal melting of shell particles of heterocoagulates with a coverage more than 60% by 152 or 217 nm shells resulted in the disappearance of zero‐point conductive particles.

Preview the Products on Display at Display Week 2023

Preview the Products on Display at Display Week 2023

Development of novel low-mass module concepts based on MALTA monolithic pixel sensors

The MALTA CMOS monolithic silicon pixel sensors has been developed in the Tower 180 nm CMOS imaging process. It includes an asynchronous readout scheme and complies with the ATLAS inner tracker requirements for the HL-LHC. Several 4-chip MALTA modules have been built using Al wedge wire bonding to demonstrate the direct transfer of data from chip-to-chip and to read out the data of the entire module via one chip only. Novel technologies such as Anisotropic Conductive Films (ACF) and nanowires have been investigated to build a compact module. A lightweight flex with 17 μm trace spacing has been designed, allowing compact packaging with a direct attachment of the chip connection pads to the flex using these interconnection technologies. This contribution shows the current state of our work towards a flexible, low material, dense and reliable packaging and modularization of pixel detectors.