Anisotropic Conductive Adhesive Research Articles

Enabling Low Pressure, Low Temperature, and Particle Control for Anisotropic Conductive Adhesives

AbstractAnisotropic conductive adhesives (ACAs) are the preferred interconnection technology for applications that employ large dies, flexible substrates, and ultra fine‐pitch interconnects. Conventional ACAs require relatively high bonding pressures and temperatures, and ultra fine‐pitch applications challenge the trade‐off between low interconnect resistance and risk of short circuits. This study introduces an ACA‐like interconnection technology that addresses these limitations, allowing for low‐pressure, low‐temperature assembly processes with enhanced particle control at the interconnects. Conductive particles are deposited onto a patterned carrier and subsequently transferred to electrical pads using either non‐conductive film or Ag sintering. The Ag sintering process is performed at a low temperature (140 °C) and low bonding pressure (1 N for a 10 × 10 mm2 chip). The capability of controlling the position and number of conductive particles within individual interconnects is demonstrated. This presents possibilities for achieving ultra‐fine pitch interconnects with negligible risk of short circuits, without compromising electrical resistance. It is demonstrated that interconnect resistance can be tuned by varying the number of conductive particles (achieving a resistance as low as 33 mΩ with 25 particles per interconnect). This approach is applicable to scenarios where the bonding force and temperature must remain low due to the nature of substrate materials.

Pixel detector hybridisation and integration with anisotropic conductive adhesives

A reliable and cost-effective interconnect technology is required for the development of hybrid pixel detectors. The interconnect technology needs to be adapted for the pitch and die sizes of the respective applications. This contribution presents recent results of a newly developed in-house single-die interconnection process based on Anisotropic Conductive Adhesives (ACA). The ACA interconnect technology replaces solder bumps with conductive micro-particles embedded in an epoxy layer applied as either film or paste. The electro-mechanical connection between the sensor and ASIC is achieved via thermocompression of the ACA using a flip-chip device bonder. The ACA technology can also be used for ASIC-PCB/FPC integration, replacing wire bonding or large-pitch solder bumping techniques. A specific pixel-pad topology is required to enable the connection via micro-particles and create cavities into which excess epoxy can flow. This pixel-pad topology is achieved with an in-house Electroless Nickel Immersion Gold (ENIG) process. The ENIG and ACA processes are qualified with a variety of different ASICs, sensors, and dedicated interconnect test structures, with pad diameters ranging from 12 μm to 140 μm and pitches between 20 μm and 1.3 mm. The produced assemblies are characterized electrically, with radioactive-source exposures, and in tests with high-momentum particle beams. This contribution introduces the developed interconnect and plating processes and showcases different hybrid assemblies produced and tested with the above-mentioned methods. A focus is placed on recent optimization of the plating and interconnect processes, resulting in an improved plating uniformity and interconnect yield.

Fabrication of PS-DVB@Cu core-shell microsphere for anisotropic conductive adhesives by electroless plating with copper nanoparticles as seeds

Polymer@metal core-shell microspheres are widely used in anisotropic conductive adhesives (ACAs). Styrene-divinylbenzene copolymer (PS-DVB) copper core-shell microspheres (PS-DVB@Cu) with high conductivity and anti-oxidation performance were successfully prepared by combining copper nanoparticles (Cu NPs) and electroless plating copper. The Cu NPs served as catalytic seeds, substituting noble metals to promote electroless deposition. Using environmentally benign ammonia borane as the reductant to replace toxic formaldehyde, the dense copper film was deposited on the surface of PS-DVB microspheres. The thickness of the Cu layer and bulk electrical resistivity of PS-DVB@Cu were tuned by controlling the concentration of the reducing agent. The minimum resistivity of PS-DVB@Cu was 3.27×10−6 Ω·m and merely slightly changed in the air for two months. The prepared PS-DVB@Cu-based ACAs could be effectively applied to anisotropic conductive connections in small-scale circuits with satisfactory anisotropic conductivity.

Effect of flip-chip bonding parameters on the property of passive UHF RFID tags with screen-printed antenna on fabric

The substrate material plays an important role in industrialized flip-chip technology of the passive ultra-high frequency radio frequency identification with antenna printed on flexible film. This work presents the flip-chip fabrication and performance evaluation of ultra-high frequency radio frequency identification with screen-printed antenna on common woven fabrics. The antennas were manufactured by screen printing a stretchable silver-based conductor directly on a common woven nylon fabric. The electrical and mechanical connection between the screen-printed antennas and microchips was formed by flip-chip technology, which bridged the antenna with the microchip under a heating contactor with additive anisotropic conductive adhesives, and the effects of heating contactor parameters (contactor pressure, temperature and duration time) on the performance of radio frequency identification tag prototypes were investigated, and the influence mechanism was discussed in terms of the conductivity, the resonance frequency shift and the morphology. The experiment results show that the flip-chip bonding conditions set in the case of screen-printed antenna on nylon fabric is 0.7 N contactor pressure, 120°C heating temperature and 0.6 s heating duration time, different from the case of previous plastic-like thin film substrate, and this is mainly attributed to the compressive and rough surface of the fabric-based antennas. In addition, the flip-chip heating temperature should be consistent with that of screen-printed conductive ink to ensure a stable electrical connection between the antenna and microchip, as well as good performance of fabric-based ultra-high frequency radio frequency identification tags. These conclusions provide a tool for manufacturing large-scale fabric-based ultra-high frequency radio frequency identification tags and characterizing the process quality.

Flip chip bonding on stretchable printed substrates; the effects of stretchable material and chip encapsulation

Stretchable printed electronics have recently opened up new opportunities and applications, including soft robotics, electronic skins, human-machine interfaces, and healthcare monitoring. Stretchable hybrid systems (SHS) leverage the benefits of low-cost fabrication of printed electronics with high-performance silicon technologies. However, direct integration of silicon-based devices on conventional stretchable substrates such as thermoplastic polyurethane (TPU) and polydimethylsiloxane (PDMS) is extremely challenging due to their restricted low-temperature processing. In this study, a recently developed thermoset, stretchable substrate (BeyolexTM) with superior thermal and mechanical properties was employed to realize SHS via direct flip chip bonding. Here, ultra-thin chips (UTC) with a fine-pitch, daisy-chain structure was flip-chip bonded by using anisotropic conductive adhesives, while the complementary circuitry was facilitated via screen-printed, stretchable silver tracks. The bonded samples successfully passed reliability assessments after being subjected to cyclic 30% stretch tests for 200 cycles. The potential benefits of chip encapsulation after integration with the stretchable substrate to withstand larger strains were demonstrated by both mechanical simulation and experimental results.

Shell structure control of monodisperse polystyrene‐silver composite microspheres and synthesis of epoxy resin‐based anisotropic conductive adhesives

AbstractMonodisperse polystyrene‐silver (PS‐Ag) composite microspheres were used to enhance the electrical conductivity of epoxy resin‐based anisotropic conductive adhesives (ACAs). Specially designed PS‐Ag composite microspheres were used as conductive elements to construct anisotropic conductive arrays. In addition, the surface microstructure and morphology of PS‐Ag composite microspheres were controlled by adjusting the silver precursor concentration and implemented electroless plating method. The size and conductivity of the final composite microspheres used for ACAs were 2.572 μm and 81,967 S cm−1, respectively. In addition, diglycidyl ether of bisphenol‐A (DGEBA) epoxy resin and polyamide resin were used as the matrix resin and curing agent, respectively, to predict the optimum curing process parameters based on curing kinetics. Based on this premise, carboxyl‐terminated butadiene‐acrylonitrile (CTBN) and PS‐Ag composite microspheres were used as the toughening agent and conductive filler, respectively, and a facile strategy was developed to prepare ACAs (viz., PS‐Ag/Ep‐ACAs). As the PS‐Ag composite microspheres exhibited a submicron structure, PS‐Ag/Ep‐ACAs creatively exhibited satisfactory anisotropic electrical and mechanical properties. Therefore, the research strategy and fabrication methods proposed demonstrate immense potential for the fabrication of large‐scale ACAs in practical applications.

Study on the synthetic mechanism of monodispersed polystyrene-nickel composite microspheres and its application in facile synthesis of epoxy resin-based anisotropic conductive adhesives

Monodispersed polystyrene-nickel (PS-Ni) composite microspheres were used to enhance the conductivity of epoxy-based anisotropic conductive adhesive (ACA) for the first time. Specifically designed PS-Ni composite microspheres were used as conductive elements to construct anisotropic conductive arrays. The surface microstructure and morphology of the prepared PS-Ni composite microspheres were tuned by controlling the implementation method of electroless plating and concentration of Ni precursor. The structural composition, thermal properties, and electrical properties of PS-Ni composite microspheres were analyzed. In addition, diglycidyl ether of bisphenol-A (DGEBA) epoxy resin and low molecular weight polyamide resin were used as the matrix resin and curing agent, respectively. The optimal process parameters were employed and predicted based on the curing kinetics. A simple strategy was developed for the preparation of anisotropic conductive binders (viz. PS-Ni/EP-ACAs) via physical doping by addition of terminal carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer and additives. PS-Ni/EP-ACAs exhibited satisfactory anisotropic electrical properties in fabricated components due to submicron structures composed of PS-Ni composite microspheres with excellent integrated properties in the anisotropic conducting array. Therefore, the theoretical concept and fabrication mean proposed in this study showed great potential and practical application in large-scale ACAs preparation.

Failure Mechanisms in Flip‐Chip Bonding on Stretchable Printed Electronics

Accelerating progress in printed electronics technology transforms the future of the manufacturing process in industrial applications, consumer electronics, and healthcare products. However, real‐life applications demand for circuits and systems that are robust and can stay functional under strong and frequent mechanical deformations. Herein, both empirical and analytical approaches to gain insight on the reliability of ultrathin bare dies on soft and highly stretchable printed circuits are used. To this end, a set of conductive ink and adhesive variants are used to develop a stretchable wireless temperature logger as the test device. The electromechanical performance of the conductive inks is first verified through the screening tests and the most suitable candidates are selected to use with different anisotropic conductive adhesives (ACAs) for chip bonding process. The performance of the test devices (per ink–adhesive combination) is tested through cyclic elongation of 60 samples to provide statistical results. Different failure modes are visualized through cross‐sectional images using broad ion beam (BIB) milling and scanning electron microscopy (SEM). The findings, herein, recognize pad delamination, air voids at the chip‐to‐substrate interface, stiffness of the conductive adhesives, and bonding parameters (pressure and temperature) as the key contributors to the contact failures in the chip assembly.

Flip Chip integration of ultra-thinned dies in low-cost flexible printed electronics; the effects of die thickness, encapsulation and conductive adhesives

As the demand for flexible hybrid electronics has increased extensively e.g. for the internet of things (IoT), electronic skin and wearables applications, the implementation of devices based on low-cost materials such as PET, paper, and unpackaged bare dies have become crucial for economical mass production. To facilitate flexibility for instance in wearables, the rigid silicon-based components are also deemed to become ultra-thin, which raises major challenges in terms of handling and reliable integration. In this paper, the hybrid integration of ultra-thin dies on PET and paper-based printed substrates is investigated. Silicon dies with different thicknesses from 10 to 50 μm with an internal daisy-chain structure were made and flip-chip bonded to screen-printed substrates. The bonding was conducted by using two types of anisotropic conductive adhesives, i.e. anisotropic conductive films (ACF) and anisotropic conductive paste (ACP). The effect of die encapsulation on the reliability of the assembly was also assessed by employing a protective foil. For reliability analysis, a cyclic bending test was carried out to identify the failure cycle. The successful integration of ultra-thin chip on the low-cost printed flexible substrate was obtained by optimizing the bonding parameters with both ACF and ACP. It was revealed that in flexible hybrid electronics; the thinner the die, the higher is the reliability, as the thinner dies promise superior flexibility, and can withstand higher bending stresses. Moreover, it was found that encapsulation of the thin die in a foil dramatically increases the long-term reliability of the bonded chip.

Gallium–Indium–Tin Liquid Metal Nanodroplet-Based Anisotropic Conductive Adhesives for Flexible Integrated Electronics

Compared with traditional solder joint bonding, an anisotropic conductive adhesive (ACA) provides an efficient and simple method for the interconnection of small-scale electronics. The wider applic...

Investigation of the impact on the radio frequency (RF) behaviour of electrically conductive adhesives (ECAs) used in wafer-level packaging (WLP) of RF-MEMS

In this work, an anisotropic conductive adhesive (ACA), belonging to the family of electrically conductive adhesives (ECAs), exploited for the wafer-level bonding phase in packaging of RF-MEMS (MEMS for radio frequency passives), is investigated in respect to its influence on the RF behaviour of packaged Microsystem structures and CPWs (coplanar waveguides). The explored encapsulation solution is based on the wafer-level packaging (WLP) approach, in which the capping substrate (high-resistivity silicon—HRS) provided of vertical through silicon vias (TSVs) is bonded to an RF-MEMS wafer using ACA as adhesion layer. After bonding of ACA, conductive metal particles within the epoxy-based matrix either contribute to the electrical path formation or, when uncompressed, remain isolated. Influence of such floating particles on the RF performance of adjacent structures is investigated by means of finite element method (FEM) electromagnetic simulations. The results show that the influence of floating ACA conductive particles on the S-parameters of the targeted planar structures is rather limited up to 60 GHz.

Analysis of Hot Bar Soldering, Insulation Displacement Connections (IDC), and Anisotropic Conductive Adhesives (ACA), for the Automated Production of Smart Textiles.

Despite all the growth forecasts of the smart textiles market, there is no stable automated manufacturing process for attaching classic electronics to textiles. The great amount of manual production steps causes high prices, which slow down market growth. During the production process, the contacting step offers the greatest potential to reduce manual manufacturing steps. For this reason, we have analyzed various contacting methods for electronic parts on conductive yarns that have a high potential for automation. The chosen methods were thermode soldering, insulation–displacement connectors and anisotropic conductive adhesives. In order to ensure reliable mechanical contacting, the samples were tested in a peeling experiment. The examination of the contact resistances took place in the context of a resistance test using four-wire measuring technology.

Contact area measurement of micron-sized metal-coated polymer particles under compression

The contact problem of a sphere under compression by a rigid plate is pertinent to understanding electrical contact formation in anisotropic conductive adhesive (ACA) applications. However, no work has experimentally verified existing contact models for the metal-coated sphere-plate contact problem. Herein, compression tests on metal-coated polymer spheres were performed by flat punch nanoindentation with in-situ scanning electron microscopy (SEM) monitoring, and the resultant contact area was measured using SEM images. The obtained results were compared to Tatara and Abbott-Firestone models, along with finite element analysis (FEA). The measured contact area falls between Tatara and Abbott–Firestone models, indicating a mixture of elastic and plastic deformation. The contact area as a function of force is dominated by the polymer core and independent of the metal coating for the thin metal coatings tested. The work provides guidelines for bonding parameters for ACA assembly.

ACA Curing Process Optimization Based on Curing Degree Considering Shear Strength of Joints

This paper presents an optimized curing process for the shear strength achievement and retention for anisotropic conductive adhesive (ACA) flip-chip interconnections. The curing kinetics of ACA is first presented and then a curing degree $(\alpha)$ relationship between curing temperature $T$ and curing time $t$ is established. The influence of different curing degree states on the interfacial shear strength is then reported, including the effect of degree of curing on shear strength both before and after hygro-thermal aging.

Study on the Interfacial Residual Stress of Flip-Chip Joints Based on Anisotropic Conductive Adhesive

Interfacial residual stress resulting from coefficient of thermal expansion mismatch between different layers of the flip-chip joint is induced during the bonding process. The flip-chip joints suffer residual stresses that may cause cracking or delamination, which is one of the major reliability issues in electronic packaging. This paper theoretically analyzes the interfacial residual stress of the flip-chip joint based on anisotropic conductive adhesive (ACA). The residual stress model is established based on refined zigzag theory (RZT). Results show that the theoretical model and the simulation analysis are consistent. It is found that the interfacial thermal residual stress in the ACA layer is uneven and reaches its peak at the end of the bump. The RZT model is validated by experiments, and the model prediction agrees well with the test results.

Influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of solderable anisotropic conductive adhesives (SACAs)

In the present work, the influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of carbon nanotube (CNT)-filled solderable anisotropic conductive adhesives (SACAs) containing low-melting-point-alloy (LMPA) fillers was investigated. To measure the reliability-related properties of SACA assemblies, six types of SACAs with different MWCNT concentrations (from 0 to 2 wt%) were formulated, and two types of reliability tests, thermal shock (TS) and high-temperature and high-humidity (HTHH) tests, were conducted on these samples. All of the CNT-filled SACA assemblies with different MWCNT concentrations showed proper and stable electrical reliability during each aging test, due to the molten LMPA fillers forming a metallurgical interconnection between the corresponding metallization regions. Although the mechanical strength of the CNT-filled SACA joints degraded after aging tests due to excessive intermetallic compound (IMC) layer growth, the CNT-filled SACAs with an MWCNT concentration below 0.1 wt% exhibited better mechanical reliability properties than those of SACAs without MWCNTs due to the initially achieved high bonding strength of the SACA joints, enabled by the reinforcing effects of the MWCNTs within the polymer composites.

Resistance Analysis of Spherical Metal Thin Films Combining Van Der Pauw and Electromechanical Nanoindentation Methods

Although micron-sized metal-coated polymer particles are an important conductive filler material in anisotropic conductive adhesives, the resistance of the particles in an adhesive is not well understood. In this study, a van der Pauw method for spherical thin films is developed and applied to determine the resistivity of 30 μm silver-coated poly(methyl methacrylate) (PMMA) particles. The resistivity is used to interpret resistance contributions in single particle electromechanical nanoindentation measurements, which simulate the compression particles undergo in application. The resistivity was found to be coating thickness dependent for thin films in the range 60–270 nm. Estimation of the resistance of the metal shell using the measured resistivity did not account for the total resistance measured in electromechanical nanoindentation. We therefore deduce a significant contribution of contact resistance at the interfaces of the particle. The contact resistance is both coating thickness and particle deformation dependent.

Comparison of Flip-Chip Bonding Characteristics on Rigid, Flexible, and Stretchable Substrates: Part II. Flip-Chip Bonding on Compliant Substrates

The contact resistance and microstructure of the flip-chip joints processed using anisotropic conductive adhesive (ACA) were characterized on flexible printed-circuit-board (FPCB) and stretchable FPCB/polydimethylsiloxane (FPCB/PDMS) substrates. On the FPCB substrate, the contact resistance was remarkably reduced from 56.9 mΩ to 13.3 mΩ when the bonding pressure increased from 10 MPa to 200 MPa. However, at a bonding pressure of 300 MPa, it substantially increased to 74.8 mΩ with an excessive deviation of ±37.7 mΩ. On the more compliant FPCB/PDMS substrate, the contact resistance decreased from 43.2 mΩ to 31.2 mΩ when the bonding pressure increased from 10 MPa to 50 MPa. Severe distortion of the FPCB/PDMS substrate occurred at bonding pressures above 50 MPa because of the softness of the PDMS. A more compliant substrate has a lower appropriate bonding pressure for the flip-chip process.

Performance of Anisotropically Conductive Adhesive Attachments on Adhesiveless Polyimide Substrate during High-Temperature Storage Tests

ABSTRACTThe high-temperature performance of electronics packages was studied at 180°C, 200°C and 240°C for 3,000h. The structure tested consisted of a fairly large silicon chip attached with anisotropic conductive adhesive (ACA) onto an adhesiveless PI substrate. These structures showed good electrical reliability at 180°C. Several early failures were seen at the other temperatures. However, only 23% of the test samples failed at 200°C, while considerably more failures were seen at 240°C. The results showed that good high-temperature reliability can be achieved with polymer interconnections. However, the exposure time should be limited, especially at very high temperatures, to avoid failures caused by the materials becoming brittle.

Reliability of Anisotropic Conductive Adhesive Flip Chip Attached Humidity Sensors in Prolonged Hygrothermal Exposure

Sensor components may markedly differ from typical silicon chips. Consequently, versatile attachment methods are required for their interconnections. Anisotropic conductive adhesives (ACA) are interesting materials for attachments of sensors due to their versatility. In this study reliability of ACA attached humidity sensors was studied in hygrothermal conditions. The reliability of the interconnections was found to be excellent even under prolonged exposure showing that high reliability can be achieved with ACA materials in sensor applications.