Isotropic Conductive Adhesives Research Articles

The Role of Self-Assembled Monolayers in the Surface Modification and Interfacial Contact of Copper Fillers in Electrically Conductive Adhesives.

Printing of electrical circuits and interconnects using isotropic conductive adhesives (ICAs) is of great interest due to their low-temperature processing and compatibility with substrates for applications in sensors, healthcare, and flexible devices. As a lower cost alternative to silver (Ag), copper (Cu)-filled ICAs are desirable but limited by the formation of high-resistivity Cu surface oxides. To overcome this limitation, self-assembled monolayers (SAMs) of octadecanethiol (ODT) have been demonstrated to reduce the oxidation of micrometer-scale Cu powder particles for use in ICAs. However, the deposition and function of the SAM require further investigation, as described in this paper. As part of this work, the stages of the SAM deposition process, which included etching with hydrochloric acid to remove pre-existing oxides, were studied using X-ray photoelectron spectroscopy (XPS), which showed low levels of subsequent Cu oxidation when ODT coated. The treated Cu powders were combined with one- or two-part epoxy resins to make Cu-ICAs, and the effect of the Cu surface condition and weight loading on electrical conductivity was examined. When thermally cured in an inert argon atmosphere, ICAs filled with Cu protected by ODT achieved electrical conductivity up to 20 × 105 S·m-1, comparable to Ag-ICAs, and were used to make a functional circuit. To understand the function of the SAM in these Cu-ICAs, scanning and transmission electron microscopy were used to examine the internal micro- and nano-structures along with the elemental distribution at the interfaces within sections taken from cured samples. Sulfur (S), indicative of the ODT, was still detected at the internal polymer-metal interface after curing, and particle-to-particle contacts were also examined. XPS also identified S on the surface of cured Cu-ICAs even after thermal treatment. Based on the observations, electrical contact and conduction mechanisms for these Cu-filled ICAs are proposed and discussed.

A Printing Strategy for Embedding Conductor Paths into FFF Printed Parts.

A novel approach to manufacture components with integrated conductor paths involves embedding and sintering an isotropic conductive adhesive (ICA) during fused filament fabrication (FFF). However, the molten plastic is deposited directly onto the adhesive path which causes an inhomogeneous displacement of the uncured ICA. This paper presents a 3D printing strategy to achieve a homogeneous cross-section of the conductor path. The approach involves embedding the ICA into a printed groove and sealing it with a wide extruded plastic strand. Three parameter studies are conducted to obtain a consistent cavity for uniform formation of the ICA path. Specimens made of polylactic acid (PLA) with embedded ICA paths are printed and evaluated. The optimal parameters include a groove printed with a layer height of 0.1 mm, depth of 0.4 mm, and sealed with a PLA strand of 700 µm diameter. This resulted in a conductor path with a homogeneous cross-section, measuring 660 µm ± 22 µm in width (relative standard deviation: 3.3%) and a cross-sectional area of 0.108 mm2 ± 0.008 mm2 (relative standard deviation 7.2%). This is the first study to demonstrate the successful implementation of a printing strategy for embedding conductive traces with a homogeneous cross-sectional area in FFF 3D printing.

Investigation on Rheological Characterization of Isotropic Conductive Adhesives

In the field of microelectronic packaging technology, isotropic conductive adhesives (ICAs) have been used widely as interconnection materials between chips and substrates. ICAs are composed of silver powder particles and adhesive matrix. The paper analysed the rheological properties of ICAs at different temperatures in order to better apply ICAs in dispensing and jet printing processes. The viscosity, viscoelasticity, and thixotropic behaviours of ICAs were studied using various experimental methods such as oscillatory stress shear, and rotational rheology. The rheological behaviours of ICAs conforms to the Cross Model. In SAOS flow field, the ICAs has G”>G’ in LVR region, which shows the viscous component is dominant and ICAs represent viscoelastic fluid characteristics.

Effects of low melting point SnBi58 alloy on the properties of isotropic conductive adhesives

Adequate mechanical and electrical properties and long-term reliability are critical in conductive adhesive interconnects. In order to improve the electrical and mechanical properties of isotropic conductive adhesives (ICAs), reduce contact resistance and enhance the aging resistance of ICAs, the effects of low melting point SnBi58 alloy on the properties of ICAs were studied. The low melting point SnBi58 alloy formed metallurgical bonds between conductive fillers, as well as between the ICAs and a Cu substrate and not only improved the electrical conductivity and mechanical properties of ICAs, but also reduced the contact resistance shift of ICAs effectively. It was found that when the mass ratio of SnBi58 alloy in conductive fillers was maintained at 20 wt%, the bulk resistivity of ICAs reached 4.8 × 10−4 Ω cm, the shear strength of ICAs was 27.71 MPa, and the contact resistance shift of ICAs was 7% after aging. In addition, better bulk resistivities and higher shear strengths of ICAs were achieved after aging at 85 °C/85%RH.

Remotely Controlled Light/Electric/Magnetic Multiresponsive Hydrogel for Fast Actuations.

As a kind of soft smart material, hydrogel actuators have extensive development prospects, but it is still difficult for these actuators to integrate multiresponsiveness, multiple remote actuation, high strength, fast responsiveness, and programmable complex deformation. Herein, we have explored an anisotropic bilayer hydrogel actuator with an Fe3O4/co-poly(isopropylacrylamide-4-benzoylphenyl acrylate) [Fe3O4/P(NIPAM-ABP)] active layer and an isotropic conductive adhesive (ICAs) passive layer based on the layer-by-layer method. Benefiting from the fibrosis and porosity of the Fe3O4/P(NIPAM-ABP) hydrogel, the ICAs-Fe3O4/P(NIPAM-ABP) hydrogel actuator has excellent mechanical strength (tensile strength of 3.1 ± 0.3 MPa) and response speed (temperature (45 °C): bending speed of 2400.3°/s; near-infrared (NIR) light: bending speed of 356.4°/s; electricity (2 V): bending speed of 180°/s; water (10 °C): recovery speed of 30.0°/s). In addition, the good photothermal properties and magnetic conductivity of Fe3O4 nanoparticles provide precise remotely controllable light- and magnetic-actuated properties for the hydrogel actuator. The Ag microsheets with excellent conductivity (1.4 × 104 S/cm) provide remotely controllable electrical-actuated property for the hydrogel actuator. Combined with the responsiveness of P(NIPAM-ABP), the actuator can achieve short-range actuation including temperature-, ethanol-, and salt-responses. More importantly, it can achieve remote actuation including light, electrical, and magnetic responses. Finally, the Fe3O4/P(NIPAM-ABP) fibers can provide excellent anisotropic structures for the actuator to achieve precise deformational programmability. Inspired by some phenomena in nature, several actuating devices with the above characteristics have been successfully developed. This study can provide a general method for multifunctional anisotropic hydrogel actuators and will provide a new strategy for exploring smart materials suitable for complex bioinspired systems.

Insights in the enhancement of electrical conductivity and dielectric constant of epoxy-carbon nanotubes decorated with CaCO3-derived from waste eggshell hybrid composites

New insights in the development of epoxy hybrid composites using CNTs decorated with CaCO3 derived from waste eggshell (CaCO3-ESp) have been studied. The composite was produced by the solution stirring method. The dielectric constant, electrical conductivity, and morphology of the samples were determined. The dielectric constant and electrical conductivity were enhanced after the modification of the CNTs with CaCO3-ESp. The high electrical conductivity and dielectric constant obtained were attributed to the high electron mobility of CaCO3-ESp-CNTs, which helps to enhance the conductivity of the epoxy. It was found that used eggshells can be used to make sure that CNTs are spread out well when making conductive polymers for isotropic conductive adhesives, assemblies, and packaging for electronics.

Assembly of Surface-Mounted Devices on Flexible Substrates by Isotropic Conductive Adhesive and Solder and Lifetime Characterization.

The assembly of passive components on flexible electronics is essential for the functionalization of circuits. For this purpose, adhesive bonding technology by isotropic conductive adhesive (ICA) is increasingly used in addition to soldering processes. Nevertheless, a comparative study, especially for bending characterization, is not available. In this paper, soldering and conductive adhesive bonding of 0603 and 0402 components on flexible polyimide substrates is compared using the design of experiments methods (DoE), considering failure for shear strength and bending behavior. Various solder pastes and conductive adhesives are used. Process variation also includes curing and soldering profiles, respectively, amount of adhesive, and final surface metallization. Samples created with conductive adhesive H20E, a large amount of adhesive, and a faster curing profile could achieve the highest shear strength. In the bending characterization using adhesive bonding, samples on immersion silver surface finish withstood more cycles to failure than samples on bare copper surface. In comparison, the samples soldered to bare copper surface finish withstood more cycles to failure than the soldered samples on immersion silver surface finish.

Contacting Inkjet-Printed Silver Structures and SMD by ICA and Solder

Digitally printed conductive structures often need to be electrically connected to batteries, microcontrollers, or other devices. A consensus of industry and research is that such hybrid printed electronics will play a major part in the future of printed electronics. To promote the technology of hybrid printed electronics, surface mount technologies for electronic components using isotropic conductive adhesives (ICAs) and low melting tin bismuth solders on inkjet-printed silver structures on injection molded liquid crystal polymer (LCP) substrates were investigated in this publication. The special needs for inkjet-printed electronics were considered as well as the reliability of assembled surface mounted devices (SMDs) and their failure mechanisms. Connected 0603 and 1206 SMD components achieved a characteristic fatigue life of more than 3500 cycles during thermal cycling at +125 °C/−40 °C and withstood over 1000 h under a damp-heat atmosphere of +85 °C/85% RH. The coefficient of thermal expansion (CTE) of the substrate and the selection of solder have major impacts on the reliability of the assemblies.

The synergistic effect of micron spherical and flaky silver-coated copper for conductive adhesives to achieve high electrical conductivity with low percolation threshold

In the application of electronic products, it is critical for conductive adhesives to ensure high electrical conductivity and good adhesion reliability. In this study, isotropic conductive adhesives (ICAs) with high conductivity and low percolation threshold can be achieved by making full use of the synergistic effect of micron spherical and flaky silver-coated copper ([email protected]) conductive fillers. The flaky [email protected] powder prepared by electroless plating was chosen as the main conductive filler, and the spherical core-shell structure [email protected] powder was used as an auxiliary conductive filler to be dispersed in the flaky [email protected] particles, providing more possibilities for building a large number of conductive bridges. In this study, the [email protected] powder was synthesized to eliminate the disadvantages of Cu oxidation and Ag electrochemical migration, while making the ICAs low cost and favourable conductivity. Furthermore, the curing parameters were optimized for the matrix resin in corresponding experiments. The results showed that a combination of spherical and flaky [email protected] powders in ICAs could not only significantly reduce the percolation threshold and the bulk resistivity but also increase the shear strength compared with the single component. In particular, the bulk resistivity of the optimum sample was 1.95 × 10−3 Ω cm, and the shear strength was 22.69 MPa when the filler content was only 60 wt%. Hence, the ICAs filled with spherical and flaky [email protected] will pave the way for high-performance and low-cost ICAs in the electronics packaging industry.

Property Prediction of Ag-Filled Isotropic Conductive Adhesive through the Analysis of Its Curing and Decomposition Kinetics

In this study, various thermal analyses were carried out on a self-developed and commerce-oriented Ag-filled isotropic conductive adhesive (ICA) and its unfilled matrix resin through which glass transition temperature (Tg) and thermal endurance could be quantitatively predicted. An autocatalyzed kinetic model was used to describe the curing reaction, which was proven to be in good consistency with the experimental data. The activation energies for the curing reaction of the ICA and the matrix resin were determined to be 68.1 kJ/mol and 72.9 kJ/mol, respectively, which means that the reaction of the ICA was easier to occur than its unfilled matrix resin. As a result, the time–temperature profile could be calculated for any Tg requested based on the kinetic model of curing and the DiBenedetto equation. Further, the thermal decomposition stability of the ICA and its unfilled matrix resin were also studied. The activation energies for the thermal decomposition of the ICA and the matrix resin were calculated to be 134.1 kJ/mol and 152.7 kJ/mol, respectively, using the Ozawa–Flynn–Wall method, which means that the decomposition of ICA was easier to occur. The service life of the resin system at a specific temperature could therefore be calculated with their activation energy. The addition of micro-scale Ag flakes did not change the curing and decomposition mechanisms by much.

Synthesis of carboxylated-silver nanowires: Electrical conductivity enhancement of isotropic conductive adhesives and long-term stability in a mixture of solvents

Incorporation of silver nanowires in conductive adhesives leads to the reduction of resistivity due to the increased surface area and reduction of contact resistance. However, due to high surface energy, aggregation in the adhesive polymeric matrix leads to performance decline. In this research, in order to solve the aggregation issue and increase the contact surface area, silver nanowires functionalized with carboxylate groups and 21.6 μm and 130.5 nm lengths and diameters were synthesized via polyol technique with a −49.7 mV zeta potential at pH = 7 in aqueous media (energy barrier ≈ 20 KBT). Also, functionalized nanowire suspension was stored in a mixture of solvents containing isopropyl alcohol, 1, 4-dioxane, and dimethyl sulfoxide maintaining stability for more than one year at the room temperature. Due to the good dispersion of the as-prepared silver nanowires in epoxy resin, increased adhesion density, reduction of air entrapment in the adhesive matrix during the curing process, and reduced oxidation rate, the as-prepared conductive adhesive with 50 wt% silver and 5 wt% polyethylene glycol showed a resistivity of 1.895 × 10-7 Ω cm.

Microscale mechanical properties dependent on the strain rate and temperature of cured isotropic conductive adhesive

In this research, the microscale strain rate sensitivity and high-temperature mechanical properties of cured isotropic conductive adhesive (ICA) were investigated using microindentation. The indentation modulus and hardness of cured ICA with high silver content are relatively large. The slopes of contact stiffness–depth curve, modulus and hardness increase with increasing loading strain rate. The elastic modulus, hardness and creep behaviour at high temperature were characterised on the basis of the “rapid loading–holding–rapid unloading” loading mode and the semiempirical method from the generalised Kelvin model. With increasing temperature, the elastic modulus and hardness of cured ICA decrease from 3000–7000 and 100–300 MPa in the glassy state to 6–200 and 1–10 MPa, respectively, in the rubbery state. Creep compliance, which is relatively high in the rubbery state, increases with increasing holding time. On the retardation spectrum, the widened retardation peaks reflect different retardation processes with increasing retardation time.

Rolling reliability of polyurethane and polyurethane-acrylic ICAs interconnections on printed stretchable electronics

In this study, bending reliability of the surface mounted devices (SMD) attached using isotropic conductive adhesive (ICAs) on screen printed stretchable devices polyurethane substrate are investigated. The performance of polyurethane and polyurethane-acrylic ICAs and the impact of rolling speed are studied. The rolling test was performed using the customized automatic rolling test device. It was found that the polyurethane-acrylic blend gives poorer adhesion with the silver conductor and very high cohesive force. The polyurethane resin brings adhesive high adhesive force with the silver conductive layer while lower adhesion with the Tin surface of the SMD, and it exhibited poorer cohesive force. The resistance of all samples experienced a steady and slow increase period before it accelerated to failure. During the slow and steady increase phase, the increased resistance was mainly from the conductive layer, whereas during the dramatic increase phase, it was mainly from the ICA connected area. The increased rolling speed had accelerated the resistance evolution process and lowered the samples' reliability severely. It also lowered the adhesion of the polyurethane-acrylic blend with the ink layer and lowered cohesive force of polyurethane ICA.

Explicit microstructure and electrical conductivity of epoxy/carbon nanotube and green silver nanoparticle enhanced hybrid dielectric composites

Carbon nanotubes (CNTs) decorated with silver nanoparticles (AgNPs) are promising nanomaterials for improving the dielectric properties of polymer materials for energy storage and micro-capacitor applications. However, the cost of AgNPs limits their wide application. This work describes the synthesis of green silver nanoparticles (GAgNPs) from cashew leaves and their hybridization with CNTs. These new hybrid nanocomposites were developed by adding 0.1, 0.2, 0.3, 0.4 and 0.5% CNTs and 0.5% GAgNPs in an epoxy matrix. Electrical conductivity, dielectric constant, and capacitor raised as CNTs content increased from 0.1 to 0.5% with 0.5% GAgNPs. The high dielectric constant reported in this work was made possible because of the high electron mobility of GAgNPs, which helps to enhance the conductivity of the epoxy. The highest electrical conductivity and dielectric constant were obtained for hybrid nanocomposites based on 0.5% CNTs and 0.5% GAgNPs. It was established that GAgNPs modified CNTs can be used to enhanced the electrical conductivity, dielectric constant and capacitance of epoxy resins for isotropic conductive adhesives, assemblies, and electronic packaging applications.

Near-Field UHF RFID Transponder With a Screen-Printed Graphene Antenna

As a method of producing RFID tags, printed graphene provides a low-cost and eco-friendly alternative to the etching of aluminum or copper. The high resistivity of graphene, however, sets a challenge for the antenna design. In practice, it has led to using very large antennas in the UHF RFID far field tags demonstrated before. Using inductive near field as the coupling method between the reader and the tag is an alternative to the radiating far field also at UHF. The read range of such a near field tag is very short, but, on the other hand, the tag is extremely simple and small. In this paper, near field UHF RFID transponders with screen-printed graphene antennas are presented and the effect of the dimensions of the tag and the attachment method of the microchip studied. The attachment of the microchip is an important step of the fabrication process of a tag that has its impact on the final cost of a tag. Of the tags demonstrated, even the smallest one with the outer dimensions of 21 mm * 18 mm and the chip attached with isotropic conductive adhesive (ICA) was readable from a distance of 10 mm with an RF power marginal of 19 dB, which demonstrates that an operational and small graphene-based UHF RFID tag can be fabricated with low-cost industrial processes.

WITHDRAWN: Strain rate and temperature effects on the mechanical properties of cured isotropic conductive adhesive (ICA) at micro scale

WITHDRAWN: Strain rate and temperature effects on the mechanical properties of cured isotropic conductive adhesive (ICA) at micro scale

Synthesis of Highly Conductive, Uniformly Silver-Coated Carbon Nanofibers by Electroless Deposition.

Noble-metal-coated carbon-based nanoparticles, when used as electrically conductive fillers, have the potential to provide excellent conductivity without the high weight and cost normally associated with metals such as silver and gold. To this effect, many attempts were made to deposit uniform metallic layers on core nanoparticles with an emphasis on silver for its high conductivity. The results so far were disheartening with the metal morphology being better described as a decoration than a coating with small effects on the electrical conductivity of the bulk particles. We tackled in this work the specific problem of electroless deposition of silver on carbon nanofibers (CNFs) with the investigation of every step of the process. We performed X-ray photoelectron spectroscopy (XPS), transmission and scanning electron microscopy (TEM, SEM), zeta potential, and electrical conductivity measurements to identify a repeatable, reliable set of parameters allowing for a uniform and fully connected silver deposition on the surface of the CNFs. The bulk particles' specific electrical conductivity (conductivity per unit mass) undergoes a more than 10-fold increase during the deposition, reaching 2500 S·cm2/g, which indicates that the added metal mass participates efficiently to the conduction network. The particles keep their high aspect ratio through the process, which enables a percolated conduction network at very low volume loadings in a composite. No byproducts are produced during the reaction so the particles do not have to be sorted or purified and can be used as produced after the short ∼15 min reaction time. The particles might be an interesting replacement to conventional fillers in isotropic conductive adhesives, as a conductive network is obtained at a much lower loading. They might also serve as electrically conductive fillers in composites where a high conductivity is needed, such as lightning strike protection systems, or as high surface area silver electrodes.

Mechanical properties of cured isotropic conductive adhesive (ICA) under hygrothermal aging investigated by micro-indentation

Cured isotropic conductive adhesive (ICA) with different conductive fillers (micro silver flakes) contents were prepared. Based on the loading mode of “rapid loading-holding-rapid unloading” and semi-empirical method from the generalized Kelvin model, micro-indentation tests were carried out to investigate the effect of hygrothermal aging on the mechanical properties of cured ICA. On the whole, the elastic modulus and hardness of cured ICA with high silver content are relatively larger. As the hygrothermal aging time increases, both elastic modulus and hardness decrease at first and then stabilize, and the creep deformation is more evident. Indentation creep displacement–time curves for cured ICA were characterized by the generalized Kelvin model. During the holding stage, creep compliance increases slowly at first and then increases quickly with modulus loss. With the aggravated aging creep compliance increases, and creep compliance of cured ICA with high silver content increases faster at the later stage of hygrothermal aging. The widened retardation peaks with increasing retardation time reflect the different retardation processes during the holding stage. With increasing aging time, the creep rate sensitivity exponent increases and creep resistance reduces accordingly.

DTT functionalization of Ag particles for conducting adhesives

ABSTRACTWe have studied the influence of the organic molecule DL-Dithiothreitol on the properties of isotropic conductive adhesives for application in microelectronics. The Ag fillers were prepared from commercial micro flakes or submicron particles from waste jewelry. The results show that coating of the Ag recycled fillers with thiol avoided the agglomeration of the particles improving the dispersion and the electrical and mechanical properties. By comparing the samples, the better properties were obtained from adhesives with recycled Ag particles relative to the ones formulated with commercial Ag flakes and improvement can be attributed to the absence of the lubricant layer and smaller size of the particles produced from recycled silver.

Room-Temperature Curing and Grain Growth at High Humidity in Conductive Adhesives with Ultra-Low Silver Content

Isotropic conductive adhesives (ICAs) are alternatives to metallic solders as interconnects in solar modules and electronic devices, but normally require silver contents >25 vol.% and elevated curing temperatures to achieve reasonable conductivity. In this work, ICAs are prepared with a silver content of 1.0 vol.% by using polymer spheres coated with nanograined silver thin films as filler particles. In contrast to conventional ICAs, there are no organic lubricants on the silver surfaces to obstruct the formation of metallic contacts, and conductivity is achieved even when the adhesive is cured at room temperature. When exposed to long-term storage at 85°C and 85% relative humidity, the silver films undergo significant grain growth, evidenced by field-emission scanning electron microscopy observation of ion-milled cross-sections and x-ray diffraction. This has a positive effect on the electrical conductivity of the ICA through the widening of metallic contacts and decreased scattering of electrons at grain boundaries, and is explained by an electrochemical Ostwald ripening process. The effects of decoupling heat and humidity is investigated by storage at either 85°C or immersion in water. It is shown that the level of grain growth during the various post-curing treatments is dependent on the initial curing temperature.