Isotropic Conductive Adhesives Research Articles

Design and characterization of novel conductive nanocomposites based on carbon nanotube modified liquid crystalline epoxy

In this work, a series of novel isotropically conductive adhesives (ICAs) based on multiwalled carbon nanotube (MWCNT)-modified liquid crystalline epoxy (LCE) resin was prepared. Different loadings of MWCNTs were used to produce LCE composites. Transmission electron microscope, scanning electron microscope, differential scanning calorimetry, polarized light optical microscopy, X-ray diffraction, electrical conductivity measurement, and tensile and flexural measurements were used to characterize the morphology, thermal, phase change, electrical, and mechanical properties of the prepared composites. The results showed that the added MWCNTs were dispersed well in the LCE matrix. The introducing MWCNTs did not show any major influence on typical curing peaks of LCE. Electrical conductivity at room temperature of the composites was increased significantly compared with neat LCE. The conductivity value of 10−2 S cm−1 was obtained at a weight concentration of only 0.5% MWCNT. In addition, tensile strength, Young’s modulus, flexural strength, and flexural modulus of the composites were increased clearly compared with neat LCE matrix. These produced LCE composites with very low MWCNT loadings showed a great prospect of being used as ICAs.

Electrical and mechanical properties of RFID chip joints assembled on flexible substrates

Purpose – The purpose of this paper is to examine electrical and mechanical properties of radio frequency identification (RFID) chip joints assembled on a flexible substrate and made from isotropic conductive adhesives (ICAs) reinforced with graphene nanoplatelets (GPNs) or graphite nanofibers (GFNs). Design/methodology/approach – The ICAs reinforced with GPNs or GFNs were prepared and screen printed on a test pattern to investigate resistance and thickness of these adhesive layers. Differential Scanning Calorimetry (DSC) was performed to assess a curing behaviour of the prepared ICAs. Then, RFID chips were mounted with the prepared ICAs to the pattern of silver tracks prepared on foil. Shear test was carried out to evaluate mechanical durability of the created chip joints, and resistance measurements were carried out to evaluate electrical properties of the tested ICAs. Findings – The 0.5 per cent (by weight) addition of GFNs or GPNs to the ICA improved shear force values of the assembled RFID chip joints, whereas resistance of these modified adhesives increased. The DSC analysis showed that a processing temperature of the tested adhesives may range from 80 to 170°C with different curing times. It revealed a crucial influence of curing time and temperature on electrical and mechanical properties of the tested chip joints. When the chip pads were cured for too long (i.e. 60 minutes), it resulted in a resistance increase and shear force decrease of the chip joints. In turn, the increase of curing temperature from 80 to 120°C entailed improvement of electrical and mechanical properties of the assembled chips. It was also found that a failure location changed from the chip – adhesive interface towards the adhesive – substrate one when the curing temperature and time were increased. Research limitations/implications – Further investigations are required to examine changes thoroughly in the adhesive reinforced with GFNs after a growth of curing time. It could also be worth studying electrical and mechanical properties of the conductive adhesive with a different amount of GFNs or GPNs. Practical implications – The tested conductive adhesive reinforced with GFNs or GPNs can be applied in the production of RFID tags because it may enhance the mechanical properties of tags fabricated on flexible substrates. Originality/value – Influence of GFNs and GPNs on the electrical and mechanical properties of commercial ICAs was investigated. These properties were also examined depending on a curing time and temperature. New conductive materials were proposed and tested for a chip assembly process in fabrication of RFID tags on flexible substrates.

New insights into silver nanowires filled electrically conductive adhesives

Silver nanowires have drawn remarkable attention owing to their anisotropic electrical and optical properties that differ from those observed in the bulk materials and the interesting applications in various areas such as electronic devices, chemical sensors and photonics. In this paper, silver nanowires with an average diameter of 450 nm and a length of up to 100 μm were successfully synthesized by a polyol process. The experimental results indicate that the morphologies and sintering behaviors of silver nanowires are impacted by glutaric acid and the sintering temperature. The isotropically conductive adhesives (ICAs) were made by adding silver nanowires to the polymer composites consisted mainly of epoxy resin, cure agent and catalyst, and then the electrical properties were investigated based on those physical parameters such like the fraction of the silver nanowires of the total of silver fillers and the curing temperature. The in situ monitoring of the variation in electrical resistance of the ICAs and DSC explore that silver nanowires can affect the curing behavior of the ICAs. Silver nanowires can significantly improve the electrical conductivity of the ICAs when the loading of silver nanowires is low and the curing temperature is less than about 250 °C. There is little difference of the electrical resistivity between the ICAs filled with silver nanowires and micro-sized silver flakes when the loading of silver fillers in the ICAs is 75 wt%, and this phenomenon is related to the curing temperature. The silver nanowires treated by glutaric acid can improve the electrical property of the ICAs.

Self-healing isotropical conductive adhesives filled with Ag nanowires

Microsized polystyrene(PS)–Ag nanowires/dimethylbenzylamine(DMBA)/polythiol core-shell capsules were prepared by an emulsion polymerization method. A novel kind of self-healing isotropical conductive adhesives (ICAs) was developed by using these microcapsules as healing agents. When micro-cracks produced by outside force reached to these healing agents, the shell layer would be destroyed automatically and the healing agents in the core would release and react with access epoxy groups in resin substrates immediately, a new network would be formed again in bulk ICAs, which has a healing effect for mechanical strength of ICAs. On the other side, the dispersed Ag nanowires in microcapsules core would also release and fill in the broken area to set up a conductive network, which will help ICAs to obtain a good or even better electrical properties. The relationship between the content of self-healing microcapsules and the healing efficiency was studied and the healing mechanism was described in detail.

Effect of dispersion condition of multi-walled carbon nanotube (MWNT) on bonding properties of solderable isotropic conductive adhesives (ICAs)

Carbon nanotubes have been considered as reinforcements in composite materials because of their exceptional mechanical, electrical, and thermal properties. In this paper, the effect of dispersion conditions of multi-walled carbon nanotubes (MWNTs) on bonding properties of solderable isotropic conductive adhesives (ICA) was investigated. Two types of ICAs, untreated pristine MWNT-filled ICAs and acid-treated MWNT (a-MWNT)-filled ICAs were formulated with 1 wt% MWNTs and 83 wt% low-melting-point alloy (LMPA) fillers. X-ray photoelectron spectroscopy analysis was conducted to characterize the surface chemical states of pristine and a-MWNTs and verify the effectiveness of a-MWNTs. The fracture surface of the polymer matrix and solderable ICAs with a-MWNTs showed good dispersion conditions through field-emission scanning electron microscope. After the interconnection process, the a-MWNT-filled solderable ICA showed uniform dispersion of MWNTs in the polymer matrix and formed a stable metallurgical conduction path because of the good rheology-coalescence-wetting behavior of LMPA. Alternatively, pristine MWNT-filled ICA showed poor dispersion and an unstable conduction path formed by aggregated MWNTs.

An investigation of the reliability of solderable ICA with low-melting-point alloy (LMPA) filler

Conductive adhesives play a major role in the electronic packaging industry as an alternative to solder due to their potential advantages that include mild processing conditions and superior thermo-mechanical performance. In a conductive adhesive interconnection, adequate mechanical and electrical performance and long-term reliability are critical.In this paper, the reliability of solderable isotropic conductive adhesive (ICA) interconnections was investigated. Reliability testing was performed via thermal shock (−55 to 125°C, 1000 cycles) and high-temperature and high-humidity tests (85°C, 85% RH, 1000h). The interfacial microstructure of the solderable ICA was also investigated. Additionally, the fracture mode was investigated via mechanical pull strength testing before and after the reliability test. The electrical resistance of the solderable ICA interconnection showed improved stability compared to conventional ICAs, and similar stability to conventional solder paste (Sn–3Ag–0.5Cu and Sn–58Bi) due to the metallurgical interconnection formed by the molten LMPA fillers between the corresponding metallization layers. After the reliability tests, the grown IMC layer was composed of Cu6Sn5 (η-phase) and Cu3Sn (ε-phase), and the scallop-type IMC transformed into a layer-type IMC. The fracture propagated along the Cu–Sn IMC/SnBi interface and the fracture surface showed a semi-brittle fracture mode mixed with cleavage and ductile tear bands.

Spray pyrolysis of phase pure AgCu particles using organic cosolvents

Abstract

Conductive adhesive joint for extreme temperature applications

This work describes the thermo-mechanical behaviour of Polyimide Isotropic Conductive Adhesive (ICA) used on passive components assemblies in harsh thermal environment. Shear tests were carried out in an “Instron tensile” machine for thermo-mechanical characterization. The stress–strain curves have been used finite element models (FEM) parameterisation. Numerical modelling simulations have shown the most probable failure area, where accumulated strain in the adhesive is the highest. In addition, simulation results have been compared with results from aged assemblies. The comparative approach has permitted to evaluate the influence of the presence of voids in adhesive joints.

Die Shear Testing of a Novel Isotropic Conductive Adhesive—Epoxy Filled With Metal-Coated Polymer Spheres

Isotropic conductive adhesives (ICAs) filled with metal-coated polymer spheres (MPS) are introduced to improve the mechanical reliability compared with conventional ICAs filled with silver (Ag) flakes. This paper deals with the die shear performance of an MPS-based ICA; an epoxy filled with 45 vol% of Ø30 μm Ag-coated monodisperse polymer spheres. The curing kinetics of the ICA is also studied. The MPS-based ICA is compared with two ICAs filled with Ag flakes: an in-house prepared ICA (with the same epoxy matrix as the MPS-based ICA) and a commercially available ICA. Both ICAs with Ag flakes have a lower particle volume fraction than the MPS-based ICA. Loading 45 vol% of MPS into the epoxy matrix has no significant effect on the curing kinetics and the glass transition temperature (Tg) of the matrix. The MPS-based ICA has 140% higher die shear strength than the in-house prepared ICA with Ag flakes, and 23% higher die shear strength than the commercial ICA with Ag flakes. The MPS-based ICA also has higher shear strain at failure, and less scatter in shear strength between repeated tests. Hence, the MPS-based ICA has a good potential for demanding applications; it has better die shear performance and higher T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> than the commercial ICA with Ag flakes.

Conventional and microwave-assisted processing of Cu-loaded ICAs for electronic interconnect applications

Isotropically conductive adhesives (ICAs) and inks are widely used for interconnecting components and for printed circuits. Silver (Ag)-filled ICAs and inks are the most popular due to their high conductivity and good reliability. However, the price of Ag is a significant issue for the wider exploitation of these materials in low cost, high volume applications such as printed electronics. In addition, there is a need to develop systems compatible with temperature-sensitive substrates through the use of alternative materials and heating methods. Copper (Cu) is considered as a more cost-effective filler for ICAs and in this work, Cu powders were treated to remove the oxide layer and then protected with a self-assembled monolayer. The treated Cu powder was combined with one of two different adhesive resins to form ICAs that were stencil printed onto glass substrates before curing. The use of conventional and microwave-assisted heating methods under an inert atmosphere for the curing of the Cu-loaded ICAs was investigated in detail. The samples were characterised for electrical performance, microstructure and shrinkage as a function of curing temperature (80–150 °C) and time. Tracks with electrical conductivity comparable to Ag-filled adhesives were obtained for both curing methods and with both resins. It was found that curing could be accelerated and/or carried out at lower temperature with the addition of microwave radiation for one adhesive resin, but the other showed almost no absorption indicating a difference in mechanism for the two formulations.

Foil-to-foil lamination and electrical interconnection of printed components on flexible substrates

• We described and compared two foil-to-foil lamination and interconnection methods. • Anisotropic conductive adhesive (ACA) method requires less processing steps. • Through foil vias (TFV) method proved to be more robust to cyclic bending forces. • ACA and TFV methods shown to have high reliability during environmental ageing. • The methods are demonstrated for two types of foil-based humidity sensors. This paper describes and compares two integration methods to structurally laminate and interconnect foil-based components with flexible polymeric substrates. The first method uses isotropic conductive adhesives (ICA) confined in laser-ablated through foil vias (TFV), while the second one uses an anisotropic conductive adhesive (ACA). Both procedures were successfully demonstrated by interconnecting silver-based inkjet printed and gold-sputtered interdigitated capacitive humidity sensors onto flexible PEN carrier substrates, showing functionality, high process yield and low-complexity. The robustness of the assemblies was tested and compared for adhesion, bending, high humidity and temperature cycling. Confined ICA vias show higher mechanical robustness to bending, while both methods remained functional after more than 900 h of environmental ageing. Finally, the interconnections were fully validated at different levels of relative humidity (RH) by comparing the sensor response to that of a commercial sensor.

In situ preparation and sintering of silver nanoparticles for low-cost and highly reliable conductive adhesive

A facile method to prepare low-cost, highly conductive and reliable silver-coated copper (Ag-coated Cu) flakes filled isotropic conductive adhesive (ICAs) by in situ generation and sintering of silver nanoparticles (AgNPs) during the curing process is presented. The silver–triethanolamine complex is derived from the complexing reaction between AgNO3 and triethanolamine in the epoxy matrix. At the curing temperature, AgNPs are in situ generated by the thermal decomposition of silver–triethanolamine complex. The sintering of AgNPs anchored on the surface of Ag-coated Cu flakes could effectively prevent the exposed Cu from oxidizing. Compared with the ICAs without AgNPs which has a resistivity of 9.6×10−4Ωcm, the ICAs filled with AgNPs shows a much lower volume resistivity of 6.62×10−4Ωcm. Furthermore, the contact resistance of the ICAs is only 6.7% increase after aging at 85°C and 85% RH for 500h, while the increase in contact resistance of the controlled ICAs reaches to 23%. The facile approach will pave the way towards high performance and low cost ICAs for electronic packaging applications.

An Overview of Isotropic Conductive Adhesives Filled with Metal-coated Polymer Spheres

Novel isotropic conductive adhesives (ICAs) filled with metal-coated polymer spheres (MPS) have been introduced in order to improve the mechanical performance, and hence the reliability, compared to conventional ICAs filled with solid silver (Ag) particles. This paper summarizes a number of characterization studies on the electrical, the mechanical and the rheological properties, as well as a study on the reliability, of MPS-based ICAs. The feasibility of the ICAs filled with MPS has been demonstrated in terms of potential processing capabilities, improved mechanical performance and competitive electrical properties (compared to conventional ICAs with Ag flakes). Therefore, the MPS-based ICAs have a good potential for demanding environment applications, in which electronic products (e.g., hand held devices) are subjected to thermal shock and thermal cycling, mechanical vibration and mechanical shock.

Relationship between the Conductivity of Isotropic Conductive Adhesives (ICAs) and the Lubricant Coated on Silver Filler Particles

We investigated the relationship between the conductivity of isotropic conductive adhesives (ICAs) and silver (Ag) particles (filler) that are coated with different lubricants; high molecular weight fatty acid. The components of the ICAs are organic epoxy binder and silver filler. The conductivity of the ICAs was measured by a milliohm tester and the electron charge between silver particles was determined by powder charge amount measurements. The conductivity of ICAs differs depending on the kind of lubricant used, and there is a possible correlation between conductivity and the charge amount of silver particle in the samples. These results suggest that the conductivity of ICAs is not only a result of connections between the silver particles, but also of some interaction between the lubricants and silver particles. The silver filler particles in the ICAs appear to function as micro-capacitors in the cured state.

Rheological characterization of a novel isotropic conductive adhesive – Epoxy filled with metal-coated polymer spheres

Novel isotropic conductive adhesives (ICAs) filled with metal-coated polymer spheres have been introduced in order to improve the mechanical reliability compared to conventional silver-filled ICAs. The topic of this study is the rheology of an epoxy filled with monodisperse polymer spheres with or without Ag coating. In trials with uncoated spheres, the viscosity increases with increasing sphere fraction, while the sphere diameter (6 vs. 30μm) only has a minor effect. With 45vol% of spheres (giving an ICA with adequate electrical properties), the Ag coating on the spheres has a large effect on the rheology of the adhesive paste. Compared to the epoxy with uncoated spheres, the epoxy with Ag-coated spheres exhibits higher viscosity, higher storage and loss moduli, and a higher ratio of storage modulus to loss modulus (about 10times). Furthermore, the viscosity of the epoxy with coated spheres increases with time in oscillatory measurements with low to intermediate frequencies. With hardener added to the epoxy containing Ag-coated spheres, the viscosity increase at room temperature is small within the first three hours. In curing trials, the viscosity development is different for unfilled and sphere-filled epoxies. The rheological properties of the ICA with 45vol% Ag-coated spheres were found to be suitable for stencil/screen printing and dispensing processes.

Multiscale modeling of residual stresses in isotropic conductive adhesives with nano-particles

Isotropic conductive adhesives (ICAs) are promising candidates for low temperature joining technologies in microelectronics, enabling ultra-fine pitch sizes. Especially in solar and automotive applications, long-term reliability is a prerequisite in new generation electronics. It is essential that reliability predictions take processing history into account in order to correctly address premature failures as well as to make sound long-term predictions. In this paper, residual stresses that develop in a nano-Ag ICA interconnect during the assembly of a flip-chip pin grid array are investigated. A multiscale modeling framework is adopted to link the nano-sized particles to the interconnect level. This is achieved by the numerical analysis of the mechanical response during the curing process through the computational homogenization approach, in which two boundary value problems, one at each scale are formulated and solved simultaneously, in a fully nested manner. The mechanical response of the interconnect is analyzed with respect to the particle volume fraction and distribution properties. It is shown that, although the overall residual stresses at the interconnect scale decrease with increasing the amount of conductive particles, at the particle scale local stress concentrations increase, indicating the possibility of damage and decohesion that might compromise mechanical integrity and interrupt the conductive path. Hence, the multiscale scheme proved crucial for the sound analysis of the nano-particle ICA interconnect problem, where consideration of only the interconnect level would lead to misleading conclusions.

Reduction synthesis of silver nanoparticles anchored on silver micro-flakes and electrical resistivity of isotropic conductive adhesives at percolation threshold

Reduction synthesis of silver in an ethanol solution without polyvinyl pyrrolidone was performed to anchor synthesized nanoparticles on micro-sized silver flake surfaces in order to enhance the electrical conductivity of isotropic conductive adhesives (ICAs). Although some shape-separated silver particles were formed, the synthesis of particles anchored on the flake surfaces was successful. The cured ICAs containing the silver flakes anchored with 1 wt. % nanoparticles indicated enhanced electrical conductivity due to their filling the gaps between noncontacting silver flakes at the beginning region of the percolation threshold. However, the anchored nanoparticles had a detrimental effect on the conductance when the content of anchored nanoparticles or flakes was higher.

Influence Mechanism of Curing Shrinkage on Tensile Shear Strength of Isotropic Conductive Adhesive

Based on epoxy resin, Aradur9506 as curing agent and Flake silver powders, isotropic conductive adhesives(ICA) were prepared, The effects of the functional groups of epoxy resin and the addition of curing agent in isothermal cure on the resistivity of the ECA were investigated by FT-IR method, scanning electron microscopy（SEM）and other means. The results indicated that tensile shear strength declined with the increased in the number of epoxy functional groups and raised with the addition of curing agent, and the tensile shear strength was inverse relationship with the curing shrinkage

Characteristics of Graphene-Filled Solderable Isotropically Conductive Adhesive (ICA)

A new class of functionalized graphene-filled solderable isotropically conductive adhesive (ICA) has been developed using a low-meltingpoint alloy (LMPA) fillers. The mechanical and electrical characteristics of formulated ICAs were investigated and compared with those of three kinds of conventional ICAs filled with Ag particles. The functionalized graphene-filled solderable ICA formed good metallurgical interconnection between upper and corresponding lower electrode. The developed ICA exhibit lower electrical resistance and higher mechanical strength compared with those of conventional ICAs. In addition, the thermal conductivity was improved about 20% by adding functionalized graphene compared with that of solderable ICA without graphene. [doi:10.2320/matertrans.M2011365]

Carbon nanotube-filled conductive adhesives for electronic applications

Carbon nanotubes (CNTs) are being used as filler in epoxy matrix to produce isotropically conductive adhesives (ICAs). Different loadings of multiwalled carbon nanotubes were used to produce composites of different concentrations. These composites have been studied for their thermal behaviour, conductivity and impact properties. Percolation threshold is very low and conductivity value of 10 ∼ 2Scm_1 was obtained at a filler concentration of only 0.3%. Higher percentage of the epoxy shear strength is retained. Differential scanning calorimetry results do not show any major influence on typical curing peaks of epoxy. Scanning electron microscopy confirmed that CNTs are easily dispersed in the epoxy matrix and there is a strong phase interaction. These composites with very low filler loadings showed a great prospect of being used as ICAs.