Electrically Conductive Adhesives Research Articles

Electrically Conductive Adhesives in Microelectronics Packaging

Abstract Electronic packaging is integral to safeguarding electronic devices while ensuring electrical connectivity and heat dissipation. This paper reviews electrically conductive adhesives (ECAs), focusing on two main types: isotropic conductive adhesives (ICAs) and anisotropic conductive adhesives (ACAs). ECAs offer advantages over traditional solders, including lower processing temperatures, environmental friendliness, and the ability to conform to flexible substrates. The paper explores the working mechanisms of ICAs and ACAs, highlighting their limitations and recent developments aimed at improvement. Key challenges for ICAs include low electrical conductivity and moisture absorption, while ACAs face limitations in fine-pitch applications and electric field-induced particle movement. Recent advancements discussed include the use of organic monolayers, nanofiber integration, magnetic self-assembly, low-temperature sintering of nano-silver particles, and copper nanoparticle fillers. These innovations hold promise for enhancing the electrical conductivity, mechanical strength, and reliability of ECAs. Finally, the paper explores applications of ECAs in die attach, flip-chip bonding, and chip-on-flex (COF) packaging, highlighting their potential for various electronic devices.

The effects of increasing filler loading on the contact resistivity of interconnects based on silver–epoxied conductive adhesives and silver metallization pastes

AbstractOur previous work highlighted how microscopic structural effects influence the sheet and contact resistance of electrically conductive adhesives (ECAs). Herein, we delve further by investigating how the contact and bulk resistivity of several ECAs that are based on the same formulation, but with different filler content, are correlated with the filler content. Additionally, two different filler geometries — high and low surface area (HSA and LSA) fillers — are combined in different ratios to maintain a similar viscosity and therefore processability. Hence, contact and bulk resistivities are also correlated with the different geometry ratios of these two fillers. As expected, it was found that the contact and bulk resistivities decreased when the filler content was increased. However, the magnitude of the decrease was found to depend strongly on the filler geometry ratio. At extreme filler geometry ratios, when the bulk is either mostly loaded with HSA‐fillers or mostly with LSA‐fillers, the impact of changes in the filler content on the bulk and contact resistivities is markedly different. The measured data is interpreted within the context of percolation theory and it is determined that the optimum ratio of the LSA and HSA Ag‐fillers investigated in this study is approximately 60:40 (for an epoxy‐based adhesive). This work has important ramifications for the design of ECAs, where cost considerations and the need to reduce silver resource usage demand the lowest (silver) filler content, but the demands of product performance point to higher filler content.

Progress in polyacrylate‐based electrically conductive adhesives: Featured properties, preparation, applications, and perspectives

AbstractPolyacrylate‐based electrically conductive adhesives (PA‐based ECAs) have attracted much attention due to their wide applications in precise instruments and consumer electronics. To obtain high‐performance and environmentally friendly PA‐based ECAs, their main properties and corresponding mechanisms to performance, chemical constitutions and structure–function relationships should be well understood, and the applications of recent PA‐based ECAs should be comprehensively summarized. This review focuses on the progress of PA‐based ECAs in the past two decades and discusses the featured properties, preparation of PA, the selection of conductive fillers and the adhibition of the ECAs. The aim of the review is to find the challenges of high‐performance ECAs and offer some measures and foresee their future development.Highlights Preparation methods of polyacrylate (PA)‐based composites are presented. Properties of PA‐based electrically conductive adhesives (ECAs) are outlined. Selection of conductive fillers and the adhibition of ECAs are discussed. Representative applications of PA‐based ECAs are exhibited. Present main challenges and future solutions are highlighted.

Highly Stretchable Photovoltaic Module with Improved Mechanical Robustness through Elastomeric Interconnections

Stretchable photovoltaics have garnered increasing attention due to their adaptability to various surfaces and applications, meeting the rising demands for portable and flexible power sources. Herein, stretchable solar modules are demonstrated by sandwiching electrically connected rigid solar cells between top and bottom polydimethylsiloxane (PDMS) layers, which are then attached onto an elastomer with an island and trench structure. The electrical connection of solar cells is achieved by dispenser printing of electrically conductive adhesive (ECA). The fabricated stretchable solar modules have a maximum biaxial elongation of 75%, and the electrical connection of the solar cells using ECA results in a negligible efficiency loss of 1.6%. In particular, embedding of solar cells and interconnects between PDMS layers enhances mechanical stability by placing these key components at the mechanical neutral plane and the long‐term stability against humidity due to the encapsulation role of the PDMS layer. The module demonstrates only a 5% of efficiency drop after 1000 hours in the dump‐heat test (85°C/85%). The results in this study are not limited to solar cells but can be applied to various types of rigid devices, paving the way for the commercialization of stretchable electronics.

Die shear analyze of electrically conductive adhesives in GaN wafer application

Because of the demand for high-speed and broadband networks, 5G technology has paid much attention. On the one hand, due to the transit frequency of 5G, which is 450 MHz ~ 6GHz and 24 GHz ~ 52 GHz, the RF component has to deal with a higher frequency than previous communication. On the other hand, a massive high-power channel is required for some applications, like wireless communication base stations, military applications, satellite communication, and TV broadcasting. Therefore, GaN has become a good choice for high-power and high-frequency RF applications. In GaN RF chips, electrically conductive adhesive (ECA) is a key component that restricts heat, electrical conductivity, and reliability. This research studied two ECAs in the die attach process of a 5G base station RF amplifier chip, ABP 8068TA from LOCTITE Corporation and ASP 295-09P9 from Heraeus Group. During the research, ASP 295-09P9 has different die shear performance for GaN and GaAs die. After troubleshooting, operation error, such as the uneven spread of ECA and height of ECA, is excluded. By testing the wafer back surface, it is found that both GaN die and GaAs die back are pure Au, but GaAs die back's roughness is 0.049um, which is rougher than GaN's (0.032um). It is reasonable to assume that a smoother surface is a disadvantage for the necking mechanism.

Performance and durability of electrically conductive tape for shingled Si heterojunction technology cells

AbstractElectrically conductive tape (ECT) was characterized and used to assemble shingled cell strings at low temperature to achieve high reliability Pb‐ and Ag‐free interconnections. The volume resistivity for two considered ECTs are 0.13 ± 0.06 mΩ·cm and 0.47 ± 0.20 mΩ·cm and specific contact resistances, 6.85± 2.00 mΩ·cm2 and 6.30 ± 0.37 mΩ·cm2 using the emerging IEC 62788‐8‐1 Technical Specification for assessment of electrically conductive adhesives (ECA). Durability and performance of the technology in glass–glass mini modules were evaluated with temperature cycling, damp heat testing, and combined‐accelerated stress testing (CAST). Through temperature cycling (−40°C to 85°C) applying five times the mini module short‐circuit current in forward bias and in the multi climate CAST protocol, there was negligible degradation of fill factor after replacing connectors at the modules' cable leads; however, CAST resulted in short circuit current loss attributed to degradation in light collection by the cells, not the ECT. The IEC 61215‐2 85°C, 85% relative humidity damp heat testing showed susceptibility of the HJT cells to effects of humidity in the electroluminescence intensity around the module perimeter that degraded power performance by 4% (relative). Contrasting the IEC 61215‐2 qualification testing‐based damp heat testing with CAST, factors such as the optical stress of CAST may precipitate the degradation of the modules whereas the humidity levels and duration of IEC 61215‐2 damp heat testing may lead to excessive levels of humidity diffused into the modules, potentially resulting in degradation that is unrepresentative of the field.

Preparation of High Conductive Medium and Establishment of Laege Capacity Conductive Channel.

The preparation of highly conductive media and the construction of conducting channels play a crucial role in improving the electrical conductivity of electrically conductive adhesives. Therefore, a new MXene structure is reported in this paper, and the improved structure is rationally designed by computational modeling, which greatly prevents the buildup of MXene nanosheets, improves the stability of the structure, and creates a wide electron transfer channel, and the capacitance contribution of this structure is up to 86.3%. By mixing MXene modified with Ag-plated copper powder in a quantitative relationship to form high conductive media, the electrical conductivity is largely improved and the defect of low electron transfer rate of conventional conductive fillers is broken. The potential value of high conductive media is largely exploited using high throughput and machine learning methods, and here we show that the resistivity has reached 9.668 × 10-7 Ω m. The first principles investigate the conductive channels and electron transfer pathways of high-conductive media at the atomic level, further revealing the mechanism of action of high-conductive media. This study is also the first report on the application of MXene to high-conductive media.

Effect of Ti3C2Tx/Ag/MWCNTs/Ag MXene fillers on the electrical conductivity of Ag-coated Cu conductive adhesives

With the rapid development in the field of microelectronic devices, electrically conductive adhesives (ECAs), which are green and easy to connect, are urgently needed as a new alternative to tin-lead solder. Dendritic Ag-covered Cu (Cu@Ag) particles possessing low resistance and the two-dimensional structure of the conductive materials Ti3C2Tx/Ag/MWCNTs/Ag were combined with each other and used as ideal fillers for developing high-performance ECAs. The two-dimensional structure of the Ti3C2Tx/Ag/MWCNTs/Ag materials allowed for the establishment of large conductive paths within the colloid, followed by Cu@Ag particles filling the gaps within the framework as well as within the colloid, effectively increasing the conductive paths. The effect of the filling amount of the two-dimensional conductive materials Ti3C2Tx/Ag/MWCNTs/Ag on the volume resistance of the ECAs was systematically investigated. More importantly, the volume resistivity of ECAs reached 9.19 × 10−6 Ω m when the percentage of filled micron Cu@Ag particles reached 74 %. When further 0.6 g of Ti3C2Tx/Ag/MWCNTs/Ag filler materials were added, the volume resistance reached a minimum of 4.09 × 10−6 Ω m.

Electrical and mechanical reliability and failure mechanism analysis of electrically conductive adhesives

For electrically conductive adhesives (ECAs) interconnection, adequate electrical, mechanical and thermal performance are crucial for long-term service reliability. The work focused primarily on analyzing the effects of environmental aging on the reliability and failure mechanism of ECAs joints. In this paper, high temperature and humidity (85 °C, 85%RH, 1000 h), thermal shock (−40 °C to 125 °C, 1000 cycles) and thermal aging (125 °C, 1000 h) tests were performed to study the reliability of the ECAs. Additionally, the fracture modes were investigated by mechanical shear strength testing before and after the reliability tests. The interfacial microstructure of the ECAs was investigated and the fracture mechanism of ECAs was characterized. The study showed that the reliability of the ECAs was improved compared with conventional ECA1 and ECA2.

Validation of methodology to determine the contact resistivity of ECA-based bonds grounded on end–contact resistance measurements using redundant and modified TLM test structures

Accurate determination of the contact resistivity between electrically conductive adhesives (ECAs) and solar metallization paste is a key component of optimizing the electrical performance of bonds based on ECAs, such as shingled joints. This work proposed and validates a methodology that accounts for non–negligible inhomogeneities of test structures based on ECAs and silver screen-printed metallization, such as changes in width of the ECA, to improve the accuracy in the calculation of the contact resistivity. The proposed validation uses statistical theory based on the central limit theorem and the law of large numbers showing that the contact resistivity of bonds based on two different commercially available ECAs can be clearly differentiated with a minimum confidence level of 95% and a maximum error margin of 5%. Moreover, the necessary sample size to achieve such requirements is calculated and shown to be equal to 24 test structures. The validation was done using an acryl-based adhesive loaded with silver flakes (∼10−4 Ω cm), which was compared to an epoxide-based ECA filled with silver–coated copper particles (∼10−3 Ω cm). The validated methodology indicates that the contact resistivity when using the acrylic conductive adhesive is 0.1791 ± 0.0766 mΩ cm2 while when using the epoxide-based adhesive is 0.5025 ± 0.1218 mΩ cm2. Additionally, it is shown that the standard deviation of the contact resistivity is highly dependent on the composition of the conductive adhesive. Hence, the sample size varies depending on the ECA. It is expected that the presented methodology will be a powerful instrument for developers and researchers in the improvement and optimization of ECAs and ECA bonding processes.

Low-temperature metallization & interconnection for silicon heterojunction and perovskite silicon tandem solar cells

In this work, we present results on various low-temperature approaches for the metallization and interconnection of high-efficiency solar cells as silicon heterojunction (SHJ) or perovskite silicon tandems. By using fine line screen printing for the cell metallization and Ag-free or -reduced interconnection technologies, we demonstrate the potential of these approaches both for SHJ and perovskite silicon tandem cells. Furthermore, low-temperature (LT, ∼200 °C) or ultra-low-temperature (ULT, ∼150 °C) processes are utilized for metallization and interconnection to treat these temperature-sensitive solar cells with a reduced energy consumption. We compare LT soldering of SHJ cells with Pb-free alloys to state-of-the-art soldering processes and interconnection with electrically conductive adhesives (ECAs). For successful module integration of perovskite silicon tandem solar cells, these findings provide the basis to build full-size tandem modules with different interconnection technologies.

Electrically conductive adhesive-free interconnection of shingle solar cells

This work deals with the usage of electrically conductive adhesives (ECA) for the interconnection of shingle solar cells. In a detailed study on small-format shingle modules characterized by current-voltage, electroluminescence and magnetic field imaging measurements, the impact of an ECA reduction (usage of ECA dashes instead of a continuous ECA line) as well as the complete omission of ECA is investigated with regards to the electrical interconnection quality. Inter alia, it is demonstrated that lamination can improve interconnection quality in ECA-containing shingle joints and establish interconnection in ECA-free shingle joints by pressing adjacent shingles together and thereby forming a busbar-to-busbar contact of decent electrical quality. In a follow-up experiment, the approach of simply omitting the usage of ECA for shingle interconnection is tested with regards to thermal cycling, damp heat and dynamic mechanical load tests finding only minor compromises, if at all, in terms of long-term stability.

Phenol-Furfural Resin/Graphite/Ag-Based Electrically Conductive Adhesive Composites from Waste Bagasse with Enhanced Thermo-Electric Properties.

This study describes the preparation and evaluation of phenol-furfural resin (PFR) from bagasse and its nanocomposites for electrically conductive adhesive (ECA) application. PFR was prepared with furfural extracted from bagasse using a modified acid digestion method. Three different formulations of PFR nanocomposites with conductive nanoparticles, i.e., PFR-silver, PFR-graphite, and PFR-silver + graphite, were prepared using 20, 40, and 60 w/w% of fillers via the impregnation method. The resultant products were characterized using FT-IR, SEM, EDS, and XRD spectroscopy. Electrical conductivity was measured using a four-probe technique, while band gap was calculated via Tauc plots. The results exhibited a significant rise in electrical conductivity of insulating virgin PFR from 2.6 × 10-4 Scm-1 to 8.2 × 10-1 Scm-1 with a 40 and 20 w/w% blend of Ag and graphite in PFR. This synergism was exhibited because graphite and Ag NPs supply excellent junctions for building networks. Both tend to coalesce due to van der Waals forces and high surface energies. Therefore, conductive pathway numbers can be increased, and the contact area can be effectively enlarged. This ternary composite exhibited the lowest bandgap energy value, i.e., 3.1 eV. Thermogravimetric temperature values T0 and Tdeg were increased up to 120 °C and 484 °C, respectively, showing a significant increase in thermal stability. Therefore, the resultant nanocomposite material has good potential to be employed as an ECA in the electronic industry.

Influence of micro– and macrostructure when determining the contact resistivity of interconnects based on electrically conductive adhesives

The contact resistivity of interfaces in solar module interconnects has a direct impact on the series resistance of the entire module and fill factor. Thus, this impacts the performance of entire photovoltaic systems. Accurate measurement of the contact resistance is a key component of optimizing the performance of such interconnects. However, this is difficult for electrically conductive adhesive (ECA) based interconnects. This work shows that transmission line method (TLM) test structures based on ECA display non–negligible inhomogeneities leading to inaccuracies when determining the contact resistivity. Seven methods based on two models — the front– and end–contact TLM models (or a combination of both) — were investigated for four commercially available ECAs used in solar modules and their impacts on the extrapolation of the contact resistivity were quantified. It was determined that even when macroscale inhomogeneities (e.g., variations in the thickness of the ECA) are not present, microscopic structural effects influence the sheet and contact resistance. In particular, variations in the distribution of fillers significantly alter the bulk resistivity of the composites and this variability is also clearly correlated with differences in the geometry of the fillers. It is concluded that the best approach to reduce inaccuracies in the determination of the contact resistivity of ECA–based interconnects is to calculate the sheet and contact resistance locally (using three consecutive contacts) employing a redundant and modified test structure. Afterwards, the contact resistivity ought to be computed using the end–contact TLM model and the median should be assigned as the contact resistivity of the sample.

Improvement in the Thermal Conductivity of Silver Epoxy Adhesive by Treating with Water Vapor.

With the miniaturization of electronic devices, electronic packaging has become increasingly precise and complex, which presents a significant challenge in terms of heat dissipation. Electrically conductive adhesives (ECAs), particularly silver epoxy adhesives, have emerged as a new type of electronic packaging material, thanks to their high conductivity and stable contact resistance. However, while there has been extensive research on silver epoxy adhesives, little attention has been paid to improving their thermal conductivity, which is a critical requirement in the ECA industry. In this paper, we propose a straightforward method for treating silver epoxy adhesive with water vapor, resulting in a remarkable improvement in thermal conductivity to 9.1 W/(m·K), three times higher than the sample cured using traditional methods (2.7 W/(m·K)). Through research and analysis, the study demonstrates that the introduction of H2O into the gaps and holes of the silver epoxy adhesive increases the path of electron conduction, thereby improving thermal conductivity. Furthermore, this method has the potential to significantly improve the performance of packaging materials and meet the needs of high-performance ECAs.

Effect of Ti3C2Tx/Ag MXene fillers on the electrical conductivity of Ag-coated Cu conductive adhesives

In this paper, we prepared ECAs (electrical conductive adhesives) with high electrical conductivity by using Ag-coated copper powder and M − II (Ti3C2Tx/Ag powder) as conductive filler. M-Ⅰ (Ti3C2Tx) and M-Ⅱ were prepared by acid etching and hydrothermal methods, respectively, and the electrochemical properties of M-Ⅱ and the effects of different contents of M-Ⅱ on the conductivity of ECAs were investigated. M − II has a unique reticular structure which helps to create conductive pathways, and its high specific surface area provides a large number of active sites for charge storage, resulting in increased electrical conductivity. The experimental results show that the best conductivity is achieved when the M-Ⅱ powder content is 1.2%, with a volume resistivity of 4.84 × 10−6 Ω m, at which time the Ag-coated Cu powder content is 69.8%. It has been shown in many studies that only when Ag-coated copper powder content is 70%–80%, the resistivity reaches about 10−6 Ω m. This work showed that adding a small amount of M − II and reducing the amount of Ag-coated copper powder could maintain the high electrical conductivity of ECAs.

How to assess the electrical quality of solar cell interconnection in shingle solar modules

AbstractThis work deals with the resistance induced by interconnecting shingle solar cells by means of an electrically conductive adhesive (ECA), which is labeled Rint throughout the work. A new approach to measure Rint directly from shingle joints of actual strings is presented and evaluated. For non‐laminated strings and two different ECAs that are applied as a continuous line on shingles with an edge length of 158.75 mm, similar interconnection resistance Rint ≈ 2.5 mΩ is found. In a full‐size shingle module, such an interconnection resistance would result in a cell‐to‐module loss of around ΔPMPP = −0.8%. Another aspect of the present work is to increase the awareness that in case Rint is unknown, it is not trivial to assess the electrical interconnection quality (of different ECAs, different amounts of the same ECA, etc.). It is demonstrated experimentally that the string/module fill factor can be highly sensitive to current mismatch and is thus not an ideal representative of Rint. The power at maximum power point PMPP tends to indicate Rint differences even less clearly. By simulating virtual strings from a pool of slightly varying shingle I–V curves, it is shown that a more suitable representative of Rint is a quantity that can be derived from the string's/module's dark I–V curve.

A new one-component electrically conductive adhesive with excellent electrical conductivity and mechanical properties using tetra-functional polyurethane acrylate oligomers and silver particle fillers

Electrically conductive adhesives (ECAs) are mainly used in integrated circuit (IC) packaging, light-emitting diode (LED) packaging and other electronic industries to replace traditional solders and lead-free solders. The electrical conductivity and mechanical properties are contradictory with an increase in the content of conductive fillers. Therefore, there is a continuous need to improve electrical conductivity without serious damage to the mechanical properties of ECAs. In this work, a tetra-functional polyurethane acrylate oligomer was used as the main composition of silver filled one-component ECAs to improve performance. By introducing the tetra-functional oligomer, low volume resistivity with good adhesion characteristics was achieved. The volume resistivity and lap shear strength of the silver flake filled ECAs were studied systematically. The ECA with 30.6 vol% content of silver flakes exhibits low volume resistivity of 1.02 × 10−7 Ω m, with a lap shear strength of 8.995 MPa.

Electrolytic Synthesis and Tolyltriazole Modification of Dendritic Silver Powders for High‐performance Electrically Conductive Adhesives

AbstractDendritic silver particles (DSPs) were synthesized by a facile electrolysis method based on the self‐made silver methanesulfonic as an electrolyte and the surface of DSPs was coated with methyl‐1H‐benzotriazole to prepare the surface‐modified DSPs. Dendritic feature with a trunk and nano‐branches is confirmed by scanning electron microscopy. At temperatures above 200 °C, the sintering phenomenon is discovered at the nano‐branches of DSPs. As the prepared DSPs are introduced into the commercial micron‐scale silver particles‐based electrically conductive adhesives (ECAs), the electrical resistivity can be significantly reduced to 5.63×10−5 Ω cm, which is superior to the traditional conductive adhesives derived from the pure micron‐scale silver particles, implying the high application value of DSPs for high‐performance conductive adhesive.

Size-Controlled Low-Melting-Point-Alloy Particle-Incorporated Transient Liquid-Phase Epoxy Composite Conductive Adhesive with High Performances

With fast developments in the environmental-friendly industry, electrically conductive adhesive (ECA) composites with high electrical conductivity, mechanical strength, and electrochemical migration resistance as well as low cost are highly desirable for emerging power electronics applications. In this study, a category of ECAs, namely, size-controllable sonochemical low-melting-point-alloy (LMPA) particle-incorporated transient liquid-phase ECAs (TLP-ECAs), with excellent electrical conductivity, considerable mechanical strength, and high electrochemical migration resistance has been successfully demonstrated. Experimental results showed that the bulk resistivity of TLP-ECA decreased to a minimum value of 2.37 × 10–4 Ω-cm with the remaining filler content at a low level (the total content of metallic fillers, i.e., 70 wt % in TLP-ECAs), while the mechanical shear strength increased considerably (by 55.4%) compared with pure silver ECA with the same filler content, demonstrating that addition of sonochemical LMPA particles into ECA can simultaneously and considerably improve the electrical and mechanical properties. Moreover, the diffusional reaction mechanism of sonochemical LMPA particle-filled TLP-ECA was systematically investigated via differential scanning calorimetry (DSC), X-ray diffraction (XRD), and high-resolution scanning transmission electron microscopy in high-resolution high-angle annular dark-field mode (STEM-HAADF). The mechanism of the diffusion reaction of TLP-ECA with LMPA particles was rationalized, where Ag9In4 (γ phase, cP52, and P43m) and Ag3Sn (ε phase, oP8, and Pmmm) intermetallic compounds (IMC) and their corresponding nanostructures were responsible for the substantial enhancement in mechanical strength and the antielectrochemical migration property. The sonochemical method is the key to synthesize LMPA particles of desirable compositions and controllable size, thereby enhancing the overall performance of Ag-based TLP-ECAs. Accordingly, the sonochemical LMPA particle-incorporated-TLP-ECAs are the promising interconnection material candidates for power electronics packaging.