Electrically Conductive Adhesives Research Articles

Environmental influence on cracking and debonding of electrically conductive adhesives

Electrically conductive adhesives (ECAs) are starting to replace metallic solders in recent designs of photovoltaic (PV) modules. This transition represents a significant material change, and a proper understanding of the durability and reliability of the new interconnect needs to be established. This paper presents our continued work on developing a degradation model for ECA interconnects in PV modules. Here, we characterize the fracture mechanics properties of an epoxy-based ECA, for both critical and subcritical, mode I and mode II loading conditions. Emphasis is on the influence of different environmental conditions such as temperature and humidity. We use the Finite Element Method to account for residual stresses, induced by temperature changes and moisture absorption, and correct the apparent fracture toughness. We found that high moisture levels not only can weaken the fracture resistance of the ECA interconnect, but can also promote subcritical debonding at significantly lower driving forces than in dry environments.

Electrical Characteristics Analysis of Bonded Cells for Shingled Modules.

A shingled module fabricated using electrically conductive adhesive (ECA) can increase the light-receiving area and provide greater power than a conventional module fabricated using solder-coated copper ribbons. However, several issues such as damage from laser cutting and poor contact by the conductive paste may arise. In this study, a 15.675 × 3.1 cm² c-Si cut cell was fabricated using a nanosecond green laser, and cell bonding was performed using ECA to fabricate shingled modules. If the laser process was performed with high speed and low power, there was insufficient depth for cut cell fabrication. This was because the laser only had a thermal effect on the surface. The cell was processed to a depth of approximately 46 μm by the laser, and it could be seen that the laser cutting proceeded smoothly when the laser process affected more than 25% of the wafer thickness. The cut cell was bonded by ECA, and the process conditions were changed. The highest efficiency of 20.27% was obtained for a cell bonded under the conditions of a curing time of 60 s and curing temperature of 150°C. As a result, the efficiency of the bonded cell was increased by approximately 2.67% compared to the efficiency of the conventional cut cell. This was because the shadow loss due to the busbar was reduced, increasing the active area of the module by eliminating the busbar from the illuminated area.

Analysis of Cell to Module Loss Factor for Shingled PV Module

Shingled technology is the latest cell interconnection technology developed in the photovoltaic (PV) industry due to its reduced resistance loss, low-cost, and innovative electrically conductive adhesive (ECA). There are several advantages associated with shingled technology to develop cell to module (CTM) such as the module area enlargement, low processing temperature, and interconnection; these advantages further improves the energy yield capacity. This review paper provides valuable insight into CTM loss when cells are interconnected by shingled technology to form modules. The fill factor (FF) had improved, further reducing electrical power loss compared to the conventional module interconnection technology. The commercial PV module technology was mainly focused on different performance parameters; the module maximum power point (Pmpp), and module efficiency. The module was then subjected to anti-reflection (AR) coating and encapsulant material to absorb infrared (IR) and ultraviolet (UV) light, which can increase the overall efficiency of the shingled module by up to 24.4%. Module fabrication by shingled interconnection technology uses EGaIn paste; this enables further increases in output power under standard test conditions. Previous research has demonstrated that a total module output power of approximately 400 Wp may be achieved using shingled technology and CTM loss may be reduced to 0.03%, alongside the low cost of fabrication.

Recombination and Resistive Losses of Transferred Foil Contacts for Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cells have been studied extensively due to their potential to reach high energy conversion efficiencies. However, one of the limiting factors for this technology is the metallization, which uses low‐curing‐temperature silver pastes that result in fingers with higher bulk resistivity. Herein, a simple approach to fabricate the SHJ solar cell contacts is demonstrated with commercially available low‐cost silver (Ag) electrically conductive adhesive (ECA) pastes and aluminium (Al) foils. This technique can result in a transparent conductive oxide (TCO)‐free cell structure and has the potential to combine cell metallization and interconnection. Recombination and resistive loss analyses are performed on the fabricated test samples. A dark saturation current density at the contact (J0c) of 3.05 fA cm−2 for test samples before annealing is reported. The analysis of the experimental and simulated photoluminescence (PL) images shows negligible added recombination. The contact resistivity (ρc) value is 49.3 mΩ cm2 after annealing which can be optimized by specialized ECA pastes.

Effect of Hybridization on the Functional Properties of AgMF–MWCNT-Filled Electrically Conductive Adhesive

A combination of metal and non-metal filler in an epoxy resin, called a hybrid electrically conductive adhesive (ECA), is an important development in the field of highly conductive electronic interconnect materials. Carbon nanotubes (CNTs) are well known for contributing strength and stiffness to ECA and silver (Ag) has been widely used as a conductive metal. This study presents the characterization of a hybrid silver micro-flake (AgMF) with multiwalled carbon nanotubes (MWCNT) in an epoxy matrix ECA, in terms of electrical and mechanical properties. The weight percentage of AgMF used was varied, from 1 wt.% to 10 wt.%, whereas the weight percentage of MWCNT filler loading was maintained at 5 wt.%. The properties of the hybrid ECA were characterized using a four-point probe and a universal testing machine for lap shear tests. It was found that the filler hybridization lowered the performance of the ECA in terms of both electrical and mechanical properties, as compared with non-hybrid MWCNT-filled ECA. This may be attributed to the weak interaction between micro- and nano-filler particle sizes and agglomeration of the MWCNT in the epoxy matrix in the hybrid ECA system. The hybrid ECA electrical conductivity was successfully enhanced when a low ratio of AgMF and MWCNT was considered. In addition, failure analysis confirmed that the hybrid ECA with less AgMF filler loading exhibits improved adhesion strength, suggesting a superior epoxy-to-substrate bonding interface.

Highly Conductive and Highly Dispersed Polythiophene Nanoparticles for Fabricating High-Performance Conductive Adhesives

The continuous advancement of modern electronic equipment requires increasing the conductivity of electrically conductive adhesives (ECAs), but the gap between discrete silver flakes in conventiona...

Preparation of gold-decorated silver nanowires for improving conductivity of electrically conductive adhesives

Excellent electrical conductivity generally conflicts with high lap shear strength for electrically conductive adhesives (ECAs). One-dimensional silver nanowires (AgNWs) as conductive fillers can effectively solve the problem. However, insulating ligands on the nanowires will form a large contact resistance between the nanowires and limit the electrical conductivity of ECAs. In this work, we reported a one-step method to prepare Au-decorated AgNW conductive fillers. Through controlling the filler content, curing temperature, and curing time, the ECAs filled with Au-decorated AgNWs obtained exceptional electrical performance. When the filler content was 39 wt%, the resistivity reached to 7.8 × 10−5 Ω cm, which had a strong competitiveness to tin–lead solder. Therefore, the feature opens up possibilities for ECAs filled with the hybrid nanofiller to completely replace traditional solder.

Mixed Carbon Nanomaterial/Epoxy Resin for Electrically Conductive Adhesives

The increasing complexity of printed circuit boards (PCBs) due to miniaturization, increased the density of electronic components, and demanding thermal management during the assembly triggered the research of innovative solder pastes and electrically conductive adhesives (ECAs). Current commercial ECAs are typically based on epoxy matrices with a high load (>60%) of silver particles, generally in the form of microflakes. The present work reports the production of ECAs based on epoxy/carbon nanomaterials using carbon nanotubes (single and multi-walled) and exfoliated graphite, as well as hybrid compositions, within a range of concentrations. The composites were tested for morphology (dispersion of the conductive nanomaterials), electrical and thermal conductivity, rheological characteristics and deposition on a test PCB. Finally, the ECA’s shelf life was assessed by mixing all the components and conductive nanomaterials, and evaluating the cure of the resin before and after freezing for a time range up to nine months. The ECAs produced could be stored at −18 °C without affecting the cure reaction.

Regulation of multidimensional silver nanostructures for high-performance composite conductive adhesives

Electrically conductive adhesives (ECAs) are widely used in electronics and silver-based ECA is one of the most typical representatives. In this work, a trinary silver-based filler strategy was proposed to achieve ECAs with high conductivity and low silver content, where silver fractal flowers (Ag-FWs) were used to set up the conductive framework and silver nanowires (Ag-NWs) and silver nanoflakes (Ag-NFs) were added to connect the separated domains and fill the small gaps, respectively. The ECAs reached a low electrical resistivity of 6.0 × 10−5 Ω cm and high adhesion strength of 13.5 MPa when the addition of Ag-FWs, Ag-NWs and Ag-NFs were 50 wt%, 7.5 wt% and 2.5 wt%, respectively, superior to those reported ECAs of 80 wt% silver loading. Besides, the ECA circuit fabricated by 3D printer demonstrated good electrical conductivity and resistance stability. The preparation idea of ECAs brings novel opportunities for the development of the electronic packaging industries.

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.

Conducting polymer-based electrically conductive adhesive materials: design, fabrication, properties, and applications

In the past few decades, increasing demands for electrically conductive adhesives (ECAs) have led to growing interest in the design and development of innovative strategies to obtain materials with synergetic or complementary properties for various industrial as well as biomedical applications. In this context, the replacement of traditional tin/lead (Sn/Pb) solders due to their corrosion, low strength of joints, solder joint fatigue, stress-induced cracking within the interconnect, as well as environmental issues are attracted a great deal of interests, especially in industrial committees. The significant progress in polymer science as well as the advent of nanotechnology, have been led to design and development of alternative materials with higher performance over conventional adhesives. On the other hand, intrinsically conductive polymers (ICPs) offer promising materials for the replacement or reducing the content of metallic fillers (e.g., silver, gold, nickel, or copper) in ECAs due to some disadvantages of metallic fillers. For the first time, an overview of the recent progress in the design, fabrication, and applications of the ECAs based on ICPs is presented.

Synthesis of Ag-Cu based epoxy polymer composite for electrical conductivity

In this study bimetallic Ag-Cu alloy has been synthesized using chemical reduction method. Fabricated Ag-Cu nanoparticles were used as conductive filler and the epoxy resin as polymer to produce Ag-Cu epoxy composites. Conductive filler with different concentration (1 wt% 2 wt%, 3 wt%, 4 wt%, 5 wt% and 10 wt%) were added in epoxy resin to investigate the electrical properties of Ag-Cu based epoxy composites . XRD pattern of Ag-Cu alloy formed fcc phase with miller indices (0 0 2), (1 1 1) and (2 0 0). For surface morphology, scanning electron microscopy (SEM) was used to perceive structure of alloy based epoxy composites and EDX confirmed the Ag-Cu elements in samples. Surface and volume resistivity of epoxy-based composite was investigated by Mega Ohm Meter-1865. Electrical conductivity of the Ag-Cu based epoxy polymer composites was measured to enhance the flow of electron in dielectric property of epoxy resin with conductive material of Ag-Cu alloy for practical application and potential fabrication of numerous electronic devices such as electrical conductive adhesives as well as micro/nano electro-mechanical systems.

Highly dispersed polypyrrole nanotubes for improving the conductivity of electrically conductive adhesives

It is imperative to fabricate electrically conductive adhesives (ECAs) with excellent electrical performance and mechanical properties. In this article, a kind of polypyrrole nanotubes (PPy nanotubes) having a high aspect ratio and excellent dispersibility in various kinds of organic solvents were prepared and added to conventional Ag-containing adhesives. Stable suspension characteristics of PPy nanotubes in common solvents provided homogeneous dispersion of the PPy nanotubes in the composites. A small amount of PPy nanotubes can remarkably change the structures of the conductive networks of conventional ECAs and significantly improve the ECAs’ conductivity. By only adding 3 wt% PPy nanotubes, the resistivity (5.8 × 10−5 Ω ‧ cm) of the ECAs containing 55 wt% silver decreased to 1/1000 of the comparative ECAs without PPy nanotubes. This resistivity is almost five to one-tenth of the ECAs materials reported by now. Furthermore, the resulted ECAs showed excellent mechanical properties. The electrical resistivity of the new PPy nanotube-containing ECAs remained stable after they were rolled at a 6 mm bending radius for over 5000 cycles or pressed under 1200 kPa. An elastic printed circuit was fabricated using the above-described ECA-containing PPy nanotube, which demonstrates its potential application in the field of flexible electronics.

Simultaneous improvement of the electrical conductivity and mechanical properties via double-bond introduction in the electrically conductive adhesives

For electrically conductive adhesives (ECAs), high electrical conductivity generally conflicts with excellent mechanical properties because electrical conductivity increases while desirable mechanical properties decrease with the increase in loading of conductive fillers or removal of lubricants on the silver filler surface. In this work, a method was developed to improve both the electrical conductivity and mechanical properties of the ECAs by introducing double bonds. Itaconic acid (IA) was used to replace the lubricant on the silver flake (Ag-F) surface, and acrylic acid (AA) was used as part of the ECA resin matrix. IA can replace the lubricant on the Ag-F surface, similar to other short-chain dibasic acids, to improve the ECA electrical conductivity. Furthermore, IA can be polymerized with AA to form covalent bonds between the silver flakes and the resin matrix, enhancing the mechanical properties of ECA. Compared with ECA filled with commercial silver flakes, the electrical conductivity of ECA filled with IA-treated silver flakes increased by ~ 21%, and the lap shear strength increased by ~ 29%.

A review on epoxy-based electrically conductive adhesives

Electrically conductive adhesives (ECAs) are widely utilized in various electronic packaging applications including die fitting, solderless interconnections, component renovation, display interconnections, and heat dissipation, etc. ECAs seize attention of scientific researchers as green and sustainable lead-free interconnection material over conventional solders owing to their eco-friendliness, processing capability at low temperature, fewer processing steps, negligible stress on a substrate, flexibility and stretchability, and cost-effectiveness. Various researchers worked in the area of epoxy based conductive adhesives mainly focusing on the categories, conduction mechanisms, applications of conductive adhesives along with various types of conductive fillers, inherent conductive polymers and future prospective of epoxy resin in the field of conductive adhesives is presented.

Design of a solar cell electrode for a shingled photovoltaic module application

New technologies to fabricate high-output power photovoltaic (PV) modules include a cell dividing and bonding technique. This technique divides and interconnects cells into a string arranged in series and in parallel to produce a module. Therefore, we designed a 3–6 dividing front electrode structure that is suitable for the shingled module. Thus, power loss was calculated based on the number of cell divisions and the number of fingers. The simulation results indicated that increases in the number of cells to be divided decreased the number of fingers exhibiting the maximum efficiency. The number of fingers optimized for division in to 5 cells was 128. Additionally, the power conversion efficiency was 17.346%, and this corresponded to the highest efficiency among various electrode structures for division from three to six solar cells. The optimized finger number for division into 3 cells was 171, and this corresponded to the lowest efficiency of 16.855%. The multi-crystalline silicon solar cells exhibiting a finger number of 100 were fabricated to compare with the simulation results. We analyzed the characteristics and obtained results that were nearly similar to those of the simulation. For application to a shingled module, a solar cell with an appropriate electrode structure was divided into 5 cells via the laser scribing system, subsequently bonded with an electrically conductive adhesive (ECA), and the characteristics were analyzed.

Metallization of crystalline silicon solar cells for shingled photovoltaic module application

The shingled photovoltaic (PV) module is a high-power PV module technology that is manufactured cells dividing and bonding with an electrically conductive adhesive (ECA). Here, the ECA is composed of 70–80% of silver, an acrylic, and a solvent. We focused on the formation of solar cell metallization suitable for the shingled PV module. Generally, the Ag pads on the rear side of the crystalline Si solar cells don’t printed on the Al surface because of the peeling problem. Therefore, a back surface field (BSF) layer is not formed at this area. The characteristics of the solar cell can be improved by applying a rear Ag pad together with the BSF layer in the entire area of the back surface of the solar cell. The formation of the Al-BSF layer and the peeling of the Ag pad off from the interface between Al-BSF and Al were analyzed using by a scanning electron microscope. The peeling phenomenon could be removed by the adjustment of the elevation rate to the peak temperature during co-firing process. The external quantum efficiency (EQE) analysis revealed the influence of the full Al-BSF layer on the rear side of the solar cells showing higher values in the long-wavelength region. Additionally, the divided cell strips were interconnected via ECA bonding, and the characteristics were investigated. The interconnected cell exhibited the efficiency of 18.302% while the efficiencies of two cell strips were 18.039% and 18.162%, respectively.

The mixture of silver nanowires and nanosilver-coated copper micronflakes for electrically conductive adhesives to achieve high electrical conductivity with low percolation threshold

For electrically conductive adhesives (ECAs), it is critical to ensure high electrical conductivity and good adhesion reliability for practical applications in electronic products. In this study, high conductivity ECAs at low filler content were achieved by fully utilizing the synergistic effect of nano- and micro-scale fillers. Nanosilver-coated copper micronflakes (Ag@Cus) prepared by an electroless plating method were selected as the main conductive fillers. Silver nanowires (AgNWs) made by a simple one-step polyol method were dispersed as auxiliary conductive fillers in Ag@Cus, providing more possibilities for establishing a large number of conductive bridges. Furthermore, the curing process of matrix resin was predicted based on curing kinetics and was verified by corresponding experiments to determine the optimal curing parameters. The results show that the combination of AgNWs and Ag@Cus in ECAs can significantly reduce the bulk resistivity and percolation threshold compared with the single component. In particular, the bulk resistivity of ECAs filled with AgNWs and Ag@Cus (mass ratio 1:9) is as low as 9.42 × 10−5 Ω cm when the filler content is only 60 wt%.

Easily Synthesized Polyaniline@Cellulose Nanowhiskers Better Tune Network Structures in Ag-Based Adhesives: Examining the Improvements in Conductivity, Stability, and Flexibility.

It is essential to develop a novel and versatile strategy for constructing electrically conductive adhesives (ECAs) that have superior conductivity and high mechanical properties. In this work, easily synthesized polyaniline@cellulose (PANI@CNs) nanowhiskers with a high aspect ratio and excellent solubility in 1,4-dioxane were prepared and added to conventional Ag-containing adhesives. A small amount of PANI@CNs can dramatically tune the structure of the ECAs’ conductive network and significantly improve the conductivity of the ECAs. Good solubility of PANI@CNs in solvents brings excellent dispersion in the polymer matrix. Thus, a three-dimensional (3D) conducting network formed with dispersed PANI@CNs and Ag flakes can enhance the conductivity of ECAs. The conductivity of the ECAs (with 1.5 wt% PANI@CNs and 55 wt% Ag flakes) showed three orders of magnitude higher than that of the ECAs filled with 55 wt% Ag flakes and 65 wt% Ag flakes. Meanwhile, the integration of PANI@CNs with Ag flakes in polymer matrices also significantly enhanced the mechanical compliance of the resulted ECAs. The resistivity remained unchanged after rolling the PANI@CNs-containing ECAs’ film into a 4 mm bending radius for over 1500 cycles. A bendable printed circuit was fabricated using the above PANI@CNs-containing ECAs, which demonstrated their future potential in the field of flexible electronics.

Structural Design of Three-Dimensional Graphene/Nano Filler (Al2O3, BN, or TiO2) Resins and Their Application to Electrically Conductive Adhesives.

In this study, we designed a three-dimensional structure of electrically conductive adhesives (ECAs) by adding three different kinds of nano filler, including BN, TiO2, and Al2O3 particles, into a few-layered graphene (FLG)/polymer composite to avoid FLG aggregation. Three different lateral sizes of FLG (FLG3, FLG8, and FLG20) were obtained from graphite (G3, G8, and G20) by a green, facile, low-cost, and scalable jet cavitation process. The corresponding characterizations, such as Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM), verified the successful preparation of graphene flakes. Based on the results of four-point probe measurements, FLG20 demonstrated the lowest sheet resistance value of ~0.021 Ω/■. The optimized ECAs’ composition was a 60% solid content of FLG20 with the addition 2 wt.% of Al2O3. The sheet resistance value was as low as 51.8 Ω/■, which was a reduction of 73% compared to that of pristine FLG/polymer. These results indicate that this method not only paves the way for the cheaper and safer production of graphene, but also holds great potential for applications in energy-related technologies.