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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)

Applied Chemistry for Engineering Vol.35 No.6 pp.587-592
DOI : https://doi.org/10.14478/ace.2024.1072

Synthesis of Highly Reliable Conductive Pastes with Silver-Graphene Composites

Sangmin Lee, Kye Sang Yoo†

Department of Chemical & Biomolecular Engineering, Seoul National University of Science and Technology, Seoul 01811, Korea

^† Corresponding Author: Seoul National University of Science and Technology Department of Chemical & Biomolecular Engineering, Seoul 01811, Korea Tel: +82-2-970-6602 e-mail: kyoo@seoultech.ac.kr

Received October 14, 2024 ; Review November 6, 2024 ; Accepted November 11, 2024

Abstract

This study focuses on the synthesis and optimization of Ag-graphene composite particles and their incorporation into conductive pastes for electronic applications. The composites were synthesized by mixing organic solvents to facilitate graphene formation on silver particle surfaces, followed by ultrasonic irradiation. Main parameters, including solvent ratio and ultrasonic power, were adjusted to optimize the carbon matrix, targeting a thickness of less than 2 nm for effective dispersion in paste formulations. The optimal synthesis conditions were identified as 100 °C for 2 hours, using 80% ethanol and 1500 W ultrasonic power. For binder preparation, an epoxy resin was designed with specific curing agents, solvents, and additives to achieve stable room-temperature curing and minimal viscosity fluctuations. The final binder formulation (epoxy, curing agent, solvent, dispersant, and thixotropic agent in a 60:2:30:5:3 ratio) provided excellent stability and dispersion properties. The conductive paste was then produced by combining the optimized Ag-graphene particles with the epoxy binder using a three-roll dispersion process. By varying the particle-to-binder ratio, the 80:20 ratio was found to offer the best balance of shear strength and low resistivity. The paste showed stable viscosity over time, ensuring reliable printability, and scanning electron microscopy (SEM) confirmed strong particle connections, supporting its expected conductivity.

Key Words : Silver-graphene composite , Conductive paste , Epoxy binder

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1. Introduction

Epoxy conductive adhesives are widely used in the electronics industry due to their high chemical and thermal resistance, good mechanical properties, excellent adhesion to a variety of materials, compatibility with a wide range of substrates and additives, low shrinkage, availability of solvent-free formulas, bright colors, and easy viscosity control of the epoxy system[1,2]. Silver is also the most commonly used and continuously researched filler for epoxy conductive adhesives [3,4]. Silver is one of the best conductive fillers because it generally performs well and is relatively inexpensive. Not only does it have the highest room temperature electrical and thermal conductivity of any metal, but its high ductility makes it easy to manufacture into a variety of shapes (e.g., spheres, rods, flakes). Silver can therefore fulfill the high aspect ratio requirements of conductive fillers[5,6].

As such, silver has been widely used as a filler in conductive adhesives, but it suffers from electrochemical migration. Electrochemical migration occurs under certain electric field and electrolyte conditions, during which the anode metal is corroded and dissolved metal ions migrate to the cathode, resulting in electrodeposition[7]. The metal ions are typically deposited on the cathode in the form of dendrites, which can form a bridge connecting the two electrodes and cause a catastrophic short circuit. In the application of epoxy conductive silver ad-hesives, typical electrochemical migration effects occur according to the following mechanisms. Under humid application conditions, water on the anode surface can undergo electrolysis to produce hydroxide ions. When the silver filler comes into contact with these hydroxide ions, it is oxidized to silver ions. The Silver ions slowly migrate to the cathode as the current in the system moves and can be reduced to Silver at the cathode. When the precipitation of Silver around the cathode reaches a certain amount, a silver bridge is formed that causes a short circuit in the system. Also, as the concentration of Silver around the anode decreases, the resistance near the anode also increases[8-10].

In recent years, researches have been conducted to inhibit silver migration in silver-based adhesives. The first method is to remove or separate the water involved in silver migration. Removing water by heat treatment during electronic packaging can inhibit silver migration, which improves the reliability of the product[11]. However, the addition of heat treatment complicates the device assembly process, and it is impossible to keep the silver gel always dry in actual use after curing, so it is not a fundamental method to reduce the effect of silver migration. Kim et al.[12,13] found that hydrophobic treatment of polyimide substrates by plasma discharge technology can effectively slow down the occurrence of silver migration.

Another method is to separate the silver from the substrate to inhibit the ionization of silver. Coating a thin layer of resin on the surface of the circuit can prevent the silver electrode from directly contacting the substrate, thereby delaying silver migration. A gold plate was placed on the surface of the silver electrode to hinder silver migration, and the resistance of the circuit was not significantly affected due to the high conductivity of the gold plate[14]. The disadvantage of this method is that the material and process costs are high and it is not suitable for large-scale applications. Another study reported that Silver wrapped in epoxy resin showed significantly slower silver migration compared to pure silver exposed to air[15]. However, the insulating epoxy resin coated the Silver conductive particles, which adversely affected the conductivity of the entire system.

The introduction of reducing agents to isolate the silver from the external environment while protecting the silver filler from oxidation has also been studied. Coating the silver filler with copper significantly delayed the silver migration[16], but the resistance increased dramatically once the copper coating was created. In other studies, silver migration was effectively inhibited by adding blocking solvents such as organic amine compounds and imidazole compounds to the conductive resin, which are more susceptible to oxidation than silver, and the oxidation of silver was inhibited[17]. However, the introduction of reducing agents affected the bulk properties of the adhesive and the curing behavior of the matrix resin, and the effect of preventing silver migration disappeared when the reducing agent was completely consumed. Thus, developing a comprehensive, economical and simple technique to inhibit silver migration is still a challenging task. In this study, we synthesized high-reliability particles by surface-growing graphene extracted from carbon source on the surface of silver particles and studied the high-strength high-reliability paste using it. By applying physical impact to the carbon source in a wet synthesis method to induce growth at the interface of silver particles, graphene of uniform thickness was grown on the particles by controlling the synthesis conditions. Using these synthesized particles, we studied pastes with high strength and high reliability using epoxy resin and curing agent, thickeners to increase the fluidity of the paste, and dispersants to induce dispersion of the particles with the binder. The paste-like composite materials obtained through this research are expected to be used as chip bonding materials for high-impact electric vehicles and electrodes for display electronics with fine patterns.

2. Experiments

The raw materials used in Ag-graphene composite preparation were Ag powder (MITSUI MINING & SMELTING Co.) and ethanol (99.9%, SK Chemical, Korea) and butyl carbitol acetate (BCA, 98.0%, DAEJUNG Chemical, Korea) as solvents. The synthesis was performed using ultrasonic generator (Max. 1500 W, VCX-1500, Sonic, USA) for ultrasonic wet synthesis and an automatically stirred reaction tank, and a centrifuge (Combi-514R model, Hanil Science) was used for separation. A vacuum dryer (JAOTECH OV4-65) was used for drying to remove moisture from the particles. The binder was designed based on Epoxy (YD-128, Kookdo Chemical, Korea). YD-128 is a BISPHENOL-A TYPE EPOXY RESIN with excellent bonding properties and high strength. Amine-based curing agent with PN50 (Ajinomoto) was also selected and designed accordingly. BCA was used as an organic solvent, and BYK-9076 and BYK-7410 were used as dispersants and thickeners, respectively. To prepare the paste, the binder and the prepared particles were stirred in a paste mixer. Afterward, the synthesis was completed using a 3-Roll Mill (Equality KRM80B), which is used for paste dispersion.

The graphene thickness of the prepared particle composites was analyzed using a transmission electron microscope (TEM, JEM-2010/ JEOL/JP). Resistance measurements were performed to evaluate the conductivity reliability of the fabricated specimens using a cotton resistivity meter (FPP-RS8, DASOL Engineering). For viscosity measurement, a cone type viscometer (DVnext, BROOKFIELD Co.) was used as the analyzing equipment to measure the flowability. The morphology of paste composites was investigated by FE-SEM (EVO10, Carl Zeiss Co.).

3. Result and discussion

3.1. Synthesis of Ag-graphene composite

Composite particles were synthesized by mixing organic solvents capable of forming graphene on the surface of silver particles, followed by ultrasonic irradiation. Crucial factors such as the ratio of both organic solvents and ultrasonic intensity were varied to optimize particle formation. Transmission electron microscopy (TEM) was used to analyze the formation of the carbon matrix. The focus was on achieving a carbon matrix with a thickness of less than 5 nm, the target thickness, as thicker matrices could hinder particle and binder dispersion, leading to phase separation when prepared as a paste. Several organic solvents were investigated, and two were chosen for their environmental stability and availability. Carbon was separated from solvent mixture and grown on the metal particles as a result. Initially, experiments were conducted by varying the ratio of the selected both organic solvents while maintaining consistent ultrasonic power, temperature, and duration.

As shown in Figure 1, increasing the ethanol content facilitated the formation of the carbon matrix. A detailed examination of the TEM image in Figure 2 revealed a carbon matrix at the 1 nm scale. The ratio of organic solvents significantly influenced the carbon content and structure. This is due to the difference in chemical structure and boiling point of the two solvents. BCA is a solvent that combines butyl carbitol (a glycol ether) with an acetic acid group, and has a relatively complex structure that includes an ether bond and an ester group. Ethanol, on the other hand, is a simple alcohol with two carbon atoms and a hydroxyl (-OH) group. BCA also has a relatively higher boiling point than ethanol. Thus, forming a carbon layer using a physical method is not feasible at low ethanol concentrations. Ultimately, achieving clear conditions for particle synthesis allows for the production of highly reproducible materials, particularly when aiming to create pastes with desired properties. Further experiments explored the impact of varying ultrasonic power while keeping other synthesis conditions constant. As shown in Figure 3, ultrasonic power played a crucial role in the formation of carbon particles. The thickness of the carbon increased rapidly as the power increased from the first 50% to 70%, but then increased relatively slowly up to 100% power. Thus, the optimal conditions were found to be 100 °C for 2 hours, with 80% ethanol and 1500 W of ultrasonic power.

3.2. Preparation of binder materials

Conductive paste is manufactured due to the dispersion process of mixing binder and particles, and although the characteristics of the particles are important, the role of the binder is an important factor in determining high reliability. Binders are composed of various organic compounds, especially resins, activators, curing agents, dispersants, dispersants, and various additives and organics that are mixed and then formed into binders through dispersion or thermal processes. The binder is an important material that holds the particles in the paste and makes it possible to form a printable paste. It is necessary to design a binder for the paste that is resistant to shear strength and can print at room temperature for more than 6 hours, which is the goal of high reliability. The first part of a reliable and strong binder design is the underlying resin. The epoxy resin was selected for its thermal strength and good bonding properties.

The epoxy resin binder was formulated based on the content of curing agent, organic solvent, dispersant, and thixotropic agent, with a primary focus on achieving room-temperature curing. This emphasis is due to the tendency of epoxies to shift from one-component systems to two-component systems rapidly at room temperature. Without addressing this, the paste dispersion process would fail, rendering the paste ineffective as an electrode. Initially, a design with 10% curing agent relative to the epoxy content was tested, but as the hardener content increased, property changes began to manifest at room temperature. Subsequently, it was found that when the curing agent was limited to 5% of the epoxy content, stability at room temperature was achieved. The reason for mixing with two recipes is that the necessity for selecting a resin was confirmed through a previous research method, and in the case of epoxy, the use of many materials is extremely limited prior to mixing. Given the strong bonding strength of the epoxy, several experiments were conducted to assess how the resin reacted with the curing agent, particularly in terms of hardness changes. Stability was assessed using a flow meter, while viscosity was measured with the goal of designing a one-component epoxy binder that could double its viscosity within 24 hours. A viscometer was used to confirm this data. For practical industrial use, it is important that viscosity changes remain minimal, and that moisture absorption of the binder at room temperature is kept to a minimum to avoid altering the final paste properties. Regarding the curing agent, accurate measurements at room temperature were challenging when the agent exceeded 10%, but when kept at 5% or below, the curing agent remained stable and could be measured using a viscometer. Through this formulation ratio design and dispersion process, a stable binder recipe was achieved. Finally, the optimal formulation was determined to be a ratio of 60:2:30:5:3 for epoxy, curing agent, solvent, dispersant, and thixotropic agent, respectively.

3.3. Synthesis of conductive paste

The paste was produced by combining optimized particles and binders through a dispersion process. To successfully create the paste, it is crucial to verify the two key raw materials—synthesized particles and binder—through their respective synthesis and dispersion processes. The physical properties of the paste, particularly its resistance, are influenced by the content and quality of the binder and particles. A three-roll dispersion process was used to produce the paste, employing the same equipment as in the binder dispersion phase. Although the process involves simply mixing the epoxy binder with graphene-metal particles, proper dispersion of the particles within the binder is essential. Poor dispersion can lead to significant variations in properties, cause phase separation of the metal particles, and prevent consistent reproducibility of the final material.

To evaluate the differences in conductivity, reliability, and strength based on varying particle and binder content, we adjusted the content levels during the paste formulation process. Since the binder itself is insulating, excessive binder content cannot be used; however, too little binder can compromise the paste's reliability. Achieving the right balance is crucial. The graphene-metal epoxy paste developed through this process demonstrated stable oxidation resistance, reliable conductivity, and strong adhesion, which were the key objectives of the experiment. Various printing methods, such as PAD printing and screen printing, are employed depending on the application. PAD printing requires advanced technology, as the paste is first applied to a silicon pad via adhesion and then transferred to the substrate. However, screen printing is more commonly used due to its ease of use and the ability to adjust the shape of the printed electrode based on the mask pattern. In this study, screen printing was chosen to produce the test specimens. We thoroughly tested the printability of four different pastes with particle- to-binder ratios of 80:20, 70:30, 60:40, and 50:50. As shown in Figure 4, all four formulations exhibited satisfactory printability, with uniform printing results and no surface curvature after printing.

The joint strength, which has the highest influence on reliability, fluctuated greatly depending on the content of powder and binder. Since it was necessary to confirm conductivity and hardness, shear strength and resistance analysis of the paste for all content conditions were conducted. As shown in Figure 5, the paste with an 80:20 binder- to-powder ratio exhibited the highest shear strength and the lowest resistivity, making it the most suitable material. Higher shear strength indicates greater reliability in future product applications, as it reflects the material’s ability to withstand impact. However, prioritizing strength too much can lead to reduced electrical conductivity, making the paste less effective as an electrode. Since binders, particularly epoxies, are highly insulating, it is essential to strike a balance between achieving sufficient shear strength and maintaining good conductivity. By selecting the right proportion of binder and powder, a reliable paste can be formulated that offers both strength and effective electrical performance.

Viscosity stability is a fundamental property of any paste, as it directly affects the fluidity required for proper printing and curing into electrode and wiring forms. Viscosity can be assessed in several ways: at room temperature, over time (storage stability), and based on weight gain. In this study, room temperature viscosity—specifically its stability over time—was evaluated using the 80:20 binder-to-powder ratio as the reference material. As shown in Figure 6, the viscosity remained stable over time without causing print collapse. This stable viscosity graph suggests that both the binder and particle distribution are well stabilized. A cross-sectional analysis of the paste was performed to confirm the curing reaction, and scanning electron microscopy (SEM) was used to verify the connections between the particles in line with their conductive properties. As shown in Figure 7, the analysis further supports the expectation of positive results for this formulation, confirming the formation of strong connections between the particles.

4. Conclusion

In conclusion, epoxy conductive adhesives play a crucial role in the electronics industry due to their exceptional mechanical, chemical, and thermal properties. Silver, as a conductive filler, has been widely used for its high electrical and thermal conductivity. However, its susceptibility to electrochemical migration poses a significant reliability challenge, leading to extensive research into various mitigation techniques. These methods include physical and chemical approaches, such as hydrophobic treatments, resin coatings, and the incorporation of reducing agents. Despite advancements, each solution has its drawbacks, including increased costs, decreased conductivity, or compromised adhesive performance. This study introduces a novel approach by synthesizing silver particles coated with graphene through ultrasonic wet synthesis. The resulting silver-graphene particles exhibit enhanced resistance to electrochemical migration while preserving strong electrical conductivity. The optimized binder formulation and conductive paste design facilitate the production of high-reliability, high-strength materials suitable for advanced electronic applications, such as chip bonding in electric vehicles and display electronics. The findings, particularly the effectiveness of the 80:20 binder-to-particle ratio, emphasize the balance achieved between shear strength, conductivity, and viscosity stability, offering a promising solution to the persistent challenge of silver migration.

Acknowledgment

This study was supported by the Research Program funded by the SeoulTech(Seoul National University of Science and Technology)

Figures

Figure 1.The effect of ethanol ratio on the formation of carbon thickness over Ag particle. The composites were prepared at temperature of 100 °C, synthesis time of 2 h, and ultrasonic power of 1500 W.

Figure 2.TEM image of Ag-graphene composites synthesized at temperature of 100 °C, synthesis time of 2 h, ultrasonic power of 1500 W, and ethanol ratio of 80%.

Figure 3.The effect of ultrasonic power (Max. 1500 W) on the formation of carbon thickness over Ag particle. The composites were prepared at temperature of 100 °C, synthesis time of 2 h and ethanol ratio of 80%.

Figure 4.Image of screen-printed paste prepared with various particle-to-binder ratios.

Figure 5.The effect of particle-to-binder ratios on shear resistance and strength of the pastes.

Figure 6.Viscosity changes of Ag-graphene paste prepared with particle-to-binder ratios of 80:20.

Figure 7.SEM image of Ag-graphene paste prepared with particle-to-binder ratios of 80:20.

Tables

References

S. R. Mousavi, S. Estaji, H. Kiaei, M. Mansourian-Tabaei, S. Nouranian, S. H. Jafari, H. Ruckd¨aschel, M. Arjmand, and H. A. Khonakdar, A review of electrical and thermal conductivities of epoxy resin systems reinforced with carbon nanotubes and graphene-based nanoparticles, Polym. Test.,112, 107645 (2022).
S. Kumar, S. Krishnan, S. K. Samal, S. Mohanty, and K.S. Nayak, Toughening of petroleum based (DGEBA) epoxy resins with various renewable resources based flexible chains for high performance applications: A review, Ind. Eng. Chem. Res., 57, 2711-2726 (2018).
Y. Chen, Y. He, J. Guo, X. Yang, B. Guo, H. Shen, and X. Wang, A new one-component electrically conductive adhesive with excellent electrical conductivity and mechanical properties using tetra-functional polyurethane acrylate oligomers and silver particle fillers, Int. J. Adhes. Adhes., 124, 103388 (2023).
Li Yi and C.P. Wong, Recent advances of conductive adhesives as a lead-free alternative in electronic packaging: Materials, processing, reliability and applications, Mater. Sci. Eng. R Rep., 51, 1-35 (2006).
R. Aradhana, S. Mohanty, and S. K. Nayak, A review on epoxybased electrically conductive adhesives, Int. J. Adhes. Adhes., 99, 102596 (2020).
H. Derakhshankhah, R. Mohammad-Rezaei, B. Massoumi, M. Abbasian, A. Rezaei, H. Samadian, and M. Jaymand, Conducting polymer-based electrically conductive adhesive materials: Design, fabrication, properties, and applications, J. Mater. Sci., 31, 10947- 10961 (2020).
L. B. Gustavo, V. S. Benjamin, S. W. Miguel, R. G. Navor, T. V. Hugo, and S. H. Gerardo, Influence of climate factors on copper corrosion in electronic equipment and devices, Anti-Corros. Methods Mater., 57, 148-152 (2010).
M. H. Kim, T. H. Kim, J. A. Ko, S. Ko, J. M. Oh, and H. J. Park, Kinetic and thermodynamic studies of silver migration from nanocomposites, J. Food Eng., 243, 1-8 (2019).
S. Chaikin, J. Janney, F. Church, and C. McClelland, Silver migration and printed wiring, Ind. Eng. Chem., 51, 299-304 (1959).
G. Liu, Y. Liu, J. Zhao, A. Pan, B. Wang, and T. Zhu, Controllable Ag migration to form one-dimensional Ag/Ag2S@ZnS for bifunctional catalysis, ACS Appl. Energy Mater., 3, 6146-6154 (2020).
Y. Wu, L. Qian, and D. Dong, Preventive measures for conductive adhesive silver migration failure in multichip modules, Electron. Process Technol., 39, 136-139 (2018).
K. S. Kim, J. O. Bang, and S. B. Jung, Electro-chemical migration behavior of silver nanopaste screen-printed for flexible and printable electronics, Curr. Appl. Phys., 13, 190-194 (2013).
K. S. Kim, Y. T. Kwon, Y. H. Cho, and S.B. Jung, Electrochemical migration of Ag nanoink patterns controlled by atmospheric-pressure plasma, Microelectron. Eng., 106, 27-32 (2013).
G. T. Kohman, H. Hermance, and G. H. Downes, Silver migration in electrical insulation, Bell Syst. Tech. J., 34, 1115-1147 (1955).
B. I. Noh, J. W. Yoon, K. S. Kim, S. Kang and S. B. Jung, Electro-chemical migration of directly printed Ag electrodes using Ag paste with epoxy binder, Microelectron. Eng., 103, 1-6 (2013).
Xiaoyun Zhu, Y. Liu, J. Long, and X. Liu, Electrochemical migration behavior of Ag-plated Cu-filled electrically conductive adhesives, Rare Met., 31, 64-70 (2012).
M. Fujimoto, H. Koyama, M. Konagai, Y. Hosoi, K. Ishihara, S. Ohnishi, and N. Awaya, Appl. Phys. Lett., 89, 223509 (2006).