1. Introduction
With growing interest in eco-friendly technology and energy efficiency in recent years, industries such as electric vehicles, aerospace, and renewable energy have been rapidly progressing. Resultantly, high-performance power semiconductors are becoming increasingly essential for minimizing energy consumption and maximizing efficiency. The power semiconductor market was valued at $23.8 billion in 2023 and is projected to expand at an annual growth rate of 7.0% through 20291). In particular, wide-bandgap (WBG) compound semiconductors such as SiC and GaN are emerging as next-generation power semiconductor devices due to their excellent performance under high voltage, high temperature, and high frequency conditions compared to conventional silicon (Si)-based devices2). Power semiconductors are essential components for power conversion and control, playing a crucial role in efficiently managing and distributing electrical energy within energy systems.
Fig. 1 presents the power system of a general electric vehicle. Inverters and converters in electrical vehicles are essential components that handle power conversion, allowing motors and electronic devices to function within the vehicle. In slow charging using AC power, the power is converted to DC through a converter within the OBC and stored in a high-voltage battery (400- 800 V). The stored DC power is converted again by a converter to a voltage of 12V or 48V and stored in a low-voltage battery. The power is then supplied to various vehicle components or converted into AC power via an inverter to operate the electric motor3). As inverters and converters demand high voltage and power efficiency, power semiconductors with excellent durability and switching performance are used. The reliability of power semiconductors is becoming more essential, leading to increased attention on packaging technology and bonding material technology. Junctions are responsible for electrical connection and heat transfer between the device and package, with the durability and reliability of the device being directly impacted by the bonding material4).
In the packaging structure, which includes a heat sink, base plate, substrate or lead frame, and die as illustrated in Fig. 2, the bonding material in the inter-junction area must have stable mechanical properties as well as excellent electrical and thermal conductivity, owing to the high power density and operating temperature of power semiconductor packaging. Specifically, the bonding materials between the substrate and WBG die must maintain high reliability at temperatures close to 300℃, necessitating the development of new bonding technologies to overcome the limitations of current soldering methods5,6).
Conventionally, flexible solder containing lead (Pb) has been used for bonding power semiconductors. However, due to international regulations (RoHS, etc.) restricting hazardous substances in electronic products, alternative bonding technologies have been developed and commercialized7). Among alternative bonding technologies, silver (Ag)-based nano paste particles have been commercialized as a representative sinter-bonding method that utilizes metal particles. Sintering typically occurs at relatively low temperatures between 200 and 300°C and offers the advantage of forming high-strength junctions suitable for high-temperature atmo- sphere. The sintered silver structure exhibits a high melting point of approximately 961℃, exceptional thermal conductivity exceeding >200 W/(m·K), low electrical resistivity (~1×10-5 Ω·cm), and overall outstanding electrical and thermal performance8,9). However, sintered Ag also has limitations, including high porosity, susceptibility to electrochemical migration (ECM), and significant production costs. A recent study by Zhang et al.10) revealed that higher porosity in the sintered Ag bonding layer reduces bonding strength and compromises long-term reliability. Yang et al.11) found that the reliability of junctions may deteriorate in high-temperature and humid conditions due to the migration of Ag ions. An array of studies are thus underway to overcome these drawbacks.
Therefore, the sinter-bonding technique utilizing Ag particles enhances the reliability of junctions in high- temperature operating environments of power semicon- ductors. As a result, it is increasingly being adopted as a next-generation power semiconductor packaging technology, playing a key role in the development of efficient energy systems. Accordingly, this study seeks to conduct an in-depth analysis of power semiconductor bonding techniques utilizing Ag-based metal particles, while also providing a comprehensive review of recent research trends and technological characteristics.
2. Silver (Ag) Particle Sintering Method
Typically, among sinter-bonding techniques using Ag particles, the paste sintering method occurs as illustrated in Fig. 3. The Ag paste applied to a substrate undergoes a sintering process, either under pressure or without pressure, during which the Ag particles expand due to heat and pressure, creating a dense bond. The sintered Ag does not contain intermetallic compounds (IMC) and demonstrates excellent mechanical properties. The composition of the paste, including dispersant, binder, solvent, and Ag filler, plays a crucial role in determining the characteristics of the sintered junction. Sintering process parameters, including temperature, pressure, and time, greatly influence the formation of necks between particles and the bonding density. Therefore, these factors act as key elements that influence the reliability of junctions and specific mechanical properties, including shear strength, thermal resistance, and fatigue life12).
The type of metal substrate used in sinter-bonding with Ag is also a significant factor influencing the bonding characteristics. Wang et al.13) studied the interfacial reactions and microstructures with different metal substrates (Ag, Au, Cu, Ni) and found that the formation of an oxide layer on Cu and Ni substrates can reduce bonding strength.
Fig. 4 illustrates the variations in shear strength of the Ag junction with temperature under different sintering conditions. In general, the shear strength of junctions tends to increase with higher sintering temperatures. At elevated temperatures, the growth of necks between Ag nanoparticles is enhanced, leading to the formation of dense structures due to the increased density of the bonding layer16). However, excessively high sintering temperatures can compromise the reliability of junctions. When bonding with highly oxidizable metals such as Cu, oxygen atoms diffuse through the porous Ag structure at high sintering temperatures, leading to the formation and growth of an oxide layer (Cu2O, CuO) on the Cu surface. As a result, cracks and pores between the sintered layer and the oxide layer expand, causing interfacial peeling and ultimately reducing shear strength20).
Fig. 4
Effect of the sintering temperature on the average shear strength of sintered silver joint for Ag chip with Ag substrate (indicating as Ag-Ag), Cu chips with Cu substrates (indicating as Cu-Cu), Ni-, Ag-plated Cu chip with Ni-, Ag-plated Cu substrate (indicating as Cu/Ni/Ag-Cu/Ni/Ag), Ti-, Cu-plated SiC chip with Cu substrate (indicating SiC/Ti/Cu-Cu) and DBC chip with DBC substrate (DBC-DBC)14)-19)
Fig. 5 illustrates the variations in shear strength of Ag junctions based on the applied pressure under different sintering conditions. In general, as the applied pressure during sintering increases, the neck size between particles grows, effectively expanding the bonding area at junctions. This ultimately enhances sintering efficiency and bonding strength. However, excessively high applied pressure can compromise the chip structure and hinder the evaporation of organic substances in the paste, leading to residual organic matter in the Ag sintering layer, which weakens bonding strength. Therefore, selecting appropriate process conditions is essential22).
Fig. 5
Effect of the sintering pressure on the average shear strength of sintered silver joint for Ag-plated Cu chips with Ag-plated Cu substrate (indicating as Cu/Ag-Cu/Ag), Ni-, Ag-plated Cu chips with Ni-, Ag-plated Cu substrates (indicating as Cu/Ni/ Ag-Cu/Ni/Ag), Ag-plated Cu chip with Ag-plated Cu substrate (indicating as Cu/Ag-Cu/Ag) and Cu chip with Cu substrate (indicating as Cu-Cu)21-26)
2.1 Pressure sinter-bonding
In industrial settings, the sintering process for sinter-bonding typically operates within a pressure range of 5 MPa and 30 MPa27). Applying an optimal level of pressure during sintering promotes expansion at the bonding interface, allowing the process to be completed in a shorter time at a lower sintering temperature28).
Du et al.29) conducted an experiment focusing on pore characteristics, such as pore shape and distribution. Sintering was performed for three minutes at 15 MPa and 250℃ using a film composed of micron- and submicron-sized Ag particles, fabricated through a laser deposition method. The experimental results showed that the Ag film with a particle content of 76 vol% exhibited the highest thermal conductivity of 260 W/(m·K). Analysis of the pore characteristics in the formed sintering layer revealed a uniform pore distribution, reduced porosity, and enhanced pore circularity in the Ag sintering layer. These results are attributed to the sintering structure, where micron-sized particles formed a framework while smaller particles filled the pores, leading to a uniform microstructure. Therefore, instead of merely reducing porosity, the shape and distribution of pores play a more significant role in enhancing thermal conductivity.
The oxidation of Cu, which adversely impacts junction performance during sinter-bonding with a Cu substrate, is a crucial factor to consider. Jo et al.30) suggested a bonding process that prevents oxidation at the Ag-Cu interface by applying an oxygen-blocking pressure bonding system. After arranging a nano-scale porous Ag sheet on a Cu substrate and bonding to an Ag- plated die, an oxygen-blocking material (Si-O bonded polymer) was applied near the interface to inhibit the diffusion of oxygen from the air. Sintering was then performed for 10 minutes at 250°C and 280°C under a pressure of 15 MPa. The experimental results indicated that bonding strengths of 13 MPa and 39.86 MPa were observed at each temperature, with a decrease in porosity and an increase in the density of the sintering layer as the temperature rose. The TEM and EDS results revealed that no oxide layer was formed on the Ag-Cu interface, and strong interfacial bonding was observed. It proves the feasibility of achieving Ag sinter-bonding with high strength simply on Cu substrates under ambient conditions, without the need for inert or reducing gases or additional processes.
Studies have also gained attention for focusing on the sinter-bonding of a paste with a particle structure designed to improve sintering efficiency. Roh et al.31) fabricated a micro chestnut-burr-like (CBL) Ag paste by combining micron-sized CBL Ag particles with spherical Ag particles (average diameter of 1 μm) in a glycol ether-based solvent. A Cu disk plated with electroless nickel immersion gold (ENIG) was used as both the substrate and die. Pre-heating was conducted for 300 seconds at 130°C to evaporate the solvent. Sinter-bonding was then performed within a temperature range of 200 to 260°C for 10 to 60 minutes under a pressure of 0.4 MPa in a N2 atmosphere. A relatively high average shear strength of 54.6 MPa was achieved during sintering at 260°C for 10 minutes, with bonding strength improving as both sintering temperature and time were increased. As shown in Fig. 6, the CBL Ag particles with sharp edges tend to have larger surface areas compared to spherical particles, resulting in a higher sintering driving force32). Moreover, the significant difference in diffusion potential promotes atom diffusion, which could reduce bonding strength to 200℃, even under low pressure of 0.4 MPa.
In the field of power electronics, there is a recent growing demand for large-area chips, typically 10 × 10 mm² or larger, due to the need for higher power. When bonding such large-area chips, issues arising from residual organic matters in the Ag paste must be addressed. Defects such as pores can form if residual organic matter is not entirely removed, resulting in a reduction of both thermal conductivity and junction strength33-35). Additionally, the large-area chip can obstruct the decomposition of binder molecules and the volatilization of solvents at the interface, resulting in numerous voids and a decrease in bonding strength36,37). Therefore, to decompose organic matter adsorbed on the surface of Ag paste, a high sintering temperature, extended sintering time, and a pressurized environment are generally required to minimize side effects38-40).
Ma et al.41) suggested a sintering method for a nanofilm using paste. Nano- and micro-sized Ag particles and flakes were mixed to create a paste, which was then applied using a patterning technique with Kapton tape to fabricate a low-organic nano-Ag film with only 2.1% organic content. Subsequently, an Ag-coated Cu metal was bonded to the film. Subsequently, sinter- bonding was performed for 5 to 30 minutes at pressures ranging from 0 to 15 MPa and temperatures between 160°C and 250°C. Shear strength improved with increasing temperature, sintering pressure, and sintering time. The maximum shear strength of 24.01 MPa was achieved under the optimized sintering conditions of 5 MPa, 200°C, and 10 minutes. Under these conditions, almost no residual organic matter remained, while the micro and nano structures on the film surface contributed to enhanced shear strength, even at low temperatures and during rapid sintering. This type of nano-Ag film contains minimal organic matter, allowing for easier evaporation and decomposition compared to conventional Ag paste. As a result, it positively contributes to enhancing the reliability of large-area chip junctions.
2.2 Non-pressure sinter-bonding
The pressure sinter-bonding process carries the risk of chip and substrate deformation, potentially increasing process complexity. Consequently, non-pressure sinter-bonding has been proposed as an alternative method that eliminates the need for applied pressure. In this approach, chip and substrate damage is prevented, and the simplified process enhances productivity. However, the junction strength is relatively lower than that of the pressure sinter-bonding process. Therefore, several studies have explored alternative solutions, such as optimizing paste composition or refining sintering profiles42).
Wu et al.43) fabricated an insulated gate bipolar transistor (IGBT) using an optimized sintering process. The IGBT chip was bonded to a Cu substrate with a nanoparticle Ag paste, followed by pre-heating at 80°C for 20 minutes and subsequent sintering at 230°C for 90 minutes in a non-pressure environment. Their experimental results demonstrated that the shear strength of the sintered Ag bonding layer reached 34.269 MPa, showing an improvement over the conventional soldering process (Sn96.5Ag3.5), which yielded a shear strength of 25 MPa. Thermal resistance was reduced by 11.06%, and a dense sintered layer was formed with the central pores effectively eliminated. The optimized solvent evaporation process is believed to have enhanced sintering density while reducing internal porosity. The introduction of a pre-heating process enabled uniform solvent removal, resulting in superior mechanical and thermal performance compared to the conventional soldering process. Therefore, optimizing the sintering profile in a non-pressure environment to enhance the reliability of high-power modules can serve as a key technological approach.
Wu et al.44) developed a novel Ag paste for bonding a large-area (15 × 15 mm2) chip under non-pressure conditions. An Ag-finished Cu substrate and a Cu dummy chip were utilized, with sintering conducted for 30 minutes at 300°C in a non-pressure environment. The sintered Ag bonding layer exhibited an average shear strength of 29.6 MPa. Due to the difference in the coefficient of thermal expansion (CTE) between the chip and paste, stress concentrated around the edges, leading to a reduced shear strength near the edges of the sintered layer. Despite increased porosity, the shear strength distribution remained relatively uniform across the entire area. These findings suggest that uniform bonding strength can be achieved for large-area chips through a non-pressure sintering process, with effective porosity control being crucial for ensuring the reliability of the packaging.
3. Composite Particles
Several recent studies have focused on addressing the high costs and ECM issues that occur when using only one type of Ag particle. Some of these studies suggested using Cu-Ag composite particles as a solution. Chen et al.45) developed a Cu-Ag composite particle paste by adding micro-sized Cu particles to the Ag paste, and then sinter-bonded a SiC chip and a Cu substrate for one hour under the conditions of 5 MPa and 250°C in air. In their experiment, a high shear strength of 57.8 MPa was achieved without the formation of a Cu oxide layer. Cu particles were dispersed within the Ag sintered layer, with Ag acting as a protective coating over the Cu particles, resulting in almost no oxide film forming on the surface of the Cu particles. Additionally, after a rapid sintering process of 1, 3, and 5 minutes under high pressure conditions of 20 MPa, a shear strength exceeding 20 MPa was achieved at 3 minutes or longer, demonstrating its potential for industrial application. In contrast, some voids and cracks were observed due to incomplete solvent evaporation during the short sintering time; however, improved shear strength is considered achievable with optimization of the pre-heating process. This study thus aims to prove that Cu-Ag composite particles can be stably sinter-bonded under low-temperature and low- pressure conditions.
Wang et al.46) researched the microstructure and micro-mechanical behavior of sinter-bonding using a Cu-Ag composite paste, focusing on the impact of solvent type. Micron-sized Cu and Ag particles were utilized, and four alcohol-based solvents and four epoxy-based solvents were categorized based on their ratio. The Si chip and DBC substrate were inter-bonded and subjected to sintering at 20 MPa and 250°C for 10 minutes in a nitrogen (N₂) atmosphere. Immediately after sintering, the shear strength was highest for the alcohol-based solvents at 51.7 MPa, whereas the epoxy-based solvents exhibited a shear strength of 48.54 MPa. Conversely, after 400 hours of aging at 250°C, the shear strength increased to 79.92 MPa for alcohol-based solvents and 94.57 MPa for epoxy-based solvents. In thermal cycling reliability tests conducted between -55°C and 150°C, the shear strength was 34 MPa and 40 MPa, respectively. This indicates that the paste containing epoxy-based solvents demonstrated superior bonding reliability under prolonged high-temperature exposure and thermal cycling conditions. Over time, alcohol-based solvents oxidize, leading to increased porosity, whereas the epoxy in epoxy-based solvents fills the pores, enhancing structural density. Additionally, its viscosity helps maintain the dispersion of Cu and Ag particles, preventing sudden coagulation. As a result, mechanical stability is improved over the long term by promoting the formation of a dense sintered layer and preserving a uniform microstructure.
Recently, studies on Cu@Ag particles, which feature a Cu core and an Ag shell, have been drawing attention due to their superior electrical and thermal conductivity, as well as their potential for cost reduction. Wang et al.47) introduced a novel structure to address the limitations of conventional single particles by synthesizing Cu@Ag nanoparticles through a liquid-phase reduction method and formulating a low-boiling-point mixed organic solvent based on Raoult’s law. Further- more, Won et al.48) examined the influence of particle size and the addition of silane on the sintering process based on the Cu@Ag structure. Particles measuring 0.8, 1.8, and 3.9 μm, along with different types of silanes (A, B, and C), were incorporated in concentrations ranging from 0.2 to 0.45 wt%. Sintering was then conducted at 315°C for one hour in a non-pressure N₂ atmosphere. The results showed that the optimal sintering performance was achieved with a combination of 0.8 μm particles and 0.2 wt% of silane A. A denser sintered layer was achieved as smaller particle sizes provided a larger surface area, while larger particles had fewer contact points and more voids. This resulted in enhanced shear strength at the junctions and reduced internal porosity. The addition of 0.2 wt% of silane A enhanced the chemical bonding between the particles and the Cu substrate, and resulted in a strong bond. This finding is expected to contribute to future research aimed at optimizing the sintering process using Cu@ Ag particles.
4. Conclusion
This study presented a sinter-bonding technology using Ag particles that can optimize energy systems and improve the reliability of junctions in high-temperature operating environments of power semiconductors. The potential of this lead-free bonding material, demonstrating superior electrical, thermal, and mechanical properties compared to existing bonding technologies, was confirmed. Currently, the pressure sintering process is widely commercialized and in use, but active research on the non-pressure sintering process is underway to enhance productivity through simplified processes. Thus, limitations can be addressed by optimizing the particle or solvent composition, advancing surface treatment technologies, and utilizing reducing solvents. Moreover, as bonding for large-area chips becomes increasingly crucial with the rise of high-output power semiconductors, ongoing research is needed to develop optimized materials and processes to address the challenges encountered during bonding. Therefore, research focused on the development and application of Cu-Ag composite pastes and other materials, such as nanoporous sheets, will play a key role in enhancing the performance of next-generation power semiconductors.
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