Ag Sintering Research Articles

Thermal decomposition temperature-dependent bonding performance of Ag nanostructures derived from metal–organic decomposition

In wide-bandgap semiconductor power device packaging, die bonding refers to attaching the die to substrate. Thereby, the process temperature of Ag sintering for the die bonding should be low to prevent damage to fragile dies. Herein, an organic-free strategy using Ag nanostructures derived from the thermal decomposition of metal–organic decomposition (MOD) was proposed to achieve low-temperature bonding. Significant effects on bonding performance were determined by the thermal decomposition temperature, which in turn determined the organic content and sintering degree of Ag nanostructures. At a low thermal decomposition temperature of 160 °C, incomplete decomposition resulted in high organic content in the Ag nanostructures, causing large pores inside the Ag joints owing to the generation of gaseous products. Owing to the Ag particles with naked surfaces and wide size distribution, the Ag nanostructure obtained at 180 °C showed an excellent bonding performance, resulting in a high shear strength of 31.1 MPa at a low bonding temperature of 160 °C. As the thermal decomposition temperature was 200 °C, sintering among Ag particles increased the particle size, resulting in a reduction of surface energy and driving force for sintering. We think that uncovering this underlying mechanism responsible for the bonding performance will promote the application of Ag MOD in the die bonding of WBG power devices.Graphical abstract

Bonding Behavior and Quality of Pressureless Ag Sintering on (111)-Oriented Nanotwinned Cu Substrate in Ambient Air.

(111)-oriented nanotwinned Cu ((111)nt-Cu) has shown its high surface diffusion rate and better oxidation resistance over common polycrystalline Cu (C-Cu). The application of (111)nt-Cu as an interface metallization layer in Ag-sintered technology under the role of oxygen was investigated in this work, and its connecting behavior was further clarified by comparing it with C-Cu. As the sintering temperature decreasing from 300 to 200 °C, the shear strength on the (111)nt-Cu substrate was still greater than 55 MPa after sintering for 10 min. The fracture surface correspondingly changed from the interface of Ag/die to mixed fracture mode, involving the interface of the Ag/Cu substrate and Ag/die. The existence of copper oxide provided a tight connection between Ag and the (111)nt-Cu substrate at all of the studied temperatures. Although lots of small dispersed voids were seen at the interface between copper oxide and (111)nt-Cu at 300 °C, these impurity-induced voids would not necessarily be a failure position and could be improved by adjusting the sintering temperature and time; for example, 200 °C/10 min or heating to 300 °C, and then start cooling at the same time. The microstructure of Ag-Cu joint on (111)nt-Cu behaved better than that on C-Cu. The thinner copper oxide layer and the higher connection ratio of the interface between copper oxide and Ag were still found on the (111)nt-Cu connection's structure. The poor connection between copper oxide and Ag on C-Cu easily became the failure interface. By controlling the thickness of copper oxide and the content of impurity-induced voids, the use of (111)nt-Cu in advanced-packaging could be improved to a new level.

Comparative Thermo-electric Analysis of Void and Wire Diameter Variation for Two Packages TO-251 and TO-252 Using Die-attach Sintered Ag

Comparative Thermo-electric Analysis of Void and Wire Diameter Variation for Two Packages TO-251 and TO-252 Using Die-attach Sintered Ag

Exploring the process-microstructure-thermal properties relationship of resin-reinforced Ag sintering material for high-power applications via 3D FIB-SEM nanotomography

Resin-reinforced Ag sintering materials represent a promising solution for die-attach applications in high-power devices requiring enhanced reliability and heat dissipation. However, the presence of resin and intricate microstructure poses challenges to its thermal performance, and improvement strategies remain unclear. This work utilizes 3D FIB-SEM nanotomography to reconstruct the microstructure of this material under various process conditions. The analysis reveals that, even with an Ag volume fraction as low as 47.3%, Ag particles form a robust 3D network. Geometric tortuosity quantifies the effect of different sintering conditions on the Ag particle network in all spatial directions. Effective thermal conductivity is simulated based on realistic microstructure models. Results show a significant negative correlation between tortuosity and effective thermal conductivity. Increasing sintering temperature in Model B notably reduces tortuosity and enhances effective thermal conductivity. Sensitivity analysis underscores the dominant role of Ag volume fraction in regulating effective thermal conductivity. Finally, transient thermal impedance measurement of this material as a thin die-attach layer in actual high-power devices demonstrated its application potential. This article strives to explore the relationship between process, microstructure, and thermal properties of this material to provide a reference for further development.

Development of Ag@Si composite sinter joining with ultra-high resistance to thermal shock test for SiC power device: Experiment validation and numerical simulation

Owing to its cost-effectiveness and low coefficient of thermal expansion (CTE), micron Si is incorporated into sintered Ag matrix to develop an Ag@Si composite sintering strategy. The Si integration helps reduce cost and alleviate CTE-mismatch of Ag-sintered die attachments. An innovative Ag@Si composite sintering structure is achieved by low-temperature and pressure-less sintering process, where a possibly slight diffusion of native SiO2 on the Si surface toward Ag is indicated by TEM observation, enabling the integration of Ag and Si into one continuous network. Owing to the robust well-bonded composite sintering, the Ag@Si joining strategy provides mechanical/microstructural reliability far beyond the pure Ag sinter joining, demonstrating significant prospects in high-temperature interconnection applications. During the harsh thermal cycling (−50–250 °C), the mitigating effect of the Si addition on CTE mismatch is manifested through the substantial suppression of microstructure deterioration in Ag@Si joint that occurs in pure-Ag-sintered joint, while the shear strength retention rate is doubled. Detailed investigation into the mechanism for elevated performance was conducted by material property and Missies stress analysis. The incorporation of Si helps modify the CTE, elastic properties, and stress distribution of Ag@Si-sintered material, synergistically contributing to the satisfactory joining reliability.

Enabling Low Pressure, Low Temperature, and Particle Control for Anisotropic Conductive Adhesives

AbstractAnisotropic conductive adhesives (ACAs) are the preferred interconnection technology for applications that employ large dies, flexible substrates, and ultra fine‐pitch interconnects. Conventional ACAs require relatively high bonding pressures and temperatures, and ultra fine‐pitch applications challenge the trade‐off between low interconnect resistance and risk of short circuits. This study introduces an ACA‐like interconnection technology that addresses these limitations, allowing for low‐pressure, low‐temperature assembly processes with enhanced particle control at the interconnects. Conductive particles are deposited onto a patterned carrier and subsequently transferred to electrical pads using either non‐conductive film or Ag sintering. The Ag sintering process is performed at a low temperature (140 °C) and low bonding pressure (1 N for a 10 × 10 mm2 chip). The capability of controlling the position and number of conductive particles within individual interconnects is demonstrated. This presents possibilities for achieving ultra‐fine pitch interconnects with negligible risk of short circuits, without compromising electrical resistance. It is demonstrated that interconnect resistance can be tuned by varying the number of conductive particles (achieving a resistance as low as 33 mΩ with 25 particles per interconnect). This approach is applicable to scenarios where the bonding force and temperature must remain low due to the nature of substrate materials.

Flip-Chip Interconnects Based on Single Metal-Coated Polymer Spheres

In microelectronics packaging, minimizing thermomechanical stresses is crucial to achieve high reliability. This paper presents a novel approach for flip-chip interconnects, utilizing individual metal-coated polymer spheres at a low bonding temperature. The method involves selectively depositing individual conductive particles onto a polydimethylsiloxane (PDMS) carrier and then transferring the particles to electrical pads on substrate using Ag sintering followed by die placement and introducing underfill material. The deposition process achieved a high yield of 98.8% on the PDMS carrier, while the transferring process resulted in well-defined ink dots with a yield of 96.2%. The sintered Ag formed good bonds between the particles and electrical pads, leading to moderate interconnect resistance (as low as 0.57 Ω). This work has demonstrated the feasibility of interconnects based on a single metal-coated polymer sphere with low thermomechanical stress induced, thanks to the low bonding temperature (140 °C) and pressure (0.1 MPa) as well as the mechanical compliance of polymer particles.

Improvement of Bonding Strength and Thermal Shock Reliability for Ag Sinter Joining Direct on Al Substrate

Ag sinter paste joining as a proven die bonding technology have been used in the electric vehicle. However, for the Ag sinter paste joining, metallization layers on substrate are usually necessary such as Ni-P/Au or Ni-P/Ag to get a good bonding quality. The direct bonding on DBC (direct bonded copper) or DBA (Direct Bonded Aluminum) substrates without metallization layers are very attractive. In this study, we investigated the direct bonding on an aluminum (Al) substrate by Ag sinter paste and propose an abrasive blasting processes to treat the Al surface to increase the initial bonding quality. The result show that the shear strength of SiC/Al joint structure by Ag sinter paste can be improved by treating the Al surface. The shear strength of joint structure without surface treatment increased from 26.9 MPa to 32.2 MPa for a strong blasting treatment on Al surface. In addition, the results of thermal shock test (-40 ℃~150℃) also show that the blasting treatment on Al surface can improve the shear strength and reliability of SiC/Al joint structure.

Reliability of Copper Sintered Interconnects under Extreme Thermal Shock Conditions

Copper flakes-based sinter paste showed high reliability under thermal shock conditions. A dense and homogeneous interconnect was realized while sintering for 5 min at 275°C under 15 MPa bonding pressure, with the flakes showing the unique behavior of stacking over each other to realize a dense and homogeneous interconnect. Porosity under 10% could be achieved. The paste could be sintered under a constant nitrogen flow and did not need a fully inert atmosphere during sintering. Under high stress thermal shock cycling of +175/-65°C, the sintered interconnects showed a drop of 25% in the shear strength values after 1000 TST, consistent with observations made with Ag sintering. Thermal conductivity more than 200 W/mK and electrical conductivity of 20 Ms/m is achieved using the Cu sinter paste.

Power Cycling Reliability with Temperature Deviation of Pressureless Silver Sintered Joint for Silicon Carbide Power Module

SiC devices can enhance power conversion in electric vehicles. However, traditional soldering techniques are limited by their low melting temperatures. Therefore, we used pressureless Ag sintering to assemble a 1200 V/200 A SiC metal-oxide-semiconductor field-effect transistor power module and compared the long-term reliability, electrical properties, and driving performance of the module with those of a similar module assembled using the solder Sn-3.0Ag-0.5Cu (SAC305). To assess sinter joint reliability, we performed power cycling tests over two temperature ranges, 50–150°C and 50–175°C, for 15,000 cycles. Subsequently, we compared the breakdown voltage (BVDSS) and drain-source on-resistance (RDS(ON)) of the SiC power modules and performed cross-sectional analyses of the device bonding interfaces. No difference in BVDSS was found between the Ag-sintered and SAC305-soldered joints. However, the RDS(ON) exhibited minimal variation for the Ag-sintered module but significantly varied for the SAC305-soldered module, suggesting that the former better maintained its characteristics. Furthermore, the electrical characteristics of the SAC305-soldered module underwent more significant alterations with increasing temperature change during power cycling, indicating that cracks propagated throughout the SAC305 soldered joint over time. Therefore, Ag sintering was quantitatively validated as the superior die attachment technology compared to soldering for long-term reliability.

Bonding Characteristics of Solder and Sinter Joints on Active-Metal-Brazing Substrates with Nano Sputtered Ag-Cu-Ti Brazing Filler Metal

In this study, to improve the bonding interfacial characteristics of active metal brazing (AMB) substrates, a Ag-Cu-Ti brazing filler metal (BFM) layer was formed on aluminum nitride (AlN) and silicon nitride (Si<sub>3</sub>N<sub>4</sub>) ceramics by nano sputtering. The AMB substrates were manufactured by brazing bonding. The measured peel strengths of the bonding interfaces of the AlN and Si<sub>3</sub>N<sub>4</sub> ceramics were 2.35 kgf/mm and 4.26 kgf/mm, respectively. Fracture surface analysis revealed AlN crack initiation at the ceramic/BFM interface, which progressed into the ceramic interior. Silicon carbide (SiC) devices for a power module were bonded on the AMB substrates by Sn-3.0Ag-0.5Cu soldering and Ag sintering bonding. To compare the deterioration characteristics of the joint interfaces, a thermal shock test was conducted. Microstructural analysis after the thermal shock test showed that no defects occurred at the Si<sub>3</sub>N<sub>4</sub>/BFM interface, whereas delamination occurred at the AlN/BFM interface. Summarizing, in this study, the characteristics of AMB interfaces coated with a Ag-Cu-Ti BFM using the sputtering process were identified and the suitability of a SiC-based power module package was confirmed.

Physics of failure and high-temperature reliability on Ag sintered ENIG finished die-attachments at 175 °C for integration of in-wheel motor systems

This study focuses on Ag die bonding failure physics and high-temperature reliability that the die back-side and substrate surface are both finished ENIG when power modules are embedded in-wheel motor systems. The thermal reliability of the Ag sintered die-bonded structures was harsh thermal aging tested for up to 500 h at 250 °C. We systematically investigated the degradation mechanism generated during the thermal endurance test at 250 °C of the die-attach structure bonded with Ag sinter at a very low temperature of 175 °C by EBSD analysis.

Surface modification of Ag-coated Cu particles using dicarboxylic acids to enhance the electrical conductivity of sintered films by suppressing dewetting in Ag shells

A resin-free paste containing 1.5 μm Cu@Ag particles was fabricated and used to form a film with excellent electrical conductivity even after a significantly high-speed sintering process. The particle surfaces were treated using three types of dicarboxylic acids, namely OA, SA, and PA, with carboxyl groups to suppress the dewetting behavior of Ag shells generated during thermal sintering. SEM, UV–vis, FT-IR, Raman spectroscopy, XPS, and TG-DTA analysis were performed to verify the morphological changes, chemical binding states (formation of an organo-metallic surface), and thermal behavior changes on the particle surfaces after surface treatment. The PA-modified particle surface exhibited a delayed dewetting behavior and the most suppressed oxidation degree. Furthermore, the paste containing the surface-modified particles exhibited a significantly low electrical resistivity (5.05 × 10−6 Ω·cm) after sintering for just 1 min at 300 °C under N2, which is an improvement of approximately three orders of magnitude over the original paste. Consequently, the surface treatment using effective dicarboxylic acid, which simultaneously inhibited Ag dewetting and suppressed Cu oxidation, improved Ag sintering between particles and enhanced the electrical properties of the sintered film.

Ag Sinter Bonding to Si Substrate via Temporal Formation and Decomposition of Ag Carboxylate.

This paper demonstrates the in situ sinter bonding of Ag microparticle pastes to a Si substrate via the temporal formation and decomposition of Ag carboxylate on the surface of Ag microparticles. This was proposed via the investigation of Ag sinter bonding using the redox reaction between Ag2O and ethylene glycol, which achieved a bonding strength above 30 MPa even for the bonding temperature at 220 °C. Thermal analysis was used to identify the product of the redox reaction between Ag2O and ethylene glycol and determine the bonding temperature because the final reaction facilitates the interfacial sinter bonding with the substrate. Fourier-transform infrared spectroscopy and nuclear magnetic resonance results indicated the in situ formation of Ag salts of carboxylic acids, such as Ag oxalate on the surface of Ag microparticles. Therefore, the sinter bonding process enabled by the in situ formation and subsequent decomposition of these Ag salts was investigated using Ag microparticles and oxalic acid. Observations of the surface and interfacial morphology of the Ag particles after heating revealed the formation of Ag nanoparticles on the surfaces of the microparticles and the formation of sintering necks between the particles. The bonding experiments demonstrated a significant increase in strength with the addition of oxalic acid to the Ag paste due to the enhanced interfacial sinter bonding with the substrate. The in situ formation and decomposition of Ag salts are promising strategies for improving sintered bonds in electronic devices because they can provide enhanced localized sinter bonding using stable insert materials.

Development of high thermal conductivity of Ag/diamond composite sintering paste and its thermal shock reliability evaluation in SiC power modules

Diamond has the highest thermal conductivity among naturally occurring materials and a relatively low coefficient of thermal expansion (CTE), making it a promising interconnected material for next-generation semiconductor power devices. In this study, Ag/diamond composite sintering paste was developed. The agglomerated diamond particles were uniformly dispersed in the sintered Ag porous structure, and the thermal conductivity was increased from 171.4 W/(m·k) for pure Ag sintering to 288 W/(m·k) for an Ag@2%diamond (2% of the total weight of Ag) specimen. The interfacial microstructure evolution and fracture behavior were investigated in detail for a SiC/Direct Bond Copper (DBC) joint structure during an extreme thermal shock test (TST) in the range of −50 °C–250 °C. Importantly, with a moderate amount of diamond additive, the microstructural degradation and severe deformation of the DBC substrate were effectively suppressed. This is mainly attributed to the diamond addition, which can block the migration pathways of Ag atoms toward long-term thermal shock and adjust the thermo-mechanical performance of the sintered layer. This detailed study about Ag/diamond composite paste can provide new guidance for strengthening sintered Ag interconnections for SiC power modules.

Reliability Analysis of Flip-Chip Packaging GaN Chip with Nano-Silver Solder BUMP.

Gallium nitride (GaN) power devices have many benefits, including high power density, small footprint, high operating voltage, and excellent power gain capability. However, in contrast to silicon carbide (SiC), its performance and reliability can be negatively impacted by its low thermal conductivity, which can cause overheating. Hence, it is necessary to provide a reliable and workable thermal management model. In this paper, a model of a flip-chip packing (FCP) GaN chip was established, and it was assigned to the Ag sinter paste structure. The different solder bumps and under bump metallurgy (UBM) were considered. The results indicated that the FCP GaN chip with underfill was a promising method because it not only reduced the size of the package model but also reduced thermal stress. When the chip was in operation, the thermal stress was about 79 MPa, only 38.77% of the Ag sinter paste structure, lower than any of the GaN chip packaging methods currently in use. Moreover, the thermal condition of the module often has little to do with the material of the UBM. Additionally, nano-silver was found to be the most suitable bump material for FCP GaN chip. Temperature shock experiments were also conducted with different UBM materials when nano-silver was used as bump. It was found that Al as UBM is a more reliable option.

The Investigation of Bonding Reliability Improvement for Power Module Containing Sintered Ag

The Investigation of Bonding Reliability Improvement for Power Module Containing Sintered Ag

Immobilization of iodine waste via moderate temperature sintering of (Ag)iodosodalite

The treatment of radioactive iodine waste is significant for the further expansion of the nuclear industry. In this study, AgI and zeolite-4A were used to synthesize iodosodalite. AgI in its molten state can not react with zeolite 4A to synthesis iodosodalite, ion exchange is used to exchange Ag+ for Na+ in zeolite 4A and transform zeolite 4A into AgAlSiO4. AgI can enter the lattice of zeolite through the action of Ag+, and then synthesize (Ag)iodosodalite. The phase composition, morphology and the main elements valence state of the sintered matrix were investigated. The results indicate that AgI and zeolite 4A successfully synthesize (Ag)iodosodalite after ion exchange, the moderate temperature sintering method can be employed for iodine waste immobilization.

(Invited, Digital Presentation) Fatigue-Resistant of Ag Sinter Joining on Ni-P/Pd/Au Finished DBA Substrate with Thick Ni-P Layer During Thermal Shock Test

The wide band-gap (WBG) semiconductor, such as silicon carbide (SiC) and gallium nitride (GaN), are widely expected to replace conventional Si-based chips, which provide the power modules with higher working temperature larger than 250 degrees. Ag paste sinter joining emerges as a promising alternative for the die attach material in WBG power devices owing to its superior electrical/thermal conductivity as well as a high melting point of 960℃. For Ag paste sinter joining on WBG chip or substrate, most studies are focusing on Ag metallization layers by which the Ag-Ag interface delivers high bonding strength because of the chemical/structural similarity. However, with the superior anti-oxidation/corrosion property, Au metallization layers ensure the Ag-Au joint much wider usage environment than Ag-Ag joint.The study was carried out to develop a highly reliable Ag-Au sintering joint in SiC chip and Direct bonded aluminum (DBA) power modules. The DBA substrate was strengthened by an electroless nickel/electroless palladium/immersion gold (ENEPIG) metallization layer. ENEPIG plated DBA substrate was modified to possess superior mechanical strength by thickening the Ni layer with the thickness of 7µm and 20µm. The reliability of SiC/DBA die-attached module by Ag-Au joining was evaluated during a long-term thermal shock test (TST) from -50℃ to 250℃ up to 1000 cycles. In the case DBA with a Ni metallization layer of 7µm, severe plastic deformation occurred at the Ag-Au interface. By thickened ENEPIG layer with Ni-P layer to 20µm, the Ag-Au interfacial deformation derived from Al plastic deformation was successfully suppressed even after 1000 cycles. The shear strength of as-sintered Ag-Au joint was evaluated to be larger than 35 MPa, but degradation was still found to be the interfacial delamination between Ag paste and ENEPIG substrate during thermal shock test for the two ENEPIG metallization cases. The bonding interface and fracture characteristics were systematically investigated, which provide basic understanding between microstructural evolution and reliability of Ag-Au joint during TST.

Recent Studies of Transient Liquid Phase Bonding Technology for Electric Vehicles

Recently, interest in power modules is increasing in eco-friendly mobility such as electric vehicles (EVs) and fuel cell electric vehicles (FCEVs). The power module is a key component of the eco-friendly mobility that drives a motor by converting direct current (DC) supplied from the battery into alternating current (AC). However, a lot of heat is generated in the power module during its operation. To ensure the reliability of power modules, it is essential to form a joint suitable for high-temperature operation. Many pieces of research have been published on various bonding methods such as soldering, Ag sintering, Cu-Cu direct bonding, and transient liquid phase (TLP) bonding. The TLP bonding has many advantages, such as significantly less cost and a high melting temperature joint. In this review paper, various TLP bonding technologies with different materials and methods have been introduced and compared.