Search

Abstract Metastable phases such as supersaturated solid solutions, supercooling, and amorphous phases are well-known in metallurgy. They are often composed in non-equilibrium states and can be transformed into a stable phase by overcoming an energy barrier with driving forces. Particularly, it has been widely used for material strengthening and heterogeneous nucleation of precipitates in solids is mainly induced by heat treatments for supersaturated solid solutions. However, little is known about the metastable phases of the Ag–Si alloy, although it is a well-known simple binary eutectic alloy. Here, we show that the metastable phases composed of amorphous Si and supersaturated Ag solid solution are induced by the eutectic reaction under rapid cooling of Ag–Si. Furthermore, the solute Si in the Ag matrix reacts with oxygen to precipitate Ag by-products, which grow as nodules. The Ag nodules have high crystallinity and robust interfacial structures, and the nodule growth leads to the formation of cross-links between the Ag–Si particles. We also demonstrate the Ag nodule-mediated bonding where the rapidly cooled Ag–Si ribbon is directly used as a bonding medium, indicating the possibility of using it as a high-temperature bonding material with low-temperature processes.

Based on the principles of micro-alloying and composite, the graphene reinforced Sn2.5Ag0.7Cu0.1Ce composite solder was designed. Ni nanoparticle-modified reduced graphene oxide (Ni-rGO) was prepared by thermal decomposition method, and the powder-melting method was proposed for the first time to prepare Ni-rGO reinforced Sn2.5Ag0.7Cu0.1Ce composite solder. The microstructure, wettability, electrical resistivity, and mechanical properties of the composite solder were systematically studied. Results indicated that the Ni nanoparticles uniformly adhered to the rGO with a size of 26.3 nm. The effective incorporation of Ni-rGO into the solder matrix was achieved. The result broke the technical difficulty that graphene cannot be effectively added to low melting point metals. With the addition of Ni-rGO, the grain size gradually decreased, and the eutectic structures increased. Deep etching results revealed that columnar β-Sn was stacked from layered β-Sn, the addition of 0.05 wt% Ni-rGO led to an increase in sheet-like Ag3Sn within the eutectics. Additionally, Ni-rGO was found at the grain boundaries of composite solder, serving as nucleation sites for Ag3Sn and Cu6Sn5. At an addition of 0.05 wt%, the tensile strength reaches a maximum of 58.1 MPa, with elongation of 33.8%, surpassing the strength of commercial Sn3.0Ag0.5Cu solder. Therefore, the high-strength and tough composite solder was obtained. This study offered a new approach for the development of low-melting-point composite materials.

The increasing demand for high-power SiC semiconductors necessitate the development of a die attachment material that combines high-temperature resistance, reliability, and cost-effectiveness. In this study, a novel micro-sized composite, Ag-10Al paste, containing 10 wt% Al particles, was designed. A remarkable phenomenon, the ejection of ultrafine Ag nanoparticles from micron-sized Ag flakes, was observed for the first time. The phenomenon was utilized for the in-situ surface modification of Al. Subsequently, the microstructure and mechanical properties of the sintered Ag-10Al/direct bonded copper (DBC) joints were studied. Results indicated that the Ag-10Al composite exhibited superior microstructure stability compared to sintered Ag. The Ag/Al interface was systematically analyzed, revealing a unique Ag/nano Ag2O/Al2O3 amorphous/Al structure. This structure was formed through the Ag nanoparticle jetting effect of Ag flakes, achieving effective bonding between nano Ag2O and Al2O3 amorphous phases through mutual dissolution at the atomic level. Moreover, the sintered Ag-10Al joint demonstrated enhanced mechanical performance stability over the sintered Ag joint. After 1000 h aging at 300 ℃, the shear strength of the sintered Ag-10Al joint reached 34.1 MPa, meeting the requirements for power semiconductor packaging. In conclusion, the Ag-10Al composite paste was thoughtfully designed, excelling in both performance and cost-effectiveness.

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.

The interfacial microstructures and corrosion behavior of tungsten heavy alloy/superalloy dissimilar joints brazed with various Cu–Ti based multicomponent amorphous alloys were investigated by time-varying chemical bath immersion tests and electrochemical measurements. The longitudinal analysis of corrosion behavior differences in different regions of dissimilar joints was conducted, and the horizontal analysis of the influence of compositional variation in filler materials on corrosion behavior was investigated. Importantly, the brazed joint with CuTiNiZrV filler material (CT3 joint) exhibited the noblest Ecorr (−0.911 VSCE) and the highest charge transfer resistance (1610.5 Ω cm2). From the compositional and microstructural effects on the corrosion resistance, the enhancing anti-corrosion performance was mainly attributed to the tailoring of the microstructures and corrosion potential of the brazing seam, and the formation of protective compact oxide film (Fe2O3-NiO-CoO-VO).

The development of next-generation power electronics requires the semiconductor power devices with higher voltage endurance, higher electrical current density under high-temperature operations above 250 °C. Herein, we propose the uses of combined micron-sized Cu particles with Ag–amino composite to achieve a robust direct Cu–Cu bonding under a low sintering pressure and temperature (300 °C) in ambient atmosphere. The strong bonding strength of the Cu–Cu joint structure was evaluated as high as 45.3 MPa under 300 °C sintering temperature with low pressure (1 MPa). For the high temperature reliability evaluation, the sintered Cu–Ag composite Cu–Cu joints were stored at 250 °C in air. The shear strength of the joints remains above 30 MPa after 500 h ageing, suggesting an excellent mechanical stability during high temperature storage. Additionally, a SiC Schottky barrier diode with the sintered Cu–Ag composite joint structure was evaluated during a power cycle test at a junction temperature of 200 °C for 10,000 cycles. The thermal resistance of the joint structure was almost unaffected, and no large delamination was observed after the power cycle test, indicating that the Cu–Ag composite material exhibit remarkable potential in SiC power devices packaging.

Three kinds of Cu–Ag composite paste were fabricated. We used 10 μm Cu particles as the main component of the paste as they are significantly cheaper than Ag and Cu nanoparticles. To understand the bonding performance and mechanism, the flake-shape Cu–Ag pastes with a different solvent and a Cu spherical particle with a diameter of 10 μm were separately implemented for the bare Cu bonding. A detailed and systematic analysis of Cu–Ag paste sintering was performed. In addition, for the flake-shape Cu–Ag paste, the bonding strength and thermal conductivity were evaluated at different sintering temperatures between 180 and 350 °C. A robust bonding strength of 56.7 MPa was achieved for the SiC chip on bare Cu bonding under low-temperature and low-pressure (300 °C, 2 MPa) air conditions. In addition, a large area of the direct bonded copper substrate (30 mm × 30 mm) was bonded to the Cu baseplate using Cu–Ag paste. The thermal conductivity of 243.7 W/m·K was better than that of almost all the Pb-free solders and comparable with the Ag sinter paste. The results revealed that the reducing agent plays a significant role in anti-oxidation and necking growth. This study offers a deeply understand of Cu–Ag composite paste on bare Cu bonding in the air for SiC power devices for its high-temperature applications.

Aiming for a suitable die-attached substrate for Ag-sintered joint technology during harsh operating environment, three surface finishes of sputtered Ti/Ag- layers, electroless Ni-/Pt-/Ag- layers, and electroplated Ni-/electroless Ni-/Pt-/Ag- layers were developed to directly bond copper (DBC) substrates, denoted as Ti/Ag, ENPA, and E-ENPA, respectively. The effect of the metallization layers by three deposition technologies on the Ag joint structures was systematically investigated during an extreme thermal shock test (TST) ranging from −50 to 250 °C. The mechanical behaviors and interface evolution considerably varied depending on the metallization layer used. Both ENPA and E-ENPA substrates provided a more robust initial shear strength of approximately 60 MPa. More importantly, both metallization layers were productive in restraining interfacial deformation owing to the effect of the thick Ni plating. A better performance was derived from the electroplated Ni in E-ENPA metallization with crack-less and deformation-resistant, because of electroplated Ni possessed a suitable hardness and toughness, thus the optimal stress-relaxation capability. This study provides insights to the properties of Ag-sintered joints on three metallization layers, where the metallization, mainly by electroplating, exhibited superior reliability beyond that of sputtered and electroless depositions in both strength and microstructure maintenance.

Silver paste is supposed to be a most prospective die-attach material for power electronics. While, the bonding of silver paste with aluminum substrate cannot be easily achieved due to the native oxidation of aluminum. This work provides a direct bonding of aluminum with silver micro flake-based paste under 250℃ with a shear strength of 15 MPa with a propriate surface treatment method of aluminum, and the shear strength even keeps higher than 15 MPa after storing at 250℃for 1000 h. The bonding mechanism was elucidated with high resolution transmission electron microcopy. The results indicate a new route for the wide application of Al based substrate in power electronic with low cost.

This work firstly evaluated the microscale mechanical properties of sintered Ag, consisting of various Ag particles of a flake shape and spherical shape from nano to micro size. Microscale pillar specimens were fabricated by a focused ion beam process and the mechanical properties of the sintered Ag, were measured by micro-compression testing using a nano-indentation system. The microstructure of sintered Ag is dependent upon the size and the shape of the utilized Ag particles, as this significantly influences its fracture behavior and mechanical properties. Mechanical properties not only depend on the porosity of the sintered Ag, but also on the degree of necking growth of the Ag particles. To explore this further, the shear strength of Ag sinter joint using different sizes and shapes of Ag particles was also evaluated. This revealed that sintered Ag flake particles possess a much greater bonding strength above 30 MPa than do sintered Ag spherical shape particles from nano to micro sizes. TEM observation revealed that many nanoparticles with a size less than 10 nm were formed in the sintered Ag flake structure due to its high grain boundary density and dislocation density, which increased its grain bonding quality. Keywords: Ag sinter joint, Nanoparticles, Micro-compression test, Mechanical properties, Die attach modules

A novel die-attach material, Cu particle paste with self-reduction and self-protection characteristics, was designed by simply adding ascorbic acid (AA) into Cu paste. The self-reduction characteristic is due to the addition of AA into the paste, as it reduces the Cu oxide layer on the metal Cu, even at room temperature. The self-protection characteristic is due to the decomposition of AA preventing further oxidation during sintering process. These characteristics are beneficial for the sintering of Cu particles and for enhancing the bondability. The increase to bonding strength is the direct result of the concentration of AA in the Cu paste. The bonding strength of Cu joints sintered from Cu paste with 1.3 wt% AA addition was as high as 24.8 MPa, thus making it attractive as a die-attach material. On the other hand, the bonding strength of those sintered from Cu paste without AA addition was only 9.7 MPa. These results strongly suggest that the self-reduction and self-protection characteristics resulting from AA addition are necessary and crucial for sintered joints fabricated from readily oxidized Cu particle paste. This novel Cu paste is an efficient, safe, and environmentally-friendly die attachment material, especially well-suited for applications in high power semiconductor devices. Keywords: Cu particle pastes, Self-reduction, Self-protection, Ascorbic acid, Sintered Cu joints, Power electronics

Abstract The requirement of energy-storage equipment needs to develop the lithium ion battery (LIB) with high electrochemical performance. The surface modification of commercial LiFePO4 (LFP) by utilizing zeolitic imidazolate frameworks-8 (ZIF-8) offers new possibilities for commercial LFP with high electrochemical performances. In this work, the carbonized ZIF-8 (CZIF-8) was coated on the surface of LFP particles by the in situ growth and carbonization of ZIF-8. Transmission electron microscopy indicates that there is an approximate 10 nm coating layer with metal zinc and graphite-like carbon on the surface of LFP/CZIF-8 sample. The N2 adsorption and desorption isotherm suggests that the coating layer has uniform and simple connecting mesopores. As cathode material, LFP/CZIF-8 cathode-active material delivers a discharge specific capacity of 159.3 mAh g−1 at 0.1C and a discharge specific energy of 141.7 mWh g−1 after 200 cycles at 5.0C (the retention rate is approximate 99%). These results are attributed to the synergy improvement of the conductivity, the lithium ion diffusion coefficient, and the degree of freedom for volume change of LFP/CZIF-8 cathode. This work will contribute to the improvement of the cathode materials of commercial LIB.

This study introduced the SiC micro-heater chip as a novel thermal evaluation device for next-generation power modules and to evaluate the heat resistant performance of direct bonded copper (DBC) substrate with aluminum nitride (AlN-DBC), aluminum oxide (DBC-Al2O3) and silicon nitride (Si3N4-DBC) ceramics middle layer. The SiC micro-heater chips were structurally sound bonded on the two types of DBC substrates by Ag sinter paste and Au wire was used to interconnect the SiC and DBC substrate. The SiC micro-heater chip power modules were fixed on a water-cooling plate by a thermal interface material (TIM), a steady-state thermal resistance measurement and a power cycling test were successfully conducted. As a result, the thermal resistance of the SiC micro-heater chip power modules on the DBC-Al2O3 substrate at power over 200 W was about twice higher than DBC-Si3N4 and also higher than DBC-AlN. In addition, during the power cycle test, DBC-Al2O3 was stopped after 1000 cycles due to Pt heater pattern line was partially broken induced by the excessive rise in thermal resistance, but DBC-Si3N4 and DBC-AlN specimens were subjected to more than 20,000 cycles and not noticeable physical failure was found in both of the SiC chip and DBC substrates by a x-ray observation. The results indicated that AlN-DBC can be as an optimization substrate for the best heat dissipation/durability in wide band-gap (WBG) power devices. Our results provide an important index for industries demanding higher power and temperature power electronics.

The tin whisker inhibition in RE-doped Sn-Zn soldered joints was investigated. The results indicated that after aging treatment at 150 °C, with the proper addition ratio of Ga and Nd, no obvious NdSn3 phase nor tin whisker growth was observed in the soldered joints. By replacing the Sn-Nd phase with a Ga-Nd phase, the risk of tin whisker growth caused by the oxidation of RE-phase was significantly inhibited even in soldered joints with additional excessive contents of Nd. The IMC layer in soldered joints with optimal addition showed a relatively regular shape after aging treatment, making the soldered joint maintain preferable mechanical performance. The results would be available for guiding the study of tin whisker growth inhibition method.

In this work, efforts were made to prepare a thermostable die-attach structure which includes stable sintered microporous Ag and multi-layer surface metallization. Silicon carbide particles (SiCp) were added into the Ag sinter joining paste to improve the high-temperature reliability of the sintered Ag joints. The use of SiCp in the bonding structures prevented the morphological evolution of the microporous structure and maintained a stable structure after high temperature storage (HTS) tests, which reduces the risk of void formation and metallization dewetting. In addition to the Ag paste, on the side of direct bonded copper (DBC) substrates, the thermal reliability of various surface metallizations such as Ni, Ti, and Pt were also evaluated by cross-section morphology and on-resistance tests. The results indicated that Ti and Pt diffusion barrier layers played a key role in preventing interfacial degradations between sintered Ag and Cu at high temperatures. At the same time, a Ni barrier layer showed a relatively weak barrier effect due to the generation of a thin Ni oxide layer at the interface with a Ag plating layer. The changes of on-resistance indicated that Pt metallization has relatively better electrical properties compared to that of Ti and Ni. Ag metallization, which lacks barrier capability, showed severe growth in an oxide layer between Ag and Cu, however, the on-resistance showed fewer changes.

The thermal cycling life of direct bonded aluminum (DBA) and active metal brazing (AMB) substrates with two types of plating—Ni electroplating and Ni⁻P electroless plating—was evaluated by thermal shock tests between −50 and 250 °C. AMB substrates with Al2O3 and AlN fractured only after 10 cycles, but with Si3N4 ceramic, they retained good thermal stability even beyond 1000 cycles, regardless of the metallization type. The Ni layer on the surviving AMB substrates with Si3N4 was not damaged, while a crack occurred in the Ni⁻P layer. For DBA substrates, fracture did not occur up to 1000 cycles for all kind of ceramics. On the other hand, the Ni⁻P layer was roughened and cracked according to the severe deformation of the aluminum layer, while the Ni layer was not damaged after thermal shock tests. In addition, the deformation mechanism of an Al plate on a ceramic substrate was investigated both by microstructural observation and finite element method (FEM) simulation, which confirmed that grain boundary sliding was a key factor in the severe deformation of the Al layer that resulted in the cracking of the Ni⁻P layer. The fracture suppression in the Ni layer on DBA/AMB substrates can be attributed to its ductility and higher strength compared with those of Ni⁻P plating.

This paper is the first to report a large-scale directcurrent electrodeposition of columnar nanotwinned copper within through silicon via (TSV) with a high aspect ratio (~4). With this newly developed technique, void-free nanotwinned copper array could be fabricated in low current density (30 mA/cm2) and convection conditions (300 rpm), which are the preconditions for copper deposition with a uniform deep-hole microstructure. The microstructure of a whole cross-section of deposited copper array was made up of (111) orientated columnar grains with parallel nanoscale twins that had thicknesses of about 22 nm. The hardness was also uniform along the growth direction, with 2.34 and 2.68 GPa for the top and bottom of the TSV, respectively. The gelatin additive is also first reported hereas a key factor in forming nanoscale twins by adsorbing on the cathode surface, in order to enhance the overpotential for cathodic reaction during the copper deposition process.