In this work, we applied a Ag flake formed paste for sinter-joining on bare Si, SiC, and GaN surfaces. The sintered joints possess a high shear strength of over 40 MPa under a pressureless sintering ...condition at 250°C. The high bonding strength of the joints is achieved by a tight adhesion between Ag and bare surfaces, which attributed to an excellent sinter-joining ability of the Ag flake. The flakes, acquired by mechanical milling, can be rapidly sintered into a homogenous porous structure at a pleasureless and low-temperature sintering condition due to its dislocation-rich nano grain structure. During sintering, a drastic morphology reconstruction of Ag flake, from a flattened flake to a drop-like particle, can introduce robust interfacial connection structures, which is important for the robust bonding. This Ag flake paste can be regarded as a promising sinter-joining material for the connection of bare surfaces in high-temperature applications.
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Silver (Ag) sinter-joining is an ideal connection technique for wide bandgap (WBG) power electronics packaging due to its excellent high-temperature stability and excellent thermal conductivity. In ...this work, we applied Ag sinter-joining to die attach for a WBG power module and focused reliability of Ag sinter-joining under harsh thermal and power cycling conditions. The die attach structure using a Ag flake paste had an initial shear strength of over 45 MPa due to the excellent sinter-joining ability of the paste. Variation of die attach shear strength and failure mode under a harsh cycling condition (−50~250 °C) have also been systematically discussed. Thermal diffusivity of sintered Ag and thermal resistance of the die attach structure were also measured, showing a superior thermal performance to solder materials. Meanwhile, a simple SiC diode module was assembled via Ag sinter-joining and aluminum (Al) ribbon-bonding for evaluation of Ag sinter-joining reliability during a severe power cycling condition. A power cycling test with a high junction temperature of 200 °C was conducted to evaluate the reliability of Ag sinter-joining. It is found that the main failure of the SiC diode module was located at ribbon-bonding rather than the Ag sinter-joining layer degradation, based on the variation of forward voltage and junction to case thermal resistance. This investigation indicates that the Ag sinter-joining has a long lifetime under a severe operating condition of power electronics.
The fracture behaviors and Ag–Au joint interface evolution of sintered micron-sized Ag particles paste joined on an electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated ...direct-bonded aluminum (DBA) and direct-bonded copper (DBC) substrates are evaluated during an extreme thermal shock test (TST) from −50–250 °C. The die shear strength of the as-sintered Ag joint on the DBA substrate is evaluated to be 33.5 MPa at a sintering temperature of 200 °C without any assisted pressure, which decreased gradually with an increase in the thermal cycling number. The shear strength declined slightly but remained at approximately 20 MPa; subsequently, it decreased considerably to 11.2 MPa after 1000 cycles. Coarsening of the sintered Ag layer is observed as the microstructure inhomogeneity and vertical cracks increased after 1000 cycles. In addition, the Al layer induced a greater undulate deformation, resulting in a sintered Ag layer exhibiting partial compression and tension after a TST. The sintered Ag layer became dense with a significant decrease in porosity at the compression parts and large horizontal cracks appeared at the tension parts. Both of horizontal cracks and vertical cracks led to fracture mode change and shear strength decrease. The Ag–Au joint interdiffusion layer became thicker during thermal shock with the gradual diffusion of the Au atoms into the sintered Ag layer. The die shear strength of the as-sintered Ag joint on the DBC substrates is evaluated as 34.4 MPa at 200 °C sintering but decreased to 0 MPa after 250 thermal shock cycles owing to the stress-induced delamination between the Ag and Au interdiffusion layer and sintered Ag layer. This study provides insights into the interface reaction and evolution of sintered Ag on ENEPIG-finished DBA and DBC substrates for applications at high temperatures.
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•Ag-Au joint with 33.5 MPa shear strength was achieved by micron-sized Ag particle at 200 ℃ pressure-less.•Ag particle with nano-grains may play an important role in low temperature pressure-free sintering.•Ag-Au joint on DBA substrate exhibit a good thermal shock stability by relax stress on Al deformation.•Interface delamination occurred for Ag-Au joint on DBC substrate after 250 thermal shock cycling.
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•Ultrafine Ag nanoparticles ejection phenomenon of micron Ag flake is observed for the first time by in-situ TEM.•In-situ surface modification of Al particles in sintered Ag-Al ...composite is completed via Ag nanoparticles ejection phenomenon.•A specific Ag/nano Ag2O/Al2O3 amorphous/Al composite interface structure was formed via mutual dissolution at the atomic level.•Shear strength of sintered Ag-10Al joint reached 34.1 MPa after 1000 h aging, meeting the requirements for power semiconductor packaging.
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.
GaN die-attach/DBC substrate joint structure bonded by Ag sinter paste/W thin film/Ag sinter paste sandwich die-attached layer was designed and its high-temperature reliability was investigated. GaN ...chips were bonded to DBC substrate using Ag sinter paste with thin W film at 220 °C and at 0.4 MPa for 60 min. The joints structure was subjected to thermal shock testing in temperature range of −50 to 250 °C for duration up to 1000 cycles. The initial average die shear strength of the GaN/DBC joint structure was about 30 MPa, and was maintained up to 1000 cycles. These results reveal that Ag sinter paste/W thin film/Ag sinter paste sandwich die-attached layer can provide a superior thermal shock resistant. The microstructure evolution of GaN/DBC joint structure during the thermal shock testing was observed by FE-SEM and EDX analysis. In addition, FEM numerical simulation has also been implemented in this study, which revealed that the GaN/DBC joint structure with the sandwich die-attached layer shows a stress-shielding effect from the external force, and thus achieves a reliable power module structure for wide-bandgap semiconductors for its application in high-temperatures.
•GaN/DBC joint structure bonded by Ag sinter paste/W/Ag sinter paste sandwich die-attached layer was developed.•The initial average die shear strength was about 30 MPa, and was maintained up to 1000 thermal cycles from −50 °C to 250 °C.•Ag sinter paste/W thin film/Ag sinter paste sandwich die-attached layer provides a superior thermal shock resistant.•GaN/DBC joint structure with the sandwich die-attached layer has a stress-shielding effect from the external force.
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.
Thermoelectric generators (TEGs) are a promising power source for realizing a self‐powered sensor, which is a primary component of the rapidly developing Internet of Things. Here, the fabrication of ...a prototype for a compact and flexible TEG (CF‐TEG) using ultra‐fine chip mounting technique is reported. The CF‐TEG consisting of 84 p‐n pairs was fabricated on a 10 × 10 mm2 flexible substrate. A temperature difference (dT) of 150 K is successfully established for this TEG. Its maximum output voltage and power density are obtained as 2.4 V and 185 mW cm−2, respectively, which corresponds to a conversion efficiency of 1.12% at dT = 150 K. Further, the experimental results are validated through theoretical analysis in which the contact effects are taken into account. This analysis is also used to elucidate the effect of thermal and electrical contact resistance on the output power. It is observed that ≈40% output power of this device is lost by these resistances, which proves that the contact properties significantly affect the performance of the TEG device.
A high performance ultra‐lightweight and compact flexible thermoelectric power generation (CF‐TEG) device is fabricated by highly refined chip mounting techniques. The CF‐TEG consisting of 84 bismuth‐telluride (Bi‐Te) p‐n pairs on a 10 × 10 mm2 flexible substrate may open great potential for thermal energy harvesting devices in powering ubiquitous sensor networks and mobile electronics of Internet of Things (IoT).
Despite the rapid progression of silicon carbide (SiC) power devices, the thermal characteristic evaluation during power cycling at high temperature (>200 °C) is an issue. In this article, a fast and ...miniaturized evaluation system with online thermal characteristic measurement function was introduced by an n-doped 4H SiC thermal engineering group (TEG) chip. Online thermal resistance measurement of a power module structure by Ag sinter joining with micron/submicron Ag particles paste in low temperature, low pressure, and cooling system by a thermal interface material bonding was performed. High-temperature reliability was systemically investigated by power cycling tests by switching on / off the power source which is connected to the SiC-TEG chip by Au wires. The total thermal resistance of the power module from the SiC-TEG chip to the cooling system increased from 0.5 to 0.53 K/W with the enhanced power source, and remained almost same after 20 000 power cycling at a swing temperature ΔTj of 150 °C. Furthermore, the SiC-TEG power module structure with the die attached with Pb and Pb-free solders, alongwith the same power source as sinter Ag paste was also measured. The Ag sinter joint possesses the lowest thermal resistance and highest high temperature reliability during power cycling compared with Pb and Pb-free die-attach materials.
In this work, an acoustic emission (AE) technique was applied to online condition monitoring (CM) of solder fatigue in a clip-bonding SiC mosfet power assembly during short and long power cycling ...tests (PCTs). Solder fatigue caused by PCTs was identified in the clip-bonding assembly via scanning acoustic tomography and scanning electron microscopy and successfully diagnosed via the AE-based online CM. By analyzing AE signals, junction temperature, and junction-to-case thermal resistance, it can be confirmed that the AE signal shows a superior sensitivity in initial fatigue diagnosis to the thermal or electrical signals. Meanwhile, two solder fatigue processes, cracking initiation and cracking acceleration, were identified based on the cumulative AE counts and AE energy during the long PCT. An abrupt increase in AE energy happened at the end stage of the long PCT due to a severe cracking process, which can be regarded as a prewarning of a failure point of the power assembly. In addition, the relationship between transitional thermal resistance and accumulative AE energy was investigated and shows a linear relationship. These results suggest that the AE technique can be a promising online CM approach to diagnose and predict solder fatigue in power assemblies during operation.