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In this study, the thermal characteristics and structure reliability during power cycling for the four types of SiC power module fabricated using a SiC-heater chip, direct bonded aluminum (DBA) substrate, and aluminum (Al) heatsink were evaluated. Two die-attach materials, including a Sn-Ag-Cu (SAC 305) solder and an Ag paste sinter, were used to bond the SiC to DBA substrate. Furthermore, three types of substrate bonding layers, including SAC solder and Ag paste sinter, and Si grease, were used to bond the DBA substrate to the Al heatsink. The large area bonding between the DBA substrate (30 × 30 mm2) and Al heatsink was achieved. In addition, compared with the traditional SAC solder-Si grease joint structure, the SiC chip temperature decreased from 265.5 to 180.4 °C and the total thermal resistance of the joint structure decreased from 1.58 to 0.85 K/W for the Ag–Ag sinter joint at the same input power. The heat dissipation improved by 1.86 times. The results were validated and fit well using three-dimensional finite element analysis. The failure time was improved 14.5 times from 2340 cycles to 33 926 cycles for the Ag–Ag sinter joint during the power cycling test. This study will help us to create a SiC power device structure that is smaller, thinner, and possesses ultra-low thermal resistance, and high reliability.

Size effect of the nickel-based single crystal superalloy DD6 is investigated in this study by performing nanoindentation experiments and finite element (FE) simulations by emphasizing the strain rate effect. An analytical method is proposed to identify whether a material has the indentation size effect (ISE) based on incremental formulations of the indentation strain rate. By taking ISE as a critical factor, the contact area is further described by considering the curvature radius of non-ideal indenters due to wear issues in practice. In order to emphasize the strain rate effect during indentations, the material parameters dominating mechanical properties are adopted from the strain rate dependent Johnson–Cook model, particular for the nickel-based single crystal superalloy. Consequently, the dimensional ISE analysis is performed by proposing a theoretical framework to establish the relationship among the indentation strain rate, the indenter tip curvature radius and the applied load–penetration depth (P–h) curve for characterizing nanoindentation responses. By identifying the indenter curvature radii and loading time as the important external factors of hardness measurement, FE simulations are validated to achieve good agreement with experimental data. The validated FE predictions are reliably employed to calibrate the dimensionless model of hardness with different indenter curvature radii and loading rates. In the proposed form of the dimensional hardness model, it is found that the indenter curvature radius can be perfectly independent of the material constitutive properties and loading rates. It reveals the potential and advantageous determination of characteristic length of materials by using nanoindentation.

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).

Carbon fiber-reinforced thermoplastics (CFRTPs) and lightweight metals are widely used in multi-material structures owing to their high strength and excellent corrosion resistance, thus stimulating research on various heat-assisted techniques to join them. Currently, the isothermal-pressing process has been developed to realize sound interface joining between CFRTP and Al alloy under relatively uniform thermal conditions. However, a quantitative investigation of the interaction between thermal conditions, chemistry reactions, and joining strength is still lacking. This current investigation analyzed chemical reaction state and resultant bonded strength of isothermal-pressing joined interface of silanized 5052 Al alloy and carbon fiber-reinforced polyamide-6 (CF/PA6) under various thermal conditions. X-ray photoelectron spectroscopy and Raman scattering analysis showed the increasing consumption of epoxy groups on silanized 5052 Al alloy side with the increase in joining temperature and residence time. This suggested higher temperatures and sufficient high-temperature duration times could efficiently promote the interface chemical reaction, increasing interface bonding strength. As the joining temperature or residence time increased, the fracture location gradually shifted from the silane coupling layer/re-solidified resin interface to the re-solidified resin close to interface, and even it directly passed through CF/PA6 along the thickness direction. Under the current adopted conditions, a critical high-temperature duration time of ∼6.0 s at least was recommended by optimizing the joining time to achieve high-strength joining between 5052 Al alloy and CF/PA6. The findings will guide optimizing the joining conditions in heat-assisted joining processes of Al/CFRTP hybrid structures.

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.

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.

• A special oxidation-enhanced property of Cu sinter joints with Cu particle paste was discovered. • The Cu sinter joints stored in an ambient condition with oxidation showed better bonding strength (39.2 MPa) than those protected in vacuum (31.4 MPa). • Formation of Cu 2 O shell structure decreased the porosity of bonding layer and increased the necking growth between Cu particles. • Unlike using formic acid atmosphere for general Cu paste sintering, the joining process reported here is more applicable for semiconductor devices. A special oxidation-enhanced property of Cu sinter joints with Cu particle paste was discovered during thermal storage test. The Cu sinter joints stored under ambient condition with oxidation showed better bonding strength (39.2 MPa) than those protected in vacuum (31.4 MPa), compared to the initial as-bonded strength of 27.6 MPa. It was found that the oxidation decreased the porosity of bonding layer by forming Cu 2 O shell structure on the sintered Cu microstructure, which enhanced the mechanical property of the sinter joint especially after storage time of 100 h. The corresponding electrical conductivity of solder joint at 100 h is 9.6 × 10−6 Ω, which is still acceptable compared to the as-bonded value of 1.0 × 10−6 Ω. The phenomenon implies that by introducing proper oxidation, Cu sinter joints with improved bonding strength and acceptable electrical conductivity can be achieved, which will provide another potential strengthening process of Cu sinter joining in practical applications. [ABSTRACT FROM AUTHOR]

• A robustly and large-area (35 × 35 mm2) bare Cu to Cu bonding was realized by a novel Ag salt paste. • The large-area joint layer exhibit low porosity about 10% and ultra-high bonding strength over 58 MPa. • Anti-oxidation function of Ag salt paste during sintering process result in pure Cu-Ag-Cu bonding. • Cu-Ag interface possessed a strong bonding without fracture during a shear test. In this paper, by proposing a novel and low-cost Ag salt paste, a robustly and large-area (35 × 35 mm2) bare Cu to Cu bonding was realized under a low sintering pressure of 0.8 MPa and a low sintering temperature of 300 °C in air atmospheric conditions. The relationship between the bonding strength and microstructure changes of sintered Ag under various bonding conditions was investigated in detail. The large-area bonded plate exhibits low porosity about 10% and low percentage of voids, which result in ultra-high bonding strength over 58 MPa. More importantly, the introduction of reducing agent formic acid (CH 2 O 2) and the low porosity successfully improve the anti-oxidation of novel Ag salt paste during sintering process, result in pure Cu-Ag-Cu bonding. The cross-section of the Ag joints was obtained to explain the bonding pattern, in which Cu was oxidized only at the edge of the sintering interface, and no Cu oxide generated in middle bonding section. The development of novel Ag salt paste successfully addresses the energy-intensive process and low bonding strength faced by large-scale sintering, which greatly promotes the high-temperature applications of power device. [ABSTRACT FROM AUTHOR]

The present study investigates a fatigue property and its microstructural and interface geometrical effect on the resistance element welded (REWed) aluminum (Al)/high-strength steel (Fe) lap joint. The assembled Al/Fe joints were subjected under the fatigue strength up to 80% of tensile-shear loads with 0.1 of the load ratio. As a result, an exceptional fatigue strength has been secured inducing the base material fracture because there are no noticeable interfacial defects at the 10.5 kA welding condition compared to assemblies joined with 3.5–4.5 kA of welding currents. Even, maximum tensile-shear loads exceed 9 kN at the 10.5 kA welding condition. Microstructural developments of base materials and welding interfaces were thoroughly analyzed by an optical microscopy (OM) and electron backscatter diffraction (EBSD), the development of prior-austenite-grain-boundaries (PAGBs) and α'-martensite phases were observed in the welding interfaces at all conditions. Heat affected zone (HAZ) developments of AA5052 were quantitatively analyzed in terms of recrystallization and grain growth, providing significant differences in the fatigue performances and striation, crack propagation developments. In addition, with the help of a finite element (FE) computational modeling, a mechanical stress distribution and strain behavior by geometries of the joining interface are discussed in detail.

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.

In this study, Al joint systems without heat-affected zones (HAZ) were developed on Al/Fe alloy lap joints fabricated by resistance element welding (REW). The robustness of the joints with and without HAZs was quantitatively evaluated to consider another side effect in REW Al/Fe alloy systems. Compared to conventional REW, removing the Al HAZ not only alters the steel properties, but also reduces the work hardening of the Al alloy due to the difference in the distinct heat transfer paths. Furthermore, the presence of Al HAZ significantly influenced the hardness characteristics of the molten steel portion. In the cold wall configuration without Al HAZ, hardness values exceeding those of the hot wall by approximately 100 Hv or more were observed. At a welding current of 10.5 kA, the hot wall configuration demonstrated an approximate 30 % increase in tensile shear load and a 66 % increase in displacement compared to the cold wall configuration without HAZ. In the cold wall conditions, interfacial failure (IF) was not attributed to microstructural disparities but was linked to the formation of an inherent notch root when S20C melted due to the absence of Al HAZ. Contrary to previous reports, the presence of HAZ in Al alloys results the production of tough steel through REW.

Ag sinter joining as a promising alternative die attach material have received more and more attention for next generation wide band gap (WBG) power modules. Here we report a robust joint by Ag sinter joining technology for different metal interface (Au, Ag, Ni, Cu, Al) in WBG power modules. Micron-scale Ag flakes were used as the Ag fillers. Sinter Ag joint for different metal interface was implemented by a sintering process under temperatures of 250 degC in air without pressure. The die shear strength of all the joint structures was larger than 25 MPa and the sinter Ag on the Ag metallized substrate shows the maximum value about 40 MPa. The die shear strength was larger than the value of traditional Sn-Pb solders (19~24MPa). Bonding mechanism for Ag sinter joining for different metal interface also was analyzed by SEM, EDS, and TEM observation. This study will add to the understanding of Ag sinter joining and expand its application in various power electric device interconnects.

In this paper, transparent electrodes with dense Cu@Ag alloy nanowires embedded in the stretchable substrates are successfully fabricated by a high-intensity pulsed light (HIPL) technique within one step. The intense light energy not only induces rapid mutual dissolution between the Cu core and the Ag shell to form dense Cu@Ag alloy nanowires but also embeds the newly formed alloy nanowires into the stretchable substrates. The combination of alloy nanowires and embedded structures greatly improve the thermal stability of the transparent electrodes that maintain a high conductivity unchanged in both high temperature (140 °C) and high humidity (85 °C, 85% RH) for at least 500 h, which is much better than previous reports. The transparent electrodes also exhibit high electromechanical stability due to the strong adhesion between alloy nanowires and substrates, which remain stable after 1000 stretching–relaxation cycles at 30% strain. Stretchable and transparent heaters based on the alloyed and embedded electrodes have a wide outputting temperature range (up to 130 °C) and show excellent thermal stability and stretchability (up to 60% strain) due to the alloy nanowires and embedded structures. To sum up, this study proposes the combination of alloying and embedding structures to greatly improve the stability of Cu nanowire-based stretchable transparent electrodes, showing a huge application prospect in the field of stretchable and wearable electronics.

Wide band gap semiconductors such as SiC and GaN have been expected to be established as the next generation power semiconductors, which are capable to reduce huge energy-loss in electric power conversion. The application of them will increase power density resulting in operation temperature rise beyond 200 degC. In addition, both increase of current and of frequency will also increase stress on devices. In general, power devices must possess robustness and high reliability for safe control of power conversion. Heat dissipation from the hot die in operation is one of the essential key factors for establishing high performance WBG devices. Here we report the current status of high temperature packaging materials including die-attach, wiring, substrates, molding compounds, and, in addition, reliability issues including thermal shock, power cycling and transient thermal properties.

Printable and flexible Cu–Ag alloy electrodes with high conductivity and ultrahigh oxidation resistance have been successfully fabricated by using a newly developed Cu–Ag hybrid ink and a simple fabrication process consisting of low-temperature precuring followed by rapid photonic sintering (LTRS). A special Ag nanoparticle shell on a Cu core structure is first created in situ by low-temperature precuring. An instantaneous photonic sintering can induce rapid mutual dissolution between the Cu core and the Ag nanoparticle shell so that core–shell structures consisting of a Cu-rich phase in the core and a Ag-rich phase in the shell (Cu–Ag alloy) can be obtained on flexible substrates. The resulting Cu–Ag alloy electrode has high conductivity (3.4 μΩ·cm) and ultrahigh oxidation resistance even up to 180 °C in an air atmosphere; this approach shows huge potential and is a tempting prospect for the fabrication of highly reliable and cost-effective printed electronic devices.

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 laser ablation process is an indispensable tool in precision micromachining for the fabrication of micro/nanostructures with specific geometries and regulation of physicochemical properties on target surfaces. However, the absence of studies on the effect of laser parameters on texturing formation and surface microstructure makes process control and the optimization of surface texturing challenging. In this study, a nanosecond pulsed laser process was employed to create the micro-texturing on the Al alloy surface, emphasizing the effect of process parameters on surficial grain structures and texturing formation. A three-dimensional heat transfer model based on the finite element (FE) method was developed to numerically visualize the shape-dimensional evolution of micro-texturing during laser ablation. A simplified but effective moving laser heat source model was proposed to predict the dynamic spatial and temporal distributions of heat generation and transfer. Material removal, which was dominated by surface evaporation in the pulsed laser process, was achieved using the element deletion method. Under various laser parameters, the model-predicted cross-sectional groove profiles agreed well with experimental measurements. In addition, the effects of laser power and scanning speed on the groove morphologies was investigated. The experimental findings and numerical models established in this study will provide guidance for the precise control of pulsed laser parameters to achieve desired texturing structures and dimensions.

In this study, a highly reliable Ag sinter joining on Direct bonded aluminum (DBA) substrate was introduced in SiC power modules. The electroless nickel /electroless palladium/immersion gold (ENEPIG) metallization layer was coated on the DBA substrate with the electroless nickel layer of 7µm and 20 µm. The reliability of SiC/DBA die-attached module by Ag sinter joining was evaluated by thermal shock test (TST) from -50℃ to 250℃ up to 1000 cycles. In the case DBA with the Ni metallization layer of 7µm, severe ups and downs deformation occurred from the Al layer during the TST, leading to uneven variation of sintered Ag layer. The deformation was suppressed when the Ni layer increased to 20 µm. The bonding interface and fracture characteristics were systematically investigated for the both cases, which provide basic understanding the Ag-Au reliability by the design for the metallization layer on substrate.

This study focuses on the fatigue fracture response and microstructure behavior of a resistance element weld (REW) in assembling AZ31B magnesium alloy and dual phase (DP) 780 pates. With the increase of the welding current the prior austenite grain size (PAGS) increased, which resulted in a decrease in the fraction of an alpha prime martensite at the welding interface and had little influence of the dual phase at the welding interface. It is clear that the formation of a suitable hook accompanied by the phase transformation between S20C elements and DP780 steels at the interface can have a positive effect on the mechanical properties. However, the Mg melting and the formation of a partially melted zone (PMZ) affected the elongation decrease significantly. Furthermore, the insufficient penetrations of the welding interface, the severe melted Mg and the formation of porosities in the PMZ were fatal to the strength and fatigue performance. Morphologies and microstructure characteristics including phases of the REWed joint were quantitatively analyzed by electron back-scatter diffraction (EBSD) and thermodynamic calculation by coupling the phase diagrams (CALPHAD) method, respectively. The experiment results were incorporated into the finite element method (FEM) to numerically elucidate their effects against mechanical responses including fatigue behaviors. The results explained why the mechanical strength, fatigue endurance, and fracture modes are radically different for joint morphologies and microstructure developments.

We carried out in situ transmission electron microscopy (TEM) on the microstructural change of a sintered Ag die-attach layer heated to 600 degC. It was found that the addition of 2 wt% tungsten carbide (WC) particles to the sintered Ag layer significantly suppressed the microporous structure growth at elevated temperature. In the case of a pure Ag layer without anything added, microporous structure growth started from 450 degC. However, microporous structure growth was hardly observed in the sintered Ag layer with WC added up to 600 degC.

Ag sinter joining as a promising alternative die attach material have received more and more attention for next generation wide band gap (WBG) power modules. Here we report a robust joint by Ag sinter joining technology for different metal interface (Au, Ag, Ni, Cu, Al) in WBG power modules. Micron-scale Ag flakes were used as the Ag fillers. Sinter Ag joint for different metal interface was implemented by a sintering process under temperatures of 250 °C in air without pressure. The die shear strength of all the joint structures was larger than 25 MPa and the sinter Ag on the Ag metallized substrate shows the maximum value about 40 MPa. The die shear strength was larger than the value of traditional Sn–Pb solders (19~24MPa). Bonding mechanism for Ag sinter joining for different metal interface also was analyzed by SEM, EDS, and TEM observation. This study will add to the understanding of Ag sinter joining and expand its application in various power electric device interconnects.

Wide band gap semiconductors such as SiC and GaN have been expected to be established as the next generation power semiconductors, which are capable to reduce huge energy-loss in electric power conversion. The application of them will increase power density resulting in operation temperature rise beyond 200 °C. In addition, both increase of current and of frequency will also increase stress on devices. In general, power devices must possess robustness and high reliability for safe control of power conversion. Heat dissipation from the hot die in operation is one of the essential key factors for establishing high performance WBG devices. Here we report the current status of high temperature packaging materials including die-attach, wiring, substrates, molding compounds, and, in addition, reliability issues including thermal shock, power cycling and transient thermal properties.

This work introduces a bonding technology that employs a cold-rolled Ag sheet, which provides a unique reaction in air, to achieve low-pressure and low-temperature die-attachment for wide-bandgap semiconductors. The cold rolling of the Ag sheet induces grain refinement down to a sub-micron level with high residual stress, which is the driving force for hillock growth on the Ag sheet, resulting in intimate bonding. The detailed mechanism underlying the hillock joining was investigated via transmission electron microscopy (TEM). TEM images and electron-diffraction patterns showed that a large quantity of self-generated Ag nanoparticles formed in the contact area between two Ag substances.