Nano copper sintering technology has great potential to be widely applied in the wide-bandgap semiconductor packaging. In order to investigate the coalescence kinetics of copper nano particles for this application, a molecular dynamic (MD) simulation was carried out at low temperature on a special model containing two substrate and multiple particles in between. Accordingly, thorough microstructure and dislocation investigation was conducted to identify the atomic-scale evolution in the system. The corresponding findings could provide evidence on the new particle-substrate sintering mechanism. Furthermore, atomic trajectories tracking method was applied to study the rotation behavior of different sized nano particles. New rotation behavior and mechanism were described. Additionally, the study on the size effect of copper particles on the sintering process and coalescence mechanism was conducted via comparing the microstructural and dislocation distribution of 3 nm, 4 nm and 5 nm models. Finally, by comparing the MSD results at low and high temperature for each model, the dominant coalescence dynamics changes were obtained.

@en

Cu-Ag core-shell (CS) nanoparticle (NP) is considered as a cost-effective alternative material to nano silver sintering material in die attachment application. To further reduce the cost, the thickness of the Ag shell can be adjusted. Whereas the shell thickness will also affect the thermal stability of the Cu-Ag CSNPs. In this study, molecular dynamics simulation was applied to study the thickness effect on the thermal behavior of Cu-Ag CSNPs. The melting points of CSNPs and Pure NPs can be determined by the evolutions of Potential Energy (PE), and the Lindemann index (LI) of the system. The results indicated that the melting points of CS NPs were lower than monometallic NP and the melting point of CS NP is influenced by the size of the Cu core and the number of lattice mismatches. Moreover, the distribution of atoms’ LI showed that the premelting point is independent of shell thickness. However, the fraction of atoms that occurred premelting is increased with the decrease of the shell thickness. Otherwise, we also simulated the sintering process of double CS NPs with equal size.@en

This paper studied the behaviors of sintering between Ag nanoparticle (NP) and nanoflake (NF) in the same size by molecular dynamics simulation. Before the sintering simulation, the melting simulation of NF was carried out to calculate the melting points of NFs and investigate the thermostability of NF. The Lindemann index and potential energy showed that the melting points of NF were significantly size-dependent. During the heating process, the sharp corner of NF transformed to the round corner and could bend spontaneously lower than melting points. In sintering simulation, the sintering process of NF-NP showed a metastable stage before equilibrium. Under low sintering temperature (500 K), the degree of plasticity sintering mechanism of NF-NP was more prominent, which generated more defects, such as amorphous atoms, dislocations, and stacking faults, than NP-NP. The sintered products of NF-NP also presented a better neck size and shrinkage than NP-NP in the same size. A new sintering behavior was observed: NF was bent toward the NP during the sintering. The bending curvature of NF increased as the thickness or the length/width decreased. For the NF with the ratio of length/width to thickness of 5:1, bending could further significantly facilitate neck growth. At 700 K, the plasticity mechanism dominated both the sintering processes of NF-NP and NP-NP. And NF-NP showed a larger diffusivity than NP-NP. At last, we investigated the effects of crystal misorientation, and found that a tilted grain boundary generated in the neck. The NF had the trend of rotation to decrease the crystal misorientation.

@en

This paper presents a continuous-time integrated readout interface for inertial sensors with high energy efficiency and high resolution. A dual digital-to-analog conversion (dual DAC) scheme is proposed which reduces the large DC baseline capacitance while preserving a high gain in the capacitance-to-voltage converter (CVC). A continuous-time delta-sigma modulator (CTΔΣM) is used to digitize the output signal of the CVC without kT/C noise. This results in a precision capacitive readout IC with a 5.5aF resolution in 0.5ms conversion time. It consumes 1.8mW. An inertial sensing system, consisting of the readout IC and a MEMS inertial sensor, achieves a resolution of 17.02ng/√Hz.@en

Sn-Ag-x solders were used as the interfacial layers between SnBi solder and Cu substrate. The effects of Sn-Ag-x layers on the solderability, microstructure, and mechanical properties of SnBi solder joint were investigated. Experimental results indicate that all the barrier layers have positive effects on improving the wettability of SnBi solder. The relative area and grain size of β-Sn was enlarged due to the addition of Sn-Ag-x layers. Meanwhile, the addition of the interfacial layers decreased the hardness of the SnBi solder joint. The addition of Sn-Ag-x layers increased the thickness of the interfacial intermetallic compound (IMC) but had limited effects on the shear force of the SnBi solder joint. Due to the addition of the interfacial layers, the brittleness of the SnBi/Cu solder joints during the shear test was slightly suppressed.

@en

The global spread of COVID-19 has caused a huge number of confirmed cases and deaths, which in return leads to a plethora of mental disorders across the world. In order to address citizens’ psychological problems, government agencies in many countries have employed AI-based chatbots to provide mental health services. However, there is a limited understanding of the determinants affecting citizens’ user experience and user satisfaction when mental health services supported by chatbots are provided. Thus, based on the Theory of Consumption Values (TCV), this study proposes an analytical framework to investigate the factors that are important to citizens’ user experience and user satisfaction when they interact with mental health chatbots. Analysis of data collected from 295 chatbot users in Wuhan and Chongqing reveals that personalization, enjoyment, learning, and condition are positively related to user experience and user satisfaction. However, voice interaction fails to devote to citizens’ user experience and user satisfaction. Thus, government agencies and their AI service contractors should enhance the functions and systems of mental health chatbots to ensure citizens’ user experience and user satisfaction. Also, they should more positively promote the use of mental health chatbots during the public health emergency.

@en

The microstructure, shear behaviour and hardness of the SnBi/SACBN/Cu solder joint before and after isothermal ageing were investigated in comparison with the SnBi/Cu and SACBN/Cu solder joints. The experimental results indicated that the pre-soldered SACBN joint had a significant effect on the formation and growth of the β-Sn grains in the SnBi bulk solder. The brittleness of the SnBi/SACBN/Cu composite solder joint was also suppressed and its failure mode transformed from brittle failure to brittle-ductile failure after reflow. However, the shear strength and failure mode of the SnBi/SACBN/Cu composite solder joint became similar to those of the SnBi/Cu joints after 600 h isothermal ageing. The shear strength of the three kinds of solder joints decreased after isothermal ageing, but the SnBi/SACBN/Cu composite solder joint showed higher shear strength than SnBi/Cu did during ageing. The shear strength of the composite solder joint was 67.1MPa after ageing. Due to the diffusion of elements in the isothermal ageing process, the microstructure of the composite solder joint was significantly coarsened after ageing for 600 hours. This phenomenon further led to the decrease of the hardness and shear strength of the three kinds of solder joints.

@en

This study investigated the mechanical properties for two types of solder alloy: Sn-0.7Ag-0.5Cu-3.5Bi-0.05Ni (SAC0705BiNi) vs. Sn-0.7Ag-0.5Cu (SAC0705) by using nano-indentation. Two kinds of solder alloy balls with a diameter of 400 μm are soldered to Cu pads on FR-4 substrates, and then formed the ball grid array (BGA) micro solder joints of Cu/SAC0705BiNi/Cu and Cu/SAC0705/Cu. Meanwhile, the combined effect of Bi and Ni elements on the mechanical properties of the bulk of low-Ag Cu/SAC0705/Cu was discussed. Experimental results revealed that the indentation depth and area of the bulk of Cu/SAC0705BiNi/Cu solder joints were smaller than that of Cu/SAC0705/Cu under the same load and strain rate. It was observed that the indentation morphologies of the two kinds of the bulk of micro solder joints have piling-up phenomenon at lower strain rate. Under the maximum load of 20 mN and the strain rate of 2.5 × 10^{− 1} s⁻¹, the indentation hardness of the bulk of Cu/SAC0705BiNi/Cu and Cu/SAC0705/Cu solder joints was 0.449 GPa and 0.200 GPa, respectively. And the strain hardening exponent was 0.302 and 0.159, respectively. Additionally, the stress-strain relationship was developed for the bulk of Cu/SAC0705BiNi/Cu and Cu/SAC0705/Cu micro solder joints. Compare with the bulk of the low-Ag Cu/SAC0705/Cu micro solder joints, the indentation hardness, indentation modulus and strain hardening exponent of the bulk of Cu/SAC0705BiNi/Cu achieve an improved by adding Bi and Ni elements.

@en

The effects of porous copper (P-Cu) on the microstructure, hardness, and shear strength of SnBi, SnBi-0.4Ag, and SnBi-1Ag solder joints were investigated in this paper. The experimental results show that P-Cu frames distribute in the solder bulks and form triangular areas. The addition of P-Cu leads to the microstructural refinement in the enclosed areas by the P-Cu frames in the solder bulks. The average hardness here is increased due to the fine grain strengthening mechanism. The SnBi-1Ag@P-Cu solder bulk shows smaller Bi-rich grains but larger β-Sn dendrites than the other two P-Cu-enhanced solder bulks. Porous Cu exists as a frame structure in the solder joints, which hinders the initiation and propagation of cracks and has a positive effect on the improvement of joint strength. Compared to the SnBi and SnBiAg solder joints, the shear strength of the P-Cu-enhanced solder joints is increased by 15%. The average shear strength of the SnBi-0.4Ag@P-Cu solder joint is 79.34 MPa, which is the highest among all the solder joints investigated in this study.

@en

Sensitivity and selectivity are important factors for Tl₂O monolayer to be the sensitive material. In this work, the sensing performance of NO₂ on pure and X-Tl₂O (X = In, Cd, Y, Pb, Ga, Si) sheets has been detailed investigated by means of DFT. The results show that the doped systems have a better sensing performance for NO₂ and it is most evident in the In-Tl₂O substrate. And In-Tl₂O has a unique response to NO₂ with appropriate adsorption energy (−1.79 eV) and charge transfer (−0.51 e) which are far more than B-C₃N (−1.15 eV, 0.44 e). Besides, although In-Tl₂O monolayer and SO₂ have formed the chemical bond, the adsorption effect of NO₂ on the substrate hardly changes when the co-adsorption of SO₂ and NO₂ occurs on it. In addition, the reflectivity and optical absorption of the gas/In-Tl₂O (gas = NO, NO₂ and SO₂) adsorption system are calculated, and the results indicate optical absorption and reflectivity of NO₂/In-Tl₂O system are all far greater than that of other systems in the visible and near infrared regions. So it is easy to be distinguished by infrared detection. These all show that the In-Tl₂O is an excellent sensing material for NO₂ detection.

@en

High reliable packaging materials are needed for electronics when they work at harsh environments. Among which, the nanosilver material has been widely studied and applied in power electronics due to its low processing temperature and high reliability. This paper investigates the bonding properties of nanosilver sintered hermetic cavity. There are two kinds of lids used in this study, including copper lid and silicon lid. The X-ray and C-Mode Scanning Acoustic Microscopy (C-SAM) results revealed that delamination tended to happen in Cu lid sintered cavity as the recovery of deformed Cu lid was hindered by sintered dense Ag layer. However, no delamination or cracks were found in Si lid sintered cavity. Finite element analysis (FEA) method was used to investigate the effects of lid materials on the stress distribution of lid. The results indicated that the Cu lid sintered cavity showed a much higher stress than the Si lid sintered cavity under the sintering parameters of 250 °C and 10 MPa. There is no obvious change in the stress distribution areas on Cu lid with the increasing of pressures from 5 to 30 MPa. However, the distribution area of stress on Si lid expanded obviously only when the sintering pressure increased to 30 MPa. With the increase of sintering pressures from 5 to 30 MPa, the maximum stresses on Cu lid are almost the same, while increasing trend was found on Si lid.@en

The microstructure, thickness, porosity, and shear performance of the silver (Ag) sintering layers under different sintering pressures were investigated. Experimental results demonstrated that the thickness and the porosity of the sintering layer decreased when the sintering pressure varied from 5 MPa to 30 MPa. This densification phenomenon facilitated the enhancement of the Ag sintering layers. The shear strength was improved significantly from 44.19 MPa to 69.41 MPa when the sintering pressure increased from 5 MPa to 10 MPa. When the sintering pressure ranged from 10 MPa to 30 MPa, the shear strength presented a slow increase from 69.41 MPa to 73.38 MPa. According to the results of the failure analysis, fracture mode transformation was considered as the basic reason for this phenomenon. The increasing sintering pressure promoted the bonding of the nano-Ag particles during the sintering process. Consequently, the fracture of the sintered-Ag layer transformed from brittle fracture to ductile fracture because of the increasing sintering pressure. The delamination area between Cu and Ni layers coated on the bottom Mo plate was clearly enlarged with the increasing sintering pressure. The delamination between Cu and Ni layers coated on the bottom Mo plate turned to be the main failure mode when the sintering pressure was higher than 10 MPa.@en

This letter presents a 48 GHz frequency doubler in a 65 nm CMOS technology. The proposed frequency doubler is composed of a complementary push-push structure with negative resistance circuit for conversion gain enhancement. The maximum measured conversion gain reaches −6.1 dB at 48 GHz output frequency, and the 3-dB bandwidth is 40∼54 GHz. The fundamental rejection is above 29.5 dB. The size of the proposed frequency doubler chip is 0.72 × 0.36 mm2The total power consumption is 16 mW.@en

The addition of Cu nanoparticles into the solder pastes by mechanical mixing method creates a positive effect on the microstructure refinement of the LED solder joints. The grain size of-Sn and Cu6Sn5 decrease obviously due to the increasing concentration of the nanoparticles in the solder pastes. However, the addition of nanoparticles facilitates the formation of voids in the solder joints, especially when the concentration of nanoparticles is higher than 0.5 wt% in the solder pastes. Both the microstructure refinement and void percentage affect the shear strength of the solder joints. Since the increase of the void percentage is limited when the concentration of nanoparticles increases from 0 to 0.5 wt%, the microstructure refinement shows a dominant effect on the shear performance and thus improves the shear strength of the solder joints from 49.8 to 55 MPa. Further addition of nanoparticles in the solder pastes leads to a sharp increase of the void percentage. Consequently, the shear strength of the solder joints decreases from 55 to 48.8 MPa when the concentration of doped particles increases from 0.5 to 1 wt% in the solder pastes.@en

Novel Mg-(3.5, 6.5wt%)Li-(0.5, 2, 4wt%)Zn ternary alloys were developed as new kinds of biodegradable metallic materials with potential for stent application. Their mechanical properties, degradation behavior, cytocompatibility and hemocompatibility were studied. These potential biomaterials showed higher ultimate tensile strength than previously reported binary Mg-Li alloys and ternary Mg-Li-X (X=Al, Y, Ce, Sc, Mn and Ag) alloys. Among the alloys studied, the Mg-3.5Li-2Zn and Mg-6.5Li-2Zn alloys exhibited comparable corrosion resistance in Hank's solution to pure magnesium and better corrosion resistance in a cell culture medium than pure magnesium. Corrosion products observed on the corroded surface were composed of Mg(OH)₂, MgCO₃ and Ca-free Mg/P inorganics and Ca/P inorganics. In vitro cytotoxicity assay revealed different behaviors of Human Umbilical Vein Endothelial Cells (HUVECs) and Human Aorta Vascular Smooth Muscle Cells (VSMCs) to material extracts. HUVECs showed increasing nitric oxide (NO) release and tolerable toxicity, whereas VSMCs exhibited limited decreasing viability with time. Platelet adhesion, hemolysis and coagulation tests of these Mg-Li-Zn alloys showed different degrees of activation behavior, in which the hemolysis of the Mg-3.5Li-2Zn alloy was lower than 5%. These results indicated the potential of the Mg-Li-Zn alloys as good candidate materials for cardiovascular stent applications. Statement of significance: Mg-Li alloys are promising as absorbable metallic biomaterials, which however have not received significant attention since the low strength, controversial corrosion performance and the doubts in Li toxicity. The Mg-Li-Zn alloy in the present study revealed much improved mechanical properties higher than most reported binary Mg-Li and ternary Mg-Li-X alloys, with superior corrosion resistance in cell culture media. Surprisingly, the addition of Li and Zn showed increased nitric oxide release. The present study indicates good potential of Mg-Li-Zn alloy as absorbable cardiovascular stent material.

@en

Existing techniques for motion imitation often suffer a certain level of latency due to their computational overhead or a large set of correspondence samples to search. To achieve real-time imitation with small latency, we present a framework in this paper to reconstruct motion on humanoids based on sparsely sampled correspondence. The imitation problem is formulated as finding the projection of a point from the configuration space of a human's poses into the configuration space of a humanoid. An optimal projection is defined as the one that minimizes a back-projected deviation among a group of candidates, which can be determined in a very efficient way. Benefited from this formulation, effective projections can be obtained by using sparsely sampled correspondence, whose generation scheme is also introduced in this paper. Our method is evaluated by applying the human's motion captured by an RGB-depth (RGB-D) sensor to a humanoid in real time. Continuous motion can be realized and used in the example application of teleoperation.@en

Sn58Bi (SnBi) composite solder pastes were fabricated with various amounts of Sn–3.0Ag–0.5Cu (SAC) particles addition by mechanical mixing technic. The microstructure, hardness, and shear behavior of the solder joints were investigated. The experimental results indicate that the addition of SAC particles in the composite solder pastes significantly increases the concentration as well as the grain size of Sn and Bi-rich phase in the microstructure of the SnBi–SAC solder bulks. Meanwhile, the amount of submicron Bi grains increases because of increasing SAC content. The hardness of the solder bulks decreases as the percentage of SAC particles varies from 0 to 8 wt%, but increases when the doped percentage rises from 8 to 15 wt%. The shear force shows an ascending tendency because of the addition of SAC particles into the solder pastes with the range from 0 to 5 wt%. As the concentration of SAC increases to 8 wt% and even 15 wt%, large voids can be observed and the shear force of the solder joints decreases.

@en