Nanosilver pastes have been regarded as the most promising die-attach materials for high-temperature and high-power applications due to their advantages such as excellent thermal conductivity, electrical conductivity, high temperature resistance, and good shear strength. However, the common hot pressing sintering process for nanosilver pastes has the limitations of long sintering time and complicated sintering processes. Thus, laser sintering has been proposed as a rapid sintering method that attracts increasing interest due to its advantages of high energy density, fast temperature rise, easy densification, etc. In this review, the recent advances in laser sintering processes were summarized, including pressure laser sintering, backside sintering, and hybrid bimodal laser sintering. The effects of various laser sintering process parameters on joint performance, such as laser power, sintering pressure, irradiation time, and defocusing amount, were further discussed. The rapid sintering mechanism of laser sintering silver nanoparticles(AgNPs) was revealed, while microscopic explanations need to be further explored. This review provided ideas and methods for subsequent researchers to develop rapid sintering methods for power electronic packaging.@en

Micro LED display technology has been spotlighted as the most promising technology compared to LCD and OLED. Its excellent advantages include higher brightness, self-illumination, higher resolution, lower power consumption, faster response, higher integration, higher stability, thinner thickness, longer life, etc. In terms of the unique benefits, it is attracting increasing attention from industries. With the commercialization of Micro LED technology, the following hurdles are identified: wafer manufacturing, full color, bonding, and mass transfer. Among them, mass transfer is so far considered as the most severe bottleneck. Several mass transfer technologies have emerged, including fine picking and placing, roll printing, laser transferring, and fluid self-assembly, which aim to solve the mass transfer problems. However, the aforementioned first 3 types of technologies still rely on the pick-and-place process, which is limited when the Micro LED die dimension shrinks to smaller scales due to processability and equipment precision. Fluidity self-assembly, on the other hand, will not be constrained by the Micro LED size and machine accuracy in the mass transfer process, which received increasing attention from researchers. In the self-assembly of component level, gravitational attraction, magnetic /electromagnetic fields, and capillary force are considered the mainstream force to facilitate the assembly process. Therefore, the component self-assembly becomes a prospective substitute for the Micro LED mass transfer solution, which overcomes the problems of the trade-off between throughput and the placement accuracy of the pick-and-place technology.@en

With the miniaturization of electronic devices and the development of integrated circuit chips in higher density and higher frequencies, thermal management has become one of the most critical challenges in device performance and reliability. Thermal interface material is bonded between the chip and heat sink. In addition to its thermal conductivity, the mechanical property is essential. Good resilience can effectively fill the bonding gap and reduce thermal resistance, thus improve the overall thermal conductivity. In this paper, three methods are adopted to prepare BN nanosheets/polymer composites. Two polymer composites materials with BN nanosheets oriented were prepared through different suction filtration methods, and the process of homogenization and mixing prepared another sample. Then the microscopic appearance structure of the three materials was analyzed by scanning electron microscope(SEM). The anisotropic compression resilience and tensile properties were studied, respectively. The study showed that BN nanosheet-oriented materials have anisotropic mechanical properties. The better the orientation performance, the greater the difference in the anisotropic mechanical properties of the composite materials. Under the pressure of 0.08MPa, materials with better BN nanosheet orientation have excellent compression and resilience performance, but homogeneous composite materials have the best compression resilience performance. The tensile properties of the three composite materials are quite different. The tensile strength of the homogeneously composite materials can reach 119.59%, while the in-plane and through-plane tensile strength of the other two materials can reach 102.84%,38.52%, and 67.65%, 46.67%, respectively.@en

With the miniaturization of electronic devices and the development of integrated circuit chips in higher density and higher frequencies, thermal management has become one of the most critical challenges in device performance and reliability. Thermal interface material is bonded between the chip and heat sink. In addition to its thermal conductivity, the mechanical property is essential. Good resilience can effectively fill the bonding gap and reduce thermal resistance, thus improve the overall thermal conductivity. In this paper, three methods are adopted to prepare BN nanosheets/polymer composites. Two polymer composites materials with BN nanosheets oriented were prepared through different suction filtration methods, and the process of homogenization and mixing prepared another sample. Then the microscopic appearance structure of the three materials was analyzed by scanning electron microscope(SEM). The anisotropic compression resilience and tensile properties were studied, respectively. The study showed that BN nanosheet-oriented materials have anisotropic mechanical properties. The better the orientation performance, the greater the difference in the anisotropic mechanical properties of the composite materials. Under the pressure of 0.08MPa, materials with better BN nanosheet orientation have excellent compression and resilience performance, but homogeneous composite materials have the best compression resilience performance. The tensile properties of the three composite materials are quite different. The tensile strength of the homogeneously composite materials can reach 119.59%, while the in-plane and through-plane tensile strength of the other two materials can reach 102.84%,38.52%, and 67.65%, 46.67%, respectively.@en

Micro LED displays offer superior performance compared to traditional LCD and OLED displays. However, challenges in transfer technology, such as high throughput and scalability, must be addressed. Among various mass transfer techniques, stamp transfer and laser-assisted transfer are widely used for Micro LED assembly. The laser-assisted transfer technique enables high-speed and accurate transfer. Anisotropic conductive film (ACF) is commonly used for its energy absorption properties during chip transfer. However, during the subsequent thermocompression bonding process, the ACF film needs to be ruptured, which adds no value to the bonding process. To address limitations, we have developed a polymer-reinforced solder paste that demonstrates high effectiveness in absorbing impact energy during chip dropping, providing performance comparable to ACF-like materials for die receiving. It also possesses typical solder paste characteristics, enabling the formation of reliable solder joints between the chip and substrate. This material facilitates streamlined manufacturing process and providing opportunities for chip rework in subsequent stages.@en

A polymer-based wafer level integration technology suitable for miniaturized and multi-functional systems integration was developed and demonstrated in this work. Wafer scale flexible interconnects were firstly fabricated on one wafer, and then transferred to another wafer. Such transfer process involved wafer bonding and application of sacrificial materials. A sacrificial layer was firstly placed on the surface of the transfer wafer, and the sandwich interconnect structures were then manufactured on top of the sacrificial layer. With the help of the sacrificial layer, the flexible interconnects were transferred to another wafer through wafer bonding process. Contact resistance structures were fabricated with the help of wafer bonding process, connecting and aligning metal contact layer on device wafer and metal layer embedded in transferred flexible interconnects. Such transferred contact resistance was measured through designed testing structures as a demo for wafer level heterogeneous integration.@en

The pre-insertion process called anticipation is an essential component of a lane-changing manoeuvre. There is little empirical research regarding the impact of anticipation. Thus, this paper aims to explore the behaviour of the new follower (NF) in the target lane when it encounters anticipation by using new trajectory datasets. The changing magnitude of the reaction pattern is proposed to identify the NF’s behaviour. We find that the anticipation significantly affects the NF’s movement in terms of gap creation and speed reduction. Then, we conduct a detailed analysis of critical variables to reveal their relationship with the NF’s behaviour. Following this, we develop binary logistic models to predict the NF’s behaviour, resulting in a good performance. It also suggests that the NF’s behaviour is highly related to the anticipation-related variables. The transferability test results show that this model can be directly used in different locations and times with satisfactory accuracy.@en

With the increasing application of wide bandgap materials such as silicon carbide and gallium nitride in power devices, the working temperature of power devices has been pushed further. Therefore, it brings higher requirements for packaging materials. Sintered silver is a widely accepted chip connection material. However, silver suffers from high prices and electromigration. Therefore, a novel sintered material based on silver-copper core-shell structured particles raises the attention of researchers to solve this deficiency. To accelerate the development of new materials and their related processes, a four-sphere model of the silver-coated copper structure is established in this paper. The mathematical relationship between the porosity and thermal conductivity of sintered body and the actual sintering process was preliminarily established through the calculation based on a series of FEM simulations. The model was further validated through experiments. The modeling method and conclusion are utilized for future process adjustment, which is of great significance to accelerate the development, application, and reliability of new packaging materials.@en

With the trend of miniaturization and the increasing power density, the operating temperature of electronic devices keeps climbing, especially for wide band-gap semiconductors such as silicon carbide and gallium nitride. The high operating temperature up to 250℃ brings challenges to encapsulation materials since traditional encapsulation materials such as epoxy resins and silicone gels hardly bear temperatures above 200℃. Calcium aluminate cement (CAC) was proved to be a promising encapsulation material, which owns high thermal stability with its operating temperature of up to 300℃. Based on its satisfied thermal stability and low cost, the thermal conductivity of CAC was researched in this work with different ratios of 10-μm-sphere-Alumina (Al 2 O 3 ) fillers at different temperatures, which formed μm-scale CAC-Al 2 O 3 composites. In this work, we focused on the thermal conductivity of CAC-Al 2 O 3 composites aiming for encapsulation applications in power electronics packaging. The thermal conductivities of μm-scale CAC-Al 2 O 3 composites by the laser-flash method from room temperature to 350℃ were firstly measured. Results showed with an increasing content of fillers, the TC of CACAl 2 O 3 will increase accordinglyIt also illustrated that calcium aluminate cement was a high thermal stable encapsulation material with thermal conductivity over epoxy resins. Then, the Finite Element Model (FEM) was established and calibrated by experimental data for thermal conductivity simulation. The FEM model accuracy reached 90%. Such models for new filler materials are effective to minimize material development by actual experiments and characterizations, for CAC composite with different fillers. It also provides an alternative method in predicting other physical properties of composites such as coefficient of thermal expansion, porosity, etc.@en

Ultrasonic wedge bonding of aluminum (Al) wires is a widely applied interconnect technology for power electronic packaging. The joint quality of the wedge bonding is mainly affected by the process parameters and material properties. Inappropriate process parameters will lead to failure modes such as chip surface pit, metal layer peeling off, wire cracking, non-sticking to the pad, etc., which limits the long-term stability of power devices. In order to reach the desired reliability, the design of experiment (DoE) is generally deployed which is costly in terms of time and related materials. Therefore, simulation-assisted analysis is in demand to rapidly narrow down the process windows. In this paper, an ultrasonic bonding model involving thick Al wires (300 μm) was established based on the Finite Element Method (FEM), to optimize process parameters effectively with reduced time and cost. The model was designed in ANSYS utilizing the transient structural mechanics module with various stresses and ultrasonic power, to simulate the relative deformation of the bonded wires and the displacement against the substrate. The result was then verified by ultrasonic wedge bonding samples with 9 sets of process parameters. The stress distributions were simulated and analyzed with the failure modes of tensile strength tests, while the deformation of wires under various process parameters was measured and compared with shear strength tests. Further, the relationship between the failure modes of the joint and the deformation was then analyzed by Response Surface Method (RSM), and the regression equation of the wire deformation and related process parameters was established and fitted with the actual sample's data. Such analysis not only found the optimum range of the deformation of thick Al ultrasonic wire bonds but also quickly provided a range of optimized processes for Al thick wires applying ultrasonic wedge bonding techniques.@en

Conventional reinforcement learning (RL) models of variable speed limit (VSL) control systems (and traffic control systems in general) cannot be trained in real traffic process because new control actions are usually explored randomly, which may result in high costs (delays) due to exploration and learning. For this reason, existing RL-based VSL control approaches need a traffic simulator for training. However, the performance of those approaches are dependent on the accuracy of the simulators. This paper proposes a new RL-based VSL control approach to overcome the aforementioned problems. The proposed VSL control approach is designed to improve traffic efficiency by using VSLs against freeway jam waves. It applies an iterative training framework, where the optimal control policy is updated by exploring new control actions both online and offline in each iteration. The explored control actions are evaluated in real traffic process, thus it avoids that the RL model learns only from a traffic simulator. The proposed VSL control approach is tested using a macroscopic traffic simulation model to represent real world traffic flow dynamics. By comparing with existing VSL control approaches, the proposed approach is demonstrated to have advantages in the following two aspects: (i) it alleviates the impact of model mismatch, which occurs in both model-based VSL control approaches and existing RL-based VSL control approaches, via replacing knowledge from the models by knowledge from the real process, and (ii) it significantly reduces the exploration and learning costs compared to existing RL-based VSL control approaches.@en

With the development of 5G communication technology and the rise of power semiconductors, the switching frequency of the circuit keeps increasing, which pushes for miniaturization of power modules and related components. Therefore, in this paper, a one-step molding technology was proposed for a DC/DC buck converter power module. We proposed a method of using Soft Magnetic Powder filled Epoxy (SMPE) adhesive as a molding material to encapsulate a power module, which is a DC/DC buck converter power module contains several passive components, 1 power inductor, and a high-efficiency switching regulator with two integrated N-channel MOSFETs. On the basis of Finite Element Method (FEM), models were firstly established with component level moldings and checked with actual module samples for calibration. Based on the calibrated model, inductors without component level molding were then simulated. SMPE with 4~7μm insulated carbonyl ferrous powder were prepared and measured the magnetic relative permeability. Such material was investigated to pot the whole power module as a one-step molding, instead of separate molding for the power inductor and the power module. After that, thermal analysis and inductance were calculated and compared.@en

Variable speed limits (VSLs) are a common traffic control measure to resolve freeway jam waves. State-of-the-art model predictive control (MPC) approaches of VSLs are developed based on Eulerian Lighthill-Whitham and Richards (LWR) models, where the decision variables are flows between road segments. It is difficult to implement constraints on speeds that are necessary in typical real-world speed limit systems, because converting flow to speed results in nonlinear and non-convex optimization formulations. In this paper, we develop a new MPC of VSLs based on a discrete Lagrangian LWR model, in which the decision variables are average speeds of vehicle groups. This allows formulating speed constraints as control constraints rather than state constraints in the MPC problem. The optimization of vehicle groups speeds is formulated as a linear programming problem which can be solved efficiently. We further integrate the presented MPC to a hierarchical VSL control framework leveraging connected vehicles. The presented MPC decides the optimal target speed of each vehicle group led by a connected automated vehicle (CAV) at the upper macroscopic level with a prediction horizon of 20 min. At the lower microscopic level, CAVs randomly distributed in mixed traffic are regarded as actuators of the upper layer. Microscopic CAV accelerations are optimized in a short horizon of the order 5–10 s so that the human-driven vehicles following them reach the target speed from the upper layer in an efficient and smooth manner. The presented MPC and the hierarchical control approach are tested in microscopic simulation environments. Simulation results show that (i) the presented MPC resolves freeway jam waves efficiently with reasonable safety constraints implemented, and (ii) the presented hierarchical control approach can effectively resolve jam waves in a single-lane freeway, even though the penetration rate of CAVs is as low as 5%.@en

Variable speed limits (VSLs) are a common traffic control measure to resolve freeway jam waves. State-of-the-art model predictive control (MPC) approaches of VSLs are developed based on Eulerian Lighthill-Whitham and Richards (LWR) models, where the decision variables are flows between road segments. It is difficult to implement constraints on speeds that are necessary in typical real-world speed limit systems, because converting flow to speed results in nonlinear and non-convex optimization formulations. In this paper, we develop a new MPC of VSLs based on a discrete Lagrangian LWR model, in which the decision variables are average speeds of vehicle groups. This allows formulating speed constraints as control constraints rather than state constraints in the MPC problem. The optimization of vehicle groups speeds is formulated as a linear programming problem which can be solved efficiently. We further integrate the presented MPC to a hierarchical VSL control framework leveraging connected vehicles. The presented MPC decides the optimal target speed of each vehicle group led by a connected automated vehicle (CAV) at the upper macroscopic level with a prediction horizon of 20 min. At the lower microscopic level, CAVs randomly distributed in mixed traffic are regarded as actuators of the upper layer. Microscopic CAV accelerations are optimized in a short horizon of the order 5–10 s so that the human-driven vehicles following them reach the target speed from the upper layer in an efficient and smooth manner. The presented MPC and the hierarchical control approach are tested in microscopic simulation environments. Simulation results show that (i) the presented MPC resolves freeway jam waves efficiently with reasonable safety constraints implemented, and (ii) the presented hierarchical control approach can effectively resolve jam waves in a single-lane freeway, even though the penetration rate of CAVs is as low as 5%.@en

Ultrasonic wire bonding is one of the critical challenges for power semiconductor manufacturing process, especially for silicon carbide (SiC) power devices. Packaging-related strain on the dies is one of the limiting factors for SiC devices scaling towards mass-production. Furthermore, due to the high current demand for SiC power device packaging, thick bond wires are often needed, which brings major challenges for the ultrasonic wire bonding process. Thus, computational simulation methods are under development to assist the wire bonding process. This paper presents a simulation method that can quickly narrow the process window for thick bond wires on SiC power devices beforehand. A process model was created to adapt process parameters of bonding force and power. This model aims to simulate the bond deformation for a discretized bonding area. Wire deformation and equivalent plastic strain were then examined and compared. The model was further validated through experiments. Experimental validation of the wire bonding model reveals a suitable deformation of bond wires, which helps to improve thick wire bonding reliability for power electronics packaging.@en

Ultrasonic wire bonding is one of the critical challenges for power semiconductor manufacturing process, especially for silicon carbide (SiC) power devices. Packaging-related strain on the dies is one of the limiting factors for SiC devices scaling towards mass-production. Furthermore, due to the high current demand for SiC power device packaging, thick bond wires are often needed, which brings major challenges for the ultrasonic wire bonding process. Thus, computational simulation methods are under development to assist the wire bonding process. This paper presents a simulation method that can quickly narrow the process window for thick bond wires on SiC power devices beforehand. A process model was created to adapt process parameters of bonding force and power. This model aims to simulate the bond deformation for a discretized bonding area. Wire deformation and equivalent plastic strain were then examined and compared. The model was further validated through experiments. Experimental validation of the wire bonding model reveals a suitable deformation of bond wires, which helps to improve thick wire bonding reliability for power electronics packaging.@en

With the development of electronic technology towards high power, miniaturization, and system integration, power electronic packaging is facing increasing challenges, especially for die attachment. This research aims to explore silver-coated copper (Cu@Ag) paste with sufficient mechanical properties and high-temperature reliability, as an alternative solution for silver sintering with lower cost. Firstly, micro-Cu@Ag sintering pastes were investigated under four kinds of polyol-based solvent systems and two types of particle morphologies, which included sphere-type (SCu@Ag) and flake-type (FCu@Ag). Sintering performance and microstructural evolution were compared and analyzed. Notably, sintered joints employing the terpineol–polyethylene glycol solvent system and flake-type morphology displayed a denser microstructure in comparison to SCu@Ag joints. Its bonding strength reached 36.15 MPa, which was approximately 20% higher than SCu@Ag joints. Subsequently, the influence of key sintering process parameters on Cu@Ag joints was analyzed, including sintering temperature, pressure and time. Additionally, high-temperature aging and thermal cycling tests were conducted on the optimized Cu@Ag joints to assess their reliability. Finally, the micromechanical properties of Cu@Ag joints before and after high-temperature aging were further evaluated by nanoindentation including creep properties. The elastoplastic constitutive models of Cu@Ag sintered materials with different particle morphologies were constructed, providing valuable insights for reliability evaluation. The results indicated that FCu@Ag joints exhibited satisfactory creep resistance and high-temperature reliability. In conclusion, the FCu@Ag micro-paste based on the terpineol–polyethylene glycol solvent system proposed in this study demonstrated sufficient bonding strength, high reliability, and adequate mechanical properties as an attractive solution for high-temperature power electronics packaging.@en