Copper sintering has gained great attention as a die-attach technology for power electronics because of its potential cost effectiveness and high reliability under harsh working conditions. However, the mechanism of how the intrinsic pores within such sintered joints influence the thermal and electrical properties still needs further investigation. The evolution of pores within such sintered joints is difficult for in-situ observation during the sintering process and reliability tests, while the porosity level greatly affects the thermal and electrical properties. In this work, four two-dimensional (2D) models with various random pore structures were established based on the Quartet Structure Generation Set (QSGS) algorithm. Then, finite element method (FEM) simulations were conducted to simulate the heat and current conduction in the sintered materials. Subsequently, the distribution of temperature as well as the electric potential in the porous sintered materials were further discussed. Lastly, both the thermal and the electrical conductivities were calculated, followed by a concluded parabolic relationship of thermal and electrical conductivities with the porosity. These findings offer insights into optimizing and predicting copper sintered joint performance and accelerate the wide application of copper sintering.

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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

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.

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With the popularization of wide band-gap power modules in offshore wind power systems and water surface photovoltaic power stations, packaging materials face challenges of corrosion by salt, blended with high humidity. Copper-silver (Cu-Ag) composite sintered paste was proposed by researchers as a novel die-attach material for a lower cost and anti-electro migration ability. However, the potential difference between copper and silver forms galvanic corrosion in a high-humidity environment, resulting in accelerated failure combined with salt mist. To further promote the application of composite sintered materials, a copper-silver double-sphere galvanic corrosion model based on finite element simulation was proposed in this paper. The relationship between corrosion rate and time of different Cu-Ag particle size combinations under different sintering degrees was predicted by initial exchange current density. Through the electrochemical characterization of the sintered samples, the optimal combination of materials was further discussed. The accuracy of the model was also verified. The conclusions obtained from both the experiments and simulation work provide guidance for future anti-corrosion analysis, as well as the reliability improvement of novel composite sintered materials.

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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.

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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

Recent years, the sintered silver paste was introduced and further developed for power electronics packaging due to low processing temperature and high working temperature. The pressure-less sintering technology reduces the stress damage caused by the pressure to the chip, improves reliability, and is widely applied in manufacturing. Currently, most existed studies are focused on alcohol-based sintered silver pastes while resins have been demonstrated to improve the bonding properties of solder joints. Hence, the performance and sintering mechanisms with epoxy-based silver paste need to be further explored. In this work, a methodology for multifactor investigation is settled on the epoxy-based silver paste to reveal the relationship between the strength and the different influence factors. We first analyzed the characteristics of commercialized epoxy-based silver paste samples, including silver content, silver particle size, organic composition, sample viscosity, and thermal conductivity. Samples were then prepared for shear tests and microstructure analysis under different pressure-less sintering temperatures, holding time, substrate surface, and chip size. Full factor analysis results were further discussed in detail for correlation. The influence factors were ranked from strong to weak as follows: sintering temperature, substrate surface, chip size, and holding time. Finally, a thermal cycling test was carried out for reliability analysis. Epoxy residues are one of the possible reasons, which result in shear strength decreasing exponentially.

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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

Traditional packaging materials such as solder paste were studied for decades, which is possible to detect initial cracks for a reliability lifetime estimation. While novel die-attach materials such as sintered silver are developing towards higher working temperature and higher current density, it is not clear whether initial cracks are also helpful for reliability assessment. Therefore, in this work, a series of FEM simulations were established for a response surface model with power electronics chip sizes to predict sintered silver joint reliability. Impact factors for resistance were analyzed and compared such as die size and the thickness of the joint. Sintered silver layer sandwiched by copper substrate and terminals with constant current supply was generated and simulated for resistance fluctuation. Through the high-precision DC resistance measurement setup based on the four-probe method, the resistance over ${50 \mathrm{n}\Omega}$ is possible to detect, thus lead to crack growth. In order to study the geometry sensitivity of cracks, preset arc-shape cracks were modeled to simulate crack generation. The coupling of resistance and crack were analyzed through von Mise strain distribution. With a proper geometric configuration of the die-attach layer, it is possible to minimize the testing time for new joint materials through a high precision electrical resistance measurement and simulation-assisted models.@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