Sandwich Structure Research Articles

Compressive and flexural responses of auxetic sandwich panels with modified re-entrant honeycomb cores

Auxetic structures possess negative Poisson’s ratio (NPR). They exhibit enhanced indentation resistance, synclastic deformation, high fracture toughness and high energy absorption. Several sorts of auxetic structures exist, such as re-entrant, arrowhead, chiral, etc. The re-entrant structure is the most common sort of auxetic structures. This paper investigates the mechanical behavior of the re-entrant auxetic structure using experimental and numerical methods. Two modified re-entrant topologies are proposed based on the original (conventional) re-entrant topology. Using these modified topologies, two re-entrant auxetic sandwich structures are designed and 3D printed using fused deposition modeling (FDM) out of polylactic acid (PLA). Conducting compression, three and four-point bending tests, the compressive and flexural performance of the two new re-entrant auxetic sandwich structures is studied and compared with the original (conventional) re-entrant structure. More specifically, Poisson’s ratio, compressive modulus, flexural stiffness and maximum loads of the structures are focused and studied. The mechanical properties of auxetic sandwich structures are improved using modified topologies. The new re-entrant auxetic sandwich structures show 11% higher normalized compressive modulus and 9% higher normalized flexural stiffness than the original re-entrant structure.

Study on interface toughening mechanism based on modified PBO fiber for CFRP/ plastic honeycomb sandwich structure

CFRP (carbon fiber reinforced composites)/honeycomb sandwich structures are widely used in aerospace, automotive, and high-speed rail industries due to their excellent mechanical properties and lightweight characteristics. This study investigates the impact of three designed toughened interfaces on the three-point bending performance of honeycomb structures. The results show that introducing a multiscale toughened interface made of poly(p-phenylene benzobisoxazole) (PBO) fibers grafted with polydopamine (PDA) and multi-walled carbon nanotubes (CNT) increases the peak load and post-peak load of the sandwich structure by 49.7 % and 51.9 %, respectively, and the absorbed energy by 92.4 %. This toughened interface adjusts the stress relationship between the panel, interface, and honeycomb, altering the deformation process and crack propagation mode of the sandwich structure. The micro-nano fibers form fiber bridging at the interface, changing the crack extension mode between the interfaces, transforming the delamination from a single debonding to the combined action of plate core debonding and honeycomb core buckling. Additionally, experimental results show that PBO fibers treated with PDA and CNT exhibit significantly improved surface roughness, Surface area per unit mass, wettability, and tensile strength, which delay and prevent crack propagation at the interface, effectively reducing panel debonding at the bonded joints.

Preparation and properties of biomimetic bone repair hydrogel with sandwich structure.

One of the critical factors that determines the biological properties of scaffolds is their structure. Due to the mechanical and structural discrepancies between the target bone and implants, the poor internal architecture design and difficulty in degradation of conventional bone implants may cause several adverse outcomes. To date, many scaffolds, such as 3-D printed sandwich structures, have been successfully developed for the repair of bone defects; however, the steps of these methods are complex and costly. Hydrogels have emerged as a unique scaffold material for repairing bone defects because of their good biocompatibility and excellent physicochemical properties. However, studies exploring bioinspired hydrogel scaffolds with hierarchical structures are scarce. More efforts are needed to incorporate bioinspired structures into hydrogel scaffolds to achieve optimal osteogenic properties. In this study, we developed a low-cost and easily available hydrogel matrix that mimicked the natural structure of the bone's porous sandwich to promote new bone growth and tissue integration. A comprehensive evaluation was conducted on the microstructure, swelling rate, and mechanical properties of this hydrogel. Furthermore, a 3D finite element analysis was employed to model the structure-property relationship. The results indicate that the sandwich-structured hydrogel is a promising scaffold material for bone injury repair, exhibiting enhanced compressive stress, elastic modulus, energy storage modulus, and superior force transmission.

Sound transmission loss and bending properties of sandwich structures based on triply periodic minimal surfaces

Four types of triply periodic minimal surfaces sandwich structures (TPMS-SS), namely, G-SS, D-SS, D1-SS, and IWP-SS, have been proposed. The bending properties were evaluated by experiments and numerical simulations, and their sound insulation performance was investigated using theoretical, numerical and experimental methods. Firstly, the bending performance of four TPMS-SS was compared. Then, the acoustic vibration control equation of TPMS-SS was established and the theoretical solution for the sound transmission loss (STL) was derived based on Reissner's theory combined with the fluid-solid coupling condition. Next, experimental study was conducted on the STL performance of TPMS-SS using the impedance tube method, and the numerical model was verified using theoretical and experimental results. Finally, parametric studies were carried out to investigate the effects of parameters such as relative density (RD), panel thickness (hf), and sound incidence elevation angle (φ), as well as the type of structure, on the STL performance. The results show that TPMS-SSs can achieve >30 dB of STL in the frequency range of 1 Hz to 10 kHz. The STL value increases with relative density (RD), panel thickness (hf), and sound incident elevation angle (φ), while the location of the dips in the STL curves is affected by relative density and panel thickness. G-SS has the best bending performance, while D1-SS has the best acoustic performance. The results could be used for the optimization design of lightweight sandwich structures.

Optimized Sandwich and Topological Structures for Enhanced Haptic Transparency.

Humans rely on multimodal perception to form representations of the world. This implies that environmental stimuli must remain consistent and predictable throughout their journey to our sensory organs. When it comes to vision, electromagnetic waves are minimally affected when passing through air or glass treated for chromatic aberrations. Similar conclusions can be drawn for hearing and acoustic waves. However, tools that propagate elastic waves to our cutaneous afferents tend to color tactual perception due to parasitic mechanical attributes such as resonances and inertia. These issues are often overlooked, despite their critical importance for haptic devices that aim to faithfully render or record tactile interactions. Here, we investigate how to optimize this mechanical transmission with sandwich structures made from rigid, lightweight carbon fiber sheets arranged around a 3D-printed lattice core. Through a comprehensive parametric evaluation, we demonstrate how this design paradigm provides superior haptic transparency, regardless of the lattice types. Drawing an analogy with topology optimization, our solution approaches a foreseeable technological limit. It offers a practical way to create high-fidelity haptic interfaces, opening new avenues for research on tool-mediated interactions.

Flexible Transparent Films of Oriented Silver Nanowires for a Stretchable Strain Sensor.

The potential applications of stretchable strain sensors in wearable electronics have garnered significant attention. However, developing susceptible stretchable strain sensors for practical applications still poses a considerable challenge. The present study introduces a stretchable strain sensor that utilizes silver nanowires (AgNWs) embedded into a polydimethylsiloxane (PDMS) substrate. The AgNWs have high flexibility and electrical conductivity. A stretchable AgNW/Pat-PDMS conductive film was prepared by arranging nanowires on the surface of PDMS using a simple rod coating method. Depending on the orientation angle, the overlap area between nanowires varies, resulting in different levels of separation under a given strain. Due to the separation of the nanowire and the change in current path geometry, the variation in strain resistance of the sensor can be primarily attributed to these factors. Therefore, precision in strain regulation can be adjusted by altering the angle θ (0°, 60°, or 90°) of the nanowire. At the same time, the stability of the AgNW/Pattern-PDMS (AgNW/Pat-PDMS) conductive film application was verified by preparing a sandwich structure PDMS/AgNW/Pat-PDMS stretchable strain sensor. The sensor exhibited high sensitivity within the operating sensing range (gauge factor (GF) of 15 within ~120% strain), superior durability (20,000 bending cycles and 5000 stretching cycles), and excellent response toward bending.

VO2-Based Spacecraft Smart Radiator with High Emissivity Tunability and Protective Layer.

In the extreme space environment, spacecraft endure dramatic temperature variations that can impair their functionality. A VO2-based smart radiator device (SRD) offers an effective solution by adaptively adjusting its radiative properties. However, current research on VO2-based thermochromic films mainly focuses on optimizing the emissivity tunability (Δε) of single-cycle sandwich structures. Although multi-cycle structures have shown increased Δε compared to single-cycle sandwich structures, there have been few systematic studies to find the optimal cycle structure. This paper theoretically discusses the influence of material properties and cyclic structure on SRD performance using Finite-Difference Time-Domain (FDTD) software, which is a rigorous and powerful tool for modeling nano-scale optical devices. An optimal structural model with maximum emissivity tunability is proposed. The BaF2 obtained through optimization is used as the dielectric material to further optimize the cyclic resonator. The results indicate that the tunability of emissivity can reach as high as 0.7917 when the BaF2/VO2 structure is arranged in three periods. Furthermore, to ensure a longer lifespan for SRD under harsh space conditions, the effects of HfO2 and TiO2 protective layers on the optical performance of composite films are investigated. The results show that when TiO2 is used as the protective layer with a thickness of 0.1 µm, the maximum emissivity tunability reaches 0.7932. Finally, electric field analysis is conducted to prove that the physical mechanism of the smart radiator device is the combination of stacked Fabry-Perot resonance and multiple solar reflections. This work not only validates the effectiveness of the proposed structure in enhancing spacecraft thermal control performance but also provides theoretical guidance for the design and optimization of SRDs for space applications.

Synthesis of hemostatic aerogel of TEMPO-oxidized cellulose nanofibers/collagen/chitosan and in vivo/vitro evaluation

The treatment of internal hemorrhage remains challenging due to the current limited antibacterial capability, hemostatic efficacy, and biocompatibility of hemostatic materials. The TEMPO-oxidized cellulose nanofibers/collagen/chitosan (TCNF/COL/CS) hemostatic aerogel was developed in this work by physically encasing COL in a sandwich structure and electrostatically self-assembling polyanionic TCNF with polycationic CS. In vitro coagulation experiments revealed the favorable procoagulant properties of TCNF/COL/CS along with high adhesion to erythrocytes and platelets. TCNF/COL/CS significantly increased the hemostatic efficacy by 59.8 % and decreased blood loss by 62.2 % in the liver injury model when compared to Surgicel®, the most frequently used hemostatic material. Furthermore, it demonstrated outstanding biodegradability both in vitro and in vivo, and a substantial increase in resistance (96.8 % against E. coli and 95.4 % against S. aureus) compared to TCNF. The significant hemostatic and biodegradable characteristics of TCNF/COL/CS can be ascribed to its interconnected porous structure, increased porosity, and efficient water absorption, along with the synergistic effect of the three constituents. The TCNF/COL/CS aerogel shows significant potential to control internal bleeding. A novel plant-derived nanocellulose composite aerogel has been described here for the first time; it has outstanding antibacterial characteristics, higher biocompatibility, and outstanding hemostatic characteristics in vivo.

Multi-functional hybrid sandwich composite structures: Evaluating damage mechanics and tolerance using compression after impacts

Building upon previous researches that introduced an innovative foam core hybrid-sandwich composite structure for radome applications, showcasing promising performance under low-velocity impacts (LVIs), this paper delves into the assessment of damage tolerance and mechanics through Compression After Impact (CAI) testing. The primary objective is to analyze disparities in damage tolerance, damage mechanisms, displacements, deformations, energy absorption, and residual strengths resulting from LVIs followed by CAIs on dissimilar materials on opposite faces of the hybrid structure. The extent of damage is evaluated through Computed Tomography (CT) scans. During LVIs, impacts on the S glass face sheets side demonstrated different energy dispersion and absorption mechanisms, leading to variations in indent damage depths and widths across all impact energy levels, unlike the impact damages observed from the Kevlar side. During CAI testing, this difference becomes more evident, with Kevlar specimens (KS3 & KS5) showing greater indentation depths and narrower widths, and S glass specimens (SK3 & SK5) experiencing buckling. These effects are due to the unique damage absorption and dispersion properties of Kevlar and S glass. These variations prompted an in-depth investigation of the structure using CAI to comprehend damage tolerance and mechanisms under compressive loads. This study, unparalleled in existing literature, proposes hybrid sandwich structures with superior specific impact and residual strength compared to various composite sandwich structures documented in published literature, expanding their utility beyond radomes.

Integrating parametric HFGMC and isogeometric RZT[formula omitted] for multiscale damage modeling of composite structures: A numerical and experimental study

In this research effort, a novel multiscale analysis scheme is proposed for damage modeling of composite laminates, sandwich structures, and stiffened plates relying on capabilities of the parametric HFGMC and isogeometric RZT{3,2} formulations. The Ramberg Osgood (RO) model is incorporated into the micromechanics model to reflect polymer matrix material nonlinearities on the overall homogenized composite behavior. Carbon fibers are assumed to behave in a linear transversely isotropic manner. The higher order RZT{3,2} theory employed at the macro level facilitates efficient applicability of the model to thick composite laminates and soft core sandwiches. On the other hand, it generates all three-dimensional stress components and thus ensures dimensional consistency between micro and macro levels. Numerical discretization and prediction of RZT{3,2} kinematic variables are enabled by performing NURBS based isogeometric analysis (IGA) thereby enhancing modeling efficacy to a significant degree. Soft core plasticity and failure in the composite are evaluated at the macro level through the RO model and Hashin criteria, respectively. Applicability of the method is presented for thin and thick flat composite and sandwich laminates; and further extended to stiffened plates via developing a multipatch formulation. A comprehensive validation of our analysis is conducted by comparing the results with established benchmarks from the literature, experimental data, and three-dimensional finite element method (3D-FEM) simulations. Initially, a moderately thick, simply supported square laminate under transverse loading is examined, a common verification benchmark. Then, results from standard mechanical tests, including tensile, shear, and four-point bending tests on thin laminates, followed by experiments on moderately thick sandwich structures subjected to four-point bending, are presented. Finally, the analysis is extended to a stiffened plate under uniform pressure, demonstrating the method’s accuracy and applicability across diverse structural configurations.

Modelling Low-Frequency Vibration and Defect Detection in Homogeneous Plate-Like Solids

The inspection of thick-section sandwich structures with skins around core materials such as honeycomb, balsa, and foam relies on low-frequency vibration techniques to identify defects through changes in amplitude or phase response. However, current industrial methods are often limited to detecting specific types of defects, potentially overlooking others. Moreover, these methods do not gather detailed information about the defect type or depth, as they only analyse a small portion of the available data instead of the full relevant response spectrum. This paper explores the scientific basis of using low-frequency vibration in the pitch-catch variant for defect detection in homogeneous solids, through analysis of the full relevant frequency spectrum (5–50 kHz). Defects in structures lead to reduced local stiffness and mass in the affected area, causing resonance in the layer above, resulting in amplified vibrations known as local defect resonance (LDR). In this work, an aluminium plate with a 40 mm diameter circular flat-bottomed hole (FBH) at a depth of 1 mm (representing a skin defect) is excited with a chirp signal of 5–50 kHz, and the response is monitored 17 mm away from the excitation point. Finite-element analysis (FEA) is used for the numerical model, addressing challenges in creating an accurate model. The process to optimise the numerical model and the reduce model-experiment error is outlined, including challenges such as the lack of knowledge of material damping. The study emphasizes the importance of modelling the probe’s stiffness and damping effects for achieving agreement between the model and experiment. After incorporating these effects, the maximum LDR frequency error decreased from approximately 3 kHz to less than 1 kHz. In addition, this study presents a method with the potential for defect classification through comparison to modelled responses. The minimum difference error was used to quantify the resonance frequencies’ error between the model and the experiment. Since the resonant frequencies are a function of the defect’s shape, size, and depth, a relatively low root mean squared (RMS) error across the resonance frequency error spectrum indicates the defect’s characteristics. Finally, defect detection and sizing using the pitch-catch probe are explored with a wide-band excitation signal and a line scan through the mid-plane of the defect. A method for defect sizing using a pitch-catch probe is presented and experimentally validated. Accurate defect sizing is achieved with the pitch-catch probe when the defect width is at least ≥\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ge $$\\end{document} twice the 17 mm pin-spacing of the probe.

A miniature temperature sensor based on upconversion luminescence for real-time GPU temperature monitoring

This paper introduces a miniaturized optical fiber temperature sensor based on Fluorescence Intensity Ratio (FIR) technology for real-time monitoring of graphics processing unit (GPU) temperature. Utilizing a femtosecond micromachining system, miniature rectangular holes are etched on a standard multimode fiber. These holes are then filled with a mixture of Er3+/Yb3+ co-doped TeO2-Na2CO3-ZnO powder and polydimethylsiloxane (PDMS) through capillary action, forming a sandwich structure. When illuminated by a 980 nm light source, upconversion (UC) fluorescence is generated, and a mathematical model correlating the fluorescence intensity ratio of two adjacent energy levels with temperature is established. The fundamental temperature sensing characteristics of the sensor are tested in a temperature-controlled chamber, with a maximum error of ± 1 K. The sensor is also applied to a GPU with real-time temperature variations, demonstrating a maximum error of 0.9 K and a response time of 1.96 s. The sensor not only possesses the advantages of a simple structure, micro size, and convenient fabrication but also exhibits immunity to electromagnetic interference, rapid response, good stability, and excellent repeatability in real-time monitoring of GPU temperature, showing potential for large-scale application.

Constructing polyimide-based nanocomposite dielectrics with superior high‐temperature energy storage performance via using ternary structure strategy

To maintain low leakage current and energy storage stability of polyimide at high temperature and electric field, ternary structure strategy is used to construct polyimide-based nanocomposite dielectrics, relating to cross-linked structure, sandwich structure and inorganic filler structure. Based on synergistic effect, energy storage properties of crosslinked polyimide-based sandwich‐structured nanocomposite films is significantly improved, in which cross-linked polyimide composited layers with 2 wt% hexagonal boron nitride nanosheets are selected as two outer insulating layers and cross-linked polyimide composited layer with 1 wt% barium titanate nanoparticles is employed as central polarization layer. When volume ratio of middle polarization layer is 10 % (denoted as N-10 T-N), the maximum Ud of 11.6 J/cm3 and η of 87 % are achieved at room temperature. At 150 °C, the Ud of N-10 T-N is 5.1 J/cm3. This study provides the experience for the development of polyimide dielectrics with high capacitive properties over a wide temperature range.

Dual-Modal Aptasensor for Sensitive Detection of Non-Small Cell Lung Cancer Exosomes Utilizing Two-Dimensional Nanopaper Co@g-C3N4@PB.

Nonsmall cell lung cancer (NSCLC), due to its lack of early symptoms, has become one of the leading causes of cancer-related deaths globally. Exosomes, small membrane vesicles secreted by cells, are widely present in human bodily fluids. In the bodily fluids of NSCLC patients, the quantity of extracellular vesicles is double that of healthy individuals, suggesting their potential as biomarkers for screening NSCLC. This study designed a dual-modal aptasensor that integrated excellent sensitivity in electrochemical detection and portability in fluorescence detection into one device. AuNPs were functionalized with exosome-capturing probes containing thiol-modified CD63 aptamers, which were immobilized on screen-printed gold electrodes. On the other hand, the carboxylated CD63 aptamer was immobilized on the surface of PB-modified g-C3N4 loaded with Co-SANs particles (Co@g-C3N4@PB). By combining these components, a sandwich structure (AuNPs/Apt1/Exo/Apt2- Co@g-C3N4@PB) was constructed, forming a probe for specific exosome recognition. First, the samples were preliminarily assessed for their positive or negative status under a fluorescence inverted microscope. Subsequently, a more in-depth quantitative analysis was conducted on suspected positive samples using electrochemical or fluorescence analysis methods. The detection limits for electrochemical analysis and fluorescence analysis were 66.68 and 33.5particles/mL, respectively. In the analysis of clinical serum exosome samples, the developed dual-modal aptasensor effectively distinguished serum specimens from those of NSCLC patients and healthy volunteers. This highlighted the inspection capability of the dual-modal adapter sensor, especially in point-of-care testing, making it a highly suitable tool for clinical applications.

Misalignment Assembly Effect on the Impact Mechanical Response of Tandem Nomex Honeycomb-Core Sandwich Structures.

To optimize the assembly methods of honeycomb structures and enhance their design flexibility, this study investigated the impact mechanical responses of tandem honeycomb-core sandwich structures with varying misalignment assembly lengths. Impact tests were conducted across different energy levels on single-layer and tandem honeycomb-core sandwiches to observe their impact processes and failure behaviors. Our findings indicate that tandem honeycomb cores significantly enhance the impact resistance compared with single-layer configurations, even though a misaligned assembly can deteriorate this property. A finite element model was developed and validated experimentally; the model showed good agreement with the experiments, thereby allowing the simulation and evaluation of the impact responses. Herein, we reveal that specific misalignment lengths can either increase or decrease the impact resistance, providing insights into improving the resilience of tandem honeycomb-core structures. Our results not only contribute to enhancing the impact resistance of honeycomb-core sandwich structures but also offer a valuable basis for their practical applications in engineering.

Synthesis and Mechanistic Investigation of an Amphiphilic Polymer in Enhancing Extra-Heavy Oil Recovery via Viscosity Reduction.

When applied to extra-heavy oil, conventional polymer surfactants exhibit poor efficacy in reducing viscosity and have limited adaptability. In this work, a novel amphiphilic polymer named PAADB was prepared by incorporating 2-acryloylamino-2-methyl-1-propanesulfonic acid (AMPS), benzyldimethyl [2-[(1-oxoallyl) zoxy] propyl] ammonium chloride (DML), and poly(ethylene glycol) methyl ether acrylate (BEM) into the main chain of acrylamide through free radical polymerization. PAADB exhibited outstanding interfacial activity, water-phase thickening ability, and emulsifying performance. The critical micelle concentration of PAADB was approximately 2500 mg/L, with a viscosity of 84.69 mPa·s at 50 °C. Additionally, interfacial tension experienced a notable decrease from 46.53 to 14.56 mN/m. At an optimal concentration of 4000 mg/L, PAADB reduced the viscosity of extra-heavy oil by over 92% across various temperatures and by more than 93% for different types of extra-heavy oil. PAADB demonstrated excellent emulsification ability and emulsion stability, effectively dispersing crude oil to create water-in-oil droplets measuring 35.33 μm in size. Meanwhile, molecular dynamics simulations further unveiled the viscosity reduction mechanism of PAADB. The hydrophilic groups within PAADB molecules are regularly distributed on the water interface, while the hydrophobic groups infiltrate the oil molecules to form a stable interfacial film. PAADB and asphaltene spontaneously form a sandwich structure, reducing intermolecular forces and disrupting the interlayer structure of asphaltene molecules. In general, this novel amphiphilic polymer demonstrates broad applicability and potential in extra-heavy oil recovery, providing valuable insights for the development of new heavy oil viscosity reducers (HOVRs).

Dynamic response of clamped sandwich beams with foam-filled sinusoidal corrugated core subjected to low-velocity impact

Sandwich structure is a lightweight structure with high specific strength and specific stiffness, as well as excellent energy absorption and load-carrying capacity. Sandwich structures are widely used in aerospace, high-speed rail and marine applications. Low-velocity impact of a sandwich beam with foam-filled sinusoidal corrugated core is investigated by means of theoretical derivations and numerical calculations. A yield criterion is used to obtain an analytical solution of the dynamic behavior of champed sandwich beams with foam-filled sinusoidal corrugated cores under low-velocity impact. Numerical calculations are performed. The analytical results are in excellent accordance with the finite element results. Influences of the amplitude, half-wavelength and thickness of the sinusoidal corrugated plate, foam strength, metal-sheet strength, and the impact location on the dynamic behavior of the sandwich beams with foam-filled sinusoidal corrugated cores under low-velocity impact are discussed. The present analytical model can predict reasonably well the dynamic behavior of foam-filled sinusoidal corrugated core sandwich beams under low-velocity impact.

Effect of Ni Addtion on Electroless Cu for High Density Interconnect PCB Substrate

Introduction Nowadays, high-density interconnect (HDI) printed circuit board (PCB) substrates have been widely applied to high-end electronics to meet the increasing input and output (I/O) density and small footprint area. Micro-via, a tiny structure buried among build-up layers that provide electrical interconnection is a key component in the HDI substrate. However, the preparation of micro-via is facing new challenges. It is not only required that the size of the micro-via scales down to meet the increased interconnect density, but also that the micro-via should withstand severe operating conditions in different scenarios. Therefore, it is of great importance to improve the quality of micro-vias during processing and manufacturing. The micro-via structure is a sandwich structure that is processed by electroless Cu plating and Cu electrolyte plating sequentially, and thus the quality of the electroless Cu layer can significantly affect the reliability of micro-via. In this work, we mainly focused on the electroless Cu plating process and investigated the effect of the electroless Cu deposition speed on the quality of the micro-via structure. SIM and HR-TEM characterizations were employed to investigate the micro-structure inside the electroless Cu layer. Materials and Method In this study, we used the common alkaline ion catalyst method to prepare the electroless Cu layer. The electroless Cu plating and electrolyte Cu plating was conducted on a PCB Resin Coated Copper (RCC) substrate that was cleaned by soft etching in an acid condition. The surface is then pre-dipped in an anionic solution, and palladium ions were adsorbed using an alkaline ion catalyst. The electroless Cu processes adopted in a reaction bath that consists of Cu resource (CuSO4·5H2O), complexing agent (KNaC4H4O6·4H2O) and reductant (HCHO). The whole reaction was conducted in the alkaline condition with a PH of 12.5, and the thickness of the electroless Cu deposition was controlled at around 0.20 µm. NiSO4 is selected as the Ni precursor to alter the Ni concentration in the reation bath. The contents of Ni addition were selected as 50 %, 100 %, 200 % and 300% of a conventional process at a bath temperature of 27 ºC. The thickness of the plated Cu layer was measured by a gravimetric method. Results and discussion TEM observation was used to identify the structure at the interface. Fig. 1 shows the TEM observation of the sample with 50 % and 200 % Ni concentrate. There are some nanovoids found at the interface between the Cu substrate and electroless Cu, which is arguably due to hydrogen generated during electroless Cu plating. The sample with a higher reaction speed has more nanovoids. It can be due to that the higher reaction speed can generate more hydrogen bubbles in the bath that can attach to the electroless Cu layer, leading to nanovoids as shown in Fig. 1. The samples prepared at different bath temperatures are under investigation to illustrate that the reaction speed can significantly affect the Cu electroless layer quality. Conclusions In this work, we investigated the Ni effect in electroless Cu on the RCC substrate. It is found that the higher Ni incorporation can lead to massive nanovoids generation and fine Cu grains in the electroless layer, which is a potential risk for the reliability of the HDI substrate. By reducing the Ni incorporation content, the number of nanovoids can be effectively reduced. Figure 1

Enhanced Mechanical Properties with Ag Addition in Electroplated Ni-Sn Solder Joints: Sub-10 Micron Bond Thickness

Keywords: Micro-bump; Microstructure; Intermetallic compounds; Mechanical properties; thermocompression bondingThe advancement of electronic devices has garnered more extensive attention towards three-dimensional integrated circuit (3D-IC) technology due to microbump size reduction and increased power density. Microbumps play an important role in driving the development of this technology, especially in 3D chip-stacking involving multiple reflow processes. The reduction in microbump height spotlights the presence of intermetallic compounds (IMCs). In comparison to traditional ball grid array (BGA) solder joints, microbumps with smaller solder volumes exhibit a predominant fraction of IMCs at solder joints, especially with microbump dimensions potentially shrinking to less than 10 micrometers.The interface reaction within confined spaces has become a critical concern. Mechanical properties will be dominated by IMC properties rather than solder characteristics. In conventional Cu/Sn/Cu solder joints, grain coarsening and prolonged reflowing times, which result in IMC collisions from opposing substrates, forming columnar grains with a uniform orientation, make the solder joint less reliable. Furthermore, the Cu3Sn layer in Cu/Sn/Cu microbumps presents another challenges, indicating the formation of Kirkendall voids, increasing the risk of brittle interface fractures.This study investigates the mechanical reliability and microstructure of two sub-10um sandwiched structures: Cu/Ni/Sn/Ni/Cu and Cu/Ni/Sn-3.5Ag/Ni/Cu. In the 3D-IC manufacturing process, solder joints will experience multiple reflow processing. Different reflow heat treatment process to achieve different fractions of Ni3Sn4 IMCs were performed on both sandwiched structure. Despite the formation of internal voids due to volume shrinkage in Ni/Sn solder joints, the addition of silver in the solder, forming Ag3Sn, effectively fills these voids. The long-time reflowed sample showed outstanding mechanical properties via the addition of silver in solder. In this study, the influence of silver addition on the microstructure and mechanical properties of the overall joints under identical reflow conditions, the fracture path observation after shear test were investigated. The growth mechanism of Ag3Sn and its distribution in the solder would be further explored and addressed. Figure 1

Experimental study of dual nano-network, high-temperature resistant aerogel material as an integration of thermal management functions

Thermal management system is highly desirable to guarantee the performance and thermal safety of lithium-ion batteries, but it reduces the energy density of battery modules and even is unable to provide highly effective protection. Here, a thermal management function integrated material is presented based on high-temperature resistant aerogel and phase change material and is applied at both charge–discharge process and thermal runaway condition. In this sandwich structure Paraffin@SiC nanowire/Aerogel sheet (denoted as PA@SAS) system, SiC nanowires endow the middle aerogel sheet (SAS) a dual nano-network structure. The enhanced mechanical properties of SAS were studied by compressive tests and dynamic mechanical analysis. Besides, the thermal conductivity of SAS at 600 °C is only 0.042 W/(m K). The surface phase change material layers facilitate temperature uniformity of batteries (surface temperature difference less than 1.82 °C) through latent heat. Moreover, a large-format battery module with four 58 Ah LiNi0.5Co0.2Mn0.3O2 LIBs was assembled. PA@SAS successfully prevents thermal runaway propagation, yielding a temperature gap of 602 °C through the 2 mm-thick cross section. PA@SAS also exhibits excellent performance in other safety issues such as temperature rise rate, flame heat flux, etc. The lightweight property and effective insulation performance achieves significant safety enhancement with mass and volume energy density reduction of only 0.79% and 5.4%, respectively. The originality of the present research stems from the micro and macro structure design of the proposed thermal management material and the combination of intrinsic advantages of every component. This work provides a reliable design of achieving the integration of thermal management functions into an aerogel composite and improves the thermal safety of lithium-ion batteries.