Electronic Packaging Research Articles

Thermal Property Estimation of Thin-Layered Structures by Means of Thermoreflectance Measurement and Network Identification by Deconvolution Algorithm

Abstract Laser-induced forward transfer (LIFT) is a powerful tool for micro and nanoscale digital printing of metals for electronic packaging. In the metal LIFT process, the donor thin metal film is propelled to the receiving substrate and deposited on it. Morphology of the deposited metal varies with the thermodynamic responses of the donor thin film during and after the laser heating. Thus, the thermophysical properties of the multilayered donor sample are important to predict the LIFT process accurately. Here, we investigated thermophysical properties of a 100 nm-thick gold coated on 0.5 mm-thick sapphire and silicon substrates by means of the nanosecond time-domain thermoreflectance (ns-TDTR) analyzed by the network identification by deconvolution (NID) algorithm, which does not require numerical simulation or analytical solution. The NID algorithm enabled us to extract the thermal time constants of the sample from the nanosecond thermal decay of the sample surface. Furthermore, the cumulative and differential structure functions allowed us to investigate the heat flow path, giving the interfacial thermal resistance and the thermal conductivity of the substrate. After calibration of the NID algorithm using the thermal conductivity of the sapphire, the thermal conductivity of the silicon was determined to be 107–151 W/(m K), which is in good agreement with the widely accepted range of 110–148 W/(m K). Our study shows the feasibility of the structure function obtained from the single-shot TDTR experiments for thermal property estimation in laser processing and electronics packaging applications.

Nanocrystalline copper for direct copper-to-copper bonding with improved cross-interface formation at low thermal budget

Direct copper-to-copper (Cu-Cu) bonding is a promising technology for advanced electronic packaging. Nanocrystalline (NC) Cu receives increasing attention due to its unique ability to promote grain growth across the bonding interface. However, achieving sufficient grain growth still requires a high thermal budget. This study explores how reducing grain size and controlling impurity concentration in NC Cu leads to substantial grain growth at low temperatures. The fabricated NC Cu has a uniform nanograin size of around 50 nm and a low impurity level of 300 ppm. To prevent ungrown NC and void formation caused by impurity aggregation, we propose a double-layer (DL) structure comprising a normal coarse-grained (CG) layer underneath the NC layer. The CG layer, with a grain size of 1 μm and an impurity level of 3 ppm, acts as a sink, facilitating impurity diffusion from the NC layer to the CG layer. Thanks to sufficient grain growth throughout the entire NC layer, cross-interface Cu-Cu bonding becomes possible under a low thermal budget, either at 100 °C for 60 min or at 200 °C for only 5 min.

Constructing bidirectional heat flow pathways by curved alumina for enhanced thermal conductivity of epoxy composites

The polymer-based composites filled with high thermal conductive fillers have received much attention in the filed of electronic package. However, traditional Al2O3 filled polymer-based composites always hardly simultaneously achieve high thermal conductivity in both horizontal and vertical directions with a low loading. Herein, curved Al2O3 particles have been fabricated via spray assembly and high-temperature sintering and subsequently are infiltrated by epoxy (EP) to obtain Al2O3/EP composites. The formation mechanism of the curved Al2O3 is analyzed in this work. The curved Al2O3 architectures are orderly stacked in EP matrix, which achieve significantly enhanced in-plane (1.21 W/m·K) and through-plane (1.46 W/m·K) thermal conductivity with a low filler loading (19.9 vol%). Importantly, the wall structure of curved Al2O3 particle consists of continuous network structure and interconnected channels, which reduce the thermal expansion coefficient of the composites. Consequently, the design of curved Al2O3 offers a new strategy for next-generation thermal management materials.

Controllable Self-Assembly of Carbon Nanotubes on Ammonium Polyphosphate as a Game-Changer for Flame Retardancy and Thermal Conductivity in Epoxy Resin.

The optimization of flame retardancy and thermal conductivity in epoxy resin (EP), utilized in critical applications such as mechanical components and electronics packaging, is a significant challenge. This study introduces a novel, ultrasound-assisted self-assembly technique to create a dual-functional filler consisting of carbon nanotubes and ammonium polyphosphate (CNTs@APP). This method, leveraging dynamic ligand interactions and strategic solvent selection, allows for precise control over the assembly and distribution of CNTs on APP surfaces, distinguishing it from conventional blending approaches. The integration of 7.5wt.% CNTs@APP10 into EP nanocomposites results in substantial improvements in flame retardancy, as evidenced by a limiting oxygen index (LOI) value of 31.8% and achievement of the UL-94V-0 rating. Additionally, critical fire hazard indicators, including total heat release (THR), total smoke release (TSR), and the peak intensity of CO yield (PCOY), are significantly reduced by 45.9% to 77.5%. This method also leads to a remarkable 3.6-fold increase in char yield, demonstrating its game-changing potential over traditional blending techniques. Moreover, despite minimal CNTs addition, thermal conductivity is notably enhanced, showing a 53% increase. This study introduces a novel approach in the development of multifunctional EP nanocomposites, offering potential for wide range of applications.

Assessing Electronics with Advanced 3D X-ray Imaging Techniques, Nanoscale Tomography, and Deep Learning

This paper presents advanced workflows that combine 3D X-ray microscopy (XRM), nanoscale tomography, and deep learning (DL) to generate a detailed visualization of the interior of electronic devices and assemblies to enable the study of internal components for failure analysis (FA). Newly developed techniques, such as the integration of DL-based algorithms for 3D image reconstruction to improve scan quality through increased contrast and denoising, are also discussed in this article. In addition, a DL-based tool called DeepScout is presented. DeepScout uses 3D XRM scans in targeted regions of interest as training data for upscaling high-resolution in a low-resolution dataset, of a wider field of view, using a neural network model. Ultimately, these workflows can be run independently or complementary to other multiscale correlative microscopy evaluations, e.g., electron microscopy, and they will provide valuable insights into the inner workings of electronic packages and integrated circuits at multiple length scales, from macroscopic features on electronic devices (i.e., hundreds of mm) to microscopic details in electronic components (in the tens of nm). Understanding advanced electronic systems through X-ray imaging and machine learning—perhaps complemented with some additional correlative microscopy investigations—can speed development time, increase cost efficiency, and simplify FA and quality inspection of printed circuit boards (PCBs) and electronic devices assembled with new emerging technologies.

Highly Anisotropic Thermally Conductive Dielectric Polymer/Boron Nitride Nanotube Composites for Directional Heat Dissipation.

An ideal dielectric material for microelectronic devices requires a combination of high anisotropic thermal conductivity and low dielectric constant (ɛ') and loss (tan δ). Polymer composites of boron nitride nanotubes (BNNTs), which offer excellent thermal and dielectric properties, show promise for developing these dielectric polymer composites. Herein, a simple method for fabricating polymer/BNNT composites with high directional thermal conductivity and excellent dielectric properties is presented. The nanocomposites with directionally aligned BNNTs are fabricated through melt-compounding and in situ fibrillation, followed by sintering the fibrous nanocomposites. The fabricated nanocomposites show a significant enhancement in thermal properties, with an in-plane thermal conductivity (K‖) of 1.8 Wm-1K-1-a 450% increase-yielding a high anisotropy ratio (K‖/K⊥) of 36, a 1700% improvement over isotropic samples containing only 7.2 vol% BNNT. These samples exhibit a 120% faster in-plane heat dissipation compared to the through-plane within 2 s. Additionally, they display low ɛ'of ≈3.2 and extremely low tan δ of ≈0.014 at 1kHz. These results indicate that this method provides a new avenue for designing and creating polymer composites with enhanced directional heat dissipation properties along with high K‖, suitable for thermal management applications in electronic packaging, thermal interface materials, and passive cooling systems.

Microstructure and mechanical properties of high-quality AlN/Cu joints brazed by a novel AgCuTi + AlCoCrFeNi2.1 composite brazing alloy

AlN/Cu combinations offer high power electronic devices superior compatibility with large current transport and high voltage resistance, making them promising heat-dissipation candidates for electronic packaging. AlCoCrFeNi2.1 high-entropy alloy (HEA), known for its excellent mechanical properties, was incorporated into the traditional AgCuTi filler for brazing AlN and Cu in this work. The obtained AlN/Cu brazed joints exhibited homogeneous interfacial phases and high bonding strength. The maximum shear strength (277 MPa) was achieved when brazed at 850 °C for 5 min via composite filler containing 0.5 wt% AlCoCrFeNi2.1, which was 156 % higher than that brazed with single AgCuTi fillers. Enhanced mechanical properties were obtained by dispersion strengthening of intermetallic compounds (IMCs) resulting from the introduction of a medium AlCoCrFeNi2.1 concentration (less than 1 wt%) in filler alloys under tailored brazing parameters.

Investigation of dynamic characteristics and fatigue life prediction of Pb free solder material under random vibration

Electronic packages are employed in diverse industries, including automotive, aerospace, and defense. However, their susceptibility to failure arises from exposure to uncontrolled operating conditions, particularly vibrations.Therefore, an investigation has been conducted to explore the effect of vibration in fatigue life of SAC305 lead free solder material employed in Printed Circuit Board (PCB) assembly with ball grid array (BGA) 144 electronic package. Finite Element Analysis (FEA) of a printed circuit board with BGA 144 electronic package was conducted to find their dynamic characterisics like natural frequencies and their mode shapes. Experiments were conducted to validate the numerically developed model, where first, second and third modes shapes from FEA and experimental results were compared. In addition, random vibration analysis was performed using numerical simulation where individual solder balls were analysed for stress distribution and experimentswere conducted on specially fabricated PCBs using electro dynamic shaker for validation. Analysis shows that the failure was prominent at the solder balls located at the corner of the package. Further, Minor’s rule was used to estimate the fatigue life of the Pb free solder material in BGA 144 package.

On-demand jetting of high-viscosity liquid by jet tube impact

The on-demand jetting of high-viscosity liquid has significant applications in fields such as electronic packaging and bioprinting. Conventional methods for high-viscosity liquid jetting often employ a needle propelling the liquid rapidly, which demands high precision in the manufacturing and assembly of the needle and nozzle, and can potentially damage biomaterials. In this study, a novel method utilizing jet tube impact for on-demand high-viscosity liquid jetting is proposed, leveraging the inherent inertia of the liquid to generate the pressure pulse necessary for on-demand jetting. This method reduces the precision requirements for the device, enables device simplification, and avoids harm to biomaterials. The feasibility of this approach for on-demand high-viscosity liquid jetting is validated through experiments, and by combining numerical simulations, the jetting mechanism is revealed and primary factors influencing jetting performance are investigated. It is found that the water hammer pressure wave induced by the liquid inertia during the sudden velocity change of the jet tube is the predominant driving force for jetting, and the peak pressure can exceed 1 MPa and the peak jet velocity can exceed 15 m/s. An increase in the jet tube impact velocity and an extension of the acceleration duration at the same impact velocity both lead to an increase in the pressure wave amplitude. In addition, a decrease in the liquid level height shortens the period of the pressure wave. These factors all have an influence on the jetting performance. This study provides a new insight and theoretical foundation for the on-demand high-viscosity liquid jetting.

Highly thermally conductive BNNS/PANF dielectric composite boards prepared by a facile compression moulding process

The structural design of an efficient heat conduction channel is critical for heat dissipation in electronic packaging material. The build of oriented boron nitride nanosheet (BNNS) network inside polymers has attracted much attention due to its excellent thermal conductivity and dielectric properties. However, to fabricate a BNNS/polymer composite, complex method and low thermal conductive resin for binding BNNS network are commonly involved, which severely restrict its facile production and enhancement in thermal conductivity. Herein, by utilizing para-aramid nanofibers (PANF) as assistance to BNNS to form thermal conduction network, we obtained high performance composite boards with good formability and adjustable sizes via a simple compression moulding of the BNNS/PANF aerogels. The composite boards exhibit a remarkable in-plane thermal conductivity of 10.62 W/m K−1 at 50 wt% BNNS loading, outstanding practical thermal management capability, excellent thermal stability, low dielectric constant and loss, showing great potential for electronic packaging applications. This facile construction strategy provides a feasible way for the large-scale production and application of high-performance thermal management materials in the future.

Bio-inspired fabrication of thin films via surface amidation of TEMPO-oxidized cellulose nanocrystals for improved UV-shielding properties

Ultraviolet (UV) radiation impacts various aspects of human beings, and there has been significant interest in membrane materials with UV shielding. However, conventional ultraviolet shielding materials are typically petroleum-based polymers like polyethylene, which pose challenges in terms of degradation and reusability, leading to environmental pollution and other issues. The development of environmentally friendly bio-based UV shielding film materials from wood fiber raw materials holds immense importance for sustainable development. In this study, TEMPO-oxidized cellulose nanocellulose crystals (TOCNC) were utilized as the starting material to obtain modified nanocellulose suspension through amination with aniline and acetamide. Subsequently, nanocellulose films were prepared and characterized for their morphology and structure analysis. With the presence of carboxyl groups on TOCNC along with the incorporation of functional groups such as benzene rings, the absorption rate of the TOCNC composite film reaches 93.2 % across the entire ultraviolet absorption spectrum, significantly enhancing its UV shielding performance. The proposed film exhibits great potential for applications in UV-sensitive food packaging, electronic product packaging materials, plastic films, etc.

Composition design and property investigation of bismaleimide by branched crosslinking structure with low dielectric permittivity and high toughness

AbstractDue to the high‐power environments of electronic components, achieving the exceptional dielectric properties and mechanical behavior necessary for electronic packaging materials presents a significant challenge. In this study, a trifunctional maleimide (HTMI) was synthesized by reacting hexamethylene diisocyanate trimer (HDI trimer) with Maleic anhydride (MA), followed by the preparation of Bismaleimide (BMI) resin featuring a micro‐branching structure through its reaction with diallyl bisphenol A (DBA) ether and BMI. The intentionally designed micro‐branching structure resulted in an increase in the free volume within BMI, leading to an 8.8% reduction in the dielectric constant. Additionally, this micro‐branching architecture imparted superior mechanical properties to the BMI resin, as demonstrated by a 140% increase in bending strength and a 149% increase in impact strength of the cured product.

Microstructure, joining mechanism, mechanical properties and optimization of inactive solder (Sn9Zn) in low temperature ultrasonic soldering of AlN ceramics and 2024Al

With the development of the electronic packaging industry, there is an increased demand for enhanced strength in DBA substrates. Ultrasonic soldering technology allows low-temperature joining of ceramic-metal materials under ambient air conditions. In this paper, we successfully achieved the joining between AlN ceramics and 2024Al using Sn9Zn solder and ultrasonic soldering technique, while also analyzing the joining mechanism through Comsol software simulations. It was observed that under the appropriate ultrasonic amplitude and frequency, the Sn9Zn solder exhibited complete wetting with both base materials, resulting in reliable joints. Building upon this finding, the conventional Sn9Zn solder was further optimized by adding Sb element. The addition of Sb element led to the formation of Zn4Sb3 phase and β-Sn(Sb) phase with Sb acting as solute. Among them, the generation of β-Sn(Sb) solid solution phase effectively improved the joint strength. The maximum joint strength was achieved at an average value of 75.066 MPa when the Sb content reached 1 wt. %. At this point, the failure occurred within the AlN ceramic base material.

Investigation on improving soldering reliability of large-scale LCCC devices with high-lead solder ball materials

Abstract Leadless Ceramic Chip Carrier (LCCC) is commonly used in high-density electronic packaging PCB boards. Due to the different thermal expansion of materials, the solder joints of LCCC devices are prone to stress and cracking after assembly, which leads to the failure of LCCC devices’ electrical performance. In this paper, the advantages and disadvantages of four solutions were analysed, and comprehensively considered the use of ball planting to alleviate thermal mismatch stress. After experimental verification, the thermal mismatch between the LCCC devices and PCB results in a lifespan of less than 200 cycles for direct assembly of the LCCC devices. Planting balls at the bottom of LCCC devices and soldering can effectively increase the lifespan of solder joints and improve product reliability. By analysing the SMT process, the bottom high-lead ball placement of LCCC devices was implemented, and environmental testing was conducted using temperature shock testing to demonstrate that bottom ball placement can extend solder joint life and improve reliability. This study provides a reliable soldering method for large-scale LCCC devices and provides a basis for improving LCCC reliability.

Toward flexible electronics: A novel polyurethane integrating self‐healing, UV‐protective, reprocessable, and degradable properties

AbstractFlexible electronics are striving in modern society, and they impose harsh and urgent requirements for flexibility on electronic package substrates. However, traditional materials, including ceramics, metals, or polymers are lack of flexibility. Herein, a polyurethane named PU‐D1Q1VF1 is proposed via incorporating carefully selected biobased units and synergistic dynamic bonds, and the PU‐D1Q1VF1 not only meets the basic requirements of flexibility but also possesses properties of self‐heal, UV‐protection, reprocessability, and degradability. The polycaprolactone diol (PCL diol) was employed as the soft segment, and the bis(2‐hydroxyethyl) disulfide (HEDS) and lignin derived model monomer hydroquinone were selected as chain extenders. Moreover, carefully synthesized bio‐based monomer (E)‐4‐(((furan‐2‐ylmethyl)imino)methyl)‐2‐methoxyphenol (VF) was used as the capping agent, which could facilitate the self‐healing process of the PU‐D1Q1VF1.

Microstructural and functional characterization of sintered fly ash

The utilization of industry by-products, such as fly ash, was investigated. However, the research results have yet to report the functional characteristics of fly ash. This paper deals with the processing, microstructure, and functional characterization of fly ash using a novel powder metallurgy method. The elemental composition, mineralogical phases, morphology phases, electrical and dielectric properties were determined by XRF, XRD, SEM, and impendence analyzer, respectively. The effect of additives on fly ash’s microstructure and functional properties was studied. As a result, sintered fly ash possesses better microstructure and dielectric properties as a function of frequency and temperature. It is suggested that sintered fly ash can be utilized for electronic packaging and EMI shielding applications.

Synthesis of phosphorus fluorine containing polymers for flame-retardant, low surface energy and good dielectric performance epoxy resin electronic packaging materials

In order to satisfy the application of epoxy resin (EP) in high-end electronic packaging, phosphorus fluorine containing polymers were synthesized to simultaneously enhance the flame retardant, dielectric, and hydrophobic properties of EP. Through the reaction of 2-hydroxyethyl methacrylate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), a monomer named HEMA-P was synthesized. By copolymerizing (2,2,2-trifluoroethyl methacrylate and 1H, 1H, 7H-dodecafluoroheptyl methacrylate) with HEMA-P through the typical reversible addition-fragmentation chain transfer (RAFT) polymerization, two phosphorus fluorine containing polymer modifiers named 3FOP and 12FOP with different lengths of fluorine side chain were obtained. The flame-retardant level of FOP/EP composites reached V-1 level. The limited oxygen index (LOI) values of 3FOP/EP and 12FOP/EP composites (10 wt% additive amount) increased to 33.6 ± 0.5 % and 32.8 ± 0.3 %, respectively, with corresponding peak heat release rates (pHRR) decreased by 37.8 % and 33.2 % compared with neat EP, suggesting the FOP modified EP composites have good flame retardant and smoke suppression effects. The mechanical properties of the FOP/EP composites can also be improved. Notably, with the 10 wt% addition of 12FOPs, the water contact angle of 12FOP/EP composite reached 116.5°, which was 40.7 % higher than that of EP. Moreover, the dielectric constant decreased with the increasing fluorine content. The thermal stability and possible thermal degradation pathways of 3FOP and 12FOP were characterized by TG-IR, Py-GC/MS, and Raman spectroscopy. Based on experimental results, we innovatively revealed a phosphorus fluorine synergistic flame retardancy mechanism.

The Influence of a Commercial Few-Layer Graphene on Electrical Conductivity, Mechanical Reinforcement and Photodegradation Resistance of Polyolefin Blends

This work demonstrates the potentials of a commercially available few-layer graphene (FLG) in enhancing the electro-dissipative properties, mechanical strength, and UV protection of polyolefin blend composites; interesting features of electronic packaging materials. Polyethylene (PE)/ polypropylene (PP)/ FLG blend composites were prepared following two steps. Firstly, different concentrations of FLG were mixed with either the PE or PP phases. Subsequently, in the second step, this pre-mixture was melt-blended with the other phase of the blend. FLG-filled composites were characterized in terms of electrical conductivity, morphological evolution upon shear-induced deformation, mechanical properties, and UV stability of polyolefin blend composites. Premixing of FLG with the PP phase has been observed to be a better mixing strategy to attain higher electrical conductivity in PE/PP/FLG blend composite. This observation is attributed to the influential effect of FLG migration from a thermodynamically less favourable PP phase to a favourable PE phase via the PE/PP interface. Interestingly, the addition of 4 wt.% (~2 vol.%) and 5 wt.% (~2.5 vol.%) of FLG increased an electrical conductivity of ~10 orders of magnitude in PE/PP—60/40 (1.87 × 10−5 S/cm) and PE/PP—20/80 (1.25 × 10−5 S/cm) blends, respectively. Furthermore, shear-induced deformation did not alter the electrical conductivity of the FLG-filled composite, indicating that the conductive FLG network within the composite is resilient to such deformation. In addition, 1 wt.% FLG was observed to be sufficient to retain the original mechanical properties in UV-exposed polyolefin composites. FLG exhibited pronounced UV stabilizing effects, particularly in PE-rich blends, mitigating surface cracking and preserving ductility.

Thermal characterization methodology for thin bond-line interfaces with high conductive materials

Silver sintering offers a promising landscape for Pb-free die attachment in electronics packaging. However, the sintered interface properties are highly process-dependent and deviate from bulk silver properties. Conventional measurement methods do not adequately capture the die-attach application geometry. Hence, this study introduces a novel methodology for characterizing thin bond-line interfaces with high-conductive materials. The transient heat flux impedance ΔZth(t, Δx) was measured between two thermally sensitive devices interconnected using pressureless Ag-sintering material. A correction factor was derived, based on thermal half-space principles, to account for non-uniform heat spreading over the die-attach interface. Experimental findings estimate an effective conductivity of ∼115W/mk for the pressureless Ag-sintered interface. The measurement results were validated by measuring a SAC305 soldered interface, which exhibited ∼55 W/mK, and a non-conductive epoxy interface of ∼2.5 W/mK. Voids on the die-attach layer, resulting from material processing, were identified to influence the interface thermal behavior. An uncertainty analysis was further discussed, emphasizing equipment tolerances, measurement sensitivity, and geometrical and thermal anisotropies. The article concludes with a comparative summary of the proposed methodology against conventional methods, highlighting differences in working principle, thickness range, measurement parameters, and their advantages and limitations.

In-situ compression tests and analysis of strength-ductility synergy in heterogeneous structured copper-tin composite joint

Copper-tin (Cu–Sn) serves as a prevalent alloy system in electronic packaging, envisioned for future high-power device applications. However, interdiffusion in Cu–Sn yields rigid and brittle Cu6Sn5 and Cu3Sn phases, causing joint hardening and embrittlement. This paper introduces a novel, non-homogeneous Cu–Sn composite employing heterostructure design. Through room temperature in-situ compression tests and molecular dynamics (MD) simulations, we investigate mechanical property degradation origins and Cu–Sn composite strength-ductility mechanisms. Our findings reveal hierarchical heterostructures in joints stimulate dynamic stress distribution at interfaces, generating a high density of geometrically necessary dislocations (GNDs). This enhances the material's work-hardening rate and imparts robust strain-hardening capabilities. This unique structural feature has altered the interaction between dislocations and interfaces, thereby effectively promoting the formation and driving force of dislocations. The combined effects of strain inhomogeneity induced by soft and hard phases, along with a hybrid mechanism involving stacking faults (SFs), Lomer-Cottrell (L-C) locking, and dislocation bypassing and shearing, realize exceptional strength-ductility synergies in Cu–Sn composite joints. This study presents a promising paradigm for designing microstructures in high-strength, deformable, and high-temperature-resistant soldered composites.