Flexible Electronic Packaging Research Articles

Multi‐Interfacial Heterostructure Design of Carbon Fiber/Silicone Rubber‐Oriented Composites for Microwave Absorption and Thermal Management

AbstractIn order to ensure the operation and longevity of electronic devices, the design of multifunctional composites integrating microwave absorption (MA) and thermal conduction has become the key to solving the problem. However, the superior MA properties and thermal conductivity are usually incompatible in the blend system. In this study, a modified carbon fiber/silicone rubber‐oriented structure is designed based on the multiscale design concept of heterostructures with multiple interfaces. The forward deposition‐reverse growth mechanism is utilized at the microscopic level to construct multi‐interfacial heterogeneous structures on the surface of 1D carbon fibers (CFs). The magneto‐electric coupling network of the multi‐interfacial heterostructure induces interfacial polarization and enhances MA properties and heat transfer. Subsequently, the structural design of the modified CFs is carried out on a macroscopic scale using the ice template method. The directionally aligned modified CFs/silicone rubber aerogel composites are obtained by backfilling with silicone rubber (SR). The samples achieved an effective absorption bandwidth of 4.41 GHz and a maximum reflection loss of −42.29 dB with ultra‐thin thickness (1.3 mm). The thermal conductivity of the sample is improved to 200% compared to pure silicone rubber. The composites with directional alignment have promising applications in lightweight and flexible electronic packaging.

Self-cleaning and fully polymer-based super-moisture-resistant gas barrier coating films with 2D polymers for flexible electronic devices and packaging applications

Through alternate stacking of spray-coated 2DP and spin-coated PDMS, bi-layers (BL)n assemblies have been designed. The WVTR value was improved from 1.4 g m−2 day−1 for PET to the optimum value of 9 × 10−4 g m−2 day−1 for PET/(BL)4.

Transparent, Stretchable, and Self‐Healable Gas Barrier Films with 2D Nanoplatelets for Flexible Electronic Device Packaging Applications

AbstractSelf‐healable elastomeric materials (PUAxPU10–x‐PDMS, x = 4, 6, or 8) have been synthesized and utilized as substrates or encapsulation films on the top of barrier films (deposited on polyethylene terephthalate (PET) substrates) to improve their gas barrier properties for packaging applications. Hexa‐layer (HL) assembly consisting of two inorganic 2D nanoplatelets, including Mg‐Al layered double hydroxide (LDH) and hexagonal boron nitride (hBN), and two oppositely charged polyelectrolytes, that is, polyethyleneimine (PEI) and poly(sodium styrene‐4‐sulfonate) (PSS), have been developed by alternative depositions of HL (i.e., PSS/LDH/PSS/PEI/hBN/PEI) sequentially. Then, a layer‐by‐layer technique is applied to produce multiple assemblies of hexa‐layer (HL)n and reduce water vapor transmission rate (WVTR) values of elastomeric PUA8PU2‐PDMS and PET substrates. The optimum WVTR values of 7.1 × 10−3 and 8.0 × 10−3 g m−2 day−1 are achieved using barrier PET/(HL)8/PUA8PU2‐PDMS (125/3/1 µm) and PUA8PU2‐PDMS/(HL)8 (125/3 µm) films, respectively. The self‐healable barrier PUA8PU2‐PDMS and PUA8PU2‐PDMS/(HL)8 films recover their initial WVTR values after self‐healing of mechanically damaged films. Organic light‐emitting diodes (OLEDs) encapsulated with PET/(HL)8/PUA8PU2‐PDMS and PUA8PU2‐PDMS/(HL)8 have 9–10 times longer half‐lifetime values than bare OLEDs. Accordingly, this report develops transparent, stretchable, and self‐healable gas barrier films for next‐generation flexible electronic device packaging applications.

Design and synthesis of novel phenolphthalein-based poly (aryl ether) resin containing isopropyl groups

With the rapid development of 5G technology, low dielectric constant materials present a widespread application prospect in microelectronic devices. Poly (aryl ether) resins have been widely used in this field due to their excellent thermal stability, mechanical properties and electrical insulation properties, however, reducing its dielectric constant remains a challenge. In this study, novel phenolphthalein poly (aryl ether ketone) containing isopropyl groups and fluorene groups was successfully synthesized with high molecular weight via SN2 aromatic nucleophilic polycondensation reaction. At lower frequencies ranging from 10 Hz to 1 MHz and higher frequencies from 10 GHz to 42 GHz, the films exhibit the intrinsic dielectric constant in the range of 2.4–2.7. The resin has good thermal stability, mechanical properties and insulation properties. Thus, the functional poly (aryl ether ketone) possesses attractive potential applications in the field of high-performance flexible electronic packaging materials.

Novel Rubber Composites Based on Copper Particles, Multi-Wall Carbon Nanotubes and Their Hybrid for Stretchable Devices.

New technologies are constantly addressed in the scientific community for updating novel stretchable devices, such as flexible electronics, electronic packaging, and piezo-electric energy-harvesting devices. The device promoted in the present work was found to generate promising ~6V and durability of >0.4 million cycles. This stretchable device was based on rubber composites. These rubber composites were developed by solution mixing of room temperature silicone rubber (RTV-SR) and nanofiller, such as multi-wall carbon nanotube (MWCNT) and micron-sized copper particles and their hybrid. The hybrid composite consists of 50:50 of both fillers. The mechanical stretchability and compressive modulus of the composites were studied in detail. For example, the compressive modulus was 1.82 MPa (virgin) and increased at 3 per hundred parts of rubber (phr) to 3.75 MPa (MWCNT), 2.2 MPa (copper particles) and 2.75 MPa (hybrid). Similarly, the stretching ability for the composites used in fabricating devices was 148% (virgin) and changes at 3 phr to 144% (MWCNT), 230% (copper particles) and 199% (hybrid). Hence, the hybrid composite was found suitable with optimum stiffness and robust stretching ability to be useful for stretching electronic devices explored in this work. These improved properties were tested for a real-time stretchable device, such as a piezoelectric energy-harvesting device and their improved voltage output and durability were reported. In the end, a series of experiments conducted were summarized and a discussion on the best candidate with higher properties useful for prospective applications was reported.

Constructing a BNNS/aramid nanofiber composite paper via thiol-ene click chemistry for improved thermal conductivity

Benefiting from its superior insulation and outstanding environmental adaptability, aramid nanofibers (ANFs) composite paper has attracted tremendous attention in the flexible electronics and electronic packaging field. However, the extremely low thermal conductivity of aramid nanofiber paper faced a serious challenge under the rapidly increasing heat dissipation demand. Herein, boron nitride nanosheets (BNNS) were introduced to ANFs paper via a controllable vacuum-assisted filtration, and hydrogen bonding and light-induced “thiol-ene” click chemistry were proposed to improve the interfacial combination. Specifically, BNNS was prepared via the microwave-assisted hydrothermal method with subsequent sulfhydryl functionalization (f-BNNS) and s-ANFs (grafted ANFs) were prepared by deprotonation of chopped aramid fibers and then treated with KH570 coupling agent. The results showed that the through-plane thermal conductivity (TC) of f-BNNS/s-ANFs composite paper reached 0.224 W/m K with 50 wt% f-BNNS loading, exhibiting an increase of 540% and 184%, respectively, compared with pure ANFs paper and BNNS/ANFs paper. The abundant hydrogen bonding and covalent bonding via “thiol-ene” click chemistry between f-BNNS and s-ANFs contributed to enhanced interface and interlayer bonding, and the continuous interconnected thermal conductive network constructed by f-BNNS was conducive to improving TC. Meanwhile, the composite paper held excellent mechanical properties and reliable electrical insulation.

Synthesis and properties of low dielectric constant phenolphthalein-based poly (aryl ether) resin containing isopropyl groups

In this study, a novel phenolphthalein poly (ether nitrile) containing isopropyl groups (iPrPENC) was successfully synthesized via SN2 aromatic nucleophilic polycondensation. The flexible and tough iPrPENC films exhibited an intrinsic low dielectric constant value of 2.86 at the frequency of 1 MHz. The resin has good thermal stability and mechanical property. In addition, it can be dissolved in common organic solvents. Thus, the functional poly (aryl ether nitrile) possessed attractive potential applications in the field of high-performance flexible electronic packaging materials.

A transparent PEDOT:PSS/PVA-co-PE/epoxy thermoelectric composite device with excellent flexibility and environmental stability

With the rapid development of thermoelectric (TE) generators, the TE device that is transparent, flexible, and can be used for a long time under high humidity and low/high temperature has aroused great interests. In this study, a new hydrophilic substrate of poly (vinyl alcohol-co-ethylene) (PVA-co-PE) nanofiber membrane was used to support the poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and then the PEDOT:PSS/PVA-co-PE composite was encapsulated by epoxy. Due to the size effect of nanofibers, the PVA-co-PE/epoxy composite area shows a good transmittance of 72% at a wavelength of 600 nm, while the area containing the PEDOT:PSS remains black. This kind of transparent packaging structure is more conducive to the application of TE device. Moreover, the special interpenetrating structure and hydrogen bonding between the three phases of PEDOT:PSS, PVA-co-PE NM and epoxy resin greatly enhance the interfacial adhesion of PEDOT:PSS/PVA-co-PE/epoxy composite. The morphology, mechanical properties, transmittance, TE performance, bending resistance and environmental stability of the transparent composite TE device were analyzed. The new composite structure proposed in this research gives the PEDOT:PSS/PVA-co-PE/epoxy TE device outstanding flexibility and environmental stability. The TE performance remains stable after 2000 cycles repeated bending and after placing one end of the TE device in hot steam and ice water. All the results demonstrate the potential application of the transparent composite TE device in the field of flexible electronic device packaging and applications.

A comprehensive study on the effective thermal conductivity of random hybrid polymer composites

Polymer composites have a wide range of applications in frontier science and technology, such as flexible electronic packaging and thermally actuated soft robotics. However, most of the theoretical and numerical researches on the thermal properties of polymer composites have been focusing on the composites with homogenous mono-fillers. In this paper, a comprehensive study on the effective thermal conductivity of the composites containing both particulate and fibrous fillers is performed through experimental, numerical and theoretical approaches. Firstly, the PDMS composites with aluminum nitride particles and/or multiwalled carbon nanotubes are prepared. Then, a series of random digital models are constructed by the simulated annealing method and monte carlo method to characterize the microstructure of such composites. And then, the lattice Boltzmann method is used to calculate the effective thermal conductivity of the random microstructure. The present numerical results are fully verified by experiments. Finally, a phenomenological prediction model is developed by the nonlinear regression method based on the systematic numerical simulations. To show the versatility of the present model, an interesting experiment on thermal management materials is designed and performed. By virtue of this model, quantitatively controlling the temperature of electronic devices may be fulfilled.

6.1: Invited Paper: Flexible Electronics Packaging for Wearable Applications

6.1: <i>Invited Paper:</i> Flexible Electronics Packaging for Wearable Applications

High-barrier polyimide containing fluorenol moiety: Gas barrier properties and molecular simulations

An intrinsic high-barrier polyimide (9-OH-FPPI) was prepared through the polycondensation reaction of pyromellitic dianhydride (PMDA) with a novel diamine (9-OH-FPDA) containing fluorenol moiety. The synthesized polyimide displays superior barrier performances, possessing water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) low to 1.64 g·m−2·day−1 and 0.66 cm3·m−2·day−1, respectively. Meanwhile, 9-OH-FPPI also demonstrates high thermal stability with a glass transition temperature (Tg) of 410 °C, 5% weight-loss temperature (Td5%) of 577 °C under N2 and coefficient of thermal expansion (CTE) of 5.65 ppm/K. A detailed molecular simulation study as well as positron annihilation lifetime spectroscopy (PALS) and wide angle X-ray diffraction (WAXD) analysis have been carried out to reveal the barrier mechanism. The results reveal that the introduction of fluorenol moiety not only enhances the rigidity and regularity of polymer backbone, but also improves the interchain cohesion of PI matrix, which in turn lead to high crystallinity, low free volume and poor chains mobility of 9-OH-FPPI. The high crystallinity and low free volume decrease the diffusion and solubility of gases in polyimide matrix. In addition, the poor chains mobility further suppresses the gases diffusion. Simultaneous reductions of gases diffusion and solubility promote the barrier properties of 9-OH-FPPI. The as-synthesized polyimide exhibits wide application prospects in the flexible electronic packaging industry.

Transparent Flexible Ultra‐Low Permeability Encapsulation Film: Fusible Glass Fired on Heat‐Resistant Polyimide Membrane

AbstractA good packing of flexible electronics is one of the most critical points to ensure its lifetime. However, efficient and straightforward packaging technology is still very challenging. In this work, a new strategy for preparing encapsulation film is reported by “enameling” a fusible glass layer on a polymer membrane. Careful consideration has been given to the materials properties and manufacturing process, and the obtained glass‐coated polymer can combine the advantageous properties of polymers and inorganic materials unconventionally. The encapsulation films show exceptional overall properties, including transparency (&lt;5% optical deterioration), flexibility (radius of curvature: 1.5 mm), and low permeability. The film with 12 µm glass layer exhibits a water vapor transmission rate of 5.7 × 10−4 g m−2 day−1, and an oxygen transmission rate of 1.46 × 10−4 g m−2 day−1, which is 5 and 3 orders of magnitude lower than that of the polymer substrate, respectively. The fabrication process is suitable for roll‐to‐roll manufacturing, showing great advantages over the existing deposition methods like plasma‐enhanced chemical vapor deposition and atomic layer deposition in terms of reducing manufacturing costs and simplifying the process. Therefore, it is a significant contribution to the development of flexible electronics packaging technology.

High-Barrier Polyimide Containing Carbazole Moiety: Synthesis, Gas Barrier Properties, and Molecular Simulations.

A high-barrier polyimide (2,7-CPI) was synthesized through the polymerization of pyromellitic dianhydride (PMDA) and a novel diamine (2,7-CDA) containing carbazole moiety. The synthesized diamine and polyimide were fully characterized by elemental analyses, FTIR and NMR. The 2,7-CPI displays very attractive barrier performances, with oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) low to 0.14 cm3·m−2·day−1 and 0.05 g·m−2·day−1, respectively. Meanwhile, 2,7-CPI also exhibits exceptional thermal stability with a glass transition temperature (Tg) of 467 °C, 5% weight-loss temperature (Td5%) of 550 °C under N2 and coefficient of thermal expansion (CTE) of 3.4 ppm/K. The barrier performances of 2,7-CPI are compared with those of a structural analogue (2,7-CPPI) and a typical polyimide (Kapton). Their barrier performances with respect to microstructure were investigated by molecular simulations, wide angle X-ray diffraction (WAXD), and positron annihilation lifetime spectroscopy (PALS). The results show that 2,7-CPI possesses better coplanar structure and more number of intermolecular hydrogen bonds among the three PIs, which result in tight chain packing and thereby high crystallinity, low free volume, and decreased chains mobility. That is, the high crystallinity and low free volume of 2,7-CPI reduce the diffusion and solubility of gases. Meanwhile, the poor chains mobility further decreases the gases diffusion. The reduced diffusion and solubility of gases consequently promote the improvement of barrier properties for 2,7-CPI. The polyimide has a wide application prospect in the field of flexible electronic packaging industries.

Finite element analysis (FEA) modelling and experimental verification to optimise flexible electronic packaging for e-textiles

In this paper a three-dimensional model of a novel electronic package has been developed using Finite element analysis to evaluate the shear load, tensile, bending and thermal stresses. Simulations of a complete flexible flip chip electronic packaging method are performed to minimize stresses on the packaged electronic device to improve robustness and reliability. Three component under-fill adhesives (Loctite 4860, Loctite 480 and Loctite 4902) and three circuit substrate materials (Kapton, Mylar and PEEK) are compared and the optimal thickness of each is found by shear load, tensile load, bending test and thermal expansion simulations. A fixed die size of 3.5 mm × 8 mm × 0.53 mm has been simulated and evaluated experimentally under shear and bending load. The shear and bending experimental results show good agreement with the simulation results and verify the simulated optimal thickness of the adhesive layer. The Kapton substrate together with the Loctite 4902 adhesive were identified as the optimum in the simulation. The simulation of under-fill adhesive and substrate thickness identified an optimum configuration of a 0.045–0.052 mm thick substrate layer and a 0.042–0.045 mm thickness of the Loctite 4902 adhesive. The bending simulation has also been used to determine the neutral axis of the encapsulated electronic package in this paper, thus identifying the optimal material and thickness for the encapsulation layer of the package.

PVP-Mediated Galvanic Replacement Synthesis of Smart Elliptic Cu–Ag Nanoflakes for Electrically Conductive Pastes

Elliptic Cu-Ag nanoflakes were syntheszied via facile in situ galvanic replacement between prepared Cu particles and Ag ions. Alloy nanoflakes with high purity and uniformity present a size of 700 × 500 nm, with a thinness of 30 nm. Nontoxic and low-cost polyvinyl pyrrolidone was used as a dispersant and structure-directing agent, promoting the formation of the remarkable structure. Synthesized nanoflakes were utilized as a filler for conductive paste in an epoxy resin matrix. Conductive patterns on flexible substrates with a resistivity of 3.75 × 10-5 Ω·cm could be achieved after curing at 150 °C for 2 h. Compared with traditional silver microflakes, smart alloy nanoflakes provide much improved conductive interconnection, whose advantage could be attributed to their nanoscale thicknesses. It is also noteworthy that the conductive patterns are able to tolerate multiple bendings at different angles, having good conductivity even after 200 repeated bendings. Therefore, alloy nanoflakes could be a promising candidate conductive filler for flexible printing electronics, electronic packaging, and other conductive applications.

Nano-Brick Wall Architectures Account for Super Oxygen Barrier PET Film by Quadlayer Assembly of Polyelectrolytes and α-ZrP Nanoplatelets.

Nanobrick wall hybrid coating with super oxygen barrier properties were fabricated on polyethylene terephthalate (PET) film using a quadlayer (QL) assembly of polyelectrolytes and nanoplateles. A quadlayer assembly consists of three repeat units of polyacrylic acid (PAA), poly (dimethyl diallyl ammonium chloride) (PDDA) and layered α-zirconium phosphate (α-ZrP). PDDA with positive charges can assemble alternatively with both α-ZrP and PAA with negative charges to form nanobrick wall architectures on the surface of PET film via the electrostatic interaction. The lamellar structure of α-ZrP platelets and the dense QL assembly coating can greatly reduce the oxygen transmission rate (OTR) of PET film. Compared to pristine PET film, the OTR of PET (QL)19 is reduced from 57 to 0.87 cc/m2/day. Moreover, even with 19 QLs coating, PET (QL)19 composite film is still with an optical transparency higher than 90% and a haze lower than 10%. Therefore, the transparent PET (QL)n composite films with super oxygen barrier properties show great potential application in food packaging and flexible electronic packaging.

Chemical sintering of direct-written silver nanowire flexible electrodes under room temperature

Transparent and flexible electrodes on cost effective plastic substrates for wearable electronics have attract great attention recently. Due to the conductivity and flexibility in network form, metal nanowire is regarded as one of the most promising candidates for flexible electrode fabrication. Prior to application, low temperature joining of nanowire processes are required to reduce the resistance of electrodes and simultaneously maintain the dimensionality and uniformity of those nanowires. In the present work, we presented an innovative, robust and cost effective method to minimize the heat effect to plastic substrate and silver nanowires which allows silver nanowire electrodes been directly written on polycarbonate substrate and sintered by different electrolyte solutions at room temperature or near. It has been rigorously demonstrated that the resistance of silver nanowire electrodes has been reduced by 90% after chemical sintering at room temperature due to the joining of silver nanowires at junction areas. After ∼1000 bending cycles, the measured resistance of silver nanowire electrode was stable during both up-bending and down-bending states. The changes of silver nanowires after sintering were characterized using x-ray photoelectron spectroscopy and transmission electron microscopy and a sintering mechanism was proposed and validated. This direct-written silver nanowire electrode with good performance has broad applications in flexible electronics fabrication and packaging.

Effect of Nanofiber Orientation on Nanofiber Solder Anisotropic Conductive Films Joint Properties and Bending Reliability of Flex-on-Flex Assembly

Anisotropic conductive films (ACFs) have been widely used as an interconnection adhesive material due to its light weight and simple and low-temperature assembly processes. However, because of the higher demands for further miniaturization, short-circuit problem of ACFs interconnection has been a major issue, when it comes to very fine pitch assembly. Also, due to the fast development of wearable devices, demands for flexible packaging as well as flexible interconnection methods such as flex-on-flex (FOF) are growing more, where the bending characteristics are important. Nanofiber incorporated ACFs were previously introduced by our research group. The nanofiber incorporated ACFs showed excellent conductive particle movement suppression capability that not only prevents short circuit but also improves the conductive ball capture rate, which eventually improves the joint reliability of fine pitch FOF assembly. In order to maximize the conductive particle movement suppression capability of the nanofiber, nanofiber was oriented using a drum-type receiver. In addition, the bending reliability of FOF assembly using nanofiber/solder ACFs with different nanofiber orientations has been also investigated. As a result, parallel nanofiber/solder ACFs showed excellent joint properties as well as bending reliability due to the stable metallurgical solder joint formation and high conductive particle movement suppression capability, and this new type of ACF technology will provide a promising solution for future flexible electronic packaging.

유연/신축성 전자패키징 용 폴리머 재료의 기계적 물성 측정 기술 리뷰

This paper presents an overview of selected advanced measurement technologies for the mechanical properties of polymers used for flexible and stretchable electronic packaging. Over the years, a variety of flexible and stretchable electronics have been developed due to their potential applications for next generation IT industry. To achieve more flexible and wearable devices for practical applications, the usage of polymeric components has been increased significantly. Therefore, accurate measurement of mechanical properties of the polymers is necessary in order to design mechanically reliable devices. However, the measurement has been challenging due to the soft nature and thin applications of polymers. Here, we describe novel measurement technologies of mechanical properties of polymers for flexible and stretchable electronics.

Electrically conductive adhesives based on thermoplastic polyurethane filled with silver flakes and carbon nanotubes

Flexible conductive adhesive could be used in flexible device electronic packaging. In this study, carbon nanotubes (CNTs) were used as one-dimensional conductive scaffolds to construct effective electrical networks among the silver flakes in electrically conductive adhesives (ECAs) for saving noble metal fillers and reducing the cost because CNTs used as conductivity fillers could bridge the neighboring silver flakes to accelerate the electron transport. The electrical, mechanical, and thermal properties have been enhanced after adding CNTs. The electrical conductivity increased 85.6% after 4.5 wt% CNT added into the 50 wt% Ag TPU adhesives. In addition, after the Ag flakes modified by succinic acid (SA), the electrical conductivity has also been improved. The development of highly conductive, flexible, and low cost ECAs will allow them to be widely used in flexible electronic devices packaging in the future.