Intense Pulsed Light Sintering Research Articles

48‐4: Distinguished Paper: Backside Bonding for Extremely Narrow Bezel at the Bottom of Flexible Displays

We developed a novel method to minimize the bezel of flexible displays through backside bonding of a chip on film, resulting in the bezel width of less than 500 μm as compared to 1000 μm of conventional displays. The metal embedded in polyimide (MEP) layer is placed between the first and second polyimide (PI) substrates and connected to the metal lines of the backplane via the MEP contact (M‐CNT) hole. Subsequently, the nonconductive film (NCF) bonding and intense pulsed light sintering are performed using conductive ink. Conductive ink as the interconnect material capable of low‐temperature sintering is applied to avert thermal degradation and crack. At a high temperature (65 ℃) and humidity (90% relative humidity), the contact resistance was a drivable level for the display after 240 h. The normalized strain in the M‐CNT hole and MEP area were less than 0.4, indicating the absence of cracks during the NCF bonding. These results demonstrated that the backside bonding method was suitable for extremely narrow bezels of the nextgeneration flexible displays.

Backside bonding for extremely narrow bezel at the bottom of flexible displays

AbstractWe developed a novel method to minimize the bezel of flexible displays through backside bonding of a chip on film, resulting in the bezel width of less than 500 μm as compared to 1000 μm of conventional displays. The metal embedded in polyimide (MEP) layer is placed between the first and second polyimide (PI) substrates and connected to the metal lines of the backplane via the MEP contact (M‐CNT) hole. Subsequently, the nonconductive film (NCF) bonding and intense pulsed light sintering are performed using conductive ink. Conductive ink as the interconnect material capable of low‐temperature sintering is applied to avert thermal degradation and crack. At a high temperature (65°C) and humidity (90% relative humidity), the contact resistance was a drivable level for the display after 240 h. The normalized strain in the M‐CNT hole and MEP area were less than 0.4, indicating the absence of cracks during the NCF bonding. These results demonstrated that the backside bonding method was suitable for extremely narrow bezels of the next‐generation flexible displays.

High-Resolution Printing of Various Electronic Materials by Electrophotography

Electrophotography is a digital, on-demand, dry, and page printing technique that operates based on toner particles of electronic materials using an electrostatic force and generates an electrical circuit via distribution of the toner particles. We developed a 10 μm linewidth resolution with various electronic materials, including conductors, semiconductors, and insulators, without any chemical pretreatments on the substrate films, while a 5 μm resolution was also possible for limited materials. The electrical resistivity of the printed Ag–Ni after an intense pulse light sintering was comparable to that of commercial indium tin oxide transparent films.

Pattern-dependent resistivity variations in inkjet-printed conductors due to non-uniform ink drying

When fabricating inkjet-printed electronic devices and circuits, inkjet-printed conductive materials require drying and sintering to improve electrical conductivity. Electrical conductivity should be the same irrespective of pattern design, size, location, or density of adjacent patterns. However, we demonstrate that spatial variations in the drying process for inkjet-printed patterns with proximity to others cause resistivity variations. These resistivity variations are studied here experimentally for different circuit patterns and in arrays of inkjet-printed square electrodes. This variation depends not only on the location of each electrode in an array but also on the number of electrodes. This means that for the same drying temperature and duration, the array with a larger number of electrodes exhibits a larger resistivity variation. The sooner an electrode dries, the lower resistivity it achieves. The resistivity variation between an individual electrode and the center electrode in a 7 × 7 electrode array can be a factor of seven. This variation decreases for lower numbers of electrodes to a factor of three for a 3 × 3 array. Furthermore, x-ray photoelectron spectroscopy analyses provide evidence for the residual presence of carbon-based materials within electrodes after the drying process. These results confirm that the location of electrodes within an array significantly influences the amount of residual carbon-based materials, thereby contributing to resistivity variations. Although intense pulsed light sintering can decrease this variation, its optimal parameters depend on the printed designs, and our simulation results show a non-uniform temperature profile over the electrode arrays. Temperature increases more at the center of patterns than the corners, which can be useful in this case to improve resistivity uniformity. In this study, for the first time, we show how different printed shapes and designs can result in non-uniform resistivity after drying and sintering.

Voidless metal lines sintered with intense pulsed light and their applications as transparent metal-mesh electrodes

An optimal metal ink that can be sintered without void generation by intense pulsed light (IPL) was systematically formulated. This ink is a hybrid ink in which silver (Ag) nanoparticle ink and copper (Cu) precursor ink are mixed in an optimal weight ratio, and the reduced Cu fills the space between Ag nanoparticles during the sintering process. The optimization process of the ink encompasses the shape and size of nanoparticles, the type of solvent, and the mixing ratio of the Cu precursor ink. This optimized ink was filled in the engraved trenches and IPL-sintered to fabricate flexible transparent metal-mesh electrodes. If there are voids in the metal lines after IPL sintering, the metal lines may shrink under condition of high temperature and high humidity for a long time, which may deteriorate the stability of the metal electrode. It was confirmed that the hybrid ink optimized in this study hardly shrinks due to no voids inside the metal lines after IPL sintering and has excellent environmental and mechanical stability. The metal-mesh electrode produced using this ink showed superior properties compared to other competing transparent electrodes such as indium-tin oxide or Ag nanowires when used for transparent heaters and electromagnetic shielding applications.

Thermo-mechanical modeling of thermal stress during multi-cycle intense pulsed light sintering of thick conductive wires on 3D printed dark substrate

Due to its advantage of rapid, selective and no contact sintering, the intense pulsed light (IPL) sintering of the conductive wires is promising to simplify the hybrid printing of the fully printed electronics. However, this fast IPL sintering process is often plagued with reproducibility of the results and morphological deterioration in the product. Since exposure to high-intensity light generates temperature gradient inside samples, accurate prediction of the thermally induced residual stresses is the key issue in improving the reproducibility of the IPL sintering. In this study, we investigated how total light energy, light exposure cycle and cooling time affect the thermal stress. The finite element analysis was used to study the in-situ evolution and reduction method of thermal stresses during multi-cycle IPL sintering of thick conductive wires on 3D printed substrate, and the experiments were used to verify the analytic results. The position and sintering stage that generated the maximum thermal stress was found, then light exposure cycle and cooling time were varied to reduce the maximum thermal stress. Finally, a higher total energy combined with multi-cycle sintering method was proposed to reduce the maximum thermal stress and obtain good sintering performance. The proposed method is reliable for optimizing the pulsed light setting. This work provides an insight for optimizing the sintering methodology to improve the IPL sintering stability.

Amphiphilic Silver Nanoparticles for Inkjet-Printable Conductive Inks.

The large-scale manufacturing of flexible electronics is nowadays based on inkjet printing technology using specially formulated conductive inks, but achieving adequate wetting of different surfaces remains a challenge. In this work, the development of a silver nanoparticle-based functional ink for printing on flexible paper and plastic substrates is demonstrated. Amphiphilic silver nanoparticles with narrow particle size distribution and good dispersibility were prepared via a two-step wet chemical synthesis procedure. First, silver nanoparticles capped with poly(acrylic acid) were prepared, followed by an amidation reaction with 3-morpholynopropylamine (MPA) to increase their lipophilicity. Density functional theory (DFT) calculations were performed to study the interactions between the particles and the dispersion medium in detail. The amphiphilic nanoparticles were dispersed in solvents of different polarity and their physicochemical and rheological properties were determined. A stable ink containing 10 wt% amphiphilic silver nanoparticles was formulated and inkjet-printed on different surfaces, followed by intense pulsed light (IPL) sintering. Low sheet resistances of 3.85 Ω sq-1, 0.57 Ω sq-1 and 19.7 Ω sq-1 were obtained for the paper, coated poly(ethylene terephthalate) (PET) and uncoated polyimide (PI) flexible substrates, respectively. Application of the nanoparticle ink for printed electronics was demonstrated via a simple flexible LED circuit.

Flash light assisted additive manufacturing of 3D structural electronics (FLAME)

Additive integration of 3D electrical circuits with off-the-shelf electronic devices inside 3D printed polymer parts, i.e., structural electronics, can enable new paradigms in miniaturized multifunctional structures. This paper investigates a hybrid printing process called Flash Light Assisted Manufacturing of structural Electronics (FLAME) which integrates Fused Filament Fabrication (FFF) of polymers with printing and Intense Pulsed Light sintering (IPL) of silver nanoparticles. The effect of IPL parameters and nanoparticle shape on conductivity is quantified, revealing that using NWs with IPL allows greater conductivity with lesser sintering-induced polymer damage. The conductivity of planar circuits is characterized during over-FFF, i.e., during FFF of the polymer on the sintered circuit. An unexpected finding is that over-FFF increases the conductivity in a complex and non-monotonic manner that depends on the over-FFF parameters. A multi-layer IPL strategy is introduced for through-plane circuits, in which NWs are deposited in increments smaller than the circuit's total height and IPL is performed after each increment. Electromagnetic and thermal simulations reveal why through-plane circuits have lesser conductivity than planar ones and uncover the key role of multi-layer IPL in increasing the conductivity of through-plane circuits. Overall, FLAME increases the conductivity by 300 % for planar circuits and by 170 % for through-plane circuits as compared to state-of-the-art nanoparticle printing-based methods, even for low-thermal-tolerance polymers, in less than 10 s for each polymer layer. These advances break the performance-material-throughput tradeoff that plagues existing nanoparticle-based printing methods for fabricating 3D structural electronics.

Multi-objective optimization of intense pulsed light sintering process for aerosol jet printed thin film

The sintering of printed nanoparticle films is a necessary processing step for most nanoparticle inks to make the printed film functional. The sintering of nanoparticle is usually performed through thermal sintering, photonic sintering, induction sintering, etc. Intense pulsed light (IPL) sintering method is one of the most popular sintering methods for nanoparticle inks due to the fast and effective process, but it may yield mediocre performance if improper sintering parameters are used. In this work, we investigate the correlation between the two factors which are the print passes of aerosol jet printing and the sintering distance of the samples on the effect of the surface morphology and sheet resistance. A contradictory correlation between the two factors was observed and a multi-objective optimization was carried out using machine learning method to identify the most optimum conditions for both factors. We found that multi-objective optimization approach is effective in reducing the conflicting responses, thus the sintered thin film can have low sheet resistance and low surface roughness. This work provides an essential guide for achieving conductive films with electrical conductivity and low surface roughness using IPL sintering process for fast fabrication of multi-layered electronics such as electrochemical electrodes.

Multi-objective optimization of intense pulsed light sintering process for aerosol jet printed thin film

The sintering of printed nanoparticle films is a necessary processing step for most nanoparticle inks to make the printed film functional. The sintering of nanoparticle is usually performed through thermal sintering, photonic sintering, induction sintering, etc. Intense pulsed light (IPL) sintering method is one of the most popular sintering methods for nanoparticle inks due to the fast and effective process, but it may yield mediocre performance if improper sintering parameters are used. In this work, we investigate the correlation between the two factors which are the print passes of aerosol jet printing and the sintering distance of the samples on the effect of the surface morphology and sheet resistance. A contradictory correlation between the two factors was observed and a multi-objective optimization was carried out using machine learning method to identify the most optimum conditions for both factors. We found that multi-objective optimization approach is effective in reducing the conflicting responses, thus the sintered thin film can have low sheet resistance and low surface roughness. This work provides an essential guide for achieving conductive films with electrical conductivity and low surface roughness using IPL sintering process for fast fabrication of multi-layered electronics such as electrochemical electrodes.

Copper Ion Inks Capable of Screen Printing and Intense Pulsed-Light Sintering on PET Substrates

Copper Ion Inks Capable of Screen Printing and Intense Pulsed-Light Sintering on PET Substrates

Computational analysis model of intense pulsed sintering of silver nanoparticles

Intense pulsed light sintering uses a high-power pulsed xenon lamp to release large amounts of energy and promote the growth of sintered necks between nanoparticles in additive manufacturing. However, the process results in poor stability of the sintering effect on conductive patterns and easily damages the target materials. An analysis model was developed in this study to investigate the mechanism of intense pulsed sintering. The sintering process was divided into pre-sintering and formal sintering. In pre-sintering, the conductive pattern is described by the porous volatilization model based on Darcy's fluid equation to calculate the solvent phase change volatilization state, whereas a multi-scale analysis model was established for the formal sintering process. First, a molecular dynamics model was used to analyze the sintering state of nano-silver clusters in a variable temperature field to calculate the time-varying parameters. Second, from a finite element model, the intense pulsed sintering temperature field was constructed for different sintering parameters and applied to the two-phase flow model of sintering nano-silver particles to calculate the sintering process state of nano-silver particles. The relationship between the intense pulsed sintering parameters and the sintering state was revealed, thus providing a reliable theoretical basis for the sintering process to improve sintering stability.

Numerical and experimental investigations on intense pulsed light sintering of silver nanoparticle inks for printed electronics

Intense pulsed light (IPL) sintering has the advantages of high efficiency and selective heating for silver nanoparticles (AgNPs) inks, which could be widely used in printed electronics. In this contribution, the heat transfer and diffusion mechanisms of AgNPs during IPL sintering are quantitatively investigated with a mathematic model combined transient heat transfer and molecular dynamics (MD) method and experiments. The results show that only 51.522% of the IPL radiation energy is absorbed by the AgNPs. During IPL sintering, the temperature of AgNPs rises significantly, whereas the temperature of polyamide substrate keeps almost unchanged, demonstrating that IPL is feasible for thermal-sensitive substrates. The MD modeling results show that sintering neck between AgNPs forms and grows rapidly at the beginning of sintering, then remains relative stable. Accordingly, the electrical resistivity of the AgNPs drops rapidly to a stable value. In addition, the effects of IPL sintering parameters are studied and the results show that increasing IPL energy and reducing IPL duration could increase the sintering performance of AgNPs inks. Finally, an antenna is fabricated using AgNP inks and IPL sintering technology. The experimentally measured performance of the antenna agrees well with the theoretical analysis. Our simulation and experimental results demonstrate that IPL is suitable for sintering of AgNPs inks for flexible electronics.

Optimisation of Operational Parameters in Atmospheric Plasma Deposition

Nowadays additive manufacturing and 3D printing techniques are transferring in the mainstream industrial use. To compete for the share of the pie in this sector, atmospheric plasma can be used for deposition of material. In this work it will be presented the atmospheric plasma deposition of copper nanoparticles and the influence of the plasma operational parameters on the shape of the deposited copper line: the with and the density. The nanoparticles were produced by a facile aqueous-based seed-mediated chemical reduction method from copper nitrate hemi(pentahydrate), as described in article by Ankireddy et al. 1 The copper nanoparticles had bimodal size distribution with mean diameters of 3.6 and 64.6 nm. The nanoparticles were then dispersed in deionised water, caped with citric acid and used as a source for atmospheric plasma deposition. The plasma system comprised from nebuliser, nozzle and kHz power supply, as in the work of Gandhiraman et al. 2 During the plasma deposition of the particles the substrate was moving by XY step-motor stage. Parameters, that could be set and influenced the deposition were: gas mixture (different ratios of Ar and He gas flows), gas flow-rate, distance of the substrate from the plasma, plasma power, time between steps of XY stage and the number of passes/repetitions by XY stage. Different permutations of the parameters were used for the plasma deposition to determine the best parameter set for a dense deposition of a narrow line.In figure it can be seen two depositions with different parameters. The difference is obvious.[1] „Seed Mediated Copper Nanoparticle Synthesis for Fabricating Oxidation Free Interdigitated Electrodes using Intense Pulse Light Sintering for Flexible Printed Chemical Sensors“; Krishnamraju Ankireddy, Thad Druffel, Swathi Vunnam, Gregor Filipič, Ruvini Dharmadasa, Delaina A. Amos; Journal of Materials Chemistry C, 5 (2017), 11128 – 11137[2] „Plasma jet based in situ reduction of copper oxide in direct write printing“; Avishek Dey, Arlene Lopez, Gregor Filipič, Aditya Jayan, Dennis Nordlund, Jessica Koehne, Satheesh Krishnamurthy, Ram P. Gandhiraman, M. Meyyappan; Journal of Vacuum Science & Technology B, 37 (2019), 031203 Figure 1

Fabrication of solderable intense pulsed light sintered hybrid copper for flexible conductive electrodes

Additively printed circuits provide advantages in reduced waste, rapid prototyping, and versatile flexible substrate choices relative to conventional circuit printing. Copper (Cu) based inks along with intense pulsed light (IPL) sintering can be used in additive circuit printing. However, IPL sintered Cu typically suffer from poor solderability due to high roughness and porosity. To address this, hybrid Cu ink which consists of Cu precursor/nanoparticle was formulated to seed Cu species and fill voids in the sintered structure. Nickel (Ni) electroplating was utilized to further improve surface solderability. Simulations were performed at various electroplating conditions and Cu cathode surface roughness using the multi-physics finite element method. By utilizing a mask during IPL sintering, conductivity was induced in exposed regions; this was utilized to achieve selective Ni-electroplating. Surface morphology and cross section analysis of the electrodes were observed through scanning electron microscopy and a 3D optical profilometer. Energy dispersive X-ray spectroscopy analysis was conducted to investigate changes in surface compositions. ASTM D3359 adhesion testing was performed to examine the adhesion between the electrode and substrate. Solder-electrode shear tests were investigated with a tensile tester to observe the shear strength between solder and electrodes. By utilizing Cu precursors and novel multifaceted approach of IPL sintering, a robust and solderable Ni electroplated conductive Cu printed electrode was achieved.

Enhanced Conductivity in Highly Stretchable Silver and Polymer Nanocomposite Conductors.

In stretchable conductors, there is a trade-off relationship between the stretchability and conductivity which makes it difficult to increase both properties simultaneously. From a practical point of view, however, high conductivity is a more important parameter for real-world applications of wearable and mobile electronics. To obtain a highly conductive stretchable conductor, we developed a stretchable conductor composed of silver (Ag) flat-type microparticles, Ag nanoparticles and a polyester binder. The printed stretchable conductor was then sintered using the intense pulse light sintering technique. The effects of different mixing ratios of Ag flat-type particles and nanoparticles on dispersibility, printability, surface properties, conductivity, and stretchability were examined. Increasing the content of Ag-flat type particles in the composite improved dispersibility, printability, and conductivity. The stretchable conductor exhibited the outstanding conductivity of 5.5×106 S/m. Increasing the Ag nanoparticles content increased the stretchability of the conductor. As the nanoparticle content increased to 30%, the stretchable conductor showed the excellent stretchability of 210%, and withstood 2,600 repeated stretching cycles at a fixed tensile strain of 50%. The conductors also exhibited superb foldability during 10,000 repeated folding tests, up to a radius of 1 mm, without any failures.

A simple process to create micro-gaps in printed copper electrodes by sintering induced stress in flexible PET substrates

A new simple low-cost technique for fabricating micro-gaps of less than 50 µm in copper electrodes is reported, which combines conventional screen printing of copper paste and intense pulsed light (IPL) sintering. The micro-gap is created via a two-step IPL sintering. First, a continuous printed copper stripe is half sintered along its length with another half blocked by an opaque mask. Then, the second sintering is performed over the entire length of copper stripe with the opaque mask removed. At the joint between two sintering steps, a micro-gap is formed across the copper stripe due to thermal contraction of underlying polyethylene terephthalate substrate. The width of copper micro-gaps can be adjusted by IPL energy. Simple light emitting diode circuits have been made by the micro-gap electrodes, demonstrating its feasibility for practical applications.

Optimization of Hybrid Ink Formulation and IPL Sintering Process for Ink-Jet 3D Printing.

Ink-jet 3D printing technology facilitates the use of various materials of ink on each ink-jet head and simultaneous printing of multiple materials. It is suitable for manufacturing to process a complex multifunctional structure such as sensors and printed circuit boards. In this study, a complex structure of a SiO2 insulation layer and a conductive Cu layer was fabricated with photo-curable nano SiO2 ink and Intense Pulsed Light (IPL)-sinterable Cu nano ink using multi-material ink-jet 3D printing technology. A precise photo-cured SiO2 insulation layer was designed by optimizing the operating conditions and the ink rheological properties, and the resistance of the insulation layer was 2.43 × 1013 Ω·cm. On the photo-cured SiO2 insulation layer, a Cu conductive layer was printed by controlling droplet distance. The sintering of the IPL-sinterable nano Cu ink was performed using an IPL sintering process, and electrical and mechanical properties were confirmed according to the annealing temperature and applied voltage. Then, Cu conductive layer was annealed at 100 °C to remove the solvent, and IPL sintered at 700 V. The Cu conductive layer of the complex structure had an electrical property of 29 µΩ·cm and an adhesive property with SiO2 insulation layer of 5B.

Intense Pulsed Light Sintering of Screen-Printed Paste Electrode on Silicon Solar Cell for High Throughput and Cost-Effective Low Temperature Metallization

Intense Pulsed Light Sintering of Screen-Printed Paste Electrode on Silicon Solar Cell for High Throughput and Cost-Effective Low Temperature Metallization

Boron Nitride Nanotube Coatings for Thermal Management of Printed Silver Inks on Temperature Sensitive Substrates

AbstractPrinted electronics provide inexpensive and light weight electrical components to fuel emerging applications. One major challenge is the high temperature required to sinter conductive metal inks, which leads to thermal degradation of the substrate and subsequently poor performance. A boron nitride nanotube (BNNT) interfacial film is reported for thermal management in rapid processing of a printable silver molecular ink platform using intense pulsed light (IPL) sintering techniques. The inclusion of BNNT thin films of varying surface concentrations deposited between the substrate and the printed features reduces thermal damage to the substrate during sintering while simultaneously improving electrical performance, achieving a sheet resistance value as low as 140 mΩ sq−1. A wide range of sintering energies ranging from 2.0 and 3.2 J cm−2 are investigated along with printed trace widths ranging from 5 mil (0.127 mm) to 20 mil (0.508 mm). Increases in the rate of cooling and in the current carrying capacity are confirmed with the inclusion of the BNNTs. Overall the thin coating of BNNTs presents no drawbacks while significantly improving the electrical properties of IPL sintered conductive traces and thus represents a simple approach that will advance the adoption of IPL for fabricating printed electronic components.