Process Recipe and Functional Circuitry Performance on Aerosol Jet Printed Water-Based Silver Ink

Flexible electronics are gaining prominence as a viable alternative to traditional print-and-etch electronics, particularly in emerging applications like flexible displays, wearable sensors, flexible batteries, e-skin, and beyond. The ongoing focus on minimizing size and weight has led to a demand for foldable electronic products that offer high portability. The inherent flexibility of these electronics allows them to fold, flex, stretch, and twist, providing an ideal solution for applications that require such adaptability. This versatility positions flexible electronics as a key technology in the development of portable and innovative electronic devices [1]. However, understanding how device performance evolves during bending, stretching, or other mechanical cycling is crucial for advancing research in the domain of flexible printed electronics [2,3]. Further, the investigation of methods to enhance the transmission stability of the printing electronic across different operational scenarios such as vibration holds significant engineering significance in elevating the reliability of flexible electronic products [4]. Additionally, researchers are interested in reliability issues and solutions in flexible electronic under different mechanical fatigue conditions [5]. Further, the adhesion of the ink on various substrate under different oxidation level, fabrication difficulty is some of the challenges faced by the flexible printed electronics [6]. Historically, additive printing processes and inks relied on volatile organic compounds, necessitating their removal through sintering processes involving thermal, laser, or photonic methods. The shift to water-based inks marks a significant opportunity to advance sustainable and environmentally friendly electronic architectures while transitioning to a low-impact waste flow in production processes. This transition aligns with the broader goal of minimizing the environmental footprint associated with electronic manufacturing and underscores the importance of adopting eco-friendly alternatives in additive printing technologies [7]. Nevertheless, the shift to water-based inks presents several challenges. These include heightened oxidation, the need for extended thermal exposure to remove solvents and establish interconnects, particle agglomeration, and the preservation of print definition throughout the solvent removal process. Addressing these challenges becomes crucial in ensuring the successful adoption of water-based inks despite the complexities associated with factors like oxidation and maintaining print quality during the ink drying and interconnection stages [8]. The understanding of process recipes for consistent trace geometries, electrical parameters, and mechanical performance is incomplete. Water-based materials, being relatively new, lack comprehensive knowledge about how process parameters influence the desired print output. Further research is needed to address these gaps and optimize outcomes [9].

In Refs. [10–12], researchers have investigated the effects of different printing/sintering parameters on the performance of printed traces, using different inks and printing platforms. The researchers identified optimum conditions where printed traces demonstrated favorable mechanical and electrical performance. But the functional circuit realization using sustainable ink was not studied well in the past. This study offers valuable insights into improving performance in both individual traces, single component attachment, and overall circuitry through optimum parameters in the printing process. This paper discusses the potential for component attachment and the performance test to functional circuitry using water-based ink and electrically conductive adhesives on the aerosol jet printing (AJP) platform. Research has been conducted to establish a reference for printing conductive inks in AJP on rapid prototyping materials for fine-pitch electronic applications [13]. Achieving both high resolution and improved electrical/mechanical performance often presents challenges when aiming to maintain a high printing speed. In the realm of aerosol printing, generating a thick line necessitates multiple procedures, consequently expanding the width of the printed line [14]. In Ref. [15], authors investigated sustainable options for paints using a life cycle assessment, revealing that the production and supply of raw materials have the greatest impact on paints across various indicators. Lall performed a thermoforming test on conductible traces using silver ink for in-mold electronics [16]. Similarly, in Ref. [17], researchers analyzed anisotropic conductive adhesive connections and reported the frequency performance of various discrete components.

The existing literature on printing with water-based inks is limited, particularly for new inks where the optimal printing recipe for desired properties like resistivity and shear load to failure (SLF) remains unknown. This study specifically focuses on the process development and component attachment of water-based silver nanoparticle ink using AJP. The goal is to explore various printing and sintering parameters to achieve the desired electrical and mechanical properties for printed traces. Parameters include ultrasonic atomizer mass flow control (UAMFC), sheath flow control (SMFC), stage speed, and temperature and time for sintering. After determining optimal parameters, the experiment progresses to establishing connections between printed traces and discrete components using electrically conductive adhesive (ECA) adhesives, whose behavior with water-based inks is unknown. The interconnection involves printing an ECA layer, placing components on printed pads, and curing the ECA for a strong bond. Measured component values are compared to rated values to ensure accuracy and a functional differentiator circuit is tested and compared with simulated output, validating the findings.

Aerosol jet printing is an emerging technology that employs a droplet-based, noncontact, high stand-off direct-write technique for printing ink with viscosities up to 1000 cP. This broad viscosity range enhances versatility, allowing the printing of diverse circuits on various substrates. An ultrasonic atomizer is utilized to print ink with viscosities up to 30 cP. This atomizer generates a mist through ultrasonic waves produced by a transducer beneath the ink vial. The waves travel through de-ionized water, transferring energy to the ink and forming droplets. These droplets, suspended in solvent, are transported to the deposition head via a mist tube. In the deposition head, sheath gas surrounds the droplets, focusing the beam to print high-resolution lines on the substrate. The working principle of the ultrasonic atomizer for aerosol jet printing is illustrated in Fig. 1.

The experimental methodology in this study is illustrated in the workflow diagram shown in Fig. 2. The process involved key steps such as test vehicle design, variable and constant parameter identification, test matrix creation, sample printing and sintering, analysis of mechanical and electrical properties, print quality assessment, optimization of printing and sintering parameters, adhesive-based component attachment, and ultimately, comparing the output with the simulated results.

In this study, a water-based silver nanoparticle ink is employed, containing 50% solid silver content by weight. The ink features a viscosity range of 6–10 cP, with particle sizes ranging from 30 to 40 nm. The resistivity of the ink spans from 7.0 × 10⁻⁴ to 1.2 × 10⁻⁵ Ohm-cm. The detailed properties of the sustainable ink can be found in Table 1.

Table 2 outlines the material properties of the ECA ink used to attach components in test vehicle designs 2 and 3. The recommended curing condition for the ECA is 150 °C for 30 min. The ECA ink, containing silver, is compatible with conductive inks based on both copper and silver.

In this study, two distinct test vehicles are utilized: the first one for component attachment study, and the second for functional circuitry evaluations.

Figure 3 illustrates the test vehicle design employed in this process development study, comprising five horizontal and five vertical lines. Both directions of printing were considered to assess their effects. To ensure statistical validity, each sample includes ten printed lines. Resistance is measured across square pads at the end of each line using a multimeter. The average resistance of the ten lines from each sample is calculated, facilitating a comparison of measured parameters under various printing conditions.

In the process parameter study, an aerosol jet (AJ) printer was employed, offering multiple process parameters influencing the electrical and mechanical properties of printed lines. While certain parameters remained constant, others varied. The maximum ultrasonic current was fixed at 0.55 A, and a 300 μm nozzle with a 2 mm stand-off height was kept constant. Ink temperature and platen temperature were maintained at 22 °C and 30 °C, respectively. Sintering conditions (250 °C for 10 min) remained consistent across all parameter studies. Tables 3 and 4 outline the constant and varied parameters, respectively. Experimental parameter sets in Table 4 include variations in UAMFC (25, 30, 35 standard cubic centimeters per minute (SCCM)), sheath (50, 55, 60 SCCM), and stage speed (3, 3.5, 4 mm/s). The sintering temperature varied at 250, 275, and 300 °C, and the sintering time varied at 5, 10, and 15 min. Resistivity and shear load to failure values were measured independently for each sample for comparisons.

Figure 4 depicts test vehicle design 2, with (A) and (B) representing layer 1 and layer 2, respectively. The design is intended to compare measured values to rated values of commonly available components like resistors, capacitors, and inductors. Table 5 provides a description of the discrete components utilized in test vehicle design 2.

The first layer of conductive ink (Layer 1) is printed with AJP and cured in a thermal oven at 275 °C for 10 min. Subsequently, the second layer with ECA (Layer 2) is printed using a direct-write printer, and selected components are attached to the bonding pads. The printing parameters for ECA include 15 psi ink pressure, 3 mm/s print speed, 100% infill density, and 0.08 mm standoff height. After attachment, the sample undergoes sintering at 150 °C for 30 min. Figures 5(a) and 5(b) depict the printing platforms used for layer 1 and layer 2, respectively.

Test vehicle design-3 features a differentiator circuit, serving as a demonstration based on the analyses conducted on test vehicles 1 and 2. Figure 6 displays the schematic diagram of the practical differentiator circuit. Additionally, Fig. 7 illustrates the layout design for the differentiator circuit, where (A) (Layer 1) represents the printing of conductive ink, and (B) (Layer 2) represents the printing of the bonding material. Figure 7(a) shows the input/output and ground terminals of the circuit. The obtained actual output from the printed flexible circuit is then compared with the Linear Technology (LT) spice simulation to evaluate its functionality.

Table 6 shows the list of components used in the differentiator circuit, along with their rating/dimensions.

Figure 8 shows the printed sample of test vehicle design 1. Each sample is printed using a 300-μm nozzle and a stand-off height of 2 mm. The printing process parameters, including the maximum ultrasonic current, ink temperature, platen temperature, number of passes, sintering temperature, and sintering time, were fixed at 0.55 A, 22 °C, 30 °C, four passes, 250 °C, and 10 min, respectively, as mentioned earlier.

Figure 9 illustrates the effect of varying UAMFC and stage speed on the resistance of printed traces, with fixed sheath values of 60 SCCM. UAMFC and speed were varied at 25 SCCM, 30 SCCM, 35 SCCM, and 3 mm/s, 3.5 mm/s, and 4 mm/s, respectively, while keeping the sheath value constant at 60 SCCM.

The results reveal that with an increase in UAMFC for each sheath, the resistance of the printed line decreases. This is attributed to higher UAMFC resulting in more material deposition and a smaller gap between ink nanoparticles. Conversely, an increase in printing speed leads to less material being deposited per second, reducing the cross-sectional area (CSA) of printed traces, and consequently increasing resistance. Therefore, it can be inferred that a UAMFC of 25 SCCM yields relatively higher resistance compared to 30 and 35 SCCM UAMFC for this water-based silver ink in the Ultrasonic Atomizer Printer. A similar trend is observed for the other two sheaths as well.

Figure 10 demonstrates the impact of stage speed on the CSA of printed traces, with UAMFC and sheath parameters held constant at 35 SCCM and 60 SCCM, respectively. The results indicate that the CSA decreases as the stage speed increases, while keeping UAMFC, sheath, and sintering conditions constant.

Figures 12 and 13 present the height and width measurements, respectively, of printed traces obtained through WLI testing. The printing and sintering parameters were maintained constant, except for the varied stage speed at 3 mm/s, 3.5 mm/s, and 4 mm/s for samples (A), (B), and (C) in both figures. The UAMFC and sheath parameters were fixed at 35 SCCM and 60 SCCM, respectively, and the sintering condition was set at 250 °C for 10 min. The results indicate that both line height and width decrease with an increase in stage speed, as higher speeds result in less material deposition compared to lower speeds.

Optical microscopy, utilizing visible light and lenses for magnification, is a noncontact and nondestructive imaging technique employed in this study to evaluate the quality of printed traces and measure their width.

Figures 14(a)–14(c) display optical images of three samples printed at speeds of 3 mm/s, 3.5 mm/s, and 4 mm/s, respectively, with constant UAMFC, sheath, and sintering conditions (35 SCCM UAMFC, 60 SCCM, and 250 °C for 10 min). The printed traces exhibit acceptable quality, with no visible cracks or signs of improper sintering. The trace width measured using optical microscopy aligns consistently with values obtained from WLI.

Figure 15 illustrates the effect of stage speed and UAMFC on resistivity when the sheath is fixed at 60 SCCM.

The results indicate that the resistivity value decreases as UAMFC increases from 25 to 35 SCCM for all three speeds. A similar trend is observed for the other two sheaths as well. The lowest resistivity of 1.76 × 10⁻⁵ Ω-cm is achieved with 35 SCCM UAMFC, 50 SCCM sheath, and 3 mm/s stage speed.

Figure 16 demonstrates the impact of speed on SLF for various UAMFCs while maintaining a constant sheath of 60 SCCM.

The study indicates that increasing UAMFC led to a higher SLF as more force was needed to shear the traces. Conversely, increasing the printing speed resulted in a lower SLF as thinner lines were produced at higher speeds. This trend was consistent across all sheath conditions. The optimal combination of parameters, resulting in the highest SLF of 25.21 grams, was found to be a UAMFC of 35 SCCM, a sheath of 50 SCCM, and a stage speed of 3 mm/s. As speed doesn't have a significant effect on resistivity and SLF, a speed of 3.5 mm/s is chosen for the same UAMFC and sheath of 30 SCCM and 50 SCCM, respectively.

Figure 17 illustrates the effect of sintering conditions on the resistance per unit length of the printed traces.

The results showed that the resistance is lower at 275 °C compared to 250 °C/300 °C for all three sintering times. The ink may not have been sintered properly at 250 °C, while at 300 °C, over-sintering may have occurred, resulting in increased line resistance. The data suggest that 275 °C and 10 min of sintering time produce the lowest resistance. Moreover, the resistance was found to be lower for 10 min of sintering compared to 5 and 15 min for all sintering temperatures. Figure 18 shows the effect of sintering time and temperature on SLF. It is observed that the SLF increases and then decreases again when the temperature increases from 250 °C to 275 °C and then 300 °C. The higher SLF is obtained for 275 °C for each of the three sintering times. Hence 275 °C and 10 min of sintering condition have been chosen as the optimum parameters.

Figure 19 depicts the schematic of the component attachment process. The initial step involves printing the first layer of conductive ink, followed by sintering. Subsequently, the second layer of ECA is printed, and components are placed in their designated positions, followed by sintering at 150 °C for 30 min.

In Figs. 20(a) and 20(b), the printed sample is shown before and after component attachment, respectively. The first layer is printed using AJP with a 300 μm nozzle and a stand-off height of 2 mm. The study utilizes optimal printing and sintering parameters derived from test vehicle design 1, including 35 SCCM UAMFC, 50 SCCM sheath, 3.5 mm/s stage speed, and 275 °C for 10 min. The fixed parameters during the printing process include a maximum ultrasonic current of 0.55 A, ink temperature of 22 °C, platen temperature of 30 °C, and four passes. Following the sintering of the first layer, ECA is directly written on the middle pads. Figure 20(b) illustrates the attachment of resistors, capacitors, and inductors to each of these pads. Replicates are created to better understand variations that may arise under identical sample conditions.

Figures 21 through 24 display the measurements taken (using an Agilent inductance-capacitance-resistance meter U1733C) before and after the component attachments on the pads. In each plot, the two red dotted lines represent the ±5% rated value for the respective passive components. The analysis of these plots suggests that the after-attachment values are slightly higher than the before-attachment values for each passive component. This difference is attributed to the higher resistivity of printed silver compared to bulk silver, with additional resistance contributed by the ECA. Despite the increase, it's noteworthy that the after-attachment values still fall within the acceptable range of ±5% rated value. This observation leads to the conclusion that a robust interconnectivity bond has been established and it is feasible to proceed with the use of the ECA binding material for sustainable silver ink.

Assessing the mechanical strength of a component is crucial for potential failure analysis in flexible circuitry subjected to bending, flexing, or twisting. Figure 25 illustrates the schematic diagram of component attachment using ECA. In Figs. 26(a) and 26(b), shear testing of the components is depicted using a metal tip. This testing provides insights into the mechanical resilience of the components and their ability to withstand shearing forces.

Figure 27 depicts shear test results for discrete components. Since different surface mount components have their different height, shearing all the components with the same shearing height is not feasible. In this study, 20% of the height of each attached component has been used as the shear height. The test speed used for each component was 12.7 μm/s. Based on the result obtained, it can be inferred that components with lesser heights exhibit greater mechanical strength.

Figures 28(a) and 28(b) present the first layer of the printed differentiator circuit and the printed circuit with attached components, respectively. Wires are connected at each terminal to facilitate testing. Figure 29 displays the LT Spice simulation circuit for the differentiator, indicating the rated value for each component. The circuit is subjected to a sinusoidal wave as an alternating current input, with a ±15 V DC supply provided to the op-amp.

Equation (3) represents the theoretical gain of the differentiator circuit. This differentiator circuit can differentiate the input having different frequencies producing different magnitudes and phases in the output. One zero occurs at s = 0. Two poles occur at the following frequency locations:

The circuit is designed to act as a differentiator below the frequency f) and as an integrator for frequencies above f. The magnitude (20log₁₀|A|) exhibits a rate of increase at +20 dB per decade for frequencies up to f (as per Eq. (6)), and it decreases at a rate of −20 dB per decade for frequencies higher than f. Specifically, a gain of 1 is achieved at a frequency of 0.1f, which corresponds to 159.15 Hz. This information provides insights into the circuit's behavior and performance characteristics based on the frequency of the input signal. Additionally, the experimental measurements were compared with simulation at eleven different frequencies chosen in such a way that they look equally apart in bode plot with logarithmic frequency scale.

Figures 30–33 illustrate the electrical performance of water-based inks printed circuits for some of the tested frequencies: 159.15 Hz, 1 kHz, 30 kHz, and 100 kHz. These frequencies are randomly chosen to see the nature of the experimental output compared to the simulated output for different frequencies. The comparison revealed a substantial similarity between the experimental and simulated output data, indicating the feasibility of employing sustainable silver ink and ECA for component attachment. In general, the percentage error falls within ±7% and ±3% for magnitude and phase, respectively. This close alignment between experimentally observed and simulated data suggests a promising approach for the advancement of flexible circuits in the future. This approach shows promise for the advancement of flexible circuits in the future.

Figures 34 and 35 present bode plots for sustainable differentiator circuits, comparing them to simulated results. In Fig. 34, the magnitude plot reveals a close match between the sustainable ink and the simulation. Similarly, Fig. 35, representing the phase plot, exhibits similar trends of alignment. The magnitude plot has unity magnitude at a frequency of 159.15 Hz. Additionally, a 180-degree phase difference is observed at a frequency of 1591.55 Hz. These findings further emphasize the consistency between the experimental and simulated performance of the sustainable differentiator circuits, validating their effectiveness in achieving desired electrical characteristics.

Figures 36(a) and 36(b) show the optical images of full CSA and quarter CSA of one of the attached inductors in the circuits.

The printed circuit with components attached with ECA is encapsulated inside a potting epoxy and grind the surface until the middle of the attached component to see the cross section of the interconnection. In the first figure, we can see the full cross section area of an attached inductor. The substrate, silver traces, ECA, and gap between the two pads are all clearly shown in the second figure. The gap between the two pads ensures that the components are not short circuited. These images provide clear evidence of a good bonding between the component, ECA, and the silver traces.

The research study is divided into two main parts. The first part investigates the impact of printing and sintering parameters on the electrical and mechanical properties of traces printed with water-based ink. Parameters such as UAMFC, SMFC, stage speed, and sintering temperature/time are analyzed, along with their influence on trace geometry. Optimum printing/sintering parameters that give higher SLF and smaller resistance are determined. Optical microscopy and white light interferometry are used for characterization of the printed traces. The second part focuses on component attachment using ECA, involving a thorough analysis of components before and after attachment. The study assesses the effect of bonding materials on electrical characteristics, comparing measurements with manufacturer-rated values. Results indicate compliance with industry standards, highlighting ECA's ability to establish stable interconnections with water-based silver traces for reliable electrical performance. Additionally, the flexible differentiator circuit performance is compared with that of the simulated output in LT Spice circuit for benchmarking. The percentage error in magnitude and phase are observed to be within ±7% and ±3%, respectively.

The project was sponsored by the NextFlex Manufacturing Institute under PC7.3 Project titled—Sustainable Additively Printed Electronics through Water-Solvent Inks-FHE Reparability-Low Temperature Processing. This material is based, in part, on research sponsored by Air Force Research Laboratory under Agreement No. FA8650-20-2-5506, as conducted through the Flexible Hybrid Electronics Manufacturing Innovation Institute, NextFlex. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of Air Force Research Laboratory or the U.S. Government.

• Air Force Research Laboratory (Agreement No. FA8650-20-2-5506; Funder ID: 10.13039/100006602).

• Flexible Hybrid Electronics Manufacturing Innovation Institute, NextFlex.