The Performance Characterization of a Drop-on-Demand Inkjet-Printed Gold Film Under the Temperature Conditions for Airborne Equipment

Abstract

Drop-on-demand (DoD) printing is an additive manufacturing technique that utilizes functional inks containing nanoparticles (NPs) to fabricate electronic circuits or devices on a variety of substrates. One of the most promising applications for such technology is the aerospace industry, due to the capability of this method to fabricate custom low-weight geometric films. This work evaluates the performance of a gold (Au) nanoparticle (NP)-based film printed on a ceramic substrate for avionics applications, following the environmental temperature guidance of the Radio Technical Commission for Aeronautics (RTCA) DO-160. Experimental results show that the Au films, printed on alumina substrates, successfully survived the environmental temperature procedures for airborne equipment. The thermal coefficient of resistance (TCR) of the films was measured to be $2.7\times {10}^{-3} {°\mathrm{C}}^{-1}.$

1. Introduction

Current trends in Microelectromechanical Systems (MEMSs) are focusing on optimizing geometry while maximizing output. A clear example of this trend is seen in the fabrication of thin, flexible film materials that allow sensors to be lightweight, conformable, and capable of capturing data across a two-dimensional surface [1].

The additive manufacturing of electronics (AME) is a topic which has gained attention as an alternative to conventional electronics fabrication methods for its capability of integration into a broad range of material manufacturing processes [2] and its cost-saving potential compared to lithography or other subtractive methods. drop-on-demand inkjet printing is an AME approach which uses a microfluidic chamber and piezoelectric actuators to eject picolitre-scale droplets onto a target substrate to form patterned films. This inkjet printing method is notable for being non-contact and not inherently limited by the substrate material, shape, or size.

In the aeronautics industry, there is a great need for the integration of functional sensor technology throughout the aircraft. Conventionally, sensor integration is independent of other structural components, and sensors must be mounted and produced as dedicated hardware with their respective power and communication lines within the aircraft. AME offers the possibility of the structural integration of electronics into the other structural and mechanical components of aircraft systems [3]. With this greater degree of integration, the properties of a device and its constituent material, which were already important in discrete systems, become increasingly critical to the success and survival of these components, with thermomechanical stability being a chief concern.

While there is broad research on the development of drop-on-demand inkjet-printed electronics, which range from the printing of package interconnects [4] to novel sensor devices [5] with characteristics unachievable by conventional processes, some critical gaps remain to be explored which are highly relevant to aeronautics—namely, mechanical robustness and thermal stability.

On the topic of mechanical robustness, it is understood that many AME processes suffer from poor mechanical properties [6] compared to many conventional processes which employ extreme thermal, atmospheric, or chemical conditions to yield films with strong layer-to-layer adhesion and packing density. To bridge this gap, the utilization of high-thermal-budget materials such as alumina ceramics as substrates would make it possible to sinter conductive material at high enough temperatures to achieve the thermomechanical stability of the film.

For the choice of conductive material considering thermal stability, much of the current AME research focuses on the use of silver for its reasonable conductivity and limited susceptibility to corrosion compared to copper [7], which readily oxidizes at low temperatures. Unfortunately, silver does exhibit surface oxidation at ambient temperatures and will oxidize completely around 300 °C, severely impacting its conductivity and application window. For this reason, in order to utilize AME for structural integration into components subject to very high temperatures, the electronics must be either packaged with strong oxygen barrier materials or resistant to corrosion. In that sense, gold is a candidate which can fulfill the latter approach, but has not been deeply explored for AME [8], likely due to being cost-prohibitive compared to silver and the lacking demand due to the low technology readiness of AME for commercial applications.

Recent contributions in the field of testing drop-on-demand devices for engineering applications have shown the great potential of additive manufacturing for the production of conductor materials showing stable behavior at room conditions [9], with good repeatability [10], and with a promising performance at low temperatures [11]. This work focuses on characterizing the temperature coefficient of resistance (TCR) of an additively manufactured Au thin film following the temperature test procedures used in the avionics industry for the qualification and certification of the parts. This is a novel approach since, to the best of the author’s knowledge, no prior airborne equipment testing has been performed on additively manufactured gold-film devices fabricated using the drop-on-demand methodology. The results show that the TCR obtained supports the values of the Au thin films found in the literature for similar types of devices.

2. Materials and Methods

2.1. Sample Fabrication

The samples were made using a Au NP ink JG-125 (Novacentrix, Austin, TX, USA) deposited by an inkjet printer (Dimatix DMP 2835) with a jetting frequency of 1 kHz. The drop spacing was set to 35 μm, and the platen temperature was fixed at 28 °C. The dimensions of the MTI alumina substrate were 10 mm in length by 10 mm in width and 0.5 mm in height. The substrates were prepared in an ultrasonic bath (Branson 200) with DI water, acetone and ethanol rinsing, nitrogen blow drying, and ozone (FPV Ultra Heavy Duty 10,000 mg/h generator) treatment for 5 min. Figure 1 shows the fabrication and testing methodology adopted; Figure 1a shows an illustration depicting the substrate preparation in the ultrasonic bath.

Figure 1b shows details about the substrate washing and wiping as part of the pre-processing step. Figure 1c shows the printing or fabrication step for the Au film. Figure 1d shows the post-processing step to sinter the sample, and Figure 1e shows the experimental setup used to characterize the electrical behavior of the thin film at the different environmental conditions typically used in the qualification of equipment for applications in avionics.

The fabrication process took place at ambient conditions. Post-processing was performed using a Stony Lab benchtop vacuum oven at pressure of 0.08 MPa at 250 °C for one hour.

Figure 2 shows the microstructure analysis of the printed film, performed by an SEM (JEOL JCM-7000 NeoScope) using secondary electron imaging with an accelerating voltage of 15 kV for a top view of the printed film. Figure 2a,b show surface SEM images of three inkjet-printed Au nanoparticle layers deposited on the alumina substrate, sintered at 250 °C for 1 h. The microstructure reveals loosely bound and distinguishable nanoparticles, exhibiting a porous network across the film. This porous nature is commonly attributed to the removal of organic binders or solvents during the sintering process, which creates voids as the nanoparticles coalesce and partially merge. Such structural features are consistent with previously observed behaviors in nanoparticle-based films sintered at moderate temperatures [12]. At higher sintering temperatures, nanoparticles undergo enhanced densification and exhibit increased necking [13,14].

Figure 2 shows the SEM images captured after sintering and before performing the environmental test on the samples. The images depict the microstructure at $1 \mathsf{\mu }\mathrm{m}$ and $10 \mathsf{\mu }\mathrm{m}$.

2.2. Experimental Setup

The integration between thin films and conventional circuits is challenging, given that soldering methods can deteriorate the region of the film where the solder is applied. Therefore, the integration between the data acquisition equipment and the printed device was accomplished by mounting the alumina substrate on an industrial prototype board. One side of a silver metal wire (0.003″ diameter, from A-M Systems, Inc., Washington, DC, USA) was soldered to conductor traces on the board. The other side of the wire was connected to the film using an Indium metal paste (99.99% purity, from Indium Corporation, New York, NY, USA). The Indium paste was applied on the film, covering the silver wire, at room conditions.

Temperature-dependent measurements of the film’s resistance were made using the four-wire kelvin method with a National Instruments DAQ system (NI CDAQ 9184) in an environmental chamber (ESPEC).

Table 1. The devices and equipment that were used to measure the temperature and resistance of the Au film.

Equipment/Device	Model
Power Supply (DC)	NI PS-15
Data Acquisition System	CDAQ 9184
Voltage Input	NI-9215
Current Output	NI-9265
Temperature Input Module	NI-9213
Thermocouples	Type-E (Thermo Sensor®)
Electrode Material	Indium
Integrating	Ag Wire
Wiring	Belden 8441 Twisted pair Shielded wire

shows the equipment and devices utilized to characterize the Au film with respect to changes in temperature using the four-probe method.

Figure 3 shows an illustration depicting the experimental configuration used to test the Au thin film. As Figure 1e shows, the thin film is placed inside the environmental chamber. Temperature measurements are recorded every second (1 Hz). A steady and fixed electrical current of 10 mA flows though the film while the differential voltage across the device is measured at the same sample rate previously specified. Table 1 shows the devices and equipment used to perform the test.

3. Environmental Test Procedure for Airborne Equipment

The Federal Aviation Administration (FAA) relies on guidelines and standards like the DO-160, developed by the RTCA for the performance evaluation of avionic components in various environmental conditions. The environmental temperature conditions suggested by the DO-160 specify a series of minimum environmental test conditions (categories) for airborne equipment. The goal of the standard is to provide a laboratory method to qualify the performance of devices and equipment for different categories. This work focuses on testing and characterizing a thin film fabricated using Au NPs following three of the test procedures suggested by the DO-160 standard [15]. The parameters, set points, and conditions selected for the test comply with aircrafts in category C1, or equipment intended for installation in a pressurized or non-pressurized, but controlled-temperature location in an aircraft that is operated at altitudes up to 35,000 ft. Equipment and devices that comply with this category can be used in the cockpit, pressure vessel, and cargo areas.

•  
  • Test 1. Operating at High Temperature Test.

To ensure that avionics equipment can continue to operate safely and effectively in the event of a cooling system failure, the Operating at High Temperature Test evaluates the performance of avionics equipment under conditions where the cooling systems fail during flight. Figure 4 shows the test parameters for this test.

The goal of this test is to assess how avionics equipment behaves under conditions where cooling is lost during flight for 90 min or more.

Table 2. Operating at High Temperature Test.

Temperature Change	Notes
$T_0 to T_1$	Temperature changes not specified
$T_1 to T_2$	Time for equipment temperature to stabilize
$T_2 to T_3$	$90$ min minimum
$T_0$	Ambient
$T_1=T_2=T_3$	$55 °\mathrm{C}$

shows the conditions for the Operating at High Temperature Test.

•  
  • Test 2. Ground Survival High Temperature Test

Table 3. Ground Survival High Temperature Test.

Temperature Change	Notes
$T_0 to T_1$	Temperature changes not specified
$T_1 to T_2$	Time for equipment temperature to stabilize
$T_2 to T_3$	$2$ h minimum
$T_0$	Ambient
$T_1=T_2=T_3$	$85 °\mathrm{C}$

shows the conditions to perform the Ground Survival High Temperature Test. The goal of this test is to assess how avionics equipment and devices behave under environmental conditions where active cooling is not present. For this test, the initial temperature changes from the ambient conditions to a maximum temperature, depending on the level of qualification needed for the device of interest.

Figure 5 shows the temperature setpoint and step times required for the test. As shown in the table, the temperature gradient from $T_0$ to $T_1$ is not specified. Once the temperature stabilizes in the environmental chamber, the devices are tested for at least 2 h.

•  
  • Test 3. Ground Survival High Temperature and Short Temperature Test

Table 4. Ground Survival High Temperature and Short Temperature Test parameters.

Temperature Change	Note
$T_0 to T_1$	Temperature changes not specified
$T_1 to T_2$	Time for temperature stabilization plus a minimum of 2 h.
$T_2 to T_3$	$\mathrm{Minimum} \mathrm{rate} \mathrm{of} 2{\displaystyle \frac{\mathrm{℃}}{\mathrm{m}\mathrm{i}\mathrm{n}}}$
$T_3$	$30\pm 5$ min or time for stabilization and $30$ min minimum
$T_0$	Ambient
$T_1=T_2$	$85 °\mathrm{C}$
$T_3$	$55 °\mathrm{C}$

shows the conditions for the Ground Survival High Temperature and Short Temperature Test. This test is designed to characterize the performance of devices used in aircrafts at extreme conditions.

Figure 5 shows the temperature setpoint and step times required for the Ground Survival High Temperature and Short Temperature Test. The temperature gradient from $T_0$ to $T_1$ is not specified. Once the temperature stabilizes in the environmental chamber, the device is tested for at least 3 h. From $T_2$ to $T_3$, the minimum temperature rate of change is specified to $2{\displaystyle \frac{\mathrm{℃}}{\mathrm{m}\mathrm{i}\mathrm{n}}}$. The time from $T_3$ to $T_4$ is set to at least 30 min for equipment stabilization, and the test time from $T_4$ to $T_5$ is 30 min or more.

4. Results

Figure 6 shows the results obtained for the Operating at High Temperature Test. The total length of the test was 1.8 h. The figure shows that at $15 °\mathrm{C}$, the measured resistance was $0.67 \mathrm{\Omega }.$ When the temperature reached the high temperature point at $55 °\mathrm{C}$, the measured resistance was $0.78 \mathrm{\Omega }.$

Figure 7 shows the measured resistance of the Au film for the Ground Survival High Temperature Test (Test 2). The total test time was 2.9 h. The figure shows that the measured resistance was around $0.68 \mathrm{\Omega }$ at $15 °\mathrm{C}$ and increased to $0.83 \mathrm{\Omega }$ when the temperature reached $85 °\mathrm{C}$. Interestingly, the resistance variation was less than $5e-3\mathrm{\Omega }$ once the temperature stabilized at $85 °\mathrm{C}$.

Figure 8 shows the measured electrical resistance of the Au film for the Ground Survival High Temperature and Short Temperature Test (Test 3). The total test time was 9.9 h. The measured resistance at $18 °\mathrm{C}$ was $0.67 \mathrm{\Omega }$ and increased to $0.822 \mathrm{\Omega }$ when the temperature reached $85 °\mathrm{C}$. After 2 h, the temperature was set to drop to $55 °\mathrm{C}$, and the measured resistance read as $0.79 \mathrm{\Omega }.$

The TCR of a thin film depends on the difference between the thermal coefficients of the film and the substrate [16]. The expression for the TCR is shown in Equation (1).

$$TCR(T,R)={\displaystyle \frac{R-R_0}{R_0·\left(T-T_0\right)}}$$

(1)

where $T$ is the measured temperature, $T_0$ is the reference temperature, $R$ is the measured resistance as a function of the temperature, and $R_0$ is the reference resistance measured at $T_0$.

Figure 9 shows the measured resistance as a function of the temperature. The figure shows that the TCR obtained experimentally in the temperature range of $15$ to $85 (°\mathrm{C})$ was $2.7\times {10}^{-3}{(°\mathrm{C}}^{-1})$, which is in line with the TCR values reported in the literature [17,18,19,20].

Figure 10 shows the SEM images captured after performing the environmental tests on the samples. The images depict the microstructure at $1 \mathsf{\mu }\mathrm{m}$ and $10 \mathsf{\mu }\mathrm{m}$. Compared to Figure 2, the microstructure does not show any deterioration or significant changes.

5. Conclusions

The experimental characterization of a thin film manufactured with the drop-on-demand methodology using gold nanoparticles deposited on an alumina substrate was explored under the environmental testing conditions aligned with the avionics industry standards. The measured temperature coefficient of resistance (TCR) was 2.7 × 10⁻³ °C⁻¹, which corroborates the values reported in the literature corresponding to gold thin films fabricated using additively manufactured techniques. A key contribution of this work is in the utilization of qualification test procedures and standards used in the avionics industry to assess the reliability and performance of printed nanoparticle-based films.. The results show that the fabrication methodology and the integration of gold nanoparticle films onto alumina substrates are compatible with the other circuits and instruments. The maximum electric resistance variation of the thin film during the test sequences for more than 13 h was less than $5\times {10}^{-3} \mathrm{\Omega }$, demonstrating the strong potential of this technology to be used in aerospace and other high-reliability engineering applications, considering that the device was integrated into an industrial data acquisition system and connected using typical conductor wires.