Wear reliability and failure mechanism of inkjet-printed conductors on paperboard substrate

In this research, we conducted a wear test on inkjet-printed silver conductors using different loads and counter materials (two paperboards, brushed steel sheet, and sandpaper) with similar surface roughness values. The conductor’s reliability was characterized by resistance measurement, the failures and tested counter materials were analyzed using an optical microscope, profilometer, scanning electron microscope, and energy dispersive spectrometer. It was found that the counter material has a dominant impact on a conductor’s reliability and failure mechanism compared with load. The conductors were exceptionally reliable but subject to adhesive wear when tested by paperboards. They were also highly reliable when tested by brushed steel sheet although the silver became severely detached, and the conductivity was lost suddenly when a major scratch was caused by two-body and three-body abrasive wear mechanisms. Sandpaper rubbing caused the most severe silver detachment and quick loss of conductivity, as a large amount of small-size (5–15 µm) silicon carbide particles with sharp edges and corners caused a dense cutting effect via two-body abrasive wear (by cutting) mechanism. Additionally, the failures in almost all samples occurred in the areas in contact with the counter edges, suggesting that failure was accelerated by the edge effect. This study proves that inkjet-printed electronics on the investigated paperboard is exceptionally durable when rubbed by paperboards and steel sheets, and thus provides a reliable solution to intelligent packaging. To promote intelligent packaging, more paperboards, as well as approaches for reducing the edge effect can be investigated.


Introduction
The Internet of Things (IoT) is a rapidly growing technology that has demonstrated a promising future by improving quality of life for a wide range of sectors [1]. It has also drastically contributed to the realization of Industry 4.0, which enables physical assets to be integrated into digital and physical processes and creates smart manufacturing environments [2]. One of the main IoT application domains is logistics as it offers intelligent transportation and logistics system with the ubiquitous 5G mobile network [2].
Smart packaging enables smart logistics by extending shelf life, monitoring freshness, displaying information of products, and improving product and customer safety [3]. Additionally, Pacquit et al [4] also identified that smart packaging can potentially be used to identify supply chain inefficiencies, lower costs, and reduce errors. Smart packaging is also a rapidly growing market, with the biggest markets being USA, Japan, Australia, Germany, and UK [3]. It is especially expected to benefit the logistics of food, which is crucially related to human health. From this perspective, smart packaging can reduce food and economic waste, and improve health significantly [5,6].
However, smart packaging has not been widely applied in the market yet. One reason is the high cost of development and production, which is rooted in manufacturing challenges [7]. Another issue is the recyclability of packaging materials, due to which nature-derived and antimicrobial agents and biodegradable solutions are needed [3,8]. Vanderroost et al [9] reported that thin film electronics can be integrated into printed and flexible systems for monitoring products, but the durability of performance remains a challenge. Kuswandi et al [10] also reported that the reliability and robustness of smart packaging systems has been a crucial issue preventing its wide application. It has been claimed that the reliability of smart packaging throughout its lifecycle still needs further research [3].
Paper and paperboard have been widely considered as one of the most suitable materials for intelligent packaging due to its environmentally friendly nature, availability from nature, light weight, low cost, ease of recycling, and sustainability [11]. However, plain paper is insufficient for food packaging due to poor barrier properties, low heat sealability and strength [11]. Being laminated with aluminum, or polymer film, or employing some additives are the common approaches to improving its properties. For example, Leminen et al [12] reported that adding a barrier layer to paperboard trays in the forming process can achieve bonding, and it can be improved by adding additives to the paperboard. A spray coating on cardboard has also been reported to improve durability and reduce water evaporation [13]. In this study, package paperboard was coated to make a smooth surface, making it suitable for high-quality printing, which is used in premium packaging (e.g. confectionery, pharmaceuticals, cosmetics).
Printing technologies have demonstrated their functionality for presenting information on paper and paperboard, and the printing of nanoparticle silver ink has shown good printability on paper-based substrates [14]. Among printing methods, inkjet printing is a promising method for fabricating electronic systems on packaging paperboard due to its design freedom, compatibility with a wide range of printable solutions, reduced manual work, and sharp details and strict lines of printed structures [15]. It has been utilized to print antennas, sensors, and circuits on paper substrates with reliable performance [16]. Xie et al [17] proved the high stability and electrical performance of inkjet-printed nanoparticle silver ink on paper substrates under environmental impact. Ölund et al [18] also reported that inkjet-printed nanoparticle silver ink on paper substrates can demonstrate high performance. However, the reliability of such inkjet-printed nanoparticle silver ink on a paper substrate has rarely been studied.
Considering the application scenarios, a printed system on a package surface can be frequently subjected to rubbing during packaging, transportation and moving from shelves. Therefore, in this study, we investigate the reliability and failure mechanism of inkjet-printed nanoparticle silver ink conductors on packaging paperboard using wear tests. Since the surface characteristics, especially the porosity and surface roughness, of a paper material are the most crucial factors impacting the electrical performance of printed conductors on the paper substrate [18], we selected coated packaging paperboards with a smooth surface. We aim at investigating (1) the conductor's reliability under rubbing of dissimilar materials and loads; (2) the dominant location and mechanisms of failure under rubbing; and (3) the dominant factors impacting the conductor's reliability and failure. Based on these, guidelines for improved reliability against wear can be given.

Substrate and counter materials
The Aegle White 290 paperboard was used as substrate for the printed conductors. It is a fully coated Solid Bleached Sulfate (SBS) paperboard with a threelayer fiber construction consisting of bleached chemithermomechanical pulp, two bleached chemical pulp outer layers, and a single bottom layer coating [19]. It is a biodegradable and renewable boxboard material that can be used as premium packaging due to its good printability. Reported by Kohtaniemi [20], its coefficient of kinetic friction is 0.31 in both the machine direction and cross direction, and the good printability with the super small ink spread of Silverjet DGP-40LT-15C silver nanoparticle (AgNP) ink on this paperboard had been confirmed.
In this study, two SBS paperboard materials-Aegle White 290 (Kotkamills Oy, Kotka, Finland) and SE Trayforma (Stora Enso, Helsinki, Finland)brushed steel sheet, and sandpaper (ECOWET P2000, Mirka Ltd, Jeppo, Finland), were used as counter materials. The counter material was fixed onto the counter surface, and it was moved to rub against the printed conductors in the wear test. The rubbing by paperboards simulates a situation where the packages are on top of another during transportation. The metal sheet rubbing simulates a scenario where the package is moved from metal or wood shelves. Sandpaper rubbing simulates more demanding conditions during package life. Since average surface roughness (Ra) has a significant impact on the abrasive effect, the same level of Ra values was emphasized when selecting counter materials to minimize the impact of the Ra of the counter material on the reliability and failure of the tested conductors. The Ra values of the selected counter materials as measured by a Keyence 3D profilometer are presented in table 1. Based on the datasheet, the tolerance of Ra for the printing side of the Aegle White 290 paperboard is 1.6 µm at most, and therefore, the Ra of the Aegle White 290 paperboard used meets the requirement.

Sample fabrication
The pattern of the printed conductor was developed to fit the wear tester to enable convenient failure inspection and 4-wire resistance measurement. Therefore, a simple conductor with 4 measurement pads and a straight line to accommodate the 100 mmwide counter of the test device was designed, as presented in figure 1.
The experiment samples were fabricated following these steps: (1) the flexible paperboard substrate Aegle White 290 (KotkaMills, Kotka, Finland) was cleaned by an air gun before printing. (2) Its thickness was measured to be around 430 µm (420-440 µm) by Mitutoyo ABSOLUTE Digimatic ID-C standardtype indicators (No. 543-250B), the resolution of which is 1 µm. Thus, the substrate thickness was set at 430 µm for ink printing. (3) The AgNP ink (Silverjet DGP-40LT-15C, Sigma-Aldrich) was printed to form two conductors onto the substrate by using a FUJIFILM Dimatix 2850 inkjet printer with optimized printing parameters. The drop spacing of printing was 15 µm (the corresponding resolution of the print being 1693.33 dpi), and two layers of ink were printed. The platen temperature was 35 • C, and the ink was printed through a Dimatix Samba printhead, which has a 75 dpi native resolution and a 2.4 picoliter (pL) native drop volume. The cartridge settings (nozzle temperature 63 • C, voltage 38.00 V, meniscus setpoint 5.0, cartridge print height 600 µm) were determined to optimize the quality of ink jetting and the printed conductor. The waveform used for printing is presented in figure 2. The maximum jetting frequency was 5.0 kHz. (4) Afterwards, the printed samples were annealed at 130 • C for 1 h in a laboratory oven. The thickness of the cured silver conductor line was measured by cross section imaging using SEM, the average value is 0.87 µm (range: 0.3-1.3 µm, median value: 0.83 µm, N = 47). An example is presented in figure 3.

Wear test
The wear test was conducted using the wear testing device as presented in figure 4(A). Different materials-Aegle White 290 paperboard, SE Trayforma paperboard, brushed steel sheet, and 2000 grit sandpaper, were fixed to the 100 × 100 mm size counter surface of the tester using double-sided tape (shown in figure 4(B)) to rub against the printed conductors repeatedly. An electric motor, together with a crank mechanism, was used to create a reciprocating motion of the sample against the counter material. The travel distance of one cycle was 20 mm (10 mm in one direction), the speed of sample movement was 20 mm s −1 , thus, one cycle of test took one second. The sample's resistance before and after test was measured by a 4-wire measurement method using a Keysight 34461A 6.5 Digital Multimeter, as demonstrated in figure 4(C). The test was conducted in a 50% RH and 23 • C environments.
Two conductor samples were tested at the same time, as they were printed on the same substrate. The counter materials and load applied to the counter were the main variables in the wear tests. The time interval of each test was decided based on changes occurring on the sample's surface and electrical resistance. The wear test was conducted as follows: (1) it started with the #1 and #2 samples using Aegle White 290 paperboard counter material and low load (0.85 kg). However, almost no change occurred in their resistance after 25 min test. (2) Then, a higher load (3.24 kg) was used with the same counter material for testing the #3 and #4 samples with 30 min test interval until one of the samples failed electrically. (3) Since SBS paperboard is a popular material for packaging, another SBS paperboard material-SE Trayforma paperboard, was used with the same high load (3.24 kg) to test the #5 and #6 samples with a 5 min test interval. No electrical degradation occurred after a 10 min test, which guided us to use the rougher side of the paperboard as the counter material to test #7 and #8 samples with a 3.24 kg load. (4) To simulate the scenario of moving packages from a metal shelf, a brushed steel sheet was used to test the #9 and #10 and #11 and #12 samples using 0.85 kg and 3.24 kg, respectively, until a clear increase in resistance or a loss of conductivity occurred. (5) Considering the even harsher condition a package surface can experience in the packaging process, a 2000 grit sandpaper was then used to test the #13 and #14 samples with a medium load (2.12 kg), and both samples failed immediately (in 1 min test). (6) Thus, we tested another two (#15 and #16) samples using a 2000 grit sandpaper with lower load (0.85 kg) until the failure occurred.

Characterization
The initial resistance, surface quality and width of the printed lines were measured to characterize the sample quality. The width of the printed conductors was measured using an Olympus BX60M optical microscope and Olympus cellSens Entry 2.2 software. The average value of the width values measured on the left, middle, and right sides of the conductor was recorded. After the wear test, the conductor's resistance was measured by a 4-wire measurement method using a Keysight 34461A 6.5 Digital Multimeter, and the surface conditions of the tested samples were checked visually and by optical microscope. A further examination of the failures of the selected samples and counter materials was performed using a field emission scanning electron microscope (FESEM) (Zeiss ULTRAplus, Oberkochen, Germany) equipped with an energy dispersive spectrometer (EDS) (Oxford Instruments X-MaxN 80). The examined areas were cut from the selected samples and attached to an aluminum scanning electron microscope (SEM) stub by carbon glue. They were carbon-coated using a sputter coater (Leica EM ACE600, Wetzlar, Germany) before FESEM imaging to avoid sample charging during the characterization. The studied areas were also imaged with an optical profilometer (Alicona InfiniteFocus G5, Raaba, Austria). The EDS analysis was conducted on different areas of the samples by elemental spectra and maps and was processed with AZtec 6.0 software (Oxford Instrument Nanotechnology Tools Limited).

Quality of initial samples
By optical microscope and SEM observation, the surfaces of the printed AgNP conductors were highly homogeneous with fine structure, as presented in figure 5(A). The widths of 15/16 printed conductors were subjected to less than 2% variation, ranging from 1040 to 1065 µm with a mean of 1055 µm and a standard deviation of 14.24, as presented in figure 5(B). The width of the printed AgNP conductors exhibited an exceptionally low level of variation, which indicates the high consistency of the samples' geometry and high printing quality. The resistance values of 13/16 conductor samples were within 20-40 Ω, and all samples' resistance ranged within 20-60 Ω with a mean of 35.0 Ω and a standard deviation of 9.31, as presented in figure 5(C). Additionally, the resistance of the samples measured before the test were almost identical (8/16 samples were subject to ±0.1 Ω difference, 8/16 samples' resistance were identical) to the resistance measured just after sintering at another laboratory around 3 weeks earlier. This demonstrates the high stability of the samples' resistance over time. The variation of the initial resistance of printed conductors can be caused by a difference in the duration of printing because the conductors exhibited lower resistance after sintering when the ink had been exposed to the air for a shorter time. This was also confirmed by the phenomenon that the second conductor printed on the same paper substrate usually exhibited slightly (1-2 Ω) higher resistance than the first conductor.

Resistance evolution of conductors
The conductors' 4-wire resistance was measured after each test, and the results are presented in figure 6. The rubbing of both paperboards, including the rougher side of the Trayforma paperboard, caused negligible change to the resistance (up to 1.32%) of the conductors, as presented in figure 6(A). The conductors were exceptionally durable when tested by paperboards. In comparison, when tested by the brushed steel sheet, the conductors' resistance increased more although they were also quite durable electrically. For example, the resistance of sample #10 increased rapidly after a 5 min test and stabilized at around 23% resistance increase after a 15 min test, as presented in figure 6(B). The long-term stable conductivity is explained further in section 3.3. Under the high load (3.24 kg) wear test, the resistance of sample #11 increased 5% steadily at a slower rate, until a mechanical disconnection occurred in the conductor and caused loss of conductivity, which is confirmed by figure 8(E). Sandpaper caused overwhelming degradation on the conductivity almost immediately, so that conductivity was lost in a 1 min test, even under 0.85 kg load, as presented with dash lines in figure 6(B). All four sandpaper samples (#13-16) experienced this loss of conductivity without exception. The mechanism causing this is explained in section 3.3.
Since it is highly likely that the same package material is used in smart packaging, in practice rubbing would happen between two surfaces of the same material. Thus, to gain a further understanding of the evolution of the resistance and failure of the conductor, a longer-term wear test with Aegle White 290 paper with higher load (3.24 kg) was applied to the #3 and #4 samples with half-hour interval until failure occurred. Resistance was measured after each test. It was observed that the resistance of conductor #4 increased slowly in the first 7 h (just around a 1.4% increase), after which the resistance increased more progressively (around 19.4% increase after a 22.5 h wear test) until the conductor #4 lost conductivity, as presented in figure 7. The evolution of resistance of conductor #3 behaved similarly.

Failure mechanism
To reveal the mechanisms of failure that occurred during the wear test, the tested samples were first checked by an optical microscope. The silver detachment was clearly visible in the tested conductors. However, it varied based on the different counter materials. The rubbing of both paperboards caused similar and moderate detachment of silver, although vertical rubbing signs and scratches were common. That was the reason for the almost unchanged resistance. Although there could be some large areas of silver detachment, the bottom of the conductors remained, and the conduction pathway was not completely destroyed. This is confirmed by figures 8(B) and (C), in which the surface of the silver conductor was removed due to adhesive wear, but the substrate is not visible as it was covered well by an undamaged bottom layer of ink conductor. Some areas had even been damaged to a certain level, but the conduction pathway remained, as presented in figure 8(A). The tearing of the silver conductors was caused by gradual flaking of cracks due to fatigue wear under cyclic loading and releasing. Similar fatigue crack and wear has been reported by Liu et al [21] and Igwemezie et al [22] When tested by paperboards, the 3.24 kg load did not cause significant impact to the physical failure of the ink, as shown in figure 8(C), and definitely did not impact the conductivity of the conductors compared with 0.85 kg load.
However, the load caused a higher impact to the conductor's surface when tested by the brushed steel sheet. When tested with the 0.85 kg load, even the most severely damaged area was not detached completely, meaning that the bottom part of the conductor remained, and most areas did not significantly detach, of which an example is presented in figure 8(D). In comparison, when tested under 3.24 kg load, severe scratches were more commonly shown, as presented in figure 9(E). The steel sheet had been brushed through friction and followed by softening. It retained exceptionally fine lines in the direction of brushing and grooves in between, as presented in figure 9(C). The silver conductor is softer than the steel, and the high load created higher compression onto the silver conductor via the fine lines, which explains the more severe silver detachment. Besides, wire brushing has been reported to increase the microhardness of a surface layer [23]. When tested under 3.24 kg load, the compressed steel plate would strengthen the two-body abrasive wear when move against the soft silver [24]. This sign of failure which is caused or strengthened by compression is clearly  figure 8(D). Some studies [25,26] of abrasive wear surface have found a similar morphology. Gahr [27] also reported that such abrasive wear can be caused by hard particles sliding on a softer solid surface which can lead to detaching material.
The zigzag-shape failure that commonly occurred on the samples tested by the brushed steel sheet is presented in figures 9(D) and (E). This was highly likely to be caused by crossed brushed lines, which led to zigzag-shape steel lines, as presented in figure 9(C). This was because the brushing was conducted in different directions regarding the steel sheet, which created lines that crossed against each other with different angles. This was shown to be common across brushed steel sheet. For example, some zigzag-shape failure occurred with curved edges, as presented in figures 8(D) and (F). This was due to the steel debris with curved edges on the steel surface, an example of which is presented in figure 9(C). This is typical two-body abrasive wear [28,29]. Such debris could have been created by the brushing treatment. The high hardness and strength would enable them to compress and cut the softer silver conductor when rubbed under load. In addition, some small-size debris and embedded particles can flake off during cyclic rubbing, which can lead to three-body abrasive wear to the silver conductor, as presented in figure 9(G), which demonstrates intensive signs of particle ploughing and rolling as well as pits. Similar morphology caused by three-body abrasive wear in metal has been reported by Huq et al [30]. The variation in surface height and roughness is also a factor for accelerating the moving of flaked particles between the steel sheet and conductor. As shown in figure 9(D), the middle of the steel plate was around 60 µm higher than the two sides, which can cause some vibration of the plate and compression of particles between the steel plate and conductor, accelerating the three-body abrasive wear.
The rubbing of sandpaper caused the most severe detachment of the conductor in the shortest time compared with other counter materials, which led to loss of conductivity even without a distinct scratch as was shown with the other samples. An example is presented in figure 8(H). There is no dominant scratch, but a large area of the conductor has experienced severe detachment of silver ink, and even the bottom of the ink has been removed, which has caused disruption of conduction pathways. With further examination using SEM, a very dense and strong cutting effect is visible on the conductors tested by sandpaper, as presented in figure 8(I). Additionally, scratches with cutting angles were commonly observed, as presented in figure 8(J). This is due to the high density of small-size (ranged 5-15 µm) particles with sharp edges, as presented in figure 8(K). Some cracks in the sandpaper were observed, and they developed along the edges of the sandpaper particles, as the examples in figures 8(K) and (L) show. This, on the other hand, also proves that the sharp edges and corners of the sandpaper particles moved under compression during the wear test. It is worthy of concern that due to the working principle of SEM and 15 kV voltage used, the information obtained about the sandpaper particles was generated based on the reflection of secondary electrons from around 1.9 µm below the surface, and information of parts below this depth could not be detected. Therefore, the size of some sandpaper particles can be slightly larger, for example 5-20 µm. The sand particle size meets the requirements of the Federation of European Producers of Abrasives standard (43-GB-1984, R 1993) [31], which defines the mean size to be 10.3 ± 0.80 µm. Zhang et al [32] reported comparable results about the size (5-20 µm), sharp edges, and surface roughness (3.2 ± 0.2 µm) of 2000 grit silicon carbide (SiC) sandpaper particles.
The EDS analysis of the material of the sandpaper particles found that it consists of C, N, Si, O, and some other minor elements, as presented in figure 9(A). Based on the product information from the manufacturer Mikra Ltd [33], the particle of the used sandpaper can be either SiC or aluminum oxide. The EDS result shows that C and Si are dominant elements, whereas no aluminum element was found. Thus, the particle is confirmed to be SiC. SiC has the hardest, sharpest, and very narrow grains among common abrasives. The razor-sharp grains of SiC enable it to cut polymers and metals easily under light pressure. This explains the large amount of separated small pieces of silver in the tested conductor, as presented in figure 8(I). Meanwhile, the enormous number of cavities among the SiC particles can accommodate small silver pieces cut by the SiC and remove them during the repeated wear test. Many white parts were shown on the sandpaper after the test, as presented in figure 8(L). According to EDS analysis, these parts consisted of mainly C, O, Ca, N, and Sias presented in figure 9(B), which are partly from the coating material on the paperboard and partly from the sandpaper. Therefore, there is no doubt that the silver layer printed on the substrate must have been removed quite drastically. This was also observed in the optical image of figure 8(H).
A common phenomenon in almost all the tested samples is that the main scratches leading to the loss of conductivity and the most damaged areas occurred in similar locations, which correspond to the two edges of the counter, as demonstrated in figure 10(A). This was caused by the straight lines of the sample's edges, as the strength of the wooden counter together with the counter material is higher than that of the printed AgNP conductor. Specimen edges have been reported to lead to the increased wear rate [34]. Ratia-Hanby et al [35] conducted an abrasive test under load with three steels with different mechanical properties and found that edge-concentrated wear was dominant in all tested materials, claiming that the wear rates at the edge areas were several times higher than the inner parts, and that this was especially obvious in short tests with 90 • sample angle. In our wear tests, the counter materials meet this condition exactly, as presented in figure 4(B). Edge profiling with anti-wear coatings has been reported to be an effective approach for reducing stress concentration and wear rate at edges significantly [36]. In the tested paperboards, many long wrinkles are shown, and they were mostly initiated from the edge of the counter material, as presented in figure 10(B). Large and dense detached areas of silver were also commonly present near the edges of the counter paperboards, as presented in figure 10(B). Additionally, some very straight bar-shape detached silver, including deep scratches, were also found near the edges of the counter paperboards, as presented on figure 10(C). In comparison, in the middle area of the counter paperboards, only small pieces of detached silver were observed. The dense detached silver stuck onto the paperboards was caused by the wrinkles, as shown in figure 10(D). These distinct differences in silver detachment and scratches in the detached silver strongly prove that the edge effect posed a dominant influence on the failures.
To gain a deeper understanding of the evolution of the failure of the silver conductor during longerterm rubbing, a longer-time wear test was applied to two conductors with half an hour interval. After a 7 h test, most areas of the sample were just subjected to very minor or negligible silver detachment, but in some areas, significant detachment of silver can be observed, an example of which is presented in figure 11(B). The same area was subjected to significantly more severe detachment after 23.5 h of test ( figure 11(C)). This is consistent with the reliability results in figure 6 that the conductor's resistance increased just 1.4% after the first 7 h test and accelerated to a 19.4% increment after the 22.5 h test. Also, the conductivity was lost completely after the 23.5 h test. The continuous increase of the resistance was caused by the silver's continuous detachment. In the end, when the silver of one area was completely detached, the conduction pathway was damaged, leading to complete failure. During the 23.5 h test, sliding had been the dominant impact on most areas of the sample, as presented in figures 11(D) and (F). However, pure sliding or adhesion did not readily cause complete loss of conductivity since the conduction pathways remained. But the debris, and delamination of silver and small particles did cause cracks and local disruption to the conduction pathways, as presented in figures 11(E)-(G). The large amount of such issues plus the thinning effect of sliding was the main mechanism of major failure.

Outlook of future work
Overall, this study has demonstrated the high reliability of inkjet-printed conductors on packaging paperboard against rubbing. It paves a solid basis for the application of inkjet printing for smart packaging and proves the benefits of using biodegradable Aegle White 290 paperboard for packaging and labeling. The wear test results indicate that the impact of the counter material on the reliability of the conductor is stronger than the impact of counter load, although increasing the load can also accelerate the loss of conductivity. The same finding has also been reported by Kucuk [37] who demonstrated that the counter material has a significant effect on the wear behavior. Considering the entire system of the conductor to be under wear impact, these factors may influence the reliability and failure of printed conductors: (1) Counter material: in addition to the type of material investigated in this research, the impact of surface roughness and surface treatment is worthy of future investigation. (2) Combined impacts: in application scenario, packages in transportation are often subjected to a small range of rubbing and low level of vibration. The conductor's reliability and failure mechanism under a combined wear test and vibration test can be investigated in the future. In addition, the packages would also be subjected to environmental impacts, as this is often the case when packages are transported for long distances in different environments. (3) Paperboard substrate: this research reveals that the investigated paperboards are excellent materials for inkjet-printed conductors regarding wear reliability. It is worthy of investigation if other paperboards behave similarly or are subject to a different failure mechanism.
In addition, the high reliability of the 1 mm-wide conductors also indicates that it is worthy to investigate the reliability of even narrower conductors, as it can contribute to the miniaturization of electronics design and packaging.

Conclusion
In this study, we conducted wear tests to the nanoparticle silver conductors that were inkjet-printed on the flexible and biodegradable Aegle White 290 paperboard. The test was conducted using different counter/rubbing materials (two SBS paperboards, a 2000 grit sandpaper, and a brushed steel sheet) with the same level of Ra values (1.40-3.67 µm) under different loads (0.85 kg, 2.12 kg, and 3.24 kg). The test was conducted with certain time intervals depending on the reliability of the conductors, the resistance of the conductors after each test was measured to characterize the reliability, and failed conductors were imaged by optical microscope, optical profilometer, and SEM. It was found that the counter material had significantly higher impact on the conductor's reliability and failure mechanism than the load applied, although the higher load also led to the increased resistance.
(1) The conductors were exceptionally reliable when rubbed by the two paperboards with negligible change of resistance even though they had a similar and moderate level of silver detachment through the mechanism of adhesive wear. The 3.24 kg load had a minor impact on the conductor's reliability and failure compared with the 0.85 kg load. (2) When rubbed by the brushed steel sheet, the conductor's resistance did not increase significantly even though the silver conductor had been severely detached, because the silver conduction pathways remained. The conductor lost conductivity suddenly when a major scratch caused a disruption of the conduction pathway. A unique zigzag-shape failure was commonly observed, which was caused by brushing in different directions by the steel sheet. It also caused the flaking of steel particles in the cyclic wear test and led to two-body and three-body abrasive wear mechanisms. The 3.24 kg load accelerated the silver detachment and loss compared to the 0.85 kg load. (3) The rubbing of 2000 grit sandpaper led to the most severe silver detachment and loss of conductivity without a major scratch in a truly short time compared to the paperboards and steel sheet. This is due to the sharp edges and corners of the small-size (5-15 µm) SiC particles, which enabled them to cut and remove the silver conductor quickly through two-body abrasive wear (or abrasive wear by cutting) mechanism. Even when tested under a small load like 0.85 kg, the conductor was damaged very quickly, and the increased load did not exhibit increased impact.
In addition, the main scratched or damaged areas in almost all the tested samples occurred in the same locations, which correspond to the two edges of the counter and counter material. The edge effect caused increased wear rate and accelerated failure in those locations.
Overall, this study has demonstrated the high reliability of inkjet-printed electronics on biodegradable paperboard against rubbing of paperboard and metal sheet and proved the reliability benefit of using paperboard for smart packaging. The inkjet printing and paperboards have a high potential to enable smart packaging. For future work, the impact of vibration, temperature, humidity, their combined impacts, other paperboard materials with different surface properties, and narrower conductors can be investigated.

Data availability statement
The data that support the findings of this study are openly available.