Highly Conductive Ag Paste for Recoverable Wiring and Reliable

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Functional Inorganic Materials and Devices

Highly Conductive Ag Paste for Recoverable Wiring and Reliable Bonding Used in Stretchable Electronics Cai-Fu Li, Wanli Li, Hao Zhang, Jinting Jiu, Yang Yang, Lingying Li, Yue Gao, Zhi-Quan Liu, and Katsuaki Suganuma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19069 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Materials & Interfaces

Highly Conductive Ag Paste for Recoverable Wiring and Reliable Bonding Used in Stretchable Electronics

Cai-Fu Li, a, Wanli Li, a,b Hao Zhang, a Jinting Jiu, a,c Yang Yang, a,d Lingying Li, a,b Yue Gao, a,b

Zhi-Quan Liu, a,e and Katsuaki Suganuma a

a

The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Japan

b

Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka

University, Suita, Japan c

Senju Metal Industry Co., Ltd., Tokyo, Japan

d

Pacific Northwest National Laboratory, Washington, USA

e

Institute of Metal Research, Chinses Academy of Sciences, Shenyang, China

*Corresponding author, Tel: +81 06 6879 8521; e-mail: [email protected]

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Abstract Stretchable wiring and stretchable bonding between a rigid chip/component and a stretchable substrate are two key factors for stretchable electronics. In this study, a highly conductive stretchable paste has been developed with commercial Ag micro flakes and PDMS which can be used to fabricate stretchable wirings and bondings under a low curing temperature of 100 C with printing method. Herein recoverabilities as to recovery time and recovery resistance of the wirings are defined and discussed. The effect of Ag composition and the tensile strain rate on the recoverability of the wirings are also examined. The wiring with a low resistivity of 8.7  10-5 cm shows much better recoverability than nanowire-based wirings due to the flake nature of the Ag particles. When stretched to 50% and 100% of strain, the resistance of the patterned wiring increases only 10% and 110%, respectively. Moreover, the resistance of the wiring during 20% tensile cyclic test remains within 1.1 times even after 1000 cycles, thus demostrating significat durability. The paste was utilized to fabricate conductive tracks and stretchable bondings to assemble a rigid chip to fabricate a stretchable demo. When stretched to 50% of strain, resistance of the wiring was increased 90%. It is anticipated that the newly developed paste will be used to fabricate various stretchable wirings, bondings and packaging structures by a simple printing process, thus enabling mass production of stretchable electronic devices.

Keywords: conductive pastes, stretchable wirings, stretchable bonding, Ag micro flakes, recoverability


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1. Introduction Stretchable electronics have gained wide applications in various research areas such as artificial skins,1 bio-electronic 2 and healthcare 3 as well as other wearable devices.4, 5 Stretchable wiring and stretchable bonding are two of the most important factors for realizing stretchable electronics.6, 7 Stretchable wirings can be prepared mainly by two methods: structure design and new material development.8-10 Also these two approaches can be combined. In the first approach, the geometric patterns of the non-stretchable materials are modified into wavy, wrinkled or net configurations to fabricate stretchable wirings without inducing significant strains in the materials themselves.11, 12 On the other hand, new stretchable materials such as liquid metals,13, 14 conductive polymers,15 ionic conductors,16,

17

and composite conductive materials

18, 19

are being developed for

stretchable wirings. Amongst them, composite conductor composed of elastomer and conductive fillers is the most popular candidate for conductive wirings due to its simple preparation

process.

24/nanoparticles25/micro

Carbon

nanotubes,20

graphene,21,

22

and

metal

nanowire23-

flakes26, 27 are commonly used filler materials to prepare stretchable

conductors. Metal based, especially Ag-based, stretchable conductors exhibit much higher conductivity. Silver nano particles(AgNPs) and nanowires (AgNWs) have been widely adopted to fabricate stretchable wirings which show low resistivity and small resistance fluctuation when stretched.24,

28

However high cost AgNP and AgNW wirings have not been entirely

successful demonstrating longtime reliability.29 Recently, silver flakes have been utilized as a filler for the stretchable conductor30 and it was found that the resulting resistivity was as low as 2.010-4 cm.31,

32

Therefore, Ag flake-based stretchable conductor has drawn significant

recent attention for its excellent printability and performances, as well as for its relatively low cost compared with the Ag nano materials.



While improvements have been made, it remains a fact that all of today’s stretchable wirings will reflect a increase in resistance when stretched. It widely known that a large increase in resistance can cause problems to electronic devices,33, 34 hence the ability to keep the resistance unchanged during stretching is an important objective for stretchable wirings. To data, it has been a great challenge to obtain the small fluctuation standard required for long term stability (for example: fluctuation within 10% after more than 1000 stretching and releasing cycles). while, the resistance of the stretchable conductor will recover with time going on when the strain is released,27 the electronics can only endure resistance change if the resistance can recover to its original value within a very short time. Otherwise it could prove disasterous to the device. Therefore, realizing a short time period for recovery is another vital challenge for further development of stretchable conductors and devices. In the case of some stretchable conductors, such as conductors prepared from AgNWs, they are unable to recover to their original value any more after the strain has been released.24, 28, 35, 36 This unrecoverable property greatly hinders their applications. Moreover, the above mentioned recoverabilities of the stretchable conductor require systematic study for its real application in wearable electronics.3739

In addition to the stretchable wiring, stretchable conductive bonding plays a vital role in joining the rigid chip to the substrate.40 Due to the differences in properties of the stretchable and rigid components, stretchable bonding has always presented an obstacle in realizing reliable stretchable electronics including bonding material and bonding technology. In this study we report on a highly conductive paste, which can be used to fabricate stretchable wirings and bondings under a low curing temperature of 100 C with printing method. The resistance of the wiring can recover to its original value after the strain is released, and shows much better recoverability than nanowire based wirings. Moreover, the resistance of the patterned wiring remains within 1.1 times even after 1000 cycles tensile test under 20% of strain,


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showing superior stability. Finally, a reliable stretchable demo was fabricated with the paste as stretchable wiring and bonding material.

2. Results and discussion 2.1 Wiring printing and curing Commercial Ag micro flakes and liquid PDMS were mixed together in vacuum to obtain the conductive pastes in which the Ag flakes were expected to distribute randomly in PDMS (Figure 1a-b). The paste with x wt.% of Ag flakes is named as PDMS-Ag x%. The sandwiched stretchable wiring is fabricated as shown in Figure 1c-e. The fabricated wiring can be very long and shows superior flexibility and stretchability (Figure 1f). In order to optimize the curing temperature and curing time, the pastes were first cured at 100 C. The resistivity change of paste PDMS-Ag 91% with time is shown in Figure S1. The resistivity experiences a rapid decrease within the first 6 h to about 1.41  10-4 cm, and reaches about 8.7  10-5 cm when the curing time is increasd to 8 h. After that, the resistivity does not change significantly, even after 50 h. Therefore, 8 h was selected as a standard curing time at 100C. The paste was then cured at a temperature of 50C and 150C to achieve the same resistivity as that obtained when cured at 100C for 8 h. The curing time at 50C and 150C were about 180 h and 2.5 h respectively. The curing time was shortened with use of a higher curing temperature. Considering 100C is not high and is suitable for many stretchable substrates, 100C was chosen as the optimal curing temperature. This development should significantly enhance fabrication and opportunities for wearable applications. While it is noted that the stretchable wiring has a good adhesion to the substrates in the sandwiched structure, there is no void or crack between the interfaces as shown in Figure 1h. Ag flakes are randomly distributed in PDMS, thus a conductive network formed after curing as



illustrated in Figure 1g. The top view of the stretchable wiring was also observed after the step in Figure 1d. The Ag flakes are connected to one another as shown in Figure S2.

Figure 1. Preparation of the stretchable paste and conductor. Fabrication process (a) and sketch map (b) of the paste. Fabrication process of the stretchable wiring (c), (d) and (e). The sandwiched wiring (f) and the cross section (g) and (h) of the stretchable wiring.

2.2. Resistivity Electrical resistivity is a key property for stretchable conductors. When the PDMS are cured, the monomer [SiO(CH3)2] units polymerize and form long chains which hold the Ag flakes tightly together to make the composite conductive. The initial resistivity of the stretchable conductors is shown in Figure 2a and listed in Table S1. The resistivity is 3.7  10-4 cm when the weight percentage of Ag flakes is 80% (corresponding to a volume fraction of 26.9%). Resistivity increases to 2.2  10-4 cm and 0.99  10-4 cm as the content of Ag flakes


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increases to 84% and 88% respectively. The lowest resistivity is about 8.7  10-5 cm when the loading of Ag flakes reaches 91% in weight. For composite conductors, electrical conductivity follows a power-law relationship as called 3D percolation theory (see supporting information).41 The resistivity of the stretchable conductors is fitted using 3D percolation theory (Figure 2a). It is found that the threshold (Vc) is 0.137 and the fitting exponent (s) is 3.15, which is consistent with reported results (Vc is 0.111 and s is 3.55).32 Furthermore, the conductivity is also affected by the size and morphology of the fillers. Stretchable conductor fabricated with Ag round particles (1 - 2 m in diameter) and PDMS obtained only a minimum resistivity of 16.7  10-4 cm,42 which is much higher than reflected in our results. The flakes used in this work are widely used commercial products with an average diameter of about 8 m and a thickness of about 200 nm. Flakes with a large diameter and thickness ratio enable larger contact areas between the Ag fillers compared with the round particles.41 Moreover, both the contacted Ag flakes and the Ag flakes whose distances are less than the tunneling distance contribute to a conductive network in the stretchable conductor.41, 43 It is the conductive network that supplies the electron pathways, which guarantees the high conductivity of the stretchable conductors. Therefore, with the use of this paste, the lowest resistivity of about 8.7  10-5 cm can be achieved. As shown in Figure S3, this is one of the best value for stretchable conductors obtained.



Figure 2. (a) Resistivity of the conductor and (b) R/R0 of the wirings during tensile test(strain rate: 10%/min).

2.3. Stretchability Figure 2b shows the change of relative resistance (R/R0, R0 indicates the initial resistance, while R indicates the instantaneous resistance during the test.) of the stretchable wirings during uniaxially tensile test. It can be seen that the R/R0 rises when the tensile strain is increased and the rising trend depends on the composition. When stretched to 80% of strain, the R/R0 rise from 1 to about 25.1, 141, and 172 for stretchable wirings prepared with paste PDMS-Ag 84%, 88%, and 91%, respectively. It is noteworthy that the R/R0 rises from 1 to about 5.9 when stretched from 0% to 80% of strain for stretchable wirings prepared with PDMS-Ag 80% paste. In the other words, the stretchable conductor exhibits a high conductivity of 2671 S cm−1 without strain and 815 S cm−1 at a strain of 80%, which shows only the smallest resistance fluctuation during tensile test. For the application of stretchable conductors, not only the resistivity and the change of R/R0 but also the recovery properties play a role in its overall success. Therefore, the recovery properties have also been investigated and discussed. It is noteworthy that when the highly conductive stretchable paste is applied as a wiring/bonding material, it is the resistance change that affects the properties of the circuit, and not the resistivity of the paste itself. Hence, in this paper the resistance change rather than the resistivity change of the stretchable conductive material, is discussed.

2.4. Recoverability Figure 3a shows the R/R0 of a stretchable wiring during cycling test with 20% tensile strain (20% cycling test). The resistance rises when the stretchable wiring is stretched, and reaches a 8 ACS Paragon Plus Environment

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maximum resistance of about 22 times when stretched to 20% of strain for the 1st cycle. After that the resistance begins to decrease once the strain is released, which is a recovery process. And the resistance recovers to a value of R1 after the 1st cycle and Rn after the nth cycle. For most of the present stretchable wirings, the recovered resistance is larger than the initial value.26, 31-32, 35-36, 38

When the stretchable wiring is kept at 0% of strain, the resistance continues to

decrease to R0. This process is time-dependent.38 The ability that the resistance recovers to its original value is indicated as recoverability. Electronic device can endure resistance change to some extent, but only if the resistance can recover to its original value within a very short time. Therefore, the recovered resistance and the length of the time period to recover are two important parameters for stretchable wirings. Firstly, we define recovery resistance as Rn/R0 to evaluate the recoverability. The ideal recovery resistance is 1. The smaller the recovery resistance, the better the recoverability. As indicate in Figure 3a for PDMS-Ag 91%, the recovery resistance is only 1.2 after the 2nd and 1.3 after the 100th cycle compared with 1.6 for nanowires.44 Figure 3b illustrates the R/R0 of the stretchable wirings prepared with pastes PDMS-Ag 84%, 88%, and 91% under 20% cycling test. The recovery resistance is 2.5, 1.5, and 1.3 after 100 cycles for the three pastes, which indicates that the recoverability becomes better with an increaseto the Ag content. The stretchable wiring prepared with paste PDMS-Ag 91% exhibits the best recoverability. Secondly, recovery time is defined as the time period from the strain is completely released to the time the resistance recovers to a standard value. As shown in Figure 2a, the original resistance is set as the standard value. Recovery time is a two dimensional parameter that can estimate the recoverability of the stretchable wirings in recovery resistance and time, which is a direct parameter to evaluate the wiring for application. Shorter recovery time means better recoverability. For the nanowire based stretchable conductors, the resistance cannot recover to its original value after the strain is released.35-36 Figure 3c shows the recovery time of the 9 ACS Paragon Plus Environment


stretchable wirings. As seen in the figure, the wirings show longer recovery time with increasing cycles. The wiring prepared with PDMS-Ag 91% paste shows simultaneous recovery within 100 cycles, which means the recovery time is 0 s. The recovery time is about 50 s at the 500th cycle and grows to about 300 s after the 1000th cycle. And more impressively, even after 10000 cycles, the resistance of the stretchable wirings can still recover to its original value. The stretchable wiring prepared with PDMS-Ag 88% paste exhibits a slightly longer recovery time compared with PDMS-Ag 91% paste. However the recovery time is less than 10 s within 40 cycles and about 1800 s at 1000th cycles (see Figure 3c), respectively. For stretchable wiring prepared with nanowire, the resistance cannot recover to its original value even after the 1st cycle.45 These findings clearly demonstrate that our stretchable wirings possess much better recoverability than the nanowire based wirings. Considering the small recovery resistance and short recovery time, the stretchable wiring prepared with PDMS-Ag 91% paste shows the best recoverability. The recoverability of this wiring was evaluated at different cycling strains and tensile strain rates as illustrated in Figure 3d-e. The recoverability of the stretchable wiring improved when a smaller cycling strain is endured. Specifically, the recovery resistances after 100 cycles are 1.8 and 1.3 when the cycling strains change from 40% to 20% of strain, respectively. Comparatively, the recovery resistances are about 5 and 3 when the cycling strains are 40% and 20% after 100 cycles for AgNW based conductors.45 Figure 3e shows that the recovery resistance is smaller when the tensile strain rate is slower. This indicated that the recoverability of the stretchable wiring can be improved under a slow strain rate. As mentioned above, the resistance of the stretchable wiring recovers almost simultaneously with a low speed releasing process once the strain is released. If the releasing speed is very high, the stretchable wiring does not have time to recover before the strain is released totally. But special pattern can be designed to eliminate the real strain in the wirings, so that the recoverability under high speed and high strain can be improved. 10 ACS Paragon Plus Environment

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Figure 3. Recoverability of the stretchable wirings. (a) Definition of the recoverability. (b) R/R0 and (c) recovery time (R1 is set as the standard value to calculate the recovery time at a tensile strain rate of 300%/min.) of the stretchable wirings. R/R0 of the stretchable wiring prepared with paste PDMS-Ag 91% (d) under different strain levels of cycling test and (e) at different tensile strain rates. (f) R/R0 of the stretchable wirings endured discontinous cycling test. (The tensile strain rate is 10%/min for (a), (b) and (d), 300%/min for (e), and 50%/min for (f).)

When the stretchable wiring suffers from continous stretching and releasing cycles, the recovery resistance keeps increasing with increasing cycling numbers as shown in Figure S3. 11 ACS Paragon Plus Environment


Once the cycling test is interrupted, the resistance of the wiring will recover to its original value. However, if the wiring is tested again after it recovers, the recoverability of the wiring will be greatly improved. As shown in Figure 3f, the wiring first endures 1000 cycles test and keep at 0% of strain until the resistance recovers to its original value. When the wiring is tested again. Although the performances of the wiring will be inferior to the original state, yet recoverability of the wiring is greatly reduced compared with the previous cycles, which means the recoverability of the wiring is greatly improved compared with the continuous cycling test. These properties of the wiring are very suitable for its applications in stretchabkle devices which cannot tolerate reduced performance from continuous stretching and releasing for many cycles. PU is also a commonly used stretchable elastomer to prepare stretchable wiring.27, 31 Figure S4 shows the R/R0 of the stretchable wiring prepared with PDMS and PU based pastes with the same content of Ag. Clearly, the stretchable wiring prepared with PDMS based paste exhibits much smaller recovery resistance. PDMS is an elastomer that demonstrates superior recoverability compared with PU.46-47 This superiority can also be deduced from Figure S5, where PDMS and PU film with a length of 150 mm were first stretched to 100% of strain and then released. PDMS film can recover to 150 mm simultaneous, while PU film can only recover to 155 mm after 5 seconds. So the stretchable wiring prepared with PDMS based paste show significantly greater stability and recoverability than PU based wiring. The recovery resistance and the recovery time are firstly defined to evaluate the recoverability of the stretchable wirings. The PDMS based wiring show much better recoverability than the PU based wiring, which indicates PDMS to be a promising candidate for stretchable matrix. Moreover, higher Ag content, smaller cycling strain, as well as low strain rate during the cycling test, will improve the recoverability of the wiring. Furthermore, the recoverability of the wiring will be greatly improved when the wiring has endured discontinous stretchcing and releasing test, which is more likely the conditions in common life applications. 12 ACS Paragon Plus Environment

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2.5. Microstructure evolution Recoverability is closely related to the microstructure of the wirings. Therefore microstructure evolution during in-situ tensile test is investigated to illustrate the mechanism of superior recoverability both by SEM and optical microscopy (OM). For the stretchable conductor without surface layer in Figure 1d, micro voids or cracks appear during the stretching process as shown in Figure S6, however all the voids will shrink and heal after the strain is totally released. For the sandwiched stretchable wiring as shown in Figure 4, no cracks are detected during the stretching process, which confirms that the reliability of the sandwiched stretchable wiring is better than the wiring without surface layer. Once the strain is totally released, the microstructure of the conductor recovers to its original state. For stretchable wirings prepared with nanowires, a stretching or cycling test can break the nanowires or the connecting spots in the conductor, which results in a permanent resistance increase.24,

44, 45

Our composite

stretchable conductor is composed of PDMS and Ag flakes. As illustrated by the two Ag flakes, the contact area is reduced as the wiring is stretched, and the flakes can return to their original location when the strain is totally released. This explains why the resistance of the stretchable wiring shows good recoverability and stability compared with AgNWs. With the Ag micro flakes as filler material for stretchable conductors, the resistivity and the fluctuation of resistance are greatly improved compared with those using Ag round particles. PDMS provides the stretchability and recoverability which are also affected by the flakes. The Ag micro flakes provide the conductivity which is also affected by PDMS as discussed above. The combination of PDMS and Ag micro flakes proves to be a good choice for both high conductivity and superior recoverability. The composite conductor is composed of an elastomer matrix and a conductive network of Ag flakes. The conductive network includes the overlapped Ag flakes and the Ag flakes whose 13 ACS Paragon Plus Environment


distances are less than the tunneling distance. When the conductor is stretched, the distance of the nearby Ag flakes become further, thus the resistance of the wiring rises. When the strain is released, the Ag flakes return to almost their original location within a very short time. The overlapped Ag flakes will contribute almost the same conductivity compared with the initial state. But for the Ag flakes whose distances are less than the tunneling distance, the conductivity is very sensitive to the distance of the nearby Ag flakes. Only if these Ag flakes return to exactly their original location (with 0 nm error), the conductivity can recover the initial value. It need time for the the Ag flakes to returns to its exactly the original location. For the conductors with higher Ag content (for example, PDMS-Ag 91%), the amount of overlapped Ag flakes are much more than that of the lower Ag content conductors. So the recovery time is shorter with higher Ag loading. Considering the recoverability and the resistance increase during stretching, an appropriate amount of Ag flakes should be selected depending on the application conditions.


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Figure 4. Microstructure evolution of the stretchable conductor prepared with PDMS-Ag 91% during 40% tensile cycling test. (a) The R/R0 change. (b) The original state, (c) stretched by 20% and (d) 40% of strain. Released from 40% to (e) 20% and (f) 0% of strain.

2.6. Improving the wirings Although the stretchable wirings prepared with the paste show superior recoverability, the resistance of the wirings increases when stretched. The resistance of an ideal stretchable wiring should remain unchanged during the stretching cycles. Stretchable wirings with zigzag 7, 48 or horseshoe 49, 50 patterns can greatly reduce strain in themselves, which leads to a small resistance 15 ACS Paragon Plus Environment


fluctuation. Therefore, to pursue an ideal stretchable wiring, straight, horseshoe, and zigzag wirings were fabricated with dispensing method as illustrated in Figure 5a. In addition, prestrained wirings were also prepared. The shape parameters of the wirings are shown in Figure S7.

Figure 5. Specially designed stretchable wirings. Stretchable wirings (a) and R/R0 of the wirings (b) during tensile test and (c), (d) 20% cycling test. (The stretchable wirings are prepared with PDMS-Ag 88% paste. The strain rate is 10%/min.)

The dispensed wirings can be stretched to 100% of strain as illustrated in Figure 5b. For straight wiring, the R/R0 is 186 at 80% of strain, which is higher than 142 of the mask printed wirings. The width of the wiring can affect the propertity of the wiring.42 The cross section of dispensed wiring is a narrow semi-circle like shape as shown in Figure S8 with an area about 0.026 mm2. 16 ACS Paragon Plus Environment

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The mask printed wiring has a rectancular cross section with an area of about 0.12 mm2, which is about 4.6 time of the dispensed ones. The wiring with the large cross section shows much better properties. The size effect of the wiring on the properties will to be investigated in detail. The horseshoe wiring shows almost the same resistance change as the straight wiring. As in the references, if the rigid horseshoe Cu wiring is embedded in soft PDMS, the wiring has some ‘‘freedom” to move inside the substrate, reducing the strain in the Cu wiring. But if the wiring is as soft as the substrate, the strain in the wiring is essentially the same as the total strain applied to the substrate.51 Our stretchable wiring is the latter case, the stretchable wiring is as soft as the substrate, so the strain in the wiring is the same as the strain as applied to the substrate. The horseshoe wiring shows a similar resistance change as the straight wiring. As for the zigzag wiring, the R/R0 are about 1.2 and 10 at 20% and 50% of strain, respectively, which are much smaller than those of straight one. But after 60% of strain, the R/R0 is not stable and rise sharply to thousands after 80% of strain. The tensile deformation of the zigzag wiring can decompose into torsional and a much smaller tensile deformation.51 The R/R0 of the wiring is much smaller than the straight one. But there is a strain concentration at the crest/trough of the wiring, so the wiring shows a poor stability at a higher strain. As expected, the prestained zigzag wiring shows the smallest R/R0 change. Impressingly, the R/R0 are only 1.1, 1.1 and 2.1 at 20%, 50% and 100% of strain, respectively. This is a much better result in comparison to the published findings of stretchable wirings prepared with conductive fillers and elastomers.6-7 Figure 5c illustrates the R/R0 of the stretchable wirings during 20% cycling test. For the straight and horseshoe ones, the maximum R/R0 reach tens to hundreds while the recovery resistances are within 2.0. Whereas, the maximum R/R0 is greatly reduced within 3.0 for zigzag wiring. For the prestrained zigzag wiring, the R/R0 are around 1.0, which means the wiring shows stable resistance during cycling test. More importantly, the R/R0 of this wiring remains less than 1.1 during 20% cycling test even after 1000 cycles as depicted in Figure 5d. And no visible cracks 17 ACS Paragon Plus Environment


can be observed in the stretchable wiring after 1000 cycles. This indicates the prestrained zigzag stretchable wiring shows long term stability during cycling test.

2.7. Stretchable application Besides the wirings, the stretchable bonding between the rigid chip and the stretchable wiring/substrate is another important factor for stretchable electronics. At present, conductive adhesive is chiefly used as a stretchable bonding material. Since the bonding material and the stretchable conductor are different, there may exist incompatibility issues. Here, we attempted to apply the conductive paste as stretchable bonding material to bond the rigid Si chip to the stretchable wirings and the substrate. The bonding is illustrated in Figure S9a, and the shear strength is about 0.4 MPa. The shear fracture appeared in the paste but near the chip/paste interface as indicated in Figure S9b. An Si chip was selected to fabricate the stretchable device. The resistance of the chip was about 0.01 Ohm as called zero-Ohm chip. In such case, the resistance of the stretchable device comprises only the resistance of the stretchable wiring and the stretchable bondings. The sketch map of the fully printed stretchable device is illustrated in Figure 6a. A hillock is pre-located beneath the bonding area to fasten the stretchable bonding as illustrated in the inset, which is a key point to enhance the reliability of the bonding. The R/R0 of the stretchable device is less than 1.2 within 35% of strain, and it increases to only 1.9 when stretched to 50% of strain as shown in Figure 6b. The R/R0 during 20% cycling test is shown in Figure 6c. The recovery resistance increases gradually from 1 to about 1.7 after 100 cycles, and to about 10 after 650 cycles. But the resistance can recover to its original value completely. The maximum R/R0 is no larger than 5 within 130 cycles, and increases abruptly to about 60 after another 10 cycles (over 140 cycles totally). After that, the maximum R/R0 increases gradually to 160 until about


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430 cycles. The maximum R/R0 experiences another rapid increase to about 1000 during 440450 cycles. The sudden increase of the R/R0 indicates that there are micro cracks in the bonding. However, as is the same with the stretchable conductor, the micro cracks will close after the strain is released. A fully printed stretchable lighting is also fabricated with two zero-Ohm chip in series with one LED light in arrays to evaluate the assembling of series of stretchable wirings and bondings. The lighting works very well both before and after the stretching cycles and also after kneading (see Figure 6d-f). This demonstrates reliable and stable wiring and bonding. Therefore, our paste represents a good choice for stretchable conductive material and bonding material.



Figure 6. Fully printed stretchable device with zero-Ohm chip. (a) Sketch map of the bonding technology with the cross section of the bonding area in the inset. R/R0 during tensile test (b) and cycling test (c). Stretchable lighting works well (d) without deformation, when (e) stretching and (f) kneading.(The strain rate is 10%/min for (c).)

3. Conclusions A highly stretchable paste is developed with Ag flakes and PDMS which can be cured at a temperature of 100 C. The paste can be applied both as stretchable wiring and bonding material. The resistivity of the prepared wiring can reach as low as 8.7  10-5 cm. Recovery resistance and recovery time is first introduced and calculated to evaluate the recoverability of the stretchable wirings, which possess much better recoverability than AgNW based wirings. Even after 10,000 cycles at 20% cycling test , the resistance of the stretchable wiring can still recover to its original value. With the special design, the resistance of the wiring increases to only 1.1 and 2.1 times when stretched to 50% and 100% of strain. Moreover, the resistance of the wiring during 20% tensile cycling test remains within 1.1 times even after 1000 cycles, thus demonstrating long term stability. With the developed paste used as stretchable wiring and stretchable bonding material, a fully printed stretchable demo is assembled with a zero-Ohm chip. The R/R0 of the stretchable device is 1.9 when stretched to 50% of strain, showing reliable and stable wirings and bondings. The fabrication process of the stretchable device with our newly developed paste enables a fully printed and low temperature curing process, which is suitable for the mass production of stretchable electronic devices.

4. Experimental details 20 ACS Paragon Plus Environment

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Preparation of the paste and stretchable wirings. The preparation process of the pastes is shown in Figure 1a-b, which is similar to our previous work.30 Micro Ag flakes (AgC-239, Fukuda Metal Foil & Powder Company, Ltd) was selected as the filler. The fabrication steps of the wiring are illustrated in Figure 1c-e. Firstly, a PDMS was prepared on a glass matrix with a mask. The thickness and the width of the mask are 200 m and 20 mm respectively. Then, the conductive pastes were printed onto the center of the PDMS film with a mask (50 m in thickness and 3 mm in width) and cured at 100 C for 8 hours as shown in Figure 1d. After curing, liquid PDMS was casted onto the cured wiring by using another mask with a thickness of 200m and cured to obtain the sandwiched stretchable wiring as shown in Figure 1e. The wiring was peeled off from the glass substrate as shown in Figure 1f. The wiring was then cut into 40 mm in lengths to prepare the sample. When the tensile test was carried out, two tips of the sample were clipped into the holder of the test machine, the testing length of the sample was 20 mm as shown in Figure S11. Preparation of the patterned wirings. The fabrication steps of the zigzag patterned stretchable wiring are similar to common stretchable wiring. A screen printing method was substituted with a dispensing method. The conductive pastes were dispensed onto the film with a dispensing machine (MUSASHI ENGINEER, INC.) at a speed of 2 mm/s. The nozzle was 250 m in diameter. For the prestrained stretchable wiring, before dispensing, the PDMS film was stretched by 30%. And the wiring was released after the final step. The cross section of dispensed wiring is a narrow semi-circle like shape as shown in Figure S8. Preparation of the stretchable device. Firstly, liquid PDMS was printed on a glass matrix with a thickness of about 200 m and cured. Then, a PDMS hillock was prepared at the assembling area. Afterwards, the PDMS film was stretched. Next, the zigzag wiring was dispensed onto the film and chips were mounted between the two wirings as in the inset of Figure 6b. After that, the wirings and the mounted chip were cured at 100 C for 8 hours in the 21 ACS Paragon Plus Environment


chamber. After curing, liquid PDMS was cast and cured to obtain a stretchable device. And the stretchable device was released at the final step. Si chip (3 mm  3 mm  0.5 mm in size) sputtered with Ag layer (2 m in thickness) was selected as the chip for the stretchable demo. Characterization of the stretchable wirings and devices. The cross section of the stretchable wiring was prepared with direct knife cutting followed by a focused ion beam (FIB, FIB-2100, HITACHI) operated at 40 KV. The microstructures of the stretchable conductor were observed with an optical microscopy (OM) and a ﬁeld emission scanning electron microscope (FE-SEM; Hitachi SU8020) at an acceleration voltage of 2 kV. The thickness of the stretchable conductors was measured by a color 3-D laser scanning microscope (VK-9510, Keyence Corporation). The resistivity was measured by the four-point probe method (Loresta GP MCP-T610, Mitsubishi Chemical Analytech Company, Ltd.) before casting the surface layer. Tensile test of the stretchable wirings and devices. The tensile and tensile cycling test of the stretchable wirings and devices were carried out on a desk-top universal testing machine (Shimazu EZ-Test) with a self-prepared holder which can connect the sample to a digital multimeter (2110 5 1/2 DIGITAL MULTIMETER, KEITHLEY). The transient resistance during the tensile and tensile cycling test was recorded with a digital multimeter. The test lengths of the stretchable devices is 50 mm.

Supporting information Resistivity of the stretchable conductors; 3D percolation theory; The resistivity change of the paste with curing time; The top view microsctructure of the wiring; Resistivity of the previous reported stretchable wirings compared with this work; Recoverability of the stretchable conductor for more than 1000 cycles with discontinnous cycling test; Recoverability of the wirings prepared with PDMS and PU based pastes; Recoverability PDMS and PU substrate; Surface microstructures of the stretchable conductor without surface layer during 40% tensile 22 ACS Paragon Plus Environment

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cycling test; The sizes of the stretchable wirings with dispensing method; The cross section of the wiring with dispensing method; Shear test of the stretchable bonding; The size and the test size of the sample.

Acknowledgements The authors acknowledged the financial support from Huawei Innovation Research Program, and this work was supported in part by "Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials" from the Ministry of Education, Culture, Sports, Science and Technology of Japan. H. Zhang acknowledged the fund from JSPS Grant-in-Aid for Young Scientists B (Grant Number JP17K14824). Y. Yang was supported by JSPS Postdoctoral Fellowship for Foreign Researchers (ID No: PE17020), and Z.Q. Liu acknowledged the Osaka University Visiting Scholar Program (Grant No. J135104902).

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Highly Conductive Ag Paste for Recoverable Wiring and Reliable

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