Multilayer flexible circuit and method of making same, flexible electronic system

Abstract

The application relates to the technical field of flexible electronic integrated system, in particular to a multilayer flexible circuit, a preparation method thereof and a flexible electronic system. The multilayer flexible circuit comprises a flexible substrate layer and a flexible conductive layer which are arranged in a stack, and the flexible conductive layer comprises metal nanowire aerogel and a filled flexible substrate. The flexible substrate in the flexible conductive layer and the flexible substrate layer both have stretchable properties. The metal nanowire aerogel is overlapped in the flexible conductive layer to form a rich conductive network, so that the flexible conductive layer has good conductivity. The special form of the metal nanowire aerogel makes the flexible conductive layer have good electrical stability, i.e. resistance-strain insensitivity and resistance stability after long-term deformation, and when the flexible conductive layer is deformed, the resistance will not change obviously. The stack structure design makes the upper limit of the mechanical stretchability of the multilayer flexible circuit significantly improved, the resistance change rate under the same deformation significantly reduced, and the resistance also changes relatively low after long-term use.

Description

Technical Field

[0001] This application relates to the field of flexible electronic integrated system technology, and in particular to multilayer flexible circuits and their fabrication methods, and flexible electronic systems. Background Technology

[0002] Modern electronic devices are constantly striving for thinner, lighter, more portable, and more multifunctional designs, especially in wearable devices, electronic skin, flexible displays, and biomedical sensors. On the one hand, devices need to adapt to the natural movements of the human body and possess good mechanical stability, such as being resistant to breakage even under significant deformation. On the other hand, while withstanding complex deformations such as bending, folding, and even stretching, devices must also maintain stable electrical performance, such as stable electrical signal transmission, high accuracy, and precision even under significant deformation.

[0003] Products in the aforementioned wearable devices and other fields can include flexible electronic systems. Flexible electronic systems comprise flexible circuits and electronic components. Integrating various electronic components into flexible circuits results in multifunctional flexible electronic systems where components can collaborate with each other. Most existing flexible circuits have only one conductive path, while multilayer flexible circuits have multiple conductive paths. Insulating layers are typically used to separate the conductive layers, allowing electronic components to be connected to each layer and achieving electrical connections between different layers. This achieves Z-axis conductivity in the thickness direction, improving the integration and synergy of electronic components while saving space. Current technologies often involve placing circuits on the sides of the layer structure to achieve interlayer conductivity, but this method is only suitable for a small number of electronic components; alternatively, anisotropic conductive adhesives or liquid metals are injected between layers to achieve interlayer conductivity.

[0004] Flexible circuits require not only good stretchability and resistance to breakage (i.e., mechanical stability), but also conductivity, preferably high conductivity and low resistance, and good electrical stability. Existing technologies commonly use stretchable conductors made of stretchable conductive materials, which retain some conductivity even under mechanical deformations such as stretching, bending, and twisting. However, existing stretchable conductors have poor electrical stability. On the one hand, resistance increases significantly during mechanical deformation such as stretching; on the other hand, after long-term cyclic stretching, even if they return to their original shape without external force, their resistance is significantly higher than before use, leading to a decrease in the stability, accuracy, and precision of signal transmission in flexible circuits. This problem is even more pronounced in multilayer flexible circuits, where the conductive paths between layers are far less stable than those within layers. Even slight mechanical deformation can cause significant changes in the resistance of the entire multilayer flexible circuit, affecting the operation of the entire flexible electronic system. Existing technologies struggle to simultaneously meet the performance requirements of high mechanical stability, low resistance, and high electrical stability in multilayer flexible circuits from the perspectives of materials and structural design. Summary of the Invention

[0005] The purpose of this application is to provide multilayer flexible circuits and their fabrication methods, as well as flexible electronic systems, in order to solve the technical problem of poor electrical stability of multilayer flexible circuits in the prior art.

[0006] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0007] In a first aspect, this application provides a multilayer flexible circuit, including a flexible substrate layer and a flexible conductive layer stacked together, wherein the flexible conductive layer includes metal nanowire aerogel and a flexible substrate filled with metal nanowire aerogel.

[0008] First, the flexible substrate and flexible base layer in the flexible conductive layer endow the multilayer flexible circuit with stretchability. Furthermore, the metal nanowire aerogel, with its three-dimensional interwoven nanowire morphology, has a smaller impact on the stretchability of the flexible conductive layer compared to common carbon fibers and conductive particulate materials. Second, the metal nanowire aerogel has high electrical conductivity, forming a rich conductive network within the flexible conductive layer, resulting in excellent conductivity. Third, the unique morphology of the metal nanowire aerogel gives the flexible conductive layer good electrical stability, namely resistance-strain insensitivity and resistance stability after long-term deformation; the resistance does not change significantly when the flexible conductive layer deforms or after long-term use. Finally, in addition to the above material selection, there is the structural design of stacking the flexible base layer and the flexible conductive layer. Compared to a single flexible conductive layer, this stacked structure, with one layer conducting current and the other insulating, significantly increases the upper limit of the mechanical stretchability of the multilayer flexible circuit, and significantly reduces the rate of resistance change under the same deformation, resulting in lower resistance changes even after long-term use. In summary, the multilayer flexible circuit of this application achieves synergistic effects through material selection and layer structure design, enabling the multilayer flexible circuit to possess excellent mechanical stability, conductivity, and electrical stability.

[0009] Secondly, this application provides a method for fabricating the aforementioned multilayer flexible circuit, comprising the following steps:

[0010] A flexible conductive layer is obtained by filling a flexible substrate solution with metal nanowire aerogel and performing a film-forming process.

[0011] A multilayer flexible circuit is obtained by stacking a flexible conductive layer with a flexible substrate layer and then hot-pressing it.

[0012] The method described in this application first fills the substrate with metal nanowire aerogel, and then prepares a flexible conductive layer by wet film deposition. This flexible conductive layer exhibits good stretchability, conductivity, and electrical stability. Next, it is laminated with a flexible substrate and subjected to hot-pressing to form a multilayer structure. Through the synergistic effect of both, the resulting multilayer flexible circuit maintains good conductivity while significantly improving its mechanical stretchability and electrical stability. This fabrication method offers controllable processes, resulting in a multilayer flexible circuit with stable structure and properties.

[0013] Thirdly, this application provides a flexible electronic system, including the multilayer flexible circuit described above or a multilayer flexible circuit prepared by the preparation method described above; it also includes a plurality of electronic components, which are electrically connected to the flexible conductive layer.

[0014] The flexible electronic system of this application includes multilayer flexible circuits, thus possessing the characteristics of high mechanical stability, good conductivity, and high electrical stability. During strain processes such as stretching and bending, the resistance does not change significantly, which is beneficial to the stability and accuracy of electrical signal transmission in the entire flexible electronic system, and improves the stability of operation of each electronic component and their synergistic effect. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the silver nanowire aerogel filling process in Embodiment A1 of this application before and after;

[0017] in, Figure 1 (a) is a schematic diagram of the initial state of silver nanowire aerogel;

[0018] Figure 1 (b) is a schematic diagram of silver nanowire aerogel being compressed into thin sheets;

[0019] Figure 1 (c) is a schematic diagram of the silver nanowire aerogel filled with SEBS;

[0020] Figure 2 These are SEM images of the silver nanowire aerogel before and after filling in Example A1 of this application;

[0021] in, Figure 2 (a) is a SEM image of the initial state of silver nanowire aerogel;

[0022] Figure 2 (b) is a SEM image of silver nanowire aerogel compressed into thin sheets;

[0023] Figure 2 Image (c) is a SEM image of SEBS filled with silver nanowire aerogel;

[0024] Figure 3 These are the tensile upper limit test results of Embodiment A1 and Comparative Example A2 of this application;

[0025] in, Figure 3 Figure (a) shows the results of the electrical tensile limit test for both.

[0026] Figure 3 Figure (b) shows the test results of the upper limit of mechanical tensile strength for both.

[0027] Figure 4 These are the electrical stability test results of Embodiment A1, Comparative Example A1, and Comparative Example A2 of this application;

[0028] in, Figure 4 (a) shows the test results of the relationship between the resistance changes and strain of the three components;

[0029] Figure 4 (b) shows the resistance change test results of the three materials under a 50% strain tensile-recovery cycle test.

[0030] Figure 4 (c) is a graph showing the resistance change test results of Example A1 after 300 tensile-recovery cycles with 50% strain.

[0031] Figure 5 These are graphs showing the long-term electrical stability test results of Embodiment A1 and Comparative Example A2 of this application;

[0032] in, Figure 5 (a) shows the resistance change test results of the two under long-term tensile-recovery cycles with 10% and 50% strain.

[0033] Figure 5 Figure (b) shows the resistance change test results of Example A1 in (a) from 2220 to 2270 cycles;

[0034] Figure 6 These are test results for different hot-pressing temperatures and times in Examples A1 to A6 and Examples A11 to A20 of this application;

[0035] in, Figure 6 Figure (a) shows the test results of the relationship between sheet resistance and temperature in Examples A1 to A6;

[0036] Figure 6 Figure (b) shows the resistance change test results of Examples A1 to A6 and Examples A11 to A20 under a tensile-recovery cycle test at 50% strain;

[0037] Figure 7 These are the electrical stability test results for Embodiments A1, A3, and A11 of this application;

[0038] in, Figure 7 (a) is a graph showing the resistance change test results of tensile-recovery cycle test at 50% strain for Examples A1 and A3;

[0039] Figure 7 Figure (b) shows the resistance change test results of the tensile-recovery cycle test at 50% strain for Examples A1 and A11;

[0040] Figure 8 These are SEM images of the side views of the laminated structures in Embodiments A1 and A3 of this application;

[0041] in, Figure 8 Image (a) is a SEM image of sample A3 before hot pressing.

[0042] Figure 8 Image (b) is a SEM image of sample A3 after hot pressing.

[0043] Figure 8 Image (c) is a SEM image of sample A1 before hot pressing.

[0044] Figure 8 Image (d) is a SEM image of sample A1 after hot pressing.

[0045] Figure 9 These are graphs showing the test results of pressure under different hot-pressing treatments in Examples A1, A7 to A10;

[0046] in, Figure 9 (a) shows the test results of the relationship between sheet resistance and pressure;

[0047] Figure 9 (b) is a graph showing the relative pressure change of the resistance under a 50% strain tensile-recovery cycle test.

[0048] Figure 9 (c) is a graph showing the relative pressure test results of the interlayer interface strength after hot pressing.

[0049] Figure 10 The tensile property test diagrams of two samples were prepared according to the preparation method of Example A21 of this application.

[0050] in, Figure 10 (a) is a tensile property test diagram of a sample containing the word "HIT";

[0051] Figure 10 (b) is a tensile property test diagram of a sample containing the words "HITSZ";

[0052] Figure 11 This is a schematic diagram of the fabrication of a multilayer flexible circuit according to embodiment A21 of this application;

[0053] in, Figure 11 (a) is a schematic diagram of the conductive unit fabrication;

[0054] Figure 11 (b) is a schematic diagram of the fabrication of a flexible substrate and the opening of through-holes;

[0055] Figure 11 (c) is a schematic diagram showing the stacking of conductive units and a flexible substrate layer;

[0056] Figure 12 This is a schematic diagram of the flexible electronic system of embodiment B1 of this application;

[0057] in, Figure 12 (a) is a multilayer flexible circuit, namely embodiment A21;

[0058] Figure 12 (b) is a schematic diagram of the flexible electronic system after connecting the LED;

[0059] Figure 12 (c) is a schematic diagram of the flexible electronic system after it is powered on. Detailed Implementation

[0060] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0061] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0062] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items.

[0063] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0064] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass in the embodiments of this application can be a well-known unit of mass in the chemical industry, such as µg, mg, g, or kg.

[0065] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, a first feature may also be referred to as a second feature, and similarly, a second feature may also be referred to as a first feature. Thus, a feature defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0066] Electrical stability encompasses two aspects: firstly, the change in resistance of a stretchable conductor under mechanical deformation such as stretching; and secondly, the change in resistance after long-term cyclic stretching and use, when the external force is removed and the stretchable conductor returns to its original shape, compared to its initial resistance. Existing stretchable conductors exhibit significantly increased resistance under both conditions, indicating poor electrical stability. Multilayer flexible circuits, however, require good electrical stability and relatively stable resistance. Therefore, the electrical stability described in this application includes resistance-strain insensitivity and resistance stability after long-term deformation.

[0067] In this application, the flexible substrate layer is non-conductive or has extremely high resistance; or, at least the region where it is stacked with the flexible conductive layer is non-conductive or has extremely high resistance.

[0068] The first aspect of this application provides a multilayer flexible circuit, including a flexible substrate layer and a flexible conductive layer stacked together, wherein the flexible conductive layer includes metal nanowire aerogel and a flexible substrate filled with metal nanowire aerogel.

[0069] First, the flexible substrate and flexible base layer in the flexible conductive layer endow the multilayer flexible circuit with stretchability. Furthermore, the metal nanowire aerogel, with its three-dimensional interwoven nanowire morphology, has a smaller impact on the stretchability of the flexible conductive layer compared to common carbon fibers and conductive particulate materials. Second, the metal nanowire aerogel has high electrical conductivity, forming a rich conductive network within the flexible conductive layer, resulting in excellent conductivity. Third, the unique morphology of the metal nanowire aerogel gives the flexible conductive layer good electrical stability, namely resistance-strain insensitivity and resistance stability after long-term deformation; the resistance does not change significantly when the flexible conductive layer deforms or after long-term use. Finally, in addition to the above material selection, there is the structural design of stacking the flexible base layer and the flexible conductive layer. Compared to a single flexible conductive layer, this stacked structure, with one layer conducting current and the other insulating, significantly increases the upper limit of the mechanical stretchability of the multilayer flexible circuit, and significantly reduces the rate of resistance change under the same deformation, resulting in lower resistance changes even after long-term use. In summary, the multilayer flexible circuits in this application achieve synergistic effects through material selection and layer structure design, enabling them to possess excellent mechanical stability, conductivity, and electrical stability.

[0070] [About flexible conductive layers]

[0071] The flexible conductive layer comprises a metal nanowire aerogel and a flexible substrate filled with the metal nanowire aerogel. The flexible substrate provides the basic mechanical properties of the flexible conductive layer, such as flexibility and stretchability. The metal nanowire aerogel plays at least three roles: Firstly, microscopically, a single nanowire can provide a relatively long conductive path, and the nanowires interlock within the flexible conductive layer to construct a rich three-dimensional conductive network. Secondly, its interwoven nanowires and ultra-flexible, ultra-lightweight properties reduce the impact on the tensile properties of the flexible conductive layer. Thirdly, when the flexible conductive layer deforms, the metal nanowire aerogel can adaptively adjust, maintaining the interwoven three-dimensional conductive network and ensuring stable conductivity. Therefore, this flexible conductive layer exhibits excellent mechanical stability, conductivity, and conductive stability. More importantly, existing technologies often disperse the conductive material within a flexible substrate, with the substrate as the dominant element. During stretching, the conductive network of the conductive material can partially break down, leading to a significant increase in macroscopic resistance. This application fills a flexible substrate into a metal nanowire aerogel, using the metal nanowire aerogel as the main body and its interwoven nanowire microstructure to improve the problem of rapid change in resistance with deformation.

[0072] In some embodiments, the flexible substrate includes at least one of styrene-ethylene-butene-styrene block copolymer (SEBS) and styrene-butadiene-styrene block copolymer (SBS). SEBS and SBS possess high elasticity and tensile properties, allowing them to adapt to various complex bending and stretching scenarios. They can serve as flexible substrates for flexible conductive layers and also exhibit good self-healing properties, which is beneficial for improving their service life and long-term stability. In the exemplary example, different copolymer segments of SEBS result in different stretchability properties. The average molar mass of SEBS can be 45,000 g / mol, which is beneficial for improving stretchability. In the exemplary example, the SEBS can withstand a stretching ratio of 700% without breaking.

[0073] In some embodiments, the material of the metal nanowire aerogel includes at least one selected from Ag, Au, Cu, Fe, and Ni, and silver may be selected as the alternative. The diameter of the nanowires in the metal nanowire aerogel is 30 nm to 1000 nm. The density of the metal nanowire aerogel is 1 mg / cm³. 3 ~600 mg / cm 3 Metal nanowire aerogels composed of these material types or parameters are further beneficial for improving the mechanical stability, conductivity, and conductive stability of flexible conductive layers.

[0074] In some embodiments, the mass ratio of metal nanowire aerogel to flexible substrate is 1:(1.09–1.83), that is, the mass ratio of metal nanowire aerogel to flexible conductive layer can be 1:(2.09–2.83). Metal nanowire aerogel provides good conductivity, while the flexible substrate provides good tensile properties. The two materials synergistically improve conductive stability, and the mass ratio within the aforementioned range further facilitates a balance between mechanical stability, conductivity, and conductive stability. Furthermore, metal nanowire aerogels generally have extremely low density; in the aforementioned mass ratio, the volume of conventional metal nanowire aerogel would be much larger than the volume of the flexible substrate. Therefore, the metal nanowire aerogel can be a compressed metal nanowire aerogel sheet, which is beneficial for improving structural stability and also for filling the flexible substrate.

[0075] In some embodiments, the thickness of the flexible conductive layer is 30 μm to 50 μm. The thickness design of the flexible conductive layer is beneficial for improving the synergistic effect when the flexible conductive layer and the flexible substrate are subsequently stacked in the structure. This allows for the stacking of more layers while maintaining mechanical stability, conductivity, and conductivity stability, thereby increasing the number of electronic components that can be connected to the multilayer flexible circuit. A suitable thickness of the flexible conductive layer also helps reduce stress concentration within the layer and improves the stability of the stacked bond with the flexible substrate.

[0076] When there are ≥2 flexible conductive layers, the aforementioned parameters such as material and thickness of each flexible conductive layer can be the same or different.

[0077] [About the flexible substrate]

[0078] The flexible substrate layer is flexible and stretchable, and together with the flexible conductive layer, it can give multilayer flexible circuits good stretchability and conductivity stability.

[0079] In some embodiments, the flexible substrate material includes at least one of styrene-ethylene-butene-styrene block copolymer and styrene-butadiene-styrene block copolymer. SEBS and SBS have high elasticity and tensile properties, and also possess self-healing properties. In an exemplary embodiment, the average molecular weight of SEBS can be 45,000 Da, which is beneficial for improving its stretchability. In an exemplary embodiment, the stretchability of SEBS can reach 700% without breaking.

[0080] In some embodiments, the thickness of the flexible substrate layer is 40 μm to 50 μm. The thickness design of the flexible substrate layer is beneficial for improving the bonding stability when it is stacked with the flexible conductive layer, enhancing the synergistic effect between the two layers, and further improving mechanical and electrical stability, thereby increasing the integration density of the multilayer flexible circuit.

[0081] When there are ≥2 flexible substrate layers, the aforementioned parameters such as material and thickness of each flexible substrate layer can be the same or different.

[0082] The flexible substrate of the flexible conductive layer and the flexible base layer can have the same or different materials and thicknesses. Optionally, they can be made of the same material. Alternatively, the flexible base layer and the flexible conductive layer can be directly laminated together, which is beneficial for improving compatibility and bonding strength, and enhancing mechanical stability during the two-layer bonding process. The shape and area of ​​the flexible conductive layer and the flexible base layer can also be the same or different.

[0083] [Regarding the structure of the entire multilayer flexible circuit]

[0084] In some embodiments, there is one flexible substrate layer and one flexible conductive layer. The stacked flexible substrate layer and flexible conductive layer have a synergistic effect; this stacked structure allows current to pass through one layer while insulating the other, which is beneficial for improving mechanical and electrical stability. In the exemplary example, compared to the case with only one flexible conductive layer, the stacked flexible conductive layer and flexible substrate layer can increase the upper limit of mechanical tensile strain by two to three times, and the electrical stability can also be significantly improved by tens or even hundreds of times.

[0085] In some embodiments, when there are ≥2 flexible substrate layers, the flexible substrate layers and flexible conductive layers are alternately stacked. When the multilayer flexible circuit is subjected to stress and deformation, this alternating stacking arrangement facilitates mutual support between the layers, improving mechanical stability under tensile and other strain conditions. Simultaneously, the alternating stacking arrangement can fully utilize the Z-axis space in the thickness direction, improving the integration density of the multilayer flexible circuit. The number of flexible substrate layers can be, but is not limited to, any value or a range between 2, 3, 5, 8, 10, and 15 layers. When there are 2 flexible substrate layers, there can be 1 flexible conductive layer, i.e., 1 flexible conductive layer sandwiched between 2 flexible substrate layers, which further improves the conductivity stability of the multilayer flexible circuit. When there are ≥2 flexible substrate layers and ≥2 flexible conductive layers, the conductivity between different flexible conductive layers is involved, which has a significant impact on the electrical stability of the entire multilayer flexible circuit; therefore, please refer to the description below.

[0086] In some embodiments, when there are ≥2 flexible conductive layers, the flexible substrate layer and the flexible conductive layer are alternately stacked. In the flexible substrate layer sandwiched between two adjacent flexible conductive layers, a plurality of through-holes are formed along the thickness direction. The flexible conductive layer extends into at least one through-hole contained in the adjacent flexible substrate layer, allowing the adjacent flexible conductive layers to be electrically connected through the through-holes. Alternating stacking facilitates mutual support between layers, improves mechanical stability, and also increases the integration density of the multilayer flexible circuit. The number of flexible conductive layers can be, but is not limited to, any value or a range between 2, 3, 5, 8, 10, and 15 layers. The flexible substrate layer sandwiched between two adjacent flexible conductive layers has through-holes, and the number of through-holes per layer can be, but is not limited to, 1 to 20. The through-holes provide a channel for electrical connection between the adjacent flexible conductive layers, eliminating the need for liquid metal, anisotropic conductive adhesive, etc., as in existing technologies. Z-axis conduction in the thickness direction can be achieved through the extended portion of the flexible conductive layer. The extension method can include, but is not limited to, at least one of the following: the edge of the flexible conductive layer extending into the via, or the surface of the flexible conductive layer forming a protrusion extending into the via. Adjacent flexible conductive layers can be, but is not limited to, one extending into the via and directly connecting to the flat surface of the other layer, or both layers extending into the via and their extended portions directly connecting. This structural design facilitates stable mechanical and electrical connections between adjacent flexible conductive layers, ensuring both mechanical and electrical stability when the multilayer flexible circuit deforms. The flexible substrates of adjacent flexible conductive layers can be identical, which helps improve the stability of the electrical connection via. In the example, the diameter of the via can be 1 mm to 3 mm, which helps balance the conductivity between adjacent flexible conductive layers with the structural integrity and stretchability of the flexible substrate itself. The shape of the via can include, but is not limited to, regular shapes such as cylinders and prisms, as well as other irregular shapes.

[0087] When there are two or more flexible conductive layers, they are alternately stacked with a flexible substrate layer. This structural design separates the flexible conductive layers through the flexible substrate layer, and each flexible conductive layer has its own conductive path, which can connect to electronic components independently, thereby reducing mutual interference. At the same time, local electrical connections are also achieved between adjacent flexible conductive layers, realizing Z-axis conduction in the thickness direction, improving the integration and synergy of electronic components.

[0088] In the above embodiments, the following can be summarized: When there are ≥2 flexible substrate layers and / or ≥2 flexible conductive layers, the flexible substrate layers and flexible conductive layers are alternately stacked. The flexible substrate layer sandwiched between two adjacent flexible conductive layers has through-holes. The outermost flexible substrate layer of the stacked structure may or may not have through-holes. If the outermost flexible substrate layer needs to connect to electronic components, it can have through-holes, allowing the flexible conductive layer to extend into the through-holes and electrically connect to the electronic components. The outermost layer of the stacked structure can be either a flexible conductive layer or a flexible substrate layer, preferably a flexible substrate layer, to protect the flexible conductive layer. In the example, the number of flexible substrate layers is n (n≥2), then the number of flexible conductive layers can be any one of n-1, n, or n+1 layers. When the flexible conductive layer is n-1 layers, both outermost layers of the stacked structure are flexible substrate layers. These two layers may or may not have through-holes, but within the stacked structure, the flexible substrate layer sandwiched between two adjacent flexible conductive layers needs to have through-holes. When the flexible conductive layer has n layers, the two outermost layers of the stacked structure are a flexible conductive layer and a flexible substrate layer. The flexible substrate layer may or may not have through-holes, but within the stacked structure, the flexible substrate layer sandwiched between two adjacent flexible conductive layers needs to have through-holes. When the flexible conductive layer has n+1 layers, both outermost layers of the stacked structure are flexible conductive layers. Choosing either n-1 or n layers is more beneficial for protecting the flexible conductive layer.

[0089] In some embodiments, when there are ≥2 flexible conductive layers, at least one flexible conductive layer includes at least two non-contact conductive units. These conductive units extend into at least one via in an adjacent flexible substrate layer, allowing adjacent flexible conductive layers to be electrically connected via the via. This structural design saves flexible conductive material and provides the flexible conductive layer with multiple conductive paths that do not interfere with each other, allowing for the separate connection of electronic components. Compared to a single flexible conductive layer structure, where multiple electronic components connected to the same layer share conductive paths and influence each other, this conductive unit structure optimizes this phenomenon and improves the integration of multilayer flexible circuits. The distribution of these conductive units can be designed according to actual needs; they can be uniformly distributed in the flexible conductive layer or distributed according to a specific pattern or rule. Furthermore, the shape of the conductive units can be any suitable shape, including but not limited to circles, squares, and stripes. The shapes of the conductive units can be the same or different, and they can extend into the vias in adjacent flexible substrate layers to achieve electrical connection. The extension and electrical connection can be referenced in the above description of the extension and electrical connection methods and forms of flexible conductive layers. This flexible conductive layer structure containing conductive units enables multilayer flexible circuits to have high flexibility and performance, adapting to the needs of circuit connection and function implementation in different scenarios.

[0090] In some embodiments, at least some adjacent flexible conductive layers each include at least two conductive units. A conductive unit in one flexible conductive layer intersects with a conductive unit in an adjacent flexible conductive layer. This intersecting arrangement can involve one or more conductive units sequentially, with the intersection point corresponding to a through-hole. The conductive unit extends into the through-hole from the intersection point. This structure, based on the aforementioned conductive unit structure design, further incorporates this design for both adjacent flexible conductive layers. This intersecting arrangement with the intersection point corresponding to the through-hole further reduces localized stress concentration, improves the stability and reliability of the mechanical and electrical connections between adjacent flexible conductive layers, and enhances the electrical stability of the entire multilayer flexible circuit. Moreover, this design allows for flexible adjustment of the intersecting position and number of conductive units according to the functional requirements of the actual circuit, enabling precise control of electrical connections in different parts of the circuit and achieving high integration of multiple functions. In the example, the conductive unit can be strip-shaped, and the width of the strip-shaped conductive unit can be 1 mm. Figure 11 , Figure 12As shown, in this way, the strip-shaped conductive units in adjacent flexible conductive layers intersect at a certain angle. When subjected to external forces such as bending and stretching, the stress will be evenly distributed along the strip-shaped conductive units, improving the stability of the connection between the conductive units in adjacent flexible conductive layers and enhancing the electrical stability of the entire multilayer flexible circuit. Furthermore, based on the diverse requirements of different complex electronic devices for circuit functions, the position of vias (i.e., the position where conductive units intersect), the number of vias, and the number of conductive units can be set, allowing for precise control of the electrical connections in different parts of the multilayer flexible circuit.

[0091] A second aspect of this application provides a method for fabricating a multilayer flexible circuit as described in the above-described embodiments, comprising the following steps S10 to S20:

[0092] Step S10: Fill the flexible substrate solution with metal nanowire aerogel and perform film formation treatment to obtain a flexible conductive layer;

[0093] Step S20: The flexible conductive layer and the flexible substrate layer are stacked and hot-pressed to obtain a multilayer flexible circuit.

[0094] The preparation method of this application first fills the metal nanowire aerogel, and then prepares a flexible conductive layer by wet film deposition. This flexible conductive layer has good stretchability, conductivity, and electrical stability. Then, it is laminated with a flexible substrate and hot-pressed to form a multilayer structure. Through the synergistic effect of both, the resulting multilayer flexible circuit maintains good conductivity, while its upper limit of mechanical stretchability and electrical stability are significantly improved. This preparation method is process-controllable, and the resulting multilayer flexible circuit has stable structure and properties.

[0095] Step S10 is the step of preparing a flexible conductive layer through a wet film-forming process. The material types and parameters of the metal nanowire aerogel can be found in the description of the metal nanowire aerogel in the multilayer flexible circuit of the embodiments described in the previous application. In the exemplary example, the metal nanowire aerogel includes silver nanowire aerogel, and its preparation method and related parameters can be found in the patent publication text with publication number CN119060540A.

[0096] In some embodiments, before filling the metal nanowire aerogel with the flexible substrate solution, a step of compressing the metal nanowire aerogel is included, wherein the volume of the compressed metal nanowire aerogel is 2% to 3% of that before compression. Metal nanowire aerogels are ultralight and very soft, making conventional mixing and stirring methods difficult to use when filling them with the flexible substrate solution; methods such as dripping and immersion are required. During this process, the metal nanowire aerogel is also prone to structural collapse, leading to problems such as uneven thickness and unstable performance. Furthermore, directly filling the metal nanowire aerogel with the flexible substrate solution requires a large amount of solution, affecting the final performance. Therefore, compressing the metal nanowire aerogel beforehand is beneficial for the uniformity of subsequent filling. In an example, the metal nanowire aerogel can be laser-cut into thicknesses of 3 nm to 10 nm, and then compressed using a tensile testing machine at a pressure of 80 kPa to 400 kPa. Figure 1 .

[0097] The types of flexible substrates can be referred to the description of the flexible substrates used in the multilayer flexible circuits in the embodiments of the above application. The solvent of the flexible substrate solution may include toluene, and the solution concentration may be 5 wt% to 30 wt%. This preparation method fills the flexible substrate solution in metal nanowire aerogel, with the metal nanowire aerogel as the main body, which is beneficial to maintaining its interwoven nanowire microstructure, rather than the conventional method of dispersing conductive materials in the flexible substrate solution, with the flexible substrate as the main body, otherwise it is difficult to have good electrical stability.

[0098] Solution filling can be achieved by dropping the solution into the metal nanowire aerogel. Film formation processes include spin coating, a method with a high success rate and superior product performance. Specifically, based on the area of ​​the compressed metal nanowire aerogel, the solution drop volume is approximately 1 μL / mm². 2 The dropping speed should be such that the solvent does not evaporate too much; dropping can be completed within 20 seconds, followed by spin coating. Due to the three-dimensional interwoven structure of the metal nanowire aerogel, it can absorb and contain a large amount of solution. After filling, the solution may be excessive. Spin coating can remove the excess solution while maintaining the solution level. The spin coating speed can be selected above 500 r / min. During the process, a metal nanowire aerogel film filled with solution is formed. Removing the solvent then yields a flexible conductive layer film filled with metal nanowire aerogel in a flexible substrate. Solvent removal can be achieved through methods such as solvent evaporation using a vacuum pump.

[0099] Solution filling can also be used to immerse the metal nanowire aerogel in a solution, followed by film formation and dip coating. Dip coating is a contact solution deposition method in which a glass substrate with metal nanowire aerogel is immersed at a fixed speed in a tank containing a solution of a flexible substrate for a very short time, and then removed at a fixed speed. The solution will cover the substrate and fill the metal nanowire aerogel. Finally, excess solvent is removed by heat treatment and drying to form a uniform flexible conductive film.

[0100] Step S20 is the hot-pressing process. A flexible substrate layer can be prepared first, which can be fabricated using a wet film deposition method. Specifically, the material of the flexible substrate layer is prepared into a solution, with toluene as the solvent and a concentration of 5 wt% to 30 wt%. The solution is dropped onto the surface of a glass substrate and spin-coated to form a liquid film. The solvent is then removed by heating, resulting in a flexible substrate layer film. After obtaining the flexible substrate layer, a flexible conductive layer can be stacked on its surface, followed by hot-pressing. Hot-pressing promotes a tight bond between the flexible conductive layer and the flexible substrate layer. It has been found that hot-pressing significantly increases the upper limit of mechanical stretchability and significantly reduces the rate of change of resistance under the same deformation, resulting in low resistance even after long-term use. In some embodiments, the temperature of the hot-pressing treatment can be 190°C to 205°C, and may include, but is not limited to, any value or any two of 190°C, 195°C, 200°C, and 205°C; the time of the hot-pressing treatment can be 30 s to 90 s, and may include, but is not limited to, any value or any two of 30 s, 45 s, 60 s, 75 s, and 90 s; the pressure of the hot-pressing treatment can be 100 kPa to 210 kPa, and in an exemplary example, may include, but is not limited to, any value or any two of 100 kPa, 130 kPa, 150 kPa, 180 kPa, and 210 kPa. Using these temperature, time, and pressure parameters for hot-pressing treatment facilitates a tight bond between the flexible conductive layer and the flexible substrate layer, forming a stable multilayer structure, which is beneficial to the integrity and performance stability of the multilayer flexible circuit structure.

[0101] The material and thickness of the flexible substrate can be referred to the description of the flexible substrate in the multilayer flexible circuit of the above application embodiment.

[0102] In some embodiments, when there are ≥2 flexible conductive layers, the hot pressing process in step S20 may include the following steps S21 to S22:

[0103] Step S21: Create several through holes along the thickness direction in at least a portion of the flexible substrate layer;

[0104] Step S22: The flexible conductive layer and the flexible substrate layer are alternately stacked and hot-pressed to extend the flexible conductive layer into at least one through hole contained in the adjacent flexible substrate layer, so that the two adjacent flexible conductive layers are electrically connected through the through hole.

[0105] Steps S21 and S22 are similar to additive manufacturing, involving alternating hot-pressing of each layer. Because the flexible conductive layer possesses a certain degree of stretchability and elasticity, during the hot-pressing process, under appropriate pressure and other parameters, it extends into the through-hole and electrically connects with an adjacent flexible conductive layer through the through-hole, thus creating a multi-layer flexible circuit where each flexible conductive layer is electrically conductive. This structural design facilitates the formation of stable mechanical and electrical connections between adjacent flexible conductive layers, ensuring both mechanical and electrical stability when the multi-layer flexible circuit deforms. Similarly, this fabrication method follows the principle of creating through-holes in the flexible substrate layer sandwiched between adjacent flexible conductive layers. In the example, the quantitative relationship between the flexible conductive layer and the flexible substrate layer, as well as the structural design, can be found in the description of the multi-layer flexible circuit in the embodiments of the above application. The flexible conductive layer may include at least two non-contact conductive units, specifically as described in the description of the conductive units in the multi-layer flexible circuit in the embodiments of the above application.

[0106] In addition to steps S21 and S22 described above, in some embodiments, when there are ≥2 flexible conductive layers, steps S23 to S26 are also included:

[0107] Step S23: Create several through holes along the thickness direction in at least a portion of the flexible substrate layer;

[0108] Step S24: Stack a flexible conductive layer and a flexible substrate layer together and perform hot pressing to obtain a stacked unit;

[0109] Step S25: The two stacked units are laminated and bonded together, so that each flexible conductive layer and each flexible substrate layer are stacked alternately in sequence, and then hot-pressed to extend the flexible conductive layer into at least one through hole contained in the adjacent flexible substrate layer, so that the two adjacent flexible conductive layers are electrically connected through the through hole.

[0110] Step S26: Perform bonding and hot pressing on the remaining stacked units in sequence.

[0111] This fabrication method is based on a stacked unit consisting of a flexible conductive layer and a flexible substrate layer, which are periodically and repeatedly stacked and then hot-pressed to form a multilayer flexible circuit. The advantage of this method is that it facilitates mass production and improves the consistency of the resulting multilayer flexible circuits. Each stacked unit can be identical or differ in thickness, material, shape, etc. Especially when each stacked unit is identical, identical stacked units can be mass-produced and then subjected to a similar hot-pressing assembly process. Similarly, this fabrication method follows the principle of creating through-holes in the flexible substrate layer sandwiched between adjacent flexible conductive layers. Furthermore, the number of flexible substrate layers and flexible conductive layers in the multilayer flexible circuit fabricated in this way is the same. To fabricate multilayer flexible circuits with other structures, the method described in steps S21 to S22 above can be referenced. After fabricating the multilayer flexible circuit, the required flexible conductive layer or flexible substrate layer can be stacked on the outermost layer and then hot-pressed.

[0112] A third aspect of this application provides a flexible electronic system, including the multilayer flexible circuit described in the above-described embodiments of this application or the multilayer flexible circuit prepared by the preparation method described in the above-described embodiments of this application; it also includes a plurality of electronic components, which are electrically connected to the flexible conductive layer.

[0113] The flexible electronic system in this application includes a multilayer flexible circuit, thus possessing the characteristics of high mechanical stability, good conductivity, and high electrical stability. During strain processes such as stretching and bending, the resistance does not change significantly, which is beneficial to the stability and accuracy of electrical signal transmission in the entire flexible electronic system, and improves the stability of the operation of each electronic component and their synergistic effect.

[0114] In the flexible electronic system of this application embodiment, the types and quantities of electronic components can be flexibly configured according to specific application requirements. Electronic components may include, but are not limited to, LEDs, surface-mount resistors, integrated chips, sensor elements, memory, and wireless communication modules. These electronic components can reliably connect to the flexible conductive layer in the multilayer flexible circuit, which is beneficial for stable transmission of electrical signals and the normal operation of the entire flexible electronic system. This flexible electronic system can be applied to wearable devices, such as smartwatches and head-mounted displays. These devices need to be stretchable and responsive to flexibly follow body movements and collect various data from the body. The system can also be applied in the medical and biomedical fields, such as implantable devices and biosensors, including implantable heart monitors, electroencephalogram (EEG) sensors, and smart bandages, to provide more accurate health monitoring and treatment. Furthermore, the system can also be applied to stretchable circuit boards, which have high stretchability, allowing the circuitry to be installed at joints such as the knees and elbows, as well as on free-form surfaces such as the neck and chest.

[0115] In some embodiments, when there are ≥2 flexible conductive layers, each flexible conductive layer may undertake different circuit functions or signal transmission tasks, and adjacent flexible conductive layers are electrically connected through vias. Therefore, aligning the positions of the electronic component connections with the vias is beneficial to the stability and accuracy of current or signal transmission between layers, reducing signal interference or transmission interruptions caused by connection misalignment. Simultaneously, this design also improves the integration and reliability of the flexible electronic system.

[0116] In some embodiments, the flexible conductive layer has protrusions on its sides relative to the flexible substrate, and electronic components are electrically connected to these protrusions. These protrusions provide additional connection areas for the electronic components, increasing the flexibility of electrical connections and improving the integration of the flexible electronic system. In an exemplary embodiment, the flexible conductive layer may be strip-shaped or comprise multiple strip-shaped conductive units, at least one end of which extends from the side as a protrusion for electrical connection to the electronic components.

[0117] The following description is based on specific embodiments.

[0118] Example A1

[0119] This embodiment provides a multilayer flexible circuit and its fabrication method, including a flexible conductive layer and a flexible substrate layer stacked together. The flexible conductive layer includes silver nanowire aerogel and a SEBS 1062 substrate filled with silver nanowire aerogel, with a thickness of 45 μm; the flexible substrate layer includes SEBS 1043 material, with a thickness of 45 μm.

[0120] The preparation method includes the following steps S1 to S4:

[0121] S1: Provide silver nanowire aerogel as described in the patent publication text with publication number CN119060540A, such as Figure 1 As shown in (a), the SEM image is as follows: Figure 2 As shown in (a), silver nanowire aerogel was cut into 6 mm thick pieces using a laser, and then placed on a tensile testing machine and compressed under a pressure of 200 kPa to obtain silver nanowire aerogel sheets. The volume of the compressed silver nanowire aerogel sheets shrank to 2.63% of their original volume. Figure 1 As shown in (b), the SEM image is as follows: Figure 2 As shown in (b).

[0122] S2: Place the cleaned and thoroughly dried glass substrate on a spin coater, and place the silver nanowire aerogel sheet on the glass substrate; take a flexible substrate of type SEBS 1062, wherein the styrene content is 18 wt%, the ethylene-butene content is 82 wt%, and the average molecular weight is 70000 g / mol, and prepare a 20 wt% solution of the flexible substrate with toluene; drop this solution onto the surface of the silver nanowire aerogel sheet at a rate of approximately 1 μL / mm. 2 The solution was gradually added to fill the silver nanowire aerogel, completing the filling process in 20 seconds. Then, the silver nanowire aerogel sheet filled with the solution on the glass substrate was spin-coated at 700 r / min to remove excess solution and form a solution-filled silver nanowire aerogel film. After spin-coating, the glass substrate was transferred to a vacuum pump to evaporate the solvent. After complete evaporation of toluene, a conductive film, i.e., a flexible conductive layer, was obtained on the glass substrate. It was observed that the volume shrinkage of the flexible substrate filled with the silver nanowire aerogel sheet was 1.32% of its original volume. Figure 1 As shown in (c), the SEM image is as follows: Figure 2 As shown in (c); finally, the conductive film is cut into the required shape using a laser.

[0123] S3: Provides SEBS 1043, with a styrene content of 67 wt%, an ethylene-butene content of 33 wt%, and an average molecular weight of 45,000 g / mol. Compared to SEBS 1062, SEBS 1043 has a higher styrene content, which gives it better elasticity and flexibility. SEBS 1043 is prepared into a solution with toluene at a concentration of 5 wt%–30 wt%. A clean and thoroughly dried glass substrate is placed on a spin coater, and the solution is dropped onto the surface of the glass substrate and formed into a thin film by spin coating. After spin coating, the glass substrate is transferred to a heating stage and heated at a set temperature to completely evaporate the toluene, obtaining an SEBS 1043 thin film, i.e., a flexible substrate layer, on the glass plate.

[0124] S4: A flexible conductive layer is placed on a flexible substrate layer, and a hot press is used to perform a hot pressing treatment at 200℃ and 200 kPa for 90 s to obtain a composite conductive film, i.e., a multilayer flexible circuit with one flexible conductive layer and one flexible substrate layer. The flexible substrate layer is also called the backing layer. SEBS 1062 and SEBS 1043 both have the self-healing properties of SEBS, which can be tightly bonded during hot pressing, resulting in a more stable structure after hot pressing. Furthermore, the self-healing properties of SEBS can effectively prevent the formation of surface cracks in the film when subjected to strain. At the same time, SEBS material has a low Young's modulus, which is close to the modulus of human skin, thus giving the device good flexibility and fit. After hot pressing, the Young's modulus further decreases, which can improve wearing comfort when used with human skin.

[0125] Examples A2 to A6

[0126] The difference between these embodiments and embodiment A1 is that the temperature of the hot pressing process in step S4 is changed to 25°C, 80°C, 100°C, 150°C and 250°C in sequence, while all other aspects are the same. Embodiments A2 to A6 are compared in sequence.

[0127] Examples A7 to A10

[0128] The difference between these embodiments and embodiment A1 is that the pressure of the hot pressing process in step S4 is changed to 50 kPa, 100 kPa, 300 kPa and 400 kPa in sequence, while all other aspects are the same, thus obtaining embodiments A7 to A10 in sequence.

[0129] Examples A11 to A15

[0130] The difference between these embodiments and embodiments A1, A3 to A6 is that the hot pressing time in step S4 is changed to 30 seconds, while all other aspects remain the same, thus obtaining embodiments A11 to A15 in sequence. That is, embodiment A1 is changed to embodiment A11, embodiment A3 is changed to embodiment A12, and so on, until embodiment A6 is changed to embodiment A15.

[0131] Examples A16 to A20

[0132] The difference between these embodiments and embodiments A1, A3 to A6 is that the hot pressing time in step S4 is changed to 60 seconds, while all other aspects remain the same, thus obtaining embodiments A16 to A20 in sequence. That is, embodiment A1 is changed to embodiment A16, embodiment A3 is changed to embodiment A17, and so on, until embodiment A6 is changed to embodiment A20.

[0133] Example A21

[0134] The difference between this embodiment and embodiment A1 is that the conductive films in step S2 are all cut into strips with a width of 1 mm, serving as conductive units in the flexible conductive layer, such as... Figure 11 As shown in (a); in step S3, through-holes are created on the flexible substrate using a laser. The width of the through-holes is slightly greater than 1 mm, and multiple through-holes form a rectangular array on the flexible substrate, as shown in Figure (a). Figure 11 As shown in (b); correspondingly, in step S4, multiple strip-shaped conductive units are placed parallel to each other on one surface of the flexible substrate and arranged along a rectangular array of through holes, with each conductive unit passing through multiple through holes, as shown in Figure (b). Figure 11 As shown in (c); it also includes the following steps S5 to S7:

[0135] S5: On the side of the flexible substrate layer away from the already hot-pressed flexible conductive layer, multiple strip-shaped conductive units are placed parallel to each other on one surface of the flexible substrate layer, perpendicular to the already hot-pressed conductive units, and arranged in a rectangular array along the through holes. Each conductive unit passes through multiple through holes. After hot pressing, the conductive units in adjacent flexible conductive layers extend into the through holes, electrically connecting the adjacent flexible conductive layers. Figure 12 As shown in (a).

[0136] S6: On the other side of the flexible conductive layer away from the flexible substrate layer, the unpressed flexible substrate layer is stacked on top of it, and the through-hole corresponds to the position of the press-fitted flexible substrate layer, and the hot-pressing process is performed.

[0137] S7: Repeat steps S5 and S6 until a multilayer flexible circuit with 3 flexible conductive layers and 4 flexible substrate layers is obtained.

[0138] Comparative Example A1

[0139] The difference between this embodiment and embodiment A1 is that steps S3 and S4 are omitted. The flexible substrate layer is not prepared separately and then bonded to the flexible conductive layer by hot pressing. Instead, after the flexible conductive layer is prepared in step S2, a toluene solution of SEBS1043 is repeatedly dropped onto the surface of the flexible conductive layer. The toluene is removed by spin coating and heating, and a flexible substrate is directly formed on the back of the flexible conductive layer. The thickness is the same as that of the flexible substrate layer in embodiment A1. Instead of hot pressing, the flexible substrate and the flexible conductive layer are integrated and no obvious layered structure is formed.

[0140] Comparative Example A2

[0141] The difference between this comparative example and Example A1 is that steps S3 and S4 are omitted, and only steps S1 and S2 are used to obtain the flexible conductive layer.

[0142] Example B1

[0143] This embodiment provides a flexible electronic system, including the multilayer flexible circuit of embodiment A21, and also includes multiple LED light-emitting components. These LED light-emitting components are connected to the extension portions of conductive units in the outermost through-hole of the multilayer flexible circuit, and the strip-shaped conductive units form protrusions along the length direction on the side of the stacked structure. These protrusions are electrically connected to copper wires, such as... Figure 12 As shown in (b), after power is applied, the LED emits light, as shown in Figure (b). Figure 12 As shown in (c), it is demonstrated that it has good functionality and high integration.

[0144] Before electrical connection, LEDs or copper wires are pretreated by being placed in a surface modifier solution (such as titanate coupling agent, zirconate coupling agent, aluminate coupling agent, etc.) to promote compatibility and adhesion between the two materials with different properties. After the pretreated device is removed from the solution, it is placed on the part of the conductive film to be connected. After the solvent has completely evaporated, the connection position is achieved by hot pressing.

[0145] Performance testing

[0146] 1. Morphology of the flexible conductive layer

[0147] According to the description in Example A1 and Figure 1 , Figure 2 The comparison shows that after compression treatment, the silver nanowire aerogels are more compact, and after being filled with SEBS, the silver nanowire aerogels can still effectively maintain overlap and maintain three-dimensional conductive pathways.

[0148] 2. Comparison of Stretch Limits

[0149] The strip-shaped conductive unit from Example A21 was hot-pressed onto a flexible substrate to form two samples with the letters "HIT" and "HITSZ" respectively. These samples were then subjected to a stretching process, with the stretching direction along the plane of the flexible substrate (this direction is used for all stretching tests below). Figure 10 As shown, from Figure 10 As can be seen, the composite conductive film obtained by hot-pressing one flexible conductive layer and one flexible substrate layer can be stretched significantly, and will not break even when stretched to a deformation of 340%.

[0150] The composite conductive film of Example A1, which was hot-pressed with a flexible substrate (backside), and the comparative example A2, which only had a flexible conductive layer, were subjected to tensile tests. The results are as follows: Figure 3 As shown. Figure 3 The "single-layer tensile failure" corresponds to Example A2, and the "hot-pressed backing tensile failure" corresponds to Example A1. From Figure 3 As shown in (a), electrical failure, or the upper limit of electrical tensile strength, is determined by a significant increase in resistance. The upper limit of electrical tensile strength is higher after hot-pressing, and the increase in resistance relative to strain is also more slow. From (b), it can be seen that the upper limit of electrical tensile strength is even higher after hot-pressing, capable of withstanding a stress of 13 MPa, with a tensile strain exceeding 1400%, while a single-layer film can only withstand a stress of about 4 MPa, with a tensile strain upper limit of about 450%. Clearly, the tensile properties of the composite conductive film after hot-pressing are significantly improved. Its stretchability exceeds that of human skin, allowing it to maintain normal operation even under significant strain and better adapt to various shapes and deformations.

[0151] 3. Comparison of electrical stability

[0152] The samples from Example A1, Comparative Example A1, and Comparative Example A2 were subjected to tensile tests. The relationship between the change in resistance and strain after tensile testing was observed. The results are as follows: Figure 4 As shown in (a). These samples were subjected to a tensile-recovery cyclic test with a strain of 50%, and the change in resistance was observed. The results are as follows. Figure 4 As shown in (b). Figure 4 In examples (a) and (b), "normal SEBS filling" corresponds to Example A2, "repeated SEBS addition" corresponds to Example A1, and "hot-pressed backing" corresponds to Example A1. From Figure 4 As can be seen in (a), when strain occurs in Example A1, the change in resistance is much lower than that in Comparative Examples A1 and A2, indicating that it is very stable. Figure 4 As can be seen in (b), during more than fifty cycles of 50% strain, the resistance change of Example A1 when strain occurred was much lower than that of Comparative Examples A1 and A2, indicating very high stability. Although Comparative Example A1 involved repeated addition of SEBS, the flexible substrate was formed in situ, and the resulting flexible substrate and flexible conductive layer were integrated. However, its electrical stability was not as good as that of Example A1, which involved hot-pressing two layers. Similarly, even with only one flexible conductive layer, its electrical stability was not as good as that of the hot-pressed layer. This demonstrates that hot-pressing can bring unexpected technical benefits.

[0153] The sample from Example A1 was subjected to a 50% strain-recovery tensile cycle test. The results of 300 cycles of the test in Example A1 are as follows: Figure 4 As shown in (c), it can be seen that even after 300 cycles of 50% strain, the resistance change per cycle is only about 0.0375 Ω, and even after 300 stretch-recovery cycles, the resistance does not increase significantly.

[0154] Samples from Example A1 and Comparative Example A2 were subjected to 4000 tensile-recovery cycles at 10% strain, and samples from Example A1 and Comparative Example A2 were subjected to 4000 tensile-recovery cycles at 50% strain. The test results, simulating long-term use, were recorded in [the relevant documentation / document]. Figure 5 In this context, "single-layer conductive film" corresponds to Example A2, and "composite conductive film" corresponds to Example A1. From... Figure 5 As can be seen in (a), whether it is a 10% strain or a 50% strain test, on the one hand, the resistance value of Example A1 is significantly lower than that of Comparative Example A2; on the other hand, the resistance value of Example A1 is very stable during tensile cycles, with very low variation; furthermore, after long-term tensile cycles, the resistance value of Example A1 does not increase significantly. Figure 5(b) Example A1 demonstrates its superior performance even more clearly in the results of 2220 to 2070 tensile-recovery cycles, with a maximum resistance change of only 0.03 Ω at 50% strain, which can maintain a small resistance change under normal operating strain.

[0155] Furthermore, the initial resistance of these samples is very low, especially that of Example A1, which is only about 0.05 Ω. Therefore, the composite conductive film (two-layer multilayer flexible circuit) of Example A1 not only has low resistance, but also good resistance-strain insensitivity characteristics, as well as resistance stability after long-term stretch-recovery cycles, and high electrical stability.

[0156] 4. Comparison of hot pressing treatment parameters

[0157] 4.1 Temperature of hot pressing treatment

[0158] Points 2 and 3 above have demonstrated that hot pressing significantly improves the mechanical tensile properties and electrical stability of two-layer multilayer flexible circuits (composite conductive films). Further sheet resistance tests were conducted on samples from Examples A1 to A6, and the results are as follows... Figure 6 As shown in Figure (a), it can be seen that the sheet resistance is the lowest when the hot pressing temperature of Example A1 is 200°C, which is only 0.032 Ω / sq. The sheet resistance increases when the temperature is lower or higher than this, but it is still only at the level of 0.035 Ω / sq to 0.05 Ω / sq, which is also very low compared to the prior art.

[0159] 4.2 Combined effect of temperature and time in hot pressing treatment

[0160] Further, samples from Examples A1 to A6 and Examples A11 to A20 were subjected to a 50% tensile test to measure the change in resistance. The combined effect of hot-pressing temperature and hot-pressing time on the change in resistance was observed, and the three-dimensional graph is shown below. Figure 6 As shown in (b), Example A1 exhibits the best hot-pressing effect at 200℃. At 200℃, a hot-pressing time of 90 s results in the lowest resistance change, indicating the best electrical stability. Other temperatures, such as 80℃ and 150℃, also show relatively ideal hot-pressing effects. However, the optimal hot-pressing time is not always 90 s at different temperatures; in some cases, 30 s is optimal. This demonstrates that the influence of hot-pressing temperature and time on the results is unpredictable and requires testing to determine.

[0161] The samples from Examples A1 and A3 were further subjected to a 50% strain tensile-recovery cyclic test, with 300 cycles. The results are as follows. Figure 7As shown in Figure (a), "200℃ 90 s" corresponds to Example A1. It can be seen that the resistance of both example samples is very low, and the rate of resistance change is also very low. Under 50% strain, the resistance change value of Example A1 is only about 0.035 Ω, while that of Example A3 is about 0.01 Ω. Moreover, after 300 tensile cycles, the external force is removed, and the sample returns to its original shape; the resistance does not significantly increase compared to before the cycle test.

[0162] Furthermore, electron microscopy was performed on the side of the stacked structure of the samples from Examples A1 and A3, and the SEM images are shown below. Figure 8 As shown. Figure 8 (a) shows the sample of Example A3 before hot pressing, and (b) shows the sample of Example A3 after hot pressing. Figure 8 (c) shows the sample of Example A1 before hot pressing, and (d) shows the sample of Example A1 after hot pressing. It can be seen that the hot pressing time of 90 s in Example A1 can better achieve a tight bond between the two layers.

[0163] The samples from Examples A1 and A11 were further subjected to a 50% strain tensile-recovery cyclic test, with 300 cycles. The results are as follows. Figure 7 As shown in (b), "200℃ 90 s" corresponds to Example A1. It can be seen that the resistance of both example samples is very low, and the rate of resistance change is also very low. Under 50% strain, the resistance change of Example A1 is only about 0.035 Ω, while that of Example A11 is about 0.04 Ω. Moreover, after 300 tensile cycles, the external force is removed, and the sample returns to its original shape; the resistance is not significantly increased compared to before the cycle test.

[0164] 4.3 Pressure during hot pressing

[0165] Sheet resistance was tested on samples from Examples A1, A7 to A10, and the results are as follows: Figure 9 As shown in (b), the sheet resistance is lowest at a pressure of 200 kPa. These samples were then subjected to 200 tensile-recovery cycles at 50% strain, and the results are as follows... Figure 9 As shown in (a), it can be seen that their resistances are all very low, and the resistance change at 50% strain is also very low. However, overall, the resistance at 200 kPa of Example A1 is the lowest, and the resistance change is the lowest.

[0166] The interfacial strength of samples from Examples A1, A7 to A10 after hot pressing was tested, and the results are as follows: Figure 9 As shown in (c), the interfacial strength gradually increases with increasing pressure. However, based on the above electrical performance test results, higher pressure does not necessarily mean a tighter interfacial bond or better electrical performance. Therefore, the effect of hot-pressing pressure on the results is unpredictable and requires further testing to determine.

[0167] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A multilayer flexible circuit, characterized in that: It includes a flexible substrate layer and a flexible conductive layer stacked together, wherein the flexible conductive layer includes a metal nanowire aerogel and a flexible substrate filled with the metal nanowire aerogel; The material of the flexible substrate layer includes at least one of styrene-ethylene-butene-styrene block copolymer and styrene-butadiene-styrene block copolymer; The flexible substrate includes at least one of styrene-ethylene-butene-styrene block copolymer and styrene-butadiene-styrene block copolymer; The flexible conductive layer has ≥2 layers, and the flexible substrate layer and the flexible conductive layer are alternately stacked; in the flexible substrate layer sandwiched between two adjacent flexible conductive layers, a plurality of through holes are formed along the thickness direction, and the flexible conductive layer extends into at least one of the through holes contained in the adjacent flexible substrate layer, so that the two adjacent flexible conductive layers are electrically connected through the through holes, and the diameter of the through holes is 1 mm to 3 mm; The method for fabricating the multilayer flexible circuit includes the following steps: A flexible conductive layer is obtained by filling a flexible substrate solution with metal nanowire aerogel and performing a film-forming process. The flexible conductive layer and the flexible substrate layer are stacked and hot-pressed to obtain a multilayer flexible circuit; wherein... The flexible conductive layer consists of ≥2 layers, and includes the following steps: At least a portion of the flexible substrate layer is provided with several through holes along the thickness direction; The flexible conductive layer and the flexible substrate layer are alternately stacked and subjected to the hot pressing process, so that the flexible conductive layer extends into at least one of the through holes contained in the adjacent flexible substrate layer, and the two adjacent flexible conductive layers are electrically connected through the through holes. or, The flexible conductive layer consists of ≥2 layers, and includes the following steps: At least a portion of the flexible substrate layer is provided with several through holes along the thickness direction; A flexible conductive layer and a flexible substrate layer are stacked together and then subjected to the hot pressing process to obtain a stacked unit. The two stacked units are laminated together, so that each flexible conductive layer and each flexible substrate layer are alternately stacked in sequence, and then the hot pressing process is performed, so that the flexible conductive layer extends into at least one of the through holes contained in the adjacent flexible substrate layer, so that the two adjacent flexible conductive layers are electrically connected through the through holes. The remaining stacked units are then subjected to the bonding process and the hot pressing process in sequence.

2. The multilayer flexible circuit according to claim 1, characterized in that: At least one of the flexible conductive layers includes at least two non-contacting conductive units, the conductive units extending into at least one of the vias contained in the adjacent flexible substrate layer, so that the two adjacent flexible conductive layers are electrically connected through the vias.

3. The multilayer flexible circuit according to claim 2, characterized in that: At least two adjacent flexible conductive layers each include at least two conductive units. The conductive units in one flexible conductive layer intersect with the conductive units in the adjacent flexible conductive layer, and the intersection point corresponds to the through hole. The conductive units extend into the through hole from the intersection point.

4. The multilayer flexible circuit according to any one of claims 1 to 3, characterized in that: The mass ratio of the metal nanowire aerogel to the flexible substrate is 1:(1.09–1.83); and / or, The thickness of the flexible substrate layer is 40 μm to 50 μm; and / or, The thickness of the flexible conductive layer is 30 μm to 50 μm; and / or, The material of the metal nanowire aerogel includes at least one of Ag, Au, Cu, Fe, and Ni; and / or, The diameter of the nanowires in the metal nanowire aerogel is 30 nm to 1000 nm; and / or, The density of the metal nanowire aerogel is 1 mg / cm³. 3 ~600 mg / cm 3 .

5. The multilayer flexible circuit according to claim 1, characterized in that: The hot pressing temperature is 190℃~205℃; and / or, The hot pressing treatment time is 30 s to 90 s; and / or, The pressure of the hot pressing treatment is 100 kPa to 210 kPa; and / or, The film-forming treatment includes at least one of spin coating and dip coating; and / or Before filling the metal nanowire aerogel with the flexible substrate solution, the method further includes a step of compressing the metal nanowire aerogel, wherein the volume of the compressed metal nanowire aerogel is 2% to 3% of that before compression.

6. A flexible electronic system, characterized in that: It includes a multilayer flexible circuit as described in any one of claims 1 to 5; it also includes a plurality of electronic components, wherein the electronic components are electrically connected to the flexible conductive layer.

7. The flexible electronic system according to claim 6, characterized in that: The flexible conductive layer is ≥2 layers, and the positions of the electrical connections of the electronic components are corresponding to the through holes; and / or, The flexible conductive layer has a protrusion on its side relative to the flexible substrate layer, and the electronic component is electrically connected to the protrusion.

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