A multilayer flexible wiring board and a method for manufacturing the same

Abstract

In order to overcome the problem of the integration adaptability of the existing flexible circuit board and battery, the present application provides a multilayer flexible circuit board, comprising a plurality of first flexible sub-layers, a plurality of second flexible sub-layers and a plurality of semi-solid layers, the first flexible sub-layers comprising a positive electrode power area and a first circuit area, the positive electrode power area comprising a first flexible base film, a positive electrode metal layer and a positive electrode material layer, the first circuit area comprising a second flexible base film and a first circuit pattern located on both sides of the second flexible base film, the second flexible sub-layers comprising a negative electrode power area and a second circuit area, the negative electrode power area comprising a third flexible base film, a negative electrode metal layer and a negative electrode material layer, the second circuit area comprising a fourth flexible base film and a second circuit pattern located on both sides of the fourth flexible base film. Meanwhile, the present application also discloses a preparation method of the above-mentioned multilayer flexible circuit board.

Description

Technical Field

[0001] This invention belongs to the field of circuit board technology, specifically relating to a multilayer flexible circuit board and its preparation method. Background Technology

[0002] In the field of electronic circuits, flexible circuit boards (PCBs) are widely used in various miniaturized and irregularly shaped electronic devices due to their bendability, thinness, and strong spatial adaptability. However, compared with rigid circuit boards, which have stable structures and strong load-bearing capacity, PCBs present significant challenges in loading electrical components: the mechanical properties of their flexible substrate limit the fixation of heavy, highly integrated components and the stability of signal transmission. This results in existing PCBs having relatively limited functions, typically serving only as "connecting bridges" in circuit systems, providing electrical connections between different rigid circuit boards, and unable to function as independent components to achieve complex circuit functions.

[0003] With the rapid development of wearable electronic devices (such as smart bracelets, flexible smart clothing, and portable medical monitoring devices), the market has placed higher demands on the functional integration of flexible circuit boards. Wearable devices need to possess both flexible deformation capabilities and complete independent operation capabilities. This requires flexible circuit boards to not only realize basic circuit connections but also integrate core modules such as power supply, sensing, and control, becoming independent functional components. However, in the process of realizing this requirement, battery compatibility has become a key technical obstacle.

[0004] Existing commercial batteries (such as pouch lithium batteries and button batteries) are basically non-deformable. If flexible circuit boards are connected to these batteries through external means, the rigidity of the batteries will greatly limit the deformation range and design freedom of the flexible circuit boards, resulting in a reduction in the overall flexibility of the device and failing to meet the usage requirements of wearable devices that conform to the human body and deform flexibly.

[0005] To address the limitations of external batteries, a type of flexible circuit board solution integrating batteries has emerged in the industry. However, the technological improvements of this type of solution are limited to the "physical integration" level, typically involving directly encapsulating pre-prepared battery cells into designated areas of the flexible circuit board using a molding process. This simple integration method has two major drawbacks: First, the rigid structure of the battery cells causes noticeable local protrusions in the corresponding encapsulation areas of the flexible circuit board, compromising the overall flatness and flexible appearance of the circuit board and affecting the fit between the device and the human body. Second, the area encapsulating the battery cells completely loses its deformability due to the rigidity of the cells, forming "rigid islands" on the flexible circuit board. This disrupts the overall flexible continuity of the circuit board, making it impossible to achieve uniform deformation across the entire area, and still failing to meet the stringent requirements for overall flexibility in wearable devices. Furthermore, both external batteries and encapsulated battery cells use lead wires for electrical connections. Due to the application environment of wearable devices, the flexible circuit board is prone to long-term deformation, posing a risk of electrical connection breakage. Summary of the Invention

[0006] To address the integration and compatibility issues of existing flexible circuit boards and batteries, this invention provides a multilayer flexible circuit board and its fabrication method.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0008] On one hand, the present invention provides a multilayer flexible circuit board, comprising multiple first flexible sublayers, multiple second flexible sublayers, and multiple pre-cured layers. The first flexible sublayers, the pre-cured layers, and the second flexible sublayers are stacked sequentially. The first flexible sublayers include a positive power region and a first circuit region. The positive power region includes a first flexible base film, a positive metal layer, and a positive material layer. The first flexible base film has multiple first through-holes. The positive metal layers are formed on both sides of the first flexible base film and are electrically connected through the first through-holes. The positive material layer is located on the surface of the positive metal layer away from the first flexible base film. The first circuit region includes a second flexible base film and first circuit patterns located on both sides of the second flexible base film. The first flexible base film and the second flexible base film are different regions of the same film. The positive metal layer is electrically connected to the first circuit patterns. The second flexible sublayers include a negative power region and a second circuit region. The negative power region includes a third flexible base film, a negative metal layer, and a negative material layer. The third flexible base film has multiple second through-holes. The negative metal layers are formed on both sides of the first flexible base film and are electrically connected through the first through-holes. The negative electrode metal layers are formed on both sides of the third flexible base film, and the negative electrode metal layers on both sides are electrically connected through the second through-hole. The negative electrode material layer is located on the surface of the negative electrode metal layer away from the third flexible base film. The second circuit region includes a fourth flexible base film and a second circuit pattern located on both sides of the fourth flexible base film. The third flexible base film and the fourth flexible base film are different regions of the same film. The negative electrode metal layer is electrically connected to the second circuit pattern. The negative electrode power region and the positive electrode power region are correspondingly arranged. The semi-cured layer has a window at the position corresponding to the positive electrode power region. A polymer electrolyte layer is arranged in the window. The polymer electrolyte layer is located between the positive electrode material layer and the negative electrode material layer. The multilayer flexible circuit board has a first electrically conductive hole, a second electrically conductive hole, and a third electrically conductive hole penetrating the first flexible sublayer, the second flexible sublayer, and the semi-cured layer. The first electrically conductive hole is sequentially electrically connected to multiple positive electrode metal layers. The second electrically conductive hole is sequentially electrically connected to multiple negative electrode metal layers. The third electrically conductive hole is electrically connected to two or more of the multiple first circuit patterns and multiple second circuit patterns.

[0009] Optionally, the thickness of the first flexible base film is lower than the thickness of the second flexible base film, so as to form a first groove recessed on the surface of the second flexible base film on both sides of the first flexible base film, and the positive electrode metal layer and the positive electrode material layer are located in the first groove;

[0010] The thickness of the third flexible base film is lower than that of the fourth flexible base film, so as to form a second groove recessed into the fourth flexible base film on both sides of the third flexible base film, and the negative electrode metal layer and the negative electrode material layer are located in the second groove.

[0011] Optionally, the positive electrode metal layer extends onto the second flexible base film to form a positive electrode tab, and the first conductive via is sequentially connected to a plurality of the positive electrode tabs;

[0012] The negative electrode metal layer extends onto the fourth flexible base film to form a negative electrode tab, and the second conductive via is sequentially connected to a plurality of the negative electrode tabs.

[0013] Optionally, the first circuit pattern and the positive electrode metal layer are aluminum layers, and the second circuit pattern and the negative electrode metal layer are copper layers.

[0014] Optionally, the semi-cured layer includes one or more of epoxy resin, cyanate ester resin, and polyphenylene ether resin.

[0015] Optionally, the first flexible base film, the second flexible base film, the third flexible base film, and the fourth flexible base film are selected from polyimide, polyethylene terephthalate, polyethylene naphthalate, or polytetrafluoroethylene.

[0016] Optionally, the first flexible base film of the outermost first flexible sublayer is provided with a positive electrode material layer only on the side facing the semi-cured layer; the third flexible base film of the outermost second flexible sublayer is provided with a negative electrode material layer only on the side facing the semi-cured layer.

[0017] Furthermore, the present invention provides a method for fabricating the multilayer flexible circuit board as described above, comprising the following steps:

[0018] S1. Obtain a first base film, and form a first flexible base film and a second flexible base film in different regions of the first base film respectively. Open a first through hole in the first flexible base film. After seed layer treatment on the surface of the first flexible base film and the second flexible base film, electroplate to form a first metal layer. Perform exposure and development operation on the first metal layer to etch and form a first circuit pattern and a positive electrode metal layer. Coat the positive electrode slurry on the positive electrode metal layer and dry it to form a positive electrode material layer, and obtain the first flexible sublayer.

[0019] S2. Obtain a second base film. A third flexible base film and a fourth flexible base film are formed in different regions of the second base film. A second through hole is opened on the third flexible base film. After seed layer treatment on the surface of the third flexible base film and the fourth flexible base film, a second metal layer is formed by electroplating. The second metal layer is exposed and developed to etch a second circuit pattern and a negative electrode metal layer. A negative electrode paste is coated on the negative electrode metal layer and dried to form a negative electrode material layer, thus obtaining the second flexible sublayer.

[0020] S3. Multiple first flexible sublayers, multiple second flexible sublayers and multiple semi-cured layers are stacked sequentially, such that a single semi-cured layer is sandwiched between adjacent first flexible sublayers and second flexible sublayers. The semi-cured layer has a window corresponding to the position of the positive electrode power region. A polymer electrolyte layer is disposed in the window. The stack is hot-pressed to obtain the laminate.

[0021] S4. Drill and metallize the laminate to form a first electrically conductive via, a second electrically conductive via, and a third electrically conductive via.

[0022] Optionally, in step S1, the first base film is subjected to hot pressing to form a first groove recessed on the surface of the second flexible base film on both sides of the first flexible base film.

[0023] In step S2, the second base film is subjected to hot pressing to form a second groove on both sides of the third flexible base film that is recessed into the surface of the fourth flexible base film.

[0024] Optionally, in steps S1 and S4, the opening method of the first through hole, the second through hole, the first electrically conductive through hole, the second electrically conductive through hole, and the third electrically conductive through hole is laser drilling.

[0025] The multilayer flexible circuit board provided by the present invention integrates the positive power region (including the positive electrode material layer) with the first circuit pattern in the first flexible sublayer, and integrates the negative power region (including the negative electrode material layer) with the second circuit pattern in the second flexible sublayer. The positive and negative power regions are correspondingly arranged, and the semi-cured layer has windows to accommodate the polymer electrolyte layer, forming an integrated structure of functional circuitry and battery. The design of the first and second flexible base films of the positive and negative power regions provides them with a certain degree of bending flexibility, enabling the multilayer flexible circuit board to simultaneously possess power supply and complex signal transmission functions. It can be used as an independent functional component, completely replacing traditional flexible circuit boards with externally connected rigid batteries or simply encapsulated rigid circuit boards. The battery cell design avoids rigid islands and localized protrusions, achieving full-area flexibility and continuity on the circuit board to meet the needs of wearable devices for conforming to the human body and flexible deformation. On the other hand, the positive electrode metal layer is integrally formed with the first circuit pattern, and the negative electrode metal layer is integrally formed with the second circuit pattern. The multi-layer sub-layer design connects multiple positive electrode metal layers through the first electrical conduction hole and multiple negative electrode metal layers through the second electrical conduction hole, which can stack to increase battery capacity and adapt to the long battery life requirements of wearable devices. At the same time, the electrical conduction hole achieves stable electrical connection of the multi-layer structure, realizing the adaptation of the battery's conductive structure to the flexible circuit board. It does not require additional electrical bridging structures, avoids the risk of poor contact of traditional external batteries, and improves the overall circuit stability. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the multilayer flexible circuit board provided by the present invention;

[0027] Figure 2 This is a flowchart of the preparation process of the first flexible sublayer in step S1 provided by the present invention;

[0028] Figure 3 This is a flowchart of the preparation process of the second flexible sublayer in step S2 provided by the present invention;

[0029] Figure 4 This is a schematic diagram of the structure of the laminated body in step S3 of the present invention.

[0030] The reference numerals in the accompanying drawings are as follows:

[0031] 1. First flexible sublayer; 1a. First base film; 11. Positive power supply region; 111. First flexible base film; 1111. First through-hole; 1112. First groove; 112. Positive metal layer; 113. Positive material layer; 114. Positive electrode tab; 12. First circuit region; 121. Second flexible base film; 122. First circuit pattern; 2. Second flexible sublayer; 2a. Second base film; 21. Negative power supply region; 211. Third flexible sublayer; 2111, second through hole; 2112, second groove; 212, negative electrode metal layer; 213, negative electrode material layer; 214, negative electrode tab; 22, second circuit area; 221, fourth flexible base film; 222, second circuit pattern; 3, semi-cured layer; 31, window; 32, polymer electrolyte layer; 4, first conductive through hole; 5, second conductive through hole; 6, third conductive through hole; 7, first metal layer; 8, second metal layer. Detailed Implementation

[0032] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0033] like Figure 1As shown, an embodiment of the present invention provides a multilayer flexible circuit board, including multiple first flexible sublayers 1, multiple second flexible sublayers 2, and multiple pre-cured layers 3. The first flexible sublayers 1, the pre-cured layers 3, and the second flexible sublayers 2 are stacked sequentially. The first flexible sublayers 1 include a positive power region 11 and a first circuit region 12. The positive power region 11 includes a first flexible base film 111, a positive metal layer 112, and a positive material layer 113. Multiple first through holes 1111 are formed on the first flexible base film 111. The positive metal layers 112 are formed on both sides of the first flexible base film 111, and the positive metal layers 112 on both sides are electrically connected through the first through holes 1111. The positive electrode material layer 113 is located on the surface of the positive electrode metal layer 112 away from the first flexible base film 111. The first circuit region 12 includes a second flexible base film 121 and a first circuit pattern 122 located on both sides of the second flexible base film 121. The first flexible base film 111 and the second flexible base film 121 are different regions of the same film. The positive electrode metal layer 112 is electrically connected to the first circuit pattern 122. The second flexible sub-layer 2 includes a negative electrode power region 21 and a second circuit region 22. The negative electrode power region 21 includes a third flexible base film 211, a negative electrode metal layer 212, and a negative electrode material layer 213. The third flexible base film 211 has a plurality of second through holes 2111. Negative electrode metal layers 212 are formed on both sides of the third flexible base film 211, and the negative electrode metal layers 212 on both sides are electrically connected through the second through-hole 2111. The negative electrode material layer 213 is located on the surface of the negative electrode metal layer 212 away from the third flexible base film 211. The second circuit region 22 includes a fourth flexible base film 221 and a second circuit pattern 222 located on both sides of the fourth flexible base film 221. The third flexible base film 211 and the fourth flexible base film 221 are different regions of the same film. The negative electrode metal layer 212 is electrically connected to the second circuit pattern 222. The negative electrode power region 21 and the positive electrode power region 11 are correspondingly arranged. The semi-cured layer 3 corresponds to the positive electrode. A window 31 is provided at the position of the power supply area 11. A polymer electrolyte layer 32 is disposed in the window 31. The polymer electrolyte layer 32 is located between the positive electrode material layer 113 and the negative electrode material layer 213. A first electrical conductive hole 4, a second electrical conductive hole 5, and a third electrical conductive hole 6 are provided on the multilayer flexible circuit board, penetrating the first flexible sublayer 1, the second flexible sublayer 2, and the semi-cured layer 3. The first electrical conductive hole 4 is sequentially electrically connected to a plurality of positive electrode metal layers 112. The second electrical conductive hole 5 is sequentially electrically connected to a plurality of negative electrode metal layers 212. The third electrical conductive hole 6 is electrically connected to two or more of a plurality of first circuit patterns 122 and a plurality of second circuit patterns 222.

[0034] By integrating the positive power region 11 and the first circuit pattern 122 into the first flexible sublayer 1, and integrating the negative power region 21 and the second circuit pattern 222 into the second flexible sublayer 2, with the positive power region 11 and the negative power region 21 correspondingly positioned, and the semi-cured layer 3 having windows to accommodate the polymer electrolyte layer 32, an integrated structure of functional circuitry and battery is formed. The design of the first and second flexible base films of the positive power region 11 and the negative power region 21 gives them a certain degree of bending flexibility, enabling the multilayer flexible circuit board to simultaneously provide power supply and complex signal transmission functions. It can be used as an independent functional component, completely replacing the traditional flexible circuit board solution of externally connected rigid batteries or simply plastic-encapsulated rigid cells, avoiding rigid islands and localized issues. The key features include: achieving full-area flexibility and continuity of the circuit board to meet the needs of wearable devices for conforming to the human body and flexible deformation; on the other hand, the positive electrode metal layer 112 is integrally formed with the first circuit pattern 122, and the negative electrode metal layer 212 is integrally formed with the second circuit pattern 222. The multi-layer sub-layer design connects the multi-layer positive electrode metal layer 112 in series through the first electrical conductive hole 4 and the multi-layer negative electrode metal layer 212 in series through the second electrical conductive hole 5, which can stack to increase battery capacity and adapt to the long battery life requirements of wearable devices; at the same time, the electrical conductive holes achieve stable electrical connection of the multi-layer structure, realize the adaptation of the battery conductive structure and the flexible circuit board, eliminate the need for additional electrical bridging structures, avoid the risk of poor contact of traditional external batteries, and improve the overall circuit stability.

[0035] In some embodiments, the thickness of the first flexible base film 111 is lower than the thickness of the second flexible base film 121, so that a first groove 1112 recessed into the surface of the second flexible base film 121 is formed on both sides of the first flexible base film 111, and the positive electrode metal layer 112 and the positive electrode material layer 113 are located in the first groove 1112.

[0036] The thickness of the third flexible base film 211 is lower than the thickness of the fourth flexible base film 221, so that a second groove 2112 is formed on both sides of the third flexible base film 211 that is recessed into the fourth flexible base film 221, and the negative electrode metal layer 212 and the negative electrode material layer 213 are located in the second groove 2112.

[0037] By making the thickness of the first flexible base film 111 lower than that of the second flexible base film 121, a recessed first groove 1112 is formed, which confines the positive electrode metal layer 112 and the positive electrode material layer 113 within the first groove 1112. Similarly, the negative electrode metal layer 212 and the negative electrode material layer 213 are accommodated by the second groove 2112, so that the power supply material does not exceed the surface of the circuit area base film, which completely solves the problem of local protrusion of the cell part in the existing integration solution, ensures the overall flatness of the multilayer flexible circuit board, and greatly improves the fit between wearable devices and human skin. The first groove 1112 and the second groove 2112 form physical limits and isolation for the positive and negative electrode materials, preventing the power supply material from shifting or falling off during the deformation of the multilayer flexible circuit board, and improving the service life of the battery.

[0038] The groove structure is formed based on the thickness difference of the same base film. Without the intervention of additional rigid structures, it will not damage the overall flexibility of the circuit board and ensure that the entire area can deform uniformly.

[0039] In some embodiments, the depth of the first groove 1112 is 10-100 μm, and the depth of the first groove 1112 is consistent with the total thickness of the positive electrode metal layer 112 and the positive electrode material layer 113. For example, when the thickness of the positive electrode metal layer 112 is 3-5 μm and the thickness of the positive electrode material layer 113 is 5-40 μm, the depth of the first groove 1112 is set to 8-45 μm. This ensures that after the positive electrode metal layer 112 and the positive electrode material layer 113 are fully embedded in the groove, their surfaces are flush with the surface of the second flexible base film 121, avoiding uneven thickness of the circuit board and adapting to scenarios with strict flatness requirements, such as smartwatch straps. Correspondingly, the depth of the second groove 2112 is 10-100 μm, and the depth of the second groove 2112 is consistent with the total thickness of the negative electrode metal layer 212 and the negative electrode material layer 213.

[0040] In some embodiments, the positive electrode metal layer 112 extends to the second flexible base film 121 to form a positive electrode tab 114, and the first conductive via 4 is sequentially connected to a plurality of the positive electrode tabs 114.

[0041] The negative electrode metal layer 212 extends onto the fourth flexible base film 221 to form a negative electrode tab 214, and the second conductive through hole 5 is sequentially connected to a plurality of negative electrode tabs 214.

[0042] The positive electrode tab 114 and the negative electrode tab 214 serve as the connection carriers for the electrically conductive vias. Compared to directly connecting to the metal layer of the power supply area, this increases the contact area, reduces contact resistance, lowers current transmission loss, and improves power supply efficiency. Simultaneously, this structure eliminates the need for battery terminals or other electrical connection adapters found in existing technologies, reducing battery size and increasing energy density.

[0043] In some embodiments, the positive electrode tab 114 and the negative electrode tab 214 are rectangular or trapezoidal in shape, with dimensions of 0.3-1mm × 0.2-0.5mm. The surface of the tab is provided with a nickel plating layer, which can enhance the corrosion resistance of the positive electrode tab 114 and the negative electrode tab 214, and at the same time improve the metallization bonding force with the first electrical conduction hole 4 and the second electrical conduction hole 5, so as to prevent the positive electrode tab 114 and the negative electrode tab 214 from detaching from the first electrical conduction hole 4 and the second electrical conduction hole 5 after long-term bending. This is suitable for smart wearable devices that require frequent deformation.

[0044] In some embodiments, in order to reduce the connection impedance between each positive electrode tab 114 and improve the electrical connection stability, a plurality of first electrical conductive holes 4 may be provided. The plurality of first electrical conductive holes 4 are arranged in an array in the corresponding area of ​​the positive electrode tab 114. Correspondingly, the number of second electrical conductive holes 5 may also be a plurality, and the plurality of second electrical conductive holes 5 are arranged in an array in the corresponding area of ​​the negative electrode tab 214.

[0045] In some embodiments, the first circuit pattern 122 and the positive electrode metal layer 112 are aluminum layers, and the second circuit pattern 222 and the negative electrode metal layer 212 are copper layers.

[0046] Aluminum and copper are both flexible metal materials with good adhesion to flexible base films (such as polyimide). The high rigidity of the metal layer will not damage the deformation capability of the circuit board. Moreover, the electroplating and etching processes are mature and compatible with existing flexible circuit board manufacturing processes, without the need to introduce special materials or equipment. The aluminum layer has excellent oxidation resistance and compatibility with cathode materials (such as good interface stability with cathode materials such as lithium iron phosphate and ternary materials), which can reduce oxidation and corrosion in the cathode area and improve the cycle life of the battery. The copper layer has high conductivity, which can reduce current transmission loss in the anode area and the circuit area and improve the overall conductivity efficiency of the circuit.

[0047] In some embodiments, the thickness of the first circuit pattern 122 and the positive electrode metal layer 112 are equal, and the thickness of the second circuit pattern 222 and the negative electrode metal layer 212 are equal.

[0048] In some embodiments, the semi-cured layer 3 includes one or more of epoxy resin, cyanate ester resin, and polyphenylene ether resin.

[0049] Epoxy resin, cyanate ester resin, and polyphenylene ether resin have strong temperature resistance (e.g., epoxy resin has a long-term temperature resistance of 120-150℃) and strong chemical corrosion resistance. They can resist the effects of human sweat and environmental humidity during the use of wearable devices, protect the internal positive and negative electrode materials and circuit patterns, and improve the environmental adaptability and service life of the circuit board.

[0050] In some embodiments, the first flexible base film 111, the second flexible base film 121, the third flexible base film 211 and the fourth flexible base film 221 are selected from polyimide, polyethylene terephthalate, polyethylene naphthalate or polytetrafluoroethylene.

[0051] Polyimide (PI) and polyethylene terephthalate (PET) are both highly flexible materials that can withstand tens of thousands of bends, meeting the long-term deformation requirements of wearable devices. They also possess excellent tensile strength and tear resistance, supporting the positive and negative electrode material layer 213 and the metal circuit layer.

[0052] In some embodiments, the first flexible base film 111 of the outermost first flexible sublayer 1 is provided with a positive electrode material layer 113 only on the side facing the semi-cured layer 3; the third flexible base film 211 of the outermost second flexible sublayer 2 is provided with a negative electrode material layer 213 only on the side facing the semi-cured layer 3.

[0053] In some embodiments, the positive electrode material layer 113 is disposed on both sides of the first flexible base film 111 of the first flexible sublayer 1 in the inner layer, and the negative electrode material layer 213 is disposed on both sides of the third flexible base film 211 of the second flexible sublayer 2 in the inner layer.

[0054] Another embodiment of the present invention provides a method for fabricating a multilayer flexible circuit board as described above, comprising the following steps:

[0055] S1, such as Figure 2 As shown, a first base film 1a is obtained, and a first flexible base film 111 and a second flexible base film 121 are formed in different regions of the first base film 1a. A first through hole 1111 is opened on the first flexible base film 111. After seed layer treatment on the surface of the first flexible base film 111 and the second flexible base film 121, a first metal layer 7 is formed by electroplating. The first metal layer 7 is exposed and developed to etch a first circuit pattern 122 and a positive electrode metal layer 112. A positive electrode paste is coated on the positive electrode metal layer 112 and dried to form a positive electrode material layer 113, thus obtaining the first flexible sublayer 1.

[0056] S2, such as Figure 3 As shown, a second base film 2a is obtained, and a third flexible base film 211 and a fourth flexible base film 221 are formed in different regions of the second base film 2a, respectively. A second through hole 2111 is opened on the third flexible base film 211. After seed layer treatment on the surface of the third flexible base film 211 and the fourth flexible base film 221, a second metal layer 8 is formed by electroplating. The second metal layer 8 is exposed and developed to etch and form a second circuit pattern 222 and a negative electrode metal layer 212. A negative electrode paste is coated on the negative electrode metal layer 212 and dried to form a negative electrode material layer 213, thus obtaining the second flexible sublayer 2.

[0057] S3, such as Figure 4 As shown, multiple first flexible sublayers 1, multiple second flexible sublayers 2, and multiple semi-cured layers 3 are stacked sequentially, such that a single semi-cured layer 3 is sandwiched between adjacent first flexible sublayers 1 and second flexible sublayers 2. The semi-cured layer 3 has a window 31 at the position corresponding to the positive electrode power region 11, and a polymer electrolyte layer 32 is disposed in the window 31. The stack is obtained by hot pressing.

[0058] S4. Drill and metallize the laminate to form a first electrically conductive hole 4, a second electrically conductive hole 5, and a third electrically conductive hole 6.

[0059] By employing a process of "first preparing flexible sublayers containing power and circuit regions (S1, S2), then stacking and hot-pressing + opening-hole metallization (S3, S4)," the core battery structure (positive and negative electrodes, electrolyte) and circuit board structure are manufactured simultaneously. This replaces the existing step-by-step process of first making the battery cell and then encapsulating it on the circuit board. This avoids problems such as structural misalignment and loose bonding caused by later assembly, improves the stability of the integrated structure, ensures the thickness consistency of the multilayer flexible circuit board at various locations, and enhances the adaptability of the multilayer flexible circuit board in various application environments.

[0060] In some embodiments, in step S1, the first base film 1a is subjected to hot pressing to form a first groove 1112 recessed on the surface of the second flexible base film 121 on both sides of the first flexible base film 111.

[0061] In step S2, the second base film 2a is subjected to hot pressing to form second grooves 2112 recessed on the surface of the fourth flexible base film 221 on both sides of the third flexible base film 211.

[0062] The first groove 1112 and the second groove 2112 are prepared by hot pressing. Compared with laser cutting and mechanical milling, this method can better maintain the integrity of the flexible base film, eliminate the risk of base film cracking caused by cutting stress, and ensure that the positive and negative electrode materials can be completely contained in the grooves without protrusion.

[0063] In some embodiments, in steps S1 and S4, the opening method of the first through hole 1111, the second through hole 2111, the first electrically conductive through hole 4, the second electrically conductive through hole 5, and the third electrically conductive through hole 6 is laser drilling.

[0064] The present invention will be further illustrated by the following examples.

[0065] Example 1

[0066] This embodiment illustrates the multilayer flexible circuit board and its fabrication method disclosed in this invention, including the following steps:

[0067] Step S1: Prepare the first flexible sublayer

[0068] Take a 30μm thick PI film (first base film) and hot press it at 150℃ and 1MPa for 60s to form a 10μm deep first groove on both sides of the first flexible base film region.

[0069] A laser is used to create a first through-hole with a diameter of 50 μm in the first flexible base film region. After plasma cleaning, the inner wall of the through-hole is chemically plated with copper to a depth of 0.5 μm.

[0070] The entire surface is electroplated with aluminum to a thickness of 3μm. After exposure and development, it is etched: the aluminum layer (positive electrode metal layer + positive electrode tab) is retained in the first flexible base film area, and the second flexible base film area is etched into the first circuit pattern.

[0071] A positive electrode slurry is coated on the surface of the positive electrode metal layer and dried at 80°C for 2 hours to obtain the first flexible sublayer.

[0072] Step S2: Prepare the second flexible sublayer

[0073] A 25μm thick PI film (second base film) is hot-pressed at 150℃ and 1MPa for 60s to form an 8μm deep second groove in the third flexible base film region.

[0074] A laser is used to create a second through-hole with a diameter of 50 μm in the third flexible base film region, and the inner wall of the through-hole is chemically plated with copper to a depth of 0.5 μm.

[0075] The entire surface is electroplated with copper to a thickness of 3μm, and after exposure and development, it is etched: the copper layer (negative electrode metal layer + negative electrode tab) is retained in the third flexible base film area, and the fourth flexible base film area is etched into the second circuit pattern.

[0076] A negative electrode slurry is coated on the surface of the negative electrode metal layer and dried at 80°C for 2 hours to obtain the second flexible sublayer.

[0077] Step S3: Lamination and Hot Pressing

[0078] Stack the layers in the following order: "First flexible sublayer → Epoxy resin semi-cured layer (windowed + electrolyte layer) → Second flexible sublayer → Epoxy resin semi-cured layer (windowed + electrolyte layer) → First flexible sublayer → Epoxy resin semi-cured layer (windowed + electrolyte layer) → Second flexible sublayer → First flexible sublayer → Epoxy resin semi-cured layer (windowed + electrolyte layer) → Second flexible sublayer" to ensure alignment of the positive and negative power supply areas.

[0079] Hot pressing parameters: 120℃ for 30s → 160℃ for 60s, pressure 1.5MPa, cool to room temperature to obtain the laminate.

[0080] Step S4: Drilling and Metallization

[0081] Laser-drilled first, second, and third electrically conductive vias with a diameter of 50μm;

[0082] The through-hole process is carried out sequentially as follows: desmearing (10% NaOH solution, 50℃, 5min) → pickling (5% H2SO4 solution, room temperature, 3min) → electroless copper plating (1μm, 40℃, 20min) → electroplating copper (3μm, 2A / dm², 30min).

[0083] After surface cleaning, a PTFE protective film is applied to the outermost layer, and windows are opened to expose the external connection pads, resulting in a multilayer flexible circuit board.

[0084] Comparative Example 1

[0085] This comparative example is used to illustrate the multilayer flexible circuit board and its fabrication method disclosed in this invention, and includes the following steps:

[0086] Six PI films with circuit layers are stacked sequentially, with an epoxy resin semi-cured layer between adjacent PI films; hot pressing is performed with the following parameters: 120℃ for 30s → 160℃ for 60s, pressure 1.5MPa, and the laminate is obtained by cooling to room temperature.

[0087] Laser-assisted drilling of electrically conductive vias;

[0088] The through-hole process is carried out sequentially as follows: desmearing (10% NaOH solution, 50℃, 5min) → pickling (5% H2SO4 solution, room temperature, 3min) → electroless copper plating (1μm, 40℃, 20min) → electroplating copper (3μm, 2A / dm², 30min).

[0089] A through hole is made in the middle of the laminate and filled with a pouch cell. The positive and negative tabs of the pouch cell are then soldered to the circuit in the laminate.

[0090] A PTFE protective film is attached to both sides of the laminate, and windows are opened to expose the external connection pads, resulting in a multilayer flexible circuit board.

[0091] Performance testing

[0092] The multilayer flexible circuit board prepared above was subjected to the following performance tests:

[0093] The prepared multilayer flexible circuit board was fixed on a bending tester with a bending radius of 10 mm. It was bent 180° repeatedly at a frequency of 20 times / min. The conductivity between the battery and the circuit was tested after 8000 bends.

[0094] A laser profilometer (accuracy 0.1μm) was used to scan the entire surface of the multilayer flexible circuit board and measure the surface undulation height; 20 points were selected in different regions (power area, circuit area, edge area) to measure the thickness and calculate the thickness range.

[0095] The test results are entered into Table 1.

[0096] Table 1

[0097]

[0098] As shown in Table 1, the multilayer flexible circuit board prepared by the method provided in this invention exhibits good electrical connection stability. Through groove design and matching with flexible materials, it can withstand long-term frequent bending, meeting the flexibility requirements of wearable devices. Furthermore, compared to existing solutions, the multilayer flexible circuit board prepared by the method provided in this invention demonstrates superior flatness and thickness uniformity, and its fit meets the requirements of wearable devices.

[0099] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multilayer flexible circuit board, characterized in that, The device comprises multiple first flexible sublayers, multiple second flexible sublayers, and multiple semi-cured layers, which are sequentially stacked. Each first flexible sublayer includes a positive power region and a first circuit region. The positive power region includes a first flexible base film, a positive metal layer, and a positive material layer. Multiple first through-holes are formed on the first flexible base film. The positive metal layers are formed on both sides of the first flexible base film and electrically connected through the first through-holes. The positive material layer is located on the surface of the positive metal layer facing away from the first flexible base film. The first circuit region includes a second flexible base film and first circuit patterns located on both sides of the second flexible base film. The first and second flexible base films are different regions of the same film. The positive metal layer is electrically connected to the first circuit patterns. Each second flexible sublayer includes a negative power region and a second circuit region. The negative power region includes a third flexible base film, a negative metal layer, and a negative material layer. Multiple second through-holes are formed on the third flexible base film. The negative metal layers are formed on the third flexible base film. The negative electrode metal layers on both sides are electrically connected through the second via. The negative electrode material layer is located on the surface of the negative electrode metal layer away from the third flexible base film. The second circuit region includes a fourth flexible base film and second circuit patterns located on both sides of the fourth flexible base film. The third flexible base film and the fourth flexible base film are different regions of the same film. The negative electrode metal layer is electrically connected to the second circuit pattern. The negative electrode power region and the positive electrode power region are correspondingly arranged. The semi-cured layer has a window at the position corresponding to the positive electrode power region. A polymer electrolyte layer is arranged in the window. The polymer electrolyte layer is located between the positive electrode material layer and the negative electrode material layer. The multilayer flexible circuit board has a first electrically conductive via, a second electrically conductive via, and a third electrically conductive via penetrating the first flexible sublayer, the second flexible sublayer, and the semi-cured layer. The first electrically conductive via is sequentially electrically connected to multiple positive electrode metal layers. The second electrically conductive via is sequentially electrically connected to multiple negative electrode metal layers. The third electrically conductive via is electrically connected to two or more of the multiple first circuit patterns and multiple second circuit patterns.

2. The multilayer flexible circuit board according to claim 1, characterized in that, The thickness of the first flexible base film is lower than that of the second flexible base film, so that a first groove is formed on both sides of the first flexible base film that is recessed into the surface of the second flexible base film, and the positive electrode metal layer and the positive electrode material layer are located in the first groove; The thickness of the third flexible base film is lower than that of the fourth flexible base film, so as to form a second groove recessed into the fourth flexible base film on both sides of the third flexible base film, and the negative electrode metal layer and the negative electrode material layer are located in the second groove.

3. The multilayer flexible circuit board according to claim 2, characterized in that, The positive electrode metal layer extends onto the second flexible base film to form a positive electrode tab, and the first conductive hole is sequentially connected to a plurality of the positive electrode tabs; The negative electrode metal layer extends onto the fourth flexible base film to form a negative electrode tab, and the second conductive via is sequentially connected to a plurality of the negative electrode tabs.

4. The multilayer flexible circuit board according to claim 1, characterized in that, The first circuit pattern and the positive electrode metal layer are aluminum layers, and the second circuit pattern and the negative electrode metal layer are copper layers.

5. The multilayer flexible circuit board according to claim 1, characterized in that, The semi-cured layer includes one or more of epoxy resin, cyanate ester resin, and polyphenylene ether resin.

6. The multilayer flexible circuit board according to claim 1, characterized in that, The first flexible base film, the second flexible base film, the third flexible base film and the fourth flexible base film are selected from polyimide, polyethylene terephthalate, polyethylene naphthalate or polytetrafluoroethylene.

7. The multilayer flexible circuit board according to claim 1, characterized in that, The first flexible base film of the outermost first flexible sublayer has a positive electrode material layer only on the side facing the semi-cured layer; the third flexible base film of the outermost second flexible sublayer has a negative electrode material layer only on the side facing the semi-cured layer.

8. The method for preparing a multilayer flexible circuit board according to any one of claims 1 to 7, characterized in that, The following steps are included: S1. Obtain a first base film, and form a first flexible base film and a second flexible base film in different regions of the first base film respectively. Open a first through hole in the first flexible base film. After seed layer treatment on the surface of the first flexible base film and the second flexible base film, electroplate to form a first metal layer. Perform exposure and development operation on the first metal layer to etch and form a first circuit pattern and a positive electrode metal layer. Coat the positive electrode slurry on the positive electrode metal layer and dry it to form a positive electrode material layer, and obtain the first flexible sublayer. S2. Obtain a second base film. A third flexible base film and a fourth flexible base film are formed in different regions of the second base film. A second through hole is opened on the third flexible base film. After seed layer treatment on the surface of the third flexible base film and the fourth flexible base film, a second metal layer is formed by electroplating. The second metal layer is exposed and developed to etch a second circuit pattern and a negative electrode metal layer. A negative electrode paste is coated on the negative electrode metal layer and dried to form a negative electrode material layer, thus obtaining the second flexible sublayer. S3. Multiple first flexible sublayers, multiple second flexible sublayers and multiple semi-cured layers are stacked sequentially, such that a single semi-cured layer is sandwiched between adjacent first flexible sublayers and second flexible sublayers. The semi-cured layer has a window corresponding to the position of the positive electrode power region. A polymer electrolyte layer is disposed in the window. The stack is hot-pressed to obtain the laminate. S4. Drill and metallize the laminate to form a first electrically conductive via, a second electrically conductive via, and a third electrically conductive via.

9. The method for preparing a multilayer flexible circuit board according to claim 8, characterized in that, In step S1, the first base film is subjected to hot pressing to form a first groove on both sides of the first flexible base film that is recessed into the surface of the second flexible base film. In step S2, the second base film is subjected to hot pressing to form a second groove on both sides of the third flexible base film that is recessed into the surface of the fourth flexible base film.

10. The method for preparing a multilayer flexible circuit board according to claim 8, characterized in that, In steps S1 and S4, the opening method for the first through hole, the second through hole, the first electrically conductive through hole, the second electrically conductive through hole, and the third electrically conductive through hole is laser drilling.

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