Flexible multilayer through-hole free circuit board structure, method of making and use thereof

Abstract

The application provides a flexible multilayer through-hole-free circuit board structure, a preparation method and application thereof. The circuit board structure comprises a second layer, a first layer and a third layer; each layer comprises a transparent insulating filling layer and a circuit grid; the circuit grid of the second layer is electrically connected with the circuit grid of the third layer through the circuit grid of the first layer. The application realizes vertical interconnection of the flexible transparent circuit board by constructing multilayer embedded conductive grids and realizing mutual conduction of the multilayer conductive grids, discards the inherent thought of constructing vertical through-hole structure in the prior art, and the obtained flexible multilayer through-hole-free circuit board structure has the characteristics of high permeability, no obvious visual visible point, stable connection, excellent conductive performance and flexibility, and is especially suitable for manufacturing full-transparent display devices.

Description

Technical Field

[0001] This invention belongs to the field of electronic circuits and display technology, specifically relating to a flexible multilayer circuit board structure without through holes, its preparation method and application. Background Technology

[0002] Conductive networks are essential structures for large-area display devices, enabling the control of the illumination and extinguishing of arrayed pixel units to achieve graphic display. Current processes primarily focus on constructing conductive meshes on a single side of the substrate. To fabricate a double-sided structure, it is typically necessary to perform two complete patterning and metallization processes independently on both sides of the substrate (e.g., two separate coating, exposure, development, and electroplating processes). This process is cumbersome, costly, and makes it difficult to guarantee precise alignment of the mesh patterns on both sides, resulting in significant challenges in yield control. Secondly, and more critically, achieving stable, low-resistance vertical interconnection (i.e., "vertical conduction") between two conductive mesh layers is a major challenge for existing fabrication methods. A common approach is to create micropores in the substrate after fabricating the upper and lower mesh layers separately, using methods such as mechanical drilling or laser ablation, and then attempt to fill the holes with metallization. This method not only introduces additional, high-precision alignment and processing steps, but the metal filling within the holes is often incomplete and has poor adhesion, making it highly susceptible to breakage due to stress concentration in subsequent flexible bending applications, leading to interconnect failure. Another approach is to use conductive adhesive or anisotropic conductive adhesive for bonding and conduction, but this introduces additional material interfaces, increases thickness, affects the uniformity of light transmission, and its connection resistance is usually high, raising questions about long-term reliability.

[0003] Transparent electronic devices, especially transparent display devices, represent a core technological direction for future human-computer interaction interfaces. Their ultimate goal is to achieve a perfect visual effect of "visible when powered on, invisible when powered off." However, all current transparent circuit technologies based on lamination processes face a fundamental and insurmountable flaw: the visual visibility of interlayer electrical interconnections.

[0004] like Figure 1 As shown, to achieve vertical interconnection between two-layer or multi-layer circuits, existing technologies invariably rely on "through-hole" technology. Whether in rigid PCBs, flexible FPCs, or transparent ITO circuits, micro-holes need to be drilled in the dielectric layer by means of mechanical drilling or laser ablation, and then the holes are metallized through processes such as electroplating and filling with conductive paste to form "through holes".

[0005] This manufacturing process has the following inherent problems: Visual visibility: Due to limitations in processing precision and material properties, the diameter of these through-holes is typically greater than 50 micrometers, and can even reach 200 micrometers. Against a transparent background, these through-holes filled with heterogeneous metal materials will form discrete, clearly visible "dark spots" or "bright spots" due to their optical properties (such as refractive index and reflectivity) that differ greatly from the surrounding medium. This severely damages the integrity and transparency of the displayed image, and is jokingly referred to in the industry as the "starry sky effect" or "pockmark effect".

[0006] Reliability issues: Through-hole structures have an interface between the metal and the dielectric material, which is prone to cracking and detachment under thermal or mechanical stress, leading to interconnect failure.

[0007] Complex process: The processes of drilling, hole wall cleaning, and hole metallization increase the complexity and cost of the manufacturing process.

[0008] Therefore, there is an urgent need in this field for an innovative, integrated fabrication method that can simultaneously form a high-precision conductive mesh on a flexible substrate within a single or continuous production cycle through a streamlined and efficient process, and achieve stable, low-resistance vertical electrical connections between them. While those skilled in the art have long focused on reducing the diameter of through-holes, no matter how small the diameter, discrete, optically discontinuous interfaces always exist, preventing true "full transparency." Existing technologies have fallen into a dead end due to the fixed mindset of "through-holes," while the market urgently demands a new interconnection technology that can fundamentally eliminate through-holes, creating a significant contradiction that urgently needs to be resolved. Summary of the Invention

[0009] The main objective of this invention is to provide a flexible multilayer circuit board structure without through holes, its preparation method and application, so as to overcome the shortcomings of the prior art.

[0010] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a flexible multilayer circuit board structure without through holes, which includes a second layer, a first layer and a third layer stacked sequentially along the thickness direction; The first layer includes a first transparent insulating filler layer and a first circuit grid embedded in the first transparent insulating filler layer; the second layer includes a second transparent insulating filler layer and a second circuit grid embedded in the second transparent insulating filler layer; and the third layer includes a third transparent insulating filler layer and a third circuit grid embedded in the third transparent insulating filler layer. The second circuit grid is electrically connected to the third circuit grid at least through the first circuit grid.

[0011] Secondly, the present invention also provides a method for preparing the above-mentioned flexible multilayer perforated circuit board structure, comprising: The first circuit grid is placed or formed on the substrate; A transparent polymer is filled into the gaps in the lines of the first circuit grid and cured to form a first transparent insulating filler layer, which is then combined to form the first layer. The first layer is removed, and patterned photoresist layers are formed on both sides of the first layer. The patterns of the patterned photoresist layers are complementary to the second circuit grid and the third circuit grid, respectively, and a portion of the surface of the first circuit grid is exposed. A second circuit grid and a third circuit grid are deposited in the gaps of the patterned photoresist layers on both sides, respectively. The patterned photoresist layer is removed, and transparent polymers are filled and cured in the second and third circuit grids to form a second transparent insulating filler layer and a third transparent insulating filler layer, which together form the second and third layers, respectively.

[0012] Thirdly, the present invention also provides an application of the above-mentioned flexible multilayer non-through-hole circuit board structure, namely a flexible fully transparent display device, which includes the above-mentioned flexible multilayer non-through-hole circuit board structure and pixel unit. The electrodes of the pixel unit are connected to the third circuit grid of the flexible multilayer non-through-hole circuit board structure, and can establish electrical connections with the external control circuit through at least the first circuit grid and the second circuit grid in sequence.

[0013] Compared with the prior art, the beneficial effects of the present invention include at least the following: This invention achieves vertical interconnection of flexible transparent circuit boards by constructing a multi-layer embedded conductive mesh and utilizing the interconnection of the multi-layer conductive mesh. It abandons the inherent idea of ​​constructing a vertical through-hole structure in the prior art. The resulting flexible multi-layer through-hole-free circuit board structure has the characteristics of strong transparency, no obvious visual visible points, stable connection, excellent conductivity and flexibility, and is especially suitable for making fully transparent display devices.

[0014] The above description is merely an overview of the technical solution of the present invention. In order to enable those skilled in the art to better understand the technical means of this application and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described below in conjunction with detailed drawings. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, 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 recorded in 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 perforated metallized structure shown in the background art of this invention; Figure 2 This is a schematic diagram of a flexible multilayer perforated circuit board structure provided by a typical embodiment of the present invention. Detailed Implementation

[0017] In view of the shortcomings of the prior art, the inventors of this invention, through long-term research and extensive practice, have proposed the technical solution of this invention. The following will further explain and illustrate this technical solution, its implementation process, and its principles.

[0018] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0019] See Figure 2 As shown, an embodiment of the present invention first provides a flexible multilayer through-hole-free circuit board structure, which includes a second layer, a first layer, and a third layer stacked sequentially along the thickness direction; the first layer includes a first transparent insulating fill layer and a first circuit grid embedded in the first transparent insulating fill layer, the second layer includes a second transparent insulating fill layer and a second circuit grid embedded in the second transparent insulating fill layer, and the third layer includes a third transparent insulating fill layer and a third circuit grid embedded in the third transparent insulating fill layer; wherein, the second circuit grid is electrically connected to the third circuit grid through the first circuit grid.

[0020] In some implementations, the linewidths of the first, second, and third circuit grids are all below 20 μm.

[0021] In some implementations, the thickness of any one of the first, second, and third layers is less than 20 μm.

[0022] In the above preferred embodiment, the line width and thickness of the circuit grid are both controlled below 20μm, preferably within the range of 1-20μm, in order to make them indistinguishable to the naked eye and thus achieve a fully transparent effect. Of course, depending on the specific application, the range of this value may vary. For example, for some larger transparent display devices, the viewing distance may be far, and the line width may still be invisible even if the size range is exceeded.

[0023] In some implementations, the materials of the first circuit grid, the second circuit grid, and the third circuit grid include any one or a combination of two or more of nickel, copper, or zinc.

[0024] In some embodiments, the materials of the first transparent insulating filler layer, the second transparent insulating filler layer, and the third transparent insulating filler layer include any one or a combination of two or more of polyimide, polyethylene terephthalate, polycarbonate, and pyrene.

[0025] This invention also provides a method for fabricating the above-mentioned flexible multilayer circuit board structure without through holes, which includes the following steps: The first circuit grid is placed or formed on the substrate; A transparent polymer is filled into the gaps in the lines of the first circuit grid and cured to form a first transparent insulating filler layer, which is then combined to form the first layer. Remove the first layer and form patterned photoresist layers on both sides of the first layer (when more layers are inserted as described below, it is also equivalent to indirectly forming on "both sides" of the first layer). The patterns of the patterned photoresist layers are complementary to the second circuit grid and the third circuit grid, respectively, and expose a portion of the surface of the first circuit grid. A second circuit grid and a third circuit grid are deposited in the gaps of the patterned photoresist layers on both sides, respectively. The patterned photoresist layer is removed, and transparent polymers are filled and cured in the second and third circuit grids to form a second transparent insulating filler layer and a third transparent insulating filler layer, which together form the second and third layers, respectively.

[0026] In some embodiments, the preparation method specifically includes the following steps: The metal thin film is patterned and etched to obtain the first circuit grid. The first circuit grid is placed on the surface of the substrate, and a liquid transparent polymer solution is dropped on it. After self-leveling, a curing reaction is carried out to form the first transparent insulating filler layer with the same thickness as the first circuit grid. The first layer is separated from the surface of the substrate to obtain an independent first layer for forming the second and third layers.

[0027] In some implementations, the formation of the first patterned photoresist layer and the second patterned photoresist layer specifically includes: Photoresist is applied by spin coating, scraping, or slot coating and then baked to cure, forming a complete photoresist layer. The photoresist layer is patterned by exposure and then cleaned and developed to form the first patterned photoresist layer or the second patterned photoresist layer.

[0028] In some implementations, the second and third circuit grids are formed by electrochemical deposition.

[0029] The preparation method provided by this invention does not adopt a bottom-to-top layer-by-layer deposition method to form a three-layer (or more) stacked structure. Instead, it adopts a preparation sequence of first forming an intermediate layer (i.e., the first layer), and then expanding the first layer on both sides to form the second and third layers (located on the front and back sides of the first layer). Of course, if a vertical interconnection structure with more layers is required, a fourth layer, a fifth layer, etc. can be inserted between the second or third layer and the first layer, respectively. (For ease of description, regardless of whether it is a three-layer or more-layer structure, this invention refers to the two outermost layers as the second and third layers, respectively. The remaining layers can be understood as insertion layers inserted between the first layer and the second or third layer, which also have a circuit grid to achieve interconnection.)

[0030] The above-mentioned fabrication sequence is specifically designed to match the specific circuit structure. For example, the first circuit network interconnects the second and third circuit networks, so its span in the planar direction is relatively large. If a layer-by-layer accumulation sequence is used for fabrication, that is, the connection end of the second circuit network is used as the conductive surface for electrochemical deposition (which is more efficient and cheaper than various metal deposition methods), it is difficult to ensure the formation of uniform first circuit grid lines over a large span due to the current distribution characteristics of electrochemical deposition, which will lead to various adverse problems. The fabrication method provided by this invention avoids the above problems by changing the stacking sequence. First, the middle first circuit grid is formed by pattern etching or pattern deposition. Then, based on the two ends of the first circuit grid, the second and third circuit grids on both sides can be formed simultaneously by electrochemical deposition on both sides in one step. This not only improves the deposition efficiency and saves costs, but also ensures the deposition uniformity of the second and third circuit grids.

[0031] This invention also provides a flexible, fully transparent display device as an application of the flexible multilayer perforated circuit board structure provided in any of the above embodiments. It includes the flexible multilayer perforated circuit board structure provided in any of the above embodiments and a pixel unit. The electrodes of the pixel unit are connected to the third circuit grid of the flexible multilayer perforated circuit board structure and can establish an electrical connection with an external control circuit through at least the first circuit grid and the second circuit grid in sequence.

[0032] The technical solution of the present invention will be further described in detail below through several embodiments and in conjunction with the accompanying drawings. However, the selected embodiments are only for illustrating the present invention and do not limit the scope of the present invention.

[0033] Example 1 This embodiment illustrates the fabrication process of a flexible multilayer circuit board structure without through holes, as detailed below.

[0034] Step 1: Etch a 1-20µm thick metal sheet into the desired circuit grid shape, with the grid linewidth controlled within 1-20µm. The metal sheet material includes, but is not limited to, any one or a combination of at least two of nickel, copper, or zinc. Step 2: A transparent insulating filler layer is prepared within the circuit grid. The thickness of the transparent insulating filler layer is equal to the thickness of the metal sheet, thus completing the fabrication of the first transparent flexible circuit board. The transparent insulating filler layer includes any one or a combination of at least two of the following: polyimide, polyethylene terephthalate layer, polycarbonate layer, or parylene layer. It should be noted that parylene is a protective polymer material and is a generic name for a unique series of parylene polymers.

[0035] Taking polyimide as an example, the specific process of the second step is as follows: the metal circuit grid from the first step is laid flat on a flat silicon wafer, a certain amount of polyimide adhesive is dripped into the gaps of the metal circuit grid, and after self-leveling, it is heated and cured at 180°C. The first layer is then peeled off.

[0036] Step 3: Coat both sides of the first transparent flexible circuit board with photoresist, expose and develop the second and third circuit grid shapes. Coating photoresist and developing is a standard industry practice. Spin coating, scraping coating or slot coating can be used. The line width of the patterned second and third circuit grid shapes is controlled within 1-20um.

[0037] Step 4: Place the first layer of transparent flexible circuit board into a metal plating bath to electroplate the second and third layers of circuit mesh, then wash away the photoresist. The photoresist can be removed by rinsing with acetone.

[0038] Step 5: Deposit a transparent insulating filler layer within the second and third circuit grid layers. The thickness of the transparent insulating filler layer is equal to the thickness of the second and third circuit grid layers, thus realizing the fabrication of a three-layer transparent flexible circuit board without through holes. The metal plating bath can be any one or a combination of at least two of nickel, copper, or zinc plating solutions, and the plating thickness is 1-20 μm.

[0039] Furthermore, if it is necessary to prepare a fourth or fifth layer, etc., other repetitive steps can be inserted before preparing the second or third layer.

[0040] More preferably, the preparation sequence of the transparent insulating filler layer in the fifth step is generally to first prepare the second transparent insulating filler layer, and then prepare the third transparent insulating filler layer after curing. This makes it easier to control the film thickness through adhesive coating, because there is a first flexible circuit board, and there is no need to remove the second transparent insulating filler layer at this time. Of course, it is also possible to use a method of coating both sides at the same time, as long as the film thickness can be controlled well, or the film thickness can be made too thick and then polished or etched to expose the circuit grid.

[0041] Example 2 This embodiment provides a complex multi-pixel fully transparent display device based on the structure provided in Embodiment 1 above, which can achieve the effect of displaying when powered on and being fully transparent when powered off. Only RGB pixels need to be soldered to the pads of the third circuit grid on the flexible circuit board. The control circuit is connected through the first circuit grid, the second circuit grid (and possibly other circuit grids), and the displayed image can be controlled by connecting and disconnecting the power to the control circuit.

[0042] Based on the above embodiments, it is clear that the embodiments of the present invention achieve vertical interconnection of flexible transparent circuit boards by constructing a multi-layer embedded conductive mesh and utilizing the interconnection of the multi-layer conductive mesh. This abandons the inherent idea of ​​constructing a vertical through-hole structure in the prior art. The resulting flexible multi-layer through-hole-free circuit board structure has the characteristics of strong transparency, no obvious visual visible points, stable connection, excellent conductivity and flexibility, and is especially suitable for making fully transparent display devices.

[0043] It should be understood that the above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A flexible multilayer circuit board structure without through holes, characterized in that, It includes a second layer, a first layer, and a third layer that are stacked sequentially along the thickness direction; The first layer includes a first transparent insulating filler layer and a first circuit grid embedded in the first transparent insulating filler layer; the second layer includes a second transparent insulating filler layer and a second circuit grid embedded in the second transparent insulating filler layer; and the third layer includes a third transparent insulating filler layer and a third circuit grid embedded in the third transparent insulating filler layer. The second circuit grid is electrically connected to the third circuit grid at least through the first circuit grid. The line widths of the first, second, and third circuit grids are all below 20 μm, and the planar span of the first circuit grid is greater than that of the second and third circuit grids.

2. The flexible multilayer perforated circuit board structure according to claim 1, characterized in that, The thickness of any one of the first, second, and third layers is less than 20 μm.

3. The flexible multilayer perforated circuit board structure according to claim 1, characterized in that, The materials of the first circuit grid, the second circuit grid, and the third circuit grid include any one or a combination of two or more of nickel, copper, or zinc.

4. The flexible multilayer perforated circuit board structure according to claim 1, characterized in that, The materials of the first transparent insulating filler layer, the second transparent insulating filler layer and the third transparent insulating filler layer include any one or a combination of two or more of polyimide, polyethylene terephthalate, polycarbonate and pyrene.

5. The method for preparing the flexible multilayer through-hole-free circuit board structure according to any one of claims 1-4, characterized in that, include: The first circuit grid is placed or formed on the substrate; A transparent polymer is filled into the gaps in the lines of the first circuit grid and cured to form a first transparent insulating filler layer, which is then combined to form the first layer. The first layer is removed, and patterned photoresist layers are formed on both sides of the first layer. The patterns of the patterned photoresist layers are complementary to the second circuit grid and the third circuit grid, respectively, and a portion of the surface of the first circuit grid is exposed. A second circuit grid and a third circuit grid are deposited in the gaps of the patterned photoresist layers on both sides, respectively. The patterned photoresist layer is removed, and transparent polymers are filled and cured in the second and third circuit grids to form a second transparent insulating filler layer and a third transparent insulating filler layer, which together constitute the second and third layers.

6. The preparation method according to claim 5, characterized in that, Specifically, it includes: The metal thin film is patterned and etched to obtain the first circuit grid. The first circuit grid is placed on the surface of the substrate, and a liquid transparent polymer solution is dropped on it. After self-leveling, a curing reaction is carried out to form the first transparent insulating filler layer with the same thickness as the first circuit grid. The first layer is separated from the surface of the substrate to obtain an independent first layer for forming the second and third layers.

7. The preparation method according to claim 5, characterized in that, The patterned photoresist layer is formed in the following ways: Photoresist is applied by spin coating, scraping, or slot coating and then baked to cure, forming a complete photoresist layer. The complete photoresist layer is patterned by exposure and then cleaned and developed to form the patterned photoresist layer.

8. The preparation method according to claim 5, characterized in that, The second and third circuit grids are formed by electrochemical deposition.

9. A flexible, fully transparent display device, characterized in that, Includes the flexible multilayer hole-free circuit board structure and pixel unit as described in any one of claims 1-4; The electrodes of the pixel unit are connected to the third circuit grid of the flexible multilayer non-through-hole circuit board structure, and can establish electrical connections with the external control circuit through at least the first circuit grid and the second circuit grid in sequence.

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