Flexible Circuit Boards and Their Manufacturing Methods

Abstract

This invention provides a high aspect ratio flexible circuit board and its manufacturing method. The high aspect ratio flexible circuit board includes a plastic substrate, a metal seed layer, and a copper circuit layer. The plastic substrate includes a first surface and a second surface opposite to each other. A first patterned groove is formed on the first surface and / or a second patterned groove is formed on the second surface. The upper limit of the total depth of the first patterned groove and the second patterned groove is the thickness of the plastic substrate minus 5 µm. The metal seed layer is formed on the first surface and / or the second surface. The copper circuit layer is formed on the metal seed layer corresponding to the first patterned groove and / or the second patterned groove. The copper circuit layer includes exposed circuit portions protruding from the plastic substrate and embedded circuit portions embedded in the plastic substrate. The height of the exposed circuit portions does not exceed 45 µm, and the height / width ratio of the copper circuit layer is in the range of 1.5 to 3.75. By using the high aspect ratio flexible circuit board as described above, the problem of dry film photoresist falling off the high aspect ratio flexible circuit board due to limited standing area can be avoided.

Description

Technical Field

[0001] This invention relates to a flexible circuit board, and more particularly to a flexible circuit board with a high aspect ratio. Prior Technology

[0002] Flexible printed circuit boards (FPCBs) are mainly made of polyimide and copper foil. They are flexible, heat-resistant, and thin. FPCBs used in voice coil motors are characterized by high aspect ratio and fine pitch. In conventional manufacturing processes, FPCBs are made by combining photolithography with pattern plating. Summary of the Invention

[0003] However, conventional high aspect ratio flexible circuit boards still have the following unresolved issues. In the circuit patterning process of high aspect ratio flexible circuit boards, especially designs with line spacing no greater than 60 µm, the limited standing area of ​​the dry film photoresist on the circuit board makes it prone to detachment during exposure and development processes. This can lead to errors in the circuit pattern formed during subsequent copper plating processes, resulting in short circuits. Therefore, how to prevent the dry film photoresist from detaching from high aspect ratio flexible circuit boards due to limited standing area remains an unsolved problem.

[0004] The present invention addresses the aforementioned problems by providing a high aspect ratio flexible circuit board, comprising: a plastic substrate including a first surface and a second surface, wherein a first patterned groove is formed on the first surface and / or a second patterned groove is formed on the second surface, and the upper limit of the total depth of the first patterned groove and the second patterned groove is the thickness of the plastic substrate minus 5. µm; a metal seed layer formed on the first surface and / or the second surface; and a copper circuit layer formed on the metal seed layer corresponding to the first patterned groove and / or the second patterned groove, the copper circuit layer including exposed circuit portions exposed outside the plastic substrate and embedded circuit portions embedded in the plastic substrate; wherein, the height of the exposed circuit portions does not exceed 45µm, the height of the embedded circuit portions is the depth of the first patterned groove or the depth of the second patterned groove, the width of the exposed circuit portions is greater than or equal to the width of the first patterned groove or the width of the second patterned groove, and the height / width ratio of the copper circuit layer is in the range of 1.5 to 3.75.

[0005] The high aspect ratio flexible circuit board described above, wherein the plastic substrate is a polyimide film and the thickness of the polyimide film is 12 to 150 µm.

[0006] In the high aspect ratio flexible circuit board described above, the first patterned groove and / or the second patterned groove are formed on the plastic substrate by laser irradiation.

[0007] In the high aspect ratio flexible circuit board described above, the composition of the metal seed layer is selected from the group consisting of nickel, copper, chromium, and titanium.

[0008] The high aspect ratio flexible circuit board described above has vias with a diameter of 20 to 50 µm.

[0009] The high aspect ratio flexible circuit board described above is a single-sided, double-sided, or multi-layer board.

[0010] In the high aspect ratio flexible circuit board described above, the bottom width of the first patterned groove and / or the second patterned groove is smaller than the top width, and the ratio of the bottom width to the top width of the first patterned groove and / or the second patterned groove is between 0.05 and 0.8.

[0011] To achieve the above and other objectives, the present invention provides a method for manufacturing a high aspect ratio flexible circuit board, comprising the following steps: providing a plastic substrate having opposing first and second surfaces; forming a first patterned groove on the first surface of the plastic substrate and / or forming a second patterned groove on the second surface of the plastic substrate using a laser, wherein the upper limit of the total depth of the first and second patterned grooves is the thickness of the plastic substrate minus 5. µm; a metal seed layer is formed on the first surface and / or the second surface by chemical plating or sputtering; a thin copper layer is formed on the metal seed layer on the first surface and / or the second surface by electroplating; and a copper circuit layer is formed on the thin copper layer on the first surface and / or the second surface by a semi-additive process. The copper circuit layer is partially embedded in the interior of the plastic substrate. The copper circuit layer includes exposed circuit portions that protrude from the plastic substrate and embedded circuit portions that are embedded in the plastic substrate. The height of the exposed circuit portions does not exceed 45µm, and the width of the exposed circuit portions is greater than or equal to the width of the first patterned groove or the width of the second patterned groove. The height / width ratio of the copper circuit layer is in the range of 1.5 to 3.75.

[0012] In the manufacturing method of the high aspect ratio flexible circuit board described above, the plastic substrate is a polyimide film, and the thickness of the polyimide film is 12 to 150 µm.

[0013] The manufacturing method of the high aspect ratio flexible circuit board as described above, wherein the wavelength of the laser is 150 to 1100 nm.

[0014] The manufacturing method of the high aspect ratio flexible circuit board as described above further includes, at the same time or after forming a first patterned groove on the plastic substrate and / or forming a second patterned groove on the second surface of the plastic substrate: drilling a via on the plastic substrate with a laser, the via having a diameter of 20 to 50 µm.

[0015] In the manufacturing method of the high aspect ratio flexible circuit board described above, the composition of the metal seed layer is selected from the group consisting of nickel, copper, chromium, and titanium.

[0016] In the manufacturing method of the high aspect ratio flexible circuit board described above, the thickness of the thin copper layer is 0.2 to 3 µm.

[0017] In the manufacturing method of the high aspect ratio flexible circuit board described above, the bottom width of the first patterned groove and / or the second patterned groove is smaller than the top width, and the ratio of the bottom width to the top width of the first patterned groove and / or the second patterned groove is between 0.05 and 0.8.

[0018] By using the high aspect ratio flexible circuit board as described above, the problem of dry film photoresist falling off the high aspect ratio flexible circuit board due to limited standing area can be avoided. Simple Explanation of the Diagram

[0019] Figure 1 is a cross-sectional schematic diagram of the high aspect ratio flexible circuit board of this embodiment 1. Figure 2 is a flowchart of the manufacturing process of the high aspect ratio flexible circuit board of this embodiment 1. Figure 3 is a three-dimensional schematic diagram of the fabrication of the high aspect ratio flexible circuit board in this embodiment 1. Figure 4 is a cross-sectional schematic diagram of the high aspect ratio flexible circuit board of this embodiment 2. Figure 5 shows the structural schematic diagrams of various test cases in the resistance quality test of high aspect ratio flexible circuit boards. Figure 6 is a cross-sectional photograph of the patterned groove after processing in Test Example 19 of the comparative test of laser processing methods for high aspect ratio flexible circuit boards. Figure 7 is a cross-sectional photograph of the patterned groove after processing in Test Example 20 of the comparative test of laser processing methods for high aspect ratio flexible circuit boards. Figure 8 is a cross-sectional photograph of the patterned groove after processing in Test Example 21 of the comparative test of laser processing methods for high aspect ratio flexible circuit boards. Figure 9 is a cross-sectional photograph of the patterned groove after processing in Test Example 22 of the comparative test of laser processing methods for high aspect ratio flexible circuit boards. Implementation

[0020] The following describes the implementation of the present invention through specific embodiments. Those skilled in the art can understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different embodiments, and various details in this specification can also be modified and changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0021] Unless otherwise stated herein, the terms "A to B" used in the specification and the appended claims have the meaning of "more than A and less than B". For example, the term "10 to 40% by weight" has the meaning of "more than 10% by weight and less than 40% by weight".

[0022] Example 1: High Aspect Ratio Flexible Circuit Board The high aspect ratio flexible circuit board 1 of this embodiment is shown in Figure 1. The high aspect ratio flexible circuit board 1 includes a plastic substrate 11, a metal seed layer 12, and a copper circuit layer 13. The plastic substrate 11 includes a first surface 111 and a second surface 112 opposite to each other. A first patterned groove 113 is formed on the first surface 111, and the upper limit of the total depth of the first patterned groove 113 is the thickness of the plastic substrate 11 minus 5 µm. The metal seed layer 12 is formed on the first surface 111. The copper circuit layer 13 is formed on the metal seed layer 12 corresponding to the first patterned groove 113. The copper circuit layer 13 includes an exposed circuit portion 131 exposed outside the plastic substrate 11 and an embedded circuit portion 132 embedded in the plastic substrate 11. The height of the exposed circuit portion 131 does not exceed 45µm, the height of the embedded circuit portion 132 is the depth of the first patterned groove 113, the width of the exposed circuit portion 131 is greater than or equal to the width of the first patterned groove 113, and the height / width ratio of the copper circuit layer 13 is in the range of 1.5 to 3.75.

[0023] Plastic substrate 11 In this embodiment 1, the plastic substrate 11 is a polyimide film, and the thickness of the polyimide film ranges from 12 to 150 µm, for example, 12 µm, 20 µm, 30 µm, 40 µm, 50 µm, 60 µm, 70 µm, 80 µm, 90 µm, 100 µm, 110 µm, 120 µm, 130 µm, 140 µm, or 150 µm. Furthermore, the polyimide substrate can also be made of transparent polyimide, for example, polyimide with a light transmittance greater than 87%. In addition, the polyimide substrate can be a commercially available product, such as the polyimide film manufactured by Damai Technology, model TX6 series. In other embodiments, the plastic substrate 11 can use other existing materials that can be used as plastic substrates for flexible circuit boards, and is not limited to this embodiment 1, as needed. Furthermore, flexible circuit boards can be single-sided, double-sided, or multi-layered.

[0024] Metal seed layer 12 The composition of the metal seed layer 12 may be selected from at least one of nickel, copper, chromium and titanium, and may include other materials that can be formed on the plastic substrate 11.

[0025] Method for manufacturing a high aspect ratio flexible circuit board (Example 1) First, referring to Figures 2 and 3, Figure 2 is a manufacturing process flowchart of the high aspect ratio flexible circuit board of Embodiment 1. Figure 3 is a three-dimensional schematic diagram of the processing of the high aspect ratio flexible circuit board of Embodiment 1. The manufacturing process of the high aspect ratio flexible circuit board of Embodiment 1 includes at least the following steps: providing a plastic substrate (corresponding to S1 in Figure 3), which has a first surface. Forming a first patterned groove (corresponding to S2 in Figure 3) on the first surface of the plastic substrate by laser, wherein the upper limit of the total depth of the first patterned groove is the thickness of the plastic substrate minus 5 µm. Forming a metal seed layer on the first surface by chemical plating or sputtering. Forming a thin copper layer (generally between 0.2 and 3 µm thick) (corresponding to S3 in Figure 3) on the metal seed layer on the first surface by electroplating. A copper circuit layer (corresponding to S4 in Figure 3) is formed on the thin copper layer of the first surface through a semi-additive process. The copper circuit layer is partially embedded in the interior of the plastic substrate. The copper circuit layer includes an exposed circuit portion that is exposed outside the plastic substrate and an embedded circuit portion that is embedded in the plastic substrate. The height of the exposed circuit portion does not exceed 45µm, and the width of the exposed circuit portion is greater than or equal to the width of the first patterned groove. The height / width ratio of the copper circuit layer is in the range of 1.5 to 3.75. The manufacturing process of the high aspect ratio flexible circuit board in this embodiment 1 is described in detail below.

[0026] First, a roll of polyimide film was used as the plastic substrate 11. The polyimide film was purchased from Taimide Tech. Inc., model number TX6-025.

[0027] Next, the plastic substrate 11 is continuously processed using a UV laser to create the first patterned groove 113 (in other embodiments, vias with a diameter of 20 to 50 µm can be drilled simultaneously or sequentially). The wavelength of the aforementioned laser is 355 nm (selectable within the range of 150 to 1100 nm, with the applicable laser wavelength value adjusted in 100 nm increments), and the energy and number of processing revolutions are adjusted according to the pattern depth. The energy for drilling is fixed at 1 W, and the number of processing revolutions is 3. The UV laser processing machine used in this embodiment 1 was purchased from MKS Instruments, Inc., model ESI / 5335.

[0028] After the aforementioned processing is completed, the substrate is cleaned by using plasma to clean the surface and vias of the plastic substrate 11 to remove splashes and contaminants caused by laser processing. The plasma energy is 8~10W, and the processing speed is 3m / min. In this embodiment 1, the aforementioned plasma processing is performed using a roll-to-roll plasma treatment machine (purchased from Creating Nano Technologies Inc.).

[0029] After cleaning the plastic substrate 11, electroless nickel plating (or sputtering) is performed on the laser-processed plastic substrate 11. The surface of the plastic substrate 11 and the inside of the vias are plated simultaneously to form the metal seed layer 12. The electroless nickel plating process is carried out at a speed of 1.2 m / min, and the nickel bath temperature is 34°C. The resulting metal seed layer 12 has a thickness of approximately 70~100 µm. The electroless nickel plating is performed using a wound nickel plating wire (purchased from FUJICHEM PRECISION MACHINERIES, INC.) and SLP series electroless nickel plating reagents (purchased from OKUNO Chemical Industries Co., Ltd., Japan).

[0030] After the metal seed layer 12 is formed on the plastic substrate 11, it is electroplated with copper sulfate plating solution to form a base copper layer. The processing speed of the base copper plating is 0.9 m / min, and the current density of each plating tank is controlled at 0.6~1.2 ASD, finally obtaining a base copper layer with a thickness of 1 µm. The aforementioned base copper plating is performed using coiled copper plating wire (purchased from JX Nippon Mining & Metals Corporation), formula a (containing copper sulfate plating solution, sulfuric acid, and copper oxide powder) (purchased from All-In-Line Chemicals Enterprise Co., Ltd.) and formula b plating additives (containing brightener and leveling agent) (purchased from JCU TAIWAN Corporation).

[0031] After the plastic substrate 11 is plated with a base copper layer, it undergoes a roughening surface treatment. A hydrochloric acid-based micro-etching solution is used to roughen the surface of the base copper layer to increase the adhesion of the photoresist dry film. The copper etching depth for the roughening surface treatment is 0.2~1.0 µm. A horizontal roughening line (purchased from FUJICHEM PRECISION MACHINERIES, INC.) and a roughening process solution (purchased from MEC TAIWAN COMPANY LTD., model UT-4100) are used for the roughening surface treatment.

[0032] After the aforementioned roughening surface treatment, a lamination process is performed, in which negative dry film photoresist is hot-pressed onto the surface of the aforementioned base copper layer. The thickness of the negative dry film photoresist is determined by the line height and linewidth. The lamination operation temperature is 90~110℃, and the pressure is 1~3 Kgf / cm2. The aforementioned lamination process uses a roll-to-roll laminator (purchased from C SUN MFG. LTD., model RTR-500PH), and the negative dry film photoresist used is an exposure-developing type dry film (purchased from Thirty Power Technology Corporation, model RESONAC / RD-1260).

[0033] After lamination, exposure is performed using a laser direct exposure (LDI) machine (purchased from Hua Zhan Optoelectronics Co., Ltd., model ORC / RDI-10Duo) to expose the circuit pattern onto the aforementioned negative dry film photoresist. The exposure energy is controlled at 15-19 divisions (41-division film).

[0034] Following the exposure process, a development process is performed. Sodium bicarbonate developer is used to wash away the unpolymerized dry film, exposing the underlying copper substrate, which serves as the base for subsequent copper plating. The exposure process uses approximately 1% sodium bicarbonate (purchased from All-In-Line Chemicals Enterprise Co., Ltd.) and a development time of 40–120 seconds. The exposure process uses horizontal development lines (purchased from FUJICHEM PRECISION MACHINERIES, INC.).

[0035] After development, copper plating is performed to form a preliminary copper circuit layer. A copper sulfate plating solution is used to plate the exposed base copper layer with copper to form the circuit, and the holes of the aforementioned vias are also plated with copper. The acid-to-copper weight ratio of the copper sulfate plating solution is approximately 2:1, and the copper plating current density is 0.8~1.5 ASD. The aforementioned copper plating process uses a vertical copper plating wire (purchased from Command Machinery Co., Ltd.), formula a (copper sulfate plating solution, sulfuric acid, copper oxide powder) (purchased from All-In-Line Chemicals Enterprise Co., Ltd.), and formula b (plating additives including gloss and leveling agent) (purchased from JCU TAIWAN Corporation).

[0036] After the initial copper circuit layer is formed, a stripping process is performed. An organic stripping solution is used to remove the remaining negative dry film photoresist, exposing the underlying copper layer. At this point, there is copper underneath and alongside the initial copper circuit layer. The stripping process is carried out at a temperature of 55°C, with an organic stripping solution concentration of 10-20% and a processing speed of 1-2 m / min. The stripping process uses a horizontal stripping line (purchased from FUJICHEM PRECISION MACHINERIES, INC.) and a dry film photoresist remover (purchased from Chemflux Co., Ltd., model CXW-PS312).

[0037] After the film stripping process, a final rapid etching process is performed. This rapid etching process uses a sulfuric acid and hydrogen peroxide solution to remove the exposed bottom copper layer and metal seed layer, allowing the initial copper circuit layer to form an independent circuit pattern copper circuit layer 13. This allows the fabrication of the high aspect ratio flexible circuit board 1 of this embodiment 1. The above method forms the copper circuit layer through a semi-additive process. The operating temperature for the rapid etching process is 30°C, and the processing speed is 0.5~1.5 m / min. The rapid etching process uses horizontal rapid etching lines (purchased from FUJICHEM PRECISION MACHINERIES, INC.) and a sulfuric acid and hydrogen peroxide-based micro-etching solution (purchased from JCU TAIWAN Corporation, model SAC2).

[0038] Example 2: High aspect ratio flexible circuit board 2 The high aspect ratio flexible circuit board 2 of this embodiment is shown in Figure 4. Its structure is generally similar to that of the high aspect ratio flexible circuit board 1 of embodiment 1, the main difference being that copper lines are formed on both the first and second surfaces of the high aspect ratio flexible circuit board 2. The high aspect ratio flexible circuit board 2 includes a plastic substrate 21, two metal seed layers 22, and two copper line layers 23. The plastic substrate 21 includes a first surface 211 and a second surface 212 opposite to each other. A first patterned groove 213 is formed on the first surface 211, and a second patterned groove 214 is formed on the second surface 212. The upper limit of the total depth of the first patterned groove 213 and the second patterned groove 214 is the thickness of the plastic substrate 21 minus 5 µm. The two metal seed layers 22 are formed on the first surface 211 and the second surface 212, respectively. The two copper circuit layers 23 are respectively formed on the metal seed layers 22 corresponding to the first patterned groove 213 and the second patterned groove 214. The copper circuit layer 23 includes an exposed circuit portion 231 exposed outside the plastic substrate 21 and an embedded circuit portion 232 embedded in the plastic substrate 21. The height of the exposed circuit portion 231 does not exceed 45µm, the height of the embedded circuit portion 232 is the depth of the corresponding first patterned groove 213 or second patterned groove 214, the width of the exposed circuit portion 231 is greater than or equal to the width of the corresponding first patterned groove 213 or second patterned groove 214, and the height / width ratio of the copper circuit layer 23 is in the range of 1.5 to 3.75.

[0039] The manufacturing process of the high aspect ratio flexible circuit board 2 is basically the same as that of Embodiment 1. The only difference is that Embodiment 2 uses UV laser to perform continuous processing on the plastic substrate 21 to create the first patterned groove 213 and the second patterned groove 214, so it will not be described again.

[0040] High aspect ratio flexible circuit board resistor quality testing The following tests will be conducted on single-sided and double-sided high aspect ratio flexible circuit boards (HIPS) as shown in Tables 1 and 2 below, manufactured according to the high aspect ratio flexible circuit board manufacturing methods described in Examples 1 and 2 above. The resistance quality of the different HIPS flexible circuit boards will be tested. The methods and instruments used for the structural measurement and resistance quality testing of the aforementioned different HIPS flexible circuit boards are described below.

[0041] The coating thickness was measured using an X-ray fluorescence spectrometer (XRF), purchased from TechMax Technical Group. The model was Olympus / Vanta C series. The specific measurement method was as follows: a 510cm x 5cm test sample was placed at the X-ray emission point of the X-ray fluorescence spectrometer. The spectrometer cover was closed, and the coating thickness was measured by selecting "Coating mode" and choosing the conditions and parameters in the operating software, then pressing the start button.

[0042] The thickness of the electroplated copper was measured using a copper thickness meter, purchased from Shin Shen Instruments Co., Ltd., model Oxford Instruments / CMI760. The specific measurement method involved placing the sample on the measurement stage and uniformly contacting the four-point probes with the copper surface of the flexible copper foil substrate (FCCL) plated with a thin copper layer at the measurement points to measure the copper layer thickness.

[0043] The circuit height was measured using a laser conjugate focal length interferometer (CFL) purchased from Keyence Taiwan Co., Ltd., model VK-X3050. The specific measurement method involved placing the high aspect ratio flexible circuit board on the microscope's observation stage, selecting an appropriate magnification lens, activating the white light measurement mode, and pressing start to obtain the circuit height.

[0044] The circuit width was measured using a metallographic microscope, purchased from Lin Trading Co., Ltd., model Nikon / LV100ND. The specific measurement method involved placing a high aspect ratio flexible circuit board on the observation stage of the metallographic microscope, examining the circuit through a 20x objective lens, and then measuring the circuit width using measurement software.

[0045] The line resistance was measured using a resistance meter (LCR Meter), purchased from Chawei Co., Ltd., model LCR700. The specific measurement method was to zero the resistance meter, then press the probe onto the test point according to the measurement point to obtain the line resistance value.

[0046] The structural measurements and overall quality of the aforementioned high aspect ratio flexible circuit boards are shown in Tables 1-1 to 1-3 below, and can be compared with the schematic diagram of the test example in Figure 5.

[0047] Circuit board structures in Tables 1-1 to 1-2, - Test Example 1-18 Table 1-1 Test case Product Type plastic substrate Thickness C First surface line width L1 Second surface line width L2 First surface groove depth A1 Second surface groove depth A2 The first surface exposed line height B1 The second surface exposes the line height B2. The overall line height H1 on the first surface 1 single-sided 25 30 0 0 0 40 0 40 2 single-sided 25 29.7 0 6 0 37.5 0 43.5 3 single-sided 25 30.2 0 10 0 35 0 45 4 single-sided 25 30.5 0 14 0 32 0 46 5 single-sided 25 29.6 0 20 0 30 0 50 6 single-sided 25 30.1 0 twenty two 0 28.5 0 50.5 7 single-sided 25 30 0 0 0 40 0 40 8 single-sided 25 29.7 0 5 0 39 0 44 9 single-sided 25 29.6 0 9 0 38.5 0 47.5 10 single-sided 25 30.1 0 16 0 40 0 56 11 single-sided 25 30.3 0 20 0 41 0 61 12 single-sided 25 30.3 0 twenty two 0 41 0 63 13 Double-sided 50 30 30 0 0 40 40 40 14 Double-sided 50 30 29.8 19 19 39 38 58 15 Double-sided 50 24.8 25 0 0 40 39 40 16 Double-sided 50 25.2 25.3 18 19 41 42 59 17 Double-sided 50 21.2 22.5 0 0 43 41 43 18 Double-sided 50 21.5 twenty two twenty three twenty two 44 43 67 19 Double-sided 25 30 30 0 0 50 50 50 20 Double-sided 75 20 20 35 35 45 45 80 Table 1-2 Test case Product Type Second surface overall line height H2 Insulation layer thickness D Aspect Ratio of the First Surface Line Aspect Ratio of the Second Surface Lines 1 single-sided 0 25 1.33 N / A 2 single-sided 0 19 1.46 N / A 3 single-sided 0 15 1.49 N / A 4 single-sided 0 11 1.51 N / A 5 single-sided 0 5 1.69 N / A 6 single-sided 0 3 1.68 N / A 7 single-sided 0 25 1.33 N / A 8 single-sided 0 20 1.48 N / A 9 single-sided 0 16 1.60 N / A 10 single-sided 0 9 1.86 N / A 11 single-sided 0 5 2.01 N / A 12 single-sided 0 3 2.08 N / A 13 Double-sided 40 50 1.33 1.33 14 Double-sided 57 12 1.93 1.91 15 Double-sided 39 50 1.61 1.56 16 Double-sided 61 13 2.34 2.41 17 Double-sided 41 50 2.03 1.82 18 Double-sided 65 5 3.12 2.95 19 Double-sided 50 25 1.67 1.67 20 Double-sided 80 5 4.00 4.00 Table 1-3 Overall Quality Table of Test Examples 1-18 Test case Product Type First surface circuit resistance Second surface line resistance Production yield Electrical performance Manufacturing costs Overall evaluation 1 single-sided 2.02 N / A generally generally N / A △ 2 single-sided 2.00 N / A Slightly higher generally Slightly lower ○ 3 single-sided 1.99 N / A Slightly higher generally reduce ◎ 4 single-sided 2.01 N / A high generally reduce ◎ 5 single-sided 2.01 N / A high generally reduce ◎ 6 single-sided 1.99 N / A High rate of exposed wire defects Slightly lower N / A ✕ 7 single-sided 2.02 N / A generally generally N / A △ 8 single-sided 1.96 N / A generally Slightly lower Slightly increased ○ 9 single-sided 1.89 N / A generally Slightly lower Slightly increased ○ 10 single-sided 1.66 N / A generally reduce Slightly increased ◎ 11 single-sided 1.54 N / A generally reduce Slightly increased ◎ 12 single-sided 1.51 N / A High rate of exposed wire defects reduce N / A ✕ 13 Double-sided 2.2 2.2 generally generally N / A △ 14 Double-sided 1.8 1.8 generally reduce Slightly increased ◎ 15 Double-sided 2.7 2.7 generally rise N / A ✕ 16 Double-sided 2.1 2.0 generally Slightly lower Slightly increased ◎ 17 Double-sided 2.9 2.9 Slightly lower rise N / A ✕ 18 Double-sided 2.2 2.2 Slightly lower generally Slightly increased ◎ 19 Double-sided 12 12 The line has a short circuit problem. N / A N / A ✕ 20 Double-sided 11 11 A dip in the circuit leads to increased resistance. N / A N / A ✕ Overall rating symbols: ✕ (poor); △ (fair); ○ (good); ◎ (excellent)

[0048] Based on the test results of Examples 1-6 above, it can be seen that under the conditions of the same plastic substrate thickness (C), the same first surface line width (L1), and the same line cross-sectional area, when the depth of the first surface groove (A1) increases, the height of the exposed line on the first surface (B1) decreases accordingly. This directly affects the thickness of the solder resist layer. Since the price of the solder resist layer decreases as its thickness decreases, material costs can be effectively reduced.

[0049] As can be seen from the test results of the above-mentioned test examples 7-12, under the same plastic substrate thickness (C), the same first surface line width (L1), and the same first surface exposed line height (B1), when the first surface groove depth (A1) increases, the overall line height (H1) of the first surface increases, directly reducing the first surface line resistance (R1). Without changing the coil density of the flexible circuit board, the product can withstand a larger current, making the operation efficiency of the coil-type product higher.

[0050] According to the test results of the above-mentioned test examples 13-18, under the same plastic substrate thickness (C), the same exposed line height (B1) on the first surface and the same exposed line height (B2) on the second surface, when the groove depth (A1) on the first surface and the groove depth (A2) on the second surface increase, the line width (L1) on the first surface and the line width (L2) on the second surface can become smaller, which directly affects the line density and can achieve the effect of reducing product size. In this way, the utilization rate of the plate can be improved and the cost can be reduced accordingly.

[0051] Furthermore, according to the overall quality table of Test Examples 1-18 above, the thickness (D) of the insulating plastic substrate at the bottom of the groove of the aforementioned high aspect ratio flexible circuit board (i.e., the insulation layer thickness D) must be greater than or equal to 5 μm. If the thickness of the double-sided board is less than this number, there will be a problem of short circuit between the upper and lower layers (refer to Test Example 12). If the thickness of the single-sided board is less than this number, there will be a problem of exposed circuits leading to leakage (refer to Test Example 6). The overall evaluation in Tables 1-3 above is based on the three indicators of production yield, electrical performance, and manufacturing cost. It can also be seen from the overall evaluation that the overall evaluation of Test Examples 6 and 12 is poor.

[0052] The exposed portion of the copper circuit layer should have a height equal to or less than 45 μm. If it exceeds this value, the dry film will be difficult to remove, resulting in residual film defects and causing short circuit defects (refer to test example 19). As can be seen from the overall evaluation, the overall evaluation of test example 19 is poor.

[0053] The ratio of the overall height to the width of the copper circuit layer should be less than 3.75. A ratio greater than this will cause poor circulation, leading to air bubbles forming on the circuit, resulting in vias or depressions and increased circuit resistance (see Test Example 20). As can be seen from the overall evaluation, Test Example 20 has a poor overall performance. The ratio of the overall height to the width of the copper circuit layer should be greater than 1.5. A ratio less than this, as seen in the results of Test Example 13, results in mediocre resistance and yield on the circuit board, and a poor overall performance, making it unsuitable for use in voice coil motors.

[0054] Comparative Test of Laser Processing Methods for High Aspect Ratio Flexible Circuit Boards The following tests were conducted using different processing methods and laser wavelengths to measure the changes in the bottom and top widths of the patterned grooves after processing. Table 3 shows the parameters of different processing methods and the corresponding bottom and top widths of the resulting patterned grooves. Figure 6-9 further shows cross-sectional photographs of the processed patterned grooves.

[0055] Table 2, Bottom and Top Widths of Patterned Grooves in Experiments 21-24 Table 2 Test case twenty one twenty two twenty three twenty four Processing method Excimer laser (with photomask) Femtosecond IR laser Picosecond UV laser (with photomask) Nanosecond UV laser Laser wavelength (nm) 248 1064 355 355 Groove depth (µm) 12.7 22.9 15.3 22.5 Groove top width (µm) 19.2 25.3 24.6 19.8 Groove bottom width (µm) 13.4 10.4 9.1 1.6 Bottom width / Top width 0.70 0.41 0.37 0.08

[0056] According to the results in Table 3, the bottom width of the grooves after laser processing is smaller than the top width, and the ratio of bottom width to top width is between 0.05 and 0.8. The width of the exposed lines must be greater than or equal to the top width of the patterned groove.

[0057] The high aspect ratio flexible circuit board provided in the above embodiments, through its recessed design, effectively reduces the aspect ratio of the dry film and allows for the fabrication of vias in a single step. Furthermore, the lower depth between the lines on the top of the circuit board effectively reduces the processing difficulty of the solder mask and minimizes the risk of air bubbles. This high aspect ratio flexible circuit board processing method allows for the use of thicker plastic substrates, improving workability and mitigating common bending damage issues during flexible circuit board production. A thicker plastic substrate provides better material rigidity and higher dimensional stability. In other words, the high aspect ratio flexible circuit board design in the above embodiments embeds the circuitry within the plastic substrate, solving the problem of dry film photoresist detachment due to limited standing area. Moreover, this semi-embedded design not only effectively reduces the aspect ratio of the dry film development and improves copper plating yield, but also strengthens the connection between the circuitry and the plastic substrate, resulting in a highly reliable high aspect ratio flexible circuit board.

[0058] The high aspect ratio flexible circuit board provided in the above embodiments also has the following advantages. 1. Excellent dimensional stability; 2. Good operability, the material is stiff enough, relatively resistant to tensile stress, and not prone to deformation or bending damage; 3. Can form fine linewidths and high copper thickness. Because the circuit is semi-embedded, it can achieve fine linewidths and high copper thickness when combined with dry film; 4. Reduces the need and cost of solder resist. Thinner solder resist can be used, which can reduce costs and reduce bubble problems; 5. Circuits and vias can be fabricated in one step. Combined with metallization and semi-additive process (mSAP), copper plating of vias and copper plating of circuits can be completed in one step, without the need for additional copper metallization of vias, which can effectively reduce costs; 6. Resistant to electroless nickel-gold (ENIG) corrosion. Because the metal seed layer is within the plastic base layer, there will be no corrosion abnormalities caused by the gold-replacing solution attacking the bottom of the circuit; 7. Excellent high-speed circuit transmission performance. Because the circuit groove is laser-grooved, the inner surface of the groove is smooth and there are no roughness issues, so there will be no problem of rough surfaces affecting signal transmission.

[0059] Although various embodiments have been described for illustrative purposes, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

[0060] 1: High aspect ratio flexible circuit board 11: Plastic substrate 12: Metal Seed Layer 13: Copper circuit layer 111: First Surface 112: Second Surface 113: First patterned groove 131: Exposed wiring section 132: Embedded wiring section 2: High aspect ratio flexible circuit board 21: Plastic substrate 211: First Surface 212: Second Surface 213: First patterned groove 214: Second patterned groove 22: Metal Seed Layer 23: Copper Circuit Layer 231: Exposed wiring section 232: Embedded wiring section A1: Depth of the first surface trench A2: Depth of the second surface groove B1: Height of exposed circuit on the first surface B2: Height of exposed circuit on the second surface C: Thickness of plastic substrate D: Insulation layer thickness H1: Overall line height on the first surface H2: Overall line height on the second surface L1: Width of the first surface line L2: Width of the second surface line S1~S4: Process

[0061] none

Claims

1. A high aspect ratio flexible circuit board, comprising: a plastic substrate including opposing first and second surfaces, wherein a first patterned groove is formed on the first surface and / or a second patterned groove is formed on the second surface, the upper limit of the total depth of the first and second patterned grooves being the thickness of the plastic substrate minus 5 µm; a metal seed layer formed on the first and / or second surfaces; and a copper circuit layer formed on the metal seed layer, the copper circuit layer including exposed circuit portions protruding from the plastic substrate and embedded circuit portions embedded in the plastic substrate; wherein... The height of the exposed circuit portion does not exceed 45µm, the height of the embedded circuit portion is the depth of the first patterned groove or the depth of the second patterned groove, the width of the exposed circuit portion is greater than or equal to the width of the first patterned groove or the width of the second patterned groove, and the height / width ratio of the copper circuit layer is in the range of 1.5 to 3.

75.

2. The high aspect ratio flexible circuit board as described in claim 1, wherein, The plastic substrate is a polyimide film with a thickness of 12 to 150 µm.

3. The high aspect ratio flexible circuit board as described in claim 1, wherein, The first patterned groove and / or the second patterned groove are formed on the plastic substrate by laser irradiation.

4. The high aspect ratio flexible circuit board as described in claim 1, wherein, The composition of the metal seed layer is selected from the group consisting of nickel, copper, chromium, and titanium.

5. The high aspect ratio flexible circuit board as described in claim 1, wherein, The flexible circuit board has vias with a diameter of 20 to 50 µm.

6. The high aspect ratio flexible circuit board as described in claim 1, wherein, The flexible circuit board can be a single-sided board, a double-sided board, or a multi-layer board.

7. The high aspect ratio flexible circuit board as described in claim 1, wherein, The bottom width of the first patterned groove and / or the second patterned groove is smaller than the top width, and the ratio of the bottom width to the top width of the first patterned groove and / or the second patterned groove is between 0.05 and 0.

8.

8. A method for manufacturing a high aspect ratio flexible circuit board, comprising the following steps: providing a plastic substrate having opposing first and second surfaces; forming a first patterned groove on the first surface of the plastic substrate and / or forming a second patterned groove on the second surface of the plastic substrate by laser, wherein the upper limit of the total depth of the first patterned groove and the second patterned groove is the thickness of the plastic substrate minus 5 µm; forming a metal seed layer on the first surface and / or the second surface by chemical plating or sputtering; A thin copper layer is formed on the metal seed layer by electroplating; and a copper circuit layer is formed on the thin copper layer on the first surface and / or the second surface by a semi-additive process. The copper circuit layer is partially embedded in the interior of the plastic substrate. The copper circuit layer includes exposed circuit portions that protrude from the plastic substrate and embedded circuit portions that are embedded in the plastic substrate. The height of the exposed circuit portions does not exceed 45µm, and the width of the exposed circuit portions is greater than or equal to the width of the first patterned groove or the width of the second patterned groove. The height / width ratio of the copper circuit layer is in the range of 1.5 to 3.

75.

9. A method for manufacturing a high aspect ratio flexible circuit board as described in claim 8, wherein, The plastic substrate is a polyimide film with a thickness of 12 to 150 µm.

10. A method for manufacturing a high aspect ratio flexible circuit board as described in claim 8, wherein, The laser has a wavelength of 150 to 1100 nm.

11. A method for manufacturing a high aspect ratio flexible circuit board as described in claim 8, wherein, While or after forming the first patterned groove on the plastic substrate and / or forming the second patterned groove on the second surface of the plastic substrate, the method further includes: drilling a through hole on the plastic substrate with a laser, the through hole having a diameter of 20 to 50 µm.

12. A method for manufacturing a high aspect ratio flexible circuit board as described in claim 8, wherein, The composition of the metal seed layer is selected from the group consisting of nickel, copper, chromium, and titanium.

13. A method for manufacturing a high aspect ratio flexible circuit board as described in claim 8, wherein, The thickness of the thin copper layer is 0.2 to 3 µm.

14. A method for manufacturing a high aspect ratio flexible circuit board as described in claim 8, wherein, The bottom width of the first patterned groove and / or the second patterned groove is smaller than the top width, and the ratio of the bottom width to the top width of the first patterned groove and / or the second patterned groove is between 0.05 and 0.8.

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