Manufacturing method of asymmetric copper thickness circuit

Abstract

The invention discloses an asymmetric copper thickness circuit manufacturing method, which comprises a pattern transfer and electroplating step and a double-sided step-by-step etching step, the pattern transfer and electroplating operation step comprises pattern film pressing, exposure, development, pattern electroplating and anti-plating film removal, and the double-sided step-by-step etching operation step comprises first etching and second etching; the pattern film pressing, the first etching and the second etching are completed by adopting the same anti-plating film, the same yellow light exposure machine, the same developing machine and the same film stripping machine. According to the method, the copper layer of the conventional single-sided copper-clad base material is pressed outwards, a special asymmetric copper thickness base material and new equipment are not needed, complex control over electroplating thickening is avoided, the circuit precision and the copper thickness uniformity are guaranteed through two-time independent etching, and the method has the advantages of being low in cost, good in compatibility, high in precision and suitable for industrial mass production.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board technology, and in particular to a method for fabricating asymmetric copper thickness circuits. Background Technology

[0002] Asymmetric copper-thickness FPCs (flexible printed circuit boards) can simultaneously meet the differentiated requirements of products in terms of current carrying capacity, signal transmission characteristics, and flexibility in different areas due to the difference in copper layer thickness on the upper and lower surfaces. They are widely used in consumer electronics, automotive electronics, and other fields. Asymmetric copper-thickness FPCs use thin copper on the bending surfaces to increase the bending resistance, while using thick copper on the non-bending / current-carrying surfaces to reduce impedance and improve return current, shielding, and current-carrying performance.

[0003] Existing methods for manufacturing asymmetric copper-thickness FPCs mainly rely on direct processing of substrates with copper foil of different thicknesses on both sides, or on localized thickening through electroplating. The former is limited by the supply of substrates, resulting in fewer copper thickness combinations to choose from, and the substrate cost is high; the latter requires additional processes such as electroplating masks and electroplating, making the process complex, difficult to control the uniformity of electroplating, and prone to reducing circuit accuracy, and also posing environmental pressures related to the treatment of electroplating wastewater.

[0004] Chinese patent CN108811353A discloses a method for etching a PCB with different copper thicknesses on both sides. This method requires first pattern electroplating and then tin plating, using tin as a resist layer. Then, a peelable blue adhesive is used to alternately protect the other side, and etching is performed in stages. The additional blue film as a protective material increases the process cost. It also adds multiple steps—electroplating, tin plating, protective adhesive, and tin / adhesive removal—making manufacturing cumbersome.

[0005] Chinese patent CN104427776A discloses a method for manufacturing a thick copper-plated printed circuit board. The steps for fabricating the first conductive line on the thick copper surface include: outer layer pattern transfer, pattern tin plating, and alkaline etching; the steps for fabricating the second conductive line on the thin copper surface include: outer layer pattern transfer and acid etching. This method requires separate front and back-side processes, a combination of acid and alkaline etching, and tin plating, resulting in a long process time, multiple raw material types, and high manufacturing costs.

[0006] Therefore, it is necessary to design a new method for fabricating asymmetric thick copper structures to solve the above problems. Summary of the Invention

[0007] The main objective of this invention is to provide a method for fabricating asymmetric thick copper circuits. By using the same anti-plating film simultaneously for pattern lamination, first etching, and second etching, the thickness of the copper layer can be flexibly adjusted based on the original thickness without adding electroplating equipment. This allows for flexible material selection, reduces investment in production equipment, and improves the compatibility of the production process.

[0008] The present invention achieves the above objectives through the following technical solution: a method for fabricating asymmetric thick copper lines, comprising a pattern transfer and electroplating step and a double-sided step etching step; The graphic transfer and electroplating operation steps include: 1) Pattern lamination: Prepare a double-sided copper-clad perforated board, one side of which is a first copper layer and the other side is a second copper layer. Apply an anti-plating film evenly to both sides of the double-sided copper-clad perforated board substrate. 2) Exposure: The copper-clad vias with the resist film applied are exposed according to the preset circuit pattern film to ensure that the resist film of the second copper layer is fully cured, while only the resist film of the non-circuit areas of the first copper layer is cured. 3) Development: The exposed copper-clad vias are developed with a developer to remove the unexposed resist film, forming a resist layer consistent with the circuit pattern and exposing the circuit area of ​​the first copper layer. The residual developer is then cleaned to remove it. 4) Pattern electroplating: The developed copper-clad perforated board is subjected to pattern electroplating to ensure that the circuit area of ​​the first copper layer reaches the expected thickness of the circuit on that side, while the second copper layer maintains its original thickness, thus obtaining a copper-clad board with two sides. 5) Removal of resist coating: The resist coating on the circuit surface of the two copper-clad laminates after electroplating is removed by stripping solution. After stripping, the two copper-clad laminates are cleaned to remove the residual stripping solution and expose the copper layer area to be etched. The steps involved in double-sided step-by-step etching include: 1) First etching: After cleaning the surfaces of the two copper-clad laminates, a resist film is attached to both sides of the two copper-clad laminates. The resist film on the first copper layer is exposed according to the preset circuit pattern to ensure that the resist film in the circuit area on the first copper layer is cured and the resist film in the non-circuit area can be removed by development. The resist film on the second copper layer is exposed to the whole surface to make it completely cured. The resist film in the non-circuit area on the first copper layer is removed by development to expose the non-circuit copper surface. Then the non-circuit copper surface is etched to form the first circuit layer. Then the cured resist film on both sides is stripped. 2) Second etching: After cleaning the surfaces of the two copper-clad laminates, the resist film is then attached to both sides of the two copper-clad laminates. The resist film on the second copper layer is partially exposed according to its preset circuit pattern to ensure that the resist film in the second copper circuit area is cured. The resist film on the first circuit layer is exposed to the whole surface to ensure that it is completely cured. The resist film in the non-circuit area on the second copper layer is developed and removed to expose the non-circuit copper surface. Then the non-circuit copper surface is etched to form the second circuit layer. Finally, the cured resist film on both sides is stripped. The pattern lamination, first etching, and second etching are completed using the same anti-coating film and the same yellow light exposure machine, developing machine, and stripping machine.

[0009] Specifically, the copper-clad perforated plate is an intermediate plate used to manufacture asymmetric copper-thickness FPCs.

[0010] Specifically, the steps include: S1. Front-end cutting and yellow light area treatment: Prepare the top substrate, bottom substrate and intermediate layer. The top substrate and the bottom substrate are both copper-clad substrates. Complete the cutting and surface protection of the top substrate and the bottom substrate. S2. Drilling and Plasma Treatment: Positioning holes are machined and substrate surfaces are activated on the top and bottom substrates. S3. Overlay and lamination: The top substrate and the bottom substrate are precisely overlaid and laminated into one piece through the middle layer with the copper layer facing outward, to obtain a copper-clad board on both sides. S4. Secondary drilling and through-hole processing: Through-hole forming and hole wall activation are completed on the two-sided copper-clad original board to obtain a two-sided copper-clad hole board. S5. Pattern transfer and electroplating: A circuit pattern resist layer is formed on the surface of the copper-clad perforated plate to obtain a copper-clad plate on both sides. S6. Double-sided step-by-step etching: The two-sided copper-clad laminate is processed into a two-sided circuit board using a double-sided step-by-step etching method. S7. Final Inspection: The inspection confirms that both sides of the produced circuit board are qualified products.

[0011] Specifically, the operation steps for the front-end cutting and yellow light area processing include: 1) Cutting sheet material: The top substrate and the bottom substrate are cut into sheets according to the product design dimensions to obtain sheet material of the preset size; 2) Applying carrier film to the yellow light area: The cut top substrate and bottom substrate are respectively sent into the yellow light area and a carrier film is applied to the surface of the two substrates. The carrier film completely covers the first copper layer and the second copper layer. 3) Cutting in the yellow light area: Based on the product's shape and processing requirements, the top and bottom substrates with the carrier film applied are precisely divided in the yellow light area, with the cutting accuracy controlled within ±0.1mm.

[0012] Specifically, the drilling and plasma treatment operation steps include: 1) First drilling: The top substrate, which has been cut by yellow light, is initially drilled to make positioning holes according to the preset positioning hole positions. The position deviation of the positioning holes is ≤ ±0.05mm. 2) Plasma treatment before stacking: The top substrate with drilled positioning holes and the bottom substrate after cutting and tidying are subjected to surface plasma treatment respectively. Plasma treatment parameters: plasma power 1000-1500W, treatment time 30-60s, treatment pressure 0.1-0.2MPa.

[0013] Specifically, the overlapping and pressing operations include: 1) Stacking: Using the top substrate as the base, a layer of pure adhesive is evenly laid on the opposite side of the first copper layer, and then the bottom substrate that has been treated with plasma is precisely bonded to the pure adhesive surface with the opposite side of the second copper layer. 2) Pre-compression: The stacked substrates are pre-compressed. Pre-compression parameters: temperature 80-100℃, pressure 0.5-1.0MPa, time 30-60s; 3) White light zone stacking: The pre-pressed substrate is sent into the white light zone, and the bottom substrate is precisely positioned with a positioning accuracy of ≤ ±0.03mm; 4) Main lamination: The precisely positioned substrate is fed into the main lamination machine for main lamination. The main lamination parameters are: temperature 160-180℃, pressure 2.0-3.0MPa, and time 60-90s. During the lamination process, pressure transmission and waste removal operations are performed simultaneously to remove the pure glue and excess waste that overflows during the lamination process. After lamination, the substrate is trimmed to remove excess parts at the edges, ensuring that the product's external dimensions meet the design requirements, resulting in a double-sided copper-clad substrate.

[0014] Furthermore, the final step of the main lamination operation involves X-ray inspection of the interlayer alignment accuracy of the substrate. If the alignment deviation is found to exceed the allowable range, rework is carried out in a timely manner.

[0015] Specifically, the operation steps for secondary drilling and through-hole treatment include: 1) Second drilling: The two copper-clad boards that have been pressed together are drilled a second time according to the through hole positions and diameters designed in the product. The positional deviation between the through holes and the circuit is ≤ ±0.05mm. 2) Through-hole plasma: The copper-clad substrate with drilled through holes on both sides is fed into the plasma treatment equipment to perform plasma treatment on the hole walls; 3) Black hole process: The copper-clad original board that has undergone through hole plasma treatment is subjected to black hole conductivity treatment to obtain a copper-clad perforated board. The temperature of the black hole solution is controlled at 30-40℃ and the processing time is 10-15min.

[0016] Furthermore, the copper thickness is checked at the end of the double-sided step-by-step etching process to ensure that the copper thickness deviation of the top layer is ≤ ±1μm and the copper thickness deviation of the bottom layer is ≤ ±1μm. If the requirements are not met, the etching parameters are adjusted in time and rework is carried out.

[0017] The beneficial effects of the technical solution of this invention are: 1. Cost advantage: It does not rely on special double-sided asymmetric thickness copper substrates. Asymmetric copper thickness can be achieved by using conventional single-sided copper-clad substrates. The material selection is flexible and the substrate procurement cost is greatly reduced. No new special equipment is required. It can rely on existing FPC single-sided circuit fabrication equipment, reduce the investment in production equipment, and easily achieve industrial mass production.

[0018] 2. Precision Advantage: By avoiding the complex control of the electroplating thickening process, the copper thickness on both sides is precisely controlled through two independent single-sided etching processes. The difference in copper thickness can be finely adjusted by selecting the substrate or by a single electroplating process, resulting in high stability and circuit accuracy that meets the requirements of high-precision impedance matching.

[0019] 3. Compatibility advantages: The process flow is compatible with the existing FPC production system, without the need for major adjustments to the production process. Operators do not require special training and can quickly get started, resulting in high production efficiency. Attached Figure Description

[0020] Figure 1 This is a diagram showing the material changes in a double-sided step-by-step etching method. Figure 2 This is a logic block diagram of an asymmetric copper thickness circuit fabrication method.

[0021] The diagram is marked as follows: 101 - Double-sided copper-clad via board, 102 - Double-sided copper-clad laminate, 103 - Double-sided circuit board. 1-Top substrate, 11-First copper layer, 11a-First circuit layer, 12-First insulating layer; 2-Substrate layer, 21-Second copper layer, 21a-Second circuit layer, 22-Second insulating layer; 3-Intermediate layer; 4-Resistant to coating. Detailed Implementation

[0022] This invention discloses a method for etching asymmetric copper thickness circuits. The core of this method achieves asymmetric copper thickness through two single-sided circuit fabrication processes, eliminating the need for special double-sided copper substrates with varying thicknesses. Utilizing conventional single-sided copper-clad substrates and existing single-sided circuit fabrication equipment, it enables high-precision and high-stability asymmetric copper thickness circuit formation, significantly reducing substrate costs and production input. This method is suitable for products requiring specific copper thickness variations and high-precision impedance matching. The following detailed embodiments illustrate the specific implementation of this method. Those skilled in the art can flexibly adjust the parameters of each process according to the copper thickness required for product functionality, all of which fall within the scope of this invention.

[0023] Example: Raw material preparation A conventional single-sided copper-clad substrate is selected as the top substrate 1. The first copper layer 11 is the copper layer on the top substrate 1 (original thickness 18μm). A single-sided copper-clad auxiliary material is selected as the bottom substrate 2. The second copper layer 21 is the copper layer on the bottom substrate 2 (original thickness 12μm). Both meet the substrate standards for FPC production, ensuring that the substrate surface is free of oxidation, scratches, and impurities. At the same time, pure adhesive (for interlayer bonding, with a thickness suitable for the bonding requirements of the two substrates), carrier film 5 (for protecting the substrate surface and preventing scratches and contamination during processing), anti-plating film 4 (for anti-corrosion protection during pattern transfer), black hole liquid (for conductive treatment of through holes) and other auxiliary materials are prepared. All raw materials must undergo incoming inspection and can only be used after passing the inspection.

[0024] Existing FPC single-sided circuit fabrication equipment is used, including a cutting machine, photolithography machine, etching machine (DES equipment), laminating machine, drilling machine, plasma treatment equipment, black hole treatment equipment, pattern electroplating equipment, and AOI automatic optical inspection equipment, without the need for additional specialized equipment. Each piece of equipment is pre-tested: the cutting machine's cutting accuracy is adjusted to ensure a dimensional error ≤ ±0.1mm; the photolithography machine's exposure parameters (exposure energy, exposure time) are adjusted to ensure clear and distortion-free circuit patterns; the etching machine's etching solution concentration, etching temperature, and etching speed are adjusted, with preset etching parameters for 18μm and 12μm copper thicknesses to ensure a copper thickness deviation ≤ ±0.5μm after etching; the laminating machine's lamination temperature, pressure, and time are adjusted to ensure tight interlayer bonding without bubbles or delamination; the drilling machine's drilling accuracy is adjusted to ensure a hole diameter deviation of ≤ ±0.05mm for positioning holes and through holes; and the AOI equipment's inspection parameters are adjusted to ensure accurate identification of defects such as short circuits, open circuits, and line width deviations.

[0025] like Figure 1 and Figure 2 As shown, the steps for fabricating asymmetric copper-thickness FPC circuits are as follows: S1. Front-end trimming and yellow light area processing The core of this step is to complete the cutting and surface protection of the top substrate 1 and the bottom substrate 2, laying the foundation for subsequent processing. The specific operations are as follows: S11. Cutting the coil: Prepare the top substrate 1 and the bottom substrate 2. Both the top substrate 1 and the bottom substrate 2 are copper-clad substrates. Put the top substrate 1 and the bottom substrate 2 into the cutting machine respectively, and cut the coil according to the product design size to obtain the sheet substrate of the preset size. Avoid scratching the copper layer on the surface of the substrate during the cutting process. After cutting, clean the sheet and remove burrs and debris.

[0026] S12, Applying carrier film to the yellow light area: The cut top substrate 1 and bottom substrate 2 are respectively sent into the yellow light area (to prevent accidental exposure and ensure the accuracy of the circuit pattern). The carrier film 5 is applied to the surface of the two substrates using an automatic film application device. The carrier film 5 must be applied flat, without bubbles or offset, to ensure complete coverage of the copper layer surface of the substrate (i.e., the outer side of the first copper layer 11 and the second copper layer 21), which plays a role in protecting the copper layer and facilitating subsequent yellow light cutting and pattern transfer.

[0027] S13. Cutting in the yellow light area: Based on the product's dimensions and processing requirements, precisely divide the top substrate 1 and bottom substrate 2, which have the carrier film 5 attached, in the yellow light area. The cutting accuracy is controlled within ±0.1mm to ensure the positioning accuracy of subsequent lamination and pressing processes. After cutting, remove excess waste and arrange qualified sheets.

[0028] S2, Drilling and Plasma Treatment The core of this step is to complete the machining of positioning holes and the activation of the substrate surface to improve interlayer adhesion. The specific operations are as follows: S21. Initial Drilling (Preliminary Positioning Hole Drilling): The top substrate 1, which has been cut by yellow light, is fed into the drilling machine. According to the preset positioning hole position, the initial positioning hole is drilled. The diameter of the positioning hole is determined according to the product design, generally 0.3-0.5mm. During the drilling process, ensure that the positioning hole is free of burrs and skew, and that the hole wall is smooth. The positional deviation of the positioning hole is ≤±0.05mm, providing a reference for the precise positioning of subsequent stacking and pressing.

[0029] S22. Plasma Treatment Before Lamination: The top substrate 1 with drilled positioning holes and the bottom substrate 2 after cutting and finishing are respectively sent to the plasma treatment equipment for surface plasma treatment. Plasma treatment parameters: plasma power 1000-1500W, treatment time 30-60s, treatment pressure 0.1-0.2MPa. The plasma treatment removes oil, oxide layer and impurities from the substrate surface, increases the surface roughness of the substrate, thereby improving the interlayer bonding force during subsequent lamination and avoiding delamination and debonding after pressing.

[0030] S3, Lamination and Pressing The core of this step is to precisely overlap and press the top substrate 1 and the bottom substrate 2 together using pure adhesive (the middle layer 3 in this example is pure adhesive, but a multilayer board with adhesive on both sides can also be used) to obtain a copper-clad board on both sides, ensuring the alignment accuracy between layers. The specific operation is as follows: S31. Stacking: On a clean stacking workbench, using the top substrate 1 as a base, a layer of pure adhesive is evenly laid on the opposite side of the first copper layer 11. The pure adhesive must be laid flat, without bubbles or offset, and the coverage area should be consistent with the substrate. Then, the bottom substrate 2, which has been treated with plasma, is precisely attached to the pure adhesive surface with the opposite side of the second copper layer 21, ensuring that the positioning holes of the bottom substrate 2 and the top substrate 1 are aligned. Impurities and bubbles should be avoided during the stacking process.

[0031] S32. Pre-pressing: The stacked substrates (top substrate 1 + middle layer 3 + bottom substrate 2) are fed into the pre-pressing machine for pre-pressing. Pre-pressing parameters: temperature 80-100℃, pressure 0.5-1.0MPa, time 30-60s. Pre-pressing initially fixes the stacked structure and removes a small amount of air between the layers, preparing for the subsequent main pressing.

[0032] S33, White Light Area Stacking (Bottom Layer): The pre-pressed substrate is sent into the white light area (ordinary white light). With the help of white light positioning equipment, the bottom substrate 2 is precisely positioned and the position of the bottom substrate 2 is adjusted to ensure that it is completely aligned with the circuit area and positioning holes of the top substrate 1. The positioning accuracy is ≤ ±0.03mm to avoid misalignment in subsequent circuit etching.

[0033] S34. Main Lamination: The precisely positioned substrate is fed into the main lamination machine for main lamination. The main lamination parameters are: temperature 160-180℃, pressure 2.0-3.0MPa, and time 60-90s. During the lamination process, pressure transfer and waste removal operations are performed simultaneously to remove the pure glue and excess waste that overflows during the lamination process. After lamination, the substrate is trimmed to remove excess parts at the edges, ensuring that the product's dimensions meet the design requirements, resulting in a double-sided copper-clad board. Finally, the interlayer alignment accuracy of the substrate is checked using X-Ray alignment inspection equipment. If the alignment deviation exceeds the allowable range, rework is performed in a timely manner to ensure that the interlayer alignment accuracy meets the requirements of subsequent circuit fabrication.

[0034] S4. Secondary drilling and through-hole treatment The core of this step is to complete the through-hole forming and hole wall activation and conductivity treatment to ensure the conductivity of the through-hole, thus obtaining the double-sided copper-clad via plate 101. The specific operation is as follows: S41. Second Drilling (Through Hole Forming): The laminated copper-clad substrates are fed into a drilling machine. According to the through hole position and diameter designed for the product, a second drilling is performed. The through hole diameter is determined according to product requirements, generally 0.2-0.4mm. During the drilling process, ensure that the through hole is free of burrs and skew, and that the hole wall is smooth. The positional deviation between the through hole and the circuit should be ≤±0.05mm to avoid short circuits or open circuits between the through hole and the circuit.

[0035] S42. Through-hole plasma: The copper-clad substrate with drilled through holes on both sides is sent into the plasma treatment equipment to perform plasma treatment on the hole walls. The treatment parameters are the same as those for the plasma treatment before stacking. The plasma treatment removes the oxide layer and impurities on the hole walls and activates the hole wall surface, laying the foundation for the conductivity treatment of the subsequent black hole process.

[0036] S43. Black Hole Process: The double-sided copper-clad substrate, after undergoing through-hole plasma treatment, is fed into the black hole processing equipment for black hole conductivity treatment, resulting in a double-sided copper-clad via board 101. The black hole solution temperature is controlled at 30-40℃, and the processing time is 10-15 minutes to ensure that the black hole solution uniformly covers the through-hole walls, forming a uniform conductive layer. The thickness of the conductive layer is controlled at 0.1-0.2μm to ensure the conductivity of the through holes and meet the requirements of subsequent electroplating and circuit continuity.

[0037] S5, Graphic Transfer and Electroplating The core of this step is to form a resist layer for the circuit pattern and increase the copper thickness of the circuit through pattern electroplating to obtain a copper-clad laminate 102 on both sides. The specific operation is as follows: S51. Pattern lamination: One side of the double-sided copper-clad perforated plate 101 is the first copper layer 11, and the other side is the second copper layer 21. The double-sided copper-clad perforated plate 101, which has been treated with black holes, is sent into the lamination machine. The anti-plating film 4 is evenly applied to both sides of the double-sided copper-clad perforated plate 101. The lamination parameters are: temperature 110-130℃, pressure 0.8-1.2MPa, time 20-30s. This ensures that the anti-plating film 4 is tightly applied, without bubbles or displacement, and completely covers the substrate surface and the wall of the through hole.

[0038] S52. Exposure: The copper-clad via 101 with the resist film applied on both sides is fed into a photolithography machine. Exposure is performed according to the preset circuit pattern film to ensure complete curing of the resist film on the second copper layer, while only the resist film on the non-circuit areas of the first copper layer is cured. Exposure parameters: Exposure energy 80-120 mJ / cm², exposure time 10-20 s. Ensure the circuit pattern is clearly transferred to the resist film. Avoid underexposure or overexposure during exposure to prevent blurring or defects in the subsequent development process.

[0039] S53. Development: The exposed copper-clad via 101 is sent to the developing machine and developed using developing solution. The developing solution temperature is controlled at 25-35℃ and the developing time is 30-60s. The unexposed resist film 4 is removed, forming a resist layer consistent with the circuit pattern and exposing the circuit area of ​​the first copper layer. After development, the copper-clad via 101 is cleaned to remove residual developing solution and ensure that the edges of the resist layer are neat and free of residue.

[0040] S54. Pattern Electroplating: The developed double-sided copper-clad through-hole board 101 is sent into the pattern electroplating equipment for pattern electroplating treatment. The electroplating solution is copper sulfate electroplating solution. The electroplating parameters are: current density 2-3A / dm², electroplating temperature 20-30℃, and electroplating time is finely adjusted according to the required copper thickness to ensure that the circuit area of ​​the first copper layer reaches the expected thickness of the circuit on that side, thus obtaining the double-sided copper-clad board 102. During the electroplating process, it is ensured that the circuit surface is uniform, without nodules or pinholes, and that the electroplating layer inside the through hole is uniformly covered to ensure conductivity.

[0041] S55. Removal of the resist coating: The electroplated copper-clad laminate 102 is fed into a stripping machine. A stripping solution is used to remove the cured resist coating from the surface. The temperature of the stripping solution is controlled at 40-50℃, and the stripping time is 10-20 seconds. After stripping, both sides of the copper-clad laminate 102 are cleaned to remove residual stripping solution, exposing the copper layer area to be etched. Stripping after pattern electroplating is to eliminate interference from the electroplated resist layer, ensuring that both DES etching processes are performed with a clean copper surface for re-etching, thus ensuring etching accuracy. The opposing surfaces of the two copper-clad laminates 102 are respectively provided with a first copper layer 11 and a second copper layer 21. A first insulating layer 12 is provided inside the first copper layer 11, which originally belonged to the top substrate 1. A second insulating layer 22 is provided inside the second copper layer 21, which originally belonged to the bottom substrate 2.

[0042] S6, Double-sided step-by-step etching This step is the core process for achieving asymmetric copper thickness. By performing two independent single-sided etching operations, the copper thickness of the top and bottom layers is controlled separately, resulting in a two-sided circuit board 103. The specific operation is as follows: S61, First Etching: S611. First pretreatment: The copper-clad laminates 102 on both sides are pickled to remove the surface oxide layer and impurities of the first copper layer 11. The pickling time is 5-10 seconds. After pickling, the surface is cleaned to ensure that there is no residual acid on the surface of the copper-clad laminates 102 on both sides.

[0043] S612, First lamination and exposure: The resist film 4 is laminated on both sides of the copper-clad laminate 102 again, and then sent into the photolithography machine for targeted exposure. The resist film on the first copper layer 11 is exposed according to the preset circuit pattern to ensure that the resist film in the circuit area on the first copper layer 11 is cured and the resist film in the non-circuit area can be removed by development. The resist film on the second copper layer 21 is exposed to the whole surface to make it completely cured and cannot be removed by development, thereby protecting the underlying copper layer from etching.

[0044] S613, First DES Process: The exposed double-sided copper clad laminate 102 is sent to a DES etching equipment for sequential development, etching, and stripping. Development removes the unexposed resist film 4 on the first copper layer 11, exposing the non-line copper surface to be etched on the first copper layer 11. During etching, preset 18μm copper thickness etching parameters are used (etching solution concentration 100-120g / L, etching temperature 35-45℃, etching speed 1-2m / min). The copper layer in the non-line area of ​​the first copper layer 11 is etched away down to the first insulating layer 12, while retaining the copper layer in the line area, thus forming the first line layer 11a, ensuring that the top layer copper thickness is 18μm after etching. After etching, the resist film on both sides is stripped and cleaned to obtain a double-sided copper clad laminate 102 with a top copper thickness of 18μm and a bottom copper thickness that is still the initial thickness. At this time, the top copper layer is the first line layer 11a with several lines, and the bottom copper layer is the second copper layer 21 with uniform thickness.

[0045] S62, Second Etching: S621. Secondary pretreatment: The copper-clad laminates 102 on both sides are acid-washed again to remove the surface oxide layer and impurities of the second copper layer 2, ensuring the etching effect. The acid-washing parameters are the same as those before the first etching.

[0046] S622, Secondary lamination and exposure: The resist film 4 is laminated again on both sides of the copper-clad laminate 102, and then sent to the photolithography machine for reverse targeted exposure. The bottom layer is exposed normally according to the preset circuit pattern to ensure that the resist film 4 in the circuit area is retained, while the resist film 4 in the non-circuit area can be removed by development. The top layer is exposed to the whole surface so that the resist film 4 in all areas of the top layer is exposed and cured, and cannot be removed by development, thereby protecting the 18μm copper-thick circuit of the top layer from etching.

[0047] S623, Secondary DES Processing: The exposed copper-clad laminates 102 are sent to a DES etching equipment for sequential development, etching, and stripping. Development removes the unexposed resist film 4 from the bottom layer, exposing the non-circuit copper layer to be etched. During etching, preset 12μm copper thickness etching parameters are used (etching solution concentration 80-100g / L, etching temperature 30-40℃, etching speed 1.5-2.5m / min) to remove the copper layer in the non-circuit area of ​​the bottom layer, while retaining the copper layer in the circuit area, ensuring that the copper thickness of the bottom circuit is 12μm after etching. After etching, stripping is performed to remove the resist film 4 from the top and bottom layers. After cleaning, a double-sided circuit board 103 (in this embodiment, an asymmetric copper thickness FPC) with a top layer (i.e., the first circuit layer 11a) of 18μm copper thickness and a bottom layer (i.e., the second circuit layer 21a) of 12μm copper thickness is obtained.

[0048] S63. Post-etching inspection: After etching is completed, use a thickness measuring instrument to check the copper thickness of the two sides of the circuit to ensure that the copper thickness deviation of the top layer is ≤ ±1μm and the copper thickness deviation of the bottom layer is ≤ ±1μm. At the same time, check whether the edges of the circuit are neat and free of burrs to avoid over-etching or under-etching. If it does not meet the requirements, adjust the etching parameters in time and rework.

[0049] In the above description, the first copper layer 11 refers to the uppermost copper layer in the double-sided copper-clad laminate 102 during the double-sided circuit etching process, used to form the first circuit layer 11a; the second copper layer 21 refers to the lowermost copper layer in the asymmetric copper-thickness circuit substrate during the double-sided circuit etching process, used to form the second circuit layer 21a; the materials of the first circuit layer 11a and the second circuit layer 21a are still copper. The distinction between the first copper layer 11 and the second copper layer 21 is mainly to illustrate the anisotropy of the two circuit layers (first circuit layer 11a and second circuit layer 21a), and not to restrict the two etching operations to be performed in the order of top layer first and bottom layer second.

[0050] Double-sided etching is a common method for adding layers to two sides of the circuit. Whether the substrate is a flexible or rigid circuit board, whether it has a single-layer or multi-layer conductive structure, and the initial copper thickness are irrelevant to the implementation. While theoretically, sequential etching of double-sided circuits can also be used on substrates with symmetrical copper thicknesses, simultaneous development and etching of symmetrical copper thicknesses results in faster formation. Therefore, double-sided etching is more suitable for forming circuits with asymmetrical copper thicknesses (i.e., the first copper layer 11 and the second copper layer 12 have different thicknesses). Through the two etching steps, independent control of the different copper thicknesses on both sides is achieved, precisely meeting the high-precision impedance matching requirements of the product. This avoids the complex control required by traditional electroplating thickening processes, improving circuit accuracy and production stability. The resist film 4 and pattern transfer used in this step are the same as those used in the electroplating step, so the principles of exposure, development, and etching are the same. The related equipment used (i.e., the photoluminescence exposure machine, developing machine, and stripping machine used in step S5) is also the same, so no additional specialized equipment is needed.

[0051] S7, Final Inspection Both sides of the circuit board 103 are sent to the AOI-1 automatic optical inspection equipment for final inspection. The inspection includes checking for defects such as short circuits, open circuits, line width deviations, and uneven copper thickness; verifying the conductivity and unblocked state of vias; and checking for delamination and air bubbles between layers. Inspection standards: Short circuits and open circuits are fatal defects and are not allowed; line width deviation ≤ ±0.03mm; copper thickness deviation ≤ ±0.5μm; via conductivity 100%; no obvious delamination or air bubbles. The inspection confirms that the produced circuit board 103 is a qualified product. Qualified products are sent to the next process; unqualified products are reworked and re-inspected until they pass.

[0052] The advantages of this invention are: 1. Cost advantage: It does not rely on special double-sided asymmetric thickness copper substrates. Asymmetric copper thickness can be achieved by using conventional single-sided copper-clad substrates. The material selection is flexible and the substrate procurement cost is greatly reduced. No new special equipment is required. It can rely on existing FPC single-sided circuit fabrication equipment, reduce the investment in production equipment, and easily achieve industrial mass production.

[0053] 2. Precision Advantage: By avoiding the complex control of the electroplating thickening process, the copper thickness on both sides is precisely controlled through two independent single-sided etching processes. The difference in copper thickness can be finely adjusted by selecting the substrate or by a single electroplating process, resulting in high stability and circuit accuracy that meets the requirements of high-precision impedance matching.

[0054] 3. Compatibility advantages: The process flow is compatible with the existing FPC production system, without the need for major adjustments to the production process. Operators do not require special training and can quickly get started, resulting in high production efficiency.

[0055] 4. Cleanliness control of each process: Key processes such as the yellow light area, the overlay area, and the etching area must be kept clean to avoid the introduction of impurities and dust, and to prevent problems such as short circuits and poor etching.

[0056] 5. Positioning accuracy control: The positioning accuracy of processes such as drilling, lamination, and exposure must be strictly controlled, especially the exposure positioning of the two etching processes, to ensure that the lines are not misaligned and the copper thickness is accurately controlled.

[0057] 6. Etching parameter control: The concentration, temperature and speed of the etching solution for the two etching processes must be strictly implemented according to the preset parameters. The performance of the etching solution should be tested regularly, and the etching solution should be replenished or replaced in a timely manner to ensure stable etching results.

[0058] 7. Raw material control: All raw materials must undergo incoming inspection to ensure that the performance of materials such as base material, intermediate layer, and anti-coating film meets the requirements, and to avoid affecting the product qualification rate due to raw material quality problems.

[0059] The above descriptions are merely some embodiments of the present invention. Those skilled in the art can make various modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A method for fabricating asymmetric copper thickness circuits, characterized in that: This includes pattern transfer and electroplating steps, and double-sided step-by-step etching steps; The graphic transfer and electroplating operation steps include: 1) Pattern lamination: Prepare a double-sided copper-clad perforated board, one side of which is a first copper layer and the other side is a second copper layer. Apply an anti-plating film evenly to both sides of the double-sided copper-clad perforated board substrate. 2) Exposure: The copper-clad vias with the resist film applied are exposed according to the preset circuit pattern film to ensure that the resist film of the second copper layer is fully cured, while only the resist film of the non-circuit areas of the first copper layer is cured. 3) Development: The exposed copper-clad vias are developed with a developer to remove the unexposed resist film, forming a resist layer consistent with the circuit pattern and exposing the circuit area of ​​the first copper layer. The residual developer is then cleaned to remove it. 4) Pattern electroplating: The developed copper-clad perforated board is subjected to pattern electroplating to ensure that the circuit area of ​​the first copper layer reaches the expected thickness of the circuit on that side, while the second copper layer maintains its original thickness, thus obtaining a copper-clad board with two sides. 5) Removal of resist coating: The resist coating on the circuit surface of the two copper-clad laminates after electroplating is removed by stripping solution. After stripping, the two copper-clad laminates are cleaned to remove the residual stripping solution and expose the copper layer area to be etched. The steps involved in double-sided step-by-step etching include: 1) First etching: After cleaning the surfaces of the two copper-clad laminates, a resist film is attached to both sides of the two copper-clad laminates. The resist film on the first copper layer is exposed according to the preset circuit pattern to ensure that the resist film in the circuit area on the first copper layer is cured and the resist film in the non-circuit area can be removed by development. The resist film on the second copper layer is exposed to the whole surface to make it completely cured. The resist film in the non-circuit area on the first copper layer is removed by development to expose the non-circuit copper surface. Then the non-circuit copper surface is etched to form the first circuit layer. Then the cured resist film on both sides is stripped. 2) Second etching: After cleaning the surfaces of the two copper-clad laminates, the resist film is then attached to both sides of the two copper-clad laminates. The resist film on the second copper layer is partially exposed according to its preset circuit pattern to ensure that the resist film in the second copper circuit area is cured. The resist film on the first circuit layer is exposed to the whole surface to ensure that it is completely cured. The resist film in the non-circuit area on the second copper layer is developed and removed to expose the non-circuit copper surface. Then the non-circuit copper surface is etched to form the second circuit layer. Finally, the cured resist film on both sides is stripped. The pattern lamination, first etching, and second etching are completed using the same anti-coating film and the same yellow light exposure machine, developing machine, and stripping machine.

2. The method for fabricating asymmetric copper thickness circuits according to claim 1, characterized in that: The double-sided copper-clad perforated plate is an intermediate plate used to manufacture asymmetric copper-thickness FPCs.

3. The method for fabricating asymmetric copper thickness circuits according to claim 1, characterized in that, step include: S1. Front-end cutting and yellow light area treatment: Prepare the top substrate, bottom substrate and intermediate layer. The top substrate and the bottom substrate are both copper-clad substrates. Complete the cutting and surface protection of the top substrate and the bottom substrate. S2. Drilling and Plasma Treatment: Positioning holes are machined and substrate surfaces are activated on the top and bottom substrates. S3. Overlay and lamination: The top substrate and the bottom substrate are precisely overlaid and laminated into one piece through the middle layer with the copper layer facing outward, to obtain a copper-clad board on both sides. S4. Secondary drilling and through-hole processing: Through-hole forming and hole wall activation are completed on the two-sided copper-clad original board to obtain a two-sided copper-clad hole board. S5. Pattern transfer and electroplating: A circuit pattern resist layer is formed on the surface of the copper-clad perforated plate to obtain a copper-clad plate on both sides. S6. Double-sided step-by-step etching: The two copper-clad laminates are processed into two-sided circuit boards; S7. Final Inspection: Inspect and confirm whether the two circuit boards are qualified products.

4. The method for fabricating asymmetric copper thickness circuits according to claim 3, characterized in that: The steps for front-end trimming and yellow light area processing include: 1) Cutting sheet material: The top substrate and the bottom substrate are cut into sheets according to the product design dimensions to obtain sheet material of the preset size; 2) Applying carrier film to the yellow light area: The cut top substrate and bottom substrate are respectively sent into the yellow light area and a carrier film is applied to the surface of the two substrates. The carrier film completely covers the first copper layer and the second copper layer. 3) Cutting in the yellow light area: Based on the product's shape and processing requirements, the top and bottom substrates with the carrier film applied are precisely divided in the yellow light area, with the cutting accuracy controlled within ±0.1mm.

5. The method for fabricating asymmetric copper thickness circuits according to claim 4, characterized in that: The drilling and plasma treatment procedures include: 1) First drilling: The top substrate, which has been cut by yellow light, is initially drilled to make positioning holes according to the preset positioning hole positions. The position deviation of the positioning holes is ≤ ±0.05mm. 2) Plasma treatment before stacking: The top substrate with drilled positioning holes and the bottom substrate after cutting and tidying are subjected to surface plasma treatment respectively. Plasma treatment parameters: plasma power 1000-1500W, treatment time 30-60s, treatment pressure 0.1-0.2MPa.

6. The method for fabricating asymmetric copper thickness circuits according to claim 3, characterized in that: The overlapping and pressing operations include: 1) Stacking: Using the top substrate as the base, a layer of pure adhesive is evenly laid on the opposite side of the first copper layer, and then the bottom substrate that has been treated with plasma is precisely bonded to the pure adhesive surface with the opposite side of the second copper layer. 2) Pre-compression: The stacked substrates are pre-compressed. Pre-compression parameters: temperature 80-100℃, pressure 0.5-1.0MPa, time 30-60s; 3) White light zone stacking: The pre-pressed substrate is sent into the white light zone, and the bottom substrate is precisely positioned with a positioning accuracy of ≤ ±0.03mm; 4) Main lamination: The precisely positioned substrate is fed into the main lamination machine for main lamination. The main lamination parameters are: temperature 160-180℃, pressure 2.0-3.0MPa, and time 60-90s. During the lamination process, pressure transmission and waste removal operations are performed simultaneously to remove the pure glue and excess waste that overflows during the lamination process. After lamination, the substrate is trimmed to remove excess parts at the edges, ensuring that the product's external dimensions meet the design requirements, resulting in a double-sided copper-clad substrate.

7. The method for fabricating asymmetric copper thickness circuits according to claim 6, characterized in that: The final step of the main lamination operation involves X-ray inspection of the interlayer alignment accuracy of the substrate. If the alignment deviation is found to exceed the allowable range, rework is carried out in a timely manner.

8. The method for fabricating asymmetric copper thickness circuits according to claim 3, characterized in that: The operation steps for secondary drilling and through-hole treatment include: 1) Second drilling: The two copper-clad boards that have been pressed together are drilled a second time according to the through hole positions and diameters designed in the product. The positional deviation between the through holes and the circuit is ≤ ±0.05mm. 2) Through-hole plasma: The copper-clad substrate with drilled through holes on both sides is fed into the plasma treatment equipment to perform plasma treatment on the hole walls; 3) Black hole process: The copper-clad original board that has undergone through hole plasma treatment is subjected to black hole conductivity treatment to obtain a copper-clad perforated board. The temperature of the black hole solution is controlled at 30-40℃ and the processing time is 10-15min.

9. The method for fabricating asymmetric copper thickness circuits according to claim 3, characterized in that: The final step of the double-sided step etching process involves copper thickness testing to ensure that the copper thickness deviation of the top layer is ≤ ±1μm and the copper thickness deviation of the bottom layer is ≤ ±1μm. If the requirements are not met, the etching parameters are adjusted in time and rework is carried out.

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