A transparent FPC and its preparation method

Abstract

This invention provides a transparent FPC and its preparation method. By first performing overall browning followed by laser ablation of the browning film on the pads, this invention improves the yield of electroless nickel-gold surface treatment. The defect rate of the transparent FPC prepared using this method is ≤1%. The laser ablation of the browning film on the pads solves the problem of gold leakage during surface treatment due to residual browning film on the pads; simultaneously, it avoids the impact of strong micro-etching and browning stripping solutions on the transparency of the transparent substrate.

Description

Technical Field

[0001] This invention relates to the field of circuit board manufacturing technology, and in particular to a transparent FPC and its preparation method. Background Technology

[0002] Transparent FPCs (Flexible Printed Circuits) for displays use PET / CPI transparent substrates. If the internal copper traces are in a mirror-like bright copper state, they will produce reflections and glare, causing bright spots and visual stray light on the display screen, which seriously affects the display effect. Therefore, it is necessary to perform a matte treatment on the copper traces to reduce the reflection of the traces. The conventional process for manufacturing transparent FPCs is as follows: after the trace etching is completed, the entire board is directly browned, and brown oxide films are generated simultaneously in the trace area and the functional pad area. In order to ensure the solderability of the pads and the requirements of subsequent SMT placement and bonding, selective micro-etching / brown stripping chemicals must be used to remove the brown film on the surface of the pads after the protective film is laminated.

[0003] Currently, the conventional pretreatment process before FPC surface treatment (after lamination of the protective film) mainly involves brush grinding combined with conventional micro-etching. This combined process can only remove surface contaminants and ordinary oxide layers, and it has the problem of incomplete removal of the browned composite film layer on the solder pads after high-temperature lamination. The residual browned film and high-temperature oxide layer will directly hinder the deposition of metal plating layers in subsequent surface treatment processes such as tin plating and gold plating, thereby causing quality defects such as missed plating of solder pads and poor adhesion of plating layers, which will significantly reduce product yield and reliability.

[0004] Currently, there is no precise solution in the industry for removing the brown film caused by high-temperature aging of pads in transparent display FPCs. Simply increasing the brush pressure or extending the conventional micro-etching time can easily cause secondary problems such as damage to the fine circuitry of the FPC, excessive copper loss, and scratches on the substrate. It is difficult to balance the brown film removal effect with the product's appearance and electrical performance. Furthermore, with the increasing functionality of electronic components and the denser circuitry in circuit areas, the spacing between pads becomes extremely small. In these tiny-pitch pads and dense circuit areas, the brown coating is difficult to completely remove. Residual brown coating can directly lead to incomplete plating, plating rejection, or poor plating quality in subsequent surface treatments such as immersion nickel gold, immersion tin, and OSP. If strong micro-etching and brown coating removal chemicals are used to remove the brown coating from the pads, it will cause copper thickness loss on the pads and thinning of the side etching of fine circuits, affecting electrical performance and structural dimensions. Moreover, the transparent FPC substrate has weak chemical corrosion resistance, and prolonged immersion in strong chemicals can easily cause whitening of the substrate, watermarks, and decreased light transmittance, affecting appearance and optical performance. Using manual tape or masking film to protect the pads results in low alignment accuracy, easy residue, and is not suitable for mass production of high-precision micro pads, leading to low yield and high cost.

[0005] Therefore, there is an urgent need to develop a method that can accurately and quickly remove the brown film from FPC.

[0006] Therefore, this invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to address the aforementioned deficiencies in the prior art, particularly the problems of residual brown coating film, copper erosion on solder pads, damage to transparent substrates, and low masking accuracy. This invention provides a method for accurately and quickly removing the brown coating film from FPCs, specifically by first browning the entire surface and then laser ablation the brown coating film on the solder pads, thereby improving the gold plating yield.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a transparent FPC, comprising the following steps: S1: Fabricate conductive circuit patterns on FCCL substrate to form conductive circuits and obtain FPC board surface; S2: The surface of the conductive circuit is browned to generate a matte browned film. S3: Remove the brown film in the pad area by laser ablation to expose the bare copper of the pad; S4: A protective film is laminated onto the FPC board surface, and the protective film has windows at the positions corresponding to the pad areas to expose the pads; S5: Perform electroless nickel-gold surface treatment on the exposed pad areas.

[0009] As an embodiment of the present invention, in step S1, the fabrication of the circuit pattern includes the following process steps in sequence: film application, exposure, development, etching, film removal, and AOI optical inspection.

[0010] As an embodiment of the present invention, in step S2, before the browning treatment, the following pretreatment step is also included: cleaning and roughening the copper surface and substrate surface of the FPC board.

[0011] As an embodiment of the present invention, step S4 includes the following steps for pressing the protective film: cutting the protective film, pre-applying and fixing, rolling the protective film, curing, and visual inspection.

[0012] In an embodiment of the present invention, the curing is hot-press curing.

[0013] As an embodiment of the present invention, the curing temperature is 160~180℃.

[0014] As an embodiment of the present invention, the curing pressure is 110~140 kgf / cm². 2 .

[0015] As an embodiment of the present invention, the curing time is 160~200s.

[0016] As an embodiment of the present invention, in step S5, the electroless nickel-gold surface treatment includes the following steps: acid pickling activation, electroless nickel plating, electroless gold plating, and appearance inspection.

[0017] In an embodiment of the present invention, in step S3, the wavelength of the laser used for laser ablation is 350~365nm.

[0018] In an embodiment of the present invention, in step S3, the ablation power of the laser ablation is 2~3W.

[0019] As an embodiment of the present invention, in step S3, the single ablation time of the laser ablation is 120~150s.

[0020] As an embodiment of the present invention, in step S3, the minimum spacing between any two pads is ≤150μm.

[0021] As an embodiment of the present invention, in step S5, the defect rate of the electroless nickel-gold surface treatment is ≤1%.

[0022] In a second aspect, the present invention also provides a transparent FPC, which is prepared by the preparation method described in the first aspect of the present invention.

[0023] Compared with the prior art, the beneficial effects of the present invention are: This invention improves the yield of electroless nickel-gold by first browning the entire film and then laser ablation the browned film on the solder pads. The defect rate of transparent FPCs prepared using the method of this invention is ≤1%. Attached Figure Description

[0024] Figure 1 The image shows the actual solder pads on the FPC board. It can be seen that there are solder pads with unplated areas and solder pads with electroless nickel-gold plating. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0027] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar to or equivalent to those described herein may be used in the implementation or testing of this invention.

[0028] In a first aspect, some embodiments of the present invention provide a method for preparing a transparent FPC, comprising the following steps: S1: Fabricate conductive circuit patterns on FCCL substrate to form conductive circuits and obtain FPC board surface; S2: The surface of the conductive circuit is browned to generate a matte browned film. S3: Remove the brown film in the pad area by laser ablation to expose the bare copper of the pad; S4: A protective film is laminated onto the FPC board surface, and the protective film has windows at the positions corresponding to the pad areas to expose the pads; S5: Perform electroless nickel-gold surface treatment on the exposed pad areas.

[0029] This invention improves the yield of electroless nickel-gold surface treatment by first performing overall browning followed by laser ablation of the browning film on the pads. The defect rate of transparent FPCs prepared using this method is ≤1%. The laser ablation of the browning film on the pads solves the problem of gold leakage during surface treatment due to residual browning film on the pads; at the same time, it avoids the impact of strong micro-etching and browning stripping solutions on the transparency performance of the transparent substrate.

[0030] FCCL (abbreviation for Flexible Copper Clad Laminate) is a substrate material used to process flexible printed circuit boards (FPCs), consisting of copper foil (conductor layer) and insulating base film (dielectric layer). Based on its structure, it can be divided into three-layer type (3L-FCCL: copper foil + adhesive + base film) and adhesive-free two-layer type (2L-FCCL: copper foil + base film directly laminated).

[0031] The framework of the copper foil pads is precisely preserved from the copper foil on the FCCL using photolithography and etching processes, while the remaining copper foil is removed, thus forming the conductive circuit pattern. The fabrication of the conductive circuit pattern includes the following process steps: film application, exposure, development, etching, film removal, and AOI optical inspection.

[0032] In some embodiments of the present invention, the fabrication of the conductive circuit pattern specifically includes the following steps: ① Film application: A photosensitive dry film is applied to the copper foil surface of the FCCL to serve as a photosensitive carrier for subsequent exposure and imaging; ② Exposure: Selective exposure is performed on the FCCL board according to the circuit design; ③Developing: The exposed FCCL is immersed in the developing solution. The dry film in the light-receiving area is photocured and insoluble in the developing solution, thus preserving the cured dry film to protect the conductive lines. The light-shielding area remains soluble and dissolves in the developing solution to remove the dry film in the non-circuit area, exposing the excess bare copper. Therefore, the conductive line pattern on the drawing can be copied onto the dry film to finally form a circuit mask pattern. ④ Etching: Remove excess copper foil without dry film protection by etching, leaving only the conductive lines protected by the dry film to form the circuit pattern; ⑤ Film Removal: Remove the photosensitive dry film from the surface of the circuit to expose the pure copper circuit. ⑥AOI optical inspection: Detects defects in pattern transfer, such as short circuits, open circuits, line width deviations, gaps, residual copper, and misalignment, intercepting defects and controlling yield.

[0033] In some embodiments of the present invention, step S2, before the browning treatment, further includes the following pretreatment step: cleaning and roughening the copper surface and substrate surface of the FPC board. The purpose of cleaning the copper surface and substrate surface of the FPC board is to remove oil, oxide layer, dust, and impurities; roughening the surface is to improve the adhesion between the protective film and the FPC board surface, preventing subsequent edge lifting and delamination.

[0034] In some embodiments of the present invention, the roughening is a slight roughening, specifically using a mild micro-etching process. After the workpiece is acid-washed and activated, it is immersed in a sulfuric acid-sodium persulfate solution and treated at 25-35°C for 30-60 seconds, with the etching depth controlled at 0.5-1.0 μm. After completion, it undergoes multiple stages of water washing and drying to uniformly and slightly roughen the copper surface, as long as the conductive circuit pattern is not damaged.

[0035] In some embodiments of the present invention, browning is usually a pretreatment method to improve the bonding force between inner and outer layers before laminating multilayer boards. Based on the characteristic that the copper surface becomes black and matte after browning, the present invention adopts browning in order to generate a uniform matte browning film on the circuit surface and achieve the effect of matting and low reflectivity.

[0036] In some embodiments of the present invention, in step S3, the brown film in the pad area is removed by selective laser ablation, thus achieving the effect that the circuit area is covered by brown film and the pad area is bare copper.

[0037] In some embodiments of the present invention, in step S3, the wavelength of the laser used for laser ablation is 350~365nm, preferably 355nm UV light.

[0038] In some embodiments of the present invention, in step S3, the ablation power of the laser ablation is 2~3W, preferably 3W.

[0039] In some embodiments of the present invention, in step S3, the single ablation time of the laser ablation is 120-150 seconds. Using a short-duration, multiple-interval laser ablation method can further improve the removal accuracy of the brown film and avoid over-removal that could damage the circuitry.

[0040] In some embodiments of the present invention, in step S3, the minimum spacing between any two pads is ≤150μm.

[0041] In some embodiments of the present invention, in step S4, the function of the press-fit protective film is to provide physical protection, electrical insulation and structural support for the FPC.

[0042] In some embodiments of the present invention, the pressing protective film includes the following steps: cutting the protective film, pre-applying and fixing, rolling the protective film, curing, and visual inspection. The protective film is cut and opened in the corresponding pad area to expose the pad area, which not only facilitates precise alignment and bonding but also does not obstruct the photochemical nickel-gold treatment on the pads. Pre-applying and fixing prevents the protective film from shifting, deviating, or tilting during the actual rolling process, laying a good foundation for a smooth film application across the entire board. Rolling the protective film removes air between the protective film and the FPC board surface, ensuring a tight, bubble-free, and wrinkle-free bond, and uniformly wetting the FPC board surface with the adhesive layer, improving adhesion strength. Curing is achieved through hot-press curing, allowing the adhesive layer to fully wet, cure, and set, enhancing resistance to bending, peeling, and aging, and preventing edge lifting and peeling during subsequent production and use. Visual inspection mainly checks for defects such as misalignment, bubbles, wrinkles, dust inclusions, and improper bonding, rejecting problematic boards while allowing qualified boards to proceed to the next process.

[0043] In some embodiments of the present invention, the protective film is made of a transparent material, including at least one of polymethyl methacrylate (PET) and polyimide (CPI).

[0044] In some embodiments of the present invention, the curing is thermo-press curing.

[0045] As an embodiment of the present invention, the curing temperature is 160~180℃.

[0046] As an embodiment of the present invention, the curing pressure is 110~140 kgf / cm². 2 The preferred value is 120 kgf / cm³. 2 .

[0047] As an embodiment of the present invention, the curing time is 160~200s, preferably 180s.

[0048] In some embodiments of the present invention, step S5, the chemical nickel-gold surface treatment includes the following steps: acid pickling and activation, chemical nickel plating, chemical gold plating, and visual inspection. Acid pickling and activation are performed to remove contaminants from the copper surface of the solder pads and enhance the adhesion between the plating layer and the copper surface. A nickel layer is deposited on the copper surface of the solder pads using the chemical nickel-gold method to prevent gold-copper diffusion. A thin gold plating layer is then applied to the nickel layer surface to improve the oxidation resistance and conductivity of the FPC board. Visual inspection involves using a microscope to visually check for missed plating on the solder pads and using 3M tape to test the adhesion between the plating layer and the solder pads. Only after passing these tests can the next process proceed.

[0049] In a second aspect, the present invention provides a transparent FPC, which is prepared by the preparation method described in the first aspect of the present invention. The defect rate of the coating of the transparent FPC prepared using the method of the present invention is ≤1%.

[0050] To further illustrate the present invention, the following detailed description is provided in conjunction with embodiments, but these should not be construed as limiting the scope of protection of the present invention. The sources of the raw materials used in the embodiments and comparative examples of the present invention are as follows. Unless otherwise specified, all raw materials used are commercially available, and the same raw materials were used in parallel experiments.

[0051] The following are specific embodiments of the present invention.

[0052] Example 1 A transparent FPC is provided, which is prepared by a method including the following steps: S0: Material feeding: The FCCL substrate uses a 12μm copper thickness; The transparent protective film uses a 25μm thick single-sided CPI substrate; S1: Fabricating conductive circuit patterns on FCCL substrate ① Dry film application: A 20μm thick dry film is applied to the copper foil surface of the FCCL as a photosensitive carrier for subsequent exposure and imaging; ② Exposure: Selective exposure is performed on the board surface according to the circuit design, with an energy of 40~50mJ; ③Developing: Remove the dry film from non-circuit areas to expose excess bare copper. Sodium carbonate solution is used as the developing solution. ④ Etching: Remove excess copper foil without dry film protection by etching, leaving only the conductive lines protected by the dry film to form the circuit pattern; ⑤ Film removal: Use an organic film removal solution to remove the dry film covering the film; ⑥AOI optical inspection: detects defective PCS in the circuit fabrication process. Only qualified FPC boards can proceed to the next process. S2: Browning Cleaning: A light micro-etching process is used. After acid pickling and activation, the workpiece is immersed in a sulfuric acid-sodium persulfate solution and treated at 25~35℃ for 30~60 seconds, with the etching depth controlled at 0.5~1.0μm. After completion, the workpiece undergoes multi-stage water washing and drying to uniformly and slightly roughen the copper surface. Browning: The cleaned and roughened substrate with etched lines is subjected to browning treatment. The browning solution (composed of sulfuric acid, hydrogen peroxide, trace amounts of chloride ions, imidazole organic film-forming agents and chelating stabilizers) is heated to 50°C. The substrate is immersed in the browning solution for 60 seconds to make the copper surface covered with a uniform browning film. S3: Laser ablation to remove brown film Using 355nm nanosecond UV light with a power of 3W, the brown film is ablated by scanning 3 times (40~50s / time). The brown film in the pad area is removed by laser ablation to expose the bare copper of the pad. In this embodiment, the minimum distance between any two pads of the board is 150μm. S4: Press-fit protective film ① After the protective film is opened, match the FPC board shape and the opening position to ensure size compliance and facilitate accurate alignment and bonding; ② Pre-fitting and fixing; ③ Rolling protective film: to expel air between the protective film and the FPC board surface, so that the protective film is completely sealed without bubbles or wrinkles, and the adhesive layer of the protective film is evenly wetted on the FPC board surface, improving the bonding strength; ④ Hot-press curing: The curing temperature is 160℃, and the curing pressure is 120 kgf / cm². 2 The curing time is 180 seconds; ⑤ Inspect for defects such as misalignment, bubbles, wrinkles, dust accumulation, and poor bonding. Reject defective boards, while qualified boards can proceed to the next process. S5: Perform electroless nickel-gold surface treatment on the exposed pad areas. First, deposit an 8μm thick nickel layer on the copper surface of the solder pads to prevent gold-copper diffusion; Then, a 0.06μm thick gold layer is plated on the nickel layer surface.

[0053] The coating yield and adhesion of the nickel-gold layer on the transparent FPC prepared in this embodiment were tested: ① Microscopic visual inspection: Inspect for incomplete solder pad plating; the defect rate is 1%. Figure 1The paper presents two physical phenomena: nickel-gold deposition on pads and nickel-gold plating defects. The defect rate (%) = number of missing pads / total number of pads on the FPC × 100%.

[0054] ② Adhesion test: Clean the pads to be tested, apply 3M tape and press it to remove air bubbles. After standing, peel it vertically and evenly, and check whether the plating is peeling off. The peeling rate of the pads tested by 3M in this embodiment is 1%. Peeling rate (%) = number of pads peeling or flaking / total number of pads on the FPC × 100%.

[0055] Comparative Example 1 A transparent FPC is provided, prepared according to the method of Example 1, except that the method for removing the browning film in step S3 is as follows: Brown film removal: Use a needle brush (pressure: 0.6 kg / cm², speed: 3 m / min) to remove oil, fingerprints, slight oxidation, and dust, and lightly roughen the copper surface. Then, chemically roughen (sulfuric acid + hydrogen peroxide, 25°C, 60 seconds, micro-etching amount is 1.2 μm) to remove high-temperature oxide scale, residual brown film, and aged oxide layer, and uniformly roughen the brown film of the FPC pads. The plating defect rate of the nickel-gold layer on the transparent FPC prepared in this comparative example was 22.5%, and the peeling rate was 30%.

[0056] The results above show that: This invention improves the yield of electroless nickel-gold surface treatment by first performing overall browning followed by laser ablation of the browning film on the pads. The defect rate of transparent FPCs prepared using this method is ≤1%. The laser ablation of the browning film on the pads solves the problem of gold leakage during surface treatment due to residual browning film on the pads; at the same time, it avoids the impact of strong micro-etching and browning stripping solutions on the transparency performance of the transparent substrate.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a transparent FPC, characterized in that, Includes the following steps: S1: Fabricate conductive circuit patterns on FCCL substrate to form conductive circuits and obtain FPC board surface; S2: The surface of the conductive circuit is browned to generate a matte browned film. S3: Remove the brown film in the pad area by laser ablation to expose the bare copper of the pad; S4: A protective film is laminated onto the FPC board surface, and the protective film has windows at the positions corresponding to the pad areas to expose the pads; S5: Perform electroless nickel-gold surface treatment on the exposed pad areas.

2. The method for preparing a transparent FPC according to claim 1, characterized in that, In step S1, the fabrication of the circuit pattern includes the following process steps in sequence: film application, exposure, development, etching, film removal, and AOI optical inspection.

3. The method for preparing a transparent FPC according to claim 1, characterized in that, In step S2, before the browning process, the following pretreatment step is also included: cleaning and roughening the copper surface and substrate surface of the FPC board.

4. The method for preparing a transparent FPC according to claim 1, characterized in that, In step S4, the pressing of the protective film includes the following steps: cutting the protective film, pre-applying and fixing, rolling the protective film, curing, and visual inspection.

5. The method for preparing a transparent FPC according to claim 4, characterized in that, The curing is thermo-press curing, and the curing process satisfies at least one of the following characteristics: (1) The curing temperature is 160~180℃; (2) The curing pressure is 110~140 kgf / cm 2 ; (3) The curing time is 160~200s.

6. The method for preparing a transparent FPC according to claim 1, characterized in that, In step S5, the electroless nickel-gold surface treatment includes the following steps: acid pickling activation, electroless nickel plating, electroless gold plating, and appearance inspection.

7. The method for preparing a transparent FPC according to claim 1, characterized in that, In step S3, the laser ablation process satisfies at least one of the following characteristics: (1) The wavelength of the laser used for laser ablation is 350~365nm; (2) The ablation power of the laser ablation is 2~3W; (3) The single ablation time of the laser ablation is 120~150s.

8. The method for preparing a transparent FPC according to claim 1, characterized in that, In step S3, the minimum spacing between any two pads is ≤150μm.

9. The method for preparing a transparent FPC according to claim 1, characterized in that, In step S5, the defect rate of the electroless nickel-gold surface treatment is ≤1%.

10. A transparent FPC, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 9.

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