Copper pillar fabrication method for flexible circuit board

Abstract

The scheme discloses a copper column manufacturing method for a flexible circuit board, and belongs to the technical field of flexible circuit board manufacturing. The method comprises the following steps: pre-treating a flexible copper-clad plate to form a conductive through hole; manufacturing a circuit pattern on a first surface while keeping a complete copper layer on a second surface; performing multiple times of film pasting, exposure, development and soft plate VCP electroplating circulation in sequence on the area of the first surface where the copper column is needed to be manufactured until the height of the copper column reaches a predetermined value; and manufacturing a circuit pattern on the second surface after removing the residual dry film. The scheme precisely controls the height of the copper column by manufacturing the circuit on the surfaces in sequence and multiple times of electroplating circulation, avoids the damage of the whole-plate electroplating thickening to the bending performance of the base material, and supports the double-sided copper column structure. The height of the copper column can reach 50-300 mu m, the diameter precision can reach plus or minus 3 mu m, and the scheme is suitable for application scenarios such as chip packaging, Z-direction interconnection and high-frequency signal transmission.

Description

Technical Field

[0001] This invention relates to the field of PCB manufacturing, and in particular to a method for fabricating copper pillars for flexible circuit boards. Background Technology

[0002] In existing technologies, the common method for fabricating copper pillars on flexible circuit boards is mainly based on improvements to the copper pillar electroplating process of traditional rigid printed circuit boards. The main process includes steps such as material preparation, drilling, hole metallization, overall board electroplating for thickness enhancement, film lamination, exposure, development, pattern electroplating of copper pillars, film removal, and flash etching. In general, the copper layer is first thickened by electroplating on the entire board, and then the copper layer in non-copper pillar areas is thinned or removed through pattern transfer and flash etching processes, ultimately leaving a raised copper pillar structure on the substrate surface.

[0003] This process is widely used in rigid circuit boards, but it has the following drawbacks in the production of flexible circuit boards: First, the increased thickness of the entire board through electroplating increases the overall rigidity of the flexible substrate, severely affecting the bending performance of the FPC. Second, the flash etching process requires high etching uniformity, making it difficult to precisely control the etching amount, resulting in poor consistency in the height and diameter of the copper pillars. Third, this process cannot achieve double-sided copper pillar structures; it can only form bumps on one side. Furthermore, the multiple electroplating and etching steps increase process complexity and cost, making it unsuitable for mass production.

[0004] Furthermore, existing technologies typically employ chemical copper plating in the hole metallization process, which requires the use of palladium activator. Palladium ions tend to remain on the surface of the polyimide substrate and the hole walls, which may lead to decreased insulation performance and increased signal loss during long-term use. In addition, chemical copper plating waste liquid contains heavy metals, resulting in high environmental treatment costs.

[0005] It is evident that existing copper pillar manufacturing processes have significant shortcomings in terms of adaptability, dimensional accuracy, double-sided integration capability, bending reliability, and environmental friendliness. A copper pillar manufacturing method specifically designed for the characteristics of flexible circuit boards is needed to solve these problems. Summary of the Invention

[0006] The technical solution of this invention aims to at least partially solve one of the technical problems in related technologies. Therefore, the main objective of this invention is to provide a method for manufacturing copper pillars for flexible circuit boards, aiming to solve problems in existing flexible circuit board copper pillar manufacturing processes such as decreased substrate bending performance due to increased thickness from overall electroplating, difficulty in precisely controlling copper pillar dimensions during flash etching, support for only single-sided copper pillar structures, and damage to the polyimide substrate caused by chemical copper plating.

[0007] To achieve the above objectives, the present invention provides a method for fabricating copper pillars for flexible circuit boards, comprising: pre-treating a flexible copper-clad laminate to form conductive vias on the substrate; covering a first photoresist layer on a first surface and a second surface of the substrate respectively; after exposure and development, exposing the circuit pattern on the first surface and protecting the entire second surface; then etching away the exposed copper layer on the first surface to form the circuit on the first surface, and removing the remaining first photoresist layer; covering a second photoresist layer on the first surface and a second surface respectively; after exposure and development, exposing the area on the first surface where the copper pillars are to be fabricated and protecting the entire second surface; then depositing copper in the exposed area by electroplating to increase the height of the copper pillars; removing the second photoresist layer after the copper pillar height reaches a predetermined value; covering a third photoresist layer on the first surface and a second surface respectively; after exposure and development, exposing the circuit pattern on the second surface and protecting the entire first surface; then etching away the exposed copper layer on the second surface to form the circuit on the second surface, and removing the remaining third photoresist layer.

[0008] As a further embodiment of the present invention, the pretreatment includes sequentially performing blanking, drilling, black hole treatment, and hole-filling electroplating, wherein the black hole treatment uses carbon black deposition on the hole wall to form a conductive layer, and the hole-filling electroplating completely fills and flattens the hole.

[0009] As a further embodiment of the present invention, the black hole replaces the chemical copper plating process, thus avoiding damage to the polyimide substrate caused by palladium activation.

[0010] As a further embodiment of the present invention, the etching is acid etching, which uses an acidic etching solution to remove the exposed copper layer.

[0011] As a further embodiment of the present invention, the electroplating is performed using a vertical continuous electroplating method.

[0012] As a further embodiment of the present invention, the thickness of the copper layer thickened by each electroplating is 50 to 100 μm, the predetermined value is 50 to 300 μm, and the number of repetitions is determined according to the predetermined value and the thickness of a single thickening.

[0013] As a further embodiment of the present invention, the photoresist layer is removed by using a NaOH solution for film removal.

[0014] As a further embodiment of the present invention, the etching is acid etching, and during the etching process, the lines on the first side and the formed copper pillars are completely covered and protected by a third photoresist layer.

[0015] As a further embodiment of the present invention, after removing the remaining third photoresist layer, at least one post-processing step is included in the following steps: cover film lamination, surface treatment, shape processing, and electrical performance testing.

[0016] As a further embodiment of the present invention, the first photoresist layer, the second photoresist layer and the third photoresist layer are all photosensitive dry films.

[0017] The beneficial effects of this invention are as follows: This invention proposes a method for fabricating copper pillars for flexible circuit boards. By first fabricating the first side of the circuitry, then progressively thickening the copper pillars through cyclic electroplating, and finally fabricating the second side of the circuitry, this method avoids the increased rigidity and decreased bending performance issues associated with thickening the entire board through electroplating. It also reduces the risk of thermal stress mismatch between the copper layer and the polyimide substrate, improving the product's bending reliability. Specifically, this method uses black holes instead of chemical copper plating to avoid damage to the polyimide substrate caused by palladium activation, improving insulation performance and signal integrity. Furthermore, through multiple cyclic electroplating cycles, the height of the copper pillars can be precisely controlled, achieving a height of 50~300μm and a diameter accuracy of ±3μm. This meets the diverse requirements of advanced packaging and high reliability, improving product yield. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the technical solutions of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of part of the process method of the present invention.

[0020] Figure 2 Appendix to this invention Figure 1 A schematic diagram of the subsequent process flow.

[0021] Figure 3 Appendix to this invention Figure 2 A schematic diagram of the subsequent process flow.

[0022] Figure 4 Appendix to this invention Figure 3 A schematic diagram of the subsequent process flow.

[0023] Figure 5 This is a schematic block diagram of the main process flow of the present invention. Detailed Implementation

[0024] In the description of this invention, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0026] as follows: Please see the appendix Figure 1-5 , Among them, the appendix Figure 1 From top to bottom, they are: Cutting: Cut large rolls of flexible copper-clad laminate into sizes suitable for production (e.g., 250×300mm). Drilling holes to create through holes for interlayer interconnection; Black holes deposit a conductive carbon layer on the non-conductive hole wall, replacing traditional chemical copper plating, and are suitable for flexible boards; Hole-filling electroplating involves completely filling through holes or blind holes with electroplated copper to create a smooth surface, which facilitates subsequent lamination and circuit fabrication. Apply a film, that is, apply a layer of photosensitive dry film to the copper surface; Exposure, using an exposure machine to expose the areas where copper needs to be retained, causing them to aggregate; development, The solution washes away the unexposed dry film, exposing the copper area to be etched; Acid etching removes the original copper foil that is not protected by the dry film, forming a preliminary circuit (this step only creates the circuit on the side where the copper pillar needs to be made; the other side retains the copper surface for conduction during subsequent copper pillar electroplating).

[0027] Attachment Figure 2 , in order Remove the film by using NaOH solution to remove the dry film and expose the complete circuit / copper surface; Apply a film, that is, apply a layer of photosensitive dry film to the circuit / copper surface; Exposure: Use an exposure machine to expose the area where copper needs to be retained to aggregate (the area where copper pillars need to be made should not be exposed in this step). development, The solution washes away the unexposed dry film, exposing the area where the copper pillar needs to be made. Flexible printed circuit board (VCP) is used to electroplate the copper pillar positions, thereby increasing the height of the copper pillars; From this point on, repeat the above four steps: lamination, exposure, development, and flexible VCP (until attached). Figure 3 Step 3 in the middle). Remove the film, on the attached Figure 3 Step 4: Use NaOH solution to remove the dry film and expose the complete copper surface (this step only removes the dry film from the side without the circuitry). Screen protector, with Figure 3 Step 5: Apply a layer of photosensitive dry film to the copper surface; Enter the attached Figure 4 , in order Exposure, using an exposure machine to expose the areas where copper needs to be retained, causing them to aggregate; development, The solution washes away the unexposed dry film, exposing the copper area to be etched; Acid etching removes the original copper foil that is not protected by the dry film, forming the final circuit (this step creates the circuit on the side other than the copper pillars). Remove the film using NaOH solution to remove the dry film, exposing the complete circuit / copper surface. End.

[0028] This invention relates to a method for fabricating copper pillars for flexible circuit boards, particularly suitable for creating conductive copper pillar structures with controllable height and high dimensional accuracy on flexible polyimide substrates to meet the requirements of chip packaging, Z-axis interconnection, and high-frequency signal transmission. The specific implementation of this solution is described in detail below, using a double-sided flexible copper-clad laminate as the starting substrate.

[0029] First, the material is prepared by selecting a large roll of flexible copper-clad laminate. The structure of the copper-clad laminate shown in this solution is a polyimide flexible substrate in the middle layer, with a copper foil layer on both the top and bottom surfaces. In practice, the large roll can be cut into sizes suitable for mass production, such as 250mm × 300mm work boards, depending on production needs and equipment capabilities, to facilitate clamping, conveying, and processing in subsequent processes.

[0030] Drilling involves creating through-holes on the flexible copper-clad laminate using mechanical or laser drilling methods, based on the location and size requirements of the through-holes in the circuit diagram. These through-holes are used for subsequent electrical interconnection between the upper and lower copper foil layers. It is important to control drilling parameters during the drilling process to ensure smooth hole walls, free of burrs and adhesive residue. Laser drilling is the preferred drilling method because it offers higher precision and generates less heat compared to mechanical drilling.

[0031] Next, a black hole process is performed. This solution uses black holes to replace the traditional chemical copper plating method for hole metallization. The main process is to deposit a conductive carbon layer on the non-conductive PI hole wall to make the hole wall conductive. Specifically, the process involves treating the drilled flexible copper-clad board with a hole-forming solution, immersing it in black hole solution, and drying it, so that the carbon black particles are uniformly adsorbed on the hole wall surface to form a conductive layer.

[0032] The black hole process used in this solution, compared with the traditional chemical copper plating process, does not require the use of palladium activator, thus avoiding the potential decrease in insulation performance and signal problems that may be caused by residues on the substrate surface and hole walls, and is also more environmentally friendly.

[0033] After the black hole is completed, the hole-filling electroplating is performed. The flexible copper-clad laminate that has undergone black hole treatment is placed in the electroplating tank. The conductive carbon layer on the hole wall is used as the cathode. Under the action of an electric field, copper ions are reduced and deposited in the hole, realizing the electrical connection between the upper and lower copper foils and forming a flat surface, which is convenient for subsequent film application and circuit fabrication processes.

[0034] In this process, after the electroplated copper fills the holes, the copper layer inside the holes and the copper foil on the surface of the substrate are integrated to form a complete conductive circuit.

[0035] After the through-hole plating, the first film is applied to the first side (i.e., the front side) of the flexible copper-clad laminate, as shown in the attached image. Figure 1 A photosensitive dry film is applied to the copper layer on the flexible substrate and the second side (back side).

[0036] Then, the first exposure process is carried out, using a laser imaging exposure machine to expose the board surface with the dry film applied.

[0037] Specifically, based on the data of the first circuit pattern, the circuit pattern is drawn on the dry film of the exposure machine. The dry film in the area where copper needs to be retained (the circuit pattern area) is exposed to light and polymerizes, while the dry film in the area where copper needs to be etched away is not exposed to light and remains in an unpolymerized state.

[0038] Then, the first development is performed, immersing the exposed flexible copper-clad laminate in... In sodium carbonate, the dry film areas that are not exposed to light are dissolved and washed away by the developer, while the dry film areas that are exposed to light remain on the copper surface.

[0039] After development, a first acid etching process is performed. The developed flexible copper-clad laminate is fed into the acid etching line, where an acidic etching solution (such as copper chloride, hydrochloric acid system, sulfuric acid, hydrogen peroxide, etc.) is used to etch away the copper foil on the first surface that is not protected by the dry film. The copper foil in the circuit pattern areas covered and protected by the dry film is retained. After this etching, a preliminary circuit pattern is formed on the first surface.

[0040] It should be noted that this step only creates the wiring for the side where the copper pillar will be made later. The copper foil on the other side (the second side) is completely covered and protected by the dry film throughout the process, without being etched, thus preserving the complete copper surface. This complete copper surface is used as a conductive layer in the subsequent copper pillar electroplating process.

[0041] Then, the first stripping process is performed by immersing the etched flexible copper-clad laminate in a sodium hydroxide (NaOH) solution. The NaOH dissolves and removes the dry film remaining on the copper surface. After stripping, the circuit pattern on the first side is fully exposed, and the copper surface on the second side is also fully exposed.

[0042] After the first film removal, the copper pillar manufacturing cycle begins. This cycle consists of four steps: film application, exposure, development, and flexible VCP electroplating. This process is repeated several times depending on the target height of the copper pillar.

[0043] Specifically, this involves first applying a second layer of photosensitive dry film to both the first and second surfaces, and then using an exposure machine to expose the board surface with the applied dry film a second time.

[0044] In this exposure, the area on the first surface where copper pillars need to be fabricated is not exposed (that is, the dry film in the copper pillar area remains in an unpolymerized state), while the dry film in other areas is exposed and polymerized.

[0045] The second side of the dry film undergoes a polymerization reaction upon exposure, acting as a protective layer. After exposure, it is developed a second time. The unexposed dry film is washed away, exposing the area on the first surface where copper pillars need to be formed, while the dry film on the second surface is completely preserved. After development, a flexible circuit board VCP process is performed, feeding the board into a vertical continuous electroplating line to electroplate copper onto the exposed copper pillar areas on the first surface, increasing the height of the copper pillars in that area. Each VCP electroplating can increase the thickness of the copper pillar by approximately 50–100 μm.

[0046] After one cycle, if the copper pillar height has not yet reached the target value (typically 50–300 μm), the four steps of film application, exposure, development, and flexible plate VCP are repeated, with the copper pillar height increasing with each repetition. Through multiple cycles, the final height of the copper pillar can be controlled, achieving the fabrication of copper pillars with any target height within the range of 50 μm to 300 μm. Because the thickness increase in each electroplating step is small and uniform, the diameter accuracy of the copper pillar can reach ±3 μm.

[0047] Once the copper pillar reaches the predetermined target height, a second film removal process is performed, using NaOH to remove the dry film remaining on the board surface.

[0048] It's important to note that this step only removes the dry film from the side without wiring (i.e., the second side), while the dry film on the first side has already been removed one by one during the copper pillar fabrication cycle. After the film is removed, a raised copper pillar structure is formed on the first side, while the second side retains a complete copper foil layer.

[0049] After the second film stripping is completed, the fabrication of the second-side circuitry begins. First, a third film application is performed, applying a layer of photosensitive dry film to both the first and second sides. Then, a third exposure is conducted, exposing the board surface with the applied dry film. Based on the circuitry pattern data for the second side, areas on the second side where the circuitry needs to be retained are exposed, while areas requiring copper etching are not exposed. The entire first side is exposed, serving as a protective layer covering the circuitry and existing copper pillars. After exposure, a third development process is performed. The unexposed dry film is washed away, exposing the copper areas to be etched away on the second side. The areas of the circuit pattern to be retained are protected by the dry film. After development, a second acid etching is performed to remove the copper foil not protected by the dry film on the second side, forming the circuit pattern on the second side. During etching, the circuits and copper pillars on the first side are completely covered and protected by the dry film, preventing them from being eroded by the etching solution. After etching, a third film stripping is performed, using NaOH to remove the remaining dry film from the board surface, exposing the complete circuit and copper pillar structure, resulting in the finished product, as shown in the attached image. Figure 4 The final step is shown.

[0050] This solution completes the fabrication of double-sided circuitry through the aforementioned process. The first side features raised copper pillar structures, while the second side forms a complete circuit pattern.

[0051] Optionally, after completing the above main processes, subsequent conventional post-processing procedures can be carried out according to product requirements, such as cover film lamination, surface treatment (e.g., immersion gold, OSP, tin plating, etc.), shape processing (e.g., punching or laser cutting), and electrical performance testing, to finally obtain the finished copper pillar process flexible circuit board.

[0052] It should be further noted that in the above specific embodiments, the flexible copper-clad laminate used is a double-sided copper-clad laminate, but this solution is also applicable to single-sided copper-clad laminates or other types of flexible substrates. The photosensitive dry film used is a conventional photosensitive dry film, and the exposure method is LDI laser direct imaging exposure; however, other exposure methods, such as traditional exposure methods using photomasks, can also be used. The acidic etching solution used can be a copper chloride system or sulfuric acid, hydrogen peroxide, and other commonly used acidic etching solutions in the art. The concentration of the NaOH solution used can be adjusted according to the actual dry film type, generally with a mass fraction of 1% to 5%. The concentration can also be adjusted according to the actual dry film type, usually between 0.5% and 2%. The electroplating solution for flexible VCP electroplating can use conventional copper sulfate, and the electroplating current density is usually controlled in the range of 1 to 3 A / dm².

[0053] The specific parameters and conditions of each of the above processes can be reasonably selected and adjusted by those skilled in the art based on the actual production equipment and material types, and all of these fall within the protection scope of this invention. The following points need to be explained: (1) The accompanying drawings of the embodiments of this disclosure only involve the methods and structures involved in the embodiments of this disclosure. Other methods and structures can be referred to the general design.

[0054] (2) Where there is no conflict, features of the same embodiment and different embodiments of this disclosure can be combined with each other.

[0055] The above are merely specific embodiments of this disclosure, but the scope of protection of this disclosure is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this disclosure should be included within the scope of protection of this disclosure. Therefore, the scope of protection of this disclosure should be determined by the scope of the claims.

Claims

1. A method for fabricating copper pillars for flexible circuit boards, characterized in that, Includes the following steps: S1. Pre-treat the flexible copper-clad laminate to form conductive vias on the substrate; S2. A first photoresist layer is applied to the first and second surfaces of the substrate respectively. After exposure and development, the circuit pattern on the first surface is exposed and the entire second surface is protected. Then, the exposed copper layer on the first surface is etched away to form the circuit on the first surface, and the remaining first photoresist layer is removed. S3. Cover the first and second surfaces with a second photoresist layer respectively. After exposure and development, the area on the first surface where the copper pillar needs to be made is exposed and the entire second surface is protected. Then, copper is deposited in the exposed area by electroplating to increase the height of the copper pillar. S4. Repeat step S3 several times until the height of the copper pillar reaches the predetermined value. S5. Remove the second photoresist layer; S6. Cover the first and second surfaces with a third photoresist layer respectively. After exposure and development, the circuit pattern on the second surface is exposed and the first surface is completely protected. Then, etch away the exposed copper layer on the second surface to form the circuit on the second surface, and remove the remaining third photoresist layer.

2. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, The pretreatment includes sequential processes of material cutting, drilling, black hole treatment, and hole-filling electroplating, wherein the black hole treatment involves depositing carbon black on the hole wall to form a conductive layer, and the hole-filling electroplating completely fills and flattens the hole.

3. The method for fabricating copper pillars for flexible circuit boards according to claim 2, characterized in that, The black hole process replaces the chemical copper plating process, avoiding damage to the polyimide substrate caused by palladium activation.

4. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, The etching described in step S2 is acid etching, which uses an acidic etching solution to remove the exposed copper layer.

5. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, The electroplating in step S3 adopts a vertical continuous electroplating method.

6. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, In step S3, the thickness of the copper layer thickened by each electroplating is 50-100 μm, and the predetermined value is 50-300 μm. The number of repetitions is determined based on the predetermined value and the thickness of each thickening.

7. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, In steps S5 and S6, the photoresist layer is removed using a NaOH solution for film stripping.

8. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, The etching described in step S6 is acid etching, and during the etching process, the lines on the first side and the formed copper pillars are completely covered and protected by the third photoresist layer.

9. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, After step S6, at least one post-processing step is included, which is covering film lamination, surface treatment, shape processing, and electrical performance testing.

10. The method for fabricating copper pillars for flexible circuit boards according to claim 1, characterized in that, The first photoresist layer, the second photoresist layer, and the third photoresist layer are all photosensitive dry films.

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