Flexible printed circuit board processing method and flexible printed circuit board

Abstract

The invention discloses a flexible printed circuit board processing method and a flexible printed circuit board, and the method comprises the following steps: (1) blind hole laser processing: employing a laser to perform laser processing on the surface of a circuit board to form a square blind groove hole, the groove width of the blind groove hole being 45 [mu] m, and the groove length being 900 [mu] m; (2) plasma cleaning: in a vacuum environment, gas is decomposed into ions through a high-frequency electric field, and then the ions collide with residues in the blind slotted hole of the circuit board, so that the blind slotted hole is cleaned; (3) chemical cleaning; (4) electroplating black shadows; (5) AOI detection; (6) pressing a film and exposing; (7) developing: cleaning the unpolymerized dry film by using a developing solution to expose the bottom of the blind slot; and (8) hole filling and copper plating: electroplating copper in the blind slotted hole to ensure that the blind slotted hole is uniformly filled with a plating layer. The square blind slotted hole is formed in a linear filling mode, so that after copper plating treatment, a plating layer can keep a good conduction effect with the bottom copper surface of the blind slotted hole, and the problem of poor low resistance is avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of circuit board processing technology, in particular to a flexible printed circuit board processing method and a flexible printed circuit board. BACKGROUND

[0002] At present, in the processing technology of FPC (flexible printed circuit board), the industry generally adopts laser drilling method to process blind holes on FPC substrate, among which the circular blind hole structure is the most common.

[0003] However, in the FPC product with circular blind hole structure, after subsequent copper plating treatment, the copper plating layer needs to be conductive with the bottom copper of FPC, but due to the probability of foreign matter or residue glue remaining at the bottom of the blind hole, the bottom copper and the copper plating layer are separated, and problems such as uneven copper layer accumulation occur, which further leads to poor grounding, showing a high electrical test failure rate. Although the existing technology has been improved in terms of laser parameter optimization and plating formula adjustment, it is still difficult to fundamentally solve such defects. SUMMARY

[0004] In order to overcome the above shortcomings, the purpose of the present application is to provide a flexible printed circuit board processing method and a flexible printed circuit board, which opens a square blind slot hole by linear filling, so that after copper plating treatment, the plating layer can maintain good conduction effect with the bottom copper surface of the blind slot hole, avoiding the problem of low resistance failure.

[0005] Technical scheme: The present application discloses a flexible printed circuit board processing method, comprising the following steps:

[0006] (1) Laser blind hole: a laser is used to laser a square blind slot hole on the surface of the circuit board, the slot width of the blind slot hole is 45um, the slot length is 900um, and the slot depth is 12-130um. The power and frequency of the laser are controlled to prevent the circuit board from being broken through;

[0007] (2) Plasma cleaning: in a vacuum environment, high-frequency electric field is used to decompose gas into ions, which collide with the residues in the blind slot hole of the circuit board, thereby cleaning the blind slot hole;

[0008] (3) Chemical cleaning: the circuit board is cleaned to remove contaminants, and the residues not removed by the plasma are also cleaned;

[0009] (4) Electroplating black shadow: graphite with conductivity is deposited on the inner wall of the blind slot hole;

[0010] (5) AOI detection: an automatic optical detection system is used to detect the quality of the processed blind slot hole;

[0011] (6) pressing film and exposure: dry film is attached on the circuit board to form the protective layer of the circuit pattern, and the circuit pattern designed on the dry film is transferred to the circuit board through exposure;

[0012] (7) development: using developing solution to clean the un-polymerized dry film, and exposing the bottom of the blind slot hole;

[0013] (8) filling hole copper plating: copper plating in the blind slot hole to ensure uniform filling of the plating layer in the blind slot hole.

[0014] Further, the gas used in the plasma cleaning in step (2) is a mixed gas of oxygen and argon, and the mixing ratio is 1:3, so as to improve the cleaning efficiency and reduce the damage to the surface of the circuit board.

[0015] Further, the deposition thickness of the graphite in step (4) is between 0.1um and 0.5um, so as to ensure that the copper layer is firmly attached and the resistance is uniform during the electroplating process.

[0016] Further, the thickness of the dry film used in step (6) is 25um to 40um, and the ultraviolet light band is used during exposure, so as to realize high-fidelity transfer of the pattern.

[0017] Further, in step (7), the circuit board is vertically placed during development, and the developing solution is sprayed in a direction perpendicular to the circuit board, so that the developing solution flows downward under the action of gravity, and the developing solution is not easily left in the blind slot hole.

[0018] Further, in step (8), the dimple value of the hole filling copper plating is less than 3um.

[0019] Further, after step (8), a secondary AOI detection is further included, so as to ensure that the plating layer quality meets the standard.

[0020] The application further discloses a flexible printed circuit board, which comprises a composite circuit board composed of at least two copper-clad plates and at least one blind slot hole arranged on the composite circuit board, wherein the slot width of the blind slot hole is at least 45um, the slot length is at least 900um, the slot depth is 12-130um, and the bottom of any blind slot hole is connected with the copper layer of the copper-clad plate.

[0021] The application has the following beneficial effects:

[0022] (1) The application processes a square blind slot hole by controlling a laser, which effectively avoids the breakdown of the substrate while improving the opening area of the blind slot hole and the edge definition;

[0023] (2) The present application uses blind slot hole structure to replace the traditional round hole structure, increases the contact area of the bottom copper and plated copper of the circuit board, so that a more complete metal covering layer can be formed in the subsequent copper plating hole filling process, reducing the electrical performance defects caused by poor conduction;

[0024] (3) The present application controls the graphite deposition thickness between 0.1um to 0.5um, which guarantees good electrical conductivity and avoids the problem of uneven distribution of electroplated copper caused by too thick graphite layer, and helps to form a continuous and well-adhered graphite layer;

[0025] (4) The present application introduces a vertical spray pressure developing method in the developing step, which makes the developing liquid discharge from the blind slot hole under the action of gravity, reduces the possibility of developing liquid remaining at the bottom of the blind slot hole, and reduces the adverse effects on the subsequent electroplating process caused by the remaining developing liquid. BRIEF DESCRIPTION OF DRAWINGS

[0026] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present application in any way. In addition, the shapes and proportions of the components in the drawings are only illustrative and are used to help understand the present application, and are not specific limitations on the shapes and proportions of the components of the present application. Those skilled in the art can select various possible shapes and proportions to implement the present application according to specific circumstances under the guidance of the present application. In the drawings:

[0027] Fig. 1 The flow chart of the flexible printed circuit board processing method described in the present application;

[0028] Fig. 2 The schematic diagram of the blind slot hole in the present application;

[0029] Fig. 3 The schematic diagram of the blind slot hole after copper plating in the present application. DETAILED DESCRIPTION

[0030] The present application will be further illustrated below in conjunction with the drawings and specific examples.

[0031] In the description of the present application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", "third" are only for the purpose of description, and cannot be understood as indicating or implying relative importance. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances. The embodiments will be described below according to the overall structure of the present application.

[0032] As shown in Figs. 1 to 3 , the present application discloses a flexible printed circuit board processing method, comprising the following steps:

[0033] (1) Laser blind hole: a laser is used to laser out a square blind groove hole on the surface of the circuit board, the groove width of the blind groove hole is 45 um, the groove length is 900 um, and the groove depth is 12-130 um. The laser is reciprocally scanned according to the set path, so that the laser beam covers the entire preset blind groove hole area, thereby realizing high-precision material removal, and the power and frequency of the laser are controlled to prevent the circuit board from being broken. In the processing process, the emission power of the laser can be 1-10 W, preferably 2.5 W, the frequency is 40-200 KHZ, preferably 60 KHZ, and the energy is about 15 mj, thereby controlling the heat concentration and avoiding the laser from breaking the bottom or lower layer circuit of the circuit board.

[0034] (2) Plasma cleaning: the circuit board with the blind groove hole formed after laser is placed in a vacuum plasma cleaning cavity. In a vacuum environment, the gas is decomposed into ions by a high-frequency electric field, and then collides with the residues in the blind groove hole of the circuit board, thereby removing the organic carbon residues, oil stains and other impurities formed during the laser process, so as to improve the cleanliness of the hole wall and clean the blind groove hole.

[0035] (3) Chemical cleaning: the circuit board cleaned by plasma is immersed in a cleaning tank containing chemical liquid, and the circuit board is cleaned to remove pollutants, so that the cleaning liquid fully penetrates the bottom and corner positions of the blind groove hole, and at the same time, the residues not removed by plasma are cleaned, thereby improving the cleaning degree of the whole hole surface of the blind groove hole.

[0036] (4) Electroplating black shadow: a layer of conductive material is deposited on the inner wall of the cleaned blind groove hole. Preferably, graphite with conductivity is deposited on the inner wall of the blind groove hole, thereby establishing a reliable conductive path for the subsequent copper plating process.

[0037] (5) AOI detection: using an automatic optical detection system to detect the quality of the processed blind slot hole, the detection content includes the position deviation, the appearance defect, the incomplete graphite coverage and other defect items of the blind slot hole. The automatic optical detection system quickly marks the poor area through high-speed image acquisition and algorithm comparison and analysis, and ensures the quality control before exposure and electroplating operation.

[0038] (6) Film pressing and exposure: the dry film is attached to the circuit board through the laminator to make it firmly adhere to form a protective layer of the circuit pattern, and the circuit pattern designed on the dry film is transferred to the circuit board through exposure.

[0039] (7) Development: the exposed circuit board is immersed in a developing solution to clean the un-polymerized dry film, and the bottom of the blind slot hole and the area to be electroplated are exposed. The development process ensures that the circuit edge of the circuit board is clear and has no burr.

[0040] (8) Hole filling and copper plating: copper is electroplated in the blind slot hole through an electroplating process. The circuit board is immersed in an electroplating solution containing copper ions, such as a solution containing copper sulfate, under the action of constant current or pulse current, so that copper ions are uniformly deposited on the bottom and inner wall of the blind slot hole until the hole is completely filled, to ensure that the blind slot hole is filled with a plating layer, prevent bubbles, cavities or thin plating layer from appearing in the blind slot hole, and ensure the stability of the electrical performance of the circuit board.

[0041] Through the above steps, the present application uses a laser to precisely process a square blind slot hole on the surface of a flexible printed circuit board, and then combines plasma cleaning and chemical cleaning processes to remove impurities remaining in the blind slot hole. The electroplating shadow process forms a conductive graphite layer on the inner wall of the blind slot hole, enhances the conductivity of the blind slot hole area, and provides a stable conduction path for hole filling and electroplating. The AOI automatic optical detection system quickly and accurately detects the processing quality of the blind slot hole to ensure that the blind slot hole meets the design requirements. During the circuit pattern transfer process, the dry film pressing and exposure development process protects the non-conductive area to ensure the integrity and resolution of the pattern transfer. Finally, electroplating realizes the dense filling of the copper layer inside the blind slot hole, improving the conduction reliability.

[0042] Further, in step (2), the gas in the plasma cleaning cavity is a mixed gas composed of oxygen and argon, and the volume mixing ratio of oxygen to argon is 1:3. By applying a high-frequency radio frequency electric field, the mixed gas is ionized to form a plasma in a vacuum state. Oxygen ions have good oxidation decomposition ability, and argon ions have strong physical bombardment effect, and the two work together to efficiently remove the fine particle impurities remaining in the blind slot hole. Preferably, in this step, the plasma cleaning radio frequency power is 15W, and the cleaning time is controlled to be 5-15min, so that the bottom of the blind hole and its inner wall are fully cleaned.

[0043] Further, in step (4), the graphite is uniformly deposited to the inner wall surface of the blind via hole, and the thickness of the deposited layer is controlled to be between 0.1 um and 0.5 um. In this thickness range, the graphite layer can have good conductivity and stability during the electroplating process, thereby improving the adhesion of the copper layer in the blind via hole region, effectively reducing the resistance fluctuation, and ensuring the uniformity and compactness of the copper layer in the hole.

[0044] Further, in step (6), the dry film is used as a pattern protection layer material and is uniformly attached to the surface of the circuit board by a laminating machine. The thickness of the dry film is preferably 25 um to 40 um, so that the resolution of the pattern transfer and the etching performance are considered. After the film is pressed, the circuit board with the attached dry film is subjected to pattern exposure treatment using ultraviolet light as the exposure light source, so that the pattern is fully aggregated.

[0045] Further, in step (7), the exposed circuit board is immersed in a developing solution to remove the unaggregated dry film area, thereby exposing the bottom of the blind via hole and the area to be electroplated. During the developing process, the circuit board is placed in a vertical manner, and the developing solution is sprayed in a direction perpendicular to the surface of the circuit board, so that the developing solution can flow downward along the surface of the circuit board under the action of gravity, avoiding the developing solution remaining at the bottom of the blind via hole, and improving the discharge efficiency of the developing solution. The vertical developing method optimizes the flow path of the developing solution, thereby effectively preventing the developing solution from remaining, bubbles or dry film debris in the blind via hole, and improving the cleanliness of the blind via hole.

[0046] Further, in step (8), the dimple value of the hole-filling copper plating is less than 3 um, that is, the degree of concave or convex of the copper surface after hole filling is less than 3 um, so as to ensure the flatness of the surface of the circuit board. Specifically, the developed circuit board is placed in an electroplating tank for hole-filling electroplating treatment, and copper material is deposited layer by layer in the blind via hole, thereby forming a dense and continuous conductive path. Preferably, the electroplating solution includes copper sulfate, sulfuric acid and organic additives, and is combined with pulse current to realize uniform deposition of the plating layer in the blind via hole. After electroplating, the hole-filling quality of the blind via hole is evaluated by scanning detection, and the dimple value is controlled to be less than 3 um, so as to ensure the flatness of the surface of the circuit board, facilitate the processing of subsequent processes, and improve the reliability of the circuit board and the electrical performance.

[0047] Further, after the hole-filling copper plating is completed and the necessary water washing and drying treatment is performed, the circuit board is subjected to a second AOI detection. A high-resolution AOI system is used to scan the copper-plated area of the blind via hole to detect whether the copper layer has defects such as filling integrity, surface flatness, dimple value exceeding the limit, copper layer bubbles, plating layer edge collapse or plating leakage, so as to ensure that the plating thickness and appearance quality of the circuit board meet the production requirements, and ensure the production consistency of the circuit board.

[0048] AsFig. 2 and Fig. 3 As shown in the drawings, the application also discloses a flexible printed circuit board, which comprises a composite circuit board formed by pressing at least two copper-clad plates and at least one blind slot hole arranged on the composite circuit board, the blind slot hole has a slot width of at least 45 um, a slot length of at least 900 um, and a slot depth of 12-130 um, and the bottom of any blind slot hole is connected with the copper layer of the copper-clad plate.

[0049] The existing circuit board usually adopts the processing blind hole mode, and then performs electroplating in the subsequent process, so as to make the circuit conductive, but in the traditional processing blind hole process, since the bottom contact area of the blind hole is small, there is a probability that the copper layer of the copper-clad plate is separated from the electroplated copper layer of the blind hole due to foreign matters or residual glue on the bottom of the blind hole, and then the signal is poor, that is, the electroplated copper layer cannot be connected with the copper layer of the copper-clad plate. In the application, the blind slot hole is used to replace the traditional circular blind hole, and the area of the blind slot hole is much larger than that of the blind hole, so that the blind slot hole is more easily filled during electroplating exchange, and then the phenomenon that the electroplated layer is not well connected with the copper layer of the copper-clad plate is avoided.

[0050] So far, the technical scheme of the application has been described in combination with the preferred embodiments shown in the drawings, but those skilled in the art can easily understand that the protection scope of the application is obviously not limited to these specific embodiments. Those skilled in the art can make equivalent changes or replacements to the related technical features without departing from the principles of the application, and the technical scheme after the changes or replacements will fall within the protection scope of the application.

Claims

1. A method for processing flexible printed circuit boards, characterized in that, Includes the following steps: (1) Laser blind hole: A laser is used to laser-etch square blind holes on the surface of the circuit board. The width of the blind hole is 45um, the length is 900um, and the depth is 12-130um. The power and frequency of the laser are controlled to prevent the circuit board from being damaged. (2) Plasma cleaning: In a vacuum environment, the gas is decomposed into ions by a high-frequency electric field and then collide with the residue in the blind slots of the circuit board, thereby cleaning the blind slots. (3) Chemical cleaning: Clean the circuit board to remove contaminants and remove any residue that has not been removed from the plasma. (4) Electroplating shadow: Conductive graphite is deposited on the inner wall of the blind slot; (5) AOI inspection: Use an automated optical inspection system to inspect the quality of the processed blind slots; (6) Lamination and exposure: The dry film is laminated onto the circuit board to form a protective layer for the circuit pattern, and the circuit pattern designed on the dry film is transferred to the circuit board through exposure. (7) Development: Use developer to clean the unpolymerized dry film to expose the bottom of the blind slots; (8) Filling holes with copper plating: Electroplating copper in the blind slot holes to ensure that the blind slot holes are uniformly filled with plating.

2. The flexible printed circuit board processing method according to claim 1, characterized in that, The gas used in step (2) plasma cleaning is a mixture of oxygen and argon in a ratio of 1:3, in order to improve cleaning efficiency and reduce damage to the circuit board surface.

3. The flexible printed circuit board processing method according to claim 1, characterized in that, In step (4), the graphite deposition thickness is between 0.1 μm and 0.5 μm, thereby ensuring that the copper layer adheres firmly and has uniform resistance during the electroplating process.

4. The flexible printed circuit board processing method according to claim 1, characterized in that, The dry film thickness used in step (6) is 25um to 40um, and the ultraviolet light band is used during exposure to achieve high-fidelity pattern transfer.

5. The flexible printed circuit board processing method according to claim 1, characterized in that, In step (7), during development, the circuit board is placed vertically and the developing solution is sprayed in a direction perpendicular to the circuit board, so that the developing solution flows downward under the action of gravity, and the developing solution is not easy to remain in the blind slot hole.

6. The flexible printed circuit board processing method according to claim 1, characterized in that, In step (8), the dimple value of the copper plating in the hole filling is less than 3 μm.

7. The flexible printed circuit board processing method according to claim 1, characterized in that, After step (8), a second AOI inspection is also included to ensure that the coating quality meets the standards.

8. A flexible printed circuit board, characterized in that, The composite circuit board includes at least two copper-clad laminates laminated together and at least one blind slot disposed on the composite circuit board. The blind slot has a slot width of at least 45 μm, a slot length of at least 900 μm, and a slot depth of 12-130 μm. The bottom of any one of the blind slots is connected to the copper layer of the copper-clad laminate.

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