Electroplating method of flexible circuit board
By setting blind holes on the flexible circuit board and carrying out specific electroplating processes, the problem that the PAD thickness of the bottom surface of the blind hole is greater than the copper layer thickness of the top surface of the copper layer is solved in the prior art, and the effect of meeting the design needs of special circuit boards is achieved.
Patent Information
- Application Number
- CN202411475986.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-10-22
- Publication Date
- 2025-02-28
Smart Images
Figure CN119545686A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the field of flexible circuit boards, and in particular relates to an electroplating method for a flexible circuit board. Background Art
[0002] With the rapid development of personal consumer electronic products, the types and formats of circuit boards in products are rich and varied to adapt to and meet the different functional requirements of different electronic products. In general, copper interconnection is mostly used in flexible circuits. In the copper interconnection of flexible circuit boards, the two surfaces of the substrate are copper layers, and the blind hole penetrates one of the copper layers and connects to the other copper layer. The copper layer penetrated by the blind hole is the top copper layer of the blind hole, and the corresponding copper layer not penetrated by the blind hole is regarded as the bottom copper layer of the blind hole. In the conventional preparation process of copper interconnection of flexible circuits, copper is first deposited on the substrate to fill the blind hole, and then after flash plating and dry film attachment, graphic electroplating is directly performed to realize PAD (pad) and other design graphics; or copper is first deposited on the substrate to fill the blind hole, and after flash plating, the required graphic electroplating PAD is directly electroplated above the blind hole position. The thickness of the electroplating is determined by the current density and the current area. In the prior art, the areas that do not require electroplating are covered with dry film, and direct cap plating is performed on the blind holes that are not covered by the dry film to form PAD. The plating areas on the bottom and top of the blind holes are basically the same. Therefore, after electroplating in the prior art, the PAD is consistent with the surface copper thickness.
[0003] However, in order to realize a certain beauty product, the product design requirements are special. In the flexible circuit, the copper plating thickness on the top and bottom sides of the blind hole must be designed to be inconsistent. Specifically, the PAD thickness obtained after etching the pattern on the bottom side of the blind hole must be greater than the copper layer thickness on the top side of the blind hole. More specifically, it is required that in this type of flexible circuit board, the PAD thickness on the bottom side of the blind hole is greater than the copper layer thickness on the top side of the blind hole by more than 20μm. The conventional flexible circuit copper interconnection process in the industry cannot achieve the above special requirements.
[0004] Therefore, it is still necessary to develop a process that can achieve a PAD thickness obtained after etching the pattern on the bottom surface of the blind hole that is greater than the copper layer thickness on the top surface of the blind hole to meet different product requirements. Summary of the invention
[0005] Aiming at the problem that the conventional process involved in the above-mentioned prior art cannot achieve that the thickness of the PAD obtained after etching the pattern on the bottom surface of the blind hole is greater than the thickness of the copper layer on the top surface of the blind hole, the present invention will provide a method for electroplating a flexible circuit board.
[0006] To achieve the above purpose, the following technical solutions are specifically included:
[0007] A method for electroplating a flexible circuit board comprises the following steps:
[0008] (1) preparing a substrate of a flexible circuit board, wherein two opposite surfaces of the substrate in the thickness direction are copper layers, and a blind hole is provided on one of the surfaces by laser, wherein the blind hole penetrates one of the copper layers and connects the two copper layers;
[0009] (2) plasma cleaning and hole metallization are sequentially performed on the substrate, and then a first copper electroplating is performed using a vertical continuous electroplating process to obtain a hole-filled substrate;
[0010] (3) attaching dry films to the two surfaces of the substrate after the holes are filled; then sequentially exposing, developing, etching, and stripping the dry films on the other side opposite to the blind hole to obtain a substrate with one side patterned, wherein the substrate with one side patterned has a PAD on the patterned side, and the position of the PAD corresponds to the position of the blind hole;
[0011] (4) attaching dry films to two surfaces of the patterned substrate, and sequentially exposing and developing the dry films on the side where the PAD is present to expose the PAD;
[0012] (5) A second copper electroplating process is performed using a vertical continuous electroplating process to obtain a plated flexible circuit board.
[0013] In the electroplating method of the present invention, firstly, the blind hole that has been cleaned and metallized in the hole is filled with the first electroplating copper; secondly, the circuit pattern is not etched on the top surface of the blind hole, and the circuit pattern is etched on the bottom surface of the blind hole to form a PAD to be electroplated on the bottom surface of the blind hole (PAD is a bump formed by etching the original copper layer of the substrate); finally, the second electroplating copper is performed. After the blind hole is filled, the hole is filled with copper plating, and the current will be conducted from the copper column in the blind hole on the top surface to the bottom surface. In addition, because the dry film adheres to the top surface, the copper surface of the PAD to be electroplated on the bottom surface is exposed. The copper ions in the electroplating process are deposited on the PAD surface, so that the single-sided capping and thickening of the PAD is achieved, so that the thickness of the PAD on the bottom surface of the blind hole is greater than the copper layer on the top surface.
[0014] Preferably, in step (1), the substrate further comprises a PI layer, the PI layer is located between two copper layers, and the blind hole penetrates the PI layer.
[0015] The electroplating method of the present invention is suitable for the electroplating of flexible circuit boards, and is particularly suitable for PCB flexible circuit boards. Flexible circuit boards are generally double-sided boards, with two surfaces being copper layers and a middle layer being a PI layer.
[0016] Preferably, in step (2), the plasma cleaning includes the following parameters: time is 10-20 min, power is 6000-7000 W, vacuum degree is 0.1-0.25 torr, oxygen flow rate is 2000-2300 sccm, carbon tetrafluoride flow rate is 200-300 sccm, nitrogen flow rate is 200-300 sccm, and temperature is 60-70°C.
[0017] Through the above-mentioned plasma cleaning parameters, the glue residue left on the blind hole wall after laser blasting can be efficiently removed.
[0018] Preferably, in step (2), during the in-hole metallization, a conductive layer is deposited on the inner wall of the blind hole, and the conductive layer comprises graphite colloid.
[0019] During the in-hole metallization process, graphite colloid is deposited in the blind hole. The graphite colloid has good conductivity and is adsorbed on the hole wall of the substrate that has not yet been connected (especially the hole wall corresponding to the PI layer) to form a conductive layer, so that the subsequent electroplated copper ions can be deposited on the hole wall to achieve hole filling, which can minimize defects such as plating voids.
[0020] Preferably, in step (2), the first copper electroplating includes the following parameters: current density is 1-2ASD, copper electroplating time is 30-120min, rectifier front output ratio is 70-120%, rectifier back output ratio is 70-120%, and copper cylinder spray frequency is 30-50Hz.
[0021] By adopting the above electroplating process parameters, the crystal density of the copper plating layer can be further improved, the uniformity of the grain size can be improved, and the defects of the copper plating layer can be reduced.
[0022] Preferably, in step (2), in the substrate after hole filling, the thickness of the surface copper is ≥20 μm, more preferably 20-60 μm.
[0023] After the first copper electroplating, the substrate is fully electroplated, which not only completely fills the blind holes, but also further thickens the two copper layers of the substrate on the original basis in the thickness direction, so that the copper layer thickness on each side is ≥20μm.
[0024] Preferably, in step (3), the etching solution is a copper chloride system.
[0025] In the method of the present invention, the blind hole is only provided on one side of the substrate, penetrates one of the copper layers and connects to the other copper layer, and the other copper layer is regarded as the side opposite to the side where the blind hole is provided, that is, the other side relative to the blind hole. Conventionally, the side where the blind hole is provided is also called the top side of the blind hole, and the other side relative to the blind hole is called the bottom side. Dry films are attached to both sides of the substrate after the holes are filled, and the other side relative to the blind hole is patterned, so that the designed pattern is transferred to the substrate, and a PAD is obtained on the other side relative to the blind hole.
[0026] Preferably, in step (5), in the second copper electroplating, an edge position of the substrate is exposed on the other side opposite to the side where the PAD is present, and the edge position serves as a conductive point to form a conductive point opposite to the side where the PAD is present, and the ratio of the electroplated area of the side where the PAD is present and the other side opposite to it is 2-3:1.
[0027] One side of the patterned substrate is filmed on both sides, and the dry film exposure and development are performed in sequence on the side where the PAD exists to expose the PAD and other image areas to be electroplated as the conductive points of this side for electroplating; then on the other side opposite to the PAD, only its edge position is exposed, and the middle unit is still under the protection of the dry film. The exposed edge position is used as the conductive point (area to be electroplated) of this side. Because the area of the area to be electroplated on the two sides is different, the second copper electroplating needs to set the area ratio of the electroplating area of the two sides so as to effectively electroplate and thicken the PAD position.
[0028] The electroplated flexible circuit board obtained by the electroplating method of the flexible circuit board can achieve different copper thicknesses on both sides of the blind hole based on the method of the present invention.
[0029] Preferably, the electroplated flexible circuit board includes a copper layer, a blind hole and a PAD, the blind hole is filled with copper, and the blind hole conducts the PAD and the copper layer, and the thickness of the PAD is greater than the thickness of the copper layer.
[0030] Further preferably, the PAD and the copper layer satisfy the following relationship: L1=L2+d, wherein L1 is the thickness of the PAD, L2 is the thickness of the copper layer, and d=10-30 μm.
[0031] More preferably, L1=65-85 μm, L2=50-60 μm.
[0032] Compared with the prior art, the present invention has the following beneficial effects: the electroplating method of the present invention, which sequentially undergoes the first copper electroplating, one-side patterning, and the second copper electroplating, can achieve that in the flexible circuit board after electroplating, the PAD thickness on the bottom surface of the blind hole is greater than the copper layer thickness on the top surface of the blind hole, so as to meet the requirements of special circuit board line design and broaden the application scope of the flexible circuit board. BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Figure 1 It is a process flow chart of the electroplating method of the present invention.
[0034] Figure 2 This is a schematic diagram of the structure of blind holes formed after laser treatment of the substrate.
[0035] Figure 3 It is a schematic diagram of the structure after the first copper electroplating and the blind hole filling.
[0036] Figure 4 This is a schematic diagram of the structure of the PAD and blind hole positions to be electroplated after one side is patterned.
[0037] Figure 5 It is a schematic diagram of the distribution of blind hole positions after one side is graphed, wherein the left figure is a schematic diagram of the distribution of blind hole positions, and the right figure is a magnified schematic diagram of the blind hole.
[0038] Figure 6 This is the area distribution diagram of the second copper electroplating, where the left picture is the bottom surface, the right picture is the top surface, and the pink area is the copper electroplating area.
[0039] Figure 7 Schematic diagram of the PAD position structure exposed after development.
[0040] Figure 8 This is a schematic diagram of the PAD thickened copper position structure after the second copper electroplating.
[0041] Fig. 9 This is a schematic cross-sectional view of the PAD and blind hole locations on a flexible circuit board after electroplating. DETAILED DESCRIPTION
[0042] In order to better illustrate the purpose, technical scheme and advantages of the present invention, the present invention will be further described below through specific examples. The test methods used in the examples and / or comparative examples are conventional methods unless otherwise specified; the materials, reagents, etc. used can be obtained from commercial channels unless otherwise specified.
[0043] Example 1
[0044] This embodiment provides a method for electroplating a flexible circuit board. The specific process flow is as shown in the attached Figure 1 As shown, the specific steps include:
[0045] (1) Prepare the substrate of the flexible circuit board: the cutter cuts the rolled copper of the coil into a sheet substrate, which includes a copper layer (bottom copper CU, 35μm), a PI layer (substrate PI, 50μm), and a copper layer (bottom copper CU, 35μm) in sequence; then, through the CFM window opening process, dry film is applied on both sides of the substrate, and after exposure, development, and etching, the copper layer to be laser blind hole is etched away; a high-energy ultraviolet beam is emitted by an ultraviolet laser, and its optical energy is used to destroy the molecular bonds and metal crystals of the organic matter on the substrate, forming suspended particles, atoms, molecules (or atomic groups, molecular groups) that escape and form blind holes; the schematic diagram of the blind hole is shown in the attached figure Figure 2 As shown, two opposite surfaces of the substrate are copper layers, one of which is provided with a blind hole by laser, the blind hole penetrates one of the copper layers (the top surface of the blind hole) and the PI layer, and is connected to the other copper layer (the bottom surface of the blind hole);
[0046] (2) Plasma cleaning is performed on the substrate. The radio frequency power supply of the plasma machine works in a vacuum environment, and plasma is formed in the vacuum chamber to clean the glue residue remaining on the wall of the blind hole after laser blasting, so as to achieve the effect of cleaning inside the blind hole. The specific cleaning parameters are shown in Table 1;
[0047] Table 1
[0048]
[0049] In the in-hole metallization, graphite colloid is deposited on the hole wall of the substrate through the black shadow process. The specific black shadow process parameters are shown in Table 2.
[0050] The main component of the black shadow is graphite colloid, which has good conductivity and is adsorbed on the pore walls of the substrate that have not yet been connected to form a conductive layer, so that the electroplated copper ions can be deposited on the pore walls.
[0051] Table 2 Black shadow process parameters
[0052]
[0053] Then, a vertical continuous electroplating (VCP) process is used to perform the first copper electroplating to obtain a substrate after hole filling, as shown in the schematic diagram of Figure 3; wherein, VCP uses direct current to transfer the copper ions with full charge in the plating solution to the substrate and the hole located at the cathode, and a hole filling light agent (commonly available in the market) is added to the copper cylinder to make the copper crystals precise and the blind holes filled with copper ions; the copper plating thickness is proportional to the current density and the copper plating time. In this embodiment, the copper thickness of the top surface after hole filling is required to be 55+ / -5μm, and the average copper plating thickness is 20μm. In order to prevent hole filling voids, the copper plating time is set to 60-65min so that the plating layer can fill the holes fully. The copper plating process parameters are shown in Table 3;
[0054] Table 3
[0055]
[0056] (3) The laminating machine attaches the dry film on the two surfaces of the substrate after the hole filling by hot pressing, so that the dry film is tightly bonded to the copper surface without bubbles; the dry film is a polymer compound, which can produce a polymerization reaction after being irradiated by ultraviolet rays to form a stable substance attached to the board surface, and the dry film is used to block the subsequent etching solution to achieve the function of blocking electroplating and etching;
[0057] Then, the other side (i.e., the bottom side of the blind hole) with respect to the blind hole is exposed, developed, etched, and stripped in sequence to obtain a patterned substrate; wherein, the ultraviolet rays of the exposure machine irradiate the dry film on the bottom side of the blind hole to transfer the designed circuit pattern on the bottom side of the blind hole to the board surface, and the top side of the blind hole is not designed with a pattern, and the whole side is exposed; after 0.5 hours of exposure, the developer uses a developer solution (K2CO3) to wash away the unexposed part from the dry film; the etching solution is a cupric chloride system, and the etching machine sprays the etching solution on the substrate through a nozzle, and the top side of the blind hole is covered by the dry film and resists the etching solution from biting, and the part of the bottom side of the blind hole that is not covered by the dry film is etched out with the designed pattern (including PAD), and the structural schematic diagram of the PAD and the blind hole position is as shown in Figure 4-5 As shown;
[0058] (4) Dry films are attached to the two surfaces of the substrate with one side patterned, and the side with the PAD (the bottom side of the blind hole) is exposed and developed in sequence to expose the PAD; wherein, after exposure and development, the pattern of the point hole plating (including the PAD to be electroplated and other areas to be electroplated) is transferred to the board surface, so that the bottom side is electroplated for the second time (point hole plating) according to the designed and processed pattern, and the schematic diagram of exposing the PAD to be electroplated is as shown in Figure 7 As shown; on the top surface, only the inside of the unit is covered with dry film, exposing the edge of the board as a conductive point for the electroplating clamp edge, such as Figure 6 As shown, the area to be electroplated is the pink area in the figure;
[0059] (5) A second copper electroplating process is performed using a vertical continuous electroplating process (VCP). After stripping the film, a flexible circuit board after electroplating is obtained, as shown in FIG8 . The copper ions in the plating solution cannot be deposited on the dry film due to the plating resistance of the dry film. The copper ions can be deposited on the area not covered by the dry film, so that the copper layer on the PAD surface is thickened. In addition, since only the edge part on the top surface of the blind hole is used as a conductive point, the conductive point on the bottom surface of the blind hole includes the PAD, the edge, and other graphic settings. As a result, the electroplating area ratio of the bottom surface of the blind hole and the top surface of the blind hole is different. It is necessary to input the corresponding electroplating area ratio according to the area ratio of the two surfaces. The specific copper plating parameters are shown in Table 4.
[0060] (6) Other conventional processing steps are then performed to form a complete flexible circuit board.
[0061] Table 4
[0062]
[0063] The flexible circuit board prepared in the above embodiment was sliced to observe the structure of the blind hole position after electroplating. Figure 8 As shown, it can be measured that the copper thickness of the top surface of the blind hole is 52μm, the PAD thickness of the bottom surface is 71.1μm, and the length is 263.2μm. Therefore, it can be confirmed that the method of the present invention can make the copper thickness of the top surface of the blind hole greater than the thickness of the PAD of the bottom surface, and the difference between the two is as high as more than 20μm, which can meet the requirements of actual special circuit boards.
[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.
Claims
1. A method for electroplating a flexible circuit board, characterized in that: The steps include: (1) preparing a substrate of a flexible circuit board, wherein two opposite surfaces of the substrate in the thickness direction are copper layers, and a blind hole is provided on one of the surfaces by laser, wherein the blind hole penetrates one of the copper layers and connects the two copper layers; (2) plasma cleaning and hole metallization are sequentially performed on the substrate, and then a first copper electroplating is performed using a vertical continuous electroplating process to obtain a hole-filled substrate; (3) attaching dry films to the two surfaces of the substrate after the holes are filled; then sequentially exposing, developing, etching, and stripping the dry films on the other side opposite to the blind hole to obtain a substrate with one side patterned, wherein the substrate with one side patterned has a PAD on the patterned side, and the position of the PAD corresponds to the position of the blind hole; (4) attaching dry films to two surfaces of the patterned substrate, and sequentially exposing and developing the dry films on the side where the PAD is present to expose the PAD; (5) A second copper electroplating process is performed using a vertical continuous electroplating process to obtain a plated flexible circuit board.
2. The electroplating method of a flexible circuit board according to claim 1, wherein: In step (1), the substrate further includes a PI layer, the PI layer is located between two copper layers, and the blind hole penetrates the PI layer.
3. The electroplating method of a flexible circuit board according to claim 1, wherein: In step (2), the plasma cleaning includes the following parameters: time is 10-20min, power is 6000-7000W, vacuum degree is 0.1-0.25torr, oxygen flow rate is 2000-2300sccm, carbon tetrafluoride flow rate is 200-300sccm, nitrogen flow rate is 200-300sccm, and temperature is 60-70°C.
4. The electroplating method of a flexible circuit board according to claim 1, wherein: In step (2), during the in-hole metallization, a conductive layer is deposited on the inner wall of the blind hole, and the conductive layer includes graphite colloid.
5. The electroplating method of a flexible circuit board according to claim 1, wherein: In step (2), the first copper electroplating includes the following parameters: current density is 1-2ASD, copper electroplating time is 30-120min, rectifier front output ratio is 70-120%, rectifier back output ratio is 70-120%, and copper cylinder spray frequency is 30-50Hz.
6. The electroplating method of a flexible circuit board according to claim 1, wherein: In step (2), the thickness of the surface copper in the substrate after the hole filling is ≥20 μm.
7. The electroplating method of a flexible circuit board according to claim 1, wherein: In step (5), in the second copper electroplating, an edge position of the substrate is exposed on the other side opposite to the side where the PAD is present, and the edge position serves as a conductive point to form a conductive point opposite to the side where the PAD is present, and the ratio of the electroplated area of the side where the PAD is present and the other side opposite to it is 2-3:
1.
8. A flexible circuit board after electroplating obtained by the electroplating method for a flexible circuit board according to any one of claims 1 to 7.
9. The plated flexible circuit board according to claim 8, wherein: It comprises a copper layer, a blind hole and a PAD, wherein the blind hole is filled with copper, and the blind hole conducts the PAD and the copper layer, and the thickness of the PAD is greater than the thickness of the copper layer.
10. The plated flexible circuit board according to claim 9, characterized in that: The PAD and the copper layer satisfy the following relationship: L1=L2+d, wherein L1 is the thickness of the PAD, L2 is the thickness of the copper layer, and d=10-30 μm.