External light imaging method for multilayer ultra-thick copper-based printed circuit board

Abstract

The invention relates to the technical field of printed circuit board manufacturing, and provides an external light imaging method for a multi-layer ultra-thick copper-based printed circuit board, which comprises the following steps: S1, measuring the thickness of a laminated board waiting for an external light imaging process; s2, selecting a screen printing plate; s3, a screen printing plate is installed on a printing machine; s4, the wet film is printed on the surface copper foil of the primary material plate through a printing machine and a screen printing plate; s5, pre-baking in a baking oven; s6, pasting a dry film; s7, transferring to an exposure machine, exposing the dry film and the wet film together, and enabling the exposure patterns of the dry film and the wet film to be consistent; and S8, developing to obtain a graphical dry film and a graphical wet film, and completing an external light imaging process. The method has the beneficial effects or advantages that the wet film covers the copper foil on the surface of the laminated board by utilizing a screen printing technology, and after pre-baking, the wet film provides a smoother and more uniform surface, so that a dry film can be pasted, the firmness of the dry film on the laminated board is improved, and an external light imaging process is smoothly carried out.

Description

Technical Field

[0001] The invention relates to the technical field of printed circuit board manufacturing, and in particular to an external light imaging method for a multi-layer ultra-thick copper-based printed circuit board. Background Art

[0002] With the development of science and technology, the requirements for the use of various electronic products are increasing, and the application fields of electronic products are also endless. Copper-based printed circuit boards are a type of printed circuit board with copper as the conductive layer. Laminates are made by stacking inner layers, outer copper foils, and insulating materials together, and pressing them together under high temperature and high pressure to form laminates. The market demand for multi-layer (for example, up to 10 layers), multi-layer ultra-thick copper-based printed circuit boards with an inner copper thickness of 400um or 105um, an outer copper thickness of 105um, and a total thickness of up to 4.5mm for the final product is increasing. In the actual manufacturing process of multi-layer ultra-thick copper-based printed circuit boards, the problem is how to deal with the problem of excessive total board thickness and uneven board thickness distribution caused by the thickness of the inner layer copper, and then the outer surface flatness of the laminate is insufficient, resulting in poor manufacturing in the subsequent external light imaging process, and ultimately the quality of the finished circuit board is low.

[0003] Thickness measurements during trial production revealed that the resulting laminated 10-layer PCB, after pattern lamination, was between 4.1 and 4.3 mm thick, with a thickness tolerance of 0.2 mm. This 0.2 mm thickness tolerance has proven to have a significant impact on the subsequent "external imaging" process, almost directly determining the success or failure of the product. External imaging involves transferring the design pattern onto the PCB using an external light source (such as UV light or laser).

[0004] For printed circuit board products of ordinary thickness, the outdoor imaging process uses a dry film process, that is, the dry film is directly attached to the surface copper foil of the laminate, and then exposed and developed to achieve the purpose of pattern transfer; however, for the laminate used for "multi-layer ultra-thick copper-based printed circuit boards", due to the insufficient flatness of the outer surface copper foil of the laminate, the local thickness difference is large, and the thickness tolerance of the entire laminate is even up to 0.2mm. When the dry film is directly used in the traditional outdoor imaging process, it is difficult to firmly attach the dry film to the surface copper foil of the laminate due to the unevenness of the local area of ​​the surface copper foil of the laminate. This will make it impossible to perform the outdoor imaging process, that is, it is impossible to perform further operations such as exposure and development.

[0005] Therefore, the technical field urgently needs a method for external light imaging of multi-layer ultra-thick copper-based printed circuit boards to improve the firmness of the dry film on the laminate and smoothly carry out the external light imaging process. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board, thereby improving the firmness of the dry film on the laminate and smoothly carrying out the external light imaging process.

[0007] The technical solution of the present invention is achieved as follows: a method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board comprises the following steps:

[0008] S1. Measure the thickness of laminates waiting for the external light imaging process, and select laminates with thickness tolerance exceeding a set threshold as primary material sheets;

[0009] S2. Select the screen;

[0010] S3, installing the screen on a printing press;

[0011] S4, adding a wet film on the screen, placing the primary sheet into the printing press, and using the printing press and the screen to print the wet film on the surface copper foil of the primary sheet to obtain a secondary sheet;

[0012] S5, placing the secondary material plate into an oven for pre-baking to obtain a tertiary material plate;

[0013] S6. Use an automatic film laminating machine to affix a dry film to the surface of the three-stage material plate to obtain a four-stage material plate;

[0014] S7, transferring the four-stage material plate to an exposure machine, exposing the dry film and the wet film on the four-stage material plate together, and making the exposure patterns of the dry film and the wet film consistent, thereby obtaining a five-stage material plate;

[0015] S8. Develop the five-level material plate to obtain a patterned dry film and a patterned wet film, wherein the patterned dry film covers the patterned wet film, thereby completing the external light imaging process.

[0016] Furthermore, the set threshold is 0.05 mm.

[0017] Furthermore, the screen plate is a screen plate with 21 to 36 meshes.

[0018] Furthermore, in S5, the pre-baking is specifically: the oven is gradually heated at a temperature of 45° C. to 75° C. to pre-bake the secondary material plate.

[0019] Furthermore, in the S8 , the development specifically includes: unexposed dry film and wet film are dissolved by a developer, and exposed dry film and wet film remain on the material plate.

[0020] Compared with the background technology, the beneficial effects or advantages of the present invention are as follows: the wet film is first covered on the surface copper foil of the laminate using screen printing technology, and the wet film effectively compensates for the uneven areas of the surface copper foil of the laminate. After pre-baking, the wet film provides a smoother and more uniform surface, which is conducive to the attachment of the dry film, improves the firmness of the dry film on the laminate, and smoothly performs the external light imaging process; thereafter, the production continues according to the normal process of the printed circuit board to obtain a multi-layer ultra-thick copper-based printed circuit board, thereby improving the manufacturing quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] Figure 1 The present invention is a flowchart of an external light imaging method for a multi-layer ultra-thick copper-based printed circuit board.

[0023] Figure 2 It is a schematic diagram of the positions of the laminate, copper foil, wet film and dry film in the present invention.

[0024] Reference numerals: laminate 1; copper foil 2; wet film 3; dry film 4. DETAILED DESCRIPTION

[0025] The embodiment of the present invention provides a method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board. The overall idea of ​​the technical solution is as follows:

[0026] In the manufacturing process of multi-layer ultra-thick copper-based printed circuit boards, during the lamination process, multiple inner layer boards, outer copper foils, and insulating material boards are stacked together and pressed together under high temperature and high pressure to form a laminate. Laminated boards have large thickness tolerances, that is, uneven thickness and uneven surface copper foil. The present invention first uses screen printing technology to print a layer of wet film on the surface copper foil of the laminate. The thickness of the wet film printed on the surface copper foil can vary depending on the thickness of the laminate. As a result, the wet film thickness is lower at more convex locations of the surface copper foil of the laminate, and higher at more concave locations of the surface copper foil of the laminate. In this way, the wet film effectively compensates for the uneven areas of the surface copper foil of the laminate. After pre-baking, the wet film provides a more flat and uniform surface. Then, a dry film is applied to the wet film to improve the firmness of the dry film on the laminate. Then, the exposure and development external light imaging process is performed. Finally, the normal production process of printed circuit boards is continued to obtain multi-layer ultra-thick copper-based printed circuit boards, improving the manufacturing quality.

[0027] In order to better understand the above technical solution, the above technical solution will be described in detail below with reference to the accompanying drawings and specific implementation methods.

[0028] See Figure 1 and Figure 2 , a preferred embodiment of the present invention.

[0029] A method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board comprises the following steps:

[0030] S1. Measure the thickness of the laminate 1 waiting for the external light imaging process, and select the laminate 1 whose thickness tolerance exceeds a set threshold as the primary material plate; the set threshold is 0.05 mm;

[0031] The thickness tolerance of the laminate is determined based on the measured value. The tolerance is the absolute value of the difference between the maximum and minimum dimensions. In this embodiment, a thickness tolerance exceeding 0.05mm indicates that the copper foil 2 on the laminate surface is not flat enough, making it difficult to directly apply a dry film. The threshold is adjusted by staff based on the actual number of layers and the thickness of the finished board. The threshold can also be set to 0.2mm or other values.

[0032] S2. Select a screen. Generally, a screen with a lower mesh size in the industry is used. The screen is a screen with a mesh size of 21 to 36.

[0033] For special printed circuit boards, a dedicated screen is produced according to the CAD design data. That is, within the usage range of the entire screen, a specific graphic area is only used for this special printed circuit board.

[0034] S3, installing the screen on a printing press;

[0035] After the screen is installed, the displacement of the screen is debugged to facilitate alignment with the laminate.

[0036] S4, adding a wet film 3 on the screen, placing the primary sheet into the printing press, and using the printing press and the screen to print the wet film 3 on the copper foil 2 on the surface of the primary sheet to obtain a secondary sheet;

[0037] During the printing process, the distance between the screen and the secondary material plate remains unchanged, the screen is set parallel to the surface copper foil, and the wet film 3 passes through the screen and then covers the surface copper foil 2, effectively making up for the uneven area of ​​the surface copper foil.

[0038] S5, placing the secondary material plate into an oven for pre-baking to obtain a tertiary material plate;

[0039] The pre-baking is specifically: the oven is gradually heated to 45° C. to 75° C. to pre-bake the secondary material plate. After the pre-baking, the wet film 3 is solidified, providing a smoother and more uniform surface.

[0040] S6. Use an automatic film laminating machine to affix a dry film 4 on the surface of the three-stage material plate to obtain a four-stage material plate;

[0041] The dry film 4 is attached to the surface of the wet film 3 , thereby achieving the purpose of attaching the dry film 4 to the ultra-thick laminate 1 .

[0042] S7, transferring the four-stage material plate to an exposure machine, exposing the dry film 4 and the wet film 3 on the four-stage material plate together, and making the exposure patterns of the dry film 4 and the wet film 3 consistent, thereby obtaining a five-stage material plate;

[0043] Specifically, after the dry film is successfully applied, the printed circuit board is transferred to the exposure machine and exposed under vacuum conditions with the highest exposure parameter (usually level 9). After exposure, the dry film and the wet film will be exposed together, and the exposed patterns are consistent.

[0044] S8. Develop the five-level material plate to obtain a patterned dry film and a patterned wet film, wherein the patterned dry film covers the patterned wet film, thereby completing the external light imaging process.

[0045] Specifically, after successful exposure, development processing is carried out; the development is carried out at a rate of 2.5 meters and a process condition of 30°C; after development, the unexposed dry film and wet film parts will be dissolved, and the exposed dry film and wet film parts will remain on the surface of the printed circuit board; and the dry film covers the wet film.

[0046] The material board that has completed the external light imaging process undergoes conventional processes such as electroplating and etching in sequence, and continues to be produced according to the normal process of printed circuit boards to obtain multi-layer ultra-thick copper-based printed circuit boards.

[0047] Pattern plating: A process in which metal (usually copper) is deposited electrochemically in specific areas to form circuit patterns. The core of this process is to selectively deposit and thicken the copper surface where it is to be retained, while preventing deposition in other areas covered by a protective film. Electroplating is performed on the exposed copper surface of patterned dry and wet films to increase the copper thickness. A layer of tin is then applied to the surface after the electroplating to serve as a protective layer during etching.

[0048] Film stripping: Use chemical solutions to remove dry and wet films to expose the unwanted copper surface.

[0049] Etching: Use chemical etching solution to etch away the copper not protected by tin to form the final outer layer circuit.

[0050] Tin stripping: Remove the tin layer to expose the final outer surface copper circuit.

[0051] Finally, after surface treatment, solder mask ink, final inspection and testing, the finished product is obtained.

[0052] The working principle of the present invention is described in detail below:

[0053] In the manufacturing process of multi-layer, ultra-thick copper-based printed circuit boards, laminates with thickness tolerances exceeding a set threshold must first be processed for excessive thickness tolerances or reduced before external imaging. The dry film used in the external imaging process is a solid photoresist that forms the circuit pattern through UV exposure and development. Wet film, also a type of photoresist, is a liquid. While dry and wet film have the same functional characteristics, wet film is unsuitable for subsequent electroplating processes (it cannot withstand the chemical attack of the electroplating process).

[0054] Faced with the problem of how to solve the problem of how to solve the problem of how the laminated board cannot be "imaged externally" during the manufacturing process of multi-layer ultra-thick copper-based printed boards with uneven board thickness, the present invention utilizes the same properties of dry film and wet film (for example, both are photoresists) and different properties (for example, solid and liquid), and also utilizes the uniformity characteristics of screen printing. First, a layer of wet film is printed on the surface copper foil of the laminate. Since it is screen printing, the thickness of the wet film printed on the copper foil can be different due to different board thicknesses. That is, screen printing can effectively compensate for the uneven surface copper foil caused by excessive board thickness tolerance. The final total thickness of "laminate thickness + wet film thickness" is a more uniform overall thickness, thereby effectively avoiding the uneven board thickness encountered by the laminated board when the dry film is attached in the background technology; then the present invention attaches a dry film on the wet film of the laminated board with relatively uniform overall thickness; and then performs conventional processes such as exposure, development, electroplating, and etching.

[0055] In the actual manufacturing process of copper-based printed circuit boards, for laminates with poor thickness uniformity, after wet film printing and pre-baking, the actual thickness of the laminated board is measured. The thickness tolerance of the board is within the range of 0.05mm. That is, the pre-baked wet film provides a smoother and more uniform surface, which helps to smoothly carry out the subsequent dry film application process; thus solving the problem of "the dry film cannot be applied due to excessive board thickness tolerance and uneven surface copper foil, resulting in the inability to complete the external light imaging process."

[0056] Although the specific embodiments of the present invention are described above, those skilled in the art should understand that the specific embodiments described are merely illustrative and are not intended to limit the scope of the present invention. Equivalent modifications and changes made by those skilled in the art in accordance with the spirit of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board, characterized in that: The following steps are involved: S1. Measure the thickness of laminates waiting for the external light imaging process, and select laminates with thickness tolerance exceeding a set threshold as primary material sheets; S2. Select the screen; S3, installing the screen on a printing press; S4, adding a wet film on the screen, placing the primary sheet into the printing press, and using the printing press and the screen to print the wet film on the surface copper foil of the primary sheet to obtain a secondary sheet; S5, placing the secondary material plate into an oven for pre-baking to obtain a tertiary material plate; S6. Use an automatic film laminating machine to affix a dry film to the surface of the three-stage material plate to obtain a four-stage material plate; S7, transferring the four-stage material plate to an exposure machine, exposing the dry film and the wet film on the four-stage material plate together, and making the exposure patterns of the dry film and the wet film consistent, thereby obtaining a five-stage material plate; S8. Develop the five-level material plate to obtain a patterned dry film and a patterned wet film, wherein the patterned dry film covers the patterned wet film, thereby completing the external light imaging process.

2. The method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board according to claim 1, characterized in that: The set threshold is 0.05 mm.

3. The method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board according to claim 1, characterized in that: The screen plate has a mesh size of 21 to 36.

4. The method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board according to claim 1, characterized in that: In S5, the pre-baking is specifically: the oven is gradually heated to a temperature of 45°C to 75°C to pre-bake the secondary material plate.

5. The method for external light imaging of a multi-layer ultra-thick copper-based printed circuit board according to claim 1, characterized in that: In the step S8 , the development specifically includes: unexposed dry film and wet film are dissolved by a developer, while exposed dry film and wet film remain on the material plate.

Images