A method for fabricating a solder resist layer on a Mini LED PCB

Abstract

The application discloses a manufacturing method of a Mini LED PCB resist layer, and comprises the following steps: pretreatment; first vertical low-pressure spraying, vertically hanging the PCB plate subjected to the pretreatment in a vertical low-pressure spraying machine, performing first resist ink spraying, and forming a completely covering base coating layer on the PCB surface; first pre-baking, feeding the PCB plate subjected to the first spraying into an oven, performing first-stage solidification, and converting the liquid base coating layer into a semi-solid anchoring layer; second vertical low-pressure spraying, vertically hanging the PCB plate subjected to the first pre-baking in the vertical low-pressure spraying machine again, performing second resist ink spraying, and uniformly depositing the ink on the surface of the anchoring layer until the coating layer thickness meets the design requirement; second pre-baking, feeding the PCB plate subjected to the second spraying into the oven again, performing second-stage solidification, and forming a final resist layer; exposure and development; and high-temperature solidification. The manufacturing method of the Mini LED PCB resist layer can significantly reduce the thickness range of the resist ink, so as to obtain a PCB surface with super-high flatness, and make the manufactured Mini LED meet optical requirements.

Description

Technical Field

[0001] This invention relates to the field of circuit board manufacturing technology, specifically to a method for manufacturing a solder resist layer for a Mini LED PCB. Background Technology

[0002] As a next-generation display technology, Mini LED places extremely stringent requirements on the quality of its supporting circuitry—the PCB. This is especially true for direct-display Mini LEDs, where the display effect directly depends on the flatness and uniformity of the PCB surface beneath the LEDs. Improper control of the thickness uniformity (usually measured by "thickness variation") of the solder mask layer, a protective layer covering the PCB surface, can lead to severe optical color differences and uneven brightness, significantly impacting display quality.

[0003] Currently, the conventional methods for preparing solder mask layers in the PCB industry include:

[0004] 1. Screen printing: Low cost and high ink utilization, but this method is heavily dependent on operator experience. On boards with high-density and high-drop patterns, it is very easy to produce uneven thickness and large differences, which is difficult to meet the requirements of Mini LED.

[0005] 2. Horizontal Low-Pressure Spray Coating: In this process, the PCB board is placed horizontally. During spraying, the atomized ink is mainly affected by nozzle pressure and airflow direction, depositing on the horizontal board surface. Before curing, the liquid ink's gravity is perpendicular to the board surface, easily leading to ink drips or insufficient coverage on the sidewalls of the pattern due to uneven surface tension. For high-density MiniLED PCBs with a finished copper thickness ≥35μm, there is a significant "drop" between the circuitry and the substrate. In addition, horizontal spray coating has other problems that make it unsuitable for Mini LED PCBs with high flatness requirements, including:

[0006] (1) High risk of ink entering the hole: When placed horizontally, the sprayed ink is more likely to fall directly into the through hole due to gravity or pressure, causing ink blockage in the hole (i.e., "solder resist entering the hole" problem), which affects the subsequent component soldering.

[0007] (2) Poor adaptability to high-density patterns: When faced with high-density, high-drop fine lines (such as Mini LED PCB), horizontal spraying is more likely to result in thin ink coverage or uneven distribution at line corners and dense areas.

[0008] (3) The target of the thickness difference cannot be achieved: the deposition rate of ink on the top and side walls of the circuit is different in a single spraying. Even if it is sprayed twice, the surface of the "anchor layer" formed by the first curing may still be uneven. Although the second spraying can increase the thickness, it cannot solve the fundamental unevenness caused by the first spraying from the deposition mechanism. Therefore, it is impossible to achieve the goal of controlling the thickness difference within 6μm in this invention.

[0009] 3. Electrostatic spraying: High efficiency, but low ink utilization and environmentally unfriendly, with limited ability to control coating uniformity.

[0010] 4. Dry film solder mask: Although it can achieve extremely high flatness, the raw material cost is high and it requires special lamination equipment. It is usually only used in very high-end fields such as chip packaging substrates and is not suitable for the large-scale production of cost-sensitive Mini LED PCBs.

[0011] Therefore, there is an urgent need for a method to fabricate a solder resist layer with extremely low ink thickness and poor quality for high copper thickness Mini LED PCBs while ensuring cost-effectiveness in production. Summary of the Invention

[0012] The purpose of this invention is to provide a method for manufacturing a solder resist layer on a Mini LED PCB, which can significantly reduce the thickness difference of the solder resist ink to obtain a PCB surface with ultra-high flatness, so that the manufactured Mini LED meets optical requirements.

[0013] The technical solution of this invention is:

[0014] A method for fabricating a solder resist layer on a Mini LED PCB includes the following steps:

[0015] Step S1, preprocessing;

[0016] Step S2, First vertical low-pressure spraying: The pre-treated PCB board is vertically suspended in a vertical low-pressure spraying machine to perform the first solder resist ink spraying, forming a base coating that completely covers the PCB surface.

[0017] Step S3, first pre-baking: The PCB board that has completed the first spraying is sent into the oven for the first stage of curing, so that the liquid base coating is transformed into a semi-cured anchoring layer.

[0018] Step S4, Second vertical low-pressure spraying: The PCB board that has been pre-baked in the first time is suspended vertically again in a vertical low-pressure spraying machine for the second solder resist ink spraying. The ink is evenly deposited on the surface of the anchor layer until the coating thickness meets the design requirements. The liquid ink sprayed in the second time automatically flows and levels under the action of surface tension, filling the micro-depressions and unevenness remaining on the anchor layer to form a continuous and smooth liquid film layer.

[0019] Step S5, second pre-baking: The PCB board that has completed the second spraying is sent back into the oven for the second stage of curing, so that the anchor layer and the liquid film layer are cured to form the final solder mask layer.

[0020] Step S6, exposure and development;

[0021] Step S7, high-temperature curing.

[0022] Furthermore, in step S2, the atomization pressure of the vertical low-pressure sprayer is 0.25-0.35 MPa, and the spray gun moving speed is 380-420 mm / s.

[0023] Furthermore, in the first pre-baking process, the baking temperature is 75-85℃ and the baking time is 20-30 minutes; the oven is a circulating hot air oven.

[0024] Furthermore, in step S4, the ink used for the second vertical low-pressure spraying is the same as the ink used for the first vertical low-pressure spraying.

[0025] Furthermore, the atomization pressure of the second vertical low-pressure spray is less than or equal to the atomization pressure of the first vertical low-pressure spray.

[0026] Furthermore, in the second pre-baking process, the baking temperature is 75-85℃ and the baking time is 20-30 minutes.

[0027] Furthermore, in step S7, the high-temperature curing temperature is 145-155℃, and the curing time is 55-65min.

[0028] Furthermore, the finished copper thickness of the PCB board is ≥35μm.

[0029] Compared with the prior art, the method for fabricating the Mini LED PCB solder mask layer provided by the present invention has the following advantages:

[0030] I. The method for manufacturing the solder mask layer of a Mini LED PCB provided by this invention employs a vertical low-pressure spraying process, with the PCB board suspended vertically. During spraying, atomized ink is sprayed horizontally onto the vertical board surface. Combined with optimized spray pressure, this allows ink droplets to adhere more evenly to various parts of the vertical board surface, including the sidewalls of high-line traces, line corners, and dense areas, effectively reducing the risk of ink dripping directly into holes. Before the liquid ink cures, the direction of gravity is parallel to the board surface and downwards, effectively reducing ink accumulation or sidewall dripping caused by gravity, thus contributing to the formation of a more uniform wet film. In practice, a process combining two vertical low-pressure spraying processes and two pre-baking processes is used. The first vertical low-pressure spraying forms a completely covering base coating on the PCB board surface. The first pre-baking process pre-heats and cures the base coating, forming a stable "anchor layer." The second vertical low-pressure spraying precisely compensates for and smooths the thickness of the anchor layer surface from the first spraying, ensuring the coating thickness meets design requirements and filling in uneven areas of the base coating to achieve uniform thickness. The combination of two spraying and two pre-baking processes effectively eliminates coverage dead spots and uneven thickness caused by variations in graphic height, thereby achieving extremely low thickness differences. Therefore, the method for manufacturing the Mini LED PCB solder resist layer of this invention optimizes the initial distribution of the ink from the source, significantly enhancing subsequent filling and leveling effects. The resulting Mini LED PCB board has extremely high surface flatness, effectively eliminating optical defects caused by uneven ink thickness and significantly improving the uniformity, contrast, and overall yield of the display module.

[0031] Second, the method for manufacturing the solder resist layer of Mini LED PCB provided by this invention performs particularly well in the face of the severe challenges brought by high copper thickness circuit boards with a finished copper thickness of ≥35μm. It can stably control the ink difference within 6μm, which is difficult to achieve with traditional screen printing and horizontal single-pass spraying processes, thus solving the key obstacle to the mass production of high-specification Mini LED PCB boards.

[0032] Third, the method for manufacturing the solder resist layer of Mini LED PCB provided by this invention belongs to the liquid spraying technology system. Through process optimization, it achieves a flatness effect close to that of dry film solder resist layer, but at a much lower cost, providing a feasible technical path for the large-scale, low-cost, and high-quality preparation of Mini LED PCB. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the solder resist layer thickness measurement points in this invention;

[0035] Figure 2 This is a cross-sectional schematic diagram of the solder resist layer produced by the manufacturing method of the present invention under a metallographic microscope. Detailed Implementation

[0036] To enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention, and to make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be further described below.

[0037] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0038] A method for fabricating a solder resist layer on a Mini LED PCB includes the following steps:

[0039] Step S1, Pre-processing: Clean and roughen the surface of the PCB board with the completed circuit pattern;

[0040] Specifically, volcanic ash abrasive brush lines are used to clean and micro-etch the boards to ensure that the board surface is clean and has appropriate roughness.

[0041] Step S2, First vertical low-pressure spraying: The pre-treated PCB board is vertically suspended in a vertical low-pressure spraying machine to perform the first solder resist ink spraying, forming a base coating that completely covers the PCB surface.

[0042] Specifically, the atomization pressure of the vertical low-pressure sprayer is 0.25-0.35 MPa, and the spray gun moving speed is 380-420 mm / s. Step S3, first pre-baking, the PCB board after the first spraying is sent into the oven for the first stage of curing, so that the liquid base coating is transformed into a semi-cured anchoring layer;

[0043] Specifically, a circulating hot air oven is used, with a baking temperature of 75-85℃ and a baking time of 20-30 minutes. The "base coating" formed after the first spraying is liquid, and its surface has tiny depressions, pinholes, or uneven coverage caused by PCB circuit patterns (especially large differences caused by high copper thickness). Through the first pre-baking (75-85℃, 20-30 minutes), some of the solvent in the ink evaporates, and the resin begins to undergo a preliminary cross-linking reaction (pre-curing). This pre-curing process transforms the base coating from a flowing liquid film into a semi-cured "anchor layer" with a certain mechanical strength and shape retention capability. At this point, the original macroscopic defects (such as large depressions) are partially fixed, but the microscopic surface still has a certain degree of roughness and micropores.

[0044] Step S4, Second vertical low-pressure spraying: The PCB board that has been pre-baked in the first time is suspended vertically again in a vertical low-pressure spraying machine for the second solder resist ink spraying. The ink is evenly deposited on the surface of the anchor layer until the coating thickness meets the design requirements. The liquid ink sprayed in the second time automatically flows and levels under the action of surface tension, filling the micro-depressions and unevenness remaining on the anchor layer to form a continuous and smooth liquid film layer.

[0045] The second coat serves to precisely fill and level the surface: the ink in the second coat is evenly deposited on the semi-cured "anchor layer." Because the anchor layer surface is relatively stable, the newly applied ink will not flow wildly or sink into deep pits as it would on bare copper or the initial liquid coating; instead, it adheres more evenly to the surface. Furthermore, the ink in the second coat is a fresh liquid material with excellent leveling properties. In the short period between spraying and the second pre-baking, the liquid ink automatically levels itself under surface tension, filling in any remaining microscopic depressions and unevenness on the anchor layer, forming a continuous, smooth liquid film.

[0046] Specifically, the ink used for the second vertical low-pressure spray is the same as that used for the first vertical low-pressure spray; and the atomization pressure of the second vertical low-pressure spray is less than or equal to that of the first vertical low-pressure spray. This lower or equal atomization pressure in the second spray produces a gentler spray and larger droplets, which helps reduce ink "splattering" or "over-atomization," avoids the formation of new microparticles or blistering, and allows the ink to deposit more gently, spreading better on the anchoring layer surface and further promoting leveling.

[0047] Step S5, second pre-baking: The PCB board that has completed the second spraying is sent back into the oven for the second stage of curing, so that the anchor layer and the liquid film layer are cured to form the final solder mask layer.

[0048] The second pre-baking process is the same as the first pre-baking process. The second stage of curing further solidifies the already leveled and filled composite coating (including the anchoring layer and the liquid film layer), fixing the smooth and flat surface state, thereby obtaining a final defect-free solder resist layer (meaning no obvious pinholes, depressions, orange peel, etc. that affect optical performance).

[0049] Step S6, Exposure and Development; Specifically, the solder resist layer is exposed using a high-precision LDI (Laser Direct Imaging) device, and then developed using a 1.0% sodium carbonate solution at 30°C to remove the ink from the unexposed areas.

[0050] Step S7, high-temperature curing; specifically, the curing temperature is 145-155℃, and the curing time is 55-65min.

[0051] The following detailed description of the method for fabricating the Mini LED PCB solder mask layer of the present invention is provided through specific embodiments.

[0052] Example 1: Taking the manufacture of a PCB for a 4K direct-view Mini LED display as an example, the finished copper thickness is 35μm and the solder mask thickness is 25±3μm.

[0053] A method for fabricating a solder resist layer on a Mini LED PCB includes the following steps:

[0054] Step S1, Pre-treatment: Clean and micro-etch the board using a volcanic ash abrasive brush line to ensure the board surface is clean and has appropriate roughness;

[0055] Step S2, First vertical low-pressure spraying: The pre-treated PCB board is vertically suspended in a vertical low-pressure spraying machine to perform the first solder resist ink spraying, forming a base coating that completely covers the PCB surface; wherein the atomization pressure of the vertical low-pressure spraying machine is 0.3 MPa and the spray gun moving speed is 400 mm / s.

[0056] Step S3, first pre-baking: The PCB board after the first spraying is sent into a circulating hot air oven for the first stage of curing, so that the liquid base coating is transformed into a semi-cured anchoring layer; the baking temperature is 80℃ and the baking time is 25min.

[0057] Step S4, Second vertical low-pressure spraying: The PCB board that has been pre-baked for the first time is suspended vertically again in a vertical low-pressure spraying machine. The same ink is used, and the solder resist ink is sprayed for the second time under the same or slightly lower atomization pressure to supplement the coating thickness to about 25μm and fill the defects on the surface of the base coating to make the coating thickness uniform.

[0058] Step S5, second pre-baking: The PCB board that has completed the second spraying is sent back into the oven for the second stage of curing to form the final solder mask layer; the baking temperature is 80℃ and the baking time is 25min.

[0059] Step S6, Exposure and Development: Expose the solder resist layer using a high-precision LDI (Laser Direct Imaging) device, and then develop it using a 1.0% sodium carbonate solution at 30°C to remove the ink from the unexposed areas.

[0060] Step S7, High-temperature curing: Heat-treat the PCB board in an oven at 150°C for 60 minutes to allow the solder mask layer to fully cross-link and cure.

[0061] PCB boards were prepared using the method described in Example 1, and three PCB boards were randomly selected. For each board, 16 measurement points were chosen according to a "nine-square grid" principle (the positions of the 16 measurement points are as follows...). Figure 1 As shown in the figure, the thickness of the solder resist layer was measured using a metallographic microscope.

[0062] A cross-sectional view of the solder mask layer under a metallographic microscope is shown below. Figure 2 As shown, by Figure 2 As can be seen, the solder resist layer fabrication method of the present invention produces a solder resist layer with excellent coverage uniformity.

[0063] The statistical results of the solder mask thickness of each circuit board are shown in Table 1.

[0064] Table 1: Statistical Results of Solder Mask Thickness for Each Circuit Board

[0065]

[0066] As shown in Table 1, the maximum ink thickness variation on side A (bright copper surface) is 2.3 μm; the maximum ink thickness variation on side B (etched pattern surface) is 2.1 μm. The ink thickness variation of all measured boards is consistently controlled within 3 μm, perfectly meeting the customer's most stringent requirements for the surface flatness of Mini LED PCBs.

[0067] PCB boards were prepared using the method described in Example 1, as well as traditional screen printing and horizontal low-pressure spraying methods. One PCB board was randomly selected from each method, and 16 measurement points were chosen from each board according to a "nine-square grid" principle (the positions of the 16 measurement points are as follows...). Figure 1 As shown in Table 2, the thickness of the solder resist layer was measured using a metallographic microscope. The measurement results are shown in Table 2.

[0068] Table 2: Test of solder resist layer thickness uniformity of circuit boards prepared by different processes

[0069]

[0070] As can be seen from Table 2, the solder resist thickness difference achieved by the method of the present invention is much lower than that achieved by the traditional screen printing and horizontal low-pressure spraying processes, indicating that the method of the present invention can achieve a PCB surface with ultra-high flatness compared to the traditional process.

[0071] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations made to these embodiments without departing from the principles and spirit of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for fabricating a solder resist layer on a Mini LED PCB, characterized in that, Includes the following steps: Step S1, preprocessing; Step S2, First vertical low-pressure spraying: The pre-treated PCB board is vertically suspended in a vertical low-pressure spraying machine to perform the first solder resist ink spraying, forming a base coating that completely covers the PCB surface. Step S3, first pre-baking: The PCB board that has completed the first spraying is sent into the oven for the first stage of curing, so that the liquid base coating is transformed into a semi-cured anchoring layer. Step S4, Second Vertical Low-Pressure Spraying: The PCB board, which has undergone the first pre-baking, is once again vertically suspended in a vertical low-pressure spraying machine for the second solder resist ink spraying. The ink is evenly deposited on the surface of the anchor layer until the coating thickness meets the design requirements. The liquid ink sprayed in the second spraying automatically flows and levels under the action of surface tension, filling the microscopic depressions and unevenness remaining on the anchor layer to form a continuous and smooth liquid film layer. The atomization pressure of the second vertical low-pressure spraying is less than or equal to the atomization pressure of the first vertical low-pressure spraying. Step S5, second pre-baking: The PCB board that has completed the second spraying is sent back into the oven for the second stage of curing, so that the anchor layer and the liquid film layer are cured to form the final solder mask layer. Step S6, exposure and development; Step S7, high-temperature curing.

2. The method for fabricating the Mini LED PCB solder mask layer according to claim 1, characterized in that, In step S2, the atomization pressure of the vertical low-pressure sprayer is 0.25-0.35 MPa, and the spray gun moving speed is 380-420 mm / s.

3. The method for fabricating the Mini LED PCB solder mask layer according to claim 1, characterized in that, In the first pre-baking process, the baking temperature is 75-85℃ and the baking time is 20-30 minutes; the oven is a circulating hot air oven.

4. The method for fabricating the Mini LED PCB solder mask layer according to claim 1, characterized in that, In step S4, the ink used for the second vertical low-pressure spraying is the same as the ink used for the first vertical low-pressure spraying.

5. The method for fabricating the Mini LED PCB solder mask layer according to claim 1, characterized in that, In the second pre-baking process, the baking temperature is 75-85℃ and the baking time is 20-30 minutes.

6. The method for fabricating the Mini LED PCB solder mask layer according to claim 1, characterized in that, In step S7, the high-temperature curing temperature is 145-155℃, and the curing time is 55-65min.

7. The method for manufacturing a Mini LED PCB solder mask layer according to any one of claims 1-6, characterized in that, The finished copper thickness of the PCB board is ≥35μm.

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