Manufacturing method for forming solder mask layers with different thicknesses in PCB

Abstract

The invention discloses a manufacturing method for forming solder mask layers with different thicknesses in a PCB (Printed Circuit Board). The manufacturing method comprises the following steps of: coating solder mask ink on a production board and then pre-curing; exposing the production board for the first time, wherein only the region corresponding to the thick solder mask layer in the production board is exposed at the moment; performing second exposure treatment on the production board, and simultaneously exposing areas corresponding to the thick solder mask layer and the thin solder mask layer in the production board, so that the crosslinking degree of the solder mask ink corresponding to the area of the thick solder mask layer is greater than that of the solder mask ink corresponding to the area of the thin solder mask layer; removing the unexposed solder resist ink through development, and dissolving the solder resist ink in a part of the thick solder resist layer area and the thin solder resist layer area so as to form a thick solder resist layer and a thin solder resist layer which are different in thickness on the production board; and finally, curing the developed solder resist ink through thermocuring treatment. According to the method, manufacturing of different solder mask layer thicknesses in different areas can be completed only through one solder mask process, the board production efficiency is effectively improved, and the production cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board manufacturing technology, and specifically to a method for forming solder mask layers of different thicknesses in a PCB. Background Technology

[0002] During the manufacturing process of printed circuit boards (PCBs), it is sometimes necessary to create different solder mask thicknesses in different areas of the same PCB to meet the electrical performance requirements of different areas. For example: 1. High-voltage areas: such as power modules and motor drive boards, require thick solder mask to provide strong insulation strength and creepage distance to prevent high-voltage breakdown or short circuit.

[0003] 2. High-density wiring areas: such as BGA and fine-pitch components around the processor, require thin solder mask to ensure precise windowing, avoid ink contamination of the solder pads, and ensure the reliability of the solder joints.

[0004] 3. Impedance control area: The thickness and dielectric constant of the solder mask layer will affect the impedance of high-speed signals. Sometimes it is necessary to precisely control the solder mask thickness to fine-tune the impedance.

[0005] 4. Heat dissipation requirements: High-power components sometimes require thin solder mask or even no solder mask underneath to facilitate direct heat conduction to the copper layer for dissipation.

[0006] The "different thickness solder resist" process is designed to address specific regional needs on the same board. The commonly used process flow is as follows: First printing of solder resist ink (base solder resist layer thickness) → pre-baking → exposure (defining windows for thicker solder resist areas) → development → thermosetting → second printing of solder resist ink (corresponding to the thickness of the thinner layer area) → pre-baking → exposure (exposure of the solder resist ink for the thinner layer area) → development → final curing. In other words, the thick and thin solder resist layers are produced in separate steps. However, this method requires two solder resist processes (including two printing, two development, and two thermosetting processes), which results in a long and cumbersome process, low production efficiency, and relatively high production costs. Summary of the Invention

[0007] To address the aforementioned shortcomings of existing technologies, this invention provides a method for forming solder mask layers of different thicknesses in a PCB. This method requires only one solder mask process to complete the fabrication of different solder mask layer thicknesses in different areas, effectively improving production efficiency and reducing production costs.

[0008] To address the aforementioned technical problems, this invention provides a method for forming solder mask layers of different thicknesses in a PCB, comprising the following steps: S1. After coating the production board with solder resist ink, pre-curing is performed; S2. Perform the first exposure process on the production board. During the first exposure, only the area of ​​the production board corresponding to the thick solder mask layer is exposed. S3. The production board is then exposed a second time. During the second exposure, the areas of the production board corresponding to the thick solder resist layer and the thin solder resist layer are exposed at the same time, so that the cross-linking degree of the solder resist ink in the area corresponding to the thick solder resist layer is greater than that in the area corresponding to the thin solder resist layer. S4. Then, the unexposed solder resist ink is removed by development, and the solder resist ink in the thick solder resist layer area and the thin solder resist layer area is dissolved. The solder resist ink in the thin solder resist layer area dissolves faster than the solder resist ink in the thick solder resist layer area, so as to form thick solder resist layers and thin solder resist layers of different thicknesses on the production board. S5. Finally, the developed solder resist ink is cured by heat curing.

[0009] Furthermore, in step S2, during the first exposure process, a strong exposure is performed using 2.0-2.5 times the reference exposure energy of the solder resist ink.

[0010] Furthermore, in step S2, the exposure scale during the first exposure process is 10-12 stops.

[0011] Furthermore, in step S3, during the second exposure process, the exposure is performed at 1.3-1.5 times the reference exposure energy of the solder resist ink.

[0012] Furthermore, in step S3, the exposure scale during the second exposure process is 7-9 stops.

[0013] Furthermore, in step S4, the development point during development is 80%-90%.

[0014] Furthermore, in step S4, during development, the concentration of the developer is 0.8-1.2%, the temperature is 28-32℃, and the development time is 60-80s.

[0015] Furthermore, in step S4, after development, the thickness of the thick solder resist layer is 35-40 μm, and the thickness of the thin solder resist layer is 15-20 μm.

[0016] Furthermore, in step S1, a layer of solder resist ink with a thickness of ≥50μm is coated on the production board and then pre-cured.

[0017] Furthermore, the production board is a multilayer board in which the inner core board and the outer copper foil are pressed together by a prepreg, and the multilayer board has successively undergone drilling, copper plating, full board electroplating and outer circuit fabrication processes.

[0018] Compared with the prior art, the present invention has the following beneficial effects: In this method, after coating the production board with solder resist ink, two exposures are performed to increase the cross-linking degree of the solder resist ink in the corresponding thicker areas, while the cross-linking degree of the solder resist ink in the thinner areas, which only undergo one exposure, is relatively low. This difference in cross-linking degree between the thick and thin solder resist inks results in different dissolution rates during development. The solder resist ink with lower cross-linking degree in the thinner areas dissolves faster, while the solder resist ink with higher cross-linking degree in the thicker areas dissolves slower. Therefore, within the same development time, less solder resist ink is dissolved in the thicker areas compared to the thinner areas. This allows for the production of thick and thin solder resist layers of varying thicknesses. After development, the entire production board is baked to completely cure the remaining solder resist ink. This method requires only one solder resist process to produce solder resist layers of different thicknesses in different areas, effectively improving production efficiency and reducing production costs. Generally, production efficiency can be increased by 50%, and production costs can be reduced by 40%. Detailed Implementation

[0019] To better understand the technical content of this invention, the technical solution of this invention will be further introduced and explained below in conjunction with specific embodiments.

[0020] Example This embodiment illustrates a PCB manufacturing method, which includes the fabrication process of solder mask layers of different thicknesses, comprising the following processing steps in sequence: (1) Cutting: Cut the core board according to the panel size of 520mm×620mm. The thickness of the core board is 0.5mm, and the copper layer thickness on both surfaces of the core board is 0.5OZ.

[0021] (2) Inner layer circuit fabrication (negative film process): Inner layer pattern transfer, photosensitive film is coated using a vertical coating machine, and the film thickness of the photosensitive film is controlled at 8μm. The inner layer circuit is exposed using a fully automatic exposure machine with 5-6 exposure rulers (21 exposure rulers). After development, the inner layer circuit pattern is formed; Inner layer etching, the inner layer circuit is etched out on the core board after exposure and development. The inner layer line width is measured to be 3mil; Inner layer AOI, and then the inner layer circuit is inspected for defects such as open circuits, short circuits, line gaps, and line pinholes. Defective products are scrapped, and defect-free products are sent to the next process.

[0022] (3) Lamination: The browning speed is based on the thickness of the bottom copper. The core board, prepreg, and outer copper foil are stacked in sequence as required. Then, according to the Tg of the board material, appropriate lamination conditions are selected to press the laminated board to form a multilayer board.

[0023] (4) Drilling: Drilling is performed on the multilayer board according to the design requirements based on the existing drilling technology.

[0024] (5) Copper plating: A thin layer of copper is deposited on the board surface and hole walls using chemical copper plating. The backlight test is level 10, and the copper plating thickness in the hole is 0.5μm.

[0025] (6) Full-board electroplating: Electroplating the entire board for 120 minutes at a current density of 18ASF to thicken the copper layer in the holes and the copper layer on the board surface.

[0026] (7) Fabrication of outer layer circuits (positive film process): The outer layer pattern is transferred using a fully automatic exposure machine and positive film circuit film. The outer layer circuit is exposed using an exposure ruler of 5 to 7 divisions (21 divisions). After development, the outer layer circuit pattern is formed on the multilayer board. The outer layer pattern is electroplated, and then copper and tin are plated on the multilayer board respectively. The electroplating parameters are set according to the required copper thickness. Copper plating is performed at a current density of 1.8 ASD for 60 minutes, and tin plating is performed at a current density of 1.2 ASD for 10 minutes, with a tin thickness of 3 to 5 μm. Then, the film is removed, etched, and tin is removed in sequence to etch the outer layer circuit on the multilayer board. The copper thickness of the outer layer circuit is greater than or equal to 70 μm. For the outer layer AOI, an automatic optical inspection system is used to detect whether there are defects such as open circuits, gaps, incomplete etching, and short circuits in the outer layer circuit by comparing with the CAM data.

[0027] (8) Solder resist and screen printing: After screen printing solder resist ink on the surface of the multilayer board, it undergoes pre-curing, exposure, development and heat curing treatments in sequence to cure the solder resist ink into a solder resist layer, forming thick and thin solder resist layers of varying thicknesses on the multilayer board; specifically, the solder resist ink on the TOP side and the "UL mark" is added to the TOP side characters, thereby coating a layer on the lines and substrate that do not need to be soldered to prevent bridging between lines during soldering, provide a permanent electrical environment and chemical corrosion resistance, and at the same time improve the appearance; the solder resist layer manufacturing process includes the following steps: a. After coating a layer of solder resist ink with a thickness of ≥50μm on the production board (the thickness of this coated ink is generally greater than the ink thickness required for a thick solder resist layer), pre-curing is performed, that is, baking to make the solder resist ink in a semi-cured state.

[0028] In one embodiment, the thickness of the solder resist ink after screen printing is controlled at 70-80 μm.

[0029] In one embodiment, the baking temperature during pre-curing can be selected as 75°C, and the time can be selected according to the specific thickness of the solder resist ink printed on the screen, such as 45 min, 60 min, 90 min, etc. Generally, the thicker the solder resist ink, the longer the baking time, but this is not limited here.

[0030] b. Perform the first exposure process on the production board. During the first exposure, only the area of ​​the production board corresponding to the thick solder mask layer is exposed, while the area corresponding to the thin solder mask layer and other areas that require solder mask openings are not exposed.

[0031] In one embodiment, the exposure scale during the first exposure process is 10-12 levels (21-level exposure scale).

[0032] In one embodiment, during the first exposure process, a strong exposure is performed using 2.0-2.5 times the reference exposure energy of the solder resist ink, thereby achieving high cross-linking of the solder resist ink in the corresponding thick solder resist layer area; for example, when the screen-printed solder resist ink is a green liquid photosensitive ink, the reference exposure energy range of this solder resist ink is 300-600 mJ / cm. 2 The exposure energy range for the first exposure process is 2.0-2.5 times the baseline exposure energy value; for example, when the solder resist ink used for screen printing is a mixed-color liquid photosensitive ink (i.e., non-green liquid photosensitive ink), the baseline exposure energy range for this solder resist ink is 400-800 mJ / cm. 2 The exposure energy range for the first exposure process is 2.0-2.5 times that of the baseline exposure energy value.

[0033] c. The production board is then subjected to a second exposure process. During the second exposure, the areas corresponding to the thick solder resist layer and the thin solder resist layer are exposed simultaneously. Only the areas that need to be solder resisted and opened are not exposed. In this way, the solder resist ink in the thick solder resist layer area has undergone two exposures, resulting in a higher degree of crosslinking. On the other hand, the solder resist ink in the thin solder resist layer area has only undergone one exposure, resulting in a lower degree of crosslinking. This ensures that the degree of crosslinking of the solder resist ink in the thick solder resist layer area is greater than that in the thin solder resist layer area.

[0034] In one embodiment, the exposure scale during the second exposure process is 7-9 levels (21 exposure scales).

[0035] In one embodiment, during the second exposure process, because the ink thickness in the thin solder resist layer area is relatively low, it is only necessary to use 1.3-1.5 times the reference exposure energy of the solder resist ink for exposure; for example, when the solder resist ink used for screen printing is a green liquid photosensitive ink, the reference exposure energy range of the solder resist ink is 300-600 mJ / cm. 2 The exposure energy range for the second exposure process is 1.3-1.5 times the baseline exposure energy value; for example, when the solder resist ink used for screen printing is a mixed-color liquid photosensitive ink (i.e., non-green liquid photosensitive ink), the baseline exposure energy range for this solder resist ink is 400-800 mJ / cm. 2 The exposure energy range for the first exposure process is 1.3 to 1.5 times that of the baseline exposure energy value.

[0036] d. Then, the unexposed solder resist ink is removed by development, and some of the solder resist ink in the thick and thin solder resist areas is dissolved. Because the cross-linking degree of the solder resist ink in the thick solder resist area is greater than that in the thin solder resist area, the dissolution rate of the solder resist ink in the thin solder resist area is greater than that in the thick solder resist area during development. Thus, under the same development conditions and time, thick and thin solder resist layers of different thicknesses are formed on the production board.

[0037] In one embodiment, compared to the existing conventional development process that controls the development point at 40%-60%, this embodiment delays the development point from the standard 40-60% to 80-90%, reducing the slow dissolution rate of the solder resist ink and the need for a longer effective development time to completely develop the ink in the thin solder resist layer area. This avoids excessive dissolution of the ink in the thin solder resist layer area when removing the solder resist ink in the unexposed areas, while ensuring that the ink in the thick areas retains sufficient thickness.

[0038] In one embodiment, the developing solution used during development is Na2CO. 3 The solution contains a developer concentration of 0.8-1.2% (i.e., the effective component of the developer is Na₂CO₃). 3 The mass percentage content is 0.8-1.2%, the temperature of the developing solution is 28-32℃, and the developing time is 60-80s.

[0039] In one embodiment, after development, the thickness of the thick solder resist layer is 35-40 μm, and the thickness of the thin solder resist layer is 15-20 μm.

[0040] e. Finally, the developed solder mask ink is completely cured through a heat curing process, thus completing the fabrication of different solder mask layer thicknesses in different areas of the same PCB.

[0041] (9) Surface treatment (immersion nickel and gold): The copper surface of the solder pads of the solder mask opening position is uniformly deposited with a nickel layer and a gold layer of a certain required thickness through chemical principle. The thickness of the nickel layer is 3-5μm; the thickness of the gold layer is 0.05-0.1μm.

[0042] (10) Electrical test: Test the electrical conductivity of the finished board. The test method used for this board is: flying probe test.

[0043] (11) Molding: Based on existing technology and design requirements, the PCB is made with an external tolerance of + / -0.05mm.

[0044] (12) FQC: Inspect the appearance of the PCB according to the customer's acceptance standards and the applicant's inspection standards. If there are any defects, repair them in time to ensure excellent quality control for the customer.

[0045] (13) FQA: Re-test the appearance of the PCB, the thickness of the hole copper, the thickness of the dielectric layer, the thickness of the green solder mask, the thickness of the inner layer copper, etc. to see if they meet the customer's requirements.

[0046] (14) Packaging: The PCBs are sealed and packaged according to the packaging method and quantity required by the customer, and desiccant and humidity card are placed in the packaging before shipment.

[0047] The technical solutions provided by the embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the embodiments of the present invention. The descriptions of the embodiments above are only for helping to understand the principles of the embodiments of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the embodiments of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for forming solder mask layers of different thicknesses in a PCB, characterized in that, Includes the following steps: S1. After coating the production board with solder resist ink, pre-curing is performed; S2. Perform the first exposure process on the production board. During the first exposure, only the area of ​​the production board corresponding to the thick solder mask layer is exposed. S3. The production board is then exposed a second time. During the second exposure, the areas of the production board corresponding to the thick solder resist layer and the thin solder resist layer are exposed at the same time, so that the cross-linking degree of the solder resist ink in the area corresponding to the thick solder resist layer is greater than that in the area corresponding to the thin solder resist layer. S4. Then, the unexposed solder resist ink is removed by development, and the solder resist ink in the thick solder resist layer area and the thin solder resist layer area is dissolved. The solder resist ink in the thin solder resist layer area dissolves faster than the solder resist ink in the thick solder resist layer area, so as to form thick solder resist layers and thin solder resist layers of different thicknesses on the production board. S5. Finally, the developed solder resist ink is cured by heat curing.

2. The method for forming solder mask layers of different thicknesses in a PCB according to claim 1, characterized in that, In step S2, during the first exposure process, a strong exposure is performed using 2.0-2.5 times the reference exposure energy of the solder resist ink.

3. The method for forming solder mask layers of different thicknesses in a PCB according to claim 2, characterized in that, In step S2, the exposure scale during the first exposure process is 10-12 stops.

4. The method for forming solder mask layers of different thicknesses in a PCB according to claim 1, characterized in that, In step S3, during the second exposure process, the exposure is performed at 1.3-1.5 times the reference exposure energy of the solder resist ink.

5. The method for forming solder mask layers of different thicknesses in a PCB according to claim 4, characterized in that, In step S3, the exposure scale during the second exposure process is 7-9 stops.

6. The method for forming different solder mask layer thicknesses in a PCB according to any one of claims 1-5, characterized in that, In step S4, the development point during development is 80%-90%.

7. The method for forming solder mask layers of different thicknesses in a PCB according to claim 6, characterized in that, In step S4, during development, the concentration of the developer solution used is 0.8-1.2%, the temperature is 28-32℃, and the development time is 60-80s.

8. The method for forming different solder mask layer thicknesses in a PCB according to claim 7, characterized in that, In step S4, after development, the thickness of the thick solder resist layer is 35-40 μm, and the thickness of the thin solder resist layer is 15-20 μm.

9. The method for forming solder mask layers of different thicknesses in a PCB according to claim 1, characterized in that, In step S1, a layer of solder resist ink with a thickness of ≥50μm is coated on the production board and then pre-cured.

10. The method for forming solder mask layers of different thicknesses in a PCB according to claim 1, characterized in that, The production board is a multilayer board in which the inner core board and the outer copper foil are pressed together by a prepreg, and the multilayer board has successively undergone drilling, copper plating, full board electroplating and outer circuit fabrication processes.