Non-alignment-offset through hole exposed hole exposure alignment manufacturing method and application

Abstract

The invention relates to the technical field of PCB (Printed Circuit Board) manufacturing, and discloses a through hole exposed hole exposure alignment manufacturing method without alignment offset, which comprises the following steps of: providing an electroplated PCB (Printed Circuit Board) which is provided with at least one through hole; a first positive dry film is laminated on the C / S surface of the PCB, the photosensitive wavelength of the first positive dry film is 365 nm, the transmittance of the first positive dry film to light with the wavelength of 405 nm is larger than 90%, and no coloring agent is added into the first positive dry film; and laminating a second positive dry film on the S / S surface of the PCB, wherein the photosensitive wavelength of the second positive dry film is 405nm, and the transmittance of the second positive dry film to light with the wavelength of 365nm is greater than 90%. The through hole exposed hole exposure alignment manufacturing method without alignment deviation aims at solving the problems that in a traditional negative dry film alignment exposure method, due to alignment deviation, the orifice windowing position is not accurate, a plating layer in a hole is not uniform, and a plating layer on a plate face is redundant. The through hole exposed hole exposure alignment manufacturing method without alignment deviation aims at solving the problems that in the traditional negative dry film alignment exposure method, due to alignment deviation, the hole windowing position is not accurate.

Description

Technical Field

[0001] This invention relates to the field of PCB manufacturing technology, specifically to a method and application for fabricating exposed vias without alignment offset. Background Technology

[0002] In PCB manufacturing, for products requiring thicker copper layers inside holes while strictly controlling the copper layer thickness on the board surface, a hole-only plating process is typically used. This means that during electroplating, copper is deposited only inside the through-holes, while the surface area remains unplated. To achieve this, a resist pattern needs to be created on the PCB surface, exposing the hole openings while the surface is covered by the resist layer. The traditional method uses negative dry film for exposure and development to create the resist pattern. This involves attaching negative dry film to both sides of the PCB, then using film for alignment and exposure. The hole openings remain unexposed and soluble, while the surface area is exposed and cured to form the resist layer. After development, the hole openings are exposed. This method is widely used in the manufacturing of PCB products such as radar products and high-density interconnect boards, where high copper thickness requirements for holes are necessary.

[0003] However, traditional negative dry film alignment exposure methods suffer from significant alignment accuracy defects. Since PCB boards require film alignment exposure on both the C / S and S / S sides, aligning the film pattern for small-diameter through-holes (e.g., 0.2-0.5mm) is challenging. Even with high-precision alignment equipment, human error and limitations in equipment repeatability can still cause a 0.05-0.15mm offset between the film pattern and the actual hole position. This alignment offset results in the hole opening position not being concentric with the through-hole position after development. The opening edge may intrude into the copper layer on the board surface or fail to fully expose the hole opening, causing uneven plating thickness inside the hole or excess plating on the board surface during subsequent electroplating. This alignment offset is particularly pronounced for micro-through-holes with diameters less than 0.3mm, where the proportion of the hole diameter is even larger, severely impacting product quality and yield, becoming a key technical bottleneck restricting high-precision PCB manufacturing. Summary of the Invention

[0004] The purpose of this invention is to solve the problems of inaccurate aperture opening position, uneven coating inside the aperture, and excess coating on the board surface caused by alignment misalignment in the traditional negative dry film film alignment and exposure method. The invention proposes a through-hole exposure alignment and fabrication method and its application without alignment misalignment.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A method for fabricating exposed vias without alignment offset includes the following steps: S1: Provide a pre-plated PCB board, the PCB board having at least one through hole; S2: A first positive dry film is laminated on the C / S surface of the PCB board. The first positive dry film has a photosensitive wavelength of 365nm and a transmittance of more than 90% for 405nm wavelength light. No colorant is added to the first positive dry film. S3: A second positive dry film is laminated on the S / S surface of the PCB board. The second positive dry film has a photosensitive wavelength of 405nm and a transmittance of more than 90% for 365nm wavelength light. No colorant is added to the second positive dry film. S4: The first exposure is performed from the C / S side using a 405nm wavelength light source, so that the light passes through the first positive dry film and the through hole and then irradiates the second positive dry film in the S / S hole area and a polymerization reaction occurs. S5: A 365nm wavelength light source is used to perform a second exposure from the S / S surface, so that the light passes through the second positive dry film and the through hole and then irradiates the first positive dry film in the C / S hole area and a polymerization reaction occurs. S6: Develop the PCB board after two exposures to remove the polymerized dry film and expose the orifices.

[0006] Based on the above technical solution, the present invention can be further improved as follows.

[0007] Furthermore, in S2, the first positive dry film has a photosensitive peak wavelength of 365 nm, a film thickness of 20-30 μm, and a light absorption coefficient of 0.8-1.2 μm at a wavelength of 365 nm. -1 The transmittance of light at a wavelength of 405nm is greater than 95%. The resin matrix of the first positive dry film is an acrylate polymer, and the photoinitiator is an aromatic ketone compound that is sensitive to a wavelength of 365nm. The second positive dry film in S3 has a photosensitive peak wavelength of 405 nm, a film thickness of 20-30 μm, and a light absorption coefficient of 0.9-1.3 μm at 405 nm. -1 The transmittance of light at a wavelength of 365nm is greater than 95%. The resin matrix of the second positive dry film is a methacrylate polymer, and the photoinitiator is a titanium complex or antimony complex that is sensitive to a wavelength of 405nm. The spectral transmittance curves of the first and second positive dry films exhibit plateau characteristics in their respective non-photosensitive wavelength bands.

[0008] Furthermore, the exposure energy for the first exposure in S4 is 80-120 mJ / cm². 2 The exposure time is 8-15 seconds, and the light source power density is 8-12 mW / cm². 2 The 405nm light source adopts an LED array arrangement, and the output power of a single LED chip is 3-5W; The exposure energy for the second exposure in S5 is 60-100 mJ / cm². 2 The exposure time is 6-12 seconds, and the light source power density is 10-15 mW / cm³. 2 The 365nm light source uses a high-pressure mercury lamp or an LED light source and is equipped with a wavelength filtering filter to allow light in the 365±5nm band to pass through. The energy parameters of the first and second exposures are independently adjusted according to the dry film thickness, aperture diameter, and aperture depth. Energy control is achieved by adjusting the exposure time or light source power to ensure that the light energy density penetrating the aperture and reaching the opposite aperture reaches 1.2-1.8 times the polymerization threshold energy of the dry film on that side.

[0009] Furthermore, in S2 and S3, a hot press roller lamination method is used for dry film lamination, with a lamination temperature of 95-115℃, a lamination pressure of 0.3-0.5MPa, a lamination speed of 1.0-2.0m / min, and the surface of the hot press roller is made of silicone rubber with a hardness of 70-85 Shore A. Before lamination, the PCB board surface is pretreated by micro-etching solution for 30-60 seconds or plasma cleaning for 2-5 minutes to remove surface oxides and oil. Both the first and second positive dry films have a three-layer structure, including a polyester film protective layer, a photosensitive resin layer, and a polyethylene release film. During lamination, the polyethylene release film is removed first and then hot-pressed. After lamination, the polyester film protective layer is retained until the exposure process. After polymerization, the non-photosensitive areas of the first and second positive dry films maintain transmittance to non-photosensitive wavelengths, with transmittance attenuation of less than 5%.

[0010] Furthermore, in step S6, a sodium carbonate aqueous solution with a concentration of 0.8-1.2% is used as the developing solution, the pH value of the developing solution is controlled at 10.5-11.5, the developing temperature is 25-35℃, the spraying pressure is 1.5-2.5 bar, the distance between the nozzle and the plate surface is 80-120 mm, and the developing time is 50-70 seconds. The developing process is carried out using a spray-type developing machine, with two sets of spray systems, one above the other, simultaneously developing the C / S side and the other S / S side of the PCB board. The developer circulation flow rate is 150-250 L / min. After developing, the board is rinsed with deionized water for 20-40 seconds, and the resistivity of the rinsing water is greater than 10 MΩ·cm. Then, hot air drying is performed at a temperature of 60-80℃ for 40-80 seconds. During the developing process, the conductivity of the developer is monitored. When the conductivity rises by more than 20% of the initial value, the developer is replaced with fresh developer. The developing waste liquid is discharged after neutralization treatment.

[0011] Furthermore, in S4 and S5, an LED exposure machine or an LDI laser direct imaging exposure machine is used as the light source. The LED exposure machine is equipped with an LED light source array with a wavelength of 405±5nm and an LED light source array with a wavelength of 365±5nm. The two sets of light sources can be independently controlled in terms of switching and power output. Alternatively, two independent single-wavelength LED exposure machines can be used for the first and second exposures respectively. The exposure machines are equipped with a vacuum adsorption platform with a vacuum degree of -0.06 to -0.08 MPa to ensure that the PCB board is flat and adhered during the exposure process. Both the first and second exposures adopt the whole-board exposure method, with the exposure area covering the entire board surface. There is no need to use film or mask for pattern transfer or pattern alignment. The distance between the exposure light source and the board surface is 300-500 mm, and the uniformity of illumination is controlled within ±5%. The exposure process is carried out in a nitrogen atmosphere or in air, and the temperature of the exposure chamber is controlled at 20-25℃.

[0012] Furthermore, in S1, the PCB board is a semi-finished board that has completed drilling, copper plating, and full-board electroplating processes. The through-hole diameter is 0.2-0.5mm, the through-hole depth is 0.8-2.0mm, and the hole diameter to board thickness ratio is 1:2 to 1:8. The copper layer thickness after full-board electroplating is 15-35μm, and the copper layer thickness on the hole wall is 20-40μm. After electroplating, the PCB board undergoes deburring and surface cleaning. Deburring is performed by brushing or chemical deburring, and surface cleaning involves soaking in an acidic cleaning solution for 3-5 minutes followed by rinsing with deionized water to ensure no residue remains on the board surface or inside the holes. The PCB board is a single-sided, double-sided, or multi-layer board structure, with a substrate of FR-4 epoxy fiberglass board, CEM-1 composite substrate, or polyimide flexible substrate. It is suitable for creating resist plating patterns with opening sizes equal to the hole diameter for subsequent selective electroplating processes within single holes.

[0013] Furthermore, after development in S6, circular openings are formed at the through-hole openings on the C / S and S / S surfaces, respectively. The difference between the diameter of the circular opening and the diameter of the through-hole is controlled within ±0.05mm, and the offset between the center position of the opening and the center position of the through-hole is less than 0.03mm. Except for the area around the through hole opening, the other areas on the board surface retain unpolymerized first and second positive dry films as resist coatings. The thickness of the resist coating is maintained at 18-28μm, the adhesion between the resist coating and the copper surface is greater than 8N / cm, and the surface of the resist coating is flat without bubbles, wrinkles, or pinhole defects. After development, the quality of the window opening is inspected visually or by automatic optical inspection equipment. Check whether the window opening edges are smooth and whether there is any residual film or over-corrosion. The qualified standard is that the window opening edges are neat, the copper surface of the hole is fully exposed, and the resist coating on the board surface is intact and undamaged.

[0014] Furthermore, the method further includes the following steps after S6: S7: The developed PCB board is inspected using an automatic optical inspection device. The inspection includes the diameter of the opening, the offset of the opening position, the integrity of the resist coating, and the quality of the opening edge. The inspection accuracy is ±0.01mm. Defective products are marked and rejected. S8: For PCBs that pass inspection, perform exposed hole electroplating. Place the PCB in an electroplating tank and use a copper sulfate electroplating solution. The solution composition is 60-80 g / L copper sulfate and 180-220 g / L sulfuric acid. Add gloss and leveling agents. The electroplating current density is 1.5-3.0 A / dm³. 2 The electroplating time is calculated based on the required plating thickness. During the electroplating process, the current only enters the through hole through the window area and deposits a copper layer on the hole wall. The plating thickness is 10-25μm. The area of ​​the board surface covered by the resist plating layer does not undergo electroplating reaction, and the increase in copper layer thickness is less than 2μm. S9: After electroplating, use an alkaline stripping solution to remove the resist layer. The stripping solution concentration is 2-4% sodium hydroxide solution, the stripping temperature is 40-60℃, and the stripping time is 3-8 minutes.

[0015] An application of a through-hole exposed via alignment method without alignment offset is disclosed. This method is applied to PCB manufacturing requiring only in-hole plating, including microwave RF boards for radar products, blind via boards, high-density interconnect (HDI) boards, communication boards, automotive electronics boards, and server motherboards. The technical specifications of these PCB products are: 4-16 layers, minimum line width / spacing of 0.075 / 0.075 mm, through-hole diameter of 0.2-0.5 mm, in-hole copper thickness of 25-50 μm, surface copper thickness of 18-35 μm, and the difference between in-hole and surface copper thickness controlled within 10 μm. This method is particularly suitable for products requiring high uniformity of copper thickness within the vias, with copper layer thickness differences of less than 15% at different locations within the vias, and for high-frequency, high-speed circuit boards sensitive to surface copper thickness requiring strict impedance control. Selective in-hole plating avoids impedance offset and signal loss problems caused by excessive surface copper thickness. Compared with the prior art, the technical solution of this application has the following beneficial technical effects: This invention utilizes colorless positive dry films with photosensitive wavelengths of 365nm and 405nm and high transmittance to heterogeneous wavelengths. Combined with step-by-step exposure at corresponding wavelengths, a precise "unidirectional light path" is constructed, conducted through the via itself. This design ensures that 405nm light irradiated from the C / S surface only photosensitively polymerizes the dry film at the opening of the S / S face, without accidentally exposing the dry film on the C / S surface itself. Conversely, 365nm light irradiated from the S / S surface also only acts on the opening area of ​​the C / S face. This physical optical isolation ensures that the exposure pattern is uniquely determined by the position of the via itself, achieving absolute "self-alignment." In principle, this completely eliminates the 0.05-0.15mm alignment offset caused by human operation and equipment positioning. After development, the exposed position of the via is completely concentric with the physical position of the via. This not only perfectly solves the problem of misplating of copper layers on the board surface or incomplete coverage of the via caused by window eccentricity, but also provides a perfect current distribution window for subsequent electroplating, greatly improving the uniformity and consistency of the plating layer within the via. Attached Figure Description

[0016] Figure 1 : A schematic diagram of the PCB semi-finished board structure after drilling, copper plating and full-board electroplating processes have been completed in step S1 of this invention; Figure 2 : A schematic diagram of the process structure for attaching a first positive dry film to the C / S surface using a hot-press roller lamination method in step S2 of the present invention; Figure 3 : A schematic diagram of the process structure for attaching the second positive dry film to the S / S surface in step S3 of the present invention; Figure 4 This is a schematic diagram of the overall structure of the present invention after steps S2 and S3 are completed, in which the C / S surface and S / S surface are respectively covered with different wavelength sensitive dry films. Figure 5 : A schematic diagram of the process principle of using a 405nm light source to perform the first through-hole exposure from the C / S surface in step S4 of the present invention; Figure 6 : A schematic diagram of the process principle of secondary through-hole exposure using a 365nm light source from the S / S surface in step S5 of the present invention; Figure 7 This is a schematic diagram of the structure formed by the exposure of the through hole opening after development in step S6 of the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] In this description, the term "C / S side" refers to the component side, and "S / S side" refers to the solder side; "first positive dry film" and "second positive dry film" are consistent with the names in the claims, and are only used to distinguish dry film types with different spectral response characteristics, and do not limit their suppliers and models.

[0019] like Figures 1 to 7 As shown, this invention provides a through-hole exposure alignment method without alignment misalignment. The overall process can be understood as sequentially performing lamination, dual-wavelength through-hole exposure, development, and subsequent electroplating and film stripping on a pre-finished PCB. Unlike existing methods that "align first, then open," this invention uses light transmission inside the through-hole to achieve self-alignment and polymerization of the dry film near the hole opening, thereby obtaining an exposed hole window the same size as the hole diameter.

[0020] In a typical process, the process sequence can be summarized as follows: PCB board with pre-process completed → pre-treatment drying → C / S side pressing of the first dry film → S / S side pressing of the second dry film → C / S side 405nm exposure → S / S side 365nm exposure → development → optical inspection → exposed hole electroplating → film peeling → subsequent patterning / etching processes.

[0021] Step S1: Preparation of PCB substrate Before implementing the method of the present invention, a PCB board with through-hole electroplating completed is first provided, and at least one through hole is provided on the board.

[0022] In a preferred embodiment, such as Figure 1 As shown, the PCB board has undergone drilling, chemical copper plating, and full-board electroplating processes in sequence. The through-hole diameter can be selected as 0.3mm; more generally, the hole diameter range can be arranged between 0.2 and 0.5mm, and the ratio of board thickness to hole diameter is controlled at approximately 1:2 to 1:8, corresponding to a hole depth of, for example, 0.8 to 2.0mm. After the full-board electroplating is completed, the copper layer thickness on the board surface can be controlled at approximately 15 to 35μm, and the copper layer thickness on the hole wall can be controlled at approximately 20 to 40μm, so as to reserve space for thickening in subsequent "hole-only" electroplating.

[0023] To ensure the quality of dry film bonding, the electroplated PCB board is preferably deburred mechanically or chemically, then soaked in an acidic cleaning solution for 3-5 minutes, thoroughly rinsed with deionized water, and finally dried in an oven or hot air device to ensure that the board surface and holes are basically free of oil and moisture residue. The substrate can be FR-4 epoxy fiberglass board, CEM-1 composite substrate, or polyimide flexible substrate, and the number of layers can be 4-16 to adapt to various product structures such as radar boards, HDI boards, and communication boards.

[0024] Step S2: Lamination of the first positive dry film on the C / S side After the pre-processing is completed, a first positive dry film is applied to the C / S side of the PCB. This dry film is designed to primarily respond to ultraviolet wavelengths around 365nm, while maintaining high transmittance in the 405nm band, and no colorant is added to the formulation so that subsequent light can penetrate to the vias and the dry film on the opposite side.

[0025] Under a specific set of parameters, a positive photoresist dry film with an acrylate polymer as the resin matrix can be selected. Its photosensitive peak wavelength is controlled at approximately 365 nm, and the thickness of the photosensitive resin layer is typically set to 20–30 μm. The light absorption coefficient in the 365 nm band is approximately 0.8–1.2 μm⁻ 1 The transmittance in the 405nm band is not less than 95%. The photoinitiation system can use aromatic ketone initiators that are sensitive to 365nm ultraviolet light, so that the dry film forms an obvious absorption peak near 365nm, while the transmittance curve in non-photosensitive bands (such as near 405nm) is plateau-shaped.

[0026] like Figure 2 As shown, the lamination process preferably uses hot roller lamination: the roller temperature is set at approximately 95–115℃, the roller linear pressure is approximately 0.3–0.5 MPa, and the conveying speed is approximately 1.0–2.0 m / min. The elastic coating layer of the roller can be made of silicone rubber with a hardness between 70 and 85 Shore A, to balance the bonding pressure and adaptability to slight undulations on the board surface.

[0027] The first positive dry film typically has a three-layer structure: an outer polyester (PET) protective film, a middle photosensitive resin layer, and an inner polyethylene release film. During lamination, the polyethylene release film is peeled off first, allowing the photosensitive resin layer to be directly pressed onto the copper layer of the PCBC / S surface by hot press rollers. After lamination, the PET protective film remains on the outside of the dry film until it is peeled off before entering the exposure process to prevent scratches or contamination during handling.

[0028] In the implementation of the present invention, when the first positive dry film is not irradiated under its photosensitive peak wavelength band, even after undergoing subsequent baking or exposure processes, its unexposed area still maintains high light transmittance, and its transmittance attenuation in the non-photosensitive wavelength band is preferably controlled within 5% to ensure energy transmission of subsequent through-hole light irradiation.

[0029] Step S3: Lamination of the second positive dry film on the S / S side Subsequently, a second positive dry film is applied to the S / S side of the PCB board. This dry film mainly responds to photopolymerization in the 405nm ultraviolet light band, while allowing 365nm light to pass through, and also does not contain colorants.

[0030] like Figure 3As shown, in one embodiment, a positive dry film with a methacrylate polymer as the resin matrix can be used, with a photosensitivity peak at approximately 405 nm. The thickness of the photosensitive layer can also be set to 20–30 μm, and the light absorption coefficient at 405 nm is approximately 0.9–1.3 μm⁻. 1 The transmittance in the 365nm band is not less than 95%. The photoinitiator system can be selected from titanium complexes or antimony complexes that have an absorption peak at a wavelength of 405nm, so that the absorption of 365nm light by the dry film is significantly reduced, and the corresponding spectral transmittance in this non-photosensitive band is approximately a plateau curve.

[0031] The lamination process conditions for the second dry film can be the same as or slightly adjusted from those for the first dry film. It also uses a hot-press roller structure and a three-layer dry film structure. First, the inner release film is removed, then it is attached to the S / S copper layer, while the outer PET protective film is temporarily retained. After lamination, as follows... Figure 4 As shown, the C / S and S / S surfaces are respectively covered with positive dry film systems that are sensitive to different wavelengths and are nearly transparent in each other's wavelength bands. Both dry films can still transmit a considerable proportion of light in areas not irradiated by the corresponding wavelength, providing conditions for subsequent "through-hole dual-wavelength exposure".

[0032] Step S4: First exposure of 405nm C / S plane After the dry film lamination is completed and the C / S side PET protective film is removed, the first full-plate exposure is performed on the C / S side using a light source with a main wavelength of approximately 405nm.

[0033] like Figure 5 As shown, in the preferred embodiment, an exposure machine or LDI laser direct imaging device with a 405±5nm LED array light source can be selected. The power of a single LED chip in the LED array can be 3-5W, and uniform illumination is formed through a lens or optical homogenization system, with the uniformity of illumination on the board surface controlled within ±5%. The distance between the light source and the board surface can be arranged at approximately 300-500mm, and the exposure platform uses a vacuum adsorption method to fix the PCB board, with the vacuum degree controlled at -0.06 to -0.08MPa to ensure flatness and adhesion of the board surface during the exposure process.

[0034] Under specific parameters, the energy density of the first exposure can be set to 80–120 mJ / cm². 2 The corresponding exposure time is approximately 8–15 seconds, and the power density is approximately 8–12 mW / cm³. 2The first positive dry film on the C / S surface has virtually no absorption in the 405nm wavelength range. Therefore, this path of light can pass through the first dry film with high transmittance and continue to propagate along the metallized walls or air channels within the via. When the light reaches the via exit area of ​​the S / S surface, the second positive dry film irradiating that area, due to its significant absorption peak for 405nm light, can induce photopolymerization at a precise location around the via opening, forming a cross-linked and cured dry film layer in this annular region. Meanwhile, the large area of ​​the S / S surface dry film outside the via remains unexposed or weakly exposed.

[0035] During process design, the light attenuation inside the hole can be calculated based on the thickness of the second dry film, the diameter of the through hole, and the thickness of the plate. By adjusting the first exposure time or the power of the light source, the effective energy density reaching the opening of the S / S hole can be made slightly higher than the polymerization threshold of the second dry film, for example, controlled at about 1.2 to 1.8 times the energy of this threshold, to ensure that the polymerization at the opening is sufficient while the area outside the through hole still retains the unpolymerized characteristics.

[0036] Step S5: Second exposure at 365nm on the S / S plane After the first exposure is completed and the S / S side PET protective film is removed, a second full-plate exposure is performed using a light source with a main wavelength of approximately 365nm on the S / S side.

[0037] like Figure 6 As shown, in one embodiment, the second exposure can use an LED exposure machine with a wavelength of 365±5nm, or a high-pressure mercury lamp light source equipped with a bandpass filter, to concentrate the output spectrum around 365nm. The plate surface can also be fixed using a vacuum adsorption platform to maintain similar illumination uniformity and geometric accuracy as the first exposure.

[0038] Under a set of practical parameters, the secondary exposure energy density can be set to approximately 60–100 mJ / cm². 2 The corresponding exposure time is approximately 6–12 seconds, and the power density of the board surface is 10–15 mW / cm². 2 Since the second positive dry film on the S / S surface has virtually no absorption in the 365nm wavelength range, light from the S / S surface can directly pass through the second dry film and enter the via, reaching the C / S surface aperture region. In this region, the first positive dry film on the C / S surface has strong absorption of 365nm light and can act, causing photopolymerization to occur in the annular dry film region corresponding to the via aperture, while other non-corresponding aperture positions on the C / S surface remain unpolymerized or weakly polymerized.

[0039] Similarly, the secondary exposure parameters are independently set in conjunction with the first dry film thickness, via size, and plate thickness, ensuring that the effective dose reaching the C / S face hole exceeds the polymerization initiation energy of the first dry film by a certain proportion, while the area outside the via maintains the dissolution characteristics expected of a positive dry film. Through this method of "passing through the non-photosensitive band dry film + via on one side, and then locally polymerizing on the photosensitive dry film on the other side," two congruent polymerized dry film rings are formed around the C / S and S / S faces around their respective holes.

[0040] In this method, both exposures are performed with full-plate coverage. There is no need to create window patterns that correspond one-to-one with the hole positions, nor is there a need to perform mechanical alignment operations on the equipment. The self-aligned exposure of the through hole can be completed simply by controlling the output and timing of the two wavelength light sources.

[0041] Step S6: Development and Hole Formation After double-sided exposure at different wavelengths, the exposed PCB board is sent to the developing process, where the dry film areas that need to be removed are dissolved by an alkaline solution, exposing the through holes.

[0042] like Figure 7 As shown, under typical conditions, a 0.8–1.2 wt% sodium carbonate aqueous solution can be used as the developer, with the solution pH controlled at 10.5–11.5 and the developing temperature at approximately 25–35°C. The developing equipment can employ a dual-head spray structure, simultaneously spraying and developing both the C / S and S / S surfaces. The spray pressure is set to 1.5–2.5 bar, the nozzle-to-plate distance is approximately 80–120 mm, the residence time is approximately 50–70 s, and the circulation flow rate is within the range of 150–250 L / min.

[0043] After development, the plate surface is rinsed with deionized water for 20–40 seconds. The resistivity of the rinsing water is preferably higher than 10 MΩ·cm to reduce ion residue. Then, it is dried by hot air at 60–80°C for 40–80 seconds to ensure the plate surface is dry and clean. During the production process, the conductivity of the developer can be monitored online. When the conductivity increases by more than a certain percentage (e.g., 20%) relative to the initial value, the developer is replaced with a new one, and the waste liquid is neutralized before being discharged in compliance with standards.

[0044] In a positive dry film system, fully exposed areas preferentially dissolve in the developer, while insufficiently exposed or unexposed areas retain the pattern. This invention, through two through-hole exposures, ensures that small areas near the through-hole openings on both the C / S and S / S surfaces are fully irradiated at the corresponding wavelengths, and its development behavior is inversely related to that of large-area panels. After development, an opening structure matching the metallized hole wall size is obtained at the through-hole opening location, while large areas on the panel unrelated to the holes retain a complete dry film layer, serving as a resist layer for subsequent "hole-only" electroplating.

[0045] Under preferred implementation conditions, the aperture openings on the C / S and S / S surfaces after development can be made into a basic circle, with the deviation between its diameter and the measured through-hole diameter controlled within ±0.05mm. According to the AOI or measurement data of the board surface, the offset between the geometric center of the opening and the center of the corresponding hole can be controlled within approximately 0.03mm. The remaining thickness of the dry film resist coating on the board surface can be controlled within 18–28μm, and the adhesion to the copper surface is not less than 8N / cm, with no visible bubbles, wrinkles, or pinholes on the surface.

[0046] After development and drying, an optical inspection step is provided in a preferred embodiment, equivalent to the subsequent inspection process in the claims. An automated optical inspection (AOI) device can be used to inspect the aperture diameter, aperture position deviation, resist coating integrity, and aperture edge quality board by board, with a measurement accuracy of ±0.01mm. Boards with apertures significantly deviating from the through-hole position or exhibiting defects such as over-development or residual film are marked and rejected to ensure that the aperture geometry of boards entering the exposed-hole electroplating process meets design requirements.

[0047] Exposed hole electroplating and film stripping process After development and optical inspection are completed and the exposed state of the aperture is confirmed to meet the requirements, the exposed aperture electroplating and film stripping steps can be added to this method according to the needs of the aperture-only plating process.

[0048] In one embodiment, the PCB board that has passed inspection is first clamped onto an electroplating fixture, ensuring that the opening areas at both ends of the through-holes are fully exposed to the electroplating solution, while most of the board surface remains covered by a dry film. A copper sulfate electroplating solution is prepared in the electroplating tank, with the copper sulfate concentration controlled at 60–80 g / L and the sulfuric acid concentration at approximately 180–220 g / L. Appropriate amounts of brighteners, leveling agents, and other organic additives are added as needed. The electroplating current density can be set to 1.5–3.0 A / dm³. 2 The electroplating time is calculated based on the target copper layer thickness (e.g., 10–25 μm).

[0049] Because the board surface is completely covered by the residual positive dry film, the copper surface at the via opening forms a current path with the electrolyte and electrodes. Therefore, the current flows almost exclusively through the inner wall of the via, depositing an additional copper layer on the via wall. The increase in the thickness of the copper layer on the board surface covered by the dry film can be limited to within 2μm. The copper thickness difference at different locations inside the via (inlet, middle, and outlet) can be controlled to within approximately 15% by adjusting the stirring method, current density, and fixture structure to meet the requirements of high-reliability products for uniform copper thickness in the vias.

[0050] After electroplating, the PCB board can be transferred to an alkaline stripping bath for dry film removal. The stripping solution can be a 2-4 wt% sodium hydroxide aqueous solution, applied at 40-60°C for 3-8 minutes to saponify the photosensitive resin layer and peel it off from the copper surface. After stripping, the board is rinsed again with deionized water and dried, resulting in a semi-finished PCB where the copper layer is thickened only on the inner walls of the through-holes while the board surface maintains essentially the original electroplated copper thickness. This provides a good foundation for subsequent circuit etching and impedance control.

[0051] Without departing from the core idea of ​​this invention, the above process parameters and material selections can be appropriately adjusted: Dry film material replacement: The first positive dry film is not limited to a specific commercial model, as long as it has a significant absorption peak at approximately 365 nm, high transmittance around 405 nm, and does not contain colorants that significantly affect transmittance; the second positive dry film can also be any other positive resist dry film with a response center close to 405 nm, requiring high transmittance around 365 nm. The resin matrix can be replaced by other acrylate or methacrylate copolymer systems, as long as they can form spectral characteristics compatible with this invention.

[0052] Light source and energy configuration: In addition to using LED arrays or LDI, mercury lamp exposure machines with narrow-band filtering systems can also be used on some production lines to achieve the same wavelength range of irradiation conditions. As long as a light source with a main wavelength concentrated around 405nm is used on the C / S side and a light source with a main wavelength concentrated around 365nm is used on the S / S side, and the total energy and illuminance distribution can be adjusted so that the dry film dose at the aperture meets the corresponding polymerization threshold requirements, the "through-hole self-aligned exposure" effect of this invention can be achieved.

[0053] Plate material and aperture range: For products with higher layer counts or thicker boards, the via diameter and exposure energy can be adjusted based on the specific board thickness, ensuring that sufficient energy is still output at the via opening even after light attenuates inside the via. For example, in multilayer backplane products, the via diameter can be increased to over 0.5mm, and the board thickness can be greater than 2.0mm. Accordingly, the light loss inside the via can be compensated by increasing the power density of the light source or extending the exposure time.

[0054] This paper discloses an application of a through-hole exposed via alignment method without alignment offset. The method is applicable to PCB manufacturing requiring only in-hole plating, including microwave RF boards for radar products, blind via boards, high-density interconnect (HDI) boards, communication boards, automotive electronics boards, and server motherboards. The technical specifications for these PCB products are: 4-16 layers, minimum line width / spacing of 0.075 / 0.075 mm, through-hole diameter of 0.2-0.5 mm, in-hole copper thickness of 25-50 μm, and surface copper thickness of 18-35 μm, with the difference between in-hole and surface copper thickness controlled within 10 μm. This method is particularly suitable for products requiring high uniformity of copper thickness within the vias, with copper layer thickness differences of less than 15% at different locations within the vias, and for high-frequency, high-speed circuit boards sensitive to surface copper thickness requiring strict impedance control. By selectively plating within the vias, it avoids impedance offset and signal loss problems caused by excessive surface copper thickness. In a class of high-frequency, high-speed products, the method of this invention can be applied to microwave RF boards used in radar front-end modules. Typical specifications include an 8-12 layer structure, a minimum linewidth / spacing of approximately 0.075 / 0.075 mm, a via diameter of approximately 0.25-0.30 mm, and a total thickness of 25-50 μm after copper plating in the vias, while the surface copper thickness needs to be controlled at 18-35 μm to meet impedance control requirements. Using the process of this invention, only the inner walls of the vias are additionally electroplated for thickening, with almost no increase in the surface copper thickness, which is beneficial for subsequent fine linewidth etching and impedance consistency control.

[0055] In another type of high-density interconnect (HDI) board or communication motherboard, this invention can also be used in "hole-only" scenarios. In typical products, the thickness difference between the copper in the hole and the copper on the surface can be controlled within 10μm, and the copper thickness difference at various locations within the hole can be controlled within about 15%, thus adapting to applications such as server motherboards and automotive electronic control boards where the reliability of the hole copper and the uniformity of the board surface are more stringent.

[0056] In summary, this invention, by placing positive dry films sensitive to different wavelengths and transparent to each other on both sides of the PCB, and combining this with two whole-board through-hole exposures performed by light sources from different sides and different wavelengths, allows the dry film at the hole opening to automatically undergo photopolymerization at the hole position without mechanical alignment, thereby obtaining an exposed hole window with the same size as the hole diameter and a high degree of positional overlap. Combined with alkaline development, exposed hole electroplating, and subsequent film stripping steps, this invention realizes a method for creating through-hole exposed holes without alignment offset, suitable for special processes such as "hole-only plating".

[0057] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0058] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for fabricating exposed vias without alignment misalignment, characterized in that, Includes the following steps: S1: Provide a pre-plated PCB board, the PCB board having at least one through hole; S2: A first positive dry film is laminated on the C / S surface of the PCB board. The first positive dry film has a photosensitive wavelength of 365nm and a transmittance of more than 90% for 405nm wavelength light. No colorant is added to the first positive dry film. S3: A second positive dry film is laminated on the S / S surface of the PCB board. The second positive dry film has a photosensitive wavelength of 405nm and a transmittance of more than 90% for 365nm wavelength light. No colorant is added to the second positive dry film. S4: The first exposure is performed from the C / S side using a 405nm wavelength light source, so that the light passes through the first positive dry film and the through hole and then irradiates the second positive dry film in the S / S hole area and a polymerization reaction occurs. S5: A 365nm wavelength light source is used to perform a second exposure from the S / S surface, so that the light passes through the second positive dry film and the through hole and then irradiates the first positive dry film in the C / S hole area and a polymerization reaction occurs. S6: Develop the PCB board after two exposures to remove the polymerized dry film and expose the orifices.

2. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, The first positive dry film in S2 has a photosensitive peak wavelength of 365 nm, a film thickness of 20-30 μm, and a light absorption coefficient of 0.8-1.2 μm at 365 nm. -1 The transmittance of light at a wavelength of 405nm is greater than 95%. The resin matrix of the first positive dry film is an acrylate polymer, and the photoinitiator is an aromatic ketone compound that is sensitive to a wavelength of 365nm. The second positive dry film in S3 has a photosensitive peak wavelength of 405 nm, a film thickness of 20-30 μm, and a light absorption coefficient of 0.9-1.3 μm at 405 nm. -1 The transmittance of light at a wavelength of 365nm is greater than 95%. The resin matrix of the second positive dry film is a methacrylate polymer, and the photoinitiator is a titanium complex or antimony complex that is sensitive to a wavelength of 405nm. The spectral transmittance curves of the first and second positive dry films exhibit plateau characteristics in their respective non-photosensitive wavelength bands.

3. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, The exposure energy for the first exposure in S4 is 80-120 mJ / cm². 2 The exposure time is 8-15 seconds, and the light source power density is 8-12 mW / cm². 2 The 405nm light source adopts an LED array arrangement, and the output power of a single LED chip is 3-5W; The exposure energy for the second exposure in S5 is 60-100 mJ / cm². 2 The exposure time is 6-12 seconds, and the light source power density is 10-15 mW / cm³. 2 The 365nm light source uses a high-pressure mercury lamp or an LED light source and is equipped with a wavelength filtering filter to allow light in the 365±5nm band to pass through. The energy parameters of the first and second exposures are independently adjusted according to the dry film thickness, aperture diameter, and aperture depth. Energy control is achieved by adjusting the exposure time or light source power to ensure that the light energy density penetrating the aperture and reaching the opposite aperture reaches 1.2-1.8 times the polymerization threshold energy of the dry film on that side.

4. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, In S2 and S3, dry film lamination is performed using a hot press roller lamination method. The lamination temperature is 95-115℃, the lamination pressure is 0.3-0.5MPa, the lamination speed is 1.0-2.0m / min, and the surface of the hot press roller is made of silicone rubber with a hardness of 70-85 Shore A. Before lamination, the PCB board surface is pretreated by micro-etching solution for 30-60 seconds or plasma cleaning for 2-5 minutes to remove surface oxides and oil. Both the first and second positive dry films have a three-layer structure, including a polyester film protective layer, a photosensitive resin layer, and a polyethylene release film. During lamination, the polyethylene release film is removed first and then hot-pressed. After lamination, the polyester film protective layer is retained until the exposure process. After polymerization, the non-photosensitive areas of the first and second positive dry films maintain transmittance to non-photosensitive wavelengths, with transmittance attenuation of less than 5%.

5. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, In step S6, a sodium carbonate aqueous solution with a concentration of 0.8-1.2% is used as the developing solution. The pH value of the developing solution is controlled at 10.5-11.5, the developing temperature is 25-35℃, the spraying pressure is 1.5-2.5 bar, the distance between the nozzle and the plate surface is 80-120 mm, and the developing time is 50-70 seconds. The developing process is carried out using a spray-type developing machine, with two sets of spray systems, one above the other, simultaneously developing the C / S side and the other S / S side of the PCB board. The developer circulation flow rate is 150-250 L / min. After developing, the board is rinsed with deionized water for 20-40 seconds, and the resistivity of the rinsing water is greater than 10 MΩ·cm. Then, hot air drying is performed at a temperature of 60-80℃ for 40-80 seconds. During the developing process, the conductivity of the developer is monitored. When the conductivity rises by more than 20% of the initial value, the developer is replaced with fresh developer. The developing waste liquid is discharged after neutralization treatment.

6. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, In S4 and S5, an LED exposure machine or an LDI laser direct imaging exposure machine is used as the light source. The LED exposure machine is equipped with an LED light source array with a wavelength of 405±5nm and an LED light source array with a wavelength of 365±5nm. The two sets of light sources can be independently controlled in terms of switching and power output. Alternatively, two independent single-wavelength LED exposure machines can be used for the first and second exposures respectively. The exposure machines are equipped with a vacuum adsorption platform with a vacuum degree of -0.06 to -0.08 MPa to ensure that the PCB board is flat and adhered during the exposure process. Both the first and second exposures adopt the whole-board exposure method, with the exposure area covering the entire board surface. There is no need to use film or mask for pattern transfer or pattern alignment. The distance between the exposure light source and the board surface is 300-500 mm, and the uniformity of illumination is controlled within ±5%. The exposure process is carried out in a nitrogen atmosphere or in air, and the temperature of the exposure chamber is controlled at 20-25℃.

7. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, The PCB board in S1 is a semi-finished board that has completed drilling, copper plating, and full-board electroplating processes. The through-hole diameter is 0.2-0.5mm, the through-hole depth is 0.8-2.0mm, and the hole diameter to board thickness ratio is 1:2 to 1:

8. The copper layer thickness after full-board electroplating is 15-35μm, and the copper layer thickness on the hole wall is 20-40μm. After electroplating, the PCB board undergoes deburring and surface cleaning. Deburring is performed by brushing or chemical deburring, and surface cleaning involves soaking in an acidic cleaning solution for 3-5 minutes followed by rinsing with deionized water to ensure no residue remains on the board surface or inside the holes. The PCB board is a single-sided, double-sided, or multi-layer board structure, with a substrate of FR-4 epoxy fiberglass board, CEM-1 composite substrate, or polyimide flexible substrate. It is suitable for creating resist plating patterns with opening sizes equal to the hole diameter for subsequent selective electroplating processes within single holes.

8. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, After development by S6, circular openings are formed at the through-hole openings on the C / S and S / S surfaces, respectively. The difference between the diameter of the circular opening and the diameter of the through-hole is controlled within ±0.05mm, and the offset between the center position of the opening and the center position of the through-hole is less than 0.03mm. Except for the area around the through hole opening, the other areas on the board surface retain unpolymerized first and second positive dry films as resist coatings. The thickness of the resist coating is maintained at 18-28μm, the adhesion between the resist coating and the copper surface is greater than 8N / cm, and the surface of the resist coating is flat without bubbles, wrinkles, or pinhole defects. After development, the quality of the window opening is inspected visually or by automatic optical inspection equipment. Check whether the window opening edges are smooth and whether there is any residual film or over-corrosion. The qualified standard is that the window opening edges are neat, the copper surface of the hole is fully exposed, and the resist coating on the board surface is intact and undamaged.

9. The method for fabricating exposed through-holes without alignment offset according to claim 1, characterized in that, The method further includes the following steps after S6: S7: The developed PCB board is inspected using an automatic optical inspection device. The inspection includes the diameter of the opening, the offset of the opening position, the integrity of the resist coating, and the quality of the opening edge. The inspection accuracy is ±0.01mm. Defective products are marked and rejected. S8: For PCBs that pass inspection, perform exposed hole electroplating. Place the PCB in an electroplating tank and use a copper sulfate electroplating solution. The solution composition is 60-80 g / L copper sulfate and 180-220 g / L sulfuric acid. Add gloss and leveling agents. The electroplating current density is 1.5-3.0 A / dm³. 2 The electroplating time is calculated based on the required plating thickness. During the electroplating process, the current only enters the through hole through the window area and deposits a copper layer on the hole wall. The plating thickness is 10-25μm. The area of ​​the board surface covered by the resist plating layer does not undergo electroplating reaction, and the increase in copper layer thickness is less than 2μm. S9: After electroplating, use an alkaline stripping solution to remove the resist layer. The stripping solution concentration is 2-4% sodium hydroxide solution, the stripping temperature is 40-60℃, and the stripping time is 3-8 minutes.

10. The application of the through-hole exposure alignment fabrication method without alignment offset according to any one of claims 1-9, characterized in that, The method is applied to PCB manufacturing that requires only in-hole plating, including the fabrication of microwave RF boards for radar products, blind via boards, high-density interconnect (HDI) boards, communication boards, automotive electronic boards, and server motherboards. The technical specifications of these PCB products are: 4-16 layers, minimum line width / spacing of 0.075 / 0.075 mm, via diameter of 0.2-0.5 mm, in-hole copper thickness of 25-50 μm, surface copper thickness of 18-35 μm, and the difference between in-hole and surface copper thickness controlled within 10 μm. This method is particularly suitable for products requiring high uniformity of copper thickness within the vias, with copper layer thickness differences of less than 15% at different locations within the vias, and for high-frequency, high-speed circuit boards that are sensitive to surface copper thickness and require strict impedance control. Selective in-hole plating avoids impedance shift and signal loss problems caused by excessive surface copper thickness.

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