Laser blind hole plate solder mask manufacturing method and PCB

Abstract

The application discloses a laser blind hole plate solder mask manufacturing method and a PCB. The method first carries out chemical pretreatment and low-temperature baking on the laser blind hole plate, then fills the blind hole with high-viscosity hole-filling ink, forms a double-layer solder mask structure through twice silk printing after standing, the twice silk printing can further fill the inside of the laser blind hole, the laser blind hole plate needs to be pre-baked after the twice silk printing, and finally a uniform solder mask layer is formed on the surface of the laser blind hole plate. In the manufacturing process, the opening aperture of the aluminum sheet is optimized, the expansion and shrinkage coefficient is compensated, and the hole-filling screen plate design is optimized. Four-stage step heating is adopted in the curing stage. The ink viscosity, standing time, glue scraping direction and angle are optimized and controlled, and the problem of copper exposure caused by the thin oil in the inside of the laser blind hole and the hole edge due to air dissipation in the laser blind hole is solved.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board manufacturing technology, and more specifically, to a method for fabricating solder mask on a laser-etched blind hole board and a PCB board. Background Technology

[0002] With the continuous development of the PCB industry, significant breakthroughs have been achieved in fields such as automobiles, medical devices, aerospace, and servers. Taking the automotive sector as an example, the rapid development of new energy vehicles, compared to traditional gasoline vehicles, places extremely high demands on perception accuracy with their core intelligent driving technology. Millimeter-wave radar is the core of intelligent driving systems, directly driving the generational upgrade of radar technology. Over the past decade, automotive millimeter-wave radar has fully transitioned from the early 24GHz band to the 77GHz band. 77GHz radar is smaller and has higher resolution, but it has extremely stringent requirements for PCB signal loss. At such a high frequency of 77GHz, traditional vias radiate energy like antennas due to the residual stub effect, interfering with the signal. Laser-drilled blind vias, on the other hand, connect directly from the surface layer to the inner layer, eliminating residual stubs and minimizing signal reflection and insertion loss. Currently, 5mil laser-drilled blind vias are the best engineering solution for achieving high performance and low-cost mass production of 77GHz radar, and also a standard capability for high-end manufacturing in PCB plants.

[0003] In existing technologies, the solder mask manufacturing process for laser blind vias typically involves single or double screen printing. Because a 5mil laser blind via has a diameter of 5mil and a depth of over 5mil, the small diameter and deep depth inevitably lead to air inclusions in the ink during screen printing, severely impacting the solder mask manufacturing quality. (See attached manual.) Figure 1 As shown, the red circle indicates a laser via with exposed copper. The arrow points to a magnified cross-section of the problematic laser via. The circles in the image represent air inside the laser via. Because laser vias are semi-hole structures, they are hollow and contain air. During the solder mask printing process, the ink above the laser via can escape due to the air, causing the ink to break. This prevents the formation of a uniform cover ink and solder mask layer on the board surface at the laser via, resulting in thin ink inside and around the laser via, leading to exposed copper. This poses a quality risk of short circuits during soldering at the client's location. Therefore, solving the problem of thin ink in laser vias is a major challenge currently faced by PCB manufacturers. Summary of the Invention

[0004] The present invention aims to overcome at least one of the defects of the prior art and provides a method for manufacturing solder mask on a laser blind hole board and a PCB board, so as to solve the technical problem of thin solder mask inside and at the edge of the laser blind hole due to air leakage, resulting in exposed copper.

[0005] The method for fabricating solder resist for laser-etched blind vias described in this application involves filling the inside of the laser-etched blind vias using a plugging process. After the plugging process is completed, the laser-etched blind vias are left to stand. Then, a first solder resist layer is formed on the surface of the laser-etched blind vias using a first screen printing process. After the first screen printing, the laser-etched blind vias are left to stand, and the ink portion of the first solder resist layer flows into the inside of the laser-etched blind vias. Finally, a second solder resist layer is formed on the basis of the first solder resist layer using a second screen printing process. After the second screen printing, the laser-etched blind vias are left to stand.

[0006] As a pre-filling process, via plugging pre-extracts ink into the interior of laser-sealed blind vias. After the plugging process, some residual air inside the blind vias will escape, causing holes and ink to burst around the vias. Allowing the vias to stand after plugging allows the ink to initially level within the blind vias and slowly expels any remaining air, preventing subsequent screen printing from causing air leakage and ink breakage. The first solder mask layer formed during the first screen printing forms a basic solder mask film on the board surface and further fills the interior of the blind vias using the ink's fluidity. The standing time after the first screen printing allows some of the air remaining inside the blind vias to be released further, allowing the ink to fully penetrate the ink gaps created by the post-plugging process. Simultaneously, the ink is flattened during plugging, and because some ink from the edges of the blind vias flows into them after the first screen printing, thin ink may appear at the edges, leading to exposed copper. The second solder resist layer, formed by the second screen printing, is a supplementary printing process based on the semi-cured first solder resist layer. This precisely fills the thin ink areas at the edges of the laser blind vias after the first screen printing. If ink gaps appear after the first screen printing has settled, the ink from the second screen printing can further fill the interior of the laser blind vias, ultimately achieving full filling and complete coverage of the laser blind via edges. The settling period after the second screen printing further allows the ink on the laser blind via surface to level out, ensuring uniform ink thickness across the board. This three-layer filling structure, combining via plugging and two screen printing processes, solves the problem of ink cracking caused by air leakage from the laser blind vias and avoids the problem of excessive ink thickness on the board surface caused by a single thick print. It reduces residual air within the laser blind vias at the source, gradually filling the internal cavities and ensuring uniform ink coverage on the board surface, while strictly controlling the overall ink thickness to meet high-precision mounting requirements.

[0007] Furthermore, after the via plugging process, the ink filling rate inside the laser blind via after standing is at least greater than 90%, and the thickness of the first solder resist layer is less than the thickness of the second solder resist layer.

[0008] Because laser-etched blind vias have small apertures, screen printing targets the entire surface of the via, making it impossible for ink to completely fill the interior. The via-filling process, however, only fills the vias with ink, resulting in better filling. After via-filling, air leakage reduces the ink fill rate inside the vias. After via-filling and resting, the ink fill rate inside the vias is at least 90%, ensuring initial filling while leaving only a portion of unfilled space due to air leakage. This allows sufficient space for air leakage while ensuring the first screen printing further fills this space, preventing incomplete filling and poor filling results after screen printing due to insufficient ink fill after via-filling and resting. A fill rate of over 90% also means that most cavities within the laser-etched blind vias are occupied by ink, with low residual air volume. This prevents large amounts of air from breaking through the surface ink during subsequent screen printing, providing a foundation for complete ink coverage. The thickness of the first solder mask layer is relatively smaller than that of the second solder mask layer because there are still some cavities inside the laser blind vias during the first screen printing, allowing ink to seep into them. Simultaneously, the thinner printing thickness allows the ink to flow more smoothly into the laser blind vias, filling the gaps after the via-filling process and preventing excessive ink buildup at the edges, which could lead to subsequent sagging. The second solder mask layer is relatively thicker because the laser blind vias are already largely filled after the first screen printing. The ink in the second screen printing is mainly used to cover the edges of the vias and supplement the thickness of the laser blind vias on the board surface, ensuring the overall insulation performance and coverage integrity of the solder mask layer. This differentiated design of the via-filling process and the ink thickness of the two screen printing processes balances the filling effect inside the laser blind vias with the uniformity of the surface ink thickness.

[0009] Furthermore, the settling time after the plugging process is 40% to 60% of the settling time after the first screen printing, and the settling time after the first screen printing is the same as the settling time after the second screen printing.

[0010] After the via plugging process, the ink inside the laser blind vias is under pressure, causing internal air to escape quickly. This can easily lead to ink breakage at the via opening, where the ink scatters around the laser blind via. Therefore, it's not advisable to let the via plugging process sit for an extended period; a screen printing process should be initiated within a short time to fill the laser blind vias with ink. The resting time after the plugging process should be controlled to 40% to 60% of the resting time after the first screen printing. This allows for initial ink setting and air expulsion without causing irreparable ink breakage due to prolonged resting. Maintaining the same resting time after the first and second screen printing is crucial because the ink state and residual air content inside the laser blind vias are similar in both processes. The same resting time ensures consistent ink filling and leveling, as well as effective air expulsion, which is beneficial for process control and quality stability.

[0011] Furthermore, the manufacturing method specifically includes the following steps: S1. Pretreatment and baking process: The laser blind hole board is chemically pretreated, and then baked at low temperature until the board surface cools to room temperature. S2. Hole-filling process: A hole-filling screen is used to fill the blind holes of the laser with hole-filling ink, and then the holes are left to stand after filling. S3. First screen printing and pre-baking process: Use screen printing ink to screen print the laser blind hole board surface for the first time, let it stand after printing, and then perform the first pre-baking process. S4. Second screen printing and pre-baking process: Use screen printing ink to screen print the laser blind hole board surface a second time, let it stand after printing, and then perform a second pre-baking process; S5. Exposure and Development Process: The laser blind hole plate is sequentially exposed and developed. S6. Stepped high-temperature curing process: The laser blind hole board surface is cured at high temperature using a multi-stage stepped heating method.

[0012] To prevent damage to the antenna or substrate of the laser-etched blind via board, a chemical pretreatment is used in step S1 to clean the board surface and roughen the copper surface, improving the adhesion between the copper surface and the ink. When the laser-etched blind via board is transported through the pretreatment equipment production line, the board containing the laser-etched blind vias must be placed face down to prevent water from accumulating in the blind vias, which could lead to incomplete drying and subsequent moisture evaporation causing the ink in the blind vias to break or burst. In step S2, the laser-etched blind vias are pre-filled using a plugging process and then allowed to stand to release air. Steps S3 and S4 involve two layers of screen printing for gradual coverage, combined with pre-baking for semi-curing and shaping. Step S5 involves sequentially exposing and developing the laser-etched blind via board. In step S6, the high-temperature curing process is a stepped curing process, with multiple temperature stages for gradual curing, allowing the ink inside the laser-etched blind vias to cure slowly and preventing bursting due to rapid heating. The above six steps constitute the complete solder mask manufacturing process, forming a complete control logic of moisture control, layer-by-layer filling, air removal, and slow curing. Each step is specifically designed to prevent and control the copper exposure caused by laser blind holes. The entire process is interconnected and systematically solves the problem of thin solder mask and exposed copper in laser blind holes.

[0013] Furthermore, the thickness of both the first and second solder resist layers is controlled between 10μm and 15μm, and the overall ink thickness of the laser blind hole board is controlled between 20μm and 30μm after the second screen printing is completed; the settling time after the hole plugging process is less than 15min, and the settling time after the first screen printing and the settling time after the second screen printing are at least 30min.

[0014] Due to the special requirements for chip mounting on the 77GHz radar board, to prevent poor soldering caused by the stencil being lifted during mounting, the overall ink thickness of the laser-engraved blind via board must be controlled below 37.5μm. The ink thickness cannot exceed this limit, so the ink thickness of the first and second screen printings must be controlled between 10μm and 15μm, meaning the overall ink thickness after two screen printings should be controlled between 20μm and 30μm. This two-step thin-printing method satisfies the overall ink thickness insulation protection requirements and avoids the problem of localized ink thickness exceeding the limit due to a single thick print. The resting time after plugging the vias is less than 15 minutes to allow for timely filling with screen printing ink, ensuring effective ink coverage. If the interval is too long, the plugging ink will shrink and set, and the openings at the vias cannot be effectively filled by the screen printing ink, ultimately resulting in thin ink around the via edges. The resting time after the first and second screen printings is no less than 30 minutes to allow the screen printing ink to fully level out, giving air sufficient time to slowly escape, and ensuring the screen printing ink evenly covers the via edges, preventing thin ink around the via edges.

[0015] Furthermore, a diluent is added during the production of the hole-filling ink, and a diluent and anti-sagging agent are added to the screen printing ink. The viscosity of the hole-filling ink is 8 to 20 times that of the screen printing ink.

[0016] Adding a thinner to via-filling ink aims to moderately reduce its viscosity and improve its flowability, making it easier to squeeze into the tiny laser blind vias with a diameter of only 5 mil under pressure, ensuring filling accuracy. However, excessive thinner should be avoided, as it can cause severe shrinkage and cracking, leading to a decrease in the filling accuracy within the laser blind vias. Similarly, adding a thinner to screen printing ink further reduces its viscosity and improves its flowability, allowing it to flow smoothly into the laser blind vias when covering the board surface. Screen printing ink also requires the addition of an anti-sagging agent. Because screen printing ink has a much lower viscosity than via-filling ink, its flowability is stronger. Low-viscosity screen printing ink is prone to runoff at the edge of the via due to gravity, resulting in thin ink and exposed copper at the via opening. The anti-sagging agent helps the screen printing ink adhere effectively to the via opening edge, ensuring a thicker ink layer. The viscosity difference design between the hole-filling ink and the screen printing ink ensures both the filling density inside the laser blind holes and avoids ink loss defects at the edge of the holes.

[0017] Furthermore, when manufacturing the aluminum sheet for the hole-filled mesh, the aperture of the aluminum sheet opening is 9 mil to 11 mil larger than the aperture of the laser blind hole, and a coefficient of thermal expansion and contraction is added for stretching compensation.

[0018] The aluminum sheet opening diameter is larger than the laser blind via diameter to provide sufficient ink flow channels, ensuring the ink completely covers the via opening and is smoothly squeezed into the via. If the aluminum sheet opening diameter is smaller than or equal to the laser blind via diameter, misalignment will cause the aluminum sheet opening to align with the laser blind via, resulting in some laser blind vias not being aligned with the ink flow opening and insufficient ink filling. Enlarging the opening diameter by 9mil to 11mil can cover the misalignment range and ensure effective filling of the laser blind vias. In traditional methods, the aluminum sheet is usually manufactured according to the original file for drilling laser blind vias on the outer layer. However, since the laser blind via board has expanded and contracted after lamination, manufacturing the aluminum sheet according to the original file will result in misalignment between the aluminum sheet opening and the actual laser blind via position on the board, affecting ink flow and via filling performance. Since laser blind hole alignment is based on the inner layer target, i.e. the actual expansion and contraction coefficient after lamination, the aluminum sheet must be made according to the expansion and contraction coefficient on the laser machine. That is, the expansion and contraction coefficient compensated when making the aluminum sheet is consistent with the expansion and contraction coefficient used when making the laser blind hole, so that the opening of the aluminum sheet corresponds precisely to the position of the laser blind hole on the actual board surface, further improving the alignment accuracy of the hole plugging process and reducing hole plugging defects caused by alignment deviation.

[0019] Furthermore, the multi-stage stepped heating method includes at least four different temperatures combined with different baking times for heating and baking the plate; Specifically, it includes the first temperature segment combined with the first time segment, the second temperature segment combined with the second time segment, the third temperature segment combined with the third time segment, and the fourth temperature segment combined with the fourth time segment. In the first three phases, the temperature in the latter phase is 20% to 35% higher than that in the former phase, and the temperature in the latter phase is 40% to 60% lower than that in the former phase. The temperature in the fourth segment is 40% to 60% higher than that in the third segment, and the duration of the fourth segment is 80% to 120% longer than that of the third segment.

[0020] The four-stage stepped heating and curing process allows the ink inside and outside the laser blind via to cure slowly and synchronously, preventing rapid expansion of the ink inside the via due to sudden temperature increases, which could break through the surface ink and cause ink splattering. The first three stages employ a strategy of gradually increasing temperature and gradually decreasing time. The lower temperature stage is longer, allowing the solvent in the ink to evaporate slowly and evenly, while the ink undergoes initial cross-linking and setting. As the temperature gradually increases, the curing reaction progresses, and the ink has already acquired a certain strength. The fourth stage uses high temperature and long curing time to ensure complete cross-linking and curing of the ink. At this point, the solvent inside the ink has basically evaporated, and there will be no rapid expansion at high temperature, thus allowing for prolonged high-temperature baking to ensure complete curing. By using a stepped curve of slow heating, stabilization, and final high temperature to adapt to the semi-closed structure of laser blind vias, both curing quality and the problem of exposed copper at the via edge caused by ink splattering are ensured.

[0021] Furthermore, the squeegee direction is reversed during hole-filling printing, while the squeegee direction is forward during the first and second screen printing. The squeegee angle during the first and second screen printing is 1 to 2 times larger than the squeegee angle during hole-filling printing.

[0022] During the via-filling printing process, the squeegee direction is reversed, with a negative angle design. This aims to push the via-filling ink downwards, generating greater vertical pressure, allowing the ink to smoothly penetrate the blind vias through the aluminum sheet openings, thus improving the filling effect. The reverse squeegee direction also reduces ink residue on the via-filling screen, concentrating pressure at the openings and improving ink application efficiency. During the first and second screen printing processes, the squeegee direction is forward, with a larger squeegee angle. This ensures uniform ink coating on the laser blind via board. Forward squeegeeing allows for even transfer of ink from the screen to the laser blind via board, forming a smooth solder resist layer. The larger squeegee angle reduces the amount of ink applied, precisely controlling the ink thickness in a single print and preventing excessive local ink thickness. The via-filling process and the screen printing process utilize different squeegee directions and angles, working together to achieve excellent filling and coverage effects.

[0023] Another objective of this invention is to provide a PCB board manufactured using the aforementioned laser-etched blind via solder mask fabrication method. This PCB board is particularly suitable for high-frequency, high-end PCB products. Its laser-etched blind via areas have complete solder mask coverage, eliminating defects such as thin exposed copper and ensuring high soldering reliability. Simultaneously, the solder mask thickness on the board surface is uniform, meeting the requirements for high-precision chip mounting, and effectively improving the signal stability and lifespan of high-frequency radar boards.

[0024] Through the above technical solution, the present invention can achieve at least the following technical effects: by pre-filling the inside of blind holes through the plugging process, allowing air to escape, and then filling the inside and edges of the laser blind holes layer by layer through two screen printing and allowing them to stand, and slowly forming them through step-curing, the entire process controls the air, moisture, ink flow and curing rate, which solves the technical problem of thin ink inside and at the edges of laser blind holes caused by air leakage, resulting in exposed copper, and improves the solder resist yield. Attached Figure Description

[0025] Figure 1 This is a schematic diagram illustrating the defect of exposed copper in laser blind holes due to thin oil.

[0026] Figure 2 This is a flowchart of the laser blind hole plate solder resist fabrication method of the present invention.

[0027] Figure 3 This diagram illustrates the difference between laser ablation of aluminum sheets and traditional drill bit drilling of aluminum sheets.

[0028] Figure 4 This is a schematic diagram of the laser-engraved blind hole board surface after the hole plugging process.

[0029] Figure 5 This is a schematic diagram of the laser blind hole board surface after the first screen printing.

[0030] Figure 6 This is a schematic diagram of the laser blind hole board surface after the second screen printing.

[0031] Figure 7 This is a schematic diagram showing the effect of laser blind hole solder resist coverage and cross-section after fabrication using the method of the present invention. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to specific embodiments. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0033] This embodiment uses a 5mil laser-etched blind via board for a 77GHz automotive millimeter-wave radar as the processing object. The preferred substrate is RO3003 high-frequency material, and the laser-etched blind vias have a diameter of 5mil and a depth of 5mil or more. Figure 2 As shown in the figure, this embodiment is executed sequentially according to the illustrated process. The specific parameters and key points of each process are explained in detail below.

[0034] Step S1. Pre-treatment and Baking Process: The core purpose of this step is to clean the board surface and copper surface, improve the roughness of the copper surface, enhance the adhesion between the solder resist ink and the copper surface, and control the residual moisture in the laser blind holes. In the pre-treatment process, because the high-frequency material board is not wear-resistant or the antenna design is thin, mechanical brushing can easily scratch the substrate or wear down the antenna lines, leading to abnormal signal performance. To prevent damage to the antenna or substrate, a chemical method is used to perform ultra-roughening treatment on the board surface. This involves cleaning the board surface with degreasing, micro-etching, and pickling solutions. The degreasing solution removes organic contaminants from the board surface, ensuring full contact between the solution and the copper surface. The sodium persulfate system solution in the micro-etching tank uniformly micro-etches the copper surface, forming a micro-rough copper surface structure, increasing the contact area between the copper surface and the ink, and improving ink adhesion. The dilute sulfuric acid in the pickling tank removes the oxide layer and micro-etching residue from the copper surface, ensuring a clean copper surface. During pretreatment, the laser-etched blind vias must be placed face down to prevent water from remaining inside and failing to dry properly. This can lead to ink cracking or oil splattering at the blind via caps during subsequent processes. When the laser-etched blind via boards are conveyed through the pretreatment equipment, they must also be placed face down. Because laser blind vias have a semi-closed recessed structure, if the blind via face up, chemicals and cleaning water will accumulate inside, making it difficult to dry them completely. Residual moisture inside the vias will vaporize and expand during subsequent heated processes, breaking through the ink above the blind vias and causing oil splattering defects. Placing the blind via face down allows gravity to allow chemicals and moisture to flow out naturally, reducing the risk of water accumulation and minimizing solder resist defects caused by moisture at the source.

[0035] After pretreatment, the laser-etched blind via board is immediately placed in a vertical oven at 75℃ for 15 minutes. During pretreatment, most of the moisture is removed by placing the laser blind via face down. However, a small amount of moisture may still remain in the tiny 5mil laser blind vias due to surface tension. Baking thoroughly dries any remaining moisture inside the vias, preventing poor sealing performance. After baking, the laser-etched blind via board must be allowed to cool naturally to room temperature before proceeding with the sealing process. If the sealing process is performed directly when the board surface temperature is high, the hot air inside the laser blind vias will contract upon cooling, causing voids to form within the sealing ink. When voids appear during the sealing process, subsequent screen printing processes will struggle to fill the laser blind vias with ink. Simultaneously, the hot board surface will decrease the viscosity of the sealing ink, causing ink shrinkage and reducing the filling degree within the vias.

[0036] Step S2. Hole-filling process: Due to the small diameter and long depth of laser blind holes, screen printing ink alone cannot fill them completely. Hole-filling is a core pre-process, aiming to pre-fill the laser blind holes with hole-filling ink before the actual screen printing, expelling air and laying the foundation for subsequent screen printing. The hole-filling process includes the fabrication of the hole-filling aluminum sheet, the fabrication of the hole-filling screen, the preparation of the hole-filling ink, hole-filling printing, and resting.

[0037] The aluminum sheet used for plugging holes is the core fixture for controlling the accuracy of plugging and the amount of ink applied. The aperture of the aluminum sheet is designed to be 10 mil larger than the diameter of the laser blind via, i.e., 15 mil. Simultaneously, a coefficient of thermal expansion / contraction is added to the aluminum sheet data to compensate for the stretching of the aluminum sheet pattern, reducing alignment deviations caused by board expansion / contraction. Specifically, since laser blind via alignment is based on the inner layer target, i.e., the actual expansion / contraction coefficient after lamination, if the coefficient used for laser blind vias is 0.2 mil / inch in the X direction and 0.2 mil / inch in the Y direction, the aluminum sheet must also be manufactured according to the expansion / contraction coefficient on the laser machine. For ease of operation, a range of 0.2 mil / inch can be used. For example, laser blind via boards with expansion / contraction between 0 mil / inch and 0.2 mil / inch can share a coefficient of 0.1 mil / inch for aluminum sheet manufacturing, and laser blind via boards with expansion / contraction between 0.2 mil / inch and 0.4 mil / inch can share a coefficient of 0.3 mil / inch for aluminum sheet manufacturing. Enlarging the aperture ensures that the laser-engraved blind holes are aligned with the aluminum sheet openings, guaranteeing smooth ink application for the plugging process. Compensation for the expansion / contraction coefficient allows for precise matching between the aluminum sheet openings and the actual blind hole positions, improving plugging alignment accuracy. This method uses high-power laser ablation to create aluminum sheet openings, replacing traditional drill bit methods. The laser power is controlled between 100W and 150W. Figure 3As shown, the left side shows the aluminum sheet for plugging holes made by drilling with a drill bit, and the right side shows the aluminum sheet for plugging holes made by laser ablation. Compared with the traditional drill bit for drilling aluminum sheets, laser can ensure that the opening of the aluminum sheet is round and there are no burrs or aluminum chips left on the edge of the hole to block the opening, further reducing the problem of insufficient ink filling for plugging holes and failure to cover laser blind holes.

[0038] The steps for making a screen printing plate for laser-filled holes are as follows: First, a screen printing mesh is pre-stretched, with the mesh tension controlled between 22 N / cm² and 30 N / cm². Then, the etched aluminum sheet is placed on the mesh and secured firmly with glue and tape. Finally, the mesh at the opening positions of the graphic is cut. If the mesh tension is too low, the screen printing plate will deform significantly during printing, resulting in large alignment deviations; if the tension is too high, the screen printing plate is prone to breakage. Fixing the aluminum sheet to the mesh and cutting the mesh at the corresponding positions allows the laser ink to be transferred directly to the board surface through the openings in the aluminum sheet, ensuring that the ink can be fully squeezed into the laser blind holes.

[0039] When preparing the via-filling ink, the main agent and hardener are mixed and stirred, and an additional 10ml / kg to 20ml / kg of thinner is added to control the ink viscosity at 300dPa·s. The purpose of adding thinner is to moderately reduce the viscosity of the via-filling ink, improve its fluidity, and make it easier for the ink to flow into the tiny laser blind holes under pressure, thus improving the filling fullness. However, excessive thinner should not be added. If too much thinner is added, the viscosity of the via-filling ink will be too low, causing severe shrinkage after the via-filling process, or even via cracking, which will reduce the filling effect.

[0040] When printing through-holes, a 10mm thick squeegee with a hardness of 75° is preferred. The squeegee should be placed at an angle, using a reverse scraping motion with an angle controlled between -5° and -10°. This reverse scraping and negative angle design allows the squeegee to press the through-hole ink downwards, creating vertical downward pressure to maximize the filling effect. During through-hole printing, the filling pressure should be adjusted to 80±10kgf, the filling speed controlled at 90±10mm / s, and the squeegee should only be used once to slowly squeeze the through-hole ink from the squeegee screen into the laser blind holes. Appropriate filling pressure ensures ink filling while preventing ink overflow due to excessive pressure; a suitable printing speed combined with a single scraping reduces air bubbles within the holes. The height of the squeegee screen from the board surface should be controlled between 3mm and 5mm to prevent ink from sticking to the screen and creating a trailing effect.

[0041] After the vias are plugged, they need to be left to stand and release air (stand still for 1 hour) to allow the plugging ink to flow evenly and prevent concentrated leakage of air during subsequent screen printing. When residual air in some laser blind holes escapes outward, it can break through the ink layer at the hole opening, causing the via to break. The plugging ink will then explode around the laser blind hole. Figure 4As shown. Therefore, the screen printing process must be followed immediately after plugging the hole. The screen printing ink can be used to fill the hole in time and refill the ink into the laser blind hole. Therefore, the interval between the hole plugging process and screen printing should be controlled within 15 minutes.

[0042] Step S3. First screen printing and pre-baking process: The purpose of the first screen printing (screen printing 1) is to form the basic first solder resist layer on the laser blind hole board surface, and at the same time further fill the gaps in the laser blind holes. The process of the first screen printing and pre-baking process includes screen printing stencil and ink preparation, screen printing, resting and pre-baking.

[0043] The screen printing stencil uses 62T mesh, a high T-count. A higher T-count means a denser mesh, resulting in less ink applied per stroke and a thinner, more uniform ink thickness. 62T mesh is chosen to precisely control the ink thickness per stroke within the range of 10μm to 15μm, preventing excessive ink application and exceeding the acceptable thickness. When thinning the screen printing ink, a thinner is added to control the viscosity to 20dPa·s to 30dPa·s, along with 3ml / kg to 5ml / kg of anti-sagging agent. Lower ink viscosity improves ink flow, allowing it to smoothly flow into the gaps within the laser blind holes, filling minor defects after the hole-filling process. However, low-viscosity ink is prone to sagging, resulting in thin ink at the corners of the holes. Therefore, an anti-sagging agent is added to ensure effective ink adhesion at these corners. The main components of the anti-sagging agent are polyamide ester, vinyl ester resin, and talc.

[0044] For screen printing, a 10mm thick squeegee with a hardness of 75° is preferred, with the squeegee angle controlled between 10° and 15°, using a forward squeegee method. Forward squeegeeing ensures even transfer of ink to the board surface, forming a smooth solder resist layer; a larger angle reduces the amount of ink applied per pass, allowing for precise control of ink thickness. During printing, the blind via side is printed first, followed by the non-blind via side. Printing the blind via side first ensures the ink condition and filling effect on the blind via side. The screen printing speed is controlled between 150mm / s and 200mm / s, the screen printing pressure between 30kgf and 60kgf, and the screen height from the board surface between 8mm and 14mm. The printing speed is faster than the blind via squeegee, adapting to the coating requirements of screen printing; the screen printing pressure is lower than the blind via squeegee pressure, avoiding excessive pressure that could affect the filling effect; the screen distance is suitable for high-mesh-count mesh, ensuring a smooth ink surface without mesh patterns or trailing defects.

[0045] After the first screen printing is completed, the laser blind hole board is horizontally inserted into the vehicle-mounted rack and left to stand for 30 minutes (Standing Time 2). This allows the screen printing ink to fully flow and distribute evenly, filling in areas with thin ink. It also allows sufficient time for air bubbles in the ink to slowly escape, preventing them from bursting the ink layer during subsequent pre-baking and curing due to thermal expansion. The horizontal rack ensures the board surface is level, preventing the screen printing ink from flowing to one side due to gravity and ensuring uniform ink thickness. Figure 5As shown. After settling, the first pre-baking (pre-baking 1) is performed using a tunnel oven at 75°C for 21 minutes. The purpose of pre-baking is to evaporate some of the solvent in the ink, changing the ink from a wet, flowable state to a semi-dry state, in preparation for the subsequent exposure process. Choosing a lower pre-baking temperature allows the solvent to evaporate slowly, preventing the ink in the laser blind holes from being damaged by rapid solvent evaporation due to rapid heating.

[0046] Step S4. Second Screen Printing and Pre-baking Process: The second screen printing (screen printing 2) is a supplementary printing process based on the semi-cured first screen printing. It further fills the laser blind vias, covers the thin ink areas around the vias, and ultimately ensures the overall ink thickness meets the standard. The screen, ink, squeegee parameters, and printing process parameters for the second screen printing are basically the same as for the first screen printing. The ink thickness per print is also controlled between 10μm and 15μm, and the overall ink thickness after two screen printings is controlled between 20μm and 30μm. During the first screen printing, the ink above the laser blind vias is flattened, and some of the air inside the laser blind vias that has not been released will slowly release. Some of the thinner ink at the edge of the vias will be squeezed into the laser blind vias, resulting in thin ink at the edge of the vias. During the second screen printing, the ink will refill the laser blind vias, filling the remaining gaps inside the vias and completely covering the thin ink areas at the edges of the vias, forming a uniform solder resist layer on the board surface. Figure 6 As shown.

[0047] After the second screen printing is completed, it is also left to stand for 30 minutes (standing time 3) to allow the screen printing ink to fully level and degas. Then, a second pre-baking is performed (pre-baking 2) at a temperature of 75°C for 48 minutes. The second pre-baking time is longer because the total thickness of the ink increases after two screen printings, requiring more time for the solvent to fully evaporate and ensure that the ink reaches a semi-cured state to meet the exposure requirements. The longer pre-baking time also allows the ink in the laser blind holes to be further set, avoiding defects in subsequent exposure and development processes.

[0048] Step S5. Exposure and Development Process: The laser blind hole board after the second screen printing and pre-baking process is exposed using a semi-automatic exposure machine. A halogen light source is used for exposure to cause cross-linking of the photopolymers in the ink. The exposure grid is controlled at 12±2 grids, corresponding to an exposure energy of 500mJ. The purpose of development is to remove the ink that has not been exposed to light and has not undergone cross-linking reaction using a weak alkaline solution, exposing a clean copper surface. A 0.8% K2CO3 developer is used for development, with the development temperature controlled at 31±2℃, the development time controlled at approximately 70 seconds, and the development spray pressure at 2.0 bar. Special control is required in the drying section of the development process. The drying section temperature in a conventional development line is 75℃. When the cooled laser blind hole board enters the hot drying section, the ink inside the laser blind holes is prone to thermal expansion and bursting. Therefore, in this method, the heating of the developing and drying section needs to be turned off, and the glass cover of the drying section should be opened to dissipate heat in advance, so as to reduce the drying temperature to below 50°C. Low-temperature drying allows the ink in the laser blind hole to heat up slowly, avoiding the oil explosion defect and ensuring the integrity of the solder resist layer.

[0049] Step S6. Stepped high-temperature curing process: High-temperature curing uses the high-temperature hot air circulation of the oven to heat-cur the ink on the board surface, allowing the solvent in the ink to completely evaporate and form a permanent insulating and anti-corrosion protective film. A four-stage stepped curing process is used, with the temperature gradually increasing in four stages during the baking process.

[0050] The first stage is at a temperature of 60℃ for 120 minutes: low-temperature long-term baking allows the residual solvent in the ink to evaporate slowly and evenly, while the ink gradually begins to cross-link and form a preliminary structure; at this time, the temperature is low, the solvent evaporation is mild, a large amount of gas will not be generated in the laser blind hole, and the surface ink will not be broken.

[0051] The second stage involves a temperature of 75℃ and a time of 60 minutes: the temperature increases slightly, the curing reaction progresses gradually, and the hardness and adhesion of the ink gradually increase. At this point, the ink has already acquired a certain strength.

[0052] The third stage has a temperature of 100℃ and a time of 30 minutes: the temperature is further increased, the curing reaction is accelerated, and the ink is quickly cross-linked and set; after the slow evaporation of the first two stages, most of the solvent in the ink has been discharged.

[0053] The fourth stage involves baking at 155℃ for 60 minutes: This high-temperature, long-term baking process ensures that the solvent inside the ink completely evaporates, preventing ink splattering defects and allowing the ink to fully cross-link and cure, thus guaranteeing the final cured quality.

[0054] The stepped temperature curing method is suitable for the semi-closed structure of laser blind holes. It gradually increases the temperature from low to high temperature, allowing the solvent to be released slowly and the ink to cure gradually. This solves the problem of oil explosion in laser blind holes that is easily caused by traditional one-time high-temperature curing.

[0055] After high-temperature curing, the final product is formed, and the improved laser blind hole solder mask coverage effect is as follows: Figure 7 As shown, the laser-cut blind holes are fully filled, the hole edges are completely covered, there are no defects such as thin exposed copper and no oil, and the solder mask layer on the board surface is dense and uniform, which improves the welding yield of the finished board.

[0056] It should be noted that the above embodiments use a 5mil laser-etched blind via automotive radar board as an example for detailed explanation. However, the method of this application is also applicable to laser-etched blind via boards with other apertures or other application scenarios. Any PCB product with issues such as thin solder mask exposing copper in blind vias can refer to this method for parameter adjustment. It should also be noted that the specific values ​​of each parameter during the process can be adjusted according to the actual product and requirements, such as the viscosity of the via-filling ink, the via-filling degree, the screen printing ink thickness, the settling time, and the coordination of curing temperatures and times at each stage. All equivalent substitutions and improvements made using the technical solutions and concepts of this invention should fall within the protection scope of this invention.

[0057] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the technical solutions of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A method for fabricating solder mask on a laser-etched blind hole plate, characterized in that, The laser blind hole is filled with a plugging process, and the laser blind hole plate is left to stand after the plugging process is completed. The first solder resist layer is formed on the surface of the laser blind hole board by screen printing for the first time. After the first screen printing, the laser blind hole board is left to stand, and part of the ink of the first solder resist layer flows into the laser blind hole. Finally, a second solder resist layer is formed on top of the first solder resist layer through a second screen printing process. After the second screen printing, the laser-etched blind hole plate is left to stand.

2. The method for fabricating laser-etched blind hole plate solder resist according to claim 1, characterized in that, After the via plugging process, the ink filling rate inside the laser blind via after standing is at least greater than 90%, and the thickness of the first solder resist layer is less than the thickness of the second solder resist layer.

3. The method for fabricating laser-etched blind hole plate solder resist according to claim 1, characterized in that, The settling time after the plugging process is 40% to 60% of the settling time after the first screen printing, and the settling time after the first screen printing is the same as the settling time after the second screen printing.

4. The method for fabricating laser-etched blind hole plate solder resist according to claim 1, characterized in that, The manufacturing method specifically includes the following steps: S1. Pretreatment and baking process: The laser blind hole board is chemically pretreated, and then baked at low temperature until the board surface cools to room temperature. S2. Hole-filling process: A hole-filling screen is used to fill the blind holes of the laser with hole-filling ink, and then the holes are left to stand after filling. S3. First screen printing and pre-baking process: Use screen printing ink to screen print the laser blind hole board surface for the first time, let it stand after printing, and then perform the first pre-baking process. S4. Second screen printing and pre-baking process: Use screen printing ink to screen print the laser blind hole board surface a second time, let it stand after printing, and then perform a second pre-baking process; S5. Exposure and Development Process: The laser blind hole plate is sequentially exposed and developed. S6. Stepped high-temperature curing process: The laser blind hole board surface is cured at high temperature using a multi-stage stepped heating method.

5. The method for fabricating laser-etched blind hole plate solder resist according to claim 3, characterized in that, The thickness of the first and second solder resist layers is controlled between 10μm and 15μm. After the second screen printing is completed, the overall solder paste thickness of the laser blind hole board is controlled between 20μm and 30μm. The settling time after the hole plugging process is less than 15min, and the settling time after the first screen printing and the settling time after the second screen printing are at least 30min.

6. The method for fabricating laser-etched blind hole plate solder resist according to claim 4, characterized in that, The orifice-filling ink is made with a thinner, and the screen printing ink is made with a thinner and an anti-sagging agent. The viscosity of the orifice-filling ink is 8 to 20 times that of the screen printing ink.

7. The method for fabricating laser-etched blind hole plate solder resist according to claim 4, characterized in that, When manufacturing the aluminum sheet for the hole-filled mesh, the aperture of the aluminum sheet opening is 9 mil to 11 mil larger than the aperture of the laser blind hole, and a coefficient of thermal expansion and contraction is added for stretching compensation.

8. The method for fabricating laser-etched blind hole plate solder resist according to claim 4, characterized in that, The multi-stage stepped heating method includes at least four different temperatures combined with different baking times for heating and baking the plate. Specifically, it includes the first temperature segment combined with the first time segment, the second temperature segment combined with the second time segment, the third temperature segment combined with the third time segment, and the fourth temperature segment combined with the fourth time segment. In the first three phases, the temperature in the latter phase is 20% to 35% higher than that in the former phase, and the temperature in the latter phase is 40% to 60% lower than that in the former phase. The temperature in the fourth segment is 40% to 60% higher than that in the third segment, and the duration of the fourth segment is 80% to 120% longer than that of the third segment.

9. The method for fabricating laser-etched blind hole plate solder resist according to claim 4, characterized in that, The squeegee direction is reversed during hole-filling printing, while the squeegee direction is forward during the first and second screen printing. The squeegee angle during the first and second screen printing is 1 to 2 times larger than the squeegee angle during hole-filling printing.

10. A PCB board manufactured using the laser blind hole board solder mask fabrication method according to any one of claims 1 to 9.

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