A blind hole manufacturing process of thick copper HDI circuit board

Abstract

This invention relates to the field of printed circuit board manufacturing, and in particular to a blind via fabrication process for thick copper HDI circuit boards, comprising the following steps: merging two closely spaced blind vias into a single mechanical hole for initial mechanical drilling; performing copper plating on the initial drilled hole to form a copper layer; filling the hole space with resin plugs; and performing a second mechanical drilling to drill away the copper layer formed in the first drilling, creating two independent semi-circular copper vias, which serve as the conduction paths for the two blind vias. This application enables reliable fabrication of small-pitch blind vias on thick copper boards through an innovative two-stage mechanical drilling method. The preferred diameter of the first drilling is 0.4 mm, and the preferred diameter of the second drilling is 0.3 mm, achieving reliable fabrication of blind via spacing of 0.25 mm to 0.5 mm, suitable for fabricating 6th-order HDI circuit boards with copper thicknesses of 3 oz to 8 oz.

Description

Technical Field

[0001] This invention relates to the field of technology, specifically to a blind via fabrication process for thick copper HDI circuit boards. Background Technology

[0002] In the printed circuit board (PCB) manufacturing industry, PCBs with a copper thickness of 3 oz or more are typically referred to as thick copper boards. Due to their excellent current-carrying capacity, thick copper boards are primarily used in power supply products. As electronic devices continue to evolve towards higher performance and miniaturization, some power supply boards require both thick copper boards to meet the demands of high-current transmission and high-density interconnect fabrication processes to achieve high-density wiring. This need has made the combination of thick copper boards and high-density interconnect technology a crucial technological direction in the industry.

[0003] Under current technological conditions, the fabrication of high-density interconnect circuit boards using thick copper plates primarily employs blind vias. The process involves first etching away the thick copper at the blind via location using a film-etching process to create the blind via area, and then fabricating the blind via using laser technology within this area. However, this traditional process has significant limitations: when the spacing between blind vias of two different networks is small, to ensure that the pads at the bottom of the blind via are not damaged by laser drilling or subsequent processes, the bottom pads of the blind via must be at least 0.10mm larger on each side than the diameter of the blind via as a process compensation and safety margin. This design requirement leads to a further reduction in the spacing between the bottom pads of the blind via. When the spacing of the thick copper between the bottom pads is less than the processing capacity of the etching process, the existing blind via fabrication method cannot achieve reliable processing, forming a technological bottleneck.

[0004] Furthermore, the manufacturing process for a conventional 14-layer printed circuit board with a six-layer stacked blind via interconnect structure is extremely complex, requiring multiple cyclic operations to complete. The specific process includes: drilling, electroplating, via plugging, capping electroplating, and circuit fabrication of the 7th and 8th layers of the core board; then repeating the lamination, blind via opening, laser drilling, via filling electroplating, and circuit fabrication steps six times; finally, completing the circuitry from layers 1 to 14, solder mask, and subsequent processes. This process involves numerous steps, a long production cycle, and high manufacturing costs, placing significant production pressure on companies. Summary of the Invention

[0005] In view of this, the problem to be solved by the present invention is to solve the problem of small-pitch blind via processing encountered in the fabrication of thick copper plates with a copper thickness of ≥3oz in high-density interconnect.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A blind via fabrication process for a thick copper HDI circuit board includes the following steps: S1. Mechanical drilling is performed on the area on the PCB where two adjacent blind holes need to be formed; when the distance between the two blind holes causes the bottom pad to exceed the thick copper etching capacity, the two blind holes are merged and drilled into one hole. S2. Apply copper plating and electroplating treatment to the hole wall formed by drilling to form an electroplated copper layer on the hole wall. S3. Use resin-filling ink to fill the drilled holes until the space inside the drilled holes is completely filled, and bake and cure the filled area in sections. S4. Perform secondary mechanical drilling at the hole-filling location. The diameter of the secondary drilling is smaller than that of the first drilling. The copper plating layer formed by the first drilling is drilled through the secondary drilling to form two independent semi-circular copper holes, which serve as the conduction paths for two blind holes. S5. Perform resin plugging treatment on the secondary drilled holes to fill the pores generated by the secondary drilled holes.

[0007] In S1, the diameter of the mechanically drilled hole is 0.4 mm.

[0008] In S1, the diameter of the mechanically drilled hole covers the original blind hole spacing and the radius range of each of the two blind holes, so that both original blind holes are included within the diameter range of the mechanically drilled hole.

[0009] In S2, the thickness of the copper layer in the electroplated holes is ≥24μm.

[0010] In S3, the segmented baking and curing process includes a first stage of baking at 110°C for 60 minutes and a second stage of baking at 150°C for 30 minutes.

[0011] In S4, the diameter of the secondary mechanical drilling is 0.3 mm.

[0012] The diameter of the secondary drilling is reserved with a drilling offset allowance to ensure that the copper layer of the electroplated hole can still be drilled through even when the drilling is offset.

[0013] The center-to-center distance between two adjacent blind holes is 0.25 mm to 0.5 mm.

[0014] The circuit board is a 14-layer printed circuit board with a six-layer stacked blind via interconnect structure and a copper thickness of 3 oz to 8 oz.

[0015] The advantages and positive effects of this invention are: This invention solves the technical problem of fabricating small-pitch blind vias in thick copper high-density interconnect circuit boards by sequentially implementing mechanical drilling, copper plating and electroplating, resin plugging and segmented baking and curing, secondary mechanical drilling, and re-resin plugging. It stably achieves blind via fabrication with a center-to-center distance of 0.25mm to 0.5mm between adjacent blind vias. By forming an electroplated copper layer with a thickness ≥24μm on the via wall, the conductive cross-sectional area of ​​the blind via meets electrical performance requirements, and the conduction resistance and current carrying capacity are stable and reliable. The use of resin plugging ink to completely fill the drilled holes and followed by segmented baking and curing avoids air bubbles and filling defects, ensuring a flat board surface. Secondary mechanical drilling with a smaller diameter and pre-reserved drilling offset allowance reliably breaks through the electroplated copper layer, forming two independent semi-circular copper vias as conductive paths, improving process stability and product yield. Re-filling the voids created by the secondary drilling with resin completes the blind via structure. The entire process eliminates the need for complex steps such as laser drilling and blind via etching, resulting in a simpler process and higher production efficiency. Meanwhile, the process of this invention is compatible with thick copper circuit boards with a copper thickness of 3oz to 8oz, as well as 14-layer printed circuit boards with a six-layer stacked blind via interconnect structure. It has a wide range of applications and strong compatibility, and can meet the mass production needs of thick copper high-density interconnect circuit boards. The formed electroplated copper layer is firmly connected to the inner copper circuit layer, with strong mechanical bonding force, resulting in a more stable product structure and higher reliability. Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0017] In the attached diagram: Figure 1 This is an overall flow chart of the blind via fabrication process for a thick copper HDI circuit board according to the present invention. Figure 2 This is a schematic diagram of the blind via fabrication process of a thick copper HDI circuit board according to the present invention. Detailed Implementation

[0018] 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.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0020] Reference Figures 1 to 2 The blind via fabrication process for thick copper HDI circuit boards provided by this invention breaks through the technical bottleneck of traditional processes that cannot fabricate small-pitch blind vias on thick copper plates, achieving reliable fabrication of adjacent blind vias with a center-to-center spacing of 0.25mm to 0.5mm. The complete process of the thick copper HDI circuit board blind via fabrication process of this invention includes the following steps: material preparation, inner layer circuit fabrication, lamination, drilling, copper plating, resin plugging, secondary drilling, further resin plugging, outer layer circuit fabrication, solder mask treatment, and post-processing. Compared with the conventional 6th-order HDI fabrication process, this invention eliminates complex processes such as blind via windowing, film etching, and laser drilling, and uses two mechanical drilling operations instead of the traditional laser blind via fabrication method, significantly simplifying the process flow. The process is shortened by 5 / 7, and the production cycle is reduced from approximately 45 days in the conventional scheme to approximately 15 days. The specific implementation method of this process is described in detail below with reference to the accompanying drawings.

[0021] This invention provides a blind via fabrication process for thick copper HDI circuit boards, comprising the following steps: S1. Mechanical drilling is performed on the area on the PCB where two adjacent blind holes need to be formed; when the distance between the two blind holes causes the bottom pad to exceed the thick copper etching capacity, the two blind holes are merged and drilled into one hole. Specifically, the first step is to perform mechanical drilling on the area where two adjacent blind vias need to be formed on the PCB. When the distance between the two blind vias causes the bottom pads to exceed the thick copper etching capability, the two blind vias are merged and drilled into a single mechanical hole for processing. The diameter of the first mechanical hole covers the original blind via distance and the radius of each of the two blind vias, so that both original blind vias are included within the diameter of the mechanical hole. The diameter of the first mechanical hole satisfies the following dimensional relationship: the diameter of the mechanical hole is equal to the original blind via distance plus the pad extension distance of the two blind vias, plus the radius of the two blind vias.

[0022] Specifically, in one embodiment, the original blind hole spacing is set to 0.1mm, the pad extension distance of the blind holes on both sides is 0.1mm, and the radius of the two blind holes is 0.05mm. After matching the above parameters, the diameter of the first mechanical drilling in this embodiment is 0.4mm, ensuring that the two blind holes can be merged into one mechanical drilling hole for unified processing.

[0023] S2. Apply copper plating and electroplating treatment to the hole wall formed by drilling to form an electroplated copper layer on the hole wall. Specifically, the hole walls formed by the initial mechanical drilling are subjected to copper plating and electroplating treatment to form an electroplated copper layer that meets the IPC II standard. In the copper plating process, a conductive copper seed layer is first deposited on the surface of the hole wall. This seed layer provides a growth basis for subsequent copper electroplating. The copper electroplating process continues to deposit copper layers on the basis of the copper plating layer, so that the thickness of the electroplated copper layer meets the design requirements.

[0024] The copper layer thickness of the electroplated holes is designed to match the process conditions: assuming the blind hole diameter is 0.1mm, the cross-sectional area of ​​the blind hole is... That is, 0.0078mm²; when drilling a second hole, the working condition of minimizing the remaining arc length when drilling a large-diameter hole as in the first hole is considered. The overlapping part of the second hole and the first hole is calculated as 0.15mm. This value includes the drilling offset allowance. The remaining arc length can be obtained by matching the hole diameter circumference and the cutting arc length. The minimum coating thickness is determined by combining the matching relationship between the cross-sectional area and the arc length.

[0025] Through process matching calculations and experiments, it has been confirmed that the thickness of the electroplated copper layer in the process of this invention is ≥24μm, which meets the standard requirements of IPC II level copper. This plating thickness can be achieved with conventional electroplating parameters, without the need for special high current density electroplating processes.

[0026] After electroplating is completed, a uniform electroplated copper layer is formed on the inner wall of the first mechanically drilled hole. This electroplated copper layer forms an electrical connection with the inner copper circuit layer of the PCB board. Since the present invention uses a through-hole method to form the electroplated copper layer, this electroplated copper layer is connected to each layer of copper in the inner layer. In a 14-layer printed circuit board, it is connected to each of the 12 inner copper layers, and the bonding force is 12 times that of traditional HDI laser blind vias.

[0027] S3. Use resin-filling ink to fill the drilled holes until the space inside the drilled holes is completely filled, and then bake and cure the filled area in sections.

[0028] Specifically, the resin via-filling layer completely fills the cavity formed by the initial mechanical drilling, creating a smooth surface and providing a good foundation for subsequent outer layer circuit fabrication. The resin via-filling ink used in this invention is preferably Shanrong PHP900IR-6P. The segmented baking and curing process includes two stages: the first stage involves baking at 110°C for 60 minutes, and the second stage involves baking at 150°C for 30 minutes. This segmented baking and curing process ensures that the resin via-filling ink is fully cured, forming a dense via-filling layer and preventing air bubbles or filling defects during subsequent processing.

[0029] After the resin filling is completed, the surface of the filling needs to be ground to ensure that it is flat with the PCB board surface. The grinding process can be carried out by mechanical grinding or chemical grinding. The surface roughness after grinding needs to meet the precision requirements of the outer layer circuit manufacturing.

[0030] S4. Perform secondary mechanical drilling at the hole-filling location. The diameter of the secondary drilling is smaller than that of the first drilling. The copper plating layer formed by the first drilling is drilled through the secondary drilling to form two independent semi-circular copper holes, which serve as the conduction paths for two blind holes. Specifically, the diameter of the secondary mechanical drilling needs to allow for drilling offset to ensure that the copper layer of the electroplated hole can still be drilled through even under drilling offset conditions. In one embodiment, the process design allows for drilling offset, so that even if the drilling reaches the maximum allowable offset, the copper layer of the electroplated hole can still be completely cut off, ensuring the formation of two independent semi-circular copper holes. The diameter of the secondary mechanical drilling is 0.3mm, which can be adapted to the preset drilling offset conditions to ensure the copper hole segmentation effect.

[0031] The secondary mechanical drilling uses a 0.3mm diameter drill bit. During the drilling process, the drill bit passes through the resin plugging layer and cuts the electroplated copper layer formed by the first mechanical drilling, dividing the complete circular electroplated copper layer into two independent semi-circular copper holes. These two semi-circular copper holes are located on both sides of the secondary mechanical drilling and maintain electrical connection with the inner copper circuit layer. This process is suitable for processing scenarios where the center-to-center distance between two adjacent blind holes is 0.25mm to 0.5mm. This suitable range specifically solves the problem that the bottom pads of small-pitch blind holes cannot be properly etched and separated under thick copper plates.

[0032] S5. Perform resin plugging treatment on the secondary drilled holes to fill the pores generated by the secondary drilled holes.

[0033] Specifically, after the secondary mechanical drilling is completed, a new pore space is formed between the two semi-circular copper holes, which needs to be filled with resin plugging material. The ink and baking process used for the resin plugging are consistent with the segmented baking and curing process in step S3 to ensure consistent performance of the filling layer.

[0034] After the resin filling is completed, the surface needs to be ground to restore the PCB board surface to a smooth state. At this point, the two semi-circular copper holes serve as two independent blind via conductive paths, maintaining electrical connection with the inner copper circuit layer, while the spacing between the two blind vias meets the design requirements.

[0035] After completing the drilling and plugging processes, the outer layer circuitry is fabricated, including circuit pattern transfer and etching. The outer layer circuitry is fabricated using the standard PCB industry process. First, a photosensitive dry film is applied to the PCB surface or a liquid photoresist is coated. Then, the circuit pattern is formed through exposure and development. Finally, the copper layer in the non-circuit areas is etched away to form the designed outer layer circuitry.

[0036] After the outer layer circuitry is fabricated, the two semi-circular copper vias are connected to the outer layer circuitry to achieve electrical conductivity with the inner copper circuitry layer. Since the cross-sectional area of ​​the semi-circular copper vias is calculated and designed based on the cross-sectional area of ​​blind vias, their conduction resistance and current carrying capacity are consistent with the original independent blind vias, thus meeting the electrical performance requirements of the product.

[0037] After the outer layer circuitry is fabricated, solder mask treatment and subsequent processes are performed. These processes include surface treatment, molding, and testing, ultimately completing the entire PCB fabrication. Solder mask treatment uses screen printing or liquid photoresist technology to coat the PCB surface with solder mask ink, forming a protective layer.

[0038] This process is suitable for circuit boards with copper thicknesses ranging from 3oz to 8oz, corresponding to copper thicknesses of approximately 105μm to 280μm. This process is also compatible with 14-layer printed circuit boards with a six-layer stacked blind via interconnect structure. The conventional six-layer stacked blind via interconnect structure printed circuit board manufacturing process requires multiple cycles of operation, including drilling, electroplating, via plugging, capping electroplating, and circuit fabrication of the L7 / 8 layer core board, and then repeating the steps of lamination, blind via opening, laser drilling, via filling electroplating, and circuit fabrication for a total of 6 times, finally completing the L1 / 14 layer circuit, solder mask and post-processing.

[0039] In contrast, the process of this invention is greatly simplified and significantly improves production efficiency: The manufacturing challenge of small-pitch blind vias in thick copper has been solved. By using a two-stage mechanical drilling method, two closely spaced blind vias are made into two semi-circular cylindrical holes. This has led to the development of a manufacturing process for small-pitch blind vias in thick copper, enabling the reliable production of blind via products with a center-to-center spacing of 0.25mm to 0.5mm.

[0040] This invention reduces the need for drilling blind vias in thick copper films, significantly improving the manufacturing capabilities of thick copper HDI. Traditional processes require multiple steps such as film application, exposure, development, and etching to achieve blind via openings, while this invention only requires two mechanical drilling and resin plugging processes, greatly simplifying the workflow.

[0041] The electrical performance remains consistent. The cross-sectional area of ​​the semi-circular copper hole is calculated based on the cross-sectional area of ​​the blind hole. The conduction resistance and current carrying capacity are consistent with the original, meeting the electrical performance requirements of the product.

[0042] The mechanical bonding strength is significantly improved. The arc-shaped electroplated copper layer of the through hole is an integral structure, which is connected to each of the 12 inner copper layers. The bonding strength is 12 times that of conventional HDI laser blind holes.

[0043] The production cycle has been significantly shortened, by about one month, from about 45 days in the conventional plan to about 15 days.

[0044] With significant cost advantages, process costs account for approximately 30% of conventional solutions, resulting in overall cost savings of about 21%, creating substantial economic benefits for enterprises.

[0045] Furthermore, taking a specific 14-layer printed circuit board product as an example, the practical application of the process of this invention will be explained.

[0046] The product design requirements are: 5oz copper thickness on the circuit board, 0.1mm diameter blind vias, 0.3mm center-to-center spacing between two blind vias, and a six-layer stacked blind via interconnect structure on the circuit board.

[0047] This product cannot be manufactured using traditional processes because the spacing between the pads at the bottom of the blind vias does not meet the etching requirements. However, with the process of this invention, the initial mechanical drilling diameter is designed to be 0.4mm, merging two blind vias with a spacing of 0.3mm into one mechanical drilling hole for processing. After copper plating, a copper plating layer with a thickness of ≥24μm is formed. The secondary mechanical drilling uses a 0.3mm diameter drill bit to divide the copper plating layer into two independent semi-circular copper vias.

[0048] Upon completion, the spacing between the two semi-circular copper holes is 0.3mm, meeting design requirements. The electroplated copper layer thickness is ≥24μm, conforming to IPC Class II standards. Electrical test results show that the conduction resistance and current carrying capacity meet product requirements, and mechanical bonding force test results show that the bonding force is more than 12 times that of traditional laser blind vias.

[0049] The production cycle for this product is 15 days, which is 30 days shorter than the conventional 45 days. Production costs are 30% of the conventional approach, resulting in an overall cost saving of approximately 21%.

[0050] In summary, this invention successfully solves the manufacturing problem of small-pitch blind vias in thick copper HDI circuit boards, and has significant advantages in terms of process simplification, efficiency improvement, and cost reduction, providing a brand-new technical solution for the manufacturing of thick copper HDI circuit boards.

[0051] The working principle and process of this invention are as follows: 1. Working principle When the center-to-center distance between two adjacent blind holes reaches the range of 0.25mm to 0.5mm and the thick copper area between the bottom pads is less than the etching process limit, this invention no longer drills the two blind holes independently. Instead, it merges the two blind holes into a single initial mechanical drill hole, so that the diameter of the mechanical drill hole covers the original blind hole spacing and the radius range of each of the two blind holes. This ensures that both original blind holes are included within the diameter range of the mechanical drill hole, thus avoiding the process limitation that thick copper etching cannot separate small-pitch pads from the source.

[0052] After the initial mechanical drilling, the hole walls are subjected to copper plating and electroplating to form a complete electroplated copper layer with a thickness ≥24μm, ensuring that the blind via's conductive cross-sectional area meets electrical performance requirements. Subsequently, resin-filled via ink is used to completely fill the drilled space, followed by segmented baking and curing to ensure the resin is fully cured, free of bubbles and filling defects, providing a stable support structure for subsequent secondary mechanical drilling.

[0053] After the resin has cured, a second mechanical drilling is performed at the same location. The diameter of the second mechanical drilling is smaller than that of the first mechanical drilling, with a pre-existing drilling offset allowance. This ensures that the electroplated copper layer can still be reliably drilled through even with the offset, dividing the integral electroplated copper layer into two independent semi-circular copper holes, which serve as the conduction paths for two blind holes. Finally, the gaps created by the second drilling are filled with resin again, completing the structure of the two independent blind holes.

[0054] The working principle of this invention is entirely based on the process steps and structural features defined in the claims. It does not require complex processes such as laser drilling, blind via etching, and multiple cycles of pressing. It can be stably applied to thick copper circuit boards with a copper thickness of 3oz to 8oz and 14-layer printed circuit boards with a six-layer stacked blind via interconnect structure. Under the premise of ensuring conduction resistance, current carrying capacity, and mechanical bonding force, it can achieve small-pitch blind via processing that cannot be completed by traditional processes.

[0055] 2. Work Process Step 1: Perform initial mechanical drilling on the area where two adjacent blind vias need to be formed on the PCB board. When the distance between the two blind vias causes the bottom pads to exceed the etching capacity of thick copper, the two blind vias are merged into a single drill hole, instead of being processed independently. The diameter of the mechanical drill hole is equal to the original blind via distance plus the pad extension distance of the two blind vias plus the radius of the two blind vias, thus ensuring that both original blind vias are completely contained within the diameter of the mechanical drill hole. In a preferred embodiment, the diameter of the initial mechanical drill hole is 0.4mm, which can stably cover the processing requirements of adjacent blind vias with a center-to-center distance of 0.25mm to 0.5mm, ensuring that the bottom pad areas of the two blind vias fall entirely within the same drill hole range, avoiding the inability to etch and separate the thick copper due to insufficient spacing. By merging the initial mechanical drill hole, this step bypasses the processing limits of traditional thick copper etching in the process path, laying the structural foundation for subsequent electroplating, via plugging, and separation steps.

[0056] The second step involves copper plating and electroplating of the hole walls formed by the initial mechanical drilling. The copper plating process first forms a continuous and uniform conductive copper seed layer on the hole wall surface, providing a conductive foundation for electroplating. Subsequently, the seed layer is thickened through electroplating, ultimately forming a complete, continuous, and uninterrupted electroplated copper layer on the hole wall. In this invention, the thickness of the electroplated copper layer is ≥24μm. This thickness, calculated by the process, ensures that the semi-arc-shaped copper hole still has sufficient conductive cross-sectional area after segmentation, so that the conduction resistance and current carrying capacity of the blind hole are consistent with those of conventional independent blind holes, meeting the IPC II standard requirements. After electroplating, the electroplated copper layer forms a complete electrical connection with the inner copper circuit layer of the PCB board. In a 14-layer printed circuit board with a six-layer stacked blind hole interconnection structure, the electroplated copper layer can be connected to the inner multilayer copper circuit simultaneously, improving the interlayer bonding strength and structural stability.

[0057] Step 3: The first mechanically drilled holes are filled with resin-filling ink to ensure complete resin filling of the drilled space, eliminating gaps, air bubbles, and insufficient resin, thus guaranteeing a dense internal structure. After filling, the filled areas undergo segmented baking and curing, following the curing regime defined in the claims: the first stage is baking at 110℃ for 60 minutes, and the second stage is baking at 150℃ for 30 minutes. This two-stage baking allows the resin to gradually cure from the surface inwards, reducing shrinkage stress and preventing resin cracking, air bubbles, and board warping. It also ensures a tight bond between the cured resin and the electroplated copper layer, providing stable support for secondary mechanical drilling. After the segmented baking and curing is completed, the plugging positions are kept flat on the board surface to meet the accuracy requirements of subsequent secondary drilling and outer layer circuit fabrication.

[0058] Step 4: Perform secondary mechanical drilling at the locations where the plugging and curing have been completed. The diameter of the secondary mechanical drilling is strictly smaller than that of the primary mechanical drilling. In a preferred embodiment, the diameter of the secondary mechanical drilling is 0.3 mm, forming a reasonable diameter difference with the primary 0.4 mm drilling. This ensures that the copper layer of the electroplated hole is evenly divided from the middle position. At the same time, the diameter of the secondary drilling is designed to allow for drilling offset. This allowance design ensures that even if normal process offset occurs in actual production, the copper layer of the electroplated hole can still be completely drilled through, avoiding defects such as incomplete cutting of the copper layer, local adhesion, and short circuits between different networks. Under the action of secondary mechanical drilling, the originally complete circular copper layer of the electroplated hole is divided into two independent, non-conductive, and symmetrically positioned semi-circular copper holes. The two semi-circular copper holes are located on both sides of the secondary drilling, each serving as an independent conductive path for a blind hole, achieving electrical isolation and independent transmission for two different networks.

[0059] Step 5: The intermediate holes formed by the secondary mechanical drilling are then filled and sealed with resin again to completely fill and seal the cavities created by the secondary drilling, preventing residual voids from causing defects such as bubbles, voids, and depressions in subsequent circuit fabrication and solder mask coating. The resin filling ink, filling method, and baking and curing regime used in the second resin filling are consistent with those in Step 3 to ensure that the filling materials are matched, the curing degree is consistent, and the structural bonding is stable, restoring the board surface to a complete and flat state. After the second resin filling is completed, the two independent semi-circular copper holes are stably wrapped in the cured resin, forming a blind via structure that is structurally complete, electrically independent, and has a very small spacing. The entire process of manufacturing blind vias for thick copper high-density interconnect circuit boards is now complete.

[0060] The entire process relies on differentiated aperture design, copper hole forming and segmentation, and double resin plugging and curing. It is compatible with 3oz-8oz thick copper plates and 14-layer printed circuit board processing with a six-layer stacked blind via interconnect structure, and stably achieves mass production processing of extremely small pitch blind vias of 0.25mm-0.5mm.

[0061] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of this patent.

Claims

1. A blind via fabrication process for a thick copper HDI circuit board, characterized in that, Includes the following steps: S1. Mechanical drilling is performed on the area on the PCB where two adjacent blind holes need to be formed; when the distance between the two blind holes causes the bottom pad to exceed the thick copper etching capacity, the two blind holes are merged and drilled into one hole. S2. Apply copper plating and electroplating treatment to the hole wall formed by drilling to form an electroplated copper layer on the hole wall. S3. Use resin-filling ink to fill the drilled holes until the space inside the drilled holes is completely filled, and bake and cure the filled area in sections. S4. Perform secondary mechanical drilling at the hole-filling location. The diameter of the secondary drilling is smaller than that of the first drilling. The copper plating layer formed by the first drilling is drilled through the secondary drilling to form two independent semi-circular copper holes, which serve as the conduction paths for two blind holes. S5. Perform resin plugging treatment on the secondary drilled holes to fill the pores generated by the secondary drilled holes.

2. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, In S1, the diameter of the mechanically drilled hole is 0.4 mm.

3. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, In S1, the diameter of the mechanically drilled hole covers the original blind hole spacing and the radius range of each of the two blind holes, so that both original blind holes are included within the diameter range of the mechanically drilled hole.

4. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, In S2, the thickness of the copper layer in the electroplated holes is ≥24μm.

5. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, In S3, the segmented baking and curing process includes a first stage of baking at 110°C for 60 minutes and a second stage of baking at 150°C for 30 minutes.

6. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, In S4, the diameter of the secondary mechanical drilling is 0.3 mm.

7. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, In S5, the diameter of the secondary drilling is reserved with a drilling offset allowance to ensure that the copper layer of the electroplated hole can still be drilled through even under the drilling offset state.

8. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, The center-to-center distance between two adjacent blind holes is 0.25 mm to 0.5 mm.

9. The blind via fabrication process for a thick copper HDI circuit board according to claim 1, characterized in that, The circuit board is a 14-layer printed circuit board with a six-layer stacked blind via interconnect structure and a copper thickness of 3 oz to 8 oz.

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