PCB multilayer buried hole plate and processing method thereof

Abstract

The application discloses a PCB multilayer buried hole plate and a processing method thereof, and the processing method comprises the steps of preparing an inner layer core plate, performing buried hole prefilling treatment and the like, the inner layer core plate is independently prepared in layers and is subjected to mechanical drilling and metallization, the traditional PCB multilayer buried hole plate lamination process is optimized by combining the prefilling resin partial hole filling and single vacuum hot pressing forming, the process flow is simplified, the traditional blind hole electroplating hole filling step is omitted, the pressing times are reduced to one time, the production cycle is greatly shortened, the cost is reduced and the efficiency is improved, the buried hole only needs to be thin-walled and metallized, the copper plating thickness is reduced, the electroplating uniformity is improved and the copper material cost is saved, the precision and reliability are improved, the hole wall is supported by the prefilling resin, the interlayer alignment precision is improved from ±6 mil to ±2 mil by combining the positioning rivet thermal expansion compensation mechanism, one-time gradient pressing makes the semi-hardened PP sheet completely fill the buried hole, and the layer deviation and stress accumulation caused by multiple pressing are avoided, and the plate thickness is reduced by adopting the ultrathin inner layer core plate and high-precision depth control glue filling technology.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board technology, specifically to a multilayer buried via PCB and its processing method. Background Technology

[0002] Printed circuit boards (PCBs) serve as the support and electrical connection carrier for electronic components, fixing and connecting components such as chips, resistors, and capacitors through insulating substrates and conductive lines. As electronic products evolve towards miniaturization and high performance, single-layer or ordinary multilayer PCBs can no longer meet the requirements for wiring density and signal integrity, and are gradually being replaced by multilayer buried via PCBs with multilayer interconnect structures. Manufacturing such multilayer buried via PCBs primarily relies on a multi-layer lamination process, achieving multilayer interconnection through layer-by-layer laser-drilled blind vias and lamination curing. This multilayer interconnect structure can significantly increase wiring density, resulting in shorter signal transmission paths and higher wiring space utilization.

[0003] However, existing PCB multilayer buried via board manufacturing methods typically require lamination temperatures of 175°C for at least 70 minutes to ensure complete curing of the adhesive material and increase the stability of interlayer connections. Adding the waiting time for heating and cooling, a single lamination cycle can easily exceed 4 hours. Furthermore, to achieve finer wiring density and electrical performance, PCB multilayer buried via board manufacturing requires complex technologies such as stacked via design, electroplating, and laser direct drilling. While these technologies improve board performance, they also significantly increase the complexity of the manufacturing process (e.g., multiple precise alignments, selective electroplating, and additional via filling processes). The entire process is not only time-consuming and complex, but also results in increasing board thickness with each lamination cycle. Therefore, developing new PCB multilayer buried via board manufacturing methods that optimize the lamination process, reduce the number of hot-pressing cycles, simplify procedures, and reduce board thickness are crucial for improving PCB multilayer buried via board performance, expanding production capacity, and saving costs. Summary of the Invention

[0004] To address the aforementioned challenges, one objective of this application is to provide a method for processing multilayer buried via PCB boards, comprising the following steps:

[0005] Fabrication of the inner core board: Mechanical drilling is performed on the inner core board to form buried holes penetrating the inner core board, with the axis of the buried holes perpendicular to the plane of the inner core board; the inner wall and surface of the buried holes in the inner core board are metallized; wherein...

[0006] For the inner core board at the top of the stacked structure, the outer surface copper foil far from the stack center is completely covered with dry film without pattern transfer and pattern etching. The inner surface copper foil close to the stack center is subjected to pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via.

[0007] For the inner core board at the bottom of the stacked structure, the outer surface copper foil far from the stack center is completely covered with dry film without pattern transfer and pattern etching. The inner surface copper foil close to the stack center is subjected to pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via.

[0008] For the inner core board located in the middle layer when forming a stacked structure, the copper foil on the upper and lower surfaces undergoes pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via.

[0009] Pre-filling treatment for buried holes: The buried holes are partially pre-filled with pre-filled resin by pressure injection, and the filling depth is 1 / 4 to 1 / 3 of the buried hole depth;

[0010] To form a laminated structure: each inner core board after being pre-filled with buried holes is alternately laminated with a semi-cured PP sheet, and the buried holes on each inner core board are positioned corresponding to each other.

[0011] Pre-treatment of the laminated structure: The laminated structure is placed in a vacuum oven with a vacuum degree ≤10Pa and subjected to gradient preheating, i.e., the temperature is increased from 20~30℃ to 80~100℃ at a rate of 2~3℃ / min, held for 20~40min, and then the temperature is increased to 115~125℃ and held for 20~40min. After preheating, the vacuum state is maintained and a pre-pressure of 0.5~1.5MPa is applied and held for 5~15min.

[0012] One-time pressing: Continue heating to 180~200℃, hot press the entire laminated structure, so that the adhesive of the semi-cured PP sheet melts and flows and completely fills the buried holes. After the semi-cured PP sheet is cured, the mechanical connection between adjacent inner core boards is achieved.

[0013] Overall drilling and metallization: Through holes are machined and metallized sequentially on the laminated structure. Then, the outer surface copper foil of the top and bottom inner core boards, which is far from the center of the laminate, is transferred and etched to obtain a multilayer buried via PCB.

[0014] Preferably, the prefilled resin is a modified epoxy resin, model AUS-308.

[0015] Preferably, in the step of preparing the inner core board, the thickness of the inner core board is ≤0.3mm.

[0016] Preferably, in the step of preparing the inner core board, the diameter of the buried holes is 0.15~1.0mm, and the density of buried holes on a single inner core board is ≤20000 / m².

[0017] Preferably, the thickness of the electroplated layer on the inner wall of the buried hole is ≤15μm.

[0018] Preferably, in the step of forming the laminated structure, the adhesive content of the semi-cured PP sheet is ≥65wt%.

[0019] Preferably, in the single-stage pressing step, the overall hot pressing includes:

[0020] Pre-positioning stage of riveting: Insert positioning rivets at the four corners of the laminated structure and apply a preload of 0.5~1.5MPa to ensure that the coaxiality deviation of the buried hole axis of each core board is ≤±3mil;

[0021] Fusion and pressing stage: Under the constraint of positioning rivets, the temperature is raised to 180~200℃ at 1~2℃ / min, while applying 3~5MPa pressure and holding for 60~90min, so that the semi-cured PP sheet melts and completely penetrates into the buried hole to form an insulating column.

[0022] Another objective of this application is to provide a multilayer buried via PCB, which is manufactured using the above-described processing method and includes at least two inner core layers and a semi-cured PP sheet located between adjacent inner core layers. The inner core layers are provided with buried vias, which are sequentially filled with the pre-filled resin and the molten adhesive material of the semi-cured PP sheet. The inner wall of the buried vias and the surface of the adhesive material are provided with a metallization layer to achieve electrical conductivity between adjacent inner core layers.

[0023] The beneficial effects are:

[0024] This application optimizes the traditional PCB multilayer buried via board stacking process by independently preparing the inner core board in layers, mechanically drilling and metallizing it, and combining pre-filled resin partial filling and single-stage vacuum hot pressing. It simplifies the process flow, eliminating the traditional blind via electroplating filling step, reducing the number of pressing steps to one, and significantly shortening the production cycle. It reduces costs and increases efficiency, requiring only thin-walled metallization (≤15μm) for buried vias, reducing copper plating thickness, improving plating uniformity, and saving copper material costs. It improves precision and reliability by using pre-filled resin to support the via walls, combined with a positioning rivet thermal expansion compensation mechanism, increasing interlayer alignment accuracy from ±6mil to ±2mil. A single gradient pressing allows the semi-cured PP sheet to completely fill the buried vias, avoiding layer misalignment and stress accumulation caused by multiple pressings. It reduces board thickness by using an ultra-thin inner core board (≤0.3mm) and high-precision depth-controlled filling technology, achieving an overall thinner and lighter board. Detailed Implementation

[0025] This embodiment provides a method for processing a multilayer buried via PCB, including the following steps:

[0026] (1) Preparation of inner core board: Mechanical drilling is performed on the inner core board with copper foil on both sides to form buried holes through the inner core board. The axis of the buried hole is perpendicular to the plane of the inner core board. The diameter of the buried hole is 0.15~1.0mm, and the density of buried holes on a single inner core board is ≤20000 / m². This can avoid the increased drilling difficulty and hole wall damage caused by the hole diameter being too small or the density being too high, and at the same time reduce signal interference between holes. The inner wall and surface of the buried holes of the inner core board are metallized. Specifically, copper plating process is used to plate the inner wall and surface of the buried holes with copper. The copper plating process is existing technology and will not be described in detail here. The thickness of the electroplated layer on the inner wall of the buried hole is ≤15μm. By controlling the thickness of the metal layer on the hole wall, the internal space of the buried hole is not excessively occupied due to excessive thickness, so as to reserve sufficient space for subsequent resin filling. At the same time, the thin metal layer is more likely to form a tight bond with the filling resin, reducing the risk of interface delamination.

[0027] For the inner core board at the top of the stacked structure, the outer surface copper foil far from the stack center is completely covered with dry film without pattern transfer and pattern etching. The inner surface copper foil close to the stack center is subjected to pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via.

[0028] For the inner core board at the bottom of the stacked structure, the outer surface copper foil far from the stack center is completely covered with dry film without pattern transfer and pattern etching. The inner surface copper foil close to the stack center is subjected to pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via.

[0029] For the inner core board located in the middle layer when forming a stacked structure, the copper foil on the upper and lower surfaces undergoes pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via. The thickness of all inner core boards is ≤0.3mm, which ensures that 8~12 inner core boards are stacked at one time, while maintaining the stacked structure thickness ≤2.0mm, in order to meet the ultra-thin board requirements of handheld terminals. In addition, the thinner inner core boards are also beneficial to improve the alignment accuracy during lamination, reduce the delay of signal transmission between layers, and make it easier to bond with the semi-cured PP sheet, reducing the gaps between layers. The outer surface copper foil of the top and bottom inner core boards is completely covered with dry film, and pattern transfer and etching are not performed temporarily. This protects the outer copper foil from damage (such as scratches and oxidation) in subsequent processes such as stacking and lamination. It is processed in the final step to ensure the accuracy and integrity of the outer pattern. The inner layer (including the inner surface of the middle layer and the inner surface of the top / bottom) copper foil is patterned in advance, which can complete the inner layer circuit structure before stacking, reduce the complexity of subsequent overall processing, and reduce the risk of damage to the inner layer pattern.

[0030] (2) Pre-filling treatment of buried holes: The pre-filled resin is pre-filled partially into the buried holes by pressure injection, with a filling depth of about 1 / 4 to 1 / 3 of the buried hole depth. The pre-filled resin can be modified epoxy resin of model AUS-308, purchased from Shanghai Showa Electric Materials Co., Ltd. The pressure injection method can reduce the generation of air bubbles and accurately control the filling depth, leaving space and channels for the inflow of PP glue during subsequent pressing. After the pre-filled resin is cured, it can provide support for the hole wall, reduce the impact and stress concentration of the molten semi-cured PP sheet on the fragile metallized hole wall during pressing, reduce the risk of microcracks, and protect the integrity of the hole wall. The modified epoxy resin 308 is a pre-filled resin for the via, not only because of its low viscosity, small coefficient of thermal expansion, and high thermal conductivity, but also because it can fuse well with molten semi-cured PP sheet. This allows the pre-filled resin to bond tightly with the metallized via wall and combine with the molten semi-cured PP sheet during pressing, forming a stronger and more uniform resin-metal interface and resin-resin interface. This greatly improves the mechanical strength of the buried via area (such as bending resistance and impact resistance), reduces the risk of delamination, and significantly improves the heat conduction path of the buried via area. This helps the heat generated by the chip or high power consumption area to be conducted more effectively to the inside or outside of the PCB through the buried via, reducing the temperature of local hot spots.

[0031] (3) Forming a stacked structure: Each inner core board after being pre-filled with buried holes is alternately stacked with a semi-cured PP sheet with an adhesive content of ≥65wt%, and the buried hole positions on each inner core board correspond to each other; the use of semi-cured PP sheet with high adhesive content is to avoid voids due to insufficient buried hole filling caused by insufficient adhesive content, while ensuring the interlayer insulation and mechanical connection strength; however, the volume shrinkage rate of semi-cured PP sheet with high adhesive content is large after melting, so it is compensated together with pre-filled resin to achieve 100% buried hole filling; the corresponding buried hole positions of each inner core board are to ensure that the subsequent pre-filled resin and semi-cured PP sheet can flow smoothly into and fill the buried holes after melting, avoiding misalignment that leads to filling defects;

[0032] (4) Pre-treatment of the laminated structure: The laminated structure is placed in a vacuum oven with a vacuum degree ≤10Pa and preheated in a gradient manner, i.e., the temperature is increased from 20~30℃ to 80~100℃ at 2~3℃ / min, and then held for 20~40min. Then the temperature is increased to 115~125℃ and held for 20~40min. After preheating, the vacuum state is maintained and a pre-pressure of 0.5~1.5MPa is applied and held for 5~15min. This step can simultaneously remove trace amounts of water vapor and volatiles from the core board, pre-filled resin and semi-cured PP sheet, remove air from the interlayer gaps, improve the buried hole filling rate and initially fix the laminated structure.

[0033] (5) One-time pressing: Continue heating to 180~200℃, and hot press the entire laminated structure to make the adhesive of the semi-cured PP sheet melt and flow and completely fill the buried holes. After the semi-cured PP sheet is cured, the mechanical connection between adjacent inner core boards is achieved. Specifically, the overall hot pressing includes the riveting pre-positioning stage and the fusion pressing stage. The riveting pre-positioning stage is to insert positioning rivets at the four corners of the laminated structure and apply a pre-pressure of 0.5~1.5MPa to make the coaxiality deviation of the buried hole axis of each core board ≤±3mil; the fusion pressing .... Under the constraint of the positioning rivets, the temperature is increased to 180-200℃ at a rate of 1-2℃ / min, while a pressure of 3-5MPa is applied and the temperature is maintained for 60-90min. This allows the semi-cured PP sheet to melt and completely penetrate into the buried holes to form an insulating column. Through gradient heating and gradient pressing, the bending deformation of the board after lamination is reduced, while the PP sheet adhesive is uniformly filled into the buried holes. In addition, the axial expansion of the positioning rivets during hot pressing can compensate for the interlayer displacement caused by the curing shrinkage of the pre-filled resin and the semi-cured PP sheet, so that the final interlayer alignment accuracy is stabilized within ±2mil.

[0034] (6) Overall drilling and metallization: Mechanical processing and through-hole metallization are performed sequentially on the laminated structure. Then, the outer surface copper foil of the inner core board at the top and bottom, which is far from the center of the laminate, is transferred and etched to obtain a multilayer buried hole PCB.

[0035] This embodiment provides a multilayer buried via PCB, manufactured using the above-described processing method. It includes at least two inner core layers and a semi-cured PP sheet located between adjacent inner core layers. The inner core layers have buried vias, which are sequentially filled with pre-filled resin and molten semi-cured PP sheet adhesive. A metallization layer is provided on the inner wall of the buried via and the surface of the adhesive to achieve electrical conductivity between adjacent inner core layers. After lamination, through-holes are uniformly processed and metallized. The fixed stacked structure ensures the alignment accuracy of the through-holes with the buried vias / circuits of each layer, avoiding deviations caused by interlayer displacement during pre-stack drilling. Finally, pattern transfer and etching are performed on the outer copper foil. This allows for precise formation of the outer layer circuit while protecting the outer copper foil from damage in previous processes, ultimately completing the full-layer circuit connection and ensuring the functional integrity of the PCB.

[0036] In summary, this embodiment optimizes the traditional PCB multilayer buried via board stacking process by independently preparing the inner core board in layers, performing mechanical drilling and metallization, and combining pre-filled resin partial filling and single-stage vacuum hot pressing. It simplifies the process flow, eliminating the traditional blind via electroplating filling step, reducing the number of pressing cycles to one, and significantly shortening the production cycle. It reduces costs and increases efficiency, as buried vias only require thin-wall metallization (≤15μm), reducing copper plating thickness, improving plating uniformity, and saving copper material costs. It improves precision and reliability, with pre-filled resin supporting the via walls and a positioning rivet thermal expansion compensation mechanism, increasing interlayer alignment accuracy from ±6mil to ±2mil. A single gradient pressing allows the semi-cured PP sheet to completely fill the buried vias, avoiding layer misalignment and stress accumulation caused by multiple pressings. It reduces board thickness by using an ultra-thin inner core board (≤0.3mm) and high-precision depth-controlled filling technology, achieving an overall thinner and lighter board.

[0037] The above are merely embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A method for processing a multilayer buried via PCB, characterized in that, Includes the following steps: Fabrication of the inner core board: Mechanical drilling is performed on the inner core board to form buried holes penetrating the inner core board, the axis of which is perpendicular to the plane of the inner core board; the inner wall and surface of the buried holes in the inner core board are metallized; wherein... For the inner core board at the top of the stacked structure, the outer surface copper foil far from the stack center is completely covered with dry film without pattern transfer and pattern etching. The inner surface copper foil close to the stack center is subjected to pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via. For the inner core board at the bottom of the stacked structure, the outer surface copper foil far from the stack center is completely covered with dry film without pattern transfer and pattern etching. The inner surface copper foil close to the stack center is subjected to pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via. For the inner core board located in the middle layer when forming a stacked structure, the copper foil on the upper and lower surfaces undergoes pattern transfer and pattern etching after metallization of the inner wall and surface of the buried via. Pre-filling treatment of buried holes: The buried holes are partially pre-filled with pre-filling resin by pressure injection, and the filling depth is 1 / 4 to 1 / 3 of the depth of the buried holes; The stacked structure is formed by alternately stacking the inner core boards after the buried hole pre-filling treatment with semi-cured PP sheets, and the buried holes on the inner core boards are corresponding to each other. Pre-treatment of the laminated structure: The laminated structure is placed in a vacuum oven with a vacuum degree ≤10Pa and subjected to gradient preheating, i.e., the temperature is increased from 20~30℃ to 80~100℃ at a rate of 2~3℃ / min, held for 20~40min, and then the temperature is increased to 115~125℃ and held for 20~40min; after preheating, the vacuum state is maintained and a pre-pressure of 0.5~1.5MPa is applied and held for 5~15min. One-time pressing molding: Continue heating to 180~200℃, and hot press the entire laminated structure to make the adhesive of the semi-cured PP sheet melt and flow and completely fill the buried hole. After the semi-cured PP sheet is cured, the interlayer mechanical connection of the adjacent inner core boards is realized. Overall drilling and metallization: Through holes are machined and metallized sequentially on the laminated structure. Then, the outer surface copper foil of the top and bottom inner core boards away from the center of the laminate is transferred and etched to obtain the PCB multilayer buried via board.

2. The PCB multilayer buried via board processing method according to claim 1, characterized in that, The pre-filled resin is a modified epoxy resin, model AUS-308.

3. The PCB multilayer buried via board processing method according to claim 1, characterized in that, In the step of preparing the inner core board, the thickness of the inner core board is ≤0.3mm.

4. The PCB multilayer buried via board processing method according to claim 1, characterized in that, In the step of preparing the inner core board, the diameter of the buried holes is 0.15~1.0mm, and the density of buried holes on a single inner core board is ≤20000 / m².

5. The PCB multilayer buried via board processing method according to claim 4, characterized in that, The thickness of the electroplated layer on the inner wall of the buried hole is ≤15μm.

6. The PCB multilayer buried via board processing method according to claim 1, characterized in that, In the step of forming the laminated structure, the semi-cured PP sheet has an adhesive content of ≥65wt%.

7. The PCB multilayer buried via board processing method according to claim 1, characterized in that, In the single-stage pressing step, the overall hot pressing includes: Pre-positioning stage of riveting: Insert positioning rivets at the four corners of the laminated structure and apply a preload of 0.5~1.5MPa to ensure that the coaxiality deviation of the buried hole axis of each core board is ≤±3mil; Fusion and pressing stage: Under the constraint of the positioning rivets, the temperature is raised to 180~200℃ at 1~2℃ / min, while applying a pressure of 3~5MPa and holding for 60~90min, so that the semi-cured PP sheet melts and completely penetrates into the buried hole to form an insulating column.

8. A PCB multilayer buried via board, characterized in that, Prepared using the processing method described in any one of claims 1-7, comprising at least two inner core layers and a semi-cured PP sheet located between adjacent inner core layers; the inner core layers are provided with embedded holes, the embedded holes are sequentially filled with the pre-filled resin and the adhesive material after the semi-cured PP sheet is melted, and the inner wall of the embedded holes and the surface of the adhesive material are provided with a metallization layer to achieve electrical conductivity between adjacent inner core layers.