A processing method for suspended conductors of printed circuit boards for three-dimensional vertical interconnection

Abstract

The present invention discloses a method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection, comprising the following steps: fixing the printed circuit board on a laser engraving machine platform; performing laser drilling at a position between each two conductors; stacking the holes of several printed circuit boards in correspondence with each other, and performing potting to form a three-dimensional stacked potting body; performing plasma roughening on the potting body, and then plating a metal layer on the surface of the potting body to complete the surface metallization. By laser drilling, the substrate on both sides of the conductor is completely removed, avoiding the possibility of leakage caused by the substrate exposed at the edge of the potting body when the side wall of the lead cross section is interconnected after stacking and potting. At the same time, it solves the problem of long processing cycle caused by widening and thickening the suspended copper wire to improve its structural strength, reduces the processing flow, and significantly shortens the processing cycle, which can significantly improve the yield rate.

Description

Technical Field

[0001] The invention belongs to the technical field of integrated circuit manufacturing and processing, and relates to a method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection. Background Art

[0002] The contradiction between the demand for continuous miniaturization of electronic systems and the backward system integration manufacturing technology has made the development of high-density assembly manufacturing technology, especially three-dimensional integration technology, very urgent. By stacking and interconnecting various components in the vertical direction, the system size and weight can be further reduced, which can effectively solve the bottleneck problem of volume and weight limitations in weapons and equipment or aerospace products due to strict requirements on space and payload.

[0003] 3D PoP microsystem modules utilize PoP technology to create a three-dimensional stack of multiple printed circuit boards. Each PCB has conductors extending to the edge. After potting and contour processing, the conductors exposed on the module sidewalls provide vertical interconnection between layers. Without any treatment, the PCB dielectric layer beneath the conductors presents a potential for leakage after laser processing. While the traditional "thickened suspended bridge leads" process can completely remove the PCB dielectric layer, this process is complex and requires electroplating to thicken the suspended leads, more than doubling their width and thickness. This results in poor lead shape and dimensional consistency, high stress, and edge warping and peeling. Consequently, the PCB processing yield has consistently remained below 60%, with single-batch processing cycles exceeding one month. Furthermore, the limitations of widening and thickening the leads prevent further reductions in wiring density, making it impossible to meet the high-density interconnection and rapid delivery requirements of 3D PoP microsystem packaging. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems in the prior art of requiring the suspended leads to be electroplated and thickened, resulting in poor consistency in the final lead morphology and size, high stress, and easy edge warping and peeling, and to provide a processing method for suspended wires on printed circuit boards for three-dimensional vertical interconnection.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection, comprising the following steps:

[0007] Fix the printed circuit board on the laser engraving machine platform;

[0008] Laser opening is performed between every two wires;

[0009] The openings of several printed circuit boards are aligned and stacked in sequence, and potted to form a three-dimensional stacked potting body;

[0010] Cut the potting body to expose the cross section of the printed circuit board conductors wrapped by the potting body;

[0011] After the potting body is plasma roughened, a metal layer is plated on the surface of the potting body to complete the surface metallization.

[0012] A further improvement of the present invention is:

[0013] When the thickness of the printed circuit board is greater than 0.3 mm, before laser drilling the printed circuit board, the printed circuit board is first subjected to step groove processing, and the specific steps include:

[0014] Measure the entire printed circuit board and complete the leveling of the step groove position;

[0015] Processing of stepped grooves in the flattened printed circuit board area;

[0016] Purge and clean the printed circuit board.

[0017] The specific steps for leveling the step groove position are:

[0018] Fix the printed circuit board on the CNC machine tool and measure the entire board;

[0019] Take the leveling point as the processing standard point, and lower 0.1 to 0.3 mm from the processing standard point as the Z-axis leveling depth;

[0020] Mill back and forth in the X and Y axis directions to mill the designated position of the printed circuit board into the same horizontal plane, completing the leveling of the step groove position.

[0021] The laser engraving machine is equipped with a CO2 laser, the working power of the laser is ≤20W, and the wavelength is 10.6μm.

[0022] The laser engraving machine can also be equipped with a UV laser, and the operating wavelength of the laser is 355nm.

[0023] The operating power during the plasma roughening is 5000W and the operating temperature is 50°C.

[0024] The mixed gas used in the plasma roughening includes CF4, O2 and N2, and the volume flow ratio is 1.5:8:0.5.

[0025] The distance between the laser opening and the wire is 50-100 μm.

[0026] The potting adopts liquid epoxy resin.

[0027] The metal layer is plated on the surface of the potting body by chemical plating and electroplating.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The present invention proposes a method for processing suspended conductors on a printed circuit board for three-dimensional vertical interconnection. By laser drilling, the substrate on both sides of the conductor is completely removed, thereby avoiding the possibility of leakage caused by the substrate exposed at the edge of the potting body when the side wall of the lead section is interconnected after stacking and potting. At the same time, the problem of long processing cycle caused by widening and thickening the suspended copper wire to improve its structural strength is solved. The processing process is reduced, the processing cycle is significantly shortened, and the yield can be significantly improved.

[0030] Furthermore, by performing surface plasma roughening on the potting body, the roughness of the epoxy resin potting body can be improved, and the oil, oxide scale, etc. remaining on the surface during the previous processing can be effectively removed, providing a good hydrophilic surface for subsequent composite plating; and the plasma can produce differentiated etching on the surface of the potting body, and its reaction rates to the epoxy resin and the metal lead terminal are inconsistent, which can make the exposed suspended wire cross-section form a metal terminal that is slightly protruding from the surface of the potting body, thereby increasing the connection area and adhesion of the plating on the surface of the metal terminal. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0032] Figure 1 This is a flow chart of a method for processing a suspended wire according to the present invention;

[0033] Figure 2 This is a schematic diagram of the structure of the printed circuit board to be processed in the present invention;

[0034] Figure 3 Schematic diagram of the printed circuit board flattening area in the present invention;

[0035] Figure 4 Schematic diagram of the stepped groove structure in the present invention;

[0036] Figure 5 Schematic diagram of the laser opening area structure in the present invention;

[0037] Figure 6 This is an enlarged front view of the interconnected suspended wires in the present invention;

[0038] Figure 7 This is an enlarged schematic diagram of the back side of the interconnected suspended wires of the present invention;

[0039] Figure 8 This is a schematic diagram of achieving three-dimensional vertical electrical interconnection after stacking, potting and surface composite plating of a multilayer printed circuit board in the present invention;

[0040] Figure 9 Schematic diagram of the distance between the rectangular hole between the wires and the edge wires in the present invention.

[0041] Among them: 1-laminated substrate, 2-edge interconnected suspended wires, 3-composite metal plating, 4-epoxy potting resin, 5-edge wires, 6-rectangular holes between wires. DETAILED DESCRIPTION

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0043] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0044] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0045] In the description of the embodiments of the present invention, it should be noted that if the terms "upper," "lower," "horizontal," "inner," etc. appear, the orientation or positional relationship indicated is based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the inventive product is typically placed when in use. These terms are merely for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or component referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on the present invention. In addition, the terms "first," "second," etc. are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0046] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0047] In the description of the embodiments of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0048] The present invention is described in further detail below with reference to the accompanying drawings:

[0049] See also Figure 1 , is a flow chart of the method for processing the suspended wire in the present invention, which specifically includes the following steps:

[0050] S1, fix the printed circuit board on the laser engraving machine platform.

[0051] S2, laser drilling is performed between every two conductors. The conductor area refers to the conductors that need to be interconnected between multilayer printed circuit boards.

[0052] The laser engraving machine can be equipped with a CO2 laser with an operating power of ≤20W and a wavelength of 10.6μm; it can also be equipped with a UV laser with an operating wavelength of 355nm; the distance between the laser opening and the wire is 50 to 100μm.

[0053] S3, stacking the openings of several printed circuit boards in correspondence with each other in sequence, and potting them to form a three-dimensional stacked potting body, wherein the potting is performed using liquid epoxy resin.

[0054] S4, perform precise cutting at the laser opening position to expose the cross-section of interconnecting wires of each layer of printed circuit board;

[0055] S5, after the potting body is roughened by plasma, a metal layer is plated on the surface of the potting body to complete the surface metallization.

[0056] The working power during plasma roughening is 5000W, the working temperature is 50°C, and the mixed gas used during plasma roughening includes CF4, O2 and N2, with a volume flow ratio of 1.5:8:0.5; the metal layer plating on the surface of the potting body includes chemical nickel plating and electroplating nickel gold or chemical nickel plating and electroplating copper nickel gold.

[0057] See also Figure 2-Figure 5 , is a schematic diagram of a specific process of the processing method of the suspended wire in the present invention, wherein the thickness of the printed circuit board is greater than 0.3 mm, and specifically includes the following steps:

[0058] S1, perform step groove processing on the printed circuit board.

[0059] S1.1, measure the entire printed circuit board and complete the leveling of the step groove position.

[0060] The normal production process is used to complete the processing and manufacturing of the printed circuit board. Then the printed circuit board is fixed on the CNC machine tool. First, the entire board is measured and leveled. This is used as the processing standard point. Based on the standard point, it is first lowered by 0.1mm to 0.3mm as the Z-axis leveling depth. Then, a double-pointed fish scale knife is assembled and milled back and forth in the X and Y axis directions to mill the designated position of the printed circuit board into the same horizontal plane to complete the leveling work of the step groove position.

[0061] S1.2, perform step groove processing in the flattened printed circuit board area.

[0062] After leveling is completed, according to the thickness of the printed circuit board and the processing depth of the step groove, the leveled printed circuit board area is used as the basis, and the "height after leveling - 0.1mm" is used as the depth-controlled milling height. After the cutter head is replaced with a depth-controlled milling flat head cutter, the step groove is processed.

[0063] During processing, first try to make the first piece, and use the depth gauge to measure the first piece with the step groove processed. At least 5 points of the center and edge of the step groove should be measured. The required thickness should be retained to meet the requirements of 0.07mm to 0.15mm, and there are no other abnormalities in the appearance before batch processing.

[0064] S1.3, purge and clean the printed circuit board.

[0065] The printed circuit board that has been processed with the stepped groove in the previous step is purged. If necessary, it can be gently scrubbed with a brush in anhydrous ethanol to remove the residual substrate and resin on the surface. Place it on the laser engraving machine platform and fix it with residue-free tape to ensure that the printed circuit board is flat.

[0066] S1.4, fix the printed circuit board on the laser engraving machine platform.

[0067] S1.5, laser opening is performed between every two wires.

[0068] A CO2 laser machine with a power of ≤20W and a wavelength of approximately 10.6μm is preferred. This can effectively etch epoxy resin substrates while absorbing less energy from metals like copper. Alternatively, a UV laser with a wavelength of approximately 355nm can provide better control over the processing effect.

[0069] The laser machine also features visual positioning. After the stepped grooves are processed, laser engraving is performed between each pair of conductors, completely removing the printed circuit board substrate between adjacent conductors through laser ablation. To prevent laser energy from burning the conductor edges, the laser processing position is kept 50 to 100 μm from the conductor edge, depending on the type of laser selected.

[0070] S1.6, stacking the openings of several printed circuit boards in sequence so as to correspond to each other and potting them to form a three-dimensional stacked potting body.

[0071] S1.7, after the potting body is plasma roughened, a metal layer is plated on the surface of the potting body to complete the surface metallization.

[0072] The PCBs, which have undergone step-grooving and laser-cut holes, are placed sequentially into a stacking mold from bottom to top, with the laser-engraved holes aligned for each layer. Liquid epoxy resin is then used to form a three-dimensional potting structure. Precise cuts are made at the laser-cut holes to expose the cross-sections of the interconnecting leads of each layer. Plasma etching technology is then used to roughen the epoxy resin and the interconnecting lead terminals encased in the PCB substrate. This process effectively roughens the epoxy resin and PCB substrate without reacting with the copper conductors. The plasma roughening conditions are 5000W power, 50°C temperature, and a CF4:O2:N2 volume flow ratio of 1.5:8:0.5.

[0073] After plasma roughening, the epoxy resin potting material and the wrapped printed circuit board substrate present a uniform matte and slightly dark appearance, carry a negative charge, and have good hydrophilicity. At this time, chemical nickel plating and electroplating nickel gold are carried out to complete the surface metallization, and finally realize the three-dimensional vertical electrical interconnection between the lead terminals in different layers of printed circuit boards.

[0074] See also Figure 6-Figure 7 , which is an enlarged schematic diagram of the front and back sides of the interconnected suspended wires in the present invention. The printed circuit board after processing is subjected to a 100% visual inspection under a low-power microscope, i.e., a magnification of 15× to 30×. It is required that the rectangular hole processing shape is complete, the substrate is clean, and the wires have no obvious signs of burning, oxidation discoloration, etc.

[0075] See also Figure 8, which is a schematic diagram of the three-dimensional vertical electrical interconnection achieved after stacking, potting, and surface composite plating of a multilayer printed circuit board in the present invention. After the printed circuit boards are stacked and potted in a specified order and orientation, they are precisely cut to expose the cross-sections of the suspended conductors at the edges of each layer of the printed circuit board. At this time, a plasma etching process is used to roughen the surface of the potting body. The roughening conditions are an operating power of 5000W, a temperature of 50°C, and a volume flow ratio of CF4, O2, and N2 of 1.5:8:0.5. The main functions of plasma roughening include: improving the roughness of the epoxy resin potting body, effectively removing oil stains and oxide scale remaining on the surface from previous processing, and providing a good hydrophilic surface for subsequent composite plating; plasma can produce differential etching of epoxy resin and does not react with metal, so that the exposed cross-sections of the suspended conductors can be formed into raised metal terminals, further improving the adhesion of the plating layer to the metal terminal surface. The epoxy resin potting body after plasma roughening is subjected to chemical nickel plating and electroplating of nickel gold or chemical nickel plating and electroplating of copper nickel gold to complete the surface metallization, and finally realize the three-dimensional vertical electrical interconnection between the lead terminals in different layers of printed circuit boards.

[0076] When a laser machine equipped with a CO2 laser is selected in the present invention, the wire width accuracy L1 is ±10% of the median, preferably L1 ≥ 100 μm; the distance between the wire and the rectangular hole L2 ≥ 100 μm; the rectangular hole width L3 ≥ 200 μm; the minimum value of the wire spacing L4 is L3 + 2 × L2, i.e. 400 μm.

[0077] When ultraviolet laser is used in the present invention, the wire width accuracy L1 is ±10% of the median, preferably L1≥100um; the distance between the wire and the rectangular hole L2 is ≥50μm; the rectangular hole width L3 is ≥200μm; the minimum value of the wire spacing L4 is L3+2×L2, that is, 300μm.

[0078] Compared to the existing process flow that requires thickening the suspended bridge leads, the method of the present invention provides a small amount of substrate beneath the copper wires at the edge of the printed circuit board, providing some reinforcement and support, preventing the copper wires from hanging in the air. The substrate on both sides of the wires is also completely removed, preventing leakage caused by exposed substrate at the edge of the potting body when the lead cross-sections are connected to the side walls after stacking and potting. This also solves the problem of long processing cycles caused by widening and thickening the suspended copper wires to improve their structural strength. The overall process reduces several steps compared to existing solutions, significantly shortening the processing cycle and significantly improving the yield rate.

[0079] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection, characterized in that: The following steps are involved: The printed circuit board is processed into stepped grooves, and the thickness of the printed circuit board is greater than 0.3mm; Fix the printed circuit board on the laser engraving machine platform; In the area where the step groove processing is completed, laser engraving is performed in the area between each two wires to completely remove the printed circuit board substrate in the area between adjacent wires through laser ablation; The openings of several printed circuit boards are aligned and stacked in sequence, and potted to form a three-dimensional stacked potting body; Cut the potting body to expose the cross section of the printed circuit board conductors wrapped by the potting body; After the potting body is plasma roughened, a metal layer is plated on the surface of the potting body to complete the surface metallization.

2. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The step groove processing of the printed circuit board specifically comprises the following steps: Measure the entire printed circuit board and complete the leveling of the step groove position; Processing of stepped grooves in the flattened printed circuit board area; Purge and clean the printed circuit board.

3. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 2, characterized in that: The specific steps for leveling the step groove position are: Fix the printed circuit board on the CNC machine tool and measure the entire board; Take the leveling point as the processing standard point, and lower 0.1~0.3mm from the processing standard point as the Z-axis leveling depth; Mill back and forth in the X and Y axis directions to mill the designated position of the printed circuit board into the same horizontal plane, completing the leveling of the step groove position.

4. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The laser engraving machine is equipped with a CO2 laser, the working power of the laser is ≤20W, and the wavelength is 10.6μm.

5. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The laser engraving machine can also be equipped with a UV laser, and the operating wavelength of the laser is 355nm.

6. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The operating power during the plasma roughening is 5000W and the operating temperature is 50°C.

7. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The mixed gas used in the plasma roughening includes CF4, O2 and N2, and the volume flow ratio is 1.5:8:0.

5.

8. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The distance between the laser engraved opening and the conductive wire is 50-100 μm.

9. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The potting adopts liquid epoxy resin.

10. A method for processing suspended conductors of a printed circuit board for three-dimensional vertical interconnection according to claim 1, characterized in that: The metal layer is plated on the surface of the potting body by chemical plating and electroplating.

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