PCB and preparation method thereof

Abstract

The invention relates to a PCB and a preparation method thereof, in particular to the field of PCB manufacturing, and aims to solve the problem of magnetic field leakage caused by a back drilling hole opening structure. An annular groove is formed in the bottom face of a back drilling hole in the PCB, and / or a soft magnetic material thin film is arranged on the side face of the back drilling hole; and / or i hollow circular ring copper layers surrounding the back drilling hole are arranged on the surface of the copper layer adjacent to the drilling stopping layer of the back drilling hole in the PCB, and i is greater than or equal to 8. According to the PCB provided by the invention, the surface and the periphery of the back drilling hole are optimized, so that effective electromagnetic shielding can be formed, the magnetic flux leakage defect caused by the back drilling hole is further avoided, and the high-speed signal integrity of the PCB is improved.

Description

Technical Field

[0001] This invention relates to the field of PCB manufacturing, specifically to a PCB board and its preparation method, and more particularly to a PCB and its preparation method that improves high-speed signal integrity by changing the direction and path of magnetic field propagation in back-drilled holes. Background Technology

[0002] In PCB manufacturing, back-drilling is used to remove excess copper pillars from through-holes in multilayer boards, reducing signal interference and improving circuit performance. Traditionally, after drilling, back-drilled holes are typically chemically plated with copper (PTH) and then electroplated to fill the gaps, or at least chemically plated with tin (ENIG or OSP) to prevent copper oxidation and provide some electromagnetic shielding. However, this process has the following drawbacks:

[0003] (1) Severe vertical magnetic field leakage (antenna effect): A back-drilled hole is equivalent to a metal cylinder with an opening on the back of the PCB. When the signal current flows through the transmission line inside the hole, the generated surrounding magnetic field loses the constraint of the reference plane at the bottom of the hole. The vertical magnetic field component will "leak" directly from the opening of the back-drilled hole into the external space, resulting in signal energy radiation loss, and the hole can easily become an "antenna" that interferes with other circuits.

[0004] (2) Flat magnetic field cavity resonance: The exposed back-drilled hole sidewall forms a cylindrical resonant cavity. The horizontal magnetic field will be reflected and the standing wave effect will occur in the cavity. At a certain frequency (depending on the hole depth and diameter), the magnetic field energy will accumulate in the hole, causing the impedance of the transmission line to fluctuate drastically and the eye diagram to deteriorate severely.

[0005] Currently, the main methods to solve back-drilling interference are to shorten the back-drilling depth or increase the hole spacing, but these methods cannot fundamentally solve the problem of magnetic field leakage caused by the open structure. Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide a PCB board and its manufacturing method to solve the problem of magnetic field leakage caused by the back-drilled hole opening structure.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a PCB board in which an annular groove is provided on the bottom surface of a back-drilled hole, and / or a soft magnetic material film is provided on the side surface of the back-drilled hole.

[0009] And / or,

[0010] The PCB board has i hollow circular copper layers surrounding the back drill hole on the surface of the copper layer adjacent to the back drill hole, where i ≥ 8.

[0011] The PCB board provided by this invention can form effective electromagnetic shielding by optimizing the surface and surrounding area of ​​the back drill hole, thereby avoiding magnetic leakage defects caused by the back drill hole itself and improving the high-speed signal integrity of the PCB board.

[0012] As a preferred embodiment of the present invention, the depth of the annular groove is equal to the thickness of the copper layer used for back-drilling.

[0013] As a preferred embodiment of the present invention, the width of the annular groove is 0.08-0.15 mm.

[0014] As a preferred embodiment of the present invention, the vertical distance between the outer edge of the annular groove and the sidewall of the back drill hole is 0.1-0.3 mm.

[0015] As a preferred embodiment of the present invention, the inner diameter of the hollow annular copper layer is 0.1-0.15 mm.

[0016] As a preferred embodiment of the present invention, the outer diameter of the hollow circular copper layer is 0.2-0.3 mm.

[0017] As a preferred embodiment of the present invention, the distance between the outer circle of the hollow annular copper layer and the edge of the back drill hole is 0.1-0.2 mm.

[0018] As a preferred technical solution of the present invention, the soft magnetic material thin film includes: ferrite or nanocrystalline alloy.

[0019] As a preferred embodiment of the present invention, the thickness of the soft magnetic material film is 1-10 μm.

[0020] In a second aspect, the present invention provides a method for manufacturing a PCB board as described in the first aspect, the method comprising:

[0021] Laser processing is performed on the bottom surface of the back-drilled hole to obtain an annular groove, or sputtering deposition is performed on the side surface of the back-drilled hole to obtain a thin film of soft magnetic material.

[0022] And / or, a hollow annular copper layer is prepared by etching on the surface of the copper layer adjacent to the back-drill stop layer.

[0023] Compared with existing technical solutions, the present invention has the following beneficial effects:

[0024] (1) Blocking radiation interference: The vertical magnetic field is "locked" in the hole by the soft magnetic thin film layer, which significantly reduces the ability of the back drill hole to radiate electromagnetic interference (EMI) as an "antenna". The length of the magnetic trace formed by the leakage magnetic test is ≤0.3mm.

[0025] (2) Eliminating cavity resonance: The high loss characteristics of soft magnetic materials (at high frequencies) can absorb the energy of the horizontal magnetic field reflected back and forth in the hole, damping the resonance peak and keeping the transmission impedance flat.

[0026] (3) Maintain process advantages: The core signal path remains in the state of exposed copper, which retains the advantages of heat dissipation performance and low contact resistance, while solving the problem of incomplete signal.

[0027] The present invention will now be described in further detail. However, the examples described below are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims. Detailed Implementation

[0028] To better illustrate the present invention and facilitate understanding of its technical solutions, typical but non-limiting embodiments of the present invention are as follows:

[0029] Currently, in the PCB manufacturing field, back-drilling is used to remove excess copper pillars from through-holes in multilayer boards to reduce signal interference and improve circuit performance. After drilling, back-drilled holes are typically chemically plated with copper (PTH) and electroplated to fill the gaps, or at least chemically plated with tin (ENIG or OSP) to prevent copper oxidation and provide some electromagnetic shielding. However, back-drilled holes still cause electromagnetic leakage. Methods to address back-drilling interference mainly involve shortening the drilling depth or increasing the hole spacing, but these methods cannot fundamentally solve the magnetic field leakage problem caused by the open structure. Therefore, this invention significantly reduces magnetic field leakage by optimizing the surface and adjacent areas of the back-drilled holes, as detailed below:

[0030] I. This embodiment provides a PCB board in which an annular groove is provided on the bottom surface of the back-drilled hole, and / or a soft magnetic material film is provided on the side surface of the back-drilled hole.

[0031] And / or,

[0032] The PCB board has i hollow circular copper layers surrounding the back drill hole on the surface of the copper layer adjacent to the back drill hole, where i ≥ 8.

[0033] In this invention, PCB board refers to a multilayer PCB board, which is formed by laminating three or more conductive pattern layers with insulating materials in between.

[0034] In this invention, the stop-drill layer refers to the layer where the drill bit stops drilling during the back-drilling process, that is, the unbreakable layer. The drill bit does not penetrate this layer. At this time, the bottom surface of the corresponding back-drilled hole is the surface of the stop-drill layer, that is, the conductive pattern layer such as the copper layer.

[0035] In this invention, an annular groove or dense via ring (i.e., a hollow annular copper layer) is designed around the center of the back-drilled hole. This structure alters the closed path of the horizontal magnetic field. The horizontal magnetic field, which originally surrounded the transmission line, attempted to bypass the back-drilled hole, resulting in a larger loop. By introducing the annular groove or grounding via, the magnetic field is forced to form a very small closed loop near the hole opening, thereby limiting the diffusion range of the horizontal magnetic field and eliminating inductive crosstalk caused by the elongated return path.

[0036] The depth of the annular groove is equal to the thickness of the copper layer used for back-drilling.

[0037] The width of the annular groove is 0.08-0.15 mm, for example, it can be 0.08 mm, 0.087 mm, 0.094 mm, 0.101 mm, 0.108 mm, 0.115 mm, 0.122 mm, 0.129 mm, 0.136 mm, 0.143 mm or 0.15 mm, but is not limited to the listed values. Other unlisted values ​​within this range also meet the requirements.

[0038] In this invention, the annular grooves are distributed around the center of the back drill hole.

[0039] The vertical distance between the outer edge of the annular groove and the sidewall of the back drill hole is 0.1-0.3 mm, for example, it can be 0.1 mm, 0.12 mm, 0.14 mm, 0.16 mm, 0.18 mm, 0.2 mm, 0.22 mm, 0.24 mm, 0.26 mm, 0.28 mm or 0.3 mm, but is not limited to the listed values. Other unlisted values ​​within this range also meet the requirements.

[0040] The inner diameter of the hollow annular copper layer is 0.1-0.15 mm, for example, it can be 0.1 mm, 0.105 mm, 0.11 mm, 0.115 mm, 0.12 mm, 0.125 mm, 0.13 mm, 0.135 mm, 0.14 mm, 0.145 mm or 0.15 mm, but is not limited to the listed values. Other unlisted values ​​within this range also meet the requirements.

[0041] In this invention, inner diameter refers to diameter.

[0042] The outer diameter of the hollow annular copper layer is 0.2-0.3 mm, for example, it can be 0.2 mm, 0.21 mm, 0.22 mm, 0.23 mm, 0.24 mm, 0.25 mm, 0.26 mm, 0.27 mm, 0.28 mm, 0.29 mm or 0.3 mm, but is not limited to the listed values. Other unlisted values ​​within this range are also acceptable.

[0043] In this invention, outer diameter refers to diameter.

[0044] The distance between the outer circle of the hollow annular copper layer and the edge of the back drill hole is 0.1-0.2 mm, for example, it can be 0.1 mm, 0.11 mm, 0.12 mm, 0.13 mm, 0.14 mm, 0.15 mm, 0.16 mm, 0.17 mm, 0.18 mm, 0.19 mm or 0.2 mm, but is not limited to the listed values. Other unlisted values ​​within this range also meet the requirements.

[0045] In this invention, a thin film of a soft magnetic material (such as ferrite or nanocrystalline alloy) with high magnetic permeability and low electrical conductivity is deposited on the inner wall of the back-drilled hole (non-electrical connection area) using physical vapor deposition (PVD) or sputtering. This film does not participate in signal transmission (no electrical connection), but physically covers the exposed copper wall. According to Ohm's law of magnetic circuits, magnetic field lines attempting to leak vertically will preferentially pass through the soft magnetic film with higher magnetic permeability rather than leaking through the air. This "bends" and "adheres" the vertical magnetic field lines around the hole wall, preventing them from radiating outwards, while simultaneously absorbing resonant energy and converting it into heat.

[0046] The soft magnetic material thin film includes ferrite or nanocrystalline alloy.

[0047] In this invention, the ferrite can be selected as manganese-zinc ferrite or nickel-zinc ferrite, and can be selected from commercially available products or prepared according to existing technology.

[0048] In this invention, the nanocrystalline alloy can be selected as an iron-based nanocrystalline alloy or a cobalt-based nanocrystalline alloy, and can be prepared using commercially available products or based on existing technologies.

[0049] The thickness of the soft magnetic material film is 1-10 μm, for example, it can be 1 μm, 1.9 μm, 2.8 μm, 3.7 μm, 4.6 μm, 5.5 μm, 6.4 μm, 7.3 μm, 8.2 μm, 9.1 μm or 10 μm, but is not limited to the listed values. Other unlisted values ​​within this range are also acceptable.

[0050] II. This embodiment provides a method for preparing a PCB board, the method comprising:

[0051] Laser processing is performed on the bottom surface of the back-drilled hole to obtain an annular groove, or sputtering deposition is performed on the side surface of the back-drilled hole to obtain a thin film of soft magnetic material.

[0052] And / or,

[0053] A hollow circular copper layer is prepared by etching on the surface of the copper layer adjacent to the back-drilling stop layer.

[0054] In this invention, the processing methods used in the preparation process, such as laser, sputtering deposition, etching, etc., can be selected and designed according to the conventional requirements of the relevant fields. They are all existing technologies and will not be described in detail in this invention. For example, laser processing refers to existing technologies (Zhang Xiao, Liu Kai, Wang Mingdi, et al. Etching process of copper clad laminate based on femtosecond laser [J]. Acta Optica Sinica, 2019, 39(12):8.DOI:10.3788 / AOS201939.1214003.), and sputtering deposition refers to existing technologies (Zhan Changyong, Jiang Wen, Zou Yu, et al. Morphology study of Ti film deposited by magnetron sputtering on porous silicon surface [J]. China Science and Technology Paper Online, 2014(003):007.).

[0055] In this invention, a hollow annular copper layer is prepared by etching on the surface of the copper layer adjacent to the back drill hole stop layer. This process is carried out at the corresponding position before the multilayer PCB is assembled, meaning that the hollow annular copper layer is formed before the back drill hole is formed.

[0056] III. To illustrate the effects of the PCB board provided by this invention, the following practical example is used for explanation:

[0057] Example 1

[0058] This embodiment provides a PCB board, as detailed below:

[0059] Step 1: Fabricate a 10-layer PCB. The first layer is the signal layer, and the tenth layer is the back-drilled surface (without tin plating). Drill the back-drilled holes from the tenth layer to the third layer. The diameter of the back-drilled holes is 0.8mm.

[0060] Step 2: On the copper foil of the third layer (drill stop layer), with the center of the back drill hole as the center, use laser etching to create an annular groove with a width of 0.1mm. The groove is 0.2mm away from the edge of the hole wall.

[0061] The annular groove cuts off the large loop current at the bottom of the hole, forcing the return current to form a very small-diameter ring inside the groove. According to Ampere's circuital law, the magnetic field is confined only to the inside of the groove, and external magnetic field interference is isolated.

[0062] Example 2

[0063] This embodiment provides a PCB board, as detailed below:

[0064] Step 1: Fabricate a 10-layer PCB. The first layer is the signal layer, and the tenth layer is the back-drilled surface (without tin plating). The back-drilled holes are drilled from the tenth layer to the third layer, which is the stop-drill layer. On the second layer, eight hollow circular copper rings are arranged around the back-drilled holes. The inner diameter of the ring is 0.1mm, the outer diameter is 0.2mm, the distance between the outer ring and the edge of the back-drilled hole is 0.1mm, and the diameter of the back-drilled hole is 0.8mm.

[0065] The air cavity formed by the hollow circular copper layer provides a fixed closed path for the magnetic flux, which is mainly concentrated within the annular space of the ring, reducing the probability of the magnetic flux diverging into the external space.

[0066] Example 3

[0067] This embodiment provides a PCB board, as detailed below:

[0068] Step 1: Use a CCD drilling machine to complete the back drilling. The hole diameter is 0.8mm. This is a 10-layer PCB; layer 1 is the signal layer, and layer 10 is the back drilling surface (without tin plating). The back drilling starts from layer 10 and stops at layer 3.

[0069] Step 2: After deburring and cleaning, a 2μm thick Ni-Zn Ferrite film is deposited on the inner wall surface of the back drill hole using a sputtering device.

[0070] This ferrite material has high permeability (μ>100) in the 1GHz-10GHz frequency band, which can effectively "bend" the vertical magnetic field lines back into the hole, eliminating radiation leakage.

[0071] Example 4

[0072] The only difference from Example 1 is that the depth of the annular groove is equal to 0.5 times the thickness of the copper layer at the back drill hole stop.

[0073] Example 5

[0074] The only difference from Example 1 is that the width of the annular groove is 0.05 mm.

[0075] Example 6

[0076] The only difference from Example 1 is that the width of the annular groove is 0.2 mm.

[0077] Example 7

[0078] The only difference from Example 2 is that the inner diameter of the hollow circular copper layer is 0.05 mm.

[0079] Example 8

[0080] The only difference from Example 2 is that the inner diameter of the hollow circular copper layer is 0.2 mm.

[0081] Example 9

[0082] The only difference from Example 2 is that the outer diameter of the hollow circular copper layer is 0.15 mm.

[0083] Example 10

[0084] The only difference from Example 2 is that the outer diameter of the hollow circular copper layer is 0.35 mm.

[0085] Example 11

[0086] The only difference from Example 2 is that the distance between the outer circle of the hollow circular copper layer and the edge of the back drill hole is 0.05 mm.

[0087] Example 12

[0088] The only difference from Example 2 is that the distance between the outer circle of the hollow circular copper layer and the edge of the back drill hole is 0.25 mm.

[0089] Example 13

[0090] The only difference from Example 1 is that the thickness of the soft magnetic material film is 0.05 μm.

[0091] Comparative Example 1

[0092] The only difference from Example 1 is that the annular groove is replaced with a circle of equal area.

[0093] Comparative Example 2

[0094] The only difference from Example 2 is that the hollow circular copper layer is set to 6.

[0095] Comparative Example 3

[0096] The only difference from Example 1 is that a thin film of soft magnetic material is not provided.

[0097] The PCB boards obtained in the above embodiments and comparative examples were subjected to magnetic particle testing. Specifically, the PCB board was placed in a magnetic field and magnetic powder was placed around the back drill hole. If there was magnetic leakage around the back drill hole, the magnetic powder would accumulate in the magnetic leakage area to form visible magnetic traces, which visually represent the distribution range of magnetic leakage. The magnetic traces would be straight or arc-shaped. The length of the magnetic traces was measured to determine the magnetic leakage situation. The longer the magnetic trace, the more serious the magnetic leakage. The details are shown in Table 1 below.

[0098] Table 1

[0099]

[0100] As shown in Table 1, the PCB board provided by the present invention can form an effective electromagnetic shield by optimizing the surface and surrounding area of ​​the back drill hole, thereby avoiding magnetic leakage defects caused by the back drill hole itself, and thus improving the high-speed signal integrity of the PCB board.

[0101] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0102] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0103] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A PCB board, characterized in that, The bottom surface of the back-drilled hole in the PCB board is provided with an annular groove, and / or the side surface of the back-drilled hole is provided with a soft magnetic material film. And / or, The PCB board has i hollow circular copper layers surrounding the back drill hole on the surface of the copper layer adjacent to the back drill hole, where i ≥ 8.

2. The PCB board as described in claim 1, characterized in that, The depth of the annular groove is equal to the thickness of the copper layer used for back-drilling.

3. The PCB board as described in claim 1, characterized in that, The width of the annular groove is 0.08-0.15 mm.

4. The PCB board as described in claim 1, characterized in that, The vertical distance between the outer edge of the annular groove and the sidewall of the back drill hole is 0.1-0.3 mm.

5. The PCB board as described in claim 1, characterized in that, The inner diameter of the hollow circular copper layer is 0.1-0.15 mm.

6. The PCB board as described in claim 1, characterized in that, The outer diameter of the hollow circular copper layer is 0.2-0.3 mm.

7. The PCB board as described in claim 1, characterized in that, The distance between the outer circle of the hollow annular copper layer and the edge of the back drill hole is 0.1-0.2 mm.

8. The PCB board as described in claim 1, characterized in that, The soft magnetic material thin film includes: ferrite or nanocrystalline alloy.

9. The PCB board as described in claim 1, characterized in that, The thickness of the soft magnetic material film is 1-10 μm.

10. A method for manufacturing a PCB board as described in any one of claims 1-9, characterized in that, The preparation method includes: Laser processing is performed on the bottom surface of the back-drilled hole to obtain an annular groove, or sputtering deposition is performed on the side surface of the back-drilled hole to obtain a thin film of soft magnetic material. And / or, A hollow circular copper layer is prepared by etching on the surface of the copper layer adjacent to the back-drilling stop layer.

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