PCB (Printed Circuit Board) structure with non-plated-through hole for improving impedance of surface-mounted bonding pad

Abstract

The invention provides a PCB structure with non-metallized holes for improving impedance of a surface-mounted bonding pad, the surface of a PCB board is provided with the surface-mounted bonding pad, one end of the surface-mounted bonding pad is used for connecting a transmission line, and a plurality of non-metallized holes are drilled in the PCB board below one end, opposite to the end connected with the transmission line, of the surface-mounted bonding pad. And the non-metallized hole is drilled from the bottom surface of the PCB to the wiring layer where the surface-mounted bonding pad is located. According to the invention, the relative dielectric constant of the non-plated-through hole area can be reduced to 1 at 1GHz, so that the equivalent relative dielectric constant of the dielectric layer below the surface-mounted bonding pad is reduced. And on the other hand, the reference ground plane of the surface-mounted bonding pad is hollowed in the non-plated-through hole area, so that the distance between the surface-mounted bonding pad and the reference ground is correspondingly increased. Therefore, the impedance of the stub position of the surface-mounted bonding pad can be effectively improved. Moreover, the non-plated-through hole structure is obtained through a back drilling technology, the back drilling technology is stable and mature, processing is easy, and the processing cost of the PCB is hardly increased.

Description

Technical Field

[0001] The invention relates to a PCB structure of a non-metallized hole for improving the impedance of a surface-mount pad. Background Art

[0002] As the signal rate increases, the system's requirements for signal integrity are becoming more and more stringent. Among them, signal reflection caused by impedance discontinuity has a particularly serious impact on signal quality, causing signal overshoot, ringing, and hook-back. PCIE connectors and high-speed pluggable connectors on the IO side are generally in the form of surface-mount pads. Considering the reliability of welding, surface-mount pads are generally designed to be relatively wide and have a long pad length, which easily causes the pad impedance to be much lower than the impedance of the transmission line and other positions in the channel, resulting in impedance discontinuity, which in turn causes serious channel reflection and reduces the quality of signal transmission.

[0003] like Figure 4 As shown, it is a high-speed pluggable connector on the IO side. The signal is transmitted to the surface mount pad 11 through the L-shaped solder foot 14, and then enters the chip inside the board through the microstrip line 13. To ensure the reliability of welding, the surface mount pad 11 is longer, so a signal stump 12 is formed, which reduces the impedance of the pad.

[0004] For the gold finger structure, such as Figure 5 The gold finger pad 21 is connected to the microstrip line 23, and there is also a residual pile 22 between the gold finger pad 21 and the spring 20, which reduces the impedance of the gold finger pad.

[0005] In response to the above problems, the conventional way to improve pad impedance is usually to set an anti-pad on the reference ground plane below the surface mount pad. A few will increase the pad impedance by increasing the pad spacing or reducing the pad size. In the above method, setting an anti-pad on the ground plane below the surface mount pad essentially increases the distance between the pad and the reference ground, which can improve the pad impedance to a certain extent. However, when the reference ground plane is far away from a certain distance, the pad impedance basically no longer changes. On the other hand, an anti-pad that is too large or setting anti-pads on too many ground planes will affect the wiring space of the PCB and the size of the power plane; increasing the pad spacing will also affect the wiring space, and reducing the pad size will affect the reliability of welding. Once the connector model is selected in the early stage, the size of the surface mount pad is also determined.

[0006] However, the conventional way to improve the impedance of the surface mount pad is to increase the impedance of the pad position as a whole to make it close to the transmission line impedance, while the low impedance point exists at the pad stump position. The conventional method cannot effectively improve the impedance mismatch between the stump and the front end of the pad.

[0007] Therefore, the invention patent with application publication number CN117177437A proposes a multi-layer PCB structure for improving the pad impedance of copper wire, including more than three wiring layers and dielectric layers located between the upper and lower adjacent wiring layers, wherein the topmost wiring layer is provided with pads for copper wire and connecting transmission lines, and at least one wiring layer below except the bottommost wiring layer is provided with anti-pads corresponding to the topmost layer, characterized in that a dielectric hole array is provided in the anti-pad projection area, the dielectric hole array is stored in the dielectric layer between the wiring layer where the pad is located and the wiring layer provided with the anti-pad, and the dielectric hole is filled with a medium with a low dielectric constant. The aperture of the dielectric hole is not less than 4 mil, and the sum of the projection areas of all dielectric holes is not less than 1 / 2 of the corresponding anti-pad projection area.

[0008] However, CN117177437A still has disadvantages:

[0009] 1. The processing method is difficult, uncontrollable, and costly. This patent punches dielectric holes during PCB lamination, which is similar to blind buried holes in the PCB process. However, the machinability of dielectric holes without copper foil (no copper foil in the anti-pad area) support and positioning is still uncertain, and the feasibility of filling dielectrics with low dielectric constants is also difficult to evaluate. On the other hand, when dielectric holes need to be punched in two adjacent dielectric layers, the center alignment of dielectric holes in different dielectric layers is also a difficulty. If lamination causes the dielectric holes in different layers to shift, the performance may be greatly reduced. Furthermore, the "blind buried hole" process has high processing costs and high process requirements, and there will also be special requirements for the hole spacing of the dielectric hole array.

[0010] 2. The overall impedance of the pad is improved, not the impedance of the pad stump. As mentioned above, the low point of the pad impedance mainly comes from the position of the pad stump. This patent can improve the overall impedance of the pad and reduce the impedance discontinuity between the pad and the transmission line, but it cannot effectively improve the continuity of the impedance between the pad stump and the front end of the pad. Summary of the invention

[0011] Based on this, the present invention proposes a PCB structure with a non-metallized hole for improving the impedance of a surface mount pad, which can effectively improve the impedance of the impedance stump position without reducing the pad size or increasing the pad spacing.

[0012] The technical solution adopted by the present invention is:

[0013] A PCB structure of a non-metallized hole for improving the impedance of a surface mount pad, wherein a surface mount pad is provided on the surface of the PCB board, and one end of the surface mount pad is used to connect a transmission line, and is characterized in that:

[0014] A plurality of non-metallized holes are drilled inside the PCB board below the end of the surface mount pad opposite to the end connected to the transmission line. The non-metallized holes are drilled from the bottom surface of the PCB board to the wiring layer where the surface mount pad is located.

[0015] The PCB structure with non-metallized holes for improving the impedance of the surface mount pad, wherein: at least one non-metallized hole is provided corresponding to each surface mount pad.

[0016] The PCB structure with non-metallized holes for improving the impedance of surface mount pads, wherein: the hole diameter of the non-metallized holes is at least 0.15 mm, and the hole wall spacing is at least 3 mil.

[0017] The PCB structure with non-metallized holes for improving the impedance of surface mount pads, wherein: the PCB board is formed by alternately stacking wiring layers and dielectric layers.

[0018] The PCB structure with non-metallized holes for improving the impedance of the surface mount pad, wherein: ground pads are provided on both sides of the surface mount pad, and ground holes are provided on the PCB board corresponding to the ground pad, and the ground pad is grounded through the ground holes.

[0019] The PCB structure of the non-metallized hole for improving the impedance of the surface mount pad, wherein: an anti-pad that at least covers the area of ​​the surface mount pad is also provided in the wiring layer inside the PCB board, and the anti-pad is arranged in 1-2 wiring layers below the surface mount pad.

[0020] The PCB structure of non-metallized holes for improving the impedance of surface mount pads, wherein: the arrangement range of the non-metallized holes does not exceed the solder joints of the surface mount pads, nor does it exceed the plug-in connection contact points.

[0021] The PCB structure with non-metallized holes for improving the impedance of the surface mount pad, wherein: the inside of the PCB board corresponding to at least one side of the surface mount pad is also processed with the non-metallized holes.

[0022] The PCB structure of non-metallized holes for improving the impedance of surface mount pads, wherein adjacent non-metallized holes are interconnected to form non-metallized grooves.

[0023] The PCB structure with non-metallized holes for improving the impedance of surface mount pads, wherein: the interior of the non-metallized holes is air.

[0024] The beneficial effects brought by the technical solution of the present invention are as follows:

[0025] 1. The present invention reduces the relative dielectric constant of the non-metallized hole area to 1@1GHz by setting a non-metallized hole under the surface mount pad, thereby reducing the equivalent relative dielectric constant of the dielectric layer under the surface mount pad. On the other hand, the reference ground plane of the surface mount pad is hollowed out in the non-metallized hole area, which correspondingly increases the distance between the surface mount pad and the reference ground. Therefore, the impedance of the residual position of the surface mount pad can be effectively improved.

[0026] 2. The non-metallized hole structure of the present invention is obtained through a back drilling process. The back drilling technology is stable and mature, easy to process, and hardly increases the processing cost of the PCB. In addition, the processable hole diameter and relative position are highly flexible.

[0027] 3. The non-metallized hole position of the present invention only acts on the surface mount pad stump position and does not affect the wiring space too much.

[0028] 4. The present invention provides a non-metallized PCB structure for improving the impedance of the surface mount pad, which can be used simultaneously with a solution of setting an anti-pad on the reference layer below the pad. On the basis of achieving the target impedance, the reference layer of the anti-pad can be flexibly reduced, and the power plane area can be correspondingly increased, or the risk of the surface mount pad referencing the power plane can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural schematic diagram of the present invention,

[0030] Figure 2 It is a cross-sectional view of the structure of the present invention.

[0031] Figure 3 The figure is a comparison chart of impedance simulation results of the PCB structure of the present invention and the conventional method of optimizing pad impedance using anti-pad (the anti-pad is of the most optimized size).

[0032] Figure 4 It is a schematic diagram of a pluggable connector forming a stump.

[0033] Figure 5 It is a schematic diagram of the residual pile formed by the gold finger structure.

[0034] Explanation of the reference numerals: 1-ground pad; 2-ground via; 3-transmission line; 4-non-metallized hole; 5-surface mount pad; 6-anti-pad; 7-sub-top wiring layer; 8-middle wiring layer; 9-bottom wiring layer; 10-dielectric layer; 11-surface mount pad 11; 12-stub; 13-microstrip line; 14-L-shaped solder foot; 20-spring; 21-gold finger pad; 22-stub; 23-microstrip line. DETAILED DESCRIPTION

[0035] The following is combined with Figure 1 , Figure 2 , Figure 3 , an implementation method of a PCB structure of a non-metallized hole for improving the impedance of a surface mount pad proposed by the present invention is described in detail. Obviously, the embodiments described here are only part of the embodiments of the present invention, not all of them.

[0036] In this embodiment, a PCB structure with non-metallized holes for improving the impedance of surface mount pads is provided. The PCB board material is M6G board material, which has a relative dielectric constant of 3.4@1GHz. The wiring layers (including the sub-top wiring layer 7, the middle wiring layer 8 and the bottom wiring layer 9) are alternately stacked with the dielectric layer 10, and the surface mount pads 5 are PCIE connector pads.

[0037] Figure 1 and Figure 2 They are respectively a plan view and a cross-sectional view of a PCB structure containing a non-metallized hole in the present embodiment, a surface mount pad 5 is provided on the surface of the PCB board, one end of the surface mount pad 5 is connected to a transmission line 3 for transmitting signals to a chip on the PCB; ground pads 1 are provided on both sides of the surface mount pad 5, a ground hole 2 is provided on the PCB board corresponding to the ground pad 1, and the ground pad 1 is grounded through the ground hole 2; since the surface mount pad 5 has a low impedance, an anti-pad 6 covering at least the area of ​​the surface mount pad 5 is further provided in the wiring layer inside the PCB board (in the present embodiment, the anti-pad 6 is provided in the sub-top wiring layer 7 adjacent to the surface mount pad, and the size of the anti-pad 6 is obtained by simulation optimization); all of these belong to the prior art;

[0038] Among them, in this embodiment, a plurality of non-metallized holes 4 are drilled inside the PCB board corresponding to the end (i.e., the stub) of the surface mount pad 5 opposite to the transmission line 3 (each surface mount pad 5 corresponds to three non-metallized holes 4, the hole radius is 9 mil, and the hole wall spacing is 3.5 mi), and the non-metallized holes 4 are drilled from the bottom surface of the PCB board to the bottom of the surface mount pad 5, passing through the alternately stacked wiring layers (including the sub-top wiring layer 7, the middle wiring layer 8 and the bottom wiring layer 9) and the dielectric layer 10 along the way;

[0039] Furthermore, the aperture size, number and position of the non-metallized holes 4 can be determined by a simulation method. The hole diameter of the non-metallized holes 4 is not less than 0.15 mm and can be larger than the width of the surface mount pad 5. The arrangement range of the non-metallized holes 4 does not exceed the solder joints or plug-in connection contact points of the surface mount pad 5.

[0040] In addition, the present invention can also process non-metallized holes 4 between a pair of surface mount pads 5 or at positions on both sides corresponding to the PCB board.

[0041] In addition, there may be no spacing between adjacent non-metallized holes 4, and they may interfere with each other, or even be connected to form a non-metallized groove (formed by multiple drillings).

[0042] The non-metallized hole 4 can be processed by a mature back-drilling method, drilling from the bottom wiring layer 9 to the wiring layer where the surface mount pad 5 is located. The hole after back-drilling is not filled with any medium, and the inside is air. Therefore, the relative dielectric constant of the non-metallized hole 4 area is reduced to 1.0@1GHz. At this time, the relative dielectric constant of the dielectric layer 10 below the surface mount pad 5 is between 1.0-3.4 (@1GHz). On the other hand, since the non-metallized hole 4 drills through the wiring layer (including the sub-top wiring layer 7, the middle wiring layer 8 and the bottom wiring layer 9), the reference ground distance of the residual pile of the surface mount pad 5 is relatively increased, further improving the residual pile impedance.

[0043] Figure 3 The differential signal impedance comparison diagram of the PCB structure without the structure of the present invention (with anti-pad and no non-metallized hole) and the PCB structure with the structure of the present invention is shown, and m1 and m2 respectively represent the impedance of the surface mount pad stump before and after the structure of the present invention is adopted. It can be seen that after adopting the structure of the present invention, the pad stump impedance is increased by 6.75ohm, and the impedance continuity between the pad stump and the front end of the pad is significantly improved.

[0044] In summary, the beneficial effects brought by the technical solution of the present invention are as follows:

[0045] 1. The present invention reduces the relative dielectric constant of the non-metallized hole area to 1@1GHz by setting a non-metallized hole under the surface mount pad, thereby reducing the equivalent relative dielectric constant of the dielectric layer under the surface mount pad. On the other hand, the reference ground plane of the surface mount pad is hollowed out in the non-metallized hole area, which correspondingly increases the distance between the surface mount pad and the reference ground. Therefore, the impedance of the residual position of the surface mount pad can be effectively improved.

[0046] 2. The non-metallized hole structure of the present invention is obtained through a back drilling process. The back drilling technology is stable and mature, easy to process, and hardly increases the processing cost of the PCB. In addition, the processable hole diameter and relative position are highly flexible.

[0047] 3. The non-metallized hole position of the present invention can only act on the surface mount pad stump position and will not affect the wiring space too much.

[0048] 4. The present invention provides a non-metallized PCB structure for improving the impedance of the surface mount pad, which can be used simultaneously with a solution of setting an anti-pad on the reference layer below the pad. On the basis of achieving the target impedance, the reference layer of the anti-pad can be flexibly reduced, and the power plane area can be correspondingly increased, or the risk of the surface mount pad referencing the power plane can be reduced.

[0049] The above description is only an embodiment of the present invention and is not intended to limit the present invention. Any improvements made to the method concept and technical solution of the patent of the present invention, or the direct application of the concept and technical solution of the patent of the present invention to other occasions without improvement, are within the protection scope of the present invention.

Claims

1. A PCB structure with a non-metallized hole for improving the impedance of a surface mount pad, wherein a surface mount pad is provided on the surface of the PCB board, and one end of the surface mount pad is used to connect a transmission line, characterized in that: A plurality of non-metallized holes are drilled inside the PCB board below the end of the surface mount pad opposite to the end connected to the transmission line. The non-metallized holes are drilled from the bottom surface of the PCB board to the wiring layer where the surface mount pad is located.

2. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: At least one non-metallized hole is provided corresponding to each surface mount pad.

3. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 2, characterized in that: The diameter of the non-metallized hole is at least 0.15 mm, and the hole wall spacing is at least 3 mil.

4. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: The PCB board is formed by alternately stacking wiring layers and dielectric layers.

5. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: Ground pads are arranged on both sides of the surface mount pad, and ground holes are arranged on the PCB board corresponding to the ground pads, and the ground pads are grounded through the ground holes.

6. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: An anti-pad which at least covers the area of ​​the surface-mount pad is also provided in the wiring layer inside the PCB board, and the anti-pad is arranged in 1-2 wiring layers below the surface-mount pad.

7. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: The arrangement range of the non-metallized holes does not exceed the solder joints of the surface mount pads, nor does it exceed the plug-in connection contact points.

8. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: The non-metallized hole is also processed inside the PCB board corresponding to the position of at least one side of the surface mount pad.

9. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: Adjacent non-metallized holes are connected to each other to form non-metallized grooves.

10. The PCB structure of non-metallized holes for improving surface mount pad impedance according to claim 1, characterized in that: The interior of the non-metallized hole is air.

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