CN113163624B

CN113163624B - Reverse pad design method for differential via hole and printed circuit board

Info

Publication number: CN113163624B
Application number: CN202110466626.1A
Authority: CN
Inventors: 羊杨
Original assignee: Embedway Technologies Shanghai Corp
Current assignee: Embedway Technologies Shanghai Corp
Priority date: 2021-04-28
Filing date: 2021-04-28
Publication date: 2022-10-25
Anticipated expiration: 2041-04-28
Also published as: CN113163624A

Abstract

The invention provides a method for designing reverse welding discs of differential via holes and a printed circuit board, wherein the printed circuit board comprises two differential via holes in pairs, a reflux ground hole is respectively arranged beside each differential via hole and is positioned on an extension line of a central connecting line of the two differential via holes, the two differential via holes are respectively provided with a single-fillet rectangular reverse welding disc, the two reverse welding discs are symmetrical about a perpendicular bisector of the central connecting line, a fillet of each reverse welding disc is close to the perpendicular bisector, the differential routing passes through a gradual change region between the fillets of the two reverse welding discs, the reflux ground hole is positioned outside each reverse welding disc, and the edge of each reverse welding disc is tangent to the excircle of the corresponding reflux ground hole. The anti-pad structure obtained by the anti-pad design method provided by the invention can enable the impedance of the differential via hole and the impedance of the transmission line to be properly matched, and is beneficial to reducing the reflection of high-speed signals.

Description

Reverse bonding pad design method for differential via hole and printed circuit board

Technical Field

The invention relates to the technical field of printed circuit boards, in particular to a method for designing a reverse bonding pad of a differential via hole and a printed circuit board.

Background

The differential wiring is widely applied to high-speed signal transmission, differential signals transmitted by the differential wiring are transmitted in an odd mode, noise is coupled to the differential wiring in a common mode, and a receiving end judges whether the logic state is '0' or '1' by comparing the difference value of the two voltages, so that the common mode noise is eliminated by subtracting the two voltages, and the anti-interference performance is stronger compared with a single-end wiring.

However, as transmission rates increase, the routing becomes denser and there are many factors that affect the integrity of signal transmission, such as impedance discontinuities of differential vias. Compared with a transmission line, the via hole is more sensitive to high-frequency signals, impedance discontinuity of the via hole is more severe when the frequency is higher, and further energy reflected by the signal is more, so that transmission integrity of the high-frequency signals is seriously affected. Among various factors of the via hole, the anti-pad structure has a large influence on the impedance of the via hole, and therefore, how to design an anti-pad structure of a differential via hole to enable the impedance of the differential via hole and the impedance of the transmission line to be properly matched as much as possible and reduce the reflection of a high-speed signal is a technical problem to be solved by those skilled in the art.

Disclosure of Invention

In view of the above, the present invention provides a printed circuit board and an anti-pad design method for differential via holes, the anti-pad structure obtained by the anti-pad design method and the anti-pad structure of the printed circuit board can properly match the impedance of the differential via holes and the impedance of the transmission lines, which is beneficial to reducing the reflection of high-speed signals.

In order to achieve the purpose, the invention provides the following technical scheme:

a method for designing reverse bonding pads of differential via holes is applied to the situation that a reflow ground hole is arranged beside two signal holes in a pair respectively, and the reflow ground hole is positioned on an extension line of a central connection line of the two signal holes, and comprises the following steps:

designing a connecting point of a differential routing and the signal hole to be positioned on the central connecting line, wherein the differential routing is firstly bent to a first direction perpendicular to the central connecting line from the direction of the central connecting line, extends for a certain distance and then is bent to a second direction parallel to the central connecting line from the first direction, and the extension section of the differential routing along the second direction passes through the vicinity of one backflow ground hole;

determining the excircle radius R of the signal hole, the line width T of the differential routing and the distance X between the signal hole and the adjacent extension section, and designing a rectangular reverse pad pre-digging area taking the central connecting line as a central symmetrical line, wherein the reflow ground hole is positioned outside the reverse pad pre-digging area, a first edge and an opposite second edge of the reverse pad pre-digging area are respectively tangent to the excircle of the adjacent reflow ground hole, and the distance between a third edge and an opposite fourth edge of the reverse pad pre-digging area is 2 (X-T + R);

and quarter circular arcs with the centers of the signal holes as the centers of the circles and R +0.75 (X-T) as the radiuses are respectively designed at the two signal holes, one end points of the quarter circular arcs are positioned on the central connecting line, the other end points and the extension sections are positioned on the same side of the central connecting line, and the edges of the anti-pad pre-digging areas, which are opposite to the concave sides of the quarter circular arcs, and the tangent lines of the quarter circular arcs and the end points of the quarter circular arcs jointly enclose a final anti-pad real digging area.

Optionally, in the above method for designing an anti-pad of a differential via, the differential trace is designed to implement a turning transition by 45 ° or an arc.

The utility model provides a printed circuit board, includes two mated difference via holes, there is a backward flow ground hole respectively on the difference via hole next door, backward flow ground hole is located two on the extension line of the central line of difference via hole, two difference via hole is provided with the anti-pad of single fillet rectangle respectively, two the anti-pad about the perpendicular bisector symmetry of central line, just the fillet of anti-pad is close to the perpendicular bisector, and the difference is walked the line and is passed through two gradual change district between the fillet of anti-pad, backward flow ground hole is located outside the anti-pad, just the limit of anti-pad with the excircle in backward flow ground hole is tangent.

Optionally, in the printed circuit board, the rounded corner of the anti-pad is a quarter of a circular arc centered at the center of the differential via.

Optionally, in the printed circuit board, a radius of the fillet is 8 to 12 mils larger than an outer radius of the differential via.

According to the technical scheme, the method for designing the reverse bonding pad of the differential via hole is applied to the situation that a reflow ground hole is arranged beside each of two paired signal holes, the reflow ground hole is positioned on an extension line of a central connecting line of the two signal holes, in the method for designing the reverse bonding pad, a differential wire is firstly designed to start from the central connecting line, an extension section parallel to the central connecting line is formed after two turns, then the width of a rectangular reverse bonding pad pre-digging area is designed according to size parameters of a distance between the extension section adjacent to the signal hole and the signal hole, the excircle radius of the signal hole and the line width of the differential wire, the integrity of a reference surface of the reverse bonding pad pre-digging area is ensured, the integrity of the reference surface of the reverse bonding pad pre-digging area is ensured according to the principle of being tangent to the excircle of the reflow ground hole, the integrity of the reference surface of the reverse bonding pad outer reflow ground hole is ensured, finally, the tangent line at the end point of one fourth circular arc of the signal hole is designed in the reverse bonding pad pre-digging area, the impedance of the reverse bonding pad is further improved, and the impedance of the reverse bonding pad is further reduced.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the prior art descriptions will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.

FIG. 1 is a schematic diagram of a differential trace obtained by a method for designing a reverse pad of a differential via according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a pre-cut region of an anti-pad obtained by the method for designing the anti-pad of the differential via according to the embodiment of the present invention;

fig. 3 is a schematic diagram of an anti-pad solid-excavated area obtained by the method for designing the anti-pad of the differential via according to the embodiment of the present invention.

Labeled as:

11 and 12, return ground holes; 21 and 22, signal holes; 31 and 32, differential routing; 41. an anti-pad pre-digging area; 42 and 43, reverse pad solid cut area.

Detailed Description

For the purpose of facilitating understanding, the present invention will be further described with reference to the accompanying drawings.

Referring to fig. 1, a return ground hole is formed beside each of the

signal holes

21 and 22, a return ground hole 11 is formed beside the signal hole 21, a return ground hole 12 is formed beside the signal hole 22, and the

return ground holes

11 and 12 are located on an extension of a central connecting line of the signal holes 21 and 22 (hereinafter, the "central connecting line" refers to the central connecting line of the

signal holes

21 and 22 unless otherwise specified). The distance between the

signal holes

21 and 22 is Ya, the distance between the signal hole 21 and the return ground hole 11 and the distance between the signal hole 22 and the return ground hole 12 are Yb, and the outer diameters of the

signal holes

21 and 22 are D.

Aiming at the situation of a signal hole and a reflow ground hole shown in FIG. 1, the method for designing the reverse bonding pad of the differential via hole provided by the invention comprises the following steps:

firstly, differential traces are designed, the result is shown as

differential traces

31 and 32 in fig. 1, the differential trace 31 is taken as an example to explain the design principle of the differential trace, the connection point of the differential trace 31 and the signal hole 21 is located on the central connection line, the differential trace 31 turns to a first direction perpendicular to the central connection line from the direction of the central connection line, extends for a distance and then turns to a second direction parallel to the central connection line from the first direction, and the extension section of the differential trace 31 along the second direction (if no special description is provided, the extension section in the following text refers to the extension section of the differential trace along the second direction) passes through the vicinity of the reflow ground hole 12.

It should be noted that, the anti-pad design method of the present invention only defines the portion of the differential trace close to the differential via, that is, for the portions of the

differential traces

31 and 32 not shown in fig. 1, a person skilled in the art should be able to make a flexible selection according to the general design principle and the specific requirement of the differential trace, and details are not described herein again.

After the differential routing is designed through the steps, an anti-pad pre-digging area is designed. The impedance of the via is typically predominantly capacitive, i.e., the impedance of the via is typically low compared to the impedance of the trace. The 'anti-bonding pad' is an isolated part between a copper sheet on the trace reference layer and the via hole, the larger the isolated part is, the smaller the capacitance is, therefore, in order to improve the impedance of the differential via hole, the area of the anti-bonding pad pre-digging area needs to be as large as possible. The anti-pad pre-digging area 41 obtained by the anti-pad design method of this embodiment is as shown in fig. 2, and the design process is to determine the outer circle radius R of the signal hole, the line width T of the differential trace, and the distance X between the signal hole and the adjacent extension section, and then design a rectangular anti-pad pre-digging area 41 taking the central connecting line as the central symmetrical line, where the length L and the width W of the anti-pad pre-digging area 41 are set according to the following principle, and the width W is obtained by formula 2 (X-T + R), so that a doubled line width is left between the anti-pad pre-digging area 41 and the extension section of the differential trace 32, and the integrity of the trace reference surface can be ensured, the length L needs to consider the connection integrity between the

reflow ground holes

11 and 12 and the reference surface, and the final length L is obtained by formula Ya +2d 2yb, so that the

reflow ground holes

11 and 12 are located outside the anti-pad pre-digging area 41, and further, two edges in the length direction of the anti-pad pre-digging area 41 are respectively tangent to the outer circle of the adjacent reflow ground holes.

The land pre-cut region 41 obtained through the above steps can indeed reduce the capacitance and raise the impedance of the

signal holes

21 and 22, but the portions of the

differential traces

31 and 32 in the land pre-cut region 41 have no reference plane, which results in the excessive impedance of the portions of the differential traces, causing the reflection of high-speed signals, and forming multiple reflections with the discontinuous points of the

signal holes

21 and 22, further deteriorating the quality of signal transmission, so that the land pre-cut region 41 is subsequently modified by designing quarter arcs at the

signal holes

21 and 22, respectively, with the centers of the signal holes as the centers and with R +0.75 (X-T) as the radii, one end point of the quarter arc being located on the center connection line, the other end point and the extension section being located on the same side of the center connection line, and finally, the concave side of the quarter arc facing the land pre-cut region 41, together with the end points of the quarter arc and the tangent line of the quarter arc, form land pre-cut

regions

42 and 43, as shown in fig. 3.

As can be seen from a comparison between fig. 2 and fig. 3, the finally determined

anti-pad areas

42 and 43 correspond to a portion of copper sheet compensated in the conventional rectangular anti-pad area, so that the problem of incomplete routing reference surface in the anti-pad area can be effectively solved, the influence of multiple reflections can be further reduced, and the integrity of high-speed signal transmission can be ensured. The

anti-pad dug areas

42 and 43 correspond to the

signal holes

21 and 22 respectively, so that the formed anti-pad can reduce the capacitive effect of the high-speed differential via hole, and the impedance of the via hole is improved, so that the impedance of the via hole is more properly matched with the impedance of the trace.

As shown in fig. 1, when designing the

differential traces

31 and 32, the turning transition is usually implemented at 45 °, and another commonly used turning transition manner is a circular arc transition.

The invention also provides a printed circuit board which comprises two paired differential via holes, wherein a backflow ground hole is respectively arranged beside each differential via hole, the backflow ground hole is positioned on an extension line of a central connecting line of the two differential via holes, the two differential via holes are respectively provided with a single-fillet rectangular reverse bonding pad, the two reverse bonding pads are symmetrical about a perpendicular bisector of the central connecting line of the two differential via holes, the fillet of the reverse bonding pad is close to the perpendicular bisector, the differential routing passes through a gradual change region between the fillets of the two reverse bonding pads, the backflow ground hole is positioned outside the reverse bonding pad, and the edge of the reverse bonding pad is tangent to the excircle of the backflow ground hole.

It should be noted that the differential traces and the anti-pads are located on different layers of the printed circuit board, the "transition region where the differential traces pass through between the rounded corners of the two anti-pads" refers to a region where the pitch gradually changes when viewed from the top of the printed circuit board. Taking fig. 3 as an example, the

anti-pad dug areas

42 and 43 correspond to two anti-pads, and a flared transition area is formed between the rounded corners of the two anti-pads. From a top view of the printed circuit board, the

differential traces

31 and 32 pass through the transition area.

In a specific practical application, the fillet of the anti-pad is usually a quarter of a circular arc centered at the center of the differential via. Generally, the radius of the fillet is 8-12 mil larger than the outer circle radius of the differential via hole.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for designing a reverse pad of a differential via hole is applied to the situation that a reflow ground hole is arranged beside two signal holes in a pair respectively, and the reflow ground hole is positioned on an extension line of a central connecting line of the two signal holes, and is characterized by comprising the following steps of:

designing a connecting point of a differential wiring and the signal hole to be positioned on the central connecting line, wherein the differential wiring is firstly bent to a first direction perpendicular to the central connecting line from the direction of the central connecting line, extends for a distance and then is bent to a second direction parallel to the central connecting line from the first direction, and the extension section of the differential wiring along the second direction passes through the vicinity of one backflow ground hole;

and a quarter circular arc which takes the center of the signal hole as the center of a circle and R +0.75 (X-T) as the radius is respectively designed at the two signal holes, one end point of the quarter circular arc is positioned on the central connecting line, the other end point and the extension section are positioned on the same side of the central connecting line, and the edge of the anti-pad pre-digging area, which is opposite to the concave side of the quarter circular arc, and the tangent line of the end point of the quarter circular arc and the tangent line of the anti-pad pre-digging area enclose a final anti-pad solid digging area.

2. The anti-pad design method for differential vias according to claim 1, wherein the differential traces are designed to make a turning transition in 45 ° or in a circular arc.

3. A printed circuit board, adopting the anti-pad design method for differential via holes according to claim 1, comprising two paired differential via holes, wherein a reflow ground hole is respectively arranged beside each differential via hole, the reflow ground hole is positioned on an extension line of a central connecting line of the two differential via holes, the two differential via holes are respectively provided with an anti-pad with a single round corner rectangle, the two anti-pads are symmetrical about a perpendicular bisector of the central connecting line, a round corner of the anti-pad is close to the perpendicular bisector, a differential routing passes through a gradual change region between round corners of the two anti-pads, the reflow ground hole is positioned outside the anti-pad, and the edge of the anti-pad is tangent to the excircle of the reflow ground hole.

4. The printed circuit board of claim 3, wherein the fillet of the anti-pad is a quarter of a circular arc centered at the center of the differential via.

5. The printed circuit board of claim 4, wherein the radius of the fillet is 8-12 mil larger than the outer circle radius of the differential via hole.