Signal transmission structure having salients aligned with non-reference regions

A signal transmission structure is provided. The signal transmission structure has salients. The salients are corresponding to the position of the non-reference region and protrude from a lateral side of the signal traces. When the signals are transmitted on the signal traces, the parasitic capacitance between the salients and the reference plane can improve the characteristic impedance mismatch. Hence, when the signals are transmitted in a high frequency/high speed environment, the salients of the signal transmission structure reduce the effect of the near-end and far-end crosstalk generated by the other signal trace when a signal trace passes through a non-reference region, in order to keep the good quality of the signals.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application Ser. No. 92128842, filed on Oct. 17, 2003, the full disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a signal transmission structure, and more particularly to a signal transmission structure for improving the mismatch of the characteristic impedance of the signal transmission structure when the signal trace passes through a non-reference region.

2. Description of Related Art

On a printed circuit board and packaging substrate, the signal trace used for connecting two devices or two terminals has to maintain a uniform trace width in order to keep a constant characteristic impedance when the electronic signals are transmitted on the signal trace. Especially when the signals are transmitted in a high speed and a high frequency environment, a good impedance matching design between the two terminals are required to reduce the reflection due to the impedance mismatch, i.e., to reduce the insertion loss and increase the return loss when transmitting the signals so that the quality of the signal transmission will not be affected.

FIGS. 1A and 1Bshow a top view and a side view of a conventional signal trace through a non-reference region. The signal transmission structure110at least includes a reference plane120and two signal traces130and140. The reference plane120for example is a power plane or a ground plane. The signal traces130and140have a uniform trace width. It should be noted that in the conventional circuit design, the reference plane will have a plurality of through holes due to the hole drilling or cutting between the planes, or will have a non-reference region122(such as a non-reference region opening). Hence, when the signals are transmitted on the signal traces130and140, a high impedance will occurs at the non-reference region122. Further, the coupling inductance between the signal traces130and140also increases so that the effect of the near-end crosstalk and the far-end crosstalk becomes more serious. Therefore, the signals cannot be completely transmitted from the one terminal of the signal traces130and140to the other terminal.

FIG. 2shows the relationship between the characteristic impedance and the frequency when the conventional signal trace passes through a reference plane (solid line R) and a non-reference region (solid line T), respectively. Referring toFIGS. 1A and 2, when the working frequency is higher, the characteristic impedance is higher when the conventional signal traces130and140at the same frequency pass through a non-reference region122. Hence, the impedance mismatch occurs. Therefore, when the signal traces pass through an incomplete reference plane120, the characteristic impedance of the signal traces130and140increase as the frequency increases. Hence, the difference between the original characteristic impedance and the affected characteristic impedance also increases, which causes a more serious characteristic impedance mismatch on the signal traces130and140.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a signal transmission structure for improving the characteristic impedance mismatch when the signal trace passes through a non-reference region.

Another object of the present invention is to provide a signal transmission structure to improve the effect of the near-end and far-end crosstalk generated by the other signal trace when a signal trace passes through a non-reference region, in order to keep the good quality of the signals.

The present invention provides a signal transmission structure for a circuit board, comprising: a reference plane having a non-reference region; and a first signal trace on a side of the reference plane, the first signal trace having at least one first salient corresponding to the position of the non-reference region and protruding from a lateral side of the first signal trace; and a second signal trace on the side of the reference plane, the signal trace having at least one second salient corresponding to the position of the non-reference region and protruding from a lateral side of the second signal trace, wherein the first salient and the second salient are not between the first signal trace and the second signal trace.

The present invention provides a signal transmission structure for a circuit board, comprising: a reference plane having a non-reference opening; a first signal trace on a side of the reference plane, the first signal trace having at least one salient corresponding to the position of the non-reference region and protruding from a lateral side of the first signal trace; and a second signal trace on the side of the reference plane, wherein the salient is not between the first and the second signal traces.

In the first preferred embodiment of the present invention, the reference plane is one of a power plane and a ground plane; the first and second signal traces are co-planar, the reference plane and the first and second signal traces are not co-planar, and both first and second signal traces pass through the non-reference region. Further, a portion of the first salient partially extends to above an area out of the non-reference region, and a portion of the second salient partially extends to above an area out of the non-reference region. In additional, in the second preferred embodiment of the present invention, the first and second signal traces are co-planar, the reference plane and the first and second signal traces are not co-planar, and the second signal trace does not pass through the non-reference region.

The present invention uses a signal transmission structure with at least one salient. The salient is corresponding to the position of the non-reference region and protrude from a lateral side of the signal trace. When the signals are transmitted on the signal traces, the parasitic capacitance between the salients and the reference plane can improve the characteristic impedance mismatch. Hence, when the signals are transmitted in a high frequency/high speed environment, the salients of the signal transmission structure reduce the effect of the near-end and far-end crosstalk generated by the other signal trace when a signal trace passes through a non-reference region, in order to keep the good quality of the signals.

The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages and embodiments of the invention will be apparent to those skilled in the art from the following description, accompanying drawings and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 3A and 3Bshow a top view and a cross-sectional view (along the I—I line) of a signal transmission structure for a circuit board in accordance with a preferred embodiment of the present invention. The signal transmission structure210is for a circuit board such as a printed circuit board or a packaging structure. The signal transmission structure210at least includes a reference plane220and two signal traces230and240. The signal traces230and240are on the same side of the reference plane220, and the signal traces230and240are co-planar. The signal traces230and240and the reference plane220are not co-planar. Further, the reference plane220can be a power plane or a ground plane, and a portion of reference plane220forms a non-reference region222such as a non-reference opening due to hole drilling or cutting. The signal traces230and240have a salient232and242respectively as shown inFIG. 3A. The salients232and242are not between the signal traces230and240. Hence, when the signals are transmitted on the signal traces230and240, the widened salients232and242prevents the high impedance at the non-reference region222. Therefore, when the signals pass through the salients232and242, the increased parasitic capacitance between the salients232and242and the reference plane220can improve the impedance mismatch.

Referring toFIGS. 3A and 3B, when the signals are transmitted in a high frequency/high speed environment, the salients232and242of the signal transmission structure reduces the effect of the near-end and far-end crosstalk generated by the other signal trace, and reduces the coupling inductance between the signal traces230and240for good quality of the signals so that the signals can be completely transmitted from the one terminal of the signal traces230and240to the other terminal.

To calculate the impedance of the signal traces232and234, the characteristic impedance Z is approximately equal to

Z=LC,
wherein L is the equivalent inductance of the signal trace, and C is the equivalent capacitance of the signal trace. To the conventional design, the equivalent inductance L of the signal trace will be increased when the signals pass through the non-reference region, thereby increasing the characteristic impedance Z. However, this embodiment increases the equivalent capacitance C of the signal traces230and240so that the characteristic impedance Z at the non-reference region222can be reduced because of the increase of the parasitic capacitance at the non-reference region222. Hence the characteristic impedance Z of the signal traces230and240becomes uniform in order to reach characteristic impedance match.

The salients232and242are designed to be wider so that the area the signal traces230and240and the reference plane220can be increased. Hence, the equivalent capacitance C between the signal traces230and240and the reference plane220is also increased. Hence, when the signals pass through the widened salients232and234, the parasitic capacitance increased due to the salients232and234can effectively improve the effect of the impedance mismatch of the signal traces230and240.

FIGS. 4 and 5show top views of a signal transmission structure210for a circuit board in accordance with the other preferred embodiments of the present invention. As shown inFIG. 4, the salients232and242can extend to an area above the non-reference region222. Therefore, by increasing the equivalent capacitance C of the salients232and242, the characteristic impedance Z of the signal traces230and240becomes uniform in order to reach characteristic impedance match of the signal traces230and240. In addition, as shown inFIG. 5, the signal traces230and240have salients232and242respectively. The salient232includes a left salient portion232aand a right salient portion232b.The salient242includes a left salient portion242aand a right salient portion242b.The left salient portions232aand242aare on one side of the non-reference region corresponding to an extension direction of the signal traces230and240. The right salient portions232band242bare on an opposite side of the non-reference region222corresponding to an extension direction of the signal traces230and240. Therefore, by increasing the equivalent capacitance C of the salients232and242, the characteristic impedance Z of the signal traces230and240becomes uniform in order to reach characteristic impedance match of the signal traces230and240.

FIG. 6shows a top view of a signal transmission structure210for a circuit board in accordance with another preferred embodiment of the present invention. Referring toFIG. 6, the signal traces230and240have salients232and242respectively. The salients232and242change their areas based on the location where the signal traces230and240pass through the non-reference region222. For example, as shown inFIG. 6, the signal trace230passes above the central portion of the non-reference region222, and the signal trace240passes above the edge portion of the non-reference region222. Because the variance of the high impedance is different, the areas of the salients232and242are also different in order to compensate the impedance mismatch due to the different paths where the signal traces230and240pass through the non-reference region222.

FIGS. 7A and 7Bshow a top view and a cross-sectional view (along the II—II line) of a signal transmission structure for a circuit board in accordance with the second preferred embodiment of the present invention. The signal transmission structure310is for a circuit board such as a printed circuit board or a packaging structure. The signal transmission structure310at least includes a reference plane320and two signal traces330and340. The signal traces330and340are on the same side of the reference plane320and the signal traces330and340are co-planar. The signal traces330and340and the reference plane320are not co-planar. Further, the reference plane320can be a power plane or a ground plane, and a portion of reference plane320forms a non-reference region322such as a non-reference opening due to hole drilling or cutting. The signal trace330has a salient332as shown inFIG. 7A. The salient332is corresponding to the lateral side of the signal trace330. The signal trace340is on a side of the signal trace330away from the salient332. The signal trace does not pass through above the non-reference region322. Hence, when the signals are transmitted on the signal trace330, the salient332prevents the high impedance at the non-reference region322. Therefore, when the signals pass through the salient332, the increased parasitic capacitance between the salient332and the reference plane320can improve the impedance mismatch of the signal trace330.

Referring toFIGS. 7A and 7B, When the signals are transmitted on the signal traces330and340, the parasitic capacitance between the salient332and the reference plane320can improve the characteristic impedance mismatch. Hence, when the signals are transmitted in a high frequency/high speed environment, the salient332of the signal trace330reduces the effect of the near-end and far-end crosstalk generated by the other signal trace340, and reduces the coupling inductance between the signal traces330and340, in order to keep good quality of the signals so that the signals can be completely transmitted from the one terminal of the signal traces330and340to the other terminal.

In brief, the signal transmission structure of the present invention at least includes two signal traces and a reference plane. The reference plane has a non-reference region. The two signal traces are on a side of the reference plane and each has a salient. The width of the salients is wider than the width of the signal traces. The salient are corresponding to the position of the non-reference region. The salients are respectively corresponding to the lateral sides of the two signal traces, and not between the signal traces230and240. The area of the salients is proportional to the area of the non-reference region, and the area of the salients can be equal to the area of the non-reference region. When the signals are transmitted on the signal traces, the parasitic capacitance between the salients and the reference plane can improve the characteristic impedance mismatch. Hence, when the signals are transmitted in a high frequency/high speed environment, the signal transmission structure can reduce the effect of the characteristic impedance mismatch when the signals pass the non-reference region so that the characteristic impedance can match in the two signal traces.

In light of the above, the signal transmission structure of the present invention has the following advantages:

1. The present invention can overcome the high impedance generated when the signals pass through the non-reference region by increasing the equivalent capacitance of the salient. Therefore, the characteristic impedance of the signal traces becomes uniform to reach the impedance match.

2. The present invention can reduce the coupling inductance on the other signal trace when the signals are pass through the non-reference region by using the salients in the signal traces.

3. The present invention can regulate the current path by using the salients in the signal trace to reduce the effect of the near-end and far-end crosstalk generated by the other signal trace in order to keep good quality of the signals.

4. The signal transmission structure of the present invention can be widely applied in a printed circuit board or a packaging substrate.

The above description provides a full and complete description of the preferred embodiments of the present invention. Various modifications, alternate construction, and equivalent may be made by those skilled in the art without changing the scope or spirit of the invention. Accordingly, the above description and illustrations should not be construed as limiting the scope of the invention which is defined by the following claims.