JP2011086975A - Printed circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a printed circuit board prepared with branched microstrip lines, the circuit board matching a characteristic impedance for wiring before branching with the one after branching, without a considerable change in the width of microstrip lines after branching and without a great modification in an existing manufacturing process. <P>SOLUTION: When the characteristic impedance of the microstrip lines before branching is Z (Ω) at the printed circuit board, a specific dielectric constant of solder resist to be covered on the microstrip line after branching is changed and adjusted in relation to solder resist covered on microstrip line before branching so that the independent characteristic impedance microstrip line after branching may become n-Z (Ω). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a printed circuit board provided with a branching microstrip line. Specifically, the characteristic impedance of the wiring before and after branching can be matched without greatly changing the width of the microstrip line after branching. The present invention relates to a printed circuit board that can be used.

  In a printed circuit board provided with a microstrip line, the microstrip line is usually set to a constant characteristic impedance (for example, 50Ω).

  On the other hand, particularly in bus wiring, a structure may be used in which a single microstrip line branches into a plurality of microstrip lines.

  In such a branch of the microstrip line, it is assumed that the characteristic impedance of the microstrip line before branching is set to be the same as the characteristic impedance of each of the microstrip lines after branching. FIG. 4 is a conceptual diagram showing a state in which the characteristic impedance of the microstrip line before branching and the microstrip line after branching is set to Z (Ω) in the microstrip line of the printed circuit board.

Here, looking at the characteristic impedance of the microstrip line after branching from the microstrip line before branching, the characteristic impedance of the microstrip wiring after branching is

1 / [(1 / Z) + (1 / Z)] = Z / 2

It becomes. FIG. 5 is a conceptual diagram showing a case where the characteristic impedance of the microstrip line after branching is seen from the microstrip line before branching.

  Therefore, the characteristic impedance mismatch between the microstrip wiring before branching and the microstrip wiring after branching may cause mismatch of characteristic impedance, transmission signal reflection may occur, and waveform quality of the transmission signal may deteriorate.

In this case, when the transmission signal has a low frequency, it is not so much a problem, but in recent years, DDR, DDR2
With the widespread use of such high-speed and large-capacity memories, the speed of signal lines transmitted on the microstrip line has been dramatically increased, and thus the above problem cannot be ignored.

In order to solve this problem, it is necessary to make the characteristic impedance of the wiring of the microstrip line after branching larger than the characteristic impedance of the microstrip line before branching. In FIG. 4, if the characteristic impedance of the microstrip line after branching is 2Z (Ω), the characteristic impedance of the microstrip line after branching from the microstrip line before branching is

1 / [(1 / 2Z) + (1 / 2Z)] = Z

And match.

  However, when microstrip lines with different characteristic impedances are mixed on the same layer on the same printed circuit board, the width of the microstrip line after branching is more than double the width of the microstrip line before branching. As a result, it is difficult to design or manufacture under high-density wiring.

Moreover, in patent document 1, the following means are taken as a solution of this subject. That is, according to the present invention, in the wiring board in which the signal line is formed in a predetermined pattern on one surface of the insulating layer and the conductive layer is formed on the other surface of the insulating layer, the insulating layer has a characteristic impedance of the signal line. A space region having a relative dielectric constant lower than that of other portions of the insulating layer is provided at a predetermined position facing the mismatched line portion, and this space region has a characteristic impedance mismatch among the signal lines. It is assumed that the gap is formed at a predetermined position facing the line portion. The space region is filled with a predetermined dielectric having a relative dielectric constant lower than that of other portions of the insulating layer. Further, the line portion of the signal line where the characteristic impedance is mismatched is assumed to be a branch portion of the signal line.

JP 09-260797 A

  In the said invention, in forming the space | gap part, it is supposed to form by a drill process or an etching process. However, this increases the number of processes.

  Therefore, the present invention provides a printed circuit board provided with a branching microstrip line, without greatly changing the width of the microstrip line after branching, and without greatly modifying the existing manufacturing process. It is an object to obtain a printed circuit board capable of matching the characteristic impedance of the wiring after branching.

  Means for solving the above problems include at least one microstrip line (hereinafter referred to as a pre-branch microstrip line) and a plurality of microstrip lines (hereinafter referred to as n-branch lines) formed by branching from the pre-branch microstrip line. And a printed circuit board in which a solder resist is coated on the pre-branch microstrip line and the post-branch microstrip line, the characteristic impedance of the pre-branch microstrip line is In the case of Z (Ω), the post-branch microstrip line is changed by changing the relative dielectric constant of the solder resist to be coated on the post-branch microstrip line with respect to the solder resist coated on the pre-branch microstrip line. Single characteristic in -Impedance in which it was decided to have been adjusted n · Z (Ω) and so as.

  Since the characteristic impedance of the microstrip line after branching is n · Z (Ω), the characteristic impedance of the entire microstrip line after branching as seen from the microstrip line before branching matches Z by the following equation. I understand that.

1 / (n × 1 / (n · Z)) = Z

  Therefore, the characteristic impedance of the pre-branch microstrip line matches the characteristic impedance of the entire pre-branch microstrip line, and reflection of the transmission signal flowing therethrough can be suppressed.

The figure which expanded and looked at the microstrip line formed in the printed circuit board. The figure which shows the state which coat | covered one kind of soldering resist on the microstrip line. The figure which shows the state which coat | covered the solder resist from which a dielectric constant differs on the microstrip line after a branch. The conceptual diagram which shows the state in which the characteristic impedance of the microstrip line before a branch and the microstrip line after a branch is set to Z ((ohm)). The conceptual diagram which shows the case where the characteristic impedance of the microstrip line after a branch is seen from the microstrip line before a branch.

  FIG. 1 is an enlarged view of a microstrip line formed on a printed circuit board. This microstrip line has a configuration in which one microstrip line 101 is branched into four microstrip lines 102. Both the microstrip line 101 and the microstrip line 102 are formed with the same width and the same thickness.

  Hereinafter, the microstrip line 101 is referred to as a microstrip line before branching, and the four microstrip lines 102 are referred to as microstrip lines after branching.

  In manufacturing a printed circuit board, one kind of solder resist 103 is usually coated on these microstrip lines. FIG. 2 is a diagram showing a state in which one kind of solder resist 103 is coated on these microstrip lines.

  Here, it is assumed that the characteristic impedance of the microstrip line before branching is set to 50Ω. The characteristic impedance of each individual microstrip line 102 of the microstrip line after branching is also 50Ω. However, the overall characteristic impedance of the microstrip line after branching as seen from the microstrip line before branching is as follows.

1 / (1/50 + 1/50 + 1/50 + 1/50) = 50/4 = 12.5Ω

  That is, since the characteristic impedance of the microstrip line after branching viewed from the microstrip line before branching is small, the transmission signal flowing from the microstrip line before branching to the microstrip line after branching is reflected at these branching points.

  Therefore, by coating the second solder resist 104 having a relative permittivity different from that of the solder resist 103 on the microstrip line after branching, the individual characteristic impedance of each microstrip line 102 of the microstrip line after branching is set to 200Ω. Set. FIG. 3 is a diagram illustrating a state in which the second solder resist 104 having a relative dielectric constant different from that of the solder resist 103 is coated on the branched microstrip line.

  As a result, the total characteristic impedance of the microstrip line after branching is as follows.

1 / (1/200 + 1/200 + 1/200 + 1/200) = 200/4 = 50Ω

  That is, the characteristic impedance is matched between the pre-branch microstrip line and the post-branch microstrip line.

  Even when the values of the characteristic impedances cannot be strictly matched, this method can reduce the reflection coefficient at the branch point and improve the waveform quality.

101 Microstrip line 102 Microstrip line 103 Solder resist 104 Second solder resist

Claims (1)

And at least one microstrip line (hereinafter referred to as a pre-branch microstrip line) and a plurality of microstrip lines (hereinafter referred to as post-branch microstrip lines) formed by branching into n lines from the pre-branch microstrip line. A printed circuit board provided with a solder resist on the pre-branch microstrip line and the post-branch microstrip line, when the characteristic impedance of the pre-branch microstrip line is Z (Ω), By changing the relative dielectric constant of the solder resist to be coated on the post-branch microstrip line with respect to the solder resist coated on the microstrip line before branching, the characteristic impedance of the post-branch microstrip line alone is n · Z (Ω ) A printed circuit board characterized by being adjusted.

Images