JP2011023547A - Circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit board in which characteristic impedance of a signal transmission line formed across a part where layer constitution changes, is sufficiently controlled. <P>SOLUTION: The circuit board comprises: a first layer constitution part A having a microstrip structure formed by arranging a signal transmission line at a first ground layer via an insulator layer; and a second layer constitution part B having a microstrip structure formed by covering at least a portion of the signal transmission line with a second insulator layer and overlaying the second insulating layer with a second ground layer, wherein signal transmission lines 4a and 4b are formed across the first layer constitution part and second layer constitution part. Mesh-shaped openings 2a and 2b are formed in the first ground layer 2 at the first and second layer constitution parts, and compared with the conductor residual rate of the first ground layer at the first layer constitution part, the conductor residual rate at the second layer constitution part is set to be smaller. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a circuit board having a strip structure and a microstrip structure in which a signal transmission path is formed across a portion where the layer configuration changes, such as a rigid (hard) portion and a flex (flexible) portion. .

  In recent years, a rigid-flex multilayer board having a rigid part and a flex part can be mounted on equipment by mounting electronic components on the rigid part and appropriately deforming the flexible part according to installation conditions. Used in electronic equipment.

  On the other hand, the processing speed of signals in the electronic devices described above is increasing, and circuit boards mounted with electronic components that operate in the high frequency band are characterized by the characteristics of signal transmission lines in order to suppress signal reflection and waveform distortion. It is necessary to match the impedance (hereinafter also referred to as Zo) to the input / output impedance of the device.

  In order to match the characteristic impedance of the signal transmission path described above, a microstrip structure or a strip structure in which a ground layer is opposed to a signal transmission path (strip line) having an appropriate pattern width with an insulating layer having an appropriate thickness interposed therebetween is used. It has been adopted.

The ground layer in the circuit board structure described above serves as an electrical reference plane that defines the characteristic impedance of the signal transmission path. In general, the characteristic impedance is often selected to be around 50Ω for single-ended transmission and around 100Ω for differential transmission.
Note that Zo in the circuit board described above is a ratio of reactance L per unit length of the signal transmission path and capacitance C per unit area between the signal transmission path and the ground layer (reactance L / electrostatic capacity). It is a value approximated by the square root of the capacitance C).

  Accordingly, in the case of using a circuit board having a thin multilayer structure, the layer of the signal transmission path with respect to the ground layer is naturally narrow (thin), and the value of the capacitance C between the two is the dimension between the layers. It rises almost inversely.

  Therefore, in order to obtain the desired Zo in the above-described thin multilayer circuit board, the width of the signal transmission path is narrower (thinner) than that of a conventional circuit board having a thick interlayer, A means for suppressing the increase in the capacitance C must be employed.

  Thus, in order to obtain a desired Zo, in the case where the line width of the signal transmission path is to be narrowed, there is a case where the signal wiring has to be made so thin that it is difficult to process. Even if signal wiring can be processed, the thinner the signal wiring, the higher the ratio of circuit processing accuracy and line width variation, and the Zo variation increases accordingly.

  For this reason, there arises a problem that signal reflection and waveform distortion occur in the signal wiring portion where the change of Zo is large. Furthermore, since the wiring resistance value of the signal wiring is increased, there is a problem that the higher the signal frequency supplied to the signal wiring, the worse the transmission characteristics.

  Therefore, in order to solve the above-described technical problem, the ground layer formed as a so-called solid electrode layer is removed in a mesh shape so that the opposing area between the ground layer and the signal wiring per unit area is substantially reduced. Proposals have been made to ensure the width of the signal wiring by making it smaller. This is disclosed in the following prior art documents.

JP 7-32463 A

  By the way, as described above, even if the means for removing the copper in the mesh layer is adopted and the line width of the signal transmission path can be secured to some extent, the rigid portion and the flex portion as in the rigid flex multilayer substrate as described above. However, when the strip structure and the microstrip structure are mixed, it is impossible to keep the Zo of the signal transmission line constant.

  FIG. 5 and FIG. 6 are schematic diagrams of the line width and the distance between lines of the signal transmission line which is made to keep the Zo constant when the layer structure is different as described above and the strip structure and the microstrip structure are mixed. It is shown as an example.

FIG. 5 shows an example of the line width when a single-end transmission line is configured. Reference numeral A indicates a portion formed in a microstrip structure, and reference numeral B indicates a portion formed in a strip structure.
That is, in the strip structure B portion, the ground layer is opposed so as to sandwich the signal transmission path 4 up and down, and the capacitive coupling to the signal transmission path 4 increases, so that the strip width is larger than the line width W1 of the microstrip structure portion A It is necessary to make the line width W2 of the structural portion B narrow.

  FIG. 6 shows the relationship between the line distances of a pair of signal transmission lines when a differential transmission line is configured. Similarly, reference numeral A denotes a portion formed in a microstrip structure, and reference numeral B denotes The part made into the strip structure is shown. In this case, it is necessary to form the distance S2 between the pair wirings of the strip structure portion B narrower than the distance S1 between the pair wirings 4a and 4b in the microstrip structure portion A.

  As described above, when the line width in the single-ended transmission line is changed in the portion where the layer configuration changes or the distance between the pair wires in the differential transmission line is changed, the changed portion of the layer configuration In this case, the transmission characteristics of the high frequency signal are adversely affected.

  The present invention has been made paying attention to the above-mentioned technical problems, and transmission is possible without controlling the line width and the distance between lines according to the layer configuration regardless of single-ended transmission and differential transmission. It is an object of the present invention to provide a circuit board that can sufficiently control the characteristic impedance of a line.

  The circuit board according to the present invention, which has been made to solve the above-described problems, has a microstrip structure in which a signal transmission line is disposed via an insulator layer with respect to the first ground layer. The layer structure part 1 and at least a part of the signal transmission line are covered with a second insulator layer, and a second ground layer is superimposed on the second insulator layer to form a strip structure. A circuit board comprising a second layer constituent part, wherein the signal transmission path is formed across the first layer constituent part and the second layer constituent part, the first layer constituent part and Each of the first ground layers in the second layer constituent part is provided with a mesh-shaped opening, and the first ground layer in the first layer constituent part has a conductor remaining ratio of the first ground layer in relation to the first ground layer. 1st gran in 2 layer components The conductor residual ratio of the layer is reduced, and the characteristic impedance of the signal transmission line formed across the first layer constituent part and the second layer constituent part is controlled. .

  In this case, the second ground layer in the second layer constituent portion is further provided with a mesh-like opening, and based on the conductor remaining rate due to the mesh-like openings provided in the first and second ground layers, respectively. It is desirable that the characteristic impedance of the signal transmission line in the second layer component is controlled.

  When the signal transmission line is a single-ended transmission line, the transmission line is configured to have the same line width in the first layer constituent part and the second layer constituent part. . Further, when the signal transmission path is a differential transmission path, the distance between the pair wirings constituting the differential transmission path in the first layer constituent portion and the second layer constituent portion is the same. It is comprised so that it may become.

  In addition, it is desirable that the insulator layer serving as the core of the circuit board is composed of a film-like base material.

  According to the circuit board having the above-described configuration, the ground layer in the first layer constituent part constituting the microstrip structure and the mesh-like opening provided in the ground layer in the second layer constituent part forming the strip structure are respectively provided. By setting the conductor remaining rate as described above, it is possible to control the characteristic impedance of the transmission line straddling both to substantially the same value.

  Thus, regardless of the single-ended transmission line and the differential transmission line, there is no need to control the line width and the distance between the lines according to the layer configuration, and the high frequency generated by controlling the line width and the distance between the lines. The problem of incurring transmission loss due to signal reflection or the like can be solved.

It is the laminated structure figure which showed the basic form of the circuit board concerning this invention with sectional drawing. It is sectional drawing seen from the aa line in FIG. 1 in the arrow direction. It is sectional drawing seen in the arrow direction from the bb line in FIG. It is the top view which showed a part of ground layer and signal transmission path shown in FIG. It is the schematic diagram which showed the example of a countermeasure of the characteristic impedance control in the conventional single end transmission line. It is the schematic diagram which showed the example of a countermeasure of the characteristic impedance control in the conventional differential transmission path.

Hereinafter, a circuit board according to the present invention will be described based on the embodiments shown in the drawings. A circuit board 1 shown in FIG. 1 is provided with a first ground layer 2 and a signal transmission path (hereinafter also referred to as signal wiring) 4 via an insulator layer 3 with respect to the ground layer. This structure forms a microstrip structure.
In FIG. 1, the microstrip structure portion is indicated by the symbol A, which is also referred to as a first layer constituent portion.

  Further, a part of the signal wiring 4 is covered with a second insulator layer 5 and further a second ground layer 6 is superimposed on the second insulator layer to form a strip structure. . The strip structure portion is indicated by a symbol B, which is also referred to as a second layer constituent portion.

  In the second layer component B, an insulator layer and a conductor layer (not shown) are further superimposed on the first ground layer 2 and the second ground layer 6 as necessary. Thereby, the second layer constituent part constitutes, for example, a rigid part, and a rigid flex multilayer substrate is constituted together with the flex part which is the first layer constituent part. The signal wiring 3 is disposed so as to straddle both of them.

2 and 3 are cross-sectional views as viewed from the direction of the arrows along the lines aa and bb in FIG. 1 as described above. That is, the microstrip structure portion A and the strip structure portion B are shown in cross-sectional views.
As shown in FIGS. 2 and 3, the signal wiring 4 in this embodiment constitutes a differential transmission path, and the pair wiring is indicated by reference numerals 4a and 4b, respectively.

  In this embodiment, a film-like base substrate is employed as the insulator layer 3, and a resin film, a fiber substrate, or the like is used as the base substrate that functions as the core of the circuit board 1.

Examples of the material constituting the resin film include polyimide resins such as polyimide resins, polyamide resins, and polyamideimide resins, thermosetting resins such as epoxy resins, and thermoplastic resins such as liquid crystal polymers.
Among these, a polyimide resin or a liquid crystal polymer is preferable. For example, in the case of polyimide resin, it is excellent in heat resistance and mechanical properties and is easy to obtain. In the case of a liquid crystal polymer, it is suitable for high-speed signal transmission due to its low relative dielectric constant, and it has excellent dimensional stability due to its low hygroscopicity.

  Moreover, as a fiber base material used for the insulator layer, for example, a glass fiber base material such as a glass fiber cloth or a glass non-fiber cloth, or an inorganic fiber base material such as a fiber cloth or a non-fiber cloth containing an inorganic compound other than glass, Examples thereof include organic fiber base materials composed of organic fibers such as aromatic polyamide resin, polyamide resin, aromatic polyester resin, polyester resin, polyimide resin, and fluororesin. Among these base materials, glass fiber base materials represented by glass fiber fabric are preferable in terms of strength and water absorption.

  When a fiber base material is used for the insulator layer, the fiber base material is preferably used in a state of being impregnated with a resin. As the resin impregnated in the fiber base material, a thermosetting resin such as an epoxy resin or an acrylic resin is preferably used, and among these, an epoxy resin is preferable from the viewpoint of heat resistance.

  The first ground layer 2 disposed on the back surface (the lower surface shown in FIG. 1) of the base substrate 3 constituting the insulator layer may be provided directly on the base substrate 3, but with an adhesive. The first ground layer 2 may be provided by a conductor made of a copper material. The ground layer 2 has a large number of holes formed in parallelogram openings. A specific configuration of the ground layer 5 will be described later in detail with reference to FIG.

  Further, the signal wirings 4a and 4b formed on the surface of the base substrate 2 (upper surface shown in FIG. 1) are made of a copper material, which may be directly provided on the base substrate 3, but an adhesive is used. May be provided. The signal wirings 4a and 4b are bonded to mounting pads such as semiconductor devices (not shown) at their end portions or appropriate intermediate portions, for example, the above-described rigid portions, and function as the circuit board 1.

  Also in the second insulator layer 5 covering the signal wirings 4a and 4b, a glass fiber substrate such as a glass fiber cloth or a glass non-woven cloth is impregnated with a thermosetting resin such as an epoxy resin or acrylic resin. Used in state. A second ground layer 6 is laminated on the surface of the second insulator layer 5 (upper surface shown in FIG. 1).

The conductive portion as the second ground layer 6 can be made of a copper material in the same manner as the first ground layer 2 described above, and can be printed with a conductive paste (silver paste) or a conductive shield film. It can also be formed by sticking.
The second ground layer 6 is also formed with a large number of holes preferably having parallelogram openings. A specific configuration of the second ground layer 6 will be described later in detail.

  By the way, the characteristic impedance Zo of the signal wiring 4 (pair wiring 4a, 4b) in the first layer constituent part A constituting the microstrip structure shown in FIG. 1 is the reactance L per unit length of the signal wiring as described above. And a value approximated by the square root of the ratio of the capacitance C per unit area between the signal wiring and the first ground layer 2 (reactance L / capacitance C).

  Further, the characteristic impedance Zo of the signal wiring 4 (pair wiring 4a, 4b) in the second layer constituent part B constituting the strip structure is a capacitance formed between the first ground layer 2 and the first ground layer 2. In addition to the coupling, capacitive coupling formed with the second ground layer 6 also acts.

  Accordingly, the signal wiring 4 is disposed across the first layer constituent part A constituting the microstrip structure and the second layer constituent part B constituting the strip structure, and the first and second layer constitutions are arranged. In order to prevent the characteristic impedance of the signal wiring 4 from changing in the portion, the capacitive coupling between the signal wiring 4 and the ground layers 2 and 6 is constant in both the first and second layer constituent portions. There is a need to configure it so that it does not change.

  FIG. 4 shows a specific example for solving the above-described problem of making the capacitive coupling between the pair wirings 4a and 4b and the ground layer constant. That is, FIG. 4 shows a boundary portion between the first layer constituent portion A and the second layer constituent portion B in the first ground layer 2, and the pair wirings 4 a and 4 b described above are connected to the ground layer 2. In contrast, a certain distance, that is, the thickness of the insulator layer 3 shown in FIG.

  In this case, the first ground layer 2 is provided with mesh-shaped openings (a large number of holes formed in a parallelogram) 2a and 2b, respectively, and the ground layer in the first layer component A The conductor (copper) remaining rate of the ground layer 2 in the second layer constituent portion B is smaller than the conductor (copper) remaining rate of 2.

  That is, as described above, in the second layer component B, the second ground layer 6 is also opposed to the pair wirings 4a and 4b via the insulator layer 5, and the second wiring layer B is opposed to the pair wirings 4a and 4b. Since the two ground layers 6 act as capacitive coupling, the capacitive coupling with the first ground layer 2 is reduced here.

Note that the degree of capacitive coupling that the second ground layer 6 acts on the pair wirings 4a and 4b depends on the thickness and material of the second insulator layer 5 described above. When a solid electrode made of a copper material is used as the ground layer 6, the degree of capacitive coupling due to this increases.
Therefore, when a solid electrode is used for the second ground layer 6, it is necessary to configure the first ground layer 2 so that the conductor remaining rate is smaller.

  In this case, in the case where the conductor remaining rate of the first ground layer 2 is extremely small and the second layer component B cannot function as a strip structure, the second ground layer 6 is Also, it is desirable to provide a mesh-shaped opening and set the degree of capacitive coupling acting on the pair wirings 4a and 4b by the second ground layer 6 to be low.

Therefore, although not shown in the drawing, the mesh-shaped opening provided in the second ground layer 6 is also substantially the same size (approximately the same size as the mesh-shaped opening 2b in the second layer component B shown in FIG. It is desirable to configure so that the same conductor remaining rate).
With the configuration described above, the characteristic impedance Zo of the pair wirings 4a and 4b straddling the first and second layer constituent portions A and B can be easily controlled so as to be substantially constant, The ease of adjustment of the impedance Zo can also be improved.

  Therefore, the pair wirings 4a and 4b constituting the differential transmission path shown in the figure have the same line width and the same line width regardless of the first layer constituent part A and the second layer constituent part B. The distance can be the same.

In the above-described embodiment, an example is shown in which a differential transmission path using a pair wiring is configured. However, when a single-ended transmission path is employed, the first layer component A and the second layer Regardless of the layer constituent portion B, the line width can be made the same.
Thereby, the effect as described in the column of the effect of above-described invention can be enjoyed.

In the embodiment described above, the mesh-shaped opening formed in the first and second ground layers 2 and 6 has a rhombus shape, but the opening is not limited to this and is formed in another shape. The size (opening area) may also be different within the layer constituent parts A and B.
However, the openings are diamond-shaped, and it is preferable that the opening areas have the same size in the layer components A and B because the characteristic impedances of the signal wirings 4a and 4b can be controlled with high accuracy. .

  In addition, the ground layer in the above-described embodiment may be configured to be applied with a circuit reference potential, or the operation power supply of each device may be superimposed. Therefore, the potential applied to the ground layer is not particularly limited.

DESCRIPTION OF SYMBOLS 1 Circuit board 2 Ground layer 2a, 2b Mesh-shaped opening 3 Insulator layer (base material)
4 Signal transmission path (signal wiring)
4a, 4b Pair wiring 5 Second insulator layer 6 Second ground layer A First layer constituent part (microstrip structure part)
B Second layer component (strip structure)

Claims (5)

The signal transmission path is disposed via the insulator layer with respect to the first ground layer, so that the first layer constituent part forming the microstrip structure and at least a part of the signal transmission path are the second part. And a second ground layer formed by superimposing a second ground layer on the second insulator layer to form a strip structure, and the signal transmission path is the first layer. A circuit board formed straddling the component part and the second layer component part,
Each of the first ground layers in the first layer constituent portion and the second layer constituent portion is provided with a mesh-shaped opening, and a conductor of the first ground layer in the first layer constituent portion. The conductor remaining ratio of the first ground layer in the second layer constituent part is reduced with respect to the residual ratio, and the first layer constituent part and the second layer constituent part are formed to extend over the residual ratio. A circuit board configured to control a characteristic impedance of a signal transmission line.   A mesh-shaped opening is further applied to the second ground layer in the second layer constituent portion, and based on the conductor remaining rate by the mesh-shaped opening respectively applied to the first and second ground layers, The circuit board according to claim 1, wherein the circuit board is configured to control a characteristic impedance of a signal transmission path in the second layer constituent portion.   The signal transmission path is a single-ended transmission path, and the line widths of the transmission paths in the first layer constituent portion and the second layer constituent portion are the same. The circuit board according to claim 2.   The signal transmission path is a differential transmission path, and the distance between the pair wirings constituting the differential transmission path in the first layer constituent portion and the second layer constituent portion is the same. The circuit board according to claim 1 or 2.   The circuit board according to any one of claims 1 to 4, wherein the insulator layer is formed of a film-like base substrate.

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