WO2011016192A1

WO2011016192A1 - Circuit board

Info

Publication number: WO2011016192A1
Application number: PCT/JP2010/004638
Authority: WO
Inventors: 荒井進也; 栗林宏行
Original assignee: 住友ベークライト株式会社
Priority date: 2009-08-07
Filing date: 2010-07-20
Publication date: 2011-02-10
Also published as: JP2011054919A; TW201112890A; JP5445011B2

Abstract

Signal transmission paths (3a, 3b) are formed on one surface of an insulator layer (2), and a ground layer (4) is formed on the other surface of the insulator layer (2). A shield layer (9) is laminated on the ground layer (4) via insulator layers (7, 8), and openings (4B) on which a metal foil is not placed are formed in the ground layer (4) along the signal transmission paths (3a, 3b). With this structure, the characteristic impedance of the signal transmission paths depends on a coupling capacitance formed between the signal transmission paths and the shield layer. Thus, the characteristic impedance of a wiring line can be sufficiently controlled without using a mesh-like conductor in the ground layer (4).

Description

Circuit board

The present invention relates to a ground layer and a circuit board in which signal wirings are arranged with respect to the ground layer via an insulator layer.

For example, in a circuit board on which a device operating in a high frequency band is mounted, the characteristic impedance (hereinafter also referred to as Zo) of the signal transmission path is matched to the input / output impedance of the device in order to suppress signal reflection and waveform distortion. It is necessary to let
In order to match the characteristic impedance of the signal transmission path described above, a microstrip structure is adopted 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. Yes.

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.

FIG. 2 is a cross-sectional view showing an example of a conventional circuit board in which a differential transmission path by pair wiring faces a ground layer through an insulator layer to form a microstrip structure. This circuit board 1 is configured on the basis of a double-sided board, and a part of a metal foil (copper foil) 3 adhered to one surface (front surface) of an insulator layer 2 to be a base substrate is etched, for example. By processing, signal transmission paths, that is, pair wirings 3a and 3b are formed, and the pair wirings constitute a differential transmission path.

Similarly, a metal foil (copper foil) is adhered to the other surface (back surface) of the insulator layer 2 as a solid electrode over the entire surface, and this constitutes the ground layer 4.
In addition, a coating layer 6 is laminated on the surface of the metal foil 3 and the pair wirings 3a and 3b via an adhesive layer 5, and a coating layer is also provided on the back surface of the ground layer 4 via an adhesive layer 7. 8 are stacked.

The characteristic impedance Zo of the signal transmission path (pair wiring 3a, 3b) in the circuit board 1 having the above-described configuration is the reactance L per unit length of the signal transmission path and the unit area between the signal transmission path and the ground layer. It is a value approximated by the square root of the ratio (reactance L / coupling capacitance C) of the electrostatic (coupling) capacitance C per unit.

That is, the characteristic impedance of the signal transmission path in the circuit board structure shown in FIG. 2 is determined by the line width W1 of the pair wirings 3a and 3b, the thickness t1 of the insulating layer 2 serving as the base substrate, and the insulating layer 2 It depends on the relative dielectric constant of the constituent material.

In recent years, thin rigid boards and flexible circuit boards are frequently used as circuit boards on which the above-described devices are mounted.
When such a circuit board is employed, it is natural that the thickness of the insulator layer between the ground layer and the signal transmission path must be reduced. Therefore, the value of the coupling capacitance C between the signal transmission line and the ground layer increases in inverse proportion to the dimension between the layers, which makes it difficult to appropriately control the characteristic impedance of the transmission line.

FIG. 3 is a sectional view showing an example of the conventional thin circuit board. Note that, in FIG. 3, parts that perform the same functions as those shown in FIG. 2 already described are denoted by the same reference numerals, and thus detailed description thereof is omitted.
As shown in FIG. 3, in the circuit board 1 in which the thickness of the insulator layer 2 is made thinner and the thickness is t2, in order to obtain the desired Zo, the insulator layer 2 is Although it is sufficient to select a material having a small relative dielectric constant, this has limitations.
Therefore, it is unavoidable to adopt a means for suppressing the increase in the coupling capacitance C by forming the line widths of the signal transmission paths 3a and 3b to be narrower (thinner) and to have a line width schematically indicated by W2. .

Thus, in order to obtain the desired Zo, when the line width W2 of the signal transmission paths 3a and 3b is to be narrowed, the transmission path may have to be made so thin that it is difficult to process.
Even if the transmission line can be processed, the thinner the transmission line, 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 transmission line portion where the change of Zo is large. Furthermore, since the wiring resistance value of the transmission path is increased, there is a problem that the higher the signal frequency supplied to the transmission path, the worse the transmission characteristics.

Therefore, in order to solve the technical problem described above, the ground layer 4 formed as a so-called solid electrode layer is removed in a mesh shape so that the ground layer 4 per unit area and the transmission paths 3a and 3b Proposals have been made to ensure the line width W2 of the transmission line appropriately by reducing the facing area substantially. This is disclosed in the following prior art documents.

JP 7-32463 A

Reference numeral 4A shown in FIG. 3 indicates a mesh-shaped opening laying portion applied to the ground layer 4, that is, a mesh-shaped conductor portion. FIG. 4 illustrates the ground layer 4 provided with the mesh-shaped conductor portion 4A. On the other hand, a state in which the pair of signal transmission paths 3 a and 3 b are opposed to each other is shown in a state seen through from a direction orthogonal to the plate surface of the circuit board 1.

In the mesh configuration example shown in FIG. 4, a large number of rhombus openings are formed in the ground layer 4 by lines intersecting in two directions, and the ground layer 4 side in the longitudinal direction of the signal wirings 3a and 3b is formed. The intersection angles of the lines in the two directions are configured to be substantially the same.

By the way, in the circuit board in which the layers are formed thinner and thinner as described above, if the minimum line width W2 is to be secured, the conductor remaining rate of the mesh-like conductor portion 4A shown in FIGS. 3 and 4 is set lower. I will do it.

However, if the conductor residual ratio is lowered beyond a certain level, the mesh becomes sparse, so that the Zo variation partially occurs and the signal waveform is distorted, which affects the high frequency characteristics. Therefore, there is a limit to the use of the mesh-like conductor portion 4A shown in FIGS.

The present invention has been made paying attention to the technical problems described above, and can sufficiently control the characteristic impedance of the line without using the mesh-like conductor part while ensuring the line width of the signal transmission path. It is an object of the present invention to provide a circuit board that can be suitably applied to a flexible board that can be bent.

The circuit board according to the present invention, which has been made to solve the above-described problems, is a circuit board in which a signal transmission path is formed on one surface and a ground layer is formed on the other surface with an insulator layer interposed therebetween. In addition, a shield layer is further laminated on the ground layer via an insulator layer, and the ground layer is formed as an opening where a metal foil is not laid at a position along the signal transmission path. The characteristic impedance of the signal transmission path is controlled by a coupling capacitance formed between the shield layer and the shield layer.

In this case, the shield layer is preferably composed of a conductive paste or a conductive shield film. A via hole is desirably formed in the insulator layer between the ground layer and the shield layer, and the shield layer is connected to the ground layer through the via hole.

In addition, the insulator layer interposed between the signal transmission path and the ground layer is preferably configured using a film-like base substrate.

According to the circuit board described above, the ground layer at the position along the signal transmission path is formed as an opening where no metal foil is laid, and the signal transmission path is further coupled with the shield layer stacked via the insulator layer. The characteristic impedance is controlled by the capacitance. As a result, it is possible to easily solve the problem of characteristic impedance control of the signal transmission path that occurs as the circuit board becomes thinner.

That is, according to the circuit board having the above-described configuration, it is possible to suppress the extreme thinning of the signal transmission path taken with the thinning of the circuit board, and the ground layer facing the signal transmission path is meshed. In this case, it is possible to effectively eliminate the occurrence of a partial variation of Zo due to an extreme decrease in the conductor remaining rate.

It is the laminated structure figure which showed the circuit board concerning this invention with sectional drawing. It is the laminated structure figure which showed an example of the conventional circuit board with sectional drawing. It is the laminated structure figure which showed the other example of the conventional circuit board with sectional drawing. FIG. 4 is a perspective view showing a configuration of a part of a ground layer and a signal transmission path in the circuit board shown in FIG. 3.

Hereinafter, a circuit board according to the present invention will be described based on the embodiments shown in the drawings. FIG. 1 shows a laminated structure of a circuit board according to the present invention. In FIG. 1, parts having the same functions as those shown in FIG.

That is, the circuit board 1 shown in FIG. 1 is configured on the basis of a double-sided board, and the thickness of the insulator layer 2 serving as a central base material is formed to be thinner and schematically shown by t3. Is formed.
Then, a part of the metal foil (copper foil) 3 adhered to one surface (front surface) of the insulator layer 2 serving as a central base substrate is a signal transmission path, that is, a pair wiring 3a by, for example, an etching process. , 3b, and the pair wirings constitute a differential transmission path.

Also, a metal foil (copper foil) is similarly attached as a solid electrode to the other surface (back surface) of the insulator layer 2, and this constitutes the ground layer 4. The portion of the ground layer 4 where the signal transmission paths 3a and 3b face each other is an opening 4B along the signal transmission paths 3a and 3b where the metal foil is not laid.

The surface of the metal foil 3 and the signal transmission paths 3a and 3b is also referred to as a covering layer 6 (hereinafter referred to as a first covering layer) via an adhesive layer 5 (hereinafter also referred to as a first adhesive layer). ) And a coating layer 8 (hereinafter also referred to as a second coating layer) via an adhesive layer 7 (hereinafter also referred to as a second adhesive layer) on the back surface of the ground layer 4. The insulating layer is constituted by the second adhesive layer 7 and the second covering layer 8 which are laminated.
Further, a shield layer 9 is formed on the back surface of the second cover layer 8, and the shield layer 9 is formed by forming the insulator layer composed of the second adhesive layer 7 and the second cover layer 8. It is connected to the ground layer 4 through a via hole 8a formed so as to penetrate therethrough.

With the above configuration, the signal transmission lines 3a and 3b form a microstrip structure in which the characteristic impedance is controlled by the coupling capacitance formed between the shield layer 9 and the signal transmission lines 3a and 3b.

The base base material 2 that functions as a central insulator layer also has a function of becoming a core of the circuit board 1. Examples of the material of the base substrate 2 include a resin film and a fiber substrate.

As the material constituting the resin film, for example, polyimide resins such as polyimide resin, polyamide resin, and polyamideimide resin, thermosetting resins such as epoxy resin, thermoplastic resins such as liquid crystal polymer, and the like can be used.
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.

Examples of the fiber base material used for the insulator layer include glass fiber base materials such as glass fiber cloth and glass non-woven cloth, or inorganic fiber base materials such as fiber cloth and non-fiber cloth containing inorganic compounds other than glass, aromatic And organic fiber base materials composed of organic fibers such as aromatic polyamide resins, polyamide resins, aromatic polyester resins, polyester resins, polyimide resins, and fluororesins. 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, it is preferably used in a state where the fiber base material is 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 thickness of the base substrate 2 is preferably in the range of 1 to 100 μm, more preferably in the range of 5 to 50 μm, and more preferably in the range of 10 to 30 μm.
By setting the thickness of the base substrate 2 to be equal to or greater than the lower limit value, it becomes easy to make the line width of the signal transmission path equal to or greater than the processing limit. It is possible to prevent the height from becoming too high and to retain the characteristics of a thin substrate such as a flexible circuit board that is flexible.

The signal transmission paths 3a and 3b arranged on the surface of the base substrate 2 may be provided directly on the base substrate 2 as shown in FIG. 1, or may be provided via an adhesive. Good. Then, at an end portion of each signal transmission path or an appropriate intermediate portion, it is bonded to a mounting pad such as a semiconductor device (not shown) and functions as a circuit board.

For the first adhesive layer 5 covering the signal transmission paths 3a and 3b, for example, an acrylic resin, an epoxy resin, a polyimide resin, or the like can be suitably used. Among these, an epoxy resin is preferable. Thereby, heat resistance and flexibility can be improved.

The thickness of the adhesive layer 5 is not particularly limited, but is preferably 5 to 40 μm, and particularly preferably 10 to 30 μm. By making the thickness of the adhesive layer 5 equal to or higher than the lower limit value, it is possible to suppress a decrease in circuit embeddability, and by setting the thickness to be equal to or lower than the upper limit value, it is possible to suppress an increase in the amount of spotting of the adhesive layer 5 and Reliability can be maintained.

The first covering layer 6 laminated on the surface of the adhesive layer 5 is preferably made of a resin material. Preferable resin materials constituting the coating layer 6 include, for example, polyester resins, polyimides, liquid crystal polymers, and the like. Among these, polyimide is preferable. Thereby, heat resistance and flexibility can be improved.

The thickness of the first coating layer 6 is not particularly limited, but is preferably 5 to 50 μm, and particularly preferably 10 to 30 μm. By setting the thickness of the coating layer 6 to be equal to or higher than the lower limit value, it becomes easy to maintain the strength of the resin layer within a practical range, and by setting the thickness to be equal to or lower than the upper limit value, slidability and flexibility are maximized. It is easy to make it appear.

The first covering layer 6 may be a cover layer film in which the first adhesive layer 5 is previously laminated.

As described above, the copper foil as the ground layer 4 laminated on the back surface of the base substrate 2 has the portions where the signal transmission paths 3a and 3b face each other along the signal transmission paths 3a and 3b. It is made in the opening 4B where no metal foil is laid.

A second adhesive layer 7 is provided on the back surface of the ground layer 4. Examples of the material constituting the second adhesive layer 7 include acrylic resins, epoxy resins, polyimide resins, and the like, similarly to the materials constituting the first adhesive layer 5 described above.
Among these, an epoxy resin is preferable. Thereby, heat resistance and flexibility can be improved. In addition, the material which comprises the above-mentioned 1st contact bonding layer 5 and the 2nd contact bonding layer 7 may be the same, or may differ.

The thickness of the second adhesive layer 7 is not particularly limited, but is preferably 5 to 40 μm, and particularly preferably 10 to 30 μm.
In the same manner as the first adhesive layer 5 already described, the thickness of the second adhesive layer 7 is set to be equal to or higher than the lower limit value, thereby suppressing deterioration of circuit embedding property and lower than the upper limit value. Thus, it is possible to suppress an increase in the amount of the stain on the adhesive layer 7 and to maintain the reliability of interlayer adhesion.
Further, the thickness of the first adhesive layer 5 and the thickness of the second adhesive layer 7 may be the same or different.

The second cover layer 8 provided on the back surface of the adhesive layer 7 is preferably made of a resin material. As a resin material used for this, like the above-mentioned 1st coating layer 6, a polyester-type resin, a polyimide, a liquid crystal polymer etc. are mentioned, for example. Among these, polyimide is preferable. Thereby, heat resistance and flexibility can be improved.
Further, the resin material constituting the first coating layer 6 and the resin material constituting the second coating layer 8 may be the same or different.

The thickness of the second coating layer 8 is not particularly limited, but is preferably 5 to 50 μm, and particularly preferably 10 to 30 μm.
As in the case of the first coating layer 6 already described, this makes it easy to maintain the strength of the resin layer in a practical range by setting the thickness of the second coating layer 8 to the lower limit value or more. By making it below the upper limit value, it becomes easy to maximize the slidability and flexibility.

It should be noted that the second cover layer 8 may be formed by previously laminating the second adhesive layer 7 and forming a coverlay film.

Further, a shield layer 9 is formed on the back surface of the second coating layer 8 described above, and this shield layer 9 penetrates the second adhesive layer 7 and the second coating layer 8 as described above. It is connected to the ground layer 4 through the via hole 8a formed as described above.

The shield layer 9 is preferably made of, for example, a conductive paste or a conductive shield film.
As the conductive paste used for the shield layer 9, for example, a well-known one in which conductive particles such as silver, copper, or carbon are dispersed in a resin material can be used. It can be set to about 20 μm.
Thereby, the shield layer 9 can be formed without impairing the function of the circuit board such as flexibility, and at the same time, it can be connected to the ground layer 4 through the via hole 8a.

In addition, a conductive shield film can be used for the shield layer 9, and a silver shield material obtained by evaporating a silver material on a resin film can be used. Even when a conductive shield film typified by this silver shield material is used, the shield layer 9 can be formed without impairing the function of the circuit board such as flexibility. *

With the configuration of the circuit board described above, the shield layer 9 formed on the back surface of the substrate 1 is set to the same potential as the ground layer 4, and the signal transmission paths 3 a and 3 b are at a distance t 4 between the shield layer 9. The characteristic impedance is based on the dependent coupling capacitance.

That is, even when a thin circuit board is configured as shown in FIG. 1, the characteristic impedance of the signal transmission lines 3a and 3b is determined based on the distance t4 between the shield layer 9 and this. Thus, the transmission line 3a, 3b line width can be formed with an appropriate width indicated by W2.

Furthermore, since the distance t4 between the signal transmission paths 3a and 3b and the shield layer 9 can be secured, a solid electrode can be adopted as the shield layer 9, and by meshing the ground layer as shown in FIG. Occurrence of partial variations in the generated Zo can be effectively eliminated.

In the above-described embodiment, the circuit board adopting the differential transmission line by the pair wiring is taken as an example, but this can obtain the same operation effect even when a single-end transmission line is configured. .

In addition, a reference potential of a circuit may be applied to the ground layer and the shield layer in the above-described embodiment, and an operation power source of each device mounted on the circuit board may be superimposed. Therefore, the potential applied to the ground layer and the shield layer is not particularly limited.

The circuit board according to the present invention can be suitably used for a thin circuit board such as a flexible printed wiring board or a multilayer flexible printed wiring board on which a device operating in a high frequency band is mounted.

1 Circuit board 2 Base substrate (insulator layer)
3 Metal foil (copper foil)
3a, 3b Signal transmission path (pair wiring)
4 Ground Layer 4B Metal Foil Opening 5 First Adhesive Layer 6 First Covering Layer 7 Second Adhesive Layer 8 Second Covering Layer 8a Via Hole 9 Shield Layer

Claims

A circuit board in which a signal transmission path is formed on one surface and a ground layer is formed on the other surface across an insulator layer,
The ground layer is further laminated with a shield layer through an insulator layer, and the ground layer is formed in an opening where a metal foil is not laid at a position along the signal transmission path. A circuit board, wherein the characteristic impedance of the signal transmission path is controlled by a coupling capacitance formed between the shield layer and the shield layer.
The circuit board according to claim 1, wherein the shield layer is made of a conductive paste or a conductive shield film.
3. The shield layer according to claim 1, wherein the shield layer is connected to the ground layer through a via hole formed in the insulator layer between the ground layer and the shield layer. Circuit board.
The insulator layer interposed between the signal transmission path and the ground layer is formed of a film-like base substrate, and is described in any one of claims 1 to 3. Circuit board.