JP2013077769A - Circuit board - Google Patents

Abstract

【Task】
Provided is a circuit board capable of reducing transmission loss of a high-frequency signal and suppressing crosstalk noise.
A circuit board including an insulating substrate and a wiring layer formed inside or at least on one main surface of the insulating substrate, wherein the insulating substrate has a dielectric loss tangent (tan δ) at 25 ° C. and 1 GHz of 0.004 to 0.004. 0.01 substrate, the wiring layer is a layer having a wiring conductor width of 40 μm or less, and the surface roughness (Rz) of the surface in contact with the insulating substrate is 1.5 μm or less. Circuit board.
[Selection] Figure 1

Description

  The present invention relates to a circuit board capable of reducing transmission loss of a high-frequency signal and suppressing crosstalk noise.

In recent years, with the advent of advanced information technology, information transmission has begun to increase in speed and frequency, and microwave communication and millimeter wave communication have become a reality. High frequency circuit boards are required to reduce transmission loss in order to ensure the quality of output signals.
The transmission loss mainly includes a dielectric loss due to a dielectric and a conductor loss due to a conductor. Dielectric loss is known to decrease as the dielectric constant and dielectric loss tangent (tan δ) of the dielectric decreases, and with respect to conductor loss, the higher the frequency, the more current concentrates on the surface of the line and the resistance value increases. It is known that the amount of loss increases.
Conventionally, in order to reduce transmission loss, dielectric loss has been reduced by lowering the dielectric constant and dielectric loss tangent of the dielectric.

  Also, in a high-frequency circuit board, so-called crosstalk noise, which is affected by signals from adjacent wiring, has become a problem. In particular, in recent years, crosstalk noise is likely to occur due to the reduction in transmission loss as described above and the close proximity of adjacent wirings accompanying the increase in circuit density.

Regarding the technology for preventing the crosstalk noise, Patent Document 1 includes an insulating layer and a wiring layer between these insulating layers, and an insulating layer is provided between the signal wirings serving as the wiring layers. There is described a circuit board provided with an inter-wiring insulating portion having a larger dielectric loss than that of the wiring board.
However, Patent Document 1 does not disclose a method for simultaneously reducing transmission loss. Further, when the circuit board described in this document is manufactured, there is a problem that many steps are required because it is necessary to provide an insulating portion between the signal wirings.

JP 2010-245573 A

  The present invention has been made in view of the above-described prior art, and an object of the present invention is to provide a circuit board that can reduce transmission loss of high-frequency signals and suppress crosstalk noise.

  In order to solve the above-mentioned problems, the present inventors diligently studied a circuit board including an insulating substrate and a wiring layer formed inside or on at least one main surface of the insulating substrate. As a result, in high-frequency signal transmission, it is more effective to reduce the conductor loss than to reduce the dielectric loss. In particular, as the wiring width becomes smaller in order to increase the circuit density, the tendency tends to decrease. I found it to increase. Then, by flattening the surface of the wiring layer that contacts the insulating substrate, the conductor loss is reduced, and at the same time, the dielectric loss tangent of the insulating substrate is increased to some extent, thereby maintaining the transmission loss at the conventional level and cross The present inventors have found that talk noise can be suppressed, and have completed the inventions described in (1) to (5) below.

(1) A circuit board including an insulating substrate and a wiring layer formed inside or at least on one main surface of the insulating substrate, wherein the insulating substrate has a dielectric loss tangent (tan δ) of 0 at 25 ° C. and 1 GHz. .004 to 0.01, the wiring layer has a portion with a wiring width of 40 μm or less, and the surface roughness (Rz) of the surface in contact with the insulating substrate is 1.5 μm or less. There is a circuit board.
(2) The circuit board according to (1), wherein the wiring layer is a layer having a surface roughness (Rz) of 1 μm or less on a surface in contact with the insulating substrate.
(3) The circuit board according to (1) or (2), wherein the wiring layer is a layer made of two or more wirings and having a portion having an interval between the wirings of 25 μm or less.
(4) The circuit board according to any one of (1) to (3), which has a microstrip line structure or a strip line structure.
(5) The circuit board according to any one of (1) to (4), wherein the circuit board transmits a signal having a frequency of 2 GHz or more.

  ADVANTAGE OF THE INVENTION According to this invention, the circuit board which can reduce the transmission loss of a high frequency signal and can suppress crosstalk noise is provided.

It is sectional drawing showing the structure of the circuit board of this invention. It is sectional drawing showing the structure of the circuit board of this invention. It is sectional drawing showing the structure of the circuit board of this invention. It is a figure showing the model used in the Example (simulation).

  The circuit board of the present invention is a circuit board including an insulating substrate and a wiring layer formed inside or on at least one main surface of the insulating substrate, and the insulating substrate has a dielectric loss tangent at 1 GHz at 25 ° C. tan δ) is a substrate having 0.004 to 0.01, the wiring layer has a portion with a wiring width of 40 μm or less, and the surface roughness (Rz) of the surface in contact with the insulating substrate is 1.5 μm. A circuit board which is the following layer.

The dielectric substrate used in the present invention has a dielectric loss tangent at 25 ° C. and 1 GHz of 0.004 to 0.01, preferably 0.008 to 0.01. By setting the dielectric loss tangent within the above range, crosstalk noise is suppressed in the circuit board of the present invention.
Examples of the insulating substrate include an inorganic substrate made of an inorganic material, a resin substrate made of a resin material, and a substrate made of a composite material of a fiber material and a resin material.
Examples of inorganic materials include silica, alumina, forsterite, and borosilicate glass.
Resin materials include epoxy resin, acrylic resin, polyamide resin, polyimide resin, polyester resin, cyanate resin, polyphenylene ether resin, fluororesin, styrene resin, olefin resin, cycloolefin resin, polyester resin, phenol resin, maleimide resin, and And triazine resin.
Examples of the composite material include composite materials obtained by impregnating the resin material with an inorganic or organic fiber material such as glass cloth.
The insulating substrate can contain a filler. The dielectric loss tangent can be adjusted by containing a filler. The filler may be either an inorganic filler or an organic filler, and may be used in combination of two or more. The type and blending amount of the filler are not particularly limited and can be appropriately selected so as to obtain a desired dielectric loss tangent.

The wiring layer is a layer made of wiring formed of a conductor, and has a wiring width of 40 μm or less, preferably 25 μm or less, more preferably 20 μm or less.
The wiring layer has a surface roughness (Rz) of a surface in contact with the insulating substrate of 1.5 μm or less, preferably 1 μm or less, more preferably 0.4 μm or less. The lower limit of the surface roughness (Rz) is usually 0.03 μm. By being 1.5 μm or less, the conductor loss can be sufficiently reduced. The surface roughness (Rz) refers to a ten-point average roughness obtained according to JIS B0601-1994.

  Originally, if the wiring width is narrowed, the transmission loss also increases as the conductor loss increases. However, in the present invention, the conductor loss can be greatly reduced by reducing the surface roughness (Rz) of the surface of the wiring layer in contact with the insulating substrate. Therefore, the smaller the wiring width, the more the effect of the present invention ( The effect of reducing the conductor loss is more exhibited. Further, since the wiring width is in the above range, it is easy to achieve high density of the circuit.

  When the wiring layer is composed of two or more wirings, the effect of the present invention can be obtained more remarkably when the wiring layer is a layer having a portion having a width between the wirings of 25 μm or less, preferably 20 μm or less. From the viewpoint, it is preferable. The lower limit of the width between the wirings is usually 5 μm. Originally, it is not preferable to reduce the width between two wirings because crosstalk noise is likely to occur. However, in the present invention, the crosstalk noise can be suppressed by increasing the dielectric loss tangent of the insulating substrate as described above. Thereby, high density of a circuit can be achieved.

The thickness of a wiring layer is 1-50 micrometers normally, Preferably it is 3-30 micrometers, More preferably, it is 5-20 micrometers.
The material of the conductor constituting the wiring layer is not particularly limited, and for example, copper can be used.

  The method for forming the wiring layer is not particularly limited. For example, the wiring layer can be formed by employing a subtractive method. Specifically, a metal foil having a surface roughness (Rz) of 1.5 μm or less is laminated on an insulating layer, then an etching resist film having a predetermined wiring pattern is formed on the metal foil, and an etching solution A wiring layer having a predetermined wiring pattern can be formed by removing an unnecessary portion of the metal film using, and finally removing the etching resist film.

  A metal foil having a surface roughness (Rz) of 1.5 μm or less can be obtained by smoothing the surface of the metal foil. Examples of the smoothing treatment include mechanical polishing treatment, plasma treatment, chemical polishing treatment, electrochemical dissolution treatment, and plating treatment.

Examples of the mechanical polishing treatment include buff polishing, grindstone polishing, brush polishing, and polishing with an abrasive. The mechanical polishing treatment may be either wet polishing or dry polishing.
Examples of the plasma treatment include glow discharge.
As the chemical polishing treatment, known methods such as acid treatment and alkali treatment can be used. In the acid treatment, an acid treatment liquid containing an acid such as sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid is usually used. In the alkali treatment, a base such as sodium hydroxide or potassium hydroxide, cyanate salt, pyrroline is usually used. An alkaline treatment liquid containing a salt such as an acid salt is used, and chemical polishing treatment is performed by bringing these treatment liquids into contact with the surface of the metal foil. From the viewpoint of processing efficiency and ease of management of the processing liquid, the processing temperature is preferably 10 to 70 ° C.

The electrochemical dissolution treatment is performed by immersing a metal foil as an anode (anode) in an electrolytic bath filled with a treatment solution. In the electrolytic cell, a conductive plate (cathode) is usually set on the opposite side of the metal foil surface. The cathode material is preferably one that is not easily dissolved in the electrolytic solution, and examples thereof include platinum, titanium, and stainless steel. Moreover, as a processing liquid, what was mentioned by said chemical polishing process can be used. Current density in the range of 1~100A / dm ² is preferred.

The plating treatment is a method for forming a smooth plating film by attaching a metal to the surface of a metal foil. For example, the plating treatment can be performed using a copper sulfate plating bath, a copper cyanide plating bath, a copper fluoride plating bath, a copper pyrophosphate plating bath, or a copper sulfamate plating bath. In the plating treatment, it is preferable to form a plating film having a thickness of 0.01 μm or more. In order to reduce the surface roughness, the size of the crystal grains attached to the surface is preferably 20 μm or less.
A metal foil having a surface roughness (Rz) of 1.5 μm or less can be obtained as a commercial product.

  The structure of the circuit board of the present invention is not limited as long as it has the above configuration. For example, the circuit board having the microstrip line structure shown in FIG. 1, the circuit board having the strip line structure shown in FIG. Examples include a multilayer circuit board having two or more wiring layers shown.

  Since the circuit board of the present invention has the above characteristics, it is suitably used as a circuit board for transmitting a high-frequency signal (for example, 2 GHz or more, preferably 5 to 20 GHz).

Hereinafter, the present invention will be described more specifically based on examples.
In order to show the characteristics of the circuit board of the present invention, calculation was performed as follows using a three-dimensional electromagnetic field simulator Sonnet 12.56 (manufactured by Sonnet Software Inc.). The simulation settings were set as follows.
Cell Size: X; 0.2 mm, Y; 5.0e-4 mm
Box Size: X; 10.0 mm, Y; 0.4 mm
Insulating substrate thickness: 0.0305mm on both top and bottom
Wiring thickness: 0.009mm
“Thick Metal Model” was selected and “Num. Sheets” was set to 18 in order to perform calculation considering the wiring thickness.

  Since the simulation software cannot directly input the roughness of the conductor surface, we decided to substitute the effective value of the conductivity that varies depending on the roughness of the surface. Table 1 shows "Allen F. Horn et al., Conductor Profile Effects on the Propagation Constant of Microstrip Transmission Lines, IMS2010 Dig., Pp. 868-871, Jun. 2010", "Kobayashi, Yoshihiro, Ma Microwave characteristics evaluation of formed COP substrates, Japan Institute of Electronics Packaging Vol. 9, No. 3, pp. 1-4, (Nov. 6, 2009) ”and“ Hitachi Chemical Technique No. 46 (2006-1) ” This shows the change in effective conductivity at each frequency due to surface roughness. Table 2 shows the effective conductivity derived from Table 1, and this value was input as the conductivity at each frequency.

Example 1
A perspective view of the model of Example 1 is shown in FIG. 4A, and a cross-sectional view thereof is shown in FIG. 4B.
The wiring (32a) has a width of 20 μm, a thickness of 9 μm, and a length of 10 mm. The signal is input from input port 1 (34) and output from output port 2 (35). In addition, a wiring (32b) connecting the port 3 (36) and the port 4 (37) is arranged in parallel with the wiring (32a) connecting the input port 1 (34) and the output port 2 (35). The distance between the wiring (32a) and the wiring (32b) is 20 μm.
The insulating substrate is formed of a dielectric layer (31a) and a dielectric layer (31b), both of which have a relative dielectric constant of 4.3 and tan δ at 25 ° C. and 1 GHz. 0.004. Conductor layers (33a) and (33b) functioning as ground layers are arranged above and below the insulating substrate. Here, the thicknesses of the dielectric layers (31a) and (31b) are such that the characteristic impedances of the port 1 (34), the port 2 (35), the port 3 (36), and the port 4 (37) are all 50Ω. Was set to 30.5 μm. The roughness (Rz) of the surfaces in contact with the dielectric layers (31a) and (31b) of the wirings (32a) and (32b) and the conductor layers (33a) and (33b) is 0.4 μm or less.

(Example 2)
In the model of Example 1, the dielectric layers (31a) and (31b) were the same as Example 1 except that the relative dielectric constant was changed to 4.3 and tan δ was changed to 0.01, respectively.
(Example 3)
In the model of Example 1, the roughness (Rz) of the surfaces in contact with the wiring layers (32a) and (32b) and the dielectric layers (31a) and (31b) of the conductor layers (33a) and (33b), respectively, Example 1 was repeated except that the thickness was changed to 1.5 μm.
Example 4
In the model of Example 1, the dielectric layers (31a) and (31b) were changed to a relative dielectric constant of 4.3 and tan δ to 0.01, respectively, and the wirings (32a) and (32b) and the conductor layer were changed. Example 33 was the same as Example 1 except that the roughness (Rz) of the surfaces in contact with the dielectric layers (31a) and (31b) of (33a) and (33b) was changed to 1.5 μm.

(Comparative Example 1)
In the model of Example 1, the roughness (Rz) of the surfaces in contact with the dielectric layers (31a) and (31b) of the wirings (32a) and (32b) and the conductor layers (33a) and (33b) are respectively determined. The procedure was the same as in Example 1 except that the thickness was changed to 3 μm.
(Comparative Example 2)
In the model of Example 1, the dielectric layers (31a) and (31b) are changed to a relative dielectric constant of 4.3 and tan δ to 0.003, respectively, and the wirings (32a) and (32b) and the conductor layer are changed. Example 33 was the same as Example 1 except that the roughness (Rz) of the surfaces in contact with the dielectric layers (31a) and (31b) of (33a) and (33b) was changed to 1.5 μm.
(Comparative Example 3)
In the model of Example 1, the dielectric layers (31a) and (31b) are changed to a relative dielectric constant of 4.3 and tan δ to 0.003, respectively, and the wirings (32a) and (32b) and the conductor layer are changed. Except that the roughness (Rz) of the surfaces in contact with the dielectric layers (31a) and (31b) of (33a) and (33b) was changed to 0.4 μm or less, respectively, it was the same as in Example 1. .

Table 3 shows the simulation results of Examples 1 to 4 and Comparative Examples 1 to 3. The frequency was calculated at 2, 5, 10, 20 GHz.
S11 is the reflected power at port 1, and the smaller the value, the better the matching is 50Ω. S21 is the transmission ratio of power from port 1 to port 2, and the larger the value, the smaller the transmission loss is transmitted. S41 is a power ratio in which the power input to the input port 1 leaks to the port 4 and is output. The smaller the value, the more the crosstalk noise is suppressed.

From Table 3, in Examples 1-4, the value of S21 is large and the value of S41 is small at each frequency with respect to Comparative Example 1 corresponding to the prior art. That is, it can be seen that transmission loss is small and crosstalk noise is suppressed.
In the third embodiment, the transmission loss is slightly larger than that of the second comparative example, but the crosstalk noise is greatly suppressed.
In the first embodiment, the transmission loss is slightly larger than that of the third comparative example, but the crosstalk noise is greatly suppressed.
The same experiment was conducted using an insulating substrate in which the wiring and conductor layers were formed of copper foil and the dielectric layer was formed of a cycloolefin resin under the conditions of Example 1. In general, Example 1 It was confirmed that a result similar to the result of was obtained.

1, 11, 21: Insulating substrates 2, 12, 22: wirings 3, 13, 23: ground layer 4: wiring width 5: width between two wirings 31a, 31b: dielectric layers 32a, 32b: wirings 33a, 33b : Conductor layer (ground layer)
34: Input port 1
35: Output port 2
36: Port 3
37: Port 4

Claims (5)

A circuit board including an insulating substrate and a wiring layer formed inside or on at least one main surface of the insulating substrate,
The insulating substrate is a substrate having a dielectric loss tangent (tan δ) at 1 GHz at 25 ° C. of 0.004 to 0.01,
The circuit board, wherein the wiring layer has a part having a wiring width of 40 μm or less and a surface roughness (Rz) of a surface in contact with the insulating substrate is 1.5 μm or less.   The circuit board according to claim 1, wherein the wiring layer is a layer having a surface roughness (Rz) of a surface in contact with the insulating substrate of 1 μm or less.   3. The circuit board according to claim 1, wherein the wiring layer is a layer made of two or more wirings and having a portion having an interval between the wirings of 25 μm or less.   The circuit board according to claim 1, which has a microstrip line structure or a strip line structure.   The circuit board according to claim 1, which is a circuit board through which a signal having a frequency of 2 GHz or more is transmitted.

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