JP5098794B2 - High frequency circuit board - Google Patents

Description

  The present invention relates to a high frequency circuit board capable of signal transmission with low loss in signal transmission in a high frequency region.

  In recent years, high-frequency applications such as home high-speed wireless communication and in-vehicle collision prevention radar have been actively developed. In circuit boards for the purpose of high-frequency signal processing, transmission lines such as strip lines and microstrip lines are often used in high-frequency regions up to several GHz, but signal attenuation in higher microwave regions and millimeter-wave regions. Since it is remarkable, it becomes difficult to use these lines. Therefore, a waveguide is widely used as an alternative signal propagation path. Waveguides exhibit excellent transmission characteristics, but have disadvantages such as a large line size, inability to perform fine routing, and high manufacturing costs. Therefore, there is a demand for a circuit board that has transmission characteristics not inferior to those of a waveguide, is inexpensive, and can handle fine wiring.

  In the case of using a strip line (see FIG. 2) or a microstrip line, it is common to use resin or ceramic as the material of the substrate. However, the dielectric characteristics of the substrate material are caused by transmission loss, particularly due to a material called dielectric loss. It is known to have a large effect on losses. In order to reduce the dielectric loss, it is preferable to use a material having a small relative dielectric constant and a dielectric loss tangent, and development of a material having a low dielectric constant and a low dielectric loss tangent is ongoing.

  PTFE (Teflon (registered trademark)) is widely known as a resin material having excellent dielectric properties, but various substrates have been used so far for applications requiring materials having a lower dielectric constant and lower dielectric loss tangent. Materials have been proposed. For example, Patent Documents 1 to 3 propose a substrate material using a foam material.

JP-A 62-69580 JP 7-202439 A JP-A-6-125154

  Patent Document 1 proposes a substrate material in which a copper foil is disposed on one or both sides of a foam material. By using a foam material, a substrate material having a low relative dielectric constant can be obtained, and a low-loss circuit substrate can be formed. However, since the core portion is made of only a thermoplastic resin, the mechanical strength is not sufficient. Also in Patent Document 2, since a thermoplastic film such as PET is used as the foaming material, sufficient mechanical strength cannot be obtained as in Patent Document 1. Further, in Patent Document 3, the mechanical strength is improved by using a foam cured product of a thermosetting resin, but in wiring formation by a wet process, a chemical solution may enter and remain in the foamed portion, thereby reducing reliability. There is. Therefore, the present invention provides a thermosetting insulating resin layer, a signal line conductor layer, an insulating resin layer made of a foam, and a ground conductor layer. It is an object to provide a high-frequency circuit board having excellent transmission characteristics and sufficient mechanical strength by laminating an insulating resin layer / ground conductor layer in this order.

  The present invention comprises a non-foam insulating resin layer made of a non-foamed cured product of a thermosetting resin, a signal line conductor layer, a foam insulating resin layer made of a resin foam, and a ground conductor layer. The present invention relates to a high-frequency circuit board in which a foam insulating resin layer / a signal line conductor layer / a foam insulating resin layer / a ground conductor layer are laminated in this order.

  The present invention also relates to the above high-frequency circuit board, wherein the resin foam has a relative dielectric constant at 10 GHz of 1.0 to 2.0 and a dielectric loss tangent of 0.0001 to 0.001.

  The present invention also relates to the high-frequency circuit board, wherein the foam insulating resin layer has a thickness of 0.05 to 1.0 mm.

  The present invention also relates to the above high-frequency circuit board, wherein the non-foamed cured product of the thermosetting resin has a relative dielectric constant at 10 GHz of 2.0 to 4.0 and a dielectric loss tangent of 0.001 to 0.01.

  The present invention also relates to the high-frequency circuit board, wherein the non-foam insulating resin layer has a thickness of 0.05 to 1.0 mm.

  Moreover, this invention relates to the said high frequency circuit board whose storage elastic modulus in 25-30 degreeC of the non-foaming hardened | cured material of the said thermosetting resin is 1-30 GPa.

  According to the present invention, it is possible to provide a high-frequency circuit board having sufficient mechanical strength and capable of signal transmission with low loss.

  A cross-sectional view of one embodiment of the high-frequency circuit board of the present invention is shown in FIG. The high-frequency circuit board of the present invention includes a non-foam insulating resin layer 1 made of a non-foamed cured product of a thermosetting resin, a signal line conductor layer 2, a foam insulating resin layer 3 made of a resin foam, and a ground conductor. A high-frequency circuit board having a layer 4 and laminated in the order of non-foam insulating resin layer 1 / signal line conductor layer 2 / foam insulating resin layer 3 / ground conductor layer 4. An insulating resin made of a resin foam is a material having a low dielectric constant and a low dielectric loss tangent, and a microstrip line with a small transmission loss can be formed by forming the structure.

  In the present specification, the dielectric constant and the dielectric loss tangent are values measured using a cavity resonator perturbation method dielectric constant measuring apparatus (manufactured by Kanto Electronics Application Development Co., Ltd.). The storage elastic modulus is a value measured using a dynamic viscoelasticity measuring device Rheogel-E4000 (manufactured by UBM).

  The material of the resin foam is not particularly limited, and may be any of a thermoplastic resin foam and a thermosetting resin foam cured product, for example, a foam such as polystyrene, polyethylene, polypropylene, and polyurethane. Can be suitably used. The relative dielectric constant of the foam insulating resin layer is preferably 1.0 to 2.0 at 10 GHz. If the relative dielectric constant exceeds 2.0, transmission loss may increase and a desired effect may not be obtained. In order to reduce transmission loss as much as possible, the relative dielectric constant of the foam insulating resin layer is more preferably 1.0 to 1.3. The dielectric loss tangent of the foam insulating resin layer is preferably 0.0001 to 0.001 at 10 GHz. If the dielectric loss tangent exceeds 0.001, the transmission loss may increase and the desired effect may not be obtained. Similarly, in order to reduce transmission loss as much as possible, the dielectric loss tangent of the foam insulating resin layer is more preferably 0.0001 to 0.0003.

  The resin foam preferably has a cell ratio of 70 to 99%, more preferably 90 to 98%. If the bubble ratio is less than 70%, desired dielectric characteristics cannot be obtained, and transmission loss may increase. On the other hand, if it exceeds 99%, the mechanical strength is remarkably lowered, and it may be difficult to control the thickness and form a circuit board.

  The thickness of the foam insulating resin layer made of a resin foam is preferably 0.05 to 1.0 mm. If the thickness exceeds 1.0 mm, the entire circuit board becomes thick and the cost is increased, which is not preferable. On the other hand, when it becomes thinner than 0.05 mm, the transmission loss tends to increase. From the viewpoint of achieving both thickness and transmission loss, it is more preferably 0.05 to 0.3 mm.

  On the foam insulating resin layer, the mechanical strength of the circuit board can be reinforced by disposing a non-foam insulating resin layer made of a non-foamed cured product of a thermosetting resin via a signal line conductor layer. Is possible. In the present invention, the non-foamed cured product of the thermosetting resin means a cured product obtained by thermosetting the thermosetting resin without performing foaming treatment such as addition of a foaming agent. The thermosetting resin is not particularly limited, and an epoxy resin system, a phenol resin system, an unsaturated polyester resin, or the like is preferably used. Examples of the epoxy resin thermosetting resin include polyphenol glycidyl ether type epoxy resins, phenol novolac type epoxy resins, and the like, such as bisphenol A type epoxy resins. There is no restriction | limiting in particular as a hardening | curing agent of an epoxy resin, For example, a dicyandiamide, imidazoles, aromatic amines, a phenol resin, an acid anhydride etc. can be used. The curing agent used for curing the phenol resin thermosetting resin is not particularly limited, such as hexamethylenetetramine. In addition, the non-foamed insulating resin layer is made of this thermosetting insulating resin, with the addition of resin and glass cloth and various fillers for the purpose of reducing the thermal expansion coefficient of the substrate and improving the mechanical strength. You may take the form. Examples of the filler include fused silica, glass, alumina, titania, zircon, calcium carbonate, calcium silicate, magnesium silicate, aluminum silicate, silicon nitride, boron nitride, beryllia, zirconia, barium titanate, potassium titanate, calcium titanate. Etc.

  The relative dielectric constant of the non-foam insulating resin layer is preferably 2.0 to 4.0 at 10 GHz. If the relative dielectric constant of the non-foam insulating resin layer exceeds 4.0, transmission loss increases, which is not preferable. In order to reduce transmission loss, it is desirable that the relative dielectric constant is as small as possible. However, in order to reduce the transmission loss to less than 2.0, a structure using a foam or the like must be used. From the viewpoint of achieving both transmission loss and mechanical strength, it is more preferably 2.0 to 3.7. The dielectric loss tangent of the non-foam insulating resin layer is preferably 0.001 to 0.01 at 10 GHz. If the dielectric loss tangent exceeds 0.01, transmission loss increases. In order to make it less than 0.001, it is necessary to use a foam or the like as in the case of the relative dielectric constant, which is not preferable because the mechanical strength is lowered. From the viewpoint of achieving both transmission loss and mechanical strength, it is more preferably 0.001 to 0.006.

  The thickness of the non-foam insulating resin layer is preferably 0.05 to 1.0 mm. Since the thermosetting resin faces the signal line conductor layer and is disposed on the side without the ground conductor layer, the thinner one is more effective in reducing transmission loss. However, if the thickness is excessively thin, the mechanical strength of the circuit board becomes insufficient, and the handleability also decreases. The upper limit is preferably 1.0 mm in order to suppress an increase in transmission loss. From the viewpoint of achieving both transmission loss and mechanical strength / handleability, the thickness is more preferably 0.05 to 0.3 mm.

  Furthermore, the storage elastic modulus of the non-foam insulating resin layer is preferably 1 to 30 GPa at 25 to 30 ° C. In order to obtain sufficient mechanical strength, it is desirably 1 GPa or more. Moreover, when it exceeds 30 GPa, there exists a tendency for workability to fall. From the viewpoint of achieving both mechanical strength and workability, it is more preferably 5 to 20 GPa.

  The material for the signal line conductor layer is not particularly limited, but metals such as copper, nickel, gold, silver, and aluminum can be suitably used. Speaking of general wiring board applications, copper is superior in terms of characteristics and cost. The signal line conductor layer may be formed from a plurality of metals by plating or the like.

  The material for the ground conductor layer is not particularly limited as in the case of the signal line conductor layer, and examples thereof include metals such as copper, nickel, gold, silver, and aluminum, and can be freely selected from the viewpoint of characteristics and cost. Can do. Although the thickness of a grounding conductor layer can be selected arbitrarily, 0.01-1.0 mm is preferable and 0.5-1.0 mm is more preferable. A metal foil such as a copper foil can be used for the ground conductor layer because of cost advantage. In this case, 0.01 mm is the lower limit because of versatility. On the other hand, in order to improve workability and reliability when assembling the substrate, the thickness is preferably 0.5 mm or more, and a metal plate or the like can also be used. Moreover, in order to suppress an increase in substrate thickness and cost, it is preferable to set 1.0 mm as the upper limit.

  In the high-frequency circuit board of the present invention, the conductor layer is not formed on the surface opposite to the surface having the ground conductor layer of the non-foam insulating resin layer, but this is due to the presence of the conductor layer, This is to suppress an increase in transmission loss. When a conductor layer is formed, the signal line has a stripline structure, and transmission loss generally increases as compared with a structure without a conductor layer.

  The high-frequency circuit board of the present invention is formed by, for example, forming a conductor layer on one surface of a non-foam insulating resin layer by bonding or sputtering of a conductor layer such as a metal foil, and then performing photolithography and etching on the conductor layer. The signal line conductor layer can be formed by processing to form a signal line conductor layer, and the foam insulating resin layer and the ground conductor layer can be laminated in that order on the signal line conductor layer. Also, non-foaming obtained by laminating a metal layer such as a metal foil on a prepreg obtained by impregnating a fiber reinforced material such as a glass cloth with a thermosetting resin and a curing agent used as necessary, and then heating and pressing. A signal line conductor layer is formed by using a metal-clad laminate having a metal layer on one or both sides of the body insulating resin layer and performing a process such as photolithography and etching on the metal foil layer on one side to form a signal line. You can also. When the metal-clad laminate has metal layers on both sides, the metal layer on one side is removed by etching or the like. Lamination and fixing of the foam insulating resin layer and the ground conductor layer on the ground conductor layer can be performed by fastening means such as screwing of the end portion of the substrate after lamination. An adhesive may be used for laminating the non-foam insulating resin layer and the foam insulating resin layer. However, since the transmission loss increases as the adhesive layer becomes thicker, it is important to make the thickness as thin as possible. If it is several μm or less, the influence on the transmission loss is small. Epoxy or the like can be used as the adhesive, but when the non-foamed insulating resin layer is a thermoplastic resin, it is necessary to consider resistance to solvents.

  EXAMPLES Next, the present invention will be specifically described by way of examples, but the present invention is not limited to these.

[Example 1]
A double-sided copper clad laminate having a thickness of 0.1 mm, having a copper layer on both sides of a non-foam insulating resin layer having a relative dielectric constant of 3.42 at 10 GHz and a dielectric loss tangent of 0.0057 and a storage elastic modulus of 14 GPa at 25-30 ° C. Characteristic impedance is present on one side of MCL-FX-2 (trade name, manufactured by Hitachi Chemical Co., Ltd.) (a laminated board having a copper layer with a thickness of 18 μm on each side of a non-foam insulating resin layer with a thickness of 64 μm). A signal line having a length of 20 mm was formed by photolithography and etching so as to be 50Ω, thereby forming a signal line conductor layer. At the same time, the copper on the opposite side was entirely removed by etching. On the signal line conductor layer on the non-foamed insulating resin layer, 0.5 mm thick foamed polypropylene (relative dielectric constant 1.1 at 10 GHz, dielectric loss tangent 0.0002, bubble rate: 97%) and thickness 0. 5 mm aluminum plates were stacked in order, the ends were screwed together to integrate the substrate, and the high-frequency circuit board 1 was obtained.

[Example 2]
A double-sided copper clad laminate having a thickness of 0.1 mm, having a copper layer on both sides of a non-foam insulating resin layer having a relative dielectric constant of 3.42 at 10 GHz and a dielectric loss tangent of 0.0057 and a storage elastic modulus of 14 GPa at 25-30 ° C. Characteristic impedance is present on one side of MCL-FX-2 (trade name, manufactured by Hitachi Chemical Co., Ltd.) (a laminated board having a copper layer with a thickness of 18 μm on each side of a non-foam insulating resin layer with a thickness of 64 μm). A signal line having a length of 20 mm was formed by photolithography and etching so as to be 50Ω, thereby forming a signal line conductor layer. At the same time, the copper on the opposite side was entirely removed by etching. On the signal line conductor layer on the non-foamed insulating resin layer, a 0.3 mm thick foamed polypropylene (relative dielectric constant 1.1 at 10 GHz, dielectric loss tangent 0.0002, bubble rate: 97%) and a thickness of 0. 5 mm aluminum plates were stacked in order, the ends were screwed together to integrate the substrate, and the high-frequency circuit board 2 was obtained.

[Example 3]
A high frequency circuit board 3 was obtained by the same procedure as in Example 1 except that the thickness of the expanded polypropylene was 0.1 mm.

[Example 4]
A double-sided copper clad laminate having a thickness of 0.1 mm, having a copper layer on both sides of a non-foam insulating resin layer having a relative dielectric constant of 4.2 at 10 GHz and a dielectric loss tangent of 0.021, 25 to 30 ° C. and a storage elastic modulus of 8 GPa Characteristic impedance is present on one side of MCL-E-67 (trade name, manufactured by Hitachi Chemical Co., Ltd.) (a laminated board having a 18 μm thick copper layer on each side of a 64 μm thick non-foam insulating resin layer). A signal line having a length of 20 mm was formed by photolithography and etching so as to be 50Ω, thereby forming a signal line conductor layer. At the same time, the copper on the opposite side was removed entirely by etching. On the signal line conductor layer on the non-foamed insulating resin layer, a 0.3 mm thick foamed polypropylene (relative dielectric constant 1.1 at 10 GHz, dielectric loss tangent 0.0002, bubble rate: 97%) and a thickness of 0. 5 mm aluminum plates were stacked in order, the ends were screwed together to integrate the substrate, and the high-frequency circuit board 4 was obtained.

[Example 5]
Double-sided copper-clad laminate The same procedure as in Example 2 except that the thickness of MCL-FX-2 was 0.3 mm (non-foam insulating resin layer thickness: 264 μm, each copper layer thickness: 18 μm) Thus, a high frequency circuit board 5 was obtained.

[Example 6]
Double-sided copper-clad laminate The same procedure as in Example 2 except that the thickness of MCL-FX-2 was changed to 0.5 mm (non-foam insulating resin layer thickness: 464 μm, each copper layer thickness: 18 μm). Thus, a high frequency circuit board 6 was obtained.

[Comparative Example 1]
A dielectric constant of 3.42 at 10 GHz, a dielectric loss tangent of 0.0057, a storage elastic modulus at 25-30 ° C .: a double-sided copper-clad laminate with a thickness of 0.1 mm, having a copper layer on both sides of a non-foam insulating resin layer of 14 GPa Plate MCL-FX-2 (trade name, manufactured by Hitachi Chemical Co., Ltd.) (a laminated plate having a copper layer with a thickness of 18 μm on each side of a non-foam insulating resin layer with a thickness of 64 μm), a characteristic impedance A signal line having a length of 20 mm was formed by photolithography and etching so as to be 50Ω, thereby forming a signal line conductor layer. On the signal line conductor layer on the non-foamed insulating resin layer, a 0.3 mm thick foamed polypropylene (relative dielectric constant 1.1 at 10 GHz, dielectric loss tangent 0.0002, bubble rate: 97%) and a thickness of 0. 5 mm aluminum plates were stacked in order, the ends were screwed together, and the substrate was integrated to obtain a high-frequency circuit board 7. The high-frequency circuit board 7 has a stripline structure having a cross section shown in FIG. 2, and the signal line conductor layer 2 is sandwiched between two dielectric layers 5, and an outer surface of one dielectric layer 5 is made of an aluminum plate. The ground conductor layer 4 has a structure in which a conductor layer 6 made of copper is disposed on the outer surface of the other dielectric layer 5.

[Comparative Example 2]
A dielectric constant of 3.42 at 10 GHz, a dielectric loss tangent of 0.0057, a storage elastic modulus at 25-30 ° C .: a double-sided copper-clad laminate with a thickness of 0.1 mm, having a copper layer on both sides of a non-foam insulating resin layer of 14 GPa Plate MCL-FX-2 (trade name, manufactured by Hitachi Chemical Co., Ltd.) (a laminated plate having a copper layer with a thickness of 18 μm on each side of a non-foam insulating resin layer with a thickness of 64 μm), a characteristic impedance A signal line having a length of 20 mm was formed by photolithography and etching so as to be 50Ω, and the high-frequency circuit board 8 was obtained.
An SMA connector was soldered to the signal line of the produced high-frequency circuit board and the end of the ground conductor layer, and transmission loss at 10 GHz was measured using a network analyzer PNA E8364B manufactured by Agilent Technologies. In the measurement of transmission loss, in Examples 1 to 5, an aluminum plate on foamed polypropylene was used as the ground conductor layer, and in Comparative Example 1, from the aluminum plate and the outer layer copper foil that remained without being removed by etching. Both of the conductor layers to be used were used as the ground conductor layers, and in Comparative Example 2, the copper layer on which the signal lines of the double-sided copper-clad laminate were not formed was used as the ground conductor layer.

  As shown in Table 1, in each of Examples 1 to 6, the transmission loss was 0.2 dB or less. On the other hand, in the comparative examples, it can be seen that the loss is large at about 0.36 dB. In the example and the comparative example, the arrangement of the ground conductor layer and the presence or absence of the foam insulating layer are different, and it can be seen from this result that the substrate structure in the present invention is effective in reducing transmission loss. In Examples 1 to 3, the transmission loss increases as the thickness of the foam decreases. This is because the dielectric loss between the signal line and the ground conductor layer increases as the foam thickness decreases. In Examples 2, 5, and 6, the transmission loss increases as the laminate thickness (thermosetting resin layer thickness) increases. This is because the dielectric loss due to the thermosetting resin increases as the thickness of the laminate increases. In the examples, the transmission loss of Example 1 is the smallest, which shows the effect of the foam being the thickest and the laminate being the thinnest in the Examples.

Sectional view of the circuit board in the present invention Sectional view of a circuit board using a conventional stripline

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Non-foam insulation resin layer 2 Signal line conductor layer 3 Foam insulation resin layer 4 Ground conductor layer 5 Dielectric layer 6 Conductor layer

Claims (5)

Such a non-foamed cured thermosetting resin impregnated into the fiber reinforcement Riatsumi is 0.05 to 1.0 mm, non-foam insulating resin layer a storage elastic modulus Ru 1~30GPa der at 25 to 30 ° C. A signal line conductor layer formed in advance on one side of the non-foamed insulating resin layer, a foam insulating resin layer made of a resin foam, and a ground conductor layer, and the signal line conductor layer is formed. The non-foam insulating resin layer, the foam insulating resin layer, and the ground conductor layer are laminated in the order of non-foam insulating resin layer / signal line conductor layer / foam insulating resin layer / ground conductor layer. High frequency circuit board. The high-frequency circuit board according to claim 1, wherein the non-foam insulating resin layer on which the signal line conductor layer is formed, the foam insulating resin layer, and the ground conductor layer are laminated and fixed by fastening means. . The foam insulating resin layer is made of a foam selected from the group consisting of foamed polystyrene, foamed polyethylene, foamed polypropylene, and foamed polyurethane. The relative dielectric constant at 10 GHz is 1.0 to 2.0, and the dielectric loss tangent is 0.0001. The high-frequency circuit board according to claim 1 or 2 , which is ˜0.001. The high frequency circuit board according to any one of claims 1 to 3 , wherein the foam insulating resin layer has a thickness of 0.05 to 1.0 mm. The non-foam insulating resin layer relative dielectric constant at 10GHz of 2.0 to 4.0, a high frequency circuit board according to claim 1-4 dielectric loss tangent is 0.001 to 0.01.

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