High-frequency circuit and communication module

A high-frequency circuit is configured so as to include a printed circuit board and a flexible circuit board connected to the printed circuit board, wherein the printed circuit board includes: a first dielectric layer having a first surface and a second surface, a first ground conductor being formed on the first surface; a second dielectric layer having a third surface and a fourth surface, a second ground conductor being formed on the fourth surface; and first signal lines wired between the second surface and the third surface, the flexible circuit board includes: a third dielectric layer having a fifth surface and a sixth surface, a third ground conductor being formed on the fifth surface; a fourth dielectric layer having a seventh surface and an eighth surface, a fourth ground conductor being formed on the eighth surface; and second signal lines wired between the sixth surface and the seventh surface.

TECHNICAL FIELD

The present disclosure relates to a high-frequency circuit including a printed circuit board and a flexible circuit board, and a communication module including the high-frequency circuit.

BACKGROUND ART

Patent Literature 1 below discloses an optical module in which a printed circuit board and a flexible wiring board are connected to each other via a plurality of pads.

A flexible wiring board disclosed in Patent Literature 1 below includes a radio wave absorbing layer that absorbs radio waves in order to reduce crosstalk between two transmission lines.

CITATION LIST

Patent Literatures

Patent Literature 1: JP 2016-184091 A

SUMMARY OF INVENTION

Technical Problem

The flexible wiring board disclosed in Patent Literature 1 includes a radio wave absorbing layer. However, since the plurality of pads is arranged in a connecting portion between the printed circuit board and the flexible wiring board, it is difficult for the connecting portion to include a radio wave absorbing layer capable of sufficiently absorbing radio waves. Therefore, radio waves may leak to the outside from the connecting portion between the printed circuit board and the flexible wiring board, and crosstalk may occur disadvantageously.

The present disclosure has been achieved in order to solve the above problem, and an object of the present disclosure is to obtain a high-frequency circuit and a communication module capable of suppressing leakage of an electric field from a connecting portion between a printed circuit board and a flexible circuit board.

Solution to Problem

A high-frequency circuit according to the present disclosure includes a printed circuit board and a flexible circuit board being connected to the printed circuit board, the printed circuit board includes: a first dielectric layer having a first surface and a second surface, on the first surface a first ground conductor being formed; a second dielectric layer having a third surface and a fourth surface, on the fourth surface a second ground conductor being formed; and a plurality of first signal lines wired between the second surface and the third surface, the flexible circuit board includes: a third dielectric layer having a fifth surface and a sixth surface, on the fifth surface a third ground conductor being formed; a fourth dielectric layer having a seventh surface and an eighth surface, on the eighth surface a fourth ground conductor being formed; and a plurality of second signal lines wired between the sixth surface and the seventh surface, wherein a connecting portion, a portion between the printed circuit board and the flexible circuit board, includes: a plurality of first connecting conductors each having a first end and a second end, each of the first ends being connected to the corresponding first signal line, each of the second ends being exposed from the fourth surface without conducting the second ground conductor; and a plurality of second connecting conductors each having a first end and a second end, each of the first ends being exposed from the fifth surface without conducting the third ground conductor and being connected to the second end of the corresponding first connecting conductor, each of the second ends being connected to the corresponding second signal line.

Advantageous Effects of Invention

The present disclosure can suppress leakage of an electric field from a connecting portion between a printed circuit board and a flexible circuit board.

DESCRIPTION OF EMBODIMENTS

Hereinafter, in order to describe the present disclosure in more detail, embodiments for carrying out the present disclosure will be described with reference to the attached drawings.

First Embodiment

FIG.1is a configuration diagram illustrating a communication module including a high-frequency circuit1according to a first embodiment.

FIG.2is a perspective view illustrating the high-frequency circuit1according to the first embodiment.

FIG.3is a cross-sectional view illustrating a cross section taken along line A1-A2of a printed circuit board2included in the high-frequency circuit1according to the first embodiment.

FIG.4is a cross-sectional view illustrating a cross section taken along line B1-B2of a flexible circuit board3included in the high-frequency circuit1according to the first embodiment.

FIG.5is a cross-sectional view illustrating a cross section taken along line C1-C2in a connecting portion4between the printed circuit board2and the flexible circuit board3.

FIG.5Aillustrates a state before the printed circuit board2and the flexible circuit board3are connected to each other, andFIG.5Billustrates a state where the printed circuit board2and the flexible circuit board3are connected to each other.

FIG.6Ais a plan view of the printed circuit board2illustrated inFIG.5Aas viewed from a direction indicated by arrow D1.

FIG.6Bis a plan view of the flexible circuit board3illustrated inFIG.5Aas viewed from a direction indicated by arrow D2.

The high-frequency circuit1includes the printed circuit board2and the flexible circuit board3, and transmits a signal transmitted by a communication module or a signal received by the communication module.

The printed circuit board2includes a first dielectric layer11, a first ground conductor12, a second dielectric layer13, a second ground conductor14, first signal lines15aand15b, a first ground connecting conductor16, and a conductor17. InFIG.2, only a part of the printed circuit board2is illustrated.

The printed circuit board2illustrated inFIGS.2and3includes two signal lines15aand15bas first signal lines. However, this is merely an example, and the printed circuit board2may include three or more first signal lines as the first signal lines.

In addition, the printed circuit board2may include only one first signal line as the first signal line. Even when the printed circuit board2includes only one first signal line, crosstalk may occur between the one first signal line and a signal line (not illustrated) wired outside the printed circuit board2.

The flexible circuit board3is connected to the printed circuit board2via the connecting portion4.

The flexible circuit board3includes a third dielectric layer21, a third ground conductor22, a fourth dielectric layer23, a fourth ground conductor24, second signal lines25aand25b, a second ground connecting conductor26, and a conductor27.

The flexible circuit board3illustrated inFIGS.2and4includes two signal lines25aand25bas second signal lines. However, this is merely an example, and the flexible circuit board3may include three or more second signal lines as the second signal lines.

In addition, the flexible circuit board3may include only one second signal line as the second signal line. Even when the flexible circuit board3includes only one second signal line, crosstalk may occur between the one second signal line and a signal line (not illustrated) wired outside the flexible circuit board3.

The connecting portion4connects the printed circuit board2and the flexible circuit board3to each other.

The connecting portion4includes a first connecting conductor31aand a second connecting conductor33aas connecting conductors that electrically connect the first signal line15aand the second signal line25ato each other in a non-conductive state with each of the second ground conductor14and the third ground conductor22.

In addition, the connecting portion4includes a first connecting conductor31band a second connecting conductor33bas connecting conductors that electrically connect the first signal line15band the second signal line25bto each other in a non-conductive state with each of the second ground conductor14and the third ground conductor22.

The first dielectric layer11has a first surface11aand a second surface11b.

The first ground conductor12is formed on the first surface11a.

The first ground conductor12is a sheet-like conductor formed on the first surface11a, and is connected to the ground.

The second dielectric layer13has a third surface13aand a fourth surface13b.

The second ground conductor14is formed on the fourth surface13b.

The second ground conductor14is a sheet-like conductor formed on the fourth surface13b, and is connected to the ground.

Each of the first signal line15aand the first signal line15bis wired between the second surface11band the third surface13a.

Each of the first signal line15aand the first signal line15btransmits a signal transmitted by the communication module or a signal received by the communication module.

The first ground connecting conductor16and the conductor17are arranged between the first signal line15aand the first signal line15b.

The first ground connecting conductor16is disposed between the first signal line15aand the first signal line15b.

In addition, the first ground connecting conductor16is also disposed around a side on which the first signal line15bis not disposed out of both sides of the first signal line15a.

In addition, the first ground connecting conductor16is also disposed around a side on which the first signal line15ais not disposed out of both sides of the first signal line15b.

A first end of the first ground connecting conductor16is connected to the first ground conductor12, and a second end of the first ground connecting conductor16is connected to the second ground conductor14.

In the high-frequency circuit1illustrated inFIG.2, the first ground connecting conductor16is disposed only around the first signal line15aand the first signal line15b. However, this is merely an example, and the first ground connecting conductor16may be disposed not only around the first signal line15aand the first signal line15bbut also in a region other than the region around the first signal line15aand the first signal line15b.

The conductor17is disposed between the second surface11band the third surface13a, and is connected to the first ground connecting conductor16, for example, in a state where the first ground connecting conductor16is inserted into the conductor17.

The third dielectric layer21has a fifth surface21aand a sixth surface21b.

The third ground conductor22is formed on the fifth surface21a.

The third ground conductor22is a sheet-like conductor formed on the fifth surface21a, and is connected to the ground.

The fourth dielectric layer23has a seventh surface23aand an eighth surface23b.

The fourth ground conductor24is formed on the eighth surface23b.

The fourth ground conductor24is a sheet-like conductor formed on the eighth surface23b, and is connected to the ground.

Each of the second signal line25aand the second signal line25bis wired between the sixth surface21band the seventh surface23a.

Each of the second signal line25aand the second signal line25btransmits a signal transmitted by the communication module or a signal received by the communication module.

The second ground connecting conductor26and the conductor27are arranged between the second signal line25aand the second signal line25b.

The second ground connecting conductor26is disposed between the second signal line25aand the second signal line25b.

In addition, the second ground connecting conductor26is also disposed around a side on which the second signal line25bis not disposed out of both sides of the second signal line25a.

In addition, the second ground connecting conductor26is also disposed around a side on which the second signal line25ais not disposed out of both sides of the second signal line25b.

A first end of the second ground connecting conductor26is connected to the third ground conductor22, and a second end of the second ground connecting conductor26is connected to the fourth ground conductor24.

In the high-frequency circuit1illustrated inFIG.2, the second ground connecting conductor26is disposed only around the second signal line25aand the second signal line25b. However, this is merely an example, and the second ground connecting conductor26may be disposed not only around the second signal line25aand the second signal line25bbut also in a region other than the region around the second signal line25aand the second signal line25b.

The conductor27is disposed between the sixth surface21band the seventh surface23a, and is connected to the second ground connecting conductor26, for example, in a state where the second ground connecting conductor26is inserted into the conductor27.

Each of the first connecting conductor31aand the second connecting conductor33ais a conductor for connecting the first signal line15aand the second signal line25a.

Each of the first connecting conductor31aand the second connecting conductor33acan be achieved by, for example, a through-hole via.

A first end of the first connecting conductor31ais connected to the first signal line15a.

A second end32aof the first connecting conductor31ais exposed from the fourth surface13bin a non-conductive state with the second ground conductor14.

A region where the second end32aof the first connecting conductor31ais exposed in the fourth surface13bis a region where the second ground conductor14is not formed.

A first end34aof the second connecting conductor33ais exposed from the fifth surface21ain a non-conductive state with the third ground conductor22and connected to the second end32aof the first connecting conductor31a.

A second end of the second connecting conductor33ais connected to the second signal line25a.

Each of the first connecting conductor31band the second connecting conductor33bis a conductor for connecting the first signal line15band the second signal line25bto each other.

Each of the first connecting conductor31band the second connecting conductor33bis constituted by, for example, a through-hole via.

A first end of the first connecting conductor31bis connected to the first signal line15b.

A second end32bof the first connecting conductor31bis exposed from the fourth surface13bin a non-conductive state with the second ground conductor14.

A region where the second end32bof the first connecting conductor31bis exposed in the fourth surface13bis a region where the second ground conductor14is not formed.

A first end34bof the second connecting conductor33bis exposed from the fifth surface21ain a non-conductive state with the third ground conductor22and connected to the second end32bof the first connecting conductor31b.

A second end of the second connecting conductor33bis connected to the second signal line25b.

In the connecting portion4illustrated inFIG.5B, the second ground conductor14and the third ground conductor22are connected to each other with, for example, solder.

In the connecting portion4illustrated inFIG.5B, the printed circuit board2includes the first connecting conductor31aand the first connecting conductor31b, and the flexible circuit board3includes the second connecting conductor33aand the second connecting conductor33b.

The second end32aof the first connecting conductor31aand the first end34aof the second connecting conductor33aare connected to each other with, for example, solder, and as a result, the first signal line15aand the second signal line25aare electrically connected to each other in a non-conductive state with each of the second ground conductor14and the third ground conductor22. In addition, the second end32bof the first connecting conductor31band the first end34bof the second connecting conductor33bare connected to each other with, for example, solder, and as a result, the first signal line15band the second signal line25bare electrically connected to each other in a non-conductive state with each of the second ground conductor14and the third ground conductor22.

In the connecting portion4illustrated inFIGS.5A and5B, the second end of the second connecting conductor33ais connected to the second signal line25a, and the second end of the second connecting conductor33bis connected to the second signal line25b.

However, this is merely an example, and as illustrated inFIG.7, a middle between the first end34aand the second end35aof the second connecting conductor33amay be connected to the second signal line25a, and a middle between the first end34band the second end35bof the second connecting conductor33bmay be connected to the second signal line25b.

In addition, each of the second end35aof the second connecting conductor33aand the second end35bof the second connecting conductor33bmay be exposed from the eighth surface23bin a non-conductive state with the fourth ground conductor24.

A region where each of the second end35aof the second connecting conductor33aand the second end35bof the second connecting conductor33bis exposed in the eighth surface23bis a region where the fourth ground conductor24is not formed.

FIG.7is a cross-sectional view illustrating a cross section taken along line C1-C2in the connecting portion4between the printed circuit board2and the flexible circuit board3.

FIG.7Aillustrates a state before the printed circuit board2and the flexible circuit board3are connected to each other, andFIG.7Billustrates a state where the printed circuit board2and the flexible circuit board3are connected to each other.

In the connecting portion4illustrated inFIGS.7A and7B, as a connecting conductor that electrically connects the first signal line15aand the second signal line25ato each other, a through-hole via including the first connecting conductor31aand the second connecting conductor33acan be used.

In addition, as a connecting conductor that electrically connects the first signal line15band the second signal line25bto each other, a through-hole via including the first connecting conductor31band the second connecting conductor33bcan be used.

In the connecting portion4illustrated inFIG.7B, the second ground conductor14and the third ground conductor22are connected to each other with, for example, solder, and the second end32aof the first connecting conductor31aand the first end34aof the second connecting conductor33aare connected to each other with, for example, solder. In addition, the second end32bof the first connecting conductor31band the first end34bof the second connecting conductor33bare connected to each other with, for example, solder.

Next, the operation of the high-frequency circuit1illustrated inFIG.1will be described.

For example, when a signal is input to the first signal line15aof the printed circuit board2, the input signal is transmitted through the first signal line15a.

The signal transmitted through the first signal line15ais transmitted to the second signal line25aof the flexible circuit board3via the first connecting conductor31aand the second connecting conductor33a.

The signal transmitted to the second signal line25ais transmitted through the second signal line25a.

When a signal is input to the first signal line15bof the printed circuit board2, the input signal is transmitted through the first signal line15b.

The signal transmitted through the first signal line15bis transmitted to the second signal line25bof the flexible circuit board3via the first connecting conductor31band the second connecting conductor33b.

The signal transmitted to the second signal line25bis transmitted through the second signal line25b.

For example, when a signal is transmitted through the first signal line15aof the printed circuit board2, a magnetic field as indicated by the dashed arrow inFIG.3is generated.

In addition, when a signal is transmitted through the first signal line15a, an electric field as indicated by the solid arrow inFIG.3is generated.

For example, when the generated electric field leaks to the outside of the printed circuit board2and reaches the first signal line15b, crosstalk occurs between the first signal line15aand the first signal line15b.

However, in the printed circuit board2illustrated inFIG.3, since the first ground conductor12is formed on the first surface11aof the first dielectric layer11, leakage of the generated electric field from the first surface11aof the first dielectric layer11to the outside of the printed circuit board2is reduced.

In addition, in the printed circuit board2illustrated inFIG.3, since the second ground conductor14is formed on the fourth surface13bof the second dielectric layer13, leakage of the generated electric field from the fourth surface13bof the second dielectric layer13to the outside of the printed circuit board2is reduced.

In addition, in the printed circuit board2illustrated inFIG.3, each of the first ground connecting conductor16and the conductor17is arranged between the first signal line15aand the first signal line15b, and each of the first ground connecting conductor16and the conductor17acts to cut off the generated electric field. Since each of the first ground connecting conductor16and the conductor17is arranged, the generated electric field is suppressed from directly reaching the first signal line15bwithout passing through the outside of the printed circuit board2.

Therefore, in the printed circuit board2illustrated inFIG.3, crosstalk between the first signal line15aand the first signal line15bcan be reduced.

For example, when a signal is transmitted through the second signal line25aof the flexible circuit board3, a magnetic field as indicated by the dashed arrow inFIG.4is generated.

In addition, when a signal is transmitted through the second signal line25a, an electric field as indicated by the solid arrow inFIG.4is generated.

For example, when the generated electric field leaks to the outside of the flexible circuit board3and reaches the second signal line25b, crosstalk occurs between the second signal line25aand the second signal line25b.

However, in the flexible circuit board3illustrated inFIG.4, since the third ground conductor22is formed on the fifth surface21aof the third dielectric layer21, leakage of the generated electric field from the fifth surface21aof the third dielectric layer21to the outside of the flexible circuit board3is reduced.

In addition, in the flexible circuit board3illustrated inFIG.4, since the fourth ground conductor24is formed on eighth surface23bof the fourth dielectric layer23, leakage of the generated electric field from the eighth surface23bof the fourth dielectric layer23to the outside of the flexible circuit board3is reduced.

In addition, in the flexible circuit board3illustrated inFIG.4, each of the second ground connecting conductor26and the conductor27is arranged between the second signal line25aand the second signal line25b, and each of the second ground connecting conductor26and the conductor27acts to cut off the generated electric field. Since each of the second ground connecting conductor26and the conductor27is arranged, the generated electric field is suppressed from directly reaching the second signal line25bwithout passing through the outside of the flexible circuit board3.

Therefore, in the flexible circuit board3illustrated inFIG.4, crosstalk between the second signal line25aand the second signal line25bcan be reduced.

For example, when a signal is transmitted through the first signal line15aof the printed circuit board2, an electric field is generated from the first signal line15aalso in the connecting portion4.

For example, when the generated electric field leaks to the outside of the printed circuit board2and reaches the first signal line15b, crosstalk occurs between the first signal line15aand the first signal line15b.

However, in the printed circuit board2illustrated inFIG.5B, since the first ground conductor12is formed on the first surface11aof the first dielectric layer11, leakage of the generated electric field from the first surface11aof the first dielectric layer11to the outside of the printed circuit board2is reduced.

In addition, in the printed circuit board2illustrated inFIG.5B, since the second ground conductor14is formed on the fourth surface13bof the second dielectric layer13, leakage of the generated electric field from the fourth surface13bof the second dielectric layer13to the outside of the printed circuit board2is reduced.

In the connecting portion4, there is a gap between the second end32aof the first connecting conductor31aand the second ground conductor14, and an electric field may leak from the gap to the outside. However, the generated electric field is concentrated in a region where a conductor is present, and is hardly concentrated in a region where no conductor is present. Therefore, since most of the generated electric field is concentrated in a region where the first ground conductor12and the second ground conductor14are formed, there will be only slight electric field leaking outside through the gap where the conductor is not formed.

For example, when a signal is transmitted through the second signal line25aof the flexible circuit board3, an electric field is generated from the second signal line25aalso in the connecting portion4.

For example, when the generated electric field leaks to the outside of printed circuit board2and reaches the second signal line25b, crosstalk occurs between the second signal line25aand the second signal line25b.

However, in the flexible circuit board3illustrated inFIG.5B, since the fourth ground conductor24is formed on the eighth surface23bof the fourth dielectric layer23, leakage of the generated electric field from the eighth surface23bof the fourth dielectric layer23to the outside of the flexible circuit board3is reduced.

In addition, in the flexible circuit board3illustrated inFIG.5B, since the third ground conductor22is formed on the fifth surface21aof the third dielectric layer21, leakage of the generated electric field from the fifth surface21aof the third dielectric layer21to the outside of the flexible circuit board3is reduced.

In the connecting portion4, there is a gap between the first end34aof the second connecting conductor33aand the third ground conductor22, and an electric field may leak from the gap to the outside. However, since most of the generated electric field is concentrated in a region where the third ground conductor22and the fourth ground conductor24are formed, there will be only slight electric field leaking outside through the gap where the conductor is not formed.

FIG.8is an explanatory diagram illustrating a simulation result of crosstalk caused by leakage of an electric field from the connecting portion4.

InFIG.8, the horizontal axis represents a frequency [GHz] of a signal transmitted through each of the first signal line15aand the second signal line25a, and the vertical axis represents crosstalk [dB].

The alternate long and short dash line indicates crosstalk caused from the connecting portion4of the high-frequency circuit1illustrated inFIG.2.

The solid line indicates crosstalk caused from the connecting portion4of the high-frequency circuit1when the printed circuit board2does not include the first ground conductor12or the second ground conductor14and the flexible circuit board3does not include the third ground conductor22or the fourth ground conductor24.

The broken line indicates crosstalk caused from the connecting portion4of the high-frequency circuit1when the flexible circuit board3includes the third ground conductor22and the fourth ground conductor24but the printed circuit board2does not include the first ground conductor12or the second ground conductor14.

As illustrated inFIG.8, it can be seen that the crosstalk caused from the connecting portion4of the high-frequency circuit1illustrated inFIG.2is reduced as compared with the crosstalk caused from the connecting portion4of the high-frequency circuit1when the flexible circuit board3does not include the third ground conductor22or the fourth ground conductor24.

In addition, as illustrated inFIG.8, it can be seen that the crosstalk caused from the connecting portion4of the high-frequency circuit1illustrated inFIG.2is reduced as compared with the crosstalk caused from the connecting portion4of the high-frequency circuit1when the printed circuit board2does not include the first ground conductor12or the second ground conductor14.

In the first embodiment described above, the high-frequency circuit1is configured in such a manner that the printed circuit board2includes the first dielectric layer11in which the first ground conductor12is formed on the first surface11a, the flexible circuit board3includes the fourth dielectric layer23in which the fourth ground conductor24is formed on the eighth surface23b, and the connecting portion4between the printed circuit board2and the flexible circuit board3includes connecting conductors that electrically connect the first signal lines15aand15band the second signal lines25aand25bto each other in a non-conductive state with each of the second ground conductor14and the third ground conductor22.

In addition, in the high-frequency circuit1, the connecting portion4between the printed circuit board2and the flexible circuit board3includes: the first connecting conductors31aand31beach having the first end connected to each of the first signal lines15aand15band the second end exposed from the fourth surface13bin a non-conductive state with the second ground conductor14; and the second connecting conductors33aand33beach having the first end exposed from the fifth surface21ain a non-conductive state with the third ground conductor22and connected to the second end of each of the first connecting conductors31aand31b, and the second end connected to each of the second signal lines25aand25b. Therefore, the high-frequency circuit1can suppress leakage of an electric field from the connecting portion4between the printed circuit board2and the flexible circuit board3.

Second Embodiment

In the flexible circuit board3illustrated inFIG.4, the third ground conductor22is formed on the fifth surface21aof the third dielectric layer21.

In a second embodiment, a high-frequency circuit1including a flexible circuit board3in which regions41aand41bfacing regions where second signal lines25aand25bare wired in a fifth surface21aof a third dielectric layer21are regions where a third ground conductor22is not formed will be described.

FIG.9is a cross-sectional view illustrating a cross section taken along line B1-B2of the flexible circuit board3included in the high-frequency circuit1according to the second embodiment. InFIG.9, the same reference numerals as inFIG.4indicate the same or corresponding parts, and therefore description thereof is omitted.

The region41ais a region facing a region where the second signal line25ais wired in the fifth surface21aof the third dielectric layer21, and the third ground conductor22is not formed in the region41a.

The region41bis a region facing a region where the second signal line25bis wired in the fifth surface21aof the third dielectric layer21, and the third ground conductor22is not formed in the region41b.

When the regions41aand41bwhere the third ground conductor22is not formed are present in the fifth surface21aof the third dielectric layer21, an electric field may leak from the regions41aand41bto the outside.

However, since most of the generated electric field is concentrated in a region where the third ground conductor22is formed, there will be only slight electric field leaking outside through the regions41aand41bwhere the conductor is not formed. Therefore, leakage of the electric field to the outside can be suppressed more than a flexible circuit board in which the third ground conductor22is not formed on the fifth surface21aof the third dielectric layer21.

For example, in the configuration ofFIG.4, when a signal is transmitted through the second signal line25aof the flexible circuit board3, as described above, simultaneously with generation of an electric field, a capacitive component is generated between the second signal line25aand the third ground conductor22, and a capacitive component is generated between the second signal line25aand the fourth ground conductor24. As the second signal line25a, for example, a line having an impedance of 50Ω is preferable, and in order to cancel a generated capacitive component and obtain a desired impedance, for example, it is only required to narrow the line width of the second signal line25aand increase an inductance component.

However, when the line width of the second signal line25ais narrowed, there is a high possibility that the second signal line25ais disconnected when the flexible circuit board3is bent, and therefore it may be difficult to narrow the line width of the second signal line25a.

In the flexible circuit board3illustrated inFIG.4, the third ground conductor22is formed on the entire fifth surface21aof the third dielectric layer21, but in the flexible circuit board3illustrated inFIG.9, the third ground conductor22is not formed in the regions41aand41bin the fifth surface21a.

Therefore, the area of the third ground conductor22included in the flexible circuit board3illustrated inFIG.9is smaller than the area of the third ground conductor22included in the flexible circuit board3illustrated inFIG.4.

In the flexible circuit board3illustrated inFIG.9, since the area of the third ground conductor22is smaller than that of the flexible circuit board3illustrated inFIG.4, the capacitive component generated between the second signal lines25aand25band the third ground conductor22is reduced as compared with that in the flexible circuit board3illustrated inFIG.4.

In the flexible circuit board3illustrated inFIG.9, since the generated capacitive component is reduced as compared with that in the flexible circuit board3illustrated inFIG.4, it may be possible to cancel the capacitive component without increasing the inductance components of the second signal lines25aand25bby narrowing the line widths of the second signal lines25aand25b.

In the second embodiment described above, the high-frequency circuit1is configured in such a manner that the regions41aand41bfacing regions where the second signal lines25aand25bare wired in the fifth surface21aare regions where the third ground conductor22is not formed. Therefore, the high-frequency circuit1can suppress leakage of an electric field from the connecting portion4between the printed circuit board2and the flexible circuit board3, and also can widen the line widths of the second signal lines25aand25bas compared with the high-frequency circuit1illustrated inFIG.2.

Third Embodiment

In the flexible circuit board3illustrated inFIG.4, the third ground conductor22is formed on the fifth surface21aof the third dielectric layer21.

In a third embodiment, a region41afacing a region where a second signal line25ais wired in a fifth surface21ais a region where a third ground conductor22is not formed. A high-frequency circuit1including a flexible circuit board3in which a region41cfacing a region where a second signal line25bis wired in an eighth surface23bis a region where a fourth ground conductor24is not formed will be described.

FIG.10is a cross-sectional view illustrating a cross section taken along line B1-B2of the flexible circuit board3included in the high-frequency circuit1according to the third embodiment. InFIG.10, the same reference numerals as inFIGS.4and9indicate the same or corresponding parts, and therefore description thereof is omitted.

The region41cis a region facing a region where the second signal line25bis wired in the eighth surface23bof a fourth dielectric layer23, and the fourth ground conductor24is not formed in the region41c.

The second signal line25aand the second signal line25bare signal lines adjacent to each other among two or more second signal lines.

In the flexible circuit board3illustrated inFIG.4, the third ground conductor22is formed on the entire fifth surface21aof the third dielectric layer21, and the fourth ground conductor24is formed on the entire eighth surface23bof the fourth dielectric layer23.

Meanwhile, in the flexible circuit board3illustrated inFIG.10, the third ground conductor22is not formed in the region41ain the fifth surface21a. In addition, the fourth ground conductor24is not formed in the region41cin the eighth surface23b.

Therefore, the area of the third ground conductor22included in the flexible circuit board3illustrated inFIG.10is smaller than the area of the third ground conductor22included in the flexible circuit board3illustrated inFIG.4.

In addition, the area of the fourth ground conductor24included in the flexible circuit board3illustrated inFIG.10is smaller than the area of the fourth ground conductor24included in the flexible circuit board3illustrated inFIG.4.

In the flexible circuit board3illustrated inFIG.10, since the area of the third ground conductor22is smaller than that of the flexible circuit board3illustrated in FIG.4, the capacitive component generated between the second signal line25aand the third ground conductor22is reduced as compared with that in the flexible circuit board3illustrated inFIG.4.

In addition, in the flexible circuit board3illustrated inFIG.10, since the area of the fourth ground conductor24is smaller than that of the flexible circuit board3illustrated inFIG.4, the capacitive component generated between the second signal line25band the fourth ground conductor24is reduced as compared with that in the flexible circuit board3illustrated inFIG.4.

In the flexible circuit board3illustrated inFIG.10, since the generated capacitive component is reduced as compared with that in the flexible circuit board3illustrated inFIG.4, it may be possible to cancel the capacitive component without increasing the inductance components of the second signal lines25aand25bby narrowing the line widths of the second signal lines25aand25b.

In addition, in the flexible circuit board3illustrated inFIG.10, since the region41ais formed on the fifth surface21aand the region41cis formed on the eighth surface23b, even if an electric field leaks from the region41ato the outside, it is difficult for the electric field to reach the region41c. Therefore, in the flexible circuit board3illustrated inFIG.10, occurrence of crosstalk between the second signal line25aand the second signal line25bcan be suppressed more than in the flexible circuit board3illustrated inFIG.9.

In the third embodiment described above, the region41afacing a region where the second signal line25ais wired in the fifth surface21ais a region where the third ground conductor22is not formed. The high-frequency circuit1is configured in such a manner that the region41cfacing a region where the second signal line25bis wired in the eighth surface23bis a region where the fourth ground conductor24is not formed. Therefore, the high-frequency circuit1can suppress leakage of an electric field from the connecting portion4between the printed circuit board2and the flexible circuit board3, and also can widen the line widths of the second signal lines25aand25bas compared with the high-frequency circuit1illustrated inFIG.2.

Fourth Embodiment

In the printed circuit board2illustrated inFIG.3, the first ground conductor12is formed on the first surface11aof the first dielectric layer11.

In a fourth embodiment, a high-frequency circuit1including a printed circuit board2in which regions51aand51bfacing regions where first signal lines15aand15bare wired in a first surface11aof a first dielectric layer11are regions where a first ground conductor12is not formed will be described.

FIG.11is a cross-sectional view illustrating a cross section taken along line A1-A2of the printed circuit board2included in the high-frequency circuit1according to the fourth embodiment. InFIG.11, the same reference numerals as inFIG.3indicate the same or corresponding parts, and therefore description thereof is omitted.

The region51ais a region facing a region where the first signal line15ais wired in the first surface11aof the first dielectric layer11, and the first ground conductor12is not formed in the region51a.

The region51bis a region facing a region where the first signal line15bis wired in the first surface11aof the first dielectric layer11, and the first ground conductor12is not formed in the region51b.

When the regions51aand51bin which the first ground conductor12is not formed are present in the first surface11aof the first dielectric layer11, an electric field may leak from the regions51aand51bto the outside.

However, since most of the generated electric field is concentrated in a region where the first ground conductor12is formed, there will be only slight electric field leaking outside through the regions51aand51bwhere the conductor is not formed. Therefore, leakage of the electric field to the outside can be suppressed more than in a printed circuit board in which the first ground conductor12is not formed on the first surface11aof the first dielectric layer11.

For example, when a signal is transmitted through the first signal line15aof the printed circuit board2, an electric field is generated as described above.

By generation of the electric field, a capacitive component is generated between the first signal line15aand the first ground conductor12, and a capacitive component is generated between the first signal line15aand the second ground conductor14. As the first signal line15a, for example, a line having an impedance of 50Ω is preferable, and in order to cancel a generated capacitive component and obtain a desired impedance, for example, it is only required to narrow the line width of the first signal line15aand increase an inductance component.

However, when the line width of the first signal line15ais narrowed, the line impedance of the first signal line15acannot be a desired impedance in some cases depending on manufacturing constraints and the like.

In the printed circuit board2illustrated inFIG.3, the first ground conductor12is formed on the entire first surface11aof the first dielectric layer11, but in the printed circuit board2illustrated inFIG.11, the first ground conductor12is not formed in the regions51aand51bin the first surface11a.

Therefore, the area of the first ground conductor12included in the printed circuit board2illustrated inFIG.11is smaller than the area of the first ground conductor12included in the printed circuit board2illustrated inFIG.3.

In the printed circuit board2illustrated inFIG.11, since the area of the first ground conductor12is smaller than that of the printed circuit board2illustrated inFIG.3, the capacitive component generated between the first signal lines15aand15band the first ground conductor12is reduced as compared with that in the printed circuit board2illustrated inFIG.3.

In the printed circuit board2illustrated inFIG.11, since the generated capacitive component is reduced as compared with that in the printed circuit board2illustrated inFIG.3, it may be possible to cancel the capacitive component without increasing the inductance components of the first signal lines15aand15bby narrowing the line widths of the first signal lines15aand15b.

In the fourth embodiment described above, the high-frequency circuit1is configured in such a manner that the regions51aand51bfacing regions where the first signal lines15aand15bare wired in the first surface11aare regions where the first ground conductor12is not formed. Therefore, the high-frequency circuit1can suppress leakage of an electric field from the connecting portion4between the printed circuit board2and the flexible circuit board3, and also can widen the line widths of the first signal lines15aand15bas compared with the high-frequency circuit1illustrated inFIG.2.

Fifth Embodiment

In a fifth embodiment, a high-frequency circuit1including a flexible circuit board3in which the thickness of a third dielectric layer21is different from the thickness of a fourth dielectric layer23will be described.

FIG.12Ais a cross-sectional view illustrating a cross section taken along line B1-B2of the flexible circuit board3included in the high-frequency circuit1according to the fifth embodiment. In the example ofFIG.12A, the thickness of the third dielectric layer21is larger than the thickness of the fourth dielectric layer23.

FIG.12Bis a cross-sectional view illustrating a cross section taken along line B1-B2of the flexible circuit board3included in the high-frequency circuit1according to the fifth embodiment. In the example ofFIG.12B, the thickness of the third dielectric layer21is smaller than the thickness of the fourth dielectric layer23.

When the thickness of the fourth dielectric layer23is smaller than the thickness of the third dielectric layer21without changing the thickness of the third dielectric layer21as illustrated inFIG.12A, a capacitive component generated between the second signal lines25aand25band the fourth ground conductor24is larger than that when the thickness of the fourth dielectric layer23is the same as the thickness of the third dielectric layer21.

Therefore, even when the thickness of the fourth dielectric layer23is smaller than the thickness of the third dielectric layer21, in order to cancel the capacitive component, it is necessary to narrow the line widths of the second signal lines25aand25bas compared with that when the thickness of the fourth dielectric layer23is the same as the thickness of the third dielectric layer21.

When the thickness of the fourth dielectric layer23is larger than the thickness of the third dielectric layer21without changing the thickness of the third dielectric layer21as illustrated inFIG.12B, a capacitive component generated between the second signal lines25aand25band the fourth ground conductor24is smaller than that when the thickness of the fourth dielectric layer23is the same as the thickness of the third dielectric layer21.

Therefore, even when the thickness of the fourth dielectric layer23is larger than the thickness of the third dielectric layer21, in order to cancel the capacitive component, it is necessary to widen the line widths of the second signal lines25aand25bas compared with that when the thickness of the fourth dielectric layer23is the same as the thickness of the third dielectric layer21.

Therefore, in the flexible circuit board3illustrated inFIGS.12A and12B, it is only required to determine the line widths of the second signal lines25aand25bdepending on the thickness of the third dielectric layer21and the thickness of the fourth dielectric layer23, respectively.

Here, it is described that the line widths of the second signal lines25aand25bare determined depending on the thickness of the third dielectric layer21and the thickness of the fourth dielectric layer23, respectively. However, this is merely an example, and after the line widths of the second signal lines25aand25bare determined to be desired line widths, each of the thickness of the third dielectric layer21and the thickness of the fourth dielectric layer23may be determined in such a manner that a capacitive component generated between the second signal lines25aand25band the fourth ground conductor24can be canceled.

In the flexible circuit board3illustrated inFIGS.12A and12B, the thickness of the fourth dielectric layer23is different from the thickness of the third dielectric layer21without changing the thickness of the third dielectric layer21.

However, this is merely an example, and the flexible circuit board3may be configured in such a manner that the thickness of the third dielectric layer21is different from the thickness of the fourth dielectric layer23without changing the thickness of the fourth dielectric layer23.

In the fifth embodiment described above, the high-frequency circuit1is configured in such a manner that the thickness of the third dielectric layer21is different from the thickness of the fourth dielectric layer23. Therefore, the high-frequency circuit1can suppress leakage of an electric field from the connecting portion4between the printed circuit board2and the flexible circuit board3, and also can increase the degree of freedom in designing the line widths of the second signal lines25aand25bas compared with the high-frequency circuit1illustrated inFIG.2.

Sixth Embodiment

In a sixth embodiment, a high-frequency circuit in which a connection position between a first signal line15aand a second signal line25ais different from a connection position between a first signal line15band a second signal line25bin a signal transmission direction on a flexible circuit board will be described.

FIG.13is a perspective view illustrating a high-frequency circuit1according to the sixth embodiment. InFIG.13, the same reference numerals as inFIG.2indicate the same or corresponding parts, and therefore description thereof is omitted.

A connecting portion4includes a first connecting portion4aand a second connecting portion4b.

The first connecting portion4aincludes a first connecting conductor31aand a second connecting conductor33ain order to electrically connect the first signal line15aand the second signal line25ato each other.

The second connecting portion4bincludes a first connecting conductor31band a second connecting conductor33bin order to electrically connect the first signal line15band the second signal line25bto each other.

In the high-frequency circuit1illustrated inFIG.13, the position where the first connecting portion4ais disposed is different from the position where the second connecting portion4bis disposed in a signal transmission direction in a flexible circuit board3.

When the position where the first connecting portion4ais disposed is different from the position where the second connecting portion4bis disposed, a distance between the position where the first connecting portion4ais disposed and the position where the second connecting portion4bis disposed is longer than that when the position where the first connecting portion4ais disposed is the same as the position where the second connecting portion4bis disposed.

Therefore, in the high-frequency circuit1illustrated inFIG.13, even if an electric field leaks from the first connecting portion4aor the second connecting portion4bto the outside, occurrence of crosstalk between the first signal line15aand the first signal line15bcan be suppressed more than when the position where the first connecting portion4ais disposed is the same as the position where the second connecting portion4bis disposed. In addition, occurrence of crosstalk between the second signal line25aand the second signal line25bcan be suppressed.

In the sixth embodiment described above, the high-frequency circuit1is configured in such a manner that the position where the first connecting portion4ais disposed is different from the position where the second connecting portion4bis disposed in a signal transmission direction in the flexible circuit board3. Therefore, the high-frequency circuit1can suppress leakage of an electric field from the connecting portion4between the printed circuit board2and the flexible circuit board3, and also can suppress occurrence of crosstalk more than the high-frequency circuit1illustrated inFIG.2.

Seventh Embodiment

In a seventh embodiment, a high-frequency circuit1in which openings61aand61bare formed in a first ground conductor12will be described.

FIG.14is a perspective view illustrating the high-frequency circuit1according to the seventh embodiment. InFIG.14, the same reference numerals as inFIG.2indicate the same or corresponding parts, and therefore description thereof is omitted.

FIG.15is a cross-sectional view illustrating a cross section taken along line C1-C2in a connecting portion4before a printed circuit board2and a flexible circuit board3are connected to each other. InFIG.15, the same reference numerals as inFIGS.5and7indicate the same or corresponding parts, and therefore description thereof is omitted.

The opening61ais a hole formed in the first ground conductor12.

The position where the opening61ais formed is a position facing a first signal line15ain the connecting portion4.

The opening61bis a hole formed in the first ground conductor12.

The position where the opening61bis formed is a position facing a first signal line15bin the connecting portion4.

In the high-frequency circuit1illustrated inFIG.14, the openings61aand61bare applied to the printed circuit board2in the connecting portion4illustrated inFIG.7. However, this is merely an example, and the openings61aand61bmay be applied to the printed circuit board2in the connecting portion4illustrated inFIG.5.

For example, when a signal is input to the first signal line15aof the printed circuit board2, the input signal is transmitted through the first signal line15a.

The signal transmitted through the first signal line15ais transmitted to the second signal line25aof the flexible circuit board3via the first connecting conductor31aand the second connecting conductor33a.

The signal transmitted to the second signal line25ais transmitted through the second signal line25a.

When a signal is input to the first signal line15bof the printed circuit board2, the input signal is transmitted through the first signal line15b.

The signal transmitted through the first signal line15bis transmitted to the second signal line25bof the flexible circuit board3via the first connecting conductor31band the second connecting conductor33b.

The signal transmitted to the second signal line25bis transmitted through the second signal line25b.

For example, when a signal is transmitted through the first signal line15aof the printed circuit board2, an electric field is generated from the first signal line15aalso in the connecting portion4.

For example, when the generated electric field leaks to the outside of the printed circuit board2and reaches the first signal line15b, crosstalk occurs between the first signal line15aand the first signal line15b.

However, in the printed circuit board2illustrated inFIG.15, since the first ground conductor12is formed on the first surface11aof the first dielectric layer11, leakage of the generated electric field from the first surface11aof the first dielectric layer11to the outside of the printed circuit board2is reduced.

In addition, in the printed circuit board2illustrated inFIG.15, since the second ground conductor14is formed on the fourth surface13bof the second dielectric layer13, leakage of the generated electric field from the fourth surface13bof the second dielectric layer13to the outside of the printed circuit board2is reduced.

In the printed circuit board2illustrated inFIG.15, the openings61a,61bare formed in the first ground conductor12, and an electric field may leak from the openings61aand61bto the outside. However, the generated electric field is concentrated in a region where a conductor is present, and is hardly concentrated in a region where no conductor is present. Therefore, since most of the generated electric field is concentrated in a region where the first ground conductor12is formed, there will be only slight electric field leaking outside through the openings61aand61bwhere the conductor is not formed.

An impedance of each of the first signal lines15aand15band the second signal lines25aand25bis designed to be, for example, 50Ω.

However, in the connecting portion4, by supply of solder from the second ends35aand35bof the second connecting conductors33aand33bconstituted by through-hole vias, the printed circuit board2and the flexible circuit board3are connected to each other.

In addition, the second connecting conductor33a, the first end34aof the second connecting conductor33a, and the second end35aof the second connecting conductor33aare arranged in the second signal line25a, and the second connecting conductor33b, the first end34bof the second connecting conductor33b, and the second end35bof the second connecting conductor33bare arranged in the second signal line25b.

For this reason, since a ground conductor cannot be arranged at a position facing the second signal lines25aand25b, the impedance of each of the second signal lines25aand25bmay deviate from 50Ω. When the impedance of each of the second signal lines25aand25bdeviates from 50Ω, resonance occurs due to reflection of an electrical signal, and a passing band is limited due to the occurrence of resonance. When the openings61aand61bare not formed in the first ground conductor12, a high-frequency component is coupled to the first ground conductor12, and a capacitive component tends to increase.

In the printed circuit board2illustrated inFIG.15, since the openings61aand61bare formed in the first ground conductor12, the capacitive component of the printed circuit board2in the connecting portion4is reduced as compared with that in a printed circuit board2in which the openings61aand61bare not formed.

Since the capacitive component of the printed circuit board2in the connecting portion4is reduced, the impedance of each of the first signal lines15aand15band the second signal lines25aand25bincreases. Therefore, by adjusting the sizes of the openings61aand61bin the first ground conductor12, the impedance of each of the signal lines can be close to 50Ω.

FIG.16is an explanatory diagram illustrating a simulation result of a signal passing characteristic in the high-frequency circuit1.

InFIG.16, the horizontal axis represents a frequency [GHz] of a signal transmitted through each of the first signal line15aand the second signal line25a, and the vertical axis represents an S parameter (S21) indicating a signal loss.

The solid line indicates a signal passing characteristic in the high-frequency circuit1according to the second embodiment, and the broken line indicates a signal passing characteristic in the high-frequency circuit1according to the seventh embodiment.

It can be seen fromFIG.16that the high-frequency circuit1according to the seventh embodiment can significantly reduce a signal loss in a band having a frequency equal to or more than 50 GHz as compared with the high-frequency circuit1according to the second embodiment.

FIG.17is an explanatory diagram illustrating a simulation result of crosstalk caused by leakage of an electric field from the connecting portion4.

InFIG.17, the horizontal axis represents a frequency [GHz] of a signal transmitted through each of the first signal line15aand the second signal line25a, and the vertical axis represents crosstalk [dB].

It can be seen fromFIG.17that the high-frequency circuit1according to the seventh embodiment can suppress crosstalk similarly to the high-frequency circuit1according to the second embodiment.

In the seventh embodiment described above, the high-frequency circuit1is configured in such a manner that the openings61aand61bare formed in the first ground conductor12at position facing the first signal lines15aand15bin the first ground conductor12disposed on the first surface11ain the connecting portion4between the printed circuit board2and the flexible circuit board3. Therefore, the high-frequency circuit1according to the seventh embodiment can suppress leakage of an electric field from the connecting portion4between the printed circuit board2and the flexible circuit board3, and can also reduce a signal loss as compared with the high-frequency circuit1according to the second embodiment.

Note that the present disclosure can freely combine the embodiments to each other, modify any constituent element in each of the embodiments, or omit any constituent element in each of the embodiments.

INDUSTRIAL APPLICABILITY

The present disclosure is suitable for a high-frequency circuit including a printed circuit board and a flexible circuit board, and a communication module including a high-frequency circuit.

REFERENCE SIGNS LIST

1: High-frequency circuit,2: Printed circuit board,3: Flexible circuit board,4: Connecting portion,4a: First connecting portion,4b: Second connecting portion,11: First dielectric layer,11a: First surface,11b: Second surface,12: First ground conductor,13: Second dielectric layer,13a: Third surface,13b: Fourth surface,14: Second ground conductor,15a,15b: First signal line,16: First ground connecting conductor,17: Conductor,21: Third dielectric layer,21a: Fifth surface,21b: Sixth surface,22: Third ground conductor,23: Fourth dielectric layer,23a: Seventh surface,23b: Eighth surface,24: Fourth ground conductor,25a,25b: Second signal line,26: Second ground connecting conductor,27: Conductor,31a,31b: First connecting conductor,32a,32b: Second end of first connecting conductor,33a,33b: Second connecting conductor,34a,34b: First end of second connecting conductor,35a,35b: Second end of second connecting conductor,41a,41b,41c,51a,51b: Region,61a,61b: Opening