FLEXIBLE CIRCUIT BOARD
A flexible circuit board includes: an insulative substrate having a first surface and a second surface opposite to the first surface; a microstrip line having a first signal line formed on the first surface and a first ground pattern formed on the second surface and located in an area opposite to the first signal line; a coplanar line having a second signal line formed on the first surface, and second ground patterns that are formed on the first surface and are spaced apart from both sides of the second signal line; a connection line that is formed on the first surface and connects the first signal line and the second signal line together, the connection line having an opening; and third ground patterns formed on the second surface and arranged in areas located at both sides of an area opposite to the connection line including the opening.
This application is based upon and claims the benefit of priority of the prior Japanese Patent Application Nos. 2012-150848 and 2013-114537, filed on Jul 4, 2012 and May 30, 2013, respectively, the entire contents of which are incorporated herein by reference.
BACKGROUND(i) Technical Field
The present invention relates to a flexible circuit board, and more particularly, to a flexible circuit board having a converter that interconnects a coplanar line and a microstrip line.
(ii) Related Art
A flexible printed circuit board (FPC) is used for interconnecting electronic circuits (see Japanese Patent Application Publication No. 2011-238883, for example). Pads to which lead pins of a package are connected are formed at opposite ends of the flexible printed circuit board. The pads are formed by coplanar lines, and a microstrip line is formed as a line for connecting the pads together.
In some cases, impedance matching between the coplanar line and the microstrip line may not be established in a position where a connection between these lines is made.
SUMMARYAccording to an aspect of the present invention, impedance matching between a coplanar line and a microstrip line is improved.
According to another aspect of the present invention, there is provided a flexible circuit board including: an insulative substrate having a first surface and a second surface opposite to the first surface; a microstrip line having a first signal line formed on the first surface and a first ground pattern formed on the second surface and located in an area opposite to the first signal line; a coplanar line having a second signal line formed on the first surface, and second ground patterns that are formed on the first surface and are spaced apart from both sides of the second signal line; a connection line that is formed on the first surface and connects the first signal line and the second signal line together, the connection line having an opening; and third ground patterns formed on the second surface and arranged in areas located at both sides of an area opposite to the connection line including the opening.
A description is now given of a flexible circuit board in accordance with a comparative example.
The coplanar line 54 has a signal line 16 (second signal line) formed on the first surface of the insulative substrate 10, ground patterns 20 (second ground patterns) that are formed on the first surface and are spaced apart from both sides of the signal line 16, and ground patterns 26 formed on the second surface. The ground patterns 26 are not formed in a surface area opposite to the signal line 16 including the opening 15. The ground patterns 26 are formed on the second surface of the insulative substrate 10 and are arranged in areas located at both sides of an area opposite to the connection line 16 including the opening 15. The arrangement of the ground patterns 26 is intended to suppress capacitance components formed by the signal line 16 and the ground patterns 26. The ground patterns 26 are opposite to the ground patterns 20 and may be at ground potential in use. The opposite ground patterns 20 and 26 are interconnected by via metals 28. As described above, the ground patterns 26 are formed in the surface areas opposite to the ground patterns 20, whereby the ground pattern 22 of the microstrip line 50 is electrically connected to the ground patterns 20 via the ground patterns 26. The signal line 16 and the ground patterns 20 and 26 may be made of a metal such as gold.
The converter 52 has a connection line 14 that interconnects the signal lines 12 and 16 formed on the first surface, and ground patterns 24 (third ground patterns) formed on the second surface. Since the signal line 16 is wider than the signal line 12, the connection line 14 has a trapezoidal shape. The ground patterns 24 are provided on the surface of the insulative substrate 10 opposite to the surface on which the connection line 14 is provided. The ground patterns 24 are not provided in the surface areas opposite to the connection line 14 including the opening 15. The ground patterns 24 are formed in areas at both sides of the area opposite to the connection line 14, and have shapes so as to extend along the outer pattern of the connection line 14. Capacitive components C0 are formed between the connection line 14 and the ground patterns 24. The ground patterns 24 electrically connect the ground patterns 20 and 26 together. In the converter 52, a pseudo coplanar line is formed by the connection line 14 and the ground patterns 24. The connection line 14 and the ground patterns 24 may be made of a metal such as gold.
As illustrated in
In the first embodiment, the opening 15 extends up to the signal line 16 in the area next to the converter 52. The extension of the opening 15 up to the signal line 16 reduces the capacitive component in the area next to the converter 52. It is thus possible to improve the impedance matching between the coplanar line 54 and the microstrip line 50 implemented by the converter 52. In the first embodiment, the connection line 14 in the converter 52 has a slope or tapered shape that gradually changes from the coplanar line 54 to the microstrip line 50. However, the shape of the connection line 14 is not limited to the above. For example, the connection line 14 has a stepwise shape having steps. Another exemplary shape of the connection line 14 is a single-stage stepwise shape in which an area having a width equal to that of the signal line 16 of the coplanar line 54 directly joins an area having a width equal to that of the signal line 12 of the microstrip line 50. The ground patterns 20 are arranged at both side of the signal line 16 in the coplanar line 54, while no ground pattern is arranged on the surface of the insulative substrate 10 opposite to the signal line 16. This is similarly applied to variations and other embodiments described below.
According to the first embodiment, the connection line 14 has the opening 15. The presence of the opening 15 suppresses the capacitive components in the converter 52 and improves the impedance matching between the coplanar line 54 and the microstrip line 50.
For example, the opening 15 branches the connection line 14 into branch lines 18 connected to the signal line 16. When the signal line 12 is narrower than the signal line 16, it is preferable that the branch lines 18 correspond to two edges of an isosceles triangle. It is thus possible to improve the transmission characteristics without any phase difference between the branch lines 18.
As the branch lines 18 are narrower, the opening 15 is larger and the capacitive components are reduced more considerably. The converter 52 of the first embodiment is formed so as to be symmetric about the center axis of the signal line 12. Thus, the branch lines 18 have the same transmission characteristics. It is thus possible to prevent degradation of the transmission characteristics due to the unbalance between the branch lines 18. In the first embodiment, the branch lines 18 are as wide as the signal line 12.
It is preferable that the connection line 14 and the ground patterns 24 do not overlap with each other. In order to avoid the overlapping, it is preferable that a margin is provided when the connection line 14 and the ground patterns 24 are formed. In the first embodiment, a spacing (margin) as much as 50 μm is provided between the edges of the connection line 14 and those of the ground patterns 24.
In the firs embodiment, as illustrated in
A second embodiment has an exemplary configuration in which two coplanar lines are used as differential transmission lines.
In the second embodiment, the branch line 18b is narrower than the branch line 18a. The coplanar lines 54 of the second embodiment have the ground patterns provided on both the surface of the insulative substrate 10 on which the signal lines 16 are formed and the surface opposite thereto. In the second embodiment, the distances between the signal lines 16 of the coplanar lines 54 and the ground patterns 20 are smaller than those between the signal lines 16 and the ground patterns 26. For example, the ground patterns 26 in the areas opposite to the ground patterns 20 (patterns at the ground potential in use) are away from the signal lines 16 by a distance larger than the distances between the signal lines 16 and the ground patterns 20. It is thus possible to strengthen the coupling between the signal lines 16 and the ground potential on the surface on which the signal lines 16 are formed.
A computer simulation of S21 and that of the reflection characteristic are now described. In
In the second embodiment, as illustrated in
A third embodiment has an exemplary structure in which the first and second embodiments are applied to an optical module.
In the housing 66, there are provided a light emitting element such as a laser diode, and a drive circuit that drives the light emitting element. An electric signal is transmitted from the circuit board 70 to the drive circuit via the flexible circuit board 106, the lead pins 30 and the insulator 68. The drive circuit amplifies the electric signal. The laser diode converts the amplified electric signal into a light signal, which is then output to the optical fiber 61.
According to the third embodiment, the optical module 104 is equipped with the flexible circuit board 106 and the optical elements. The optical elements have the lead pins 30 used for inputting or outputting signals. The signal lines 16 of the flexible circuit board 106 are connected to the lead pins 30. It is thus possible to suppress loss of the input/output signals applied to or output from the optical elements as RF signals in the flexible circuit board.
The openings 15 may have pentagonal shapes or structures. The openings 15 may have quadrilateral or trapezoidal shapes or structures.
The present invention is not limited to the specifically described embodiments and variations, but other embodiments and variations may be made without departing from the scope of the present invention.
Claims
1. A flexible circuit board comprising:
- an insulative substrate having a first surface and a second surface opposite to the first surface;
- a microstrip line having a first signal line formed on the first surface and a first ground pattern formed on the second surface and located in an area opposite to the first signal line;
- a coplanar line having a second signal line formed on the first surface, and second ground patterns that are formed on the first surface and are spaced apart from both sides of the second signal line;
- a connection line that is formed on the first surface and connects the first signal line and the second signal line together, the connection line having an opening; and
- third ground patterns formed on the second surface and arranged in areas located at both sides of an area opposite to the connection line including the opening.
2. The flexible circuit board according to claim 1, wherein the connection line includes branch lines defined by the opening, and one of the branch lines that is substantially as wide as the first signal line and is aligned with the first signal line.
3. The flexible circuit board according to claim 1, further including ground patterns that are provided on the second surface and are located in areas opposite to the second ground patterns.
4. The flexible circuit board according to claim 3, wherein the third ground patterns are away from the second signal line by a distance larger than that by which the second signal line is away from the second ground patterns.
5. The flexible circuit board according to claim 1, wherein the coplanar line has a characteristic impedance smaller than that of the microstrip line.
6. The flexible circuit board according to claim 1, wherein the flexible circuit board includes a plurality of sets of lines, each of which sets includes the microstrip line, the coplanar line and the connection line, and adjacent coplanar lines share the second ground pattern.
7. The flexible circuit board according to claim 1, wherein the opening of the connection line has a pentagonal structure.
8. The flexible circuit board according to claim 1, wherein the opening of the connection line has a quadrilateral structure.
9. The flexible circuit board according to claim 1, wherein the opening of the connection line has a trapezoidal structure.
10. The flexible circuit board according to claim 1, wherein the ground patterns and the third ground patterns are interconnected by via metals.
11. The flexible circuit board according to claim 3, wherein a width of the second ground patterns is larger than a width of the ground patterns.
12. The flexible circuit board according to claim 3, wherein the second ground patterns is as long as the ground patterns.
13. An optical module comprising:
- a flexible circuit board including: an insulative substrate having a first surface and a second surface opposite to the first surface, a microstrip line having a first signal line formed on the first surface and a first ground pattern formed on the second surface and located in an area opposite to the first signal line, a coplanar line having a second signal line formed on the first surface, and second ground patterns that are formed on the first surface and are spaced apart from both sides of the second signal line, a connection line that is formed on the first surface and connects the first signal line and the second signal line together, the connection line having an opening; and third ground patterns formed on the second surface and arranged in areas located at both sides of an area opposite to the connection line including the opening;
- an optical sub-assembly having a receptacle, a housing, lead pins and an insulator; and
- a circuit board connected to the optical sub-assembly via the flexible circuit board transmitting RF signals.
14. The optical module according to claim 13, wherein the lead pins are jointed to the second signal line and the second ground patterns.
15. The optical module according to claim 14, wherein the lead pins are jointed to an area except the opening of the connection line.
Type: Application
Filed: Jul 3, 2013
Publication Date: Sep 11, 2014
Patent Grant number: 9270001
Inventor: Masahiro HIRAYAMA (Yokohama-shi)
Application Number: 13/935,132
International Classification: H01P 3/02 (20060101); G02B 6/42 (20060101);