BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a -high-speed transmission board. Background Art

Conventionally, flat cables, such as .flexible flat cables (FFC) and flexible printed circuit boards (FPC) , have been used as high-speed transmission cables (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-274586) .

However, the conventional flat cables use very thin foil, such as copper foil, to form signal traces, and thus have failed to exhibit good damping characteristics. Also, in order to reduce crosstalk, shielding is effected through formation of bumps or application of plating. Formation of • bumps or application of plating increases the cost of manufacturing flat cables and impairs flexibility of flat cables.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above-mentioned conventional problems and to provide a high-, speed transmission board in which wires are embedded in a substrate that includes a plate member formed of a dielectric material, and two conductive layers formed on corresponding

opposite sides of the plate member, thereby exhibiting good damping characteristics, facilitating tailoring of ends, enhancing noise resistance, and facilitating control of characteristic impedance.

To achieve the above object, a high-speed transmission board of the present invention comprises a plate member formed of a dielectric material; two conductive layers formed on corresponding opposite sides of the plate member; and a plurality of wires embedded in the plate member.

Preferably, each of the wires comprises a core formed of a conductor, and a coating member covering the core and having electrically insulating properties.

The wires may be disposed in pairs each consisting of two wires; the wires of each pair are in contact with each other; and adjacent pairs are spaced apart from each other.

Alternatively, the wires may be disposed such that adjacent wires are in contact with each other.

Preferably, the high-speed transmission board further comprises a plurality of drain wires embedded in the plate member, and each of the drain wires is sandwiched between the wires .

According to the present invention, the high-speed transmission board is configured such that the wires are embedded in the substrate, which comprises the plate member formed of a dielectric material, and the two conductive layers formed on the corresponding opposite sides of the plate member. Thus, the high-speed transmission board of the

present invention can exhibit good damping characteristics, facilitate tailoring of ends, enhance noise resistance, and facilitate control of characteristic impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the configuration of a highspeed transmission board according to a first embodiment of the present invention;

FIG. 2 is a view showing the configuration of a highspeed transmission board according to a second embodiment of the present invention; and

FIG. 3 is a view showing the configuration of a highspeed transmission board according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED ^' EMBODIMENTS Embodiments of the present invention will now be described in detail with reference to the drawings .

FIG. 1 is a ^" view showing the configuration of a highspeed transmission board according to a first embodiment of the present invention, wherein FIG. IA is a perspective, partially cutaway view of the high-speed transmission board, and FIG. IB is a cross-sectional view of the high-speed transmission board.

In FIG. 1, reference numeral 10 denotes the high-speed transmission board of the present embodiment, and reference numeral 11 denotes signal wires, which are wires for

transmitting mainly high-frequency signals. In the present embodiment, terms for expressing direction, such as up, down, left, right, front, and rear, are used for explaining the structure and action of respective portions of the high-speed transmission board 10; however, these terms represent respective directions for the case where the high-speed transmission board 10 is used in an orientation shown in the drawings, and must be construed to represent corresponding different directions when the orientation of the high-speed transmission board 10 is changed.

Each of the signal wires 11 includes a core 11a formed of a conductor, such as metal, and an electrically insulative coating member lib covering the core 11a and formed of resin. The diameter of the cores 11a is, for example, about 0.3 mm, but may be changed as appropriate. In the illustrated example, the cores 11a each have a circular cross section, but may have a rectangular cross section. The signal wires 11 are disposed in pairs each consisting of two signal wires 11; the signal wires 11 of each pair are in contact with each other; and adjacent pairs are spaced apart from each other. Employment of such an arrangement reduces crosstalk between adjacent pairs.

Reference numerals 12 to 15 denote substrate members that constitute a substrate. The substrate members 12 to 15 include dielectric substrates 12a to 15a, respectively, formed of dielectric materials, and conductive plates 12b to 15b, respectively, formed of conductors, such as metal, and

serving as conductive layers. The conductive plates 12b to 15b are affixed to respective one sides of the dielectric substrates 12a to 15a. Each of the substrate members 12 to 15 is similar to a substrate used in an ordinary printed circuit board (PCB) , but may be a substrate used in a flexible printed circuit. The dielectric substrates 12a to 15a are formed of, for example, prepreg, which is formed by impregnating a matrix of glass fibers or the like with resin, but may be formed of any kind of dielectric material, such as polyimide resin or liquid crystal polymer. The conductive plates 12b to 15b are thin films of, for example, copper, but may be formed of any kind of plate-like conductor that adheres to the dielectric substrates 12a to 15a. The thickness of each of the substrate members 12 to 15 is, for example, about 0.25 mm, but may be modified as appropriate. The substrate members 12 to 15 may differ from one another in dimension, material, and the like; however, the following description assumes that they are virtually identical with one another in dimension, material, and the like.

The signal wires 11 are sandwiched from above and below between the substrate members 12 and 14. In this case, the substrate members 12 and 14 are bonded together such that the dielectric substrates 12a and 14a are located inside, whereas the conductive plates 12b and 14b are located outside. In the example of FIG. IA, grooves each having a semicircular cross section are formed beforehand in the dielectric substrates 12a and 14a of the substrate members 12 and 14 in

order to accommodate the corresponding signal wires 11. However, as in the case of ordinary prepreg, when the dielectric substrates 12a and 14a are flexible and readily deformable at a stage prior to curing of resin, formation of the grooves is unnecessary. Also, as in the case of ordinary prepreg, when the dielectric substrates 12a and 14a are adhesive at a stage prior to curing of resin, the dielectric substrates 12a and 14a can be bonded together by means of compression-bonding together the substrate members 12 and 14 between which the signal wires 11 are sandwiched from above and below. The dielectric substrates 12a and 14a are bonded together to become a unitary plate member.

In the present embodiment, the substrate members 13 and 15 are further bonded, from the outside, onto the substrate members 12 and 14, respectively. In this case, the substrate members 13 and 15 are bonded, from the outside, onto the substrate members 12 and 14, respectively, such that the dielectric substrate 13a is sandwiched between the inner conductive plate 12b and the outer conductive plate 13b, whereas the dielectric substrate 15a is sandwiched between the inner conductive plate 14b and the outer conductive plate 15b. As in the case of ordinary prepreg, when the dielectric substrates 13a and 15a are adhesive at a stage prior to curing of resin, the dielectric substrates 13a and 15a can be bonded to the conductive plates 12b .and 14b, respectively, by means of compression-bonding the substrate members 13 and 15 to the substrate members 12 and 14, respectively. As a

result, as shown in FIG. IB, the high-speed transmission board 10 is configured into a multilayer board such that the conductive plates 12b to 15b are arranged in four layers while the individual dielectric substrates 12a to 15a intervene therebetween. In other words, the high-speed transmission board 10 is configured such that two conductive layers implemented by the conductive plates 13b and 15b are formed on the corresponding opposite sides of a plate member which is composed of the dielectric substrates 12a to 15a and in which the signal wires 11 are embedded. In this case, the plate member has two conductive layers formed therein and implemented by the conductive plates 12b and 14b, in addition to the conductive layers formed on the opposite sides thereof.

Any of the conductive plates 12b to 15b can be formed into a wiring layer by means of forming conductive traces in a predetermined pattern through subjection to photolithography. For example, the high-speed transmission board 10 can be used as a multilayered printed circuit board by means of forming conductive traces in a predetermined pattern on the outermost conductive plates 13b and 15b thereof. In this case, desirably, the inner two conductive plates 12b and 14b are used as ground layers. If necessary, the inner two conductive plates 12b and 14b can also be formed into wiring layers through formation of conductive traces thereon. In the case where the wiring layers are formed, a desired portion of a conductive trace can be electrically connected to a desired core 11a through

formation of a conductive hole; i.e., a via, extending vertically through the dielectric substrates 12a to 15a at an appropriate position.

In the illustrated example, the four substrate members 12 to 15 are arranged in layers ^' , thereby obtaining the highspeed transmission board 10. However, any of the substrate members 12 to 15 can be omitted. For example,- the outer substrate members 13 and 15 can be omitted. In this case, the high-speed transmission board 10 is configured such that the signal wires 11 are sandwiched from above and below between the substrate members 12 and 14. In other words, the high-speed transmission board 10 is configured such that two conductive layers implemented by the conductive plates 12b and 14b are formed on the corresponding opposite sides of a plate member which is composed of the dielectric substrates 12a and 14a and in which the signal wires 11 are embedded. In this case, the plate member has the conductive layers implemented by the conductive plates ¹ 12b and 14b only on the opposite sides thereof.

The high-speed transmission board 10 can also be obtained through employment of far more substrate members arranged in layers. In this case, any number of substrate members are arranged in layers from the outside on the substrate members 13 and 15 of the high-speed transmission board 10 shown in FIG. 1. The plate member formed of a dielectric material has a plurality of conductive layers formed therein, in addition to the conductive layers formed

on the opposite sides thereof.

As described above, according to the present embodiment, the high-speed transmission board 10 includes the signal wires 11, each of which is composed of the core 11a and the coating member lib covering the core 11a and formed of resin or the like; a plate member which is composed of the dielectric substrates 12a to 15a and in which the signal wires 11 are embedded; and conductive layers implemented by the conductive plates 12b to 15b and formed on the opposite sides of and in the plate member. Accordingly, signals can be transmitted through the core wires 11a each having a sufficiently large cross-sectional area, so that good damping characteristics can be exhibited. Particularly, in transmission of high-frequency signals, the goodness of damping characteristics of the high-speed transmission board 10 becomes more noticeable as compared with the conventional flat cables or the like. Longitudinal ends of the conductive plates 12b to 15b, which function as shields, can be readily tailored. As compared with a coaxial cable having fine conductors, the high-speed transmission board 10 facilitate end tailoring, since removal of an outer coating layer and an outer conductor is not involved. Further, by virtue of a strip structure in which the conductive plates 12b to 15b are disposed on the opposite sides of a row of the signal wires 11, noise resistance is enhanced by means of using the conductive plates 12b to 15b as ground layers. Also, by means of adjusting spacing between the signal wires 11 and

spacing between the signal wires 11 and the conductive plates 12b to 15b, characteristic impedance can be readily controlled.

Next, a second embodiment of the present invention will be described. Structural features similar to those of the first embodiment are denoted by like reference numerals, and repeated description thereof is omitted. Also, repeated description of operations and effects similar to those of the first embodiment is omitted.

FIG. 2 is a view showing the configuration of a highspeed transmission board according to the second embodiment of the present invention, wherein FIG. 2A is a perspective, partially cutaway view of the high-speed transmission board, and FIG. 2B is a cross-sectional view of the high-speed transmission board.

In the first embodiment, the signal wires 11 are disposed in pairs each consisting of two signal wires 11; the signal wires 11 of each pair are in contact with each other; and adjacent pairs are spaced apart from each other. By contrast, in the present embodiment, the signal wires 11 are disposed such that adjacent wires 11 are in contact with each other. This allows a large number of signal wires 11 to be disposed in a limited space, thereby enabling a reduction in the width of the high-speed transmission board 10. Other structural features and effects are similar to those of the first embodiment, and repeated description thereof is omitted.

Next, a third embodiment of the present invention will

be described. Structural features similar to those of the first and second embodiments are denoted by like reference numerals, and repeated description thereof is omitted. Also, repeated description of operations and effects similar to those of the first and second embodiments is omitted.

FIG. 3 is a view showing the configuration of a highspeed transmission board according to the third embodiment of the present invention, wherein FIG. 3A is a perspective, partially cutaway view of the high-speed transmission board, and FIG. 3B is a cross-sectional view of the high-speed transmission board.

In the present embodiment, a drain wire 16 is disposed between the signal wires 11. Each of the drain wires 16 is in contact with the adjacent signal wire(s) 11. Each of the drain wires 16 is formed of a conductor, such as metal, and is not covered with an electrically insulative coating member; i.e., the drain wires 16 are so-called bare wires. In the illustrated example, the outside diameter of the drain wire 16 is substantially equal to that of the signal wire 11 including the coating member lib. However, the outside diameter of the drain wire 16 can be changed as needed; for example, the drain wire 16 can assume an outside diameter substantially equal to that of the core 11a.

In the illustrated example, the signal wires 11 are disposed in pairs each consisting of two signal wires 11; the signal wires 11 of each pair are in contact with each other; and each of the drain ^' wires 16 is disposed between the

adjacent pairs of ^" signal wires 11. In other words, the signal wires 11 and the drain wires 16 are disposed such that two signal wires 11 alternate with one drain wire 16. However, the arrangement of the drain wires 16 can be changed as appropriate. For example, the signal wires 11 and the drain wires 16 can be disposed in such a manner as to alternate the signal wire 11 and the drain wire 16; i.e., such that one signal wire 11 alternates with one drain wire 16.

By means of using the drain wires 16 as ground wires, noise resistance can be enhanced, and crosstalk between the signal wires 11 can be reduced. For example, when the signal wires 11 and the drain wires 16 are disposed such that the signal wire 11 alternates with the drain wire 16, every signal wire 11 is sandwiched between the ground wires. Thus, a pseudo-structure similar to a coaxial cable can be obtained, so that crosstalk between the adjacent signal wires 11 can be reduced. When each of the drain wires 16 is disposed between adjacent pairs of signal wires 11, a pair of signal wires 11 is sandwiched between the ground wires. Accordingly, a pseudo-structure similar to a two-core coaxial cable can be obtained, so that crosstalk between adjacent pairs of signal wires 11 can be reduced. Other structural features and effects are similar to those of the first embodiment, and repeated description thereof is omitted.

The present invention is not limited to the above- described embodiments. Numerous modifications and variations

of the present invention are possible in light of the spirit of the present invention, and they are not excluded from the scope of the present invention.