Wiring substrate, wiring board, and wiring substrate mounting structure

In a wiring substrate, a high-frequency component is carried on a dielectric board having a transmission line formed on its surface, a reverse surface of the dielectric board is formed with an opening in a predetermined cross-sectional shape, and a high-frequency connecting pad is formed around the opening. In the wiring board, a dielectric board penetrates a waveguide structure and has its inner wall coated with a conductor, and a high-frequency connecting pad is formed on a surface of the dielectric board. The wiring substrate is placed on the wiring board, and the respective high-frequency connecting pads are electrically connected to each other, to fabricate a module. Even when a low-cost material having a large dielectric loss tangent is used for the wiring board, a high-frequency signal can be prevented from being attenuated.

DETAILED DESCRIPTION OF THE INVENTION 1. Structure of Wiring Substrate FIGS. 1A to 1 C are diagrams for explaining an example of the structure of a wiring substrate A according to the present invention. As shown in FIGS. 1A to 1 C, a wiring substrate A has a dielectric substrate 1 having a stacked structure of dielectric layers 1 a, 1 b, and 1 c. A cover 2 is joined to a surface of the dielectric layer 1 a in the dielectric substrate 1 , thereby forming a cavity 3 hermetically sealed. A strip conductor 5 for a microstrip line is formed on a surface of the dielectric layer 1 b in the dielectric substrate 1 . A ground layer 6 for a microstrip line is formed on a surface of the dielectric layer Ic in the dielectric substrate 1 . The stripe conductor 5 and the ground layer 6 constitute the microstrip line. A carrying section on which a high-frequency component is carried is formed on a surface of the ground layer 6 , and a high-frequency component 4 is carried thereon. The high-frequency component 4 is coated with a power or control line 7 for feeding power or a control signal to the high-frequency component 4 . A high-frequency connecting pad 9 is formed on a reverse surface of the dielectric substrate 1 . A cross-sectional shape of an opening 8 in the high-frequency connecting pad 9 has the same shape as that in cross section of a waveguide structure (described later). In the wiring substrate A shown in FIG. 1 , two high-frequency connecting pads 9 for input and output signals are formed. Further, the reverse surface of the dielectric substrate 1 is coated with a power pad 11 . The power pad 11 is connected to the power or control line 7 formed on the surface of the dielectric substrate 1 by a via conductor 10 . The wiring substrate A comprises a conversion section 12 for coupling the waveguide structure and the microstrip line formed on the surface of the dielectric substrate 1 . The structure of the conversion section 12 is as follows. As shown in FIG. 1B, a slot hole 13 is formed in the ground layer 6 . The position where the slot hole 13 is formed is the center of the opening 8 in the high-frequency connecting pad 9 as viewed from the top (see FIG. 1C ). As shown in FIG. 1 A, an opened end 5 a of the stripe conductor 5 constituting the microstrip line is formed at a predetermined position so as to stand face to face with the slot hole 13 . A vertical conductor 14 for connecting the ground layer 6 and the high-frequency connecting pad 9 to each other is formed on the dielectric layer 1 c in the dielectric substrate 1 . A matching section 15 for achieving impedance matching with a waveguide is formed in a region enclosed by the vertical conductor 14 . The conversion section 12 makes it possible to electromagnetically couple the microstrip line and the waveguide structure to each other through the slot hole 13 . Beneath the slot hole 13 is filled with dielectric material. A positional relationship for electromagnetically coupling the slot hole 13 and the stripe conductor 5 to each other is the same as a conversion structure conventionally known. It is described in International Publication WO96/27913, for example. Briefly stated, the opened end 5 a of the stripe conductor 5 is formed at a position projecting by a length which is one-fourth the wavelength of a signal from the center of the slot hole 13 as viewed from the top (planview) The slot hole 13 is a long narrow hole which is rectangular, elliptical, for example, and the shape thereof is adjusted by the used frequency and the bandwidth of the frequency. The long diameter of the slot hole 13 is set to a length which is one-half (½) the wavelength of the signal, and the short diameter thereof is set to a length which is one-fifth (&frac15;) to one-fiftieth ({fraction (1/50)}) the wavelength of the signal. The wiring substrate A having the above-mentioned structure comprises the high-frequency connecting pad 9 . Accordingly, the microstrip line in the cavity 3 can be coupled to all waveguide structures. Further, the wiring substrate A has the power pad 11 . Accordingly, the wiring substrate can be surface-mounted on a wiring board having a waveguide structure, described later. As shown in FIG. 5, a dielectric layer 1 d having a waveguide structure 50 having an opening whose inner wall is coated with a conductor layer formed therein may be stacked on a reverse surface of the dielectric boards 1 a to 1 c in the wiring substrate A. The high-frequency connecting pad 9 is hollowed inward from the reverse surface of the dielectric layer 1 d. According to such a structure, it is possible to increase the thickness of the dielectric substrate 1 to increase the substrate strength without degrading high-frequency characteristics. Further, the number of wiring layers is increased, thereby making it possible to increase the degree of freedom of wiring. 2. Structure of Wiring Board A wiring board will be then described on the basis of FIGS. 2A and 2B . A wiring board B has a dielectric board 21 . A waveguide structure 22 penetrates the dielectric board 21 from its surface to its reverse surface. The cross-sectional shape of the waveguide structure 22 is the same as the cross-sectional opening shape of the high-frequency connecting pad 9 . The waveguide structure 22 has its inner wall coated with a conductor. High-frequency connecting pads 23 and 24 are formed around the waveguide structure 22 , respectively, on a surface and a reverse surface of the dielectric board 21 . Further, a power pad 25 is formed on the surface of the dielectric board 21 . The power pad 25 , together with a low-frequency component such as a resistive element or a capacitor element which is carried on the wiring board B, constitutes a power circuit or a control circuit. The power circuit or the control circuit is finally connected to an external circuit via a connecting pad 26 (see FIG. 2A ). Further, the dielectric board 21 is formed with a screw hole 27 , used when the wiring board B is connected to an external circuit such as a waveguide or a plane antenna having a waveguide port, for screwing the external circuit. 3. Structure in which Wiring Substrate A is Mounted on Wiring Board B FIG. 3 is a schematic sectional view in a case where the wiring substrate A shown in FIG. 1 is mounted on the wiring board B shown in FIG. 2 . As shown in FIG. 3 , the high-frequency connecting pad 9 on the side of the wiring substrate A and the high-frequency connecting pad 23 on the side of the wiring board B are electrically connected to each other by a brazing material 30 . Further, the power pad 11 on the side of the wiring substrate A and the power pad 25 on the side of the wiring board B are electrically connected to each other by the brazing material 30 . According to such a mounting structure, the wiring substrate A and the wiring board B can be connected to each other by a waveguide mode in the waveguide structure 22 . They are connected to each other by the waveguide mode, as compared with the conventional connection by a microstripe line, a coplanar line, or the like. Accordingly, the transmission characteristics of the waveguide mode are determined irrespective of the dielectric characteristics of the dielectric board 21 . Even if the dielectric board 21 in the wiring board B is formed of a material having bad frequency characteristics, for example, an insulating material containing organic resin as an ingredient, for example, glass epoxy, it is possible to make lossless transmission of a high-frequency signal. According to the mounting structure, a waveguide C can be brazed to the high-frequency connecting pad 24 on the reverse surface of the wiring board B. Consequently, the wiring substrate A and an external circuit such as a plane antenna having the waveguide C can be coupled to each other through the wiring board B. According to the mounting structure, it is possible to carry only the high-frequency component on the wiring substrate A, and mount the other low-frequency components on the surface and the reverse surface of the wiring board B, for example. Consequently, the wiring substrate A on which the high-frequency component is carried can be made smaller in size, as compared with that in a case where the high-frequency component and the low-frequency component are carried in the wiring substrate A, as in the conventional example, thereby making it possible to increase the density of the wiring substrate A. Further, the miniaturization of the wiring substrate A makes it possible to decrease the cost of a module and the mounting reliability thereof. 4. Structure in which a Plurality of Wiring Substrates A are Mounted on Wiring Board B A mounting structure using a plurality of wiring substrates A 1 and A 2 will be described using a schematic sectional view of FIG. 4 . According to the mounting structure shown in FIG. 4 , at least four waveguide structures 22 a, 22 b, 22 c, and 22 d are formed in the wiring board B. The wiring substrate A 1 and the wiring substrate A 2 are mounted on an upper surface of the wiring board B, as in FIG. 3 , respectively, with respect to the waveguide structures 22 a and 22 b and the waveguide structures 22 c and 22 d. Further, a wiring substrate A 3 is mounted on the waveguide structures 22 b and 22 c in the wiring board B from the reverse surface of the wiring board B. In such a mounting structure, the wiring substrate A 1 and the wiring substrate A 3 can be coupled to each other through the waveguide structure 22 b formed in the wiring board B. The wiring substrate A 3 and the wiring substrate A 2 can be coupled to each other through the waveguide structure 22 c formed in the wiring board B. The wiring substrates A 1 , A 2 , and A 3 are coupled to one another by a waveguide mode. Accordingly, the transmission loss of a signal can be reduced without being affected by the dielectric characteristics of a dielectric material for the wiring board B. Furthermore, the wiring substrate can be divided into a plurality of blocks. Accordingly, it is possible to improve mounting the reliability by miniaturizing each of the blocks. In the above-mentioned mounting structure, ends of the waveguide structures 22 a and 22 d are further connected to another high-frequency component, antenna, or the like via another wiring substrate, waveguide, or the like. In the mounting structure, the wiring substrate A 3 which performs the function of connecting the two wiring substrates A 1 and A 2 need not necessarily have a power line, a control line, or a power pad, as shown in FIGS. 1A to 1 C. A high-frequency component denoted by reference numeral 4 a in the wiring substrate A 3 may be a conversion section for connecting stripe conductors for output and input signals in the wiring substrate A 3 to each other, for example. 5. Another Embodiment Although a case where the cross-sectional shape of the waveguide structure in the wiring board B is a square is illustrated in the above-mentioned embodiment described in the items 1 to 4, the cross-sectional shape of the waveguide structure may be a circle. Particularly when the cross-sectional shape is a circle, a dielectric board can be easily processed by a drill. The waveguide structure has the merits of having a smooth processed surface and being good as a waveguide. Further, when the waveguide structure is formed in a circular shape, the shape of the opening 8 in the high-frequency connecting pad 9 in the wiring substrate A may be either a circle or a square. However, it is desirably a circle. Examples of a dielectric material forming the dielectric substrate 1 in the wiring substrate A and the dielectric board 21 in the wiring board B include a ceramic material mainly composed of Al 2 O 3 , AlN, Si 3 N 4 , or mullite, a glass ceramic material formed by sintering glass or a mixture of glass and ceramic filler, an organic resin material such as epoxy resin, polyimide resin, or fluororesin such as Teflon, and an organic resin-ceramic (including glass) composite material. Particularly, a suitable example of the dielectric substrate 1 in the wiring substrate A on which the high-frequency component is carried is one which has a small dielectric loss tangent and can be hermetically sealed. An example of a particularly desirable dielectric material is at least one type of inorganic material selected from a group consisting of alumina, AlN, and a glass ceramic material. If the dielectric substrate 1 is composed of such a hard material, it is possible to hermetically seal the carried high-frequency component, which is preferable in order to increase reliability. As the dielectric board 21 in the wiring board B, all dielectric materials can be used because the high-frequency transmission characteristics thereof are not affected by the dielectric characteristics of the dielectric board 21 according to the present invention. Consequently, the dielectric material which is as low as possible may be used. From such a point, a suitable example of an insulating metal containing organic resin and particularly, at least one type selected from a group consisting of glass cloth-fluorine resin, glass cloth-epoxy resin, and alamide cloth-epoxy resin. Such an insulating material containing organic resin is low in cost, and is easily subjected to processing of a screw hole or the like. Accordingly, it can be fixed to an external circuit such as a waveguide or an antenna by a screw, which is preferable in that the cost is reduced, and connection to the external circuit is easy. The difference in thermal expansion coefficients at room temperature between dielectric material of the wiring board B and dielectric material of the wiring substrate A is preferably not more than 10×10 −6 /K. As the most suitable combination, it is the most desirable in terms of performance and cost that the dielectric substrate 1 in the wiring substrate A is composed of alumina ceramics or a glass ceramic material, the dielectric board 21 in the wiring board B is composed of glass cloth-epoxy resin. 
 6. EXAMPLE The following experiments were conducted in order to confirm the effect of the present invention. First, as a wiring substrate A, a substrate for evaluation which is similar to the wiring substrate A shown in FIG. 1 was fabricated by a normal stacking and simultaneous sintering technique using a green sheet composed of alumina ceramics (if the green sheet is sintered, the dielectric loss tangent at a frequency of 10 GHz is 0.0006) and tungsten metallized ink. In the substrate for evaluation, there is no cavity in the wiring substrate A shown in FIG. 1 , no high frequency component is carried thereon, and two microstrip lines each having an opened terminal end for input and output signals are connected to each other. An example of a matching section was one having a structure of a microstrip line 5 , a slot hole 13 , and a matching section 15 as shown in FIGS. 1A to 1 C. After sintering, metallized surfaces of a surface and a reverse surface of a dielectric substrate were subjected to plating processing using nickel and gold. The wiring board B shown in FIG. 2 was fabricated using a glass epoxy printed board FR-4 (the dielectric loss tangent at 10 GHz is 0.023) After the printed board was formed with an opening in cross section of a waveguide by a drill, and an inner surface of the opening was subjected to copper plating processing, to form a waveguide structure. Further, a high-frequency connecting pad, a power pad, or the like on the surface and the reverse surface of the printed board were formed by patterning copper foil. Tin, silver, and copper solder paste was printed on a pad of the above-mentioned printed board by a printing method, the wiring board for evaluation was solder-mounted thereon by a reflow method, to obtain a sample for evaluation. A waveguide for measurement was connected to the sample for evaluation, and an insertion loss at a frequency of 76 GHz was measured, to measure a connection loss from a microstrip line in the wiring substrate to the opening of the waveguide in the wiring board. As a result, it is confirmed that the connection loss at 76 GHz was approximately 0.4 dB, which is a practical and sufficiently small loss in fabricating a module. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.