A MULTILAYER CIRCUIT BOARD

Abstract: A multilayer circuit board is provided having an inner porous fluororesin layer (11) containing electrical circuits, a ground conductor layer (7A) and a power supply conductor layer (5A) located adjacent the fluororesin layer on either side thereof, and a reinforcing, solid plastic dielectric layer (4A,4B) adjacent each conductor layer and being external to the inner fluororesin layer. In an alternative embodiment, the multilayer circuit board may have a plurality of inner porous fluororesin layers containing electrical circuits, ground conductors and power supply conductors adjacent each fluororesin layer, and reinforcing, solid plastic dielectric layers on the outermost surfaces of the circuit board assembly.

Description: The present invention concerns a multilayer circuit board and in particular one which can be appropriately used in high-frequency, electric signal transmission equipment such as electronic equipment, communications equipment and electronic computers. One of the present inventors has been involved in the development of high-frequency circuit boards, and has proposed a multilayer circuit board which prevents an increase in signal propagation lag time by using a porous material for a dielectric which is inserted between adjacent printed circuit boards, as disclosed in Japanese Utility Model Application Nos. 60-140292 and 60-158095. Conventionally, circuit boards which use a porous material as a substrate have been superior in terms of electrical characteristics such as propagation lag time. In such cases, however, because the substrate itself is soft and spongy, the circuit board has little resistance to bending stress when a multilayer arrangement is formed. Thus, such circuit boards have been considered inappropriate for use in multilayer configurations. JP-A-61 220 499 published 30.09.86, is a patent family member of US-A-4 710 854, which was published after the priority date of the application. These documents disclose a multilayer wiring board including low permittivity (fluorocarbonic polymer) and high permittivity insulation layers laminated between neighboring power (ground) line layers interposed by one or more signal line layers. An insulation material having better mechanical characteristics such as higher dimensional stability, rather than the perfection of relative permittivity, is used for portions of lower electric flux density such as the section between the power (ground) line layers and the section between the signal line layers. The section between a signal line layer and a power (ground) line layer is high in the electric flux density, and a low relative permittivity insulation layer is provided so that the signal propagation delay is minimized. There is a requirement for a multilayer circuit board which is superior in terms of electrical characteristics such as propagation lag time, and which is also superior in terms of mechanical strength characteristics. According to the present invention there is provided a multilayer circuit board comprising a plurality of inner, porous fluororesin layers, at least one of them containing electrical circuits, ground conductors and power supply conductors in contact with the fluororesin layers and located adjacent said at least one fluororesin layer, and reinforcing, solid plastic dielectric layers being external to said inner fluororesin layers and located on the outermost surfaces of the circuit board assembly. The dielectric layers may be of a thermosetting resin, a polyimide resin or a glass epoxy. According to another aspect of the present invention there is provided a multilayer circuit board comprising a plurality of porous fluororesin layers, ground conductors and power supply conductors in contact with the fluororesin layers, and reinforcing, solid plastic dielectric layers external to at least one inner fluororesin layer, said at least one inner, porous fluororesin layer containing electrical circuits, while said ground conductors and power supply conductors are located adjacent said at least one inner layer, and said dielectric layers are located between said at least one inner porous fluororesin layer and two outer porous fluororesin layers of the circuit board assembly. Again, the dielectric layers may be of a thermosetting resin, a polyimide resin or a glass epoxy. The invention will now be particularly described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a perspective view, partly in cross-section, of one embodiment of a multilayer circuit board according to the invention; and Figure 2 is a cross-sectional view of another embodiment of a circuit board according to the invention. A multilayer circuit board is provided having an inner porous fluororesin layer containing electrical circuits, a ground conductor layer and a power supply conductor layer located adjacent the fluororesin layer on either side thereof, and a reinforcing, solid plastic dielectric layer adjacent each conductor layer and being external to the inner fluororesin layer. In an alternative embodiment, the multilayer circuit board may have a plurality of inner porous fluororesin layers containing electrical circuits, ground conductors and power supply conductors adjacent each fluororesin layer, and reinforcing, solid plastic dielectric layers on the outermost surfaces of the circuit board assembly. In the present invention, electrical circuits are formed in a porous fluororesin layer. Accordingly, the electrical characteristics of the conductive pathways of these electrical circuits are determined by the characteristics of the porous fluororesin layer, which has a superior dielectric constant and dielectric loss tangent. As a result, electrical signals propagated through these electrical circuits have a short lag time and small energy loss. Furthermore, a reinforcing dielectric layer is laminated with the porous fluororesin layer, with a conductor layer interposed between the dielectric layer and the fluororesin layer. Accordingly, when a multilayer circuit board is formed, the reinforcing dielectric layer, which is preferably formed of a thermosetting resin, resists bending stress. Thus, a circuit board which is superior in terms of resistance to bending stress is obtained, and since the circuit board is shielded by the aforementioned conductor layer, there are no harmful effects on the electrical characteristics of the electrical circuits. In the embodiment of Figure 1, a multilayer circuit board 1 is formed by laminating, in the order listed, (a) a surface layer 2A of copper, (b) a prepreg sheet 3A of expanded, porous tetrafluoroethylene resin such as that disclosed in U.S. Patent 3,953,566 (hereinafter referred to as "EPTFE") sheet, (c) a copper-covered laminated composite sheet 4A which is formed by bonding a power supply conductor layer 5A and a ground conductor layer 7A, both of copper, to the respective surfaces of a prepreg sheet 6A of a polyimide which forms a reinforcing dielectric layer, (d) a prepreg sheet 8A of EPTFE, (e) a prepreg sheet 11 of EPTFE and which has electrical circuits 10A, 10B of copper formed on both sides, (f) a prepreg sheet 8B of EPTFE, (g) a copper-covered laminated composite sheet 4B which is formed by installing a power supply conductor layer 7B and a ground conductor layer 5B, both of copper, on the respective surfaces of a prepreg sheet 6B of a polyimide, (h) a prepreg sheet 3B of EPTFE, and (i) a surface layer 2B of copper. Here, the prepreg sheets 8A,8B and 11 form the aforementioned porous fluororesin layer. The electrical circuit layers 10A and 10B are sandwiched between the respective prepreg sheets 8A, 11 and 8B, which are EPTFE sheets having good electrical characteristics, such as specific inductive capacity and dielectric loss tangent. Accordingly, the electrical signals which are propagated through the electric circuits 10A,10B have a short lag time and small energy loss. Furthermore, the copper-covered laminated composite sheets 4A and 4B both have prepreg sheets 6A,6B of a polyimide which are laminated into integral units to form reinforcing dielectric layers. Accordingly, even if a bending force is applied to the multilayer circuit board 1, the prepreg sheets 6A,6B of a polyimide resist this bending force and support the laminated structure. As a result, there are few problems associated with distortion or contraction of the prepreg sheets of EPTFE, and the favourable characteristics of the electrical signals propagated through the electrical circuits 10A,10B are not lost. In the present invention, as described above, electrical circuits are formed in a porous fluororesin layer. Accordingly the electrical characteristics of the conductive paths of these electrical circuits are determined by this porous fluororesin, which has a superior dielectric constant and dielectric loss tangent. As a result, the electrical signals which are propagated through the aforementioned electrical circuits have a short lag time and small energy loss. Furthermore, reinforcing dielectric layers are formed which use a thermosetting resin as a constituent element. Accordingly, when a multilayer circuit board is constructed which combines these reinforcing dielectric layers with the porous fluororesin layer, the reinforcing dielectric layers resist bending stress, so that a multilayer circuit board with superior resistance to bending stress is obtained. The problem of the low mechanical strength characteristics of EPTFE, which has superior dielectric characteristics, is remedied by reinforcement with a dielectric of a thermosetting resin so that a multilayer circuit board with stable superior electrical characteristics is obtained. The present invention is not limited to the example of application described above, and various modifications can be made. For example, it would also be possible to obtain a multilayer circuit board with a superior resistance to bending stress and a superior surface hardness by using reinforcing dielectric layers both as interior layers and as outermost layers. One such alternative construction is shown in Figure 2 wherein a multilayer circuit board 12 is formed by laminating, in the order listed, (a) a surface layer 13A of copper, (b) a reinforcing dielectric layer 14A of a glass epoxy, (c) a copper-covered laminated sheet 15A which is formed by bonding a power supply layer 16A and a ground layer 18A, both of copper, to the respective surfaces of a prepreg sheet 17A of an expanded, porous tetrafluoroethylene (EPTFE) sheet, (d) a prepreg sheet 19A of an EPTFE sheet, (e) a copper-covered laminated composite sheet which is formed by installing electrical circuits 21A and 21B, both of copper, on the respective surfaces of a prepreg sheet 22 of an EPTFE sheet and which forms a porous fluororesin layer, (f) a prepreg sheet 19B of an EPTFE sheet, (g) a copper-covered laminated sheet 15B which is formed by bonding a power supply conductor layer 16B and a ground conductor layer 18B, both of copper, to the respective surfaces of a prepreg sheet 17B of an EPTFE sheet, (h) a reinforcing dielectric layer 14B of a glass epoxy resin, and (i) a surface layer 13B of copper. The electrical circuits 21A and 21B are sandwiched between prepreg sheets of EPTFE. Accordingly, the electrical signals which are propagated through these electrical circuits 21A,21B have a short lag time and small energy loss. In this case since reinforcing dielectric layers 14A,14B are installed as the outermost layers, the surface hardness of the multilayer circuit board 12 is determined by these reinforcing dielectric layers 14A,14B. As a result, the surface hardness is greatly improved compared to cases where a multilayer circuit board is constructed exclusively of prepreg sheets of EPTFE. Thus, attachment of the circuit board to equipment and mounting of electronic parts are facilitated, and reliability is improved.

Claims: A multilayer circuit board (12), comprising a plurality of inner porous fluororesin layers (20, 15A, 15B), electrical circuits (21A, 21B), ground conductors (18A, 18B) and power supply conductors (16A, 16B) in contact with the fluororesin layers (20, 15A, 15B) and reinforcing, solid plastic dielectric layers (14A, 14B) external to said inner fluororesin layers, characterized in that at least one (20) of said inner porous fluororesin layers (20, 15A, 15B) contains said electrical circuits (21A, 21B), while said ground and power supply conductors (18A, 18B; 16A, 16B) are located adjacent said at least one inner layer (20), and said dielectric layers (14A, 14B) are located on the outermost surfaces of the circuit board assembly. A multilayer circuit board (1), comprising: a plurality of porous fluororesin layers (3A, 3B, 9), electrical circuits (10A, 10B), ground conductors (7A, 7B) and power supply conductors (5A, 5B) in contact with the fluororesin layers (3A, 3B, 9), and reinforcing, solid plastic dielectric layers (6A, 6B) external to at least one inner fluororesin layer (9), characterized in that said at least one inner porous fluororesin layer (9) contains said electrical circuits (10A, 10B), while said ground and power supply conductors (5A, 5B; 7A, 7B) are located adjacent said at least one inner layer (9), and said dielectric layers are located between said at least one inner porous fluororesin layer (9) and two outer porous fluororesin layers (3A, 3B) of the circuit board assembly. The circuit board of claim 1 or 2 wherin said reinforcing, solid plastic dielectric layers (14A, 14B; 6A, 6B) are of a thermosetting resin. The circuit board of claim 1 or 2 wherein said reinforcing, solid plastic dielectric layers are of a polyimide resin. The circuit board of claim 1 or 2 wherein said reinforcing, solid plastic dielectric layers are of a glass epoxy.

Inventors: HIROSE, TETSUYA, KOBAYASHI, SATORU, SUZUKI, MASAHIRO, TANAKA, SATOSHI
Applicants: JUNKOSHA CO. LTD.
Application year: 1988
Main classification: H05K1
IPC codes: H05K1, H05K3