Composite circuit board and manufacturing method of the same

Abstract: The composite circuit board comprises a substrate (13) made of an insulating material, a thick circuit conductor (11), embedded in this substrate (13), which includes a parts-mounting section (11a) and has a predetermined circuit pattern, a thin circuit conductor (12), provided on the surface of said substrate (13), which includes a land (12a) corresponding to the parts-mounting section (11a) of said thick circuit conductor (11) and has a predetermined circuit pattern, and a conductor means (14, 14a) for electrically connecting said land and said parts-mounting section. In this circuit board, the land (12a) and the part-mounting section (11a) are connected by a conducting member provided in this through hole (14).

Description: The present invention relates to a composite circuit board having a thick circuit conductor and thin circuit conductor, and its manufacturing method according to the preamble of claims 1, 2, respectively and known from EP-A-0 052 738. A composite circuit board known from EP-A-0 052 738 has a substrate, thick circuit conductors, thin circuit conductors and conductor means for electrically connecting lands of the thin circuit conductors with parts-mounting sections of the thick circuit conductors. These conductor means are arranged in holes which extends through the substrate and the thick circuit conductors. Due to the high heat conductivity of the thick circuit conductors problems occur in mounting parts on the composite circuit board. A circuit board through which a relatively large electrical current, for example, one for driving a motor flows requires a circuit conductor having a large cross-sectional area. A conventional circuit board of this type has a structure in which a punched-out circuit conductor made by punching out a desirable circuit pattern from a metal plate (copper, aluminum, etc) is integrally fixed on an insulation substrate (see Published Unexamined Japanese Patent Application (PUJPA) No. 63-237495). In such a circuit board, to fix a punched-out circuit conductor on an insulation substrate, a copper foil pattern is formed on an insulation substrate, and a punched-out circuit conductor is soldered thereon. This is rather a complicated procedure. Further, the punched-out circuit conductor, which is relatively thick, inevitably creates a bump on the surface of the insulation substrate for its thickness, causing great irregularity on the circuit substrate surface and making matter difficult for printing of a solder resist and circuit conductor to be multi-layered. In consideration of the above problems it is the object of the present invention to provide a new and improved composite circuit board and a method for making a circuit board, onto which parts can be easily mounted and which prevent heat diffusion from the circuit conductor to other parts of the conductor. In order to solve this object there is provided a composite circuit board according to claim 1. This object is also solved by a method of manufacturing such a composite circuit board as described in claim 2. According to an aspect of the present invention, there is provided a composite circuit board comprising a substrate made of an insulating material, a thick circuit conductor embedded in the substrate, which includes a parts-mounting section and has a predetermined circuit pattern; a thin circuit conductor, provided on the surface of said substrate, which includes a land corresponding to the parts-mounting section of said thick circuit conductor and has a predetermined circuit pattern, and a conductor means for electrically connecting said land and said parts-mounting section. According to another aspect of the present invention, there is provided a method of manufacturing a composite circuit board, comprising steps of preparing an insulation substrate, embedding a plate for forming a thick circuit conductor, into at least one side of the substrate, the plate being formed into a predetermined pattern and having bridges for connecting conductor portions of the plate, forming an internal circuit plate having a thick circuit conductor with a part-mounting section by removing sections corresponding to the bridges, arranging prepreg sheet and plate for forming a thin circuit conductor in the order mentioned on that side of the internal circuit plate where the thick circuit conductor is formed, integrating the internal circuit plate, the prepreg sheet and the plate for forming a thin circuit conductor, forming a thin circuit conductor formed into a predetermined pattern and having a land at a position which corresponds to that of the part-mounting section of the thick circuit conductor, and forming a through hole for conducting the part-mounting section to the land. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a diagonal view showing the main section of a composite circuit board having a thick circuit board in its surface area; Fig. 2 is a view explaining a method of manufacturing the circuit board shown in Fig. 1; Fig. 3 is a plane view of a circuit pattern of a thick circuit conductor which is prepared by punching-out method; Fig. 4 is a diagonal view showing the main section of a composite circuit board; Fig. 5 is a cross section of a conductor means of the circuit board shown in Fig. 4; Fig. 6 is a view explaining a method of manufacturing the circuit board shown in Fig. 4; and Fig. 7 is a plane view showing of a circuit pattern of the thick circuit conductor according to the invention which is prepared by the punching-out method. Japanese Patent Application No. 1-152241 (not yet published) discloses a method of manufacturing a circuit board of this type by a hot press method, to solve the problems stated above. This method is illustrated in the following description of figures 1-3. Fig. 1 illustrates an embodiment of a composite circuit substrate manufactured by this method. The figure shows a thick circuit conductor 11 for a large current flow of, for example, a power circuit, thin circuit conductor 12 for a small current flow such as of a signal circuit, and an insulation substrate 13, and through hole 14, the inner surface of which is plated. As shown in this figure, the composite circuit bcard has a structure in which the thick circuit conductor 11 is embedded in the surface area of the insulation substrate 13, and the thin circuit conductor 12 is embedded in the insulation substrate 13. To manufacture a circuit board of this type, the method shown in Fig. 2 is employed. In this method, first, an internal circuit board 16 having the thin circuit conductor 12 formed by etching a pattern on the copper foil on both (or either) sides (or side) on an insulation sheet 15 is prepared. The appropriate number of prepreg sheets 17 and the thick circuit conductor 11 formed into a desired circuit pattern by the punching-out method are laminated on both sides of the internal circuit board 16, and heated and pressed by hot press machine 18. Thus, the insulation sheet 15 and the prepreg sheets 17 are integrated into the insulation substrate 13 shown in Fig. 1, in the surface area of which the thick circuit conductor 11 is embedded. The thickness of the thick circuit conductor is generally about 0.1-1mm, and that of the thin circuit conductor 12 is generally 35-70 µm. The thick circuit conductor 11 is punched out into a pattern shown in Fig. 3. With this pattern, a predetermined circuit pattern can be maintained through the punching-out process. More specifically, this thick circuit pattern 19 has an arrangement in which each of the thick circuit conductor 11 is formed into a predetermined circuit pattern in a square frame edge 21, and thick circuit conductors 11 located next to each other are connected by bridges 22, and so are thick circuit conductors 11 and the frame edge 21. Each of the thick circuit conductors 11 has a hole 23 for forming a through hole, and the edge frame 21 has holes 24 for locating the conductor 11. The bridges 22 is removed by cutting them off or punching them out after integrating them into the insulation substrate 13 as in Fig. 1. In this figure, numerals 25 denotes the sections where the bridges 22 have been removed. However, due to such a structure in which the thick circuit conductor is embedded in the surface area of the composite circuit board, to mount a part, the lead wire of the part is soldered to the thick circuit conductor. Since the thick circuit conductor has a large heat capacity, the heat is likely to diffuse, making the soldering work very difficult. Further, the sections where the bridges have been removed remain in the form of hole or recess, in which some foreign materials are likely to accumulate, deteriorating the insulating property of the board. Meanwhile, the composite circuit board of the present invention comprises a substrate made of an insulating material, a thick circuit conductor, embedded in this substrate, which includes a parts-mounting section and has a predetermined circuit pattern, a thin circuit conductor, provided on the surface of said substrate, which includes a land corresponding to the parts-mounting section of said thick circuit conductor and has a predetermined circuit pattern, and a conductor means for electrically connecting said land and said parts-mounting section. Due to this structure, the above-mentioned problems can be solved without extinguishing advantages such as to be easily manufactured, and suitable to be multi-layered. In other words, such a circuit board can be manufactured by the hot press method as in the case of the above composite circuit board. Therefore, it is easily manufactured and is suitable to be multi-layered. Further, when mounting parts, all that is required is to put the lead wire of a part through the through hole and solder it to the land of the thin circuit conductor, where diffusion of heat is less. Therefore, soldering can be performed easily and the parts can be accurately mounted. Moreover, since the thick circuit conductor is embedded in the insulation substrate, the section where the bridge has been cut off does not appear on the surface thereof, improving the reliability of the circuit in terms of insulation. Fig. 4 is a diagonal view showing the main section of a composite circuit board and Fig. 5 is a cross section of a through hole forming portion thereof. In Fig. 4, members having the same functions as those of Fig. 1 have the same reference numerals thereof. The composite circuit board shown in these figures comprises an insulation substrate 13, a thick circuit conductor 11 embedded in the substrate and formed into a predetermined circuit pattern, and a thin circuit conductor 12 provided on the surface of said substrate and formed into a predetermined circuit pattern. The thick circuit conductor 11 includes part-mounting sections 11a on which the parts are mounted, and the thin circuit conductor 12 has lands 12a at the positions corresponding to the part-mounting sections 11a. This composite circuit board has through holes 14 formed thereon so that the parts-mounting sections 11a and lands 12a are conducted with each other. A conducting members 14a is formed on the inner wall of each of the through holes 14 by plating. In the case of such a structure, all that is required in order to mount the parts on the thick circuit conductor 11 is to pass the lead wire of a part through a through hole 14, and solder it to a land 12a of the thin circuit conductor 12. Some of the solder comes into the through hole 14. Meanwhile, the thick circuit conductor 11 is the same as that shown in Fig. 3, and can be formed into a predetermined circuit pattern by, for example, the punching-out method. Although the thick circuit conductor 11 is embedded into the insulation substrate 13, if the thick circuit conductor 11 is formed as wide as possible, and the insulation layer between the thick and thin circuit conductors 11 and 12 is formed as thin as possible, heat emission upon conducting electricity can be suppressed, and the heat can be radiated. Such a composite circuit board can be manufactured, for example, as shown in Fig. 6. First, an internal circuit plate 27 in which thick circuit conductors 11 are embedded in both (or either) surface areas of the insulation substrate 26, is formed. To manufacture internal circuit plate 27, a thick circuit pattern, as shown in Fig. 6, which is formed by punching out a copper plate or by etching both surfaces is stacked on both surfaces of a plurality of prepreg sheets prepared by laminating. Then, the circuit pattern is hot-pressed, and the bridges are removed by punching out or cutting off, thereby forming an internal circuit plate 27. Next, the prepreg sheets 28 are stacked on both surface of the internal circuit plate 27, and a copper laminated plates 29 are stacked on the prepreg sheets 28 respectively, with a copper foil 31 facing outside. These laminated layers are heated and pressed into an integrated body by a hot press machine 18. During this step, the holes or recesses formed at the sections where the bridge has been cut off are filled with resin flown out of the prepreg sheets 28. After that, the surface copper foil 31 is etched into a desired pattern to form a thin circuit conductor 12 such as that shown in Fig. 4. The conductor 12 thus formed is worked to make holes, and the inner surface of each of the holes is plated, whereby through holes 14 are completed. In this example, a thin circuit conductor is formed by etching copper foil into a desired pattern (the subtractive method); however formation of the conductor is not limited to this method. For example, the additive method (in which a circuit pattern is formed on the surface of an insulation substrate by using only electroless plating or using both electrolytic and electroless platings) can be also employed. The following is an explanation of an embodiment of the present invention. This embodiment is exactly the same as the example mentioned previously except for the thick circuit conductor shown in figure 7. The thick circuit conductor 11 is formed of a metal which has a high heat conductivity; therefore soldering heat is likely to diffused to the circuit conductor when mounting the parts. In order for prevention of this heat diffusion, a hole 40 is formed near the part-mounting section of the thick circuit conductor 11. This hole 40 suppresses diffusion of the heat of the part-mounting section of the circuit conductor 11 to the other sections of the conductor. Thus, the temperature of the soldering section of the part-mounting section can be maintained high, and therefore soldering of the parts can be carried out more accurately.

Claims: A composite circuit board, comprising: a) a substrate (13) of an insulating material; b) a thick circuit conductor (11), embedded in said insulating substrate (13), which includes at least one parts-mounting section (11a, 23) and has a predetermined circuit pattern; c) a thin circuit conductor (12), provided on both exposed main surfaces of said insulating substrate (13), which include at least one corresponding land pair (12a) at each main surface of said insulating substrate which correspond to said at least one parts-mounting section (11a, 23) of said thick circuit conductor (11) and have a predetermined circuit pattern; and d) at least one through hole (14) extending between said at least one corresponding land pair through said insulating substrate and the corresponding at least one parts-mounting section (11a, 23), a conducting member (14a) formed on the inner wall of said at least one through hole (14) which electrically connects said at least one corresponding land pair (12a) and said corresponding at least one parts-mounting section (11a) and said conducting member leaves an opening extending between said at least one corresponding land pair for mounting parts; characterized in that e) said thick circuit conductor (11) has at least one hole (40) near said at least one parts-mounting section (11a, 23) for suppressing heat diffusion. A method of manufacturing a composite circuit board, comprising the following steps: a) preparing an insulating substrate (26); b) embedding a thick circuit plate for forming a thick circuit conductor (11), into at least one side of said insulating substrate (26), said thick circuit plate being formed into a predetermined pattern and having bridges (22) connecting portions (11) of said thick circuit conductor; c) forming an internal circuit plate (27) having said thick circuit conductor (11) with at least one parts-mounting section (11a, 23) by removing sections from said bridges (22) to isolate said portions of said thick circuit conductor; d) arranging a prepreg sheet (28) on each main surface of said internal circuit plate and then arranging a plate (29) comprising a layer for forming a thin circuit conductor (12) on each prepeg sheet with said layer exposed; e) integrating said internal circuit plate (27), said prepeg sheets (28) and said plates (29) comprising a layer for forming said thin circuit conductor (12); f) forming said thin circuit conductor (12) into a predetermined pattern including at least one corresponding land pair (12a) at a position which corresponds to that of said at least one parts-mounting section (11a, 23); g) forming at least one through hole (14) extending between said at least one corresponding land pair through said insulating substrate (13), said prepeg sheets and the corresponding at least one parts-mounting secton and forming a conducting member (14a) on the inner wall of said at least one through hole (14) which electrically connects said at least one corresponding land pair and said corresponding at least one parts-mounting section, and said conducting member leaves an opening extending between said at least one corresponding land pair for mounting parts; and characterized in that h) said thick circuit plate is being formed with at least one hole (40) for suppressing heat diffusion and positioned near said at least one parts-mounting section in said thick circuit conductor (11). A method according to claim 2, characterized in that said thick circuit plate (29) is formed by a punching-out method. A method according to claim 2, characterized in that said thin circuit conductor (12) is formed by etching. A method according to claim 2, characterized in that said integrating step is carried out by hot-pressing, by which holes or recesses resulting from removing said bridge sections (22) from said internal circuit plate (27) are filled with resin which flows out of said prepeg sheet (28).

Inventors: KOBAYASHI KENZO, MOCHIZUKI HAJIME, YAMAMOTO MASAAKI, YATABE HIROSHI, KOBAYASHI, KENZO, MOCHIZUKI, HAJIME, YAMAMOTO, MASAAKI, YATABE, HIROSHI
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1990
Main classification: H01L23
IPC codes: H05K1, H05K3, H01L23, H01L21