Printed circuit boards

Abstract: On a surface of an insulating board (1), a front wiring pattern (2) is formed. On the rear surface of the insulating board (1), opposing the front wiring pattern (2), a rear wiring pattern (3) is formed in a shape which is substantially plane-symmetrical to the front wiring pattern (2). An opening (10) which penetrates the front wiring pattern (2) and the rear wiring pattern (3) is formed. At the inner wall of the opening (10), an electrically conductive layer (8C) is formed. Inside the opening (10), solder (11), for example, is placed to form an electrically conductive filling. As a result, the front wiring pattern (2) and the rear wiring pattern (3) are positively connected to each other electrically. Therefore, the current capacity and the heat capacity of the wiring patterns are increased, and the amount of generated heat is reduced.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to printed circuit boards for use in electronic apparatus. 2. Description of the Related Art These days, most electronic components used for an electronic apparatus are densely mounted on a printed circuit board and are connected to each other through wiring patterns formed on the printed circuit board. A general printed circuit board used for an electronic apparatus is formed by a copper-coated laminated plate including an insulating board made, for example, by dipping glass cloth formed of glass fiber in epoxy resin and by putting copper foil on the front and rear surfaces of the insulating board. An insulating board having a specified thickness of about 0.1 mm to 0.2 mm is used. The copper foil generally used is relatively inexpensive and has a relatively thin thickness of 18, 35, or 70 µm. A printed circuit board formed by relatively thin copper foil has a good yield due to easy copper-foil etching and is manufactured with a relatively low cost. In a wiring pattern on a printed circuit board used for the secondary circuit of a switching power unit, for example, a large current flows when a small load resistance is connected to the output of the secondary circuit. As a result, the pattern on the printed circuit board generates a loss due to its resistance, that is copper loss, and heat is generated in the pattern. Then, the heat generated in the pattern affects the surrounding electronic components and, for example, changes the temperature characteristics of the electronic components. When the pattern is widened to reduce copper loss, it requires a larger printed circuit board, resulting in a larger electronic apparatus. EP-A-0 287 681 describes a printed circuit board having a multi-layer structure in which current-carrying capacity of the conductor sections is increased by providing a plurality of identical conductor sections (e.g. 4) stacked in the multilayer structure. The different conductor sections are connected to each other by a through-hole. US patent 5,408,053 describes a hybrid multilayer circuit element, notably a transmission line, in which conductor losses which cause high frequency attenuation are reduced by replacing single conductors by respective sets of conductor sections stacked in a multilayer structure and interconnected using conductive vias. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a printed circuit board which solves the above-described problems. The foregoing object is achieved according to the present invention through the provision of a printed circuit board, comprising: an insulating board; a front wiring pattern formed on the front surface of said insulating board, said front wiring pattern including at least one longitudinally extending section; a rear wiring pattern formed on the rear surface of said insulating board, said rear wiring pattern including at least one longitudinally extending section; one of said sections of said front wiring pattern opposing a corresponding one of said sections of said rear wiring pattern to form an opposing pair of longitudinally extending wiring pattern sections, each section of said pair being located opposite to and being substantially the same shape and size as the other section of said pair; a plurality of openings extending through said insulating board and penetrating both sections of said pair; an electrically conductive layer provided at an inner wall of each opening, electrically connecting said front and rear wiring patterns through said insulating board; and an electrically and thermally conductive material filling said openings so as to electrically and thermally couple said pair of wiring sections. The front wiring pattern and the rear wiring pattern may be disposed such that they entirely oppose each other, or only partially oppose each other. With this configuration, the electrically conductive filling with which the openings are filled positively connects the front wiring pattern with the rear wiring pattern. The current capacity of the wiring patterns is reliably increased and copper loss is reduced. In addition, the electrically conductive filling is highly heat conductive and the heat capacity of the wiring pattern becomes larger. In other words, the electrically conductive filling serves as a heat sink. As a result, heat generated by the front wiring pattern is suppressed. Therefore, effects caused by heat on electric components mounted around the wiring patterns are reduced and reliability of the electronic apparatus increases. Since there is no need to change the widths of the wiring patterns, compactness of the electronic apparatus is maintained. The openings may be provided at a plurality of positions along the longitudinal direction defined by the opposed front wiring pattern and rear wiring pattern. With this configuration, since the electrically conductive layers and the generally column-shaped electrically conductive fillings serve as current paths between the front wiring pattern and the rear wiring pattern, a current flows through both wiring patterns, and the column-shaped electrically conductive fillings work as heat sinks. The openings may alternatively be configured such that they are long and narrow and extend along said longitudinal direction, and are provided side-by-side across the transverse direction defined by the front wiring pattern and the rear wiring pattern. With this configuration, since the electrically conductive layers and the electrically conductive fillings themselves serve as current paths, the current capacity of the wiring patterns increases. In addition, the volume of the electrically conductive fillings is greater than in the first embodiment and they work more effectively as heat sinks. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a part of a perspective view of a printed circuit board according to a first embodiment of the present invention. Fig. 2A is a part of an enlarged top view of a typical wiring pattern formed on the front surface of the printed circuit board according to the embodiment of the present invention. Fig. 2B is a part of an enlarged view of a cross section taken on line X-X' shown in Fig. 2A. Fig. 3A is a part of an enlarged top view of a typical wiring pattern formed on the front surface of a printed circuit board according to another embodiment of the present invention. Fig. 3B is a part of an enlarged view of a cross section taken on line X-X' shown in Fig. 3A. DESCRIPTION OF EMBODIMENTS OF THE INVENTION A first embodiment of the present invention will be described below by referring to Figs. 1 and 2. Fig. 1 is a perspective view showing a part of a printed circuit board. Figs. 2A and 2B are enlarged views of part A of the wiring pattern shown in Fig. 1. The printed circuit board includes an insulating board 1 which has been used conventionally, a front wiring pattern 2, and a rear wiring pattern 3. The front wiring pattern 2 includes lands 4 for mounting components, terminals 5, and leads 6 for connecting lands 4 and terminals 5 to each other. The front wiring pattern 2 comprises a front copper-foil pattern 7 which is formed by etching copper foil provided on the front surface of the insulating board 1, and a plated layer 8A with which the front copper-foil pattern 7 is laminated at the front surface. The rear wiring pattern 3 is formed at the rear surface of the insulating board 1, which is opposite the front wiring pattern 2, in a shape which is substantially plane-symmetrical to the front wiring pattern 2. Like the front wiring pattern 2, the rear wiring pattern 3 is formed by a rear copper-foil pattern 9 and a plated layer 8B. The insulating board 1 is provided with a plurality of openings 10 which penetrate the front wiring pattern 2 and the rear wiring pattern 3 and are spaced along the longitudinal direction of the front wiring pattern 2. The openings 10 may have any shape, such as a circle, an ellipse, a quadrilateral, or a triangle. At the inner walls of the openings 10, electrically conductive layers, such as plated layers 8C, are provided. As a result, the front wiring pattern 2 and the rear wiring pattern 3 are electrically connected to each other through the plated layers 8C. The openings 10 are formed by using a laser beam, by cutting physically, or by punching with the use of a metal die. It is preferred that the maximum dimension of an opening be several millimeters. The openings 10 are filled, for example, with solder 11 to form column-shaped electrically conductive fillings. The solder 11 is put in the openings 10, for example, by dipping the rear wiring pattern 3 of the printed circuit board in a molten-solder bath. To prevent the solder 11 from sticking to the surface of the rear wiring pattern 3, a solder resist layer (not shown) is provided in advance. When the pattern is dipped into the solder bath, the plated layers 8C at the inner walls of the openings 10 become wet with the molten solder 11, and the molten solder 11 goes upward into the openings 10 by capillary action. Then, the printed circuit board is cooled down. The molten solder 11 becomes hard and the openings 10 are filled with the solder 11. Therefore, the solder 11 does not stick to the surface of the rear wiring pattern 3 and the printed circuit board can be made thin. As a result, the front wiring pattern 2 and the rear wiring pattern 3 are positively connected by the solder 11 electrically, and the current capacity of the wiring patterns made up of the front wiring pattern 2 and the rear wiring pattern 3 is reliably increased. Therefore, the copper loss of the wiring patterns is reduced. The heat capacity of the wiring patterns becomes larger due to the solder 11 with which the openings 10 are filled. The solder 11 serves as a heat sink. The front wiring pattern 2 is prevented from generating heat. In the above embodiment, the current capacity of the entire area of the front wiring pattern 2 is made larger. In an alternative embodiment, only a part of the front wiring pattern 2 is provided with openings 10 and conductive fillings 11, whereby the current capacity is only partially increased. Even in this case, since heat generated by a part of the front wiring pattern 2 is reduced, the amount of heat generated by the whole printed circuit board is also reduced. In the above embodiment, the rear wiring pattern 3 is formed in a plane-symmetrical manner to the front wiring pattern 2, and they are positively connected by the solder 11. However, the printed circuit board may alternatively be configured such that the rear wiring pattern 3 is formed plane-symmetrically to only a selected part of the front wiring pattern 2, where a large current flows, and this part of the front wiring pattern 2 is positively connected to the rear wiring pattern 3 by the electrically conductive fillings. In this case, heat generated by the part of the front wiring pattern 2, where a large current flows, is reduced. In addition, if the printed circuit board is allowed to be made thick, the solder 11 may be allowed to stick to the rear wiring pattern 3 by not providing the solder resist layer. In that case, the current capacity of the wiring patterns becomes even larger. A plurality of printed circuit boards which are formed in this way may be stacked to form a multilayer printed circuit board. The configuration of another embodiment of printed circuit board according to the invention will be described below by referring to Figs. 3A and 3B. Since the configuration is the same as that of the printed circuit board described above except that openings 10 have slit shapes, only the openings 10 will be described below. An insulating board 1 is provided with a plurality of long and narrow openings 10 which have a width of several millimeters or less and which extend in the longitudinal direction through a front wiring pattern 2 and a rear wiring pattern 3. The openings 10 are formed by using a laser beam, by cutting physically, or by punching with the use of a metal die. The openings 10 are also provided with electrically conductive layers, such as plated layers 8C, at the inner walls. The openings 10 are filled with solder 11 to form electrically conductive fillings. As a result, the current capacity of the wiring patterns in the printed circuit board becomes larger, and the electrically conductive fillings serve as current paths to reduce the copper loss of the wiring patterns. The solder 11 with which the openings 10 are filled has a larger volume than the electrically conductive fillings shown in Figs. 2A and 2B, and it works more effectively as a heat sink. As a result, the front wiring pattern 2 is prevented from generating heat.

Claims: A printed circuit board comprising: an insulating board (1); a front wiring pattern (2) formed on the front surface of said insulating board, said front wiring pattern including at least one longitudinally extending section; a rear wiring pattern (3) formed on the rear surface of said insulating board, said rear wiring pattern including at least one longitudinally extending section; one of said sections of said front wiring pattern opposing a corresponding one of said sections of said rear wiring pattern to form an opposing pair of longitudinally extending wiring pattern sections, each section of said pair being located opposite to and being substantially the same shape and size as the other section of said pair; a plurality of openings (10) extending through said insulating board and penetrating both sections of said pair; an electrically conductive layer (8C) provided at an inner wall of each opening (10), electrically connecting said front and rear wiring patterns through said insulating board; and an electrically and thermally conductive material (11) filling said openings (10) so as to electrically and thermally couple said pair of wiring sections. A printed circuit board according to claim 1, wherein said openings (10) are located at respective positions spaced from each other along the longitudinal direction of said opposed front and rear wiring patterns (2, 3). A printed circuit board according to claim 1, wherein said openings (10) are elongated and are disposed transversely side-by-side, extending along the longitudinal direction of said front and rear wiring patterns (2, 3).

Inventors: NISHI KOJI, TSUJI HITOSHI, NISHI, KOJI, TSUJI, HITOSHI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: H05K1
IPC codes: H05K3, H05K1