Method of manufacturing multilayered printed-wiring-board

Abstract: A method of manufacturing a multilayered printed-wiring-board comprises attaching conductive members (136a, 136b; 236a, 236b; 336; 436a, 436b; 542, 642) on conductive layers (228, 230; 328, 330; 428, 430; 528, 530) superimposing a plurality of printed-wiring boards, (220, 222; 320, 322; 420, 422; 520, 522) and electrically connecting the conductive layers (228, 230; 328, 330; 428, 430; 528, 530) with one another by the conductive members (136a, 136b; 236a, 236b; 336; 436a, 436b; 542, 642).

Description: 1. Field of the Invention This invention relates to a method of manufacturing a multilayered printed-wiring-board out of printed-wiring-boards (hereinafter called "PWB"), comprising the steps of attaching a conductive member on a partial surface of a conductive layer formed on at least one of opposite surfaces of a printed-wiring-board in a pattern superimposing a plurality of printed-wiring-boards one over another and attached on said conductive layer on one of said printed-wiring-boards confronts either said conductive layer or said conductive member on another printed-wiring-board, at least one of said plurality of printed-wiring-boards having been processed by said attaching step, and after said superimposing step, electrically connecting said conductive member to either said confronting conductive layer or said confronting conductive member to form an electrical connecting layer. 2. Description of the Related Art Heretofore, attempts have been made to install circuit elements on a substrate with high-concentration to realize an integrated circuit board. To this end, it is a known practice to use a multilayered PWB, in which insulating and conductive layers are placed alternately one over another. Another popular method is to use a mulitlayered printed-circuit-board (hereinafter called "multilayered PCB"), in which various circuit elements are installed on and/or inside a multilayered PWB. The multilayered PWB and PCB are particularly useful when realizing a compact-sized and high-concentration circuit of high frequency, such as 400 MHz band, 800 - 900 MHz band and 1.5 GHz band, in automobile and portable telephones, for example. In the manufacture of a multilayered PWB or PCB, it is known to unified a plurality of PWBs by superimposing. As is well known in the art, a PWB is produced by providing a conductive layer on the surface of an insulating substrate and forming an additional conductive layer thereover in a desired pattern according to need. A copper foil or a conductive paste, for example, is used for the conductive layer. Further, a PWB having a conductive layer on only one of opposite surfaces is called a single-face PWB, while a PWB having a conductive layer on either one of opposite surfaces is called a double-face PWB. FIG. 5 is a cross-sectional view showing a manufacturing of a multilayered PWB according to another conventional method, which will now be described in greater detail. The conventional production of a multilayered PCB is similar to that of a multilayered PWB except for an additional step of installing circuit elements, and therefore, its detailed description is omitted here for clarity. In FIG. 5 two single-face PWB 10, 12, a double-face PWB 14, and two inter-layer adhesive sheets 16, 18 are shown. Each of the single-face PWB 10, 12 is composed of an insulating substrate 20, 22, and a conductive layer 24, 25 in the form of a copper foil attached on one surface of the insulating substrate 20, 22. The conductive layer 24, 25 is free of patterning. The double-face PWB 14 is composed of an insulating substrate 26, and a conductive layer 28, 30 attached on either one of opposite surface of the insulating substrate 26. The conductive layer 28, 30 is previously formed in a pattern necessary to realize a desired circuit. The conductive layer 24, 25 has a thickness of about 18 to 35 µm, for example, while the conductive layer 28, 30 has a thickness of about 70 µm, for example. Thus the conductive layer 28, 30 is larger in thickness than the conductive layer 24, 25 for securing reliability of connection between the conductive layers 28, 30 and through-holes after the through-hole forming. The inter-layer adhesive sheet 16, 18 is a sheet commonly called "prepreg". The inter-layer adhesive sheet 16, 18 is formed of an epoxy resin material curable under a predetermined curing condition (i.e., temperature and time). In general, this material is very popular for inter-layer adhesive sheets. A method according to the precharacterizing features is known out of DE-A-33 16 017. A solder metal is provided as the conductive member. By applying heat confronting conductive layers are electrically connected. However, if the applying of heat is repeated to superimpose a plurality of printed-wiring-boards, damages or breakages at the connecting portion may occur. SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a method of manufacturing multilayered PWBs at a high rate of production of good quality products. According to the invention, there is provided a method of manufacturing a multi-layered printed-wiring-board of the related state of the art, wherein the conductive layer is made out of a conductive material having a first coefficient of thermal expansion, the conductive layer is made out of a conductive material having a second coefficient of thermal expansion, which is different to the first coefficient of thermal expansion, and applying an intermediate layer made out of a conductive material buffering thermal stress resulting from the difference between the first and the second coefficient of thermal expansion over the surface of said conductive layer before attaching said conductive layer. In the method of this invention, an electrical connecting layer connecting the conductive layers of a plurality of PWBs by the conductive member attached on a partial surface of the conductive layer of the individual PWB. It is therefore possible to avoid any defects, such as inadequate smear, inadequate penetration, etc., resulting from the forming of through-holes. Also, it is needless to make the thickness of the individual inner conductive layer larger than that of the individual outer conductive layer. Further, the outwardly facing conductive layer of the outmost PWB may be previously processed by patterning. In addition, it is possible to attach the conductive member within a desired section with improved precision, thus causing an improved pattern accuracy and a high concentration of pattern. The PWB may be either a single-face PWB or a double-face PWB. If a buffer layer is attached over the surface of the conductive layer, a thermal stress acting on the conductive layer is reduced. Also it is possible to install circuit elements on the conductive layer. Further, the conductive member may be either a solder material, e.g., a solder paste having a predetermined melting point or a conductive resin having a predetermined curing temperature. In the former case, the solder paste is attached on the conductive layer such as by printing and subsequent reflowing. In the latter case, the conductive resin is attached on the conductive layer such as by potting. When using a solder paste as the conductive member in the method, the melting point of the solder paste is preferably lower than the melting point of spacer pieces. In this case, the step of attaching the conductive member may include a step of mixing the spacer pieces in the conductive member at a mixture ratio depending on the amount of thermal shrinkage of the conductive member. The connecting step may include a pressurizing step in which a plurality of the superimposed PWBs are clamped in the direction of superimposing, and in which a predetermined pressure in the direction of superimposing is applied to the superimposed PWBs to bring the conductive member in contact with the confronting conductive layer or the confronting conductive member. If a insulating member is used, the insulating member is brought in contact with the confronting conductive layer or the confronting insulating member during the pressurizing step. Further, the connecting step may also include a heating step in which a predetermined temperature of heat is applied to the superimposed PWBs to fix the conductive member to the conductive layer or the conductive member. The heating step is performed by applying ultrasonic waves, in which case an oxidized layer at the border of the conductive layer and the conductive member is dispersed, and as a result, the conductive layer and the conductive member jointly constitute an alloy or diffuse together to connect with each other firmly. The heating step may be conducted concurrently with the pressurizing step. The insulating member may be an insulating adhesive resin having a predetermined curing temperature. In this case, the attaching of the insulating member to the conductive layer may be performed by printing. Most preferably, the conductive member may be a solder paste having a melting point higher than a curing temperature of the insulating adhesive resin. The shape of the individual spacer piece may be a sphere and should by no means be limited to a specific shape. The material of the spacer pieces is preferably metal such as copper, or ceramic. The above and other advantages, features and additional objects of this invention will be manifest to those versed in the art upon making reference to the following detailed description and the accompanying drawings in which several embodiments incorporating the principles of this invention are shown by way of illustrative example. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of components for manufacturing a multilayered PWB; FIG. 2 is a cross-sectional view of the components of FIG. 1, showing them after having been processed by pressurizing and heating; FIG. 3A is a cross-sectional view of components for manufacturing a multilayered PWB in the inventive method according to an embodiment; FIG. 3B is an enlarged cross-sectional view showing the surface of a conductive layer in the embodiment of FIG. 3A; FIG. 4 is a cross-sectional view of components for manufacturing a multilayered PCB by adapting the inventive method FIG. 5 is a cross-sectional view of components for manufacturing a multilayered PWB in a prior art method; DETAILED DESCRIPTION FIGS. 1 and 2 show various components for manufacturing a multilayered PWB in the known method. In this embodiment, two double-face PWB 110, 112 and four conductive layers 124, 128, 130, 126 are all previously processed by patterning. Each of the conductive layers 124, 128, 130, 126 is a copper foil having a thickness of about 18 - 35 µm, for example. Each of the double-face PWB 110, 112 has a through-hole 138 at a predetermined position, the inner surface of the through hole 138 being plated. The through-hole 138 electrically connects front and rear conductive layers 124, 128 or 130, 126 together. A solder paste is printed on the conductive layers 128, 130 of the double-face PWBs 110, 112 at predetermined positions, or solder bumps 136a, 136b are formed by solder reflow. Then, a polymer resin 140a, 140b is printed on the surface of the conductive layer 128, 130 at portions devoid of the solder bump 136. A binder contained in the polymer resin 140a, 140b is discharged into the atmosphere so that the polymer resin 140a, 140b is precured. As shown in FIG. 1, the foregoing components are placed one over another. Specifically, after the polymer resin 140a, 140b have been precured, the two double-face PWBs 110, 112 are placed one over another in such a manner that the solder bumps 136a on one double-face PWB 110 confront the solder bumps 136b on the other double-face PWB 112. Subsequently, these two double-face PWBs 110, 112 are processed by pressurizing and heating concurrently. For example, the double-face PWBs 110, 112 are clamped between a pair of flat plates in the direction of superimposing, under pressure and heat. The flat plates are stainless steel, for example. This pressurizing and heating are conducted separately at first and second stages. At the first stage, vapor-phase soldering is conducted at a temperature, e.g., 220°C, higher than the melting point of the solder bumps 136a, 136b for two minutes. In this first stage, the solder bumps 136a, 136b are fused to form fused solder bumps 142 (FIG. 2) which electrically connect the conductive layers 128, 130 with one another. Meanwhile, the polymer resins 140a, 140b are brought in contact with one another due to the reduced viscosity resulting from the temperature rise and also due to pressure. This first stage is followed by the second stage, in which the polymer resins 140a, 140b are cured by heating at 150°C for about one hour. As a result, an insulating 144 (FIG. 2) has been formed to insulate the conductive layers 128, 130 from one another. From the foregoing steps, a composite PWB having four conductive layers, namely, a four-layered PWB can be obtained, as shown in FIG. 2. In this first embodiment, after the two double-face PWBs 110, 112 have been superimposed into a unitary form, it is unnecessary to form any through-hole, and the PWBs 110, 112 may be previously processed by patterning. Therefore, it is possible to prevent various inferiorities, such as inadequate smear, inadequate penetration of plating liquid, etc., resulting from the forming of through-holes. Also other inferiorities due to permeation of the liquid for patterning can be prevented. In addition, it is possible to use, as the individual conductive layer, a copper foil having a small thickness. Therefore high-concentration patterning and an improved degree of circuit integration can be achieved. A green sheet may be used instead of the polymer resin 140a, 140b. The solder paste may be a high-temperature or low-temperature paste now put on the market, or may be a eutectic solder. FIGS. 3A and 3B show various components for manufacturing a multilayered PWB in the inventive method by example of an embodiment. In this embodiment, like the embodiment of FIGS. 1 and 2, the forming of solder bumps, the attaching of polymer resins, the superimposing of the components, and the pressurizing and heating are carried out. As a result, a multilayered PWB (six-layered PWB) having a cross section shown in FIG. 3A has been obtained. Further, the conductive layers are previously plated with platinum. As shown in FIG. 3B, the surfaces of the conductive layers 628, 630 are provided with platinum layers 654, 656, respectively, having a thickness of about 0.5 µm by electroplating. Therefore the solder bumps are formed on the platinum layers 654, 656. In this embodiment, the same results as to the known method can be obtained. Additionally, it is possible to prevent any circuit pattern from being ruptured due to the heat cycle, for example. Since practically a PWB and a solder bump are different in coefficient of thermal expansion, the conductive layer would be subjected to a stress due to heat cycle, etc. In this embodiment, any stress exerted on the conductive layers 628, 630 is buffered by the platinum layers 654, 656 to thereby prevent any circuit pattern from being ruptured due to such a stress. In the illustrated embodiments, the conductive layers are plated with platinum; alternatively, palladium may be used instead of platinum for the same results. Further, in the foregoing embodiments, the multilayered PWB has a relatively small number of layers. But it may have a larger number of layers. The foregoing description concerning the multilayered PWB may be said for a multilayered PCB. For example, as shown in FIG. 4, it is possible to manufacture a multilayered PCB in which circuit elements 758, 760 have been installed on or in the superimposed layers in the same method as the embodiment according to the inventive method.

Claims: A method of manufacturing a multilayered printed-wiring-board, comprising the steps of a. attaching a conductive member on a partial surface of a conductive layer formed on at least one of opposite surfaces of a printed-wiring-board in a pattern b. superimposing a plurality of printed-wiring-boards one over another in such a manner that said conductive member formed and attached on said conductive layer on one of said printed-wiring-boards confronts either said conductive layer or said conductive member on another printed-wiring-board, at least one of said plurality of printed-wiring-boards having been processed by said attaching step, and c. after said superimposing step, electrically connecting said conductive member to either said confronting conductive layer or said confronting conductive member to form an electrical connecting layer, characterized by, that the conductive layer is made out of a conductive material having a first coefficient of thermal expansion, that the conductive member is made out of a conductive material having a second coefficient of thermal expansion, which is different to the first coefficient of thermal expansion, and applying an intermediate layer made out of a conductive material buffering thermal stress resulting from the difference between the first and the second coefficient of thermal expansion over the surface of said conductive layer before attaching said conductive member. A method according to claim 1, wherein at least said attaching step, electronic components are put on the surface of said conductive layer. A method according to claim 1, wherein at least one of said plurality of printed-wiring-boards is a double-face printed-wiring-board having on either one of opposite surfaces said conductive layer in pattern. A method according to claim 1, wherein said conductive member includes a solder paste having a predetermined melting point, said attaching of said conductive member onto said conductive layer being accomplished by printing and reflowing. A method according to claim 1, wherein said conductive member includes a conductive resin having a predetermining temperature of curing, said attaching of said conductive member onto said conductive layer being accomplished by potting. A method according to claim 1, wherein said connecting step includes a pressurizing step composed of clamping said superimposed printed-wiring-boards in the direction of said superimposing, and applying to said superimposed printed-wiring-boards a predetermined pressure in the direction of said superimposing to bring said conductive member in contact with either said confronting conductive layer or said confronting conductive member. A method according to claim 1, wherein said connecting step includes a heating step of applying a predetermined temperature of heat to said superimposed printed-wiring-boards to fix said conductive member to either said confronting conductive layer or said confronting conductive member. A method according to claim 1, wherein said heating step is accomplished by applying ultrasonic waves. A method according to claim 6, wherein said connecting step further includes a heating step of applying a predetermined temperature of heat to said superimposed printed-wiring-boards to fix said conductive member to either said confronting conductive layer or said confronting conductive member, said heating step being conducted concurrently with said pressurizing step. A method according to anyone of the claims 1 to 9, wherein said intermediate layer has a thickness of about 0.5 µm. A method according to anyone of the claims 1 to 10, wherein said intermediate layer is electroplated onto the surface of the conductive layers. A method according to anyone of the claims 1 to 11, wherein said intermediate layer is made out of platinum or palladium.

Inventors: EIMURA TAKESHI C O JAPAN RADIO, KINOSHITA MASAKI C O JAPAN RAD, OHIWA TAKAO C O JAPAN RADIO CO, TAKAHASHI EIKI C O JAPAN RADIO, TESHIGAWARA OSAMU C O JAPAN RA, YAMASHITA KAZUO C O JAPAN RADI, EIMURA, TAKESHI C/O JAPAN RADIO CO., LTD., KINOSHITA, MASAKI C/O JAPAN RADIO CO., LTD., OHIWA, TAKAO C/O JAPAN RADIO CO., LTD., TAKAHASHI, EIKI C/O JAPAN RADIO CO., LTD., TESHIGAWARA, OSAMU C/O JAPAN RADIO CO., LTD., YAMASHITA, KAZUO C/O JAPAN RADIO CO., LTD.
Applicants: JAPAN RADIO CO LTD, JAPAN RADIO CO., LTD
Application year: 1989
Main classification: H05K3
IPC codes: H05K1, H05K3