Method of manufacturing multilayered printed-wiring-board

Abstract: A method of manufacturing a multilayered printed-wiring-board comprises attaching conductive members on conductive layers, superimposing a plurality of printed-wiring-boards, and electrically connecting the conductive layers with one another by the conductive members. A unitary insulating is provided between the conductive layers.

Description: This invention relates to a method of manufacturing a multilayered printed-wiring-board (hereinafter called "PWB") according to the preamble of claim 1. Such a method is disclosed in the US 3,764,436. According to this known method an insulating sheet is disposed and sandwiched between the confronting conducting layers during the superimposing step. 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 multilayered printed-circuitboard (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. FIGS. 10 through 12 are cross-sectional views showing progressive manufacturing steps of a multilayered PWB according to the typical 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. 10, 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 the through-holes after the through-hole forming described below. 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. In production, first of all, these components are placed one over another. For example, as shown in FIG. 10, the single-face PWB 10, the inter-layer adhesive sheet 16, the double-face PWB 14, the inter-layer adhesive sheet 18, and the single-face PWB 12 are placed one over another in this order. Further, the conductive layer 24 of the single-face PWB 10 faces upwardly, while the conductive layer 25 of the other single-face PWB 12 faces downwardly. Namely, these two single-face PWBs 10, 12 are placed in such a manner that their respective conductive layers 24, 25 face outwardly in opposite directions. In this superimposed position, all of the superimposed components are heated at a predetermined temperature for a predetermined time, with applying a pressure in the direction of superimposing. At that time the adjacent superimposed parts come into contact with each other. For example, all of the superimposed components are clamped between a pair of flat stainless plates in the direction of superimposing, and are pressed by a pressure of 30 kgf/cm 2 . At that time, they are heated at a temperature higher than a curing temperature of the inter-layer adhesive sheet 16, 18, e.g., 170°C, for 90 minutes. Under this pressure and heat, the inter-layer adhesive sheets 16, 18 are cured to form a pair of insulating layers 32, 34 covering the conductive layers 28, 30 of the double-face PWB 14. In addition, the single-face PWBs 10, 12 are adhered one to each surface of the double-face PWB 14 by the respective insulating layers 32, 34. Then, sub-through-holes 36 are formed through the superimposed components at predetermined positions in the direction of superimposing, as shown in FIG. 11. As the inner surface of the individual sub-through-hole 36 is plated to extract copper, the conductive layers 24, 28, 30, 25 are electrically connected at portions where the sub-through-holes 36 are formed, as shown in FIG. 12. That is, the conductive layers 24, 28, 30, 25 are electrically connected by through-holes 38 having an inner surface coated with copper. Subsequently, the conductive layers 24, 25 are processed by patterning. If the patterning was done before forming the sub-through-holes 36, an accurate pattern could not have been achieved. This is the reason why the patterning is done after having formed the through-holes. As a result, a multilayered PWB has been completed in which a desired circuit is formed as patterns of the conductive layers 24, 28, 30, 25. In this conventional case, there are four conductive layers, which thus constitute a four-layered PWB. Thus in the conventional art, a plurality of PWBs are superimposed into a unitary form, and then the conductive layers of the PWBs are electrically connected by through-holes. However, if the sub-through-holes are formed after the PWBs have been superimposed into a unitary form, good smearing is difficult to achieve. Further, in the foregoing manufacturing method, after having formed the sub-through-holes, it is necessary to take a wet processing such as plating and patterning. But if a liquid penetrates into the multilayered PWB during the wet processing, it would result in various troubles such as oxidation of the conductive layers, oxidation of the conductive layers and short-circuit between the patterns. With the foregoing conventional methods, only a limited rate of production of good quality multilayered PWBs can be achieved. The same can be said of multilayered PCBs. It is an object of this invention to provide a method of manufacturing multilayered PWBs at a high rate of production of good quality products, having a great stability after superimposing the PWBs and electrically connecting conductive layers on PWBs. This object is solved by the characterizing features of claim 1. According to a first aspect of this invention, there is provided a method of manufacturing a multi-layered 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 the conductive member formed and attached on the conductive layer on one of the printed-wiring-boards confronts either the conductive layer or the conductive member on another printed-wiring-boards, at least one of the plurality of printed-wiring-boards having been processed by the attaching step; and (c) after the superimposing step, electrically connecting the conductive member to either the confronting conductive layer or the confronting conductive member to form an electrical connecting layer. According to a second aspect of this invention, there is provided 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) attaching an electrically insulating member on a partial surface of the conductive layer at at least a conductive-member-free portion thereof; (c) superimposing a plurality of printed-wiring-boards one over another in such a manner that the conductive member attached on the conductive layer on one of the printed-wiring-boards confronts either the conductive layer or the conductive member on another printed-wiring-boards, at least one of the plurality of printed-wiring-boards having been processed by the first-named attaching step and the second-named attaching step; and (d) after the superimposing step, electrically connecting the conductive member to either the confronting conductive layer or the confronting conductive member to form an electrical connecting 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. The same results can be obtained according to the second aspect. An additional result with the method is that insulation between the conductive layers can be achieved at desired positions. 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. 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 the 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 BWBs 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 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. FIG. 1 is a cross-sectional view of components for manufacturing a multilayered PWB in a method according to a first embodiment of this invention; FIG. 2 is a cross-sectional view of the components of FIG. 1, showing them after having been processed by pressurizing and heating; FIG. 3 is a cross-sectional view of components for manufacturing a multilayered PWB in a method according to another embodiment; FIG. 4 is a cross-sectional view of the components of FIG. 3, showing them after having been processed by pressurizing and heating; FIG. 5 is a cross-sectional view of components for manufacturing a multilayered PWB in a method according to a further embodiment; FIG. 6 is an enlarged cross-sectional view showing the junction between a solder bump and a conductive layer in the embodiment of Fig. 5; FIG. 7 is a cross-sectional view of components for manufacturing a multilayered PWB in a method according to another embodiment; FIG. 8 is an enlarged cross-sectional view showing the surface of a conductive layer in the embodiment of Fig. 7; FIG. 9 is a cross-sectional view of components for manufacturing a multilayered PCB by adapting the method; FIG. 10 is a cross-sectional view of components for manufacturing a multilayered PWB in a prior art method; FIG. 11 is a cross-sectional view of the components of FIG. 10, showing them after having been processed by pressurizing and heating and also by forming through-holes; and FIG. 12 is a cross-sectional view of the components of FIG. 11, showing them after the through-holes have been plated. FIGS. 1 and 2 show various components for manufacturing a multilayered PWB in a method according to a first embodiment of this invention. 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, a unitary 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 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. Consequently, an improved rate of production of good quality products can be achieved, thus minimizing the occurrences of manufacturing troubles. In addition, it is possible to use, as the individual conductive layer, a copper foil having a thickness smaller than the prior art. 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. 3 and 4 show various components for manufacturing a multilayered PWB in a method according to another embodiment. In this embodiment, like the first embodiment, solder bumps 436a, 436b are formed on conductive layers 428, 430 of double-face PWBs 420, 422 at predetermined positions, and polymer resins 440a, 440b are attached on the double-face PWBs 420, 422, whereupon these components are placed one over another. In FIG. 3, the components are shown after the polymer resins 440a, 440b have been attached on the double-face PWBs 420, 422 and also after these double-face PWBs 420, 422 have been placed one over another. As is seen from FIG. 3, there are following differences, concerning the attaching of polymer resin, between first and fourth embodiments. In the first embodiment, the polymer resins 140a, 140b are attached on the double-face PWBs 120, 122 so as not to lie over the solder bumps 136a, 136b. In this embodiment, the polymer resins 440a, 440b are attached on the double-face PWBs 420, 422 so as to lie over the solder bumps 436a, 436b; the polymer resins 440a, 440b should be a resin having a curing temperature lower than the melting point of the solder bumps 436a, 436b. When the components of FIG. 3 are pressed in the superimposing direction by a predetermined pressure and at the same time, are heated at a temperature higher than the melting point of at least the solder bumps 436a, 436b, the solder bumps 436a, 436b are fused except the polymer resins 440a, 440b as the curing temperature of the polymer resins 440a, 440b is lower than the melting point of the solder bumps 436a, 436b. As a result, the fused solder bumps 442 have been formed to electrically connect the conductive layers 428, 430. At that time, the polymer resins 440a, 440b are in contact with one another as pressurized. When the components are heated at the curing temperature of the polymer resins 440a, 440b, the polymer resins 440a, 440b is cured into a unitary form to form a unified insulating layer 444. As the foregoing steps are thus conducted, a four-layered PWB of FIG. 4 has been manufactured. In this embodiment, the same results as the first embodiment can be obtained. Additionally, in this embodiment, the attaching of the polymer resins 440a, 440b by printing can be conducted without considering the portions to which the solder bumps 436a, 436b are attached. Therefore, with this embodiment, it is possible to manufacture a multilayered PWB easily, compared with the first embodiment. FIGS. 5 and 6 schematically show various components for manufacturing a multilayered PWB in a method according to another embodiment. In this embodiment, firstly a via hole 538 is formed in a double-face PWB 520, and the inner surface of the via hole 538 is plated. On the conductive layer 530 of another PWB 522, the solder bump containing 2% of silver, 62% of tin and 36% of lead and having a melting point of 179° is attached. Yet the double-face PWB 520 and the PWB 522 are placed one over another in such a manner that this solder bump confronts the via hole 538. Then the via hole 538 and the solder bump are electrically connected. In this embodiment, following steps are adopted to electrically connect the via hole 538 and the solder bump. Ultrasonic waves and a predetermined pressure are applied over the double-face PWB 520 and the PWB 522 in the superimposing direction indicated by arrows in FIG. 5. At that time the ultrasonic waves causes the temperature of the solder bump to rise. As the temperature of the solder bump increases over its melting point, the solder bump fuses to form a fused solder bump 542. Accordingly the conductive layer 530 and the plated inner surface of the via hole 538 confronting the conductive layer 530 are electrically connected by the fused solder bump 542, whereupon application of the ultrasonic waves is stopped. As a result, a multilayered PWB electrically connected by the fused solder bump 542 as shown in FIG. 5 has been obtained. In this embodiment, the same results as the first embodiment can be obtained. Additionally, in this embodiment, oxides 548 having formed on the conductive layer 528, for example, due to the heat is dispersed more than directly heating, as shown in FIG. 6. Therefore, the connection between the fused solder bump 542 and the conductive layer 528 is firm, thus causing a multilayered PWB which is resistant to thermal shock and low in impedance between the substrates. An auxiliary direct heating concurrent with application of the ultrasonic waves also causes the same results. FIGS. 7 and 8 show various components for manufacturing a multilayered PWB in a method according to a further embodiment. In this embodiment, like the first embodiment, the forming of solder bumps, the attaching of polymer resins, the superimposing of the components, and the pressurizing and heating are conducted. As a result, a multilayered PWB (six-layered PWB) having a cross section shown in FIG. 7 has been obtained. Further, the conductive layers are previously plated with platinum. As shown in FIG. 8, 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 the first embodiment 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. 9, 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 sixth embodiment.

Claims: A method of manufacturing a multilayered printed-wiring-board out of a plurality of printed-wiring-boards (120, 122; 420, 422) each having a conductive layer (124, 126, 128, 130; 424, 426, 428, 430) formed on at least one of their opposite surfaces in a pattern, comprising the steps of: a) attaching a conductive member (136; 436) on a partial surface of at least one conductive layer (128, 130; 428, 430) of at least one of said printed-wiring-boards (120, 122; 420, 422); b) disposing an electrically insulating member between said printed-wiring-boards (120, 122; 420, 422) to be superimposed; c) superimposing said printed-wiring-boards (120, 122; 420, 422) one over another in such a manner that said conductive member (136; 436) confronts either said conductive layer (128, 130; 428, 430) or said conductive member (136; 436) of another printed-wiring-board (120, 122; 420, 422); d) electrically connecting said conductive member (136, 436) to either said confronting conductive layer (128, 130; 428, 430) or said confronting conductive member (136; 436) by applying heat and pressure to form an electrical connecting layer; and e) curing said insulating member at a predetermined temperature; characterized in that attaching an insulative adhesive resin (140; 440) as said electrically insulating member on at least the conductive-member-free portion of the confronting surfaces of said conductive layers (128, 130; 428, 430) particularly by printing, precuring said insulative adhesive resin (140; 440) on said conductive layers (128, 130; 428, 430) before carrying out step c, and forming a unitary insulating (144; 444) between said conductive layer (128, 130; 428, 430) by curing said insulating adhesive resin (140; 440) during step e). A method according to claim 1, wherein said step of attaching said insulating member is accomplished by printing. A method according to claim 1 or 2, wherein said connecting step includes a pressurizing step composed of clamping said superimposed printed-wiring-boards (120, 122; 420, 422) in the direction of said superimposing, and applying to said superimposed printed-wiring-boards (120, 122; 420, 422) a predetermined pressure in the direction of said superimposing to bring said conductive member (136; 436) in contact with either said confronting conductive layer (128, 130; 428, 430) or said confronting conductive member (136; 436) and also to bring said electrically insulating member in contact with either said confronting conductive layer (128, 130; 428, 430) or said confronting electrically insulating member. A method according to anyone of the claims 1 to 3, wherein said connecting step includes a heating step of applying a predetermined temperature of heat to said super-imposed printed-wiring-boards (120, 122; 420, 422) to fix said conductive member (136; 436) to either said confronting conductive layer (128, 130; 428, 430) or said confronting conductive member (136; 436) and also to fix said electrically insulating member to either said confronting conductive layer (128, 130; 428, 430) or said confronting electrically insulating member. A method according to anyone of the claims 1 to 4, wherein said electrically insulating member has a curing temperature lower than a melting point of said conductive member (136; 436) said second-named attaching step being such that said electrically insulating member is attached to the partial surface of said conductive layer (128, 130; 428, 430) so as to cover said conductive member (136; 436).

Inventors: EIMURA TAKESHI, KINOSHITA MASAKI, OHIWA TAKAO, TAKAHASHI EIKI, TESHIGAWARA OSAMU, YAMASHITA KAZUO, EIMURA, TAKESHI, KINOSHITA, MASAKI, OHIWA, TAKAO, TAKAHASHI, EIKI, TESHIGAWARA, OSAMU, YAMASHITA, KAZUO, 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: H05K3, H05K1