Method of manufacturing multilayer printed wiring board

Abstract: Apply photosensitive resin to an internal conductive pattern layer 2 as well as to an inside layer which has a hole 3a filled with conductive paste 3b. Form a hole 4a communicating to the internal conductive pattern layer 2 by exposure and development. Then, form metal plated film 6a on the surface of the photosensitive resin 4 including the hole 4a. Process this metal plated film 6a into an external conductive pattern layer 6. This manufacturing method can increase the wiring density on the external conductive pattern layer 6, and thus the multilayer printed wiring board of the higher wiring density can be obtained.

Description: Field of the Invention The present invention relates to a multilayer printed wiring board (hereinafter referred to PWB) extensively used in various electronics products for consumer-use as well as industrial-use. Background of the Invention Recently, a personal computer, word-processor, video movie camera, portable telephone, etc. have been widely used, which boosts the demand for the multilayer PWB. Those electronics products require a compact size, light weight, and multi-function, and thus a more capacious accommodation for wiring as well as a higher density of surface mounting is required to the multilayer PWB. In order to satisfy this request, the multilayer PWB employing IVH (interstitial via hole) coupling method was developed. This method uses a via hole (non-through hole) to make electric contact. Typical examples of IVH coupling method are as follows: (1) a resin-made multilayer PWB in which an IVH filled with conductive paste couples all conductive pattern layers, (2) a resin-made multilayer PWB in which photosensitive resin is used as an external layer, and a metal plated IVH is used in a provided hole on the external layer so that an internal conductive pattern layer makes contact with an external conductive pattern layer. A method of manufacturing a conventional multilayer PWB is described hereafter by referring to the attached drawings. Figs. 2(a), 2(b) and 2(c) depict a method of manufacturing the multilayer PWB in which the IVH filled with the conductive paste couples all conductive pattern layers. First, as shown in Fig. 2(a), punch a hole 13c on a laminated board made of aramid nonwoven fabric which is impregnated with epoxy resin, second, fill the hole with conductive paste 13d, whereby an insulating substrate 13b is completed. Next, paste a copper foil for copper-clad laminate on both faces of the insulating substrate 13b, and then process this copper foil into an internal conductive pattern layer 13a by using a printing method or a photographic method. An inside layer 13, in which the conductive pattern layer 13a on both faces thereof is formed, is thus produced. Prepare a copper foil 11 normally used for copper-clad laminate and a pre-preg 12. The pre-preg 12 is made of aramid nonwoven fabric with a punched hole in which conductive paste 12b is filled. Then, as shown in Fig. 2(b), laminate the pre-preg 12 on both sides of the inside layer 13, and the copper foil 11 on top of the pre-preg 12. Apply heat and press to the laminated body thus produced with a heating press machine so that the resin is melted, cooled, and solidified. The copper foil 11 is processed to an external conductive pattern layer 14a through the printing method or photographic method. The multilayer PWB 14 shown in Fig. 2(c) is thus manufactured. Figs. 3(a), 3(b) and 3(c) depict a method of manufacturing the multilayer PWB in which the metal plated IVH is used. First, as shown in Fig. 3(a), by using the printing method or a photographic method, produce an inside layer 23 having an internal conductive pattern layer 23a which comprises a copper foil normally used for copper-clad laminate on both sides of an insulating substrate 23b. Then, as shown in Fig. 3(b), form a photosensitive resin film 22 on both sides of the insulating substrate 23b. Next, form a hole communicating to the internal conductive pattern layer 23a on the photosensitive resin film 22 by exposure and development. Further, drill a through hole where conductivity between the internal conductive pattern layers 23a on both sides of the inside layer 23 is required. Then, plate an inside wall of the through hole and non-through hole with metal. A metal plated non-through hole 24a (called photo via) and a metal plated through hole 24b are thus formed. Finally, as shown in Fig. 3(c), provide a copper foil 21 on the surface of photosensitive resin film 22, and process the copper foil 21 into an external conductive pattern layer 24c. The internal conductive pattern layer 23a thus makes electrical contact with the external conductive pattern layer 24c through the photo via 24a. Therefore, not only between the internal conductive pattern layers 23a, but also between the external conductive pattern layer 24c and the internal conductive pattern layer 23a, the electrical conductivity are assured, whereby the multilayer PWB 24 is obtained. The conventional multilayer PWB described above, however; has the following restrictions of increasing a wiring density on the external conductive pattern layer for obtaining the parts more densely mounted. In the case of using the IVH filled with conductive paste shown in Fig. 2, the smaller diameter of the hole 12a is required in order to increase the wiring density of the external conductive pattern layer of the pre-preg 12. When the diameter of the hole 12a is narrowed, the small hole must be fully filled with conductive paste in order to avoid losing conductivity. For this purpose, the viscosity of the conductive paste shall be lowered by increasing a quantity of solvent to be added. However, when increasing the solvent quantity, the solvent vapors due to heating in the mounting process, which causes a delamination. Because the hole 12a filled with the conductive paste is shielded, it is not possible to increase the viscosity of the conductive paste by vaporizing the solvent of the first filled conductive paste. Thus, the conductive paste of rather higher viscosity with a little solvent must be used from the first. Due to the above factor, it has been difficult to narrow the diameter of the hole 12a not more than 200 µm. In addition, due to the rather large diameter, the land formed around the hole 12a takes space, which restricts the wiring density on the external conductive pattern layer 14a from being increased. In the case of using the photo via 24a shown in Fig. 3, since the metal plated film is used instead of the conductive paste, a photo via having a smaller hole diameter ranging 50-100 µ can be formed. However, on the multilayer PWB 24, a number of through holes must be formed where conductivity is required between the internal conductive pattern layers 23a on both sides of the inside layer 23. The numerous through holes 24b hamper to design a desirable external conductive pattern layer 24c, and thus restrict the wiring density from being increased. In order to make electrical contact between the internal conductive pattern layers 23a on both sides of the inside layer, the through holes may be punched beforehand on the inside layer 23, however; when applying a photosensitive resin to the inside layer, the through holes must be filled with the photosensitive resin. It is almost impossible to fill the through holes with resin, thus this idea is not practical. Summary of the Invention The present invention aims to provide a method of manufacturing a multilayer PWB having the higher wiring density. A method of manufacturing a multilayer printed wiring board comprising the steps of: (a) forming an insulating substrate including a hole filled with conductive paste, and an internal conductive pattern layer formed on said insulating substrate, the internal conductive pattern layer making electrical contact with said conductive paste, wherein the step of forming said insulating substrate comprises the steps of impregnating thermosetting resin into aramid nonwoven fabric and forming said hole when said thermosetting resin is in semi-setting status; (b) forming a photosensitive resin film on a surface on which said internal conductive pattern layer is formed, and forming a hole on said photosensitive resin film to communicate to said internal conductive pattern layer; (c) forming a conductive member in said hole formed on the photosensitive resin film, said conductive member making electrical contact with said internal conductive pattern layer; and (d) forming an external conductive pattern layer on said photosensitive resin film, said external conductive pattern layer making electrical contact with said conductive member. According to the present invention, the conductive paste filled into the hole on the insulating substrate makes electrical contact between the internal conductive pattern layers on both sides of the inside layer, therefore, through holes are not required. In addition, the conductive member provided in the hole on the photosensitive resin film make electrical contact between the internal conductive pattern layer and the external conductive pattern layer, therefore, the diameter of the hole can be smaller than that of the conventional hole, i.e. the pre-preg hole filled with the conductive paste of the higher viscosity. As a result, a wiring density of the external conductive pattern layer can be remarkably increased, which assures the higher density of surface mounted parts. Brief Description of the Drawings Figs. 1(a), 1(b) and 1(c) are cross sections depicting a method of manufacturing a multilayer PWB used in Embodiment 1 of the present invention. Figs. 2(a), 2(b) and 2(c) are cross sections depicting a conventional method of manufacturing a multilayer PWB. Figs. 3(a), 3(b) and 3(c) are cross sections depicting another conventional method of manufacturing a multilayer PWB. Description of the Preferred Embodiments Embodiments of the present invention are described hereinafter by referring to the attached drawings. Embodiment 1 First, as shown in Fig. 1(a), laminate the aramid nonwoven fabric in which epoxy resin (thermosetting resin) is impregnated, next, make the epoxy resin semi-setting to form a through hole 3a thereon, fill the through hole 3a with conductive paste 3b to complete an insulating substrate 3. Then, dispose a copper foil (for copper-clad laminate) on both sides of the insulating substrate 3 to be bonded by thermo-compression. Process the copper foils into internal conductive pattern layers 2 by the printing method or the photographic method. The inner layer 1 is thus manufactured, i.e. the conductive pattern layers 2 on both sides are connected by the conductive paste 3b. Second, as shown in Fig. 1(b), apply photosensitive resin to both sides of the inside layer 1 to form a photosensitive resin film 4 thereon, next, on the film 4, form a hole 4a communicating to the internal conductive pattern layer 2 by exposure and development, then plate the surface of the photosensitive resin film 4 including the bottom and inside wall of the hole 4a with metal to form metal plated film 6a. Third, as shown in Fig. 1(c), process the metal plated film 6a into the external conductive pattern layer 6 by the printing method or the photographic method. The metal plated hole 4a (photo via 5) thus makes contact between the internal conductive pattern layer 2 and the external conductive pattern layer 6, whereby the multilayer PWB 7 is manufactured. When comparing the multilayer PWB manufactured in Embodiment 1 with the conventional one shown in Fig. 2, the diameter of a land around the photo via 5 on the photosensitive resin film 4 is less than a half that of the conventional one. As a result, the multilayer PWB having the higher wiring density of the external conductive pattern layer can be obtained. When inserting a part having a lead wire besides a surface mounting chip, punch a hole for the part, and provide a photo via as well as plate the hole with metal to form a through hole. Thus, providing the through hole in addition to the photo via makes it possible to accommodate the part having a lead wire. In Embodiment 1, the external conductive pattern layer 6 is formed by the metal plated film 6a, however; instead of the metal plated film 6a, it can be formed by a conductive foil pasted onto the photosensitive resin film 4 although an additional step of pasting the conductive foil is required. Embodiment 2 A method of manufacturing to be described in Embodiment 2 differs from that in Embodiment 1 in the following point: Before providing metal plating, another step is added, i.e. fill the hole 4a on the photosensitive resin film 4 with conductive paste of the lower viscosity. Because the other steps are the same, detail description thereof is omitted. Produce the inside layer 1 through the same steps of Embodiment 1, then apply the photosensitive resin 4 thereto and form the hole 4a. Fill the hole 4a with the conductive paste of rather low viscosity which is adjusted appropriately by adding solvent. After reducing the solvent, plate the surface of the photosensitive resin film 4 with metal. Finally, process the metal plated film into the external conductive pattern layer, as described in Embodiment 1. In Embodiment 2, since the conductive paste of the lower viscosity is used, the smaller photo via can be filled therewith. Since the metal plated film is formed on the photosensitive resin film after filling the photo via with the conductive paste, the surface of the metal plated film can be flat. As a result, the higher density of wiring on the less uneven external conductive pattern layer can be obtained. Embodiment 3 A method of manufacturing to be described in Embodiment 3 differs from that in Embodiment 1 in the following point: Instead of providing metal plating as described in Embodiment 1, apply conductive paste of the lower viscosity to the surface of the photosensitive resin film 4 including the hole 4a, and at the same time, form the external conductive pattern layer. Because the other steps are the same as Embodiment 1, the detail description thereof is omitted. First, produce the inside layer 1 through the same steps as defined in Embodiment 1, and apply the photosensitive resin 4 on the inside layer 1 to form the hole 4a. Then, print the external conductive pattern layer onto the surface of the photosensitive resin film 4 by using screen printing technique. This conductive pattern comprises conductive paste of rather low viscosity which is appropriately adjusted by adding solvent. The hole 4a is filled with this conductive paste at the same time. The amount of the solvent is then reduced. In this case, since the conductive paste of the lower viscosity is employed, a higher wiring density can be realized. Further, because the external conductive pattern layer has undergone a screen process printing, and is formed thus directly on the photosensitive resin surface, manufacturing process can be simplified. The present invention is not limited within the scope of the above embodiments, but it is obvious that there are various modifications. For instance, it has been described hitherto that the multilayer PWB has two internal conductive pattern layers, however; the present invention can be applicable to a case where the PWB has three internal conductive pattern layers or more. Therefore, various modifications may be made within the scope and range of equivalents of the claims and without departing from the spirit of the invention.

Claims: A method of manufacturing a multilayer printed wiring board comprising the steps of: (a) forming an insulating substrate including a hole filled with conductive paste, and an internal conductive pattern layer formed on said insulating substrate, the internal conductive pattern layer making electrical contact with said conductive paste, wherein the step of forming said insulating substrate comprises the steps of impregnating thermosetting resin into aramid nonwoven fabric and forming said hole when said thermosetting resin is in semi-setting status; (b) forming a photosensitive resin film on a surface on which said internal conductive pattern layer is formed, and forming a hole on said photosensitive resin film to communicate to said internal conductive pattern layer; (c) forming a conductive member in said hole formed on the photosensitive resin film, said conductive member making electrical contact with said internal conductive pattern layer; and (d) forming an external conductive pattern layer on said photosensitive resin film, said external conductive pattern layer making electrical contact with said conductive member. The method of manufacturing the multilayer printed wiring board as defined in Claim 1, wherein the step (c) comprises a step of forming a metal plated film. The method of manufacturing the multilayer printed wiring board as defined in Claim 2, further comprising the steps of: (e) forming a through hole running through said insulating substrate, said internal conductive pattern layer, said photosensitive resin, and said external conductive pattern layer, and (f) plating said through hole with metal. The method of manufacturing the multilayer printed wiring board as defined in Claim 2, wherein the step (d) comprises a step of forming a metal plated film. The method of manufacturing the multilayer printed wiring board as defined in Claim 1, wherein the step (c) comprises a step of forming the conductive member by using conductive paste. The method of manufacturing the multilayer printed wiring board as defined in Claim 5, wherein the step (d) comprises a step of forming the external conductive pattern layer by using conductive paste. The method of manufacturing the multilayer printed wiring board as defined in Claim 1, wherein the step of forming said insulating substrate comprising the steps of: (a-1) laminating a conductive foil onto both sides of the insulating substrate, and bonding the same thereto by thermo compression bonding method, and (a-2) processing said conductive foil on both sides of the insulating substrate into said internal conductive pattern layer. The method of manufacturing the multilayer printed wiring board as defined in Claim 1, wherein the step (b) forming a hole on said photosensitive resin film comprising the step of: (b-1) exposing said photosensitive resin film in a state of semi-setting.

Inventors: TACHIBANA MASASHI, TACHIBANA, MASASHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H05K3
IPC codes: H05K3