Method for manufacturing printed-circuit board

A method of manufacturing a PCB comprising the steps of: forming through-holes in a substrate having releasing layers on front and back faces; filling conductive paste in the through-holes; removing the releasing layers and disposing metal foil on both faces of the substrate; and heat pressing them. A diameter of the through-holes is larger than that of corresponding holes formed on the releasing layers. According to the present invention, when the conductive paste is compressed, conductive paste protruding from the surface of the substrate is trapped at the edges of the through-holes. This configuration prevents short circuits with undesirable wiring patterns. So, an enough amount of the conductive paste can protrude from the surface of the substrate. Therefore, after the compression, stable electric connections inside the conductive paste and between the conductive paste and the metal foils are ensured, thus PCBs with superior reliability can be produced.

FIELD OF THE INVENTION

The present invention relates to a printed circuit board to be used for electronic devices.

BACKGROUND OF THE INVENTION

Electronic devices of recent years have been increasingly miniaturized, reduced in weight and advanced in functionality. Against this backdrop, the trend toward multi-layered, high-density printed circuit boards (hereinafter, PCB) has been gathering its momentum. A PCB is typically produced by alternately stacking a plurality of substrates with conductive circuits formed thereon and prepreg sheets (so-called “prepreg”), bonding them under heat and pressure, forming through-holes, and plating them with copper or other metallic materials to provide an electrical connection between the surface and inner layers.

Due to an intensified demand for video cameras and mobile telecommunication devices, however, PCBs used for such devices have been ever more strongly required to be lighter, thinner and high density. In response to such demand, a manufacturing method for a new kind of PCB disclosed in Japanese Patent No. 2601128 has been put into practical application. The manufacturing method of this PCB comprises the steps of:1. forming through-holes on a porous compressible substrate sandwiched between releasing films;2. filling the through-holes with conductive paste;3. removing the releasing films;4. placing metal foils on both sides of the porous substrate and compress the laminate under heat to provide an electrical connection; and5. etching the metal foils to form circuitry.

The configuration of the foregoing conventional PCB is described with reference to the attached drawings. FIGS.8(a)-(g) are sectional view showing the manufacturing process of the conventional PCB. As FIG.8(a) shows, a porous substrate (hereinafter, prepreg)11whose dimensions are 500 mm square and T1mm in thickness, sandwiched between releasing films12, is prepared. As the prepreg11, a composite material produced by impregnating thermosetting epoxy resin into non-woven fabric made of aromatic polyamide fiber is used.

Successively, as FIG.8(b) shows, through-holes13are formed at the predetermined places of the prepreg11by a laser machining method.

The prepreg11is then placed on a table of a printer (not illustrated), and conductive paste14is printed on the releasing films12to fill the through-holes13as FIG.8(c) shows. In this process, the releasing film12coating the top face of the prepreg11, acts as a printing mask and an anti-contamination material for the prepreg11.

Successively, as shown in FIG.8(d), the releasing films12coating both faces of the prepreg11are removed at room temperature. Metal foils15made of copper or other metals are attached to both faces of the prepreg11as shown in FIG.8(e). The laminate is then pressed under heat to adhere the prepreg11and the metal foils15as illustrated in FIG.8(f). Simultaneously, in this process the prepreg11is compressed to T2mm in thickness (T1>T2) to electrically connect the metal foils15on both of its faces with the conductive paste14.

During this step, the epoxy resin composing the prepreg11and the conductive paste14are cured. In order to lower connection resistance between the conductive paste14and the metal foils15, as disclosed in the U.S. Pat. No. 2,587,596, after filling the conductive paste14and removing the releasing films12, the prepreg11is provided with protrusions of the conductive paste on both of its faces (see FIG.8(d)).

If the conductive paste14protrudes from the top and bottom faces of the prepreg11as disclosed in this method, the protrusions allow the conductive paste14to be compressed more and become denser than a method without protrusion when the metal foil15and the prepreg11are pressed under heat. Therefore each of metal powders contained in the conductive paste14contact in larger area with each other and with the metal foils15placed on both front and back faces of the prepreg11. This results in lower resistance in the connection.

Successively, as shown in FIG.8(g), after the metal foils15are provided with patterns by photo lithography, both of the metal foils15on cured faces of the prepreg11are etched to form wiring patterns16.

According to the conventional configuration, however, when compressed, the conductive paste protruding from the prepreg fails to be kept in the through-holes and overflows from them, and connects to other wiring patterns to trigger a short circuit.

The present invention aims to address the foregoing problems, and providing a PCB which achieves a reliable electrical connection between the conductive paste and the metal foils disposed on both faces of the prepreg. The PCB of the present invention allows the conductive paste to be filled into the through-holes properly, and even when the wiring patterns has high density, avoids short circuits triggered by connection between the conductive paste and an undesignated wiring pattern.

DISCLOSURE OF THE INVENTION

The method of manufacturing the PCB of the present invention comprises the steps of:1) forming through-holes in the substrate having releasing layers formed its front and back faces;2) filling conductive paste in the through-holes;3) removing the releasing layers and disposing metal foils on front and back faces of the substrate; and4) curing resin material under pressure and heat to adhere the metal foils and the substrate.

Wherein, a diameter of the through-holes formed on the front and the back faces of the substrate of the PCB is larger than that of corresponding holes formed on the releasing layers.

According to the manufacturing method of the present invention, when the conductive paste is compressed, flow of the conductive paste protruding from the surface of the substrate is trapped at the edges of the through-holes. This configuration prevents short circuits with other wiring patterns from occurring, and allows adequate conductive paste to protrude from the surface of the substrate. Therefore, even after the compression of the substrate, electric connections inside the conductive paste and between the conductive paste and the metal foil are ensured, thus, the present invention provides PCBs with superior reliability.

DESCRIPTION OF THE PREFERRED ENBODIMENT

The preferred embodiments of the present invention are described below with reference toFIGS. 1to7.

The First Preferred Embodiment

FIGS.1(a)-(g) are sectional views illustrating the manufacturing method of PCB in accordance with the first preferred embodiment of the present invention.

First, as shown in FIG.1(a), releasing films2are bonded on to both faces of a prepreg1whose sides are 500 mm each in length and 120 μm in thickness. The prepreg1is a composite material produced by impregnating thermosetting epoxy resin into non-woven fabric made of aromatic polyamide fiber. The releasing films2are produced by forming a silicone releasing layer of about 100 Å in thickness on a polymer film made of PET (polyethylene terephthalate) 19 μm in thickness. In place of PET, PI (polyimide), PEN (polyethylene naphthalate), PPS (polyphenylene sulfide), PP(polypropylen) or PPO (polyphenylene oxide) can be used as the polymer film for the releasing films2. Similarly, as with the prepreg1, a composite material produced by thermosetting epoxy resin impregnated glass fiber can be used.

Second, as shown in FIG.1(b), through-holes3are formed by a laser machining method at predetermined places of the prepreg1. It is important to ensure that the diameter of the through-holes3be larger than the diameter of the holes formed on the releasing films2.

To make the diameter of the through-holes3larger than the diameter of the holes on the releasing films2, gel time of the B stage thermosetting epoxy resin impregnated in the prepreg1is shortened. To be more precise, the gel time is set at between 50 and 150 seconds. This provides the releasing films2formed on both faces of the prepreg1with holes with a diameter suitable to the diameter of the mask during the laser processing. The prepreg1is also provided with desirable holes, since the gel time of the thermosetting epoxy resin impregnated in the prepreg1is short. That is, a heat energy generated during the laser processing is dispersed in the vicinity of the predetermined holes, thus promoting curing of the resin. As resin shrinks during curing, the diameter of holes consequently becomes larger than that of the mask. The non-woven fabric used for the prepreg1is laser processed and provided with holes of a predetermined diameter by the laser processing. Since the non-woven fabric is drawn due to the shrinkage of the resin caused by curing, diameter of holes becomes larger than that of the holes of the mask. If the non-woven fabric is not drawn when resin is cured and shrunken, the diameter of the holes formed on the non-woven cloth may become the same as the diameter of the holes of the mask. However, since the non-woven fabric is porous, when the conductive paste4is filled in the through-holes3in the following process, the conductive paste4penetrates into the spaces of the porous non-woven fabric to occupy a larger area inside the prepreg1than the diameter of the holes formed on the releasing film2. It is important to note that the foregoing effect can be achieved by the use of the releasing film2rather than liquid releasing agent.

It is also critical to ensure that the holes of the releasing film2be located inside the outer edges of the through-holes3. By adjusting relative positions between and shapes of the through-holes3and the releasing film2, the conductive paste4can be prevented from overflowing from the through-holes3during the subsequent heating and pressing process.

For the laser machining, such lasers as carbon dioxide laser, YAG laser and excimer laser can be used.

Third, as FIG.1(c) shows, the prepreg1whose through-holes have already been processed is placed on a table of a printer (not illustrated) and the conductive paste4is directly printed over the releasing film2and filled into the through-holes3. In this process, the top releasing film2acts as a printing mask and an anti-contamination member for the prepreg1. The conductive paste4can be printed from either side of the prepreg1.

Fourth, the releasing film2is removed from the prepreg1. When the releasing film2is removed, the conductive paste4protrudes from the surface of the prepreg1as shown in FIG.1(d).

Successively, as shown in FIG.1(e), metal foils4of 18 μm in thickness made of such materials as copper is attached on both faces of the prepreg1, followed by a compression under heat to adhere the prepreg1and the metal foils5as shown in FIG.1(f). In this process, the prepreg1is compressed to 80 μm in thickness to electrically connect the metal foils5disposed on both faces with the conductive paste4. Simultaneously, during this process, epoxy resin, a component of the prepreg1, and the conductive paste4are cured.

Fifth, as shown in FIG.1(g), the metal foils5on both faces are selectively etched to form wiring patterns6to complete the PCB.

As escribed above, according to the present invention, the diameter of the through-holes3on both faces of the prepreg1is larger than that of the holes of the releasing film2, which are placed inside the edges of the through-holes3. Thus the protrusions of the conductive paste4on both faces of the prepreg1after the releasing film2is removed, become smaller in diameter than the through-holes3, and exist inside the edges of the through-holes3. This configuration prevents the protrusions of the conductive paste4, which are formed to reduce the connection resistance between the metal foil5and the conductive paste4, from overflowing out of the through-holes3when the prepreg1is sandwiched between the metal foils5and compressed. According to the present invention, the conductive paste4is connected only to the part of the metal foil5, which will become the predetermined wiring patterns6by etching in the following process, and will not be connected to other wiring patterns6and cause a short circuit.

Further, according to the present embodiment, by making the diameter of the through-holes smaller than 200 μm, a PCB being superior in reliability and suitable for high density wiring, which has been difficult to produce with conventional manufacturing methods, can be manufactured.

The Second Preferred Embodiment

FIGS.2(a)-(d) show shapes of the through-holes in accordance with the second preferred embodiment of the present invention.

As shown in FIG.2(a), edges of the through-holes3on the front and back faces of the prepreg1, are processed to form border sections3awhich are ring-shaped recesses. The releasing film2is disposed such that it covers the border sections3a. In this state, the through-holes3are filled with the conductive paste4in the same manner as the first preferred embodiment. Following steps are the same as those of the first preferred embodiment 1:1) removing the releasing film2from the prepreg1(see FIG.2(b));2) placing the metal foils5on both faces of the prepreg1(see FIG.2(c)); and3) compress the laminate under heat to electrically connect the metal foils5on both faces with the conductive paste4(see FIG.2(d)). In this process, epoxy resin, a component of the prepreg1, and the conductive paste4are cured.

As thus far described, the border sections3alocated around the borders between the edges of the through-holes3and both faces of the prepreg1, are processed to become ring-shaped recesses. The releasing film2is disposed to cover the border sections3a. Thus the conductive paste4, which protrudes from both faces of the prepreg1after the releasing film2is removed, becomes smaller than the diameter of the through-holes3on front and back faces and steadily stays inside the edges of the through-holes3. This configuration traps the protrusions of the conductive paste4in the border sections3a, and prevents them from undesirably overflowing out of the through-holes3when the prepreg1is sandwiched between the metal foils5and compressed. Consequently, the conductive paste4is connected only to the part of the metal foil5, which will become the predetermined wiring patterns6by etching, and will not be connected to other wiring patterns6and cause a short circuit.

The Third Preferred Embodiment

FIGS.3(a)-(d) show shapes of the through-holes in accordance with the third preferred embodiment of the present invention.

As shown in FIG.3(a), edges of the through-holes3, in contact with front and back faces of the prepreg1, are chamfered to form border sections3b. The releasing film2is disposed such that it covers the border sections3b. In this state, the through-holes3are filled with the conductive paste4in the same manner as the first preferred embodiment. Following steps are the same as those of the first preferred embodiment 1:1) removing the releasing film2from the prepreg1(see FIG.3(b));2) placing the metal foils5on both faces of the prepreg1(see FIG.3(c)); and3) compressing the laminate under heat to electrically connect the metal foils5placed on both faces with the conductive paste4(see FIG.3(d)). In this process, epoxy resin, a component of the prepreg1, and the conductive paste4are cured.

As thus far described, according to this embodiment, the edges of the through-holes3are chamfered to form the border sections3b. The releasing film2is disposed to cover the border sections3b. Thus the conductive paste4, which protrudes from both faces of the prepreg1after the releasing film2is removed, can be made smaller than the diameter of the through-holes3on the front and back faces and steadily stays inside the circumference of the through-holes3. This traps the protrusions of the conductive paste4in the border sections3b, and prevents them from undesirably overflowing out of the through-holes3when the prepreg1is sandwiched between the metal foils5and compressed. Consequently, the conductive paste4is connected only to the part of the metal foils5, which will become the predetermined wiring patterns6by etching, and will not be connected to other wiring patterns6and cause a short circuit.

The Fourth Preferred Embodiment

FIGS.4(a)-(d) show shapes of the through-holes in accordance with the fourth preferred embodiment of the present invention.

As shown in FIG.4(a), edges of the through-holes3of this embodiment, in contact with the front and back faces of the prepreg1, are processed to form step-like border sections3c. The releasing film2is disposed such that it covers the border sections3c. In this state, the through-holes3are filled with the conductive paste4in the same manner as the first preferred embodiment. following steps are the same as those of the first preferred embodiment 1:1) removing the releasing film2from the prepreg1(see FIG.4(b));2) placing the metal foils5on both faces of the prepreg1(see FIG.4(c)); and3) compressing the laminate under heat to adhere the prepreg1and the metal foils5and electrically connect the metal foils5placed on both faces with the conductive paste4(see FIG.4(d)). In this process, epoxy resin, a component of the prepreg1, and the conductive paste4are cured.

As thus far described, according to this embodiment, the edges of the through-holes3on the front and back faces of the prepreg1are processed to form the step-like border sections3c. The releasing film2is disposed to cover the border sections3c. Thus the conductive paste4, which protrudes from both faces of the prepreg1after the releasing film2is removed, becomes smaller than the diameter of the through-holes3on the front and back faces and steadily stays inside the circumference of the through-holes3. This configuration traps the protrusions of the conductive paste4in the border sections3c, and prevents them from undesirably overflowing out of the through-holes3when the prepreg1is sandwiched between the metal foils5and compressed. Consequently, the conductive paste4is connected only to the part of the metal foils5, which will become the predetermined wiring patterns6by etching, and will not be connected to other wiring patterns6and cause a short circuit.

The Fifth Preferred Embodiment

FIGS.5(a)-(d) show shapes of the through-holes in accordance with the fifth preferred embodiment of the present invention.

As shown in FIG.5(a), the through-holes3of this embodiment is processed such that a diameter of the inside of the through-hole is smaller than that of on the front and back faces of the prepreg1. Border sections3dof the through-holes3, which are located at the front and back faces of the prepreg1, are covered with the releasing film2. In this state, the through-holes3are filled with the conductive paste4in the same manner as the first preferred embodiment. Following steps are the same as those of the first preferred embodiment 1:1) removing the releasing film2from the prepreg1(see FIG.5(b));2) placing the metal foils5on both faces of the prepreg1(see FIG.5(c)); and3) compressing the laminate under heat to adhere the prepreg1and the metal foils5and to electrically connect the metal foils5placed on both faces with the conductive paste4(see FIG.5(d)). In this process, epoxy resin, a component of the prepreg1, and the conductive paste4are cured.

As thus far described, according to this embodiment, the through-holes3of this embodiment is processed such that a diameter of the inside of the through-hole is smaller than that of on the front and back faces of the prepreg1. Furthermore, the border sections3dof the through-holes3, which are located at the front and back faces of the prepreg1, are covered with the releasing film2. Thus the conductive paste4, which protrudes from both faces of the prepreg1after the releasing film2is removed, becomes smaller than the diameter of the through-holes3on the front and back faces and steadily stays inside the circumference of the through-holes3. This configuration traps the protrusions of the conductive paste4in the border sections3d, and prevents them from undesirably overflowing out of the through-holes3when the prepreg1is sandwiched between the metal foil5and compressed. Consequently, the conductive paste4is connected only to the part of the metal foils5, which will become the predetermined wiring patterns6by etching, and will not be connected to other wiring patterns6and cause a short circuit.

The Sixth Preferred Embodiment

FIGS.6(a)-(d) show shapes of the through-holes and the releasing film2in accordance with the sixth preferred embodiment of the present invention.

As shown in FIG.6(a), in this embodiment, expanded sections2a, which are located near the openings of the releasing film2formed by a laser processing, cover the circumference of the through-holes3. To expand part of the releasing film2, a carbon dioxide laser with high heat energy should be used. Due to the heat energy, rearrangement of molecules, which occurs along the sections exposed to the laser processing, deforms the film and triggers an expansion to form the expanded sections2a. In this state, the through-holes3are filled with the conductive paste4in the same manner as the first preferred embodiment. Following steps are the same as those of the first preferred embodiment 1:1) removing the releasing film2from the prepreg1(see FIG.6(b));2) placing the metal foils5on both faces of the prepreg1(see FIG.6(c)); and3) compressing the laminate under heat to adhere the prepreg1and the metal foils5and electrically connect the metal foils5placed on both faces with the conductive paste4(see FIG.6(d)). In this process, epoxy resin, a component of the prepreg1, and the conductive paste4are cured.

As thus far described, according to this embodiment, the expanded sections2alocated near the openings of the releasing film2formed by the laser processing, cover around the edges of the through-holes3. Thus, after the releasing film2is removed, the level of the conductive paste4at around the edges of the through-holes3is recessed from the levels of both faces of the prepreg1while it is protruded at and around the center of the through-holes3. This configuration traps the protrusions of the conductive paste4in its recesses formed around the edges of the through-holes3and prevents them from undesirably overflowing out of the through-holes3when the prepreg1is sandwiched between the metal foils5and compressed. Consequently, the conductive paste4is connected only to the part of the metal foils5, which will become the predetermined wiring patterns6by etching, and will not be connected to other wiring patterns6and cause a short circuit.

The Seventh Preferred Embodiment

FIGS.7(a)-(d) show shapes of the through-holes in accordance with the seventh preferred embodiment of the present invention.

As shown in FIG.7(a), in this embodiment, expanded sections2blocated near the openings of the releasing film2formed by a laser processing cover border sections3ebetween the through-holes3and the front and back faces of the prepreg1. In this embodiment, the expanded sections2b, which is formed due to shrinkage and deformation of the releasing film2caused by heat emitted during the laser processing, push down the front and back faces of the prepreg1softened by heat to form recesses of the border section3e. This shape of the releasing film2can be achieved, similarly to the sixth preferred embodiment, by using a carbon dioxide laser with high heat energy. In this state, the through-holes3are filled with the conductive paste4in the same manner as the first preferred embodiment. Following steps are the same as those of the first preferred embodiment 1:1) removing the releasing film2from the prepreg1(see FIG.7(b));2) placing the metal foils5on both faces of the prepreg1(see FIG.7(c)); and3) compressing the laminate under heat to adhere the prepreg1and the metal foils5and electrically connect the metal foils5placed on both faces with the conductive paste4(see FIG.7(d)). In this process, epoxy resin, a component of the prepreg1, and the conductive paste4are cured. As thus far described, according to this embodiment, the conductive paste4does not totally fill up the border sections3elocated around the edges of the through-holes3. At around the center of the through-holes3is the conductive paste4protruding from the front and back faces of the prepreg1. This configuration traps the protrusions of the conductive paste4in the unfilled part of the through-holes3located around their edges and prevents it from undesirably overflowing out of the through-holes3when the prepreg1is sandwiched between the metal foils5and compressed. Consequently, the conductive paste4is connected only to the part of the metal foils5, which will become the predetermined wiring patterns6by etching, and will not be connected to other wiring patterns6and cause a short circuit.

INDUSTRIAL APPLICABILITY

As thus far described, according to the present invention, the protrusions of the conductive paste, which are formed in order to lower the connection resistance between the metal foils and the conductive paste, do not overflow out of the through-holes. The conductive paste is connected only to the part of the metal foils, which form the predetermined wiring patterns by the etching process, and do not get connected to other wiring patterns to cause a short circuit. Thus a reliable PCB suitable for high density wiring can be manufactured.