JPH11195849A - Flexible printed wiring board and method for manufacturing it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily control a circuit width at etching for forming a circuit with no degradation in bending-resistance, by providing narrow pitch wiring 1 over the surface of a flexible printed wiring board, and comprising a flexible bent part and a both-side wiring part. SOLUTION: The surface of a printed wiring board is entirely wired at a narrow pitch, comprising a flexible bent part 12 and a both-side wiring part 13, with a via hole provided at the both-side wiring part 13. The copper foil on a rear surface is opened by etching at a specified position of a both-side copper-plated laminating board, boring is performed by laser, plasma, or etching, through hole plating is applied only to the rear surface, then a circuit pattern is formed by etching and a cover lay film is pasted to obtain a printed wiring board. The copper foil on the surface where no plating is performed is thin, a narrow pitch circuit is easily formed on the surface, and a copper foil of high bending-resistance, as a starting material, is left alone.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible printed wiring board which is applicable to narrow pitch wiring and high-density mounting in repeated bending applications, and more particularly to a flexible printed wiring for wiring to a bent portion of an HDD or a CD-ROM drive. Regarding the board.

[0002]

2. Description of the Related Art A conventional method of manufacturing a flexible printed wiring board is to make a hole in a double-sided wiring portion of a copper-clad laminate with an NC lathe,
These holes are plated through holes, then etched to form circuit patterns, and a cover lay is attached. By the way, in general, an electrolytic copper foil has poor flexibility, and a rolled copper foil or an electrolytic copper foil such as an HTE foil whose crack resistance is improved by annealing is used as a copper foil used for bending. However, there is a drawback in that a double-sided board is covered with hard and brittle through-hole plated copper even if a copper foil having good flexibility is used. It is said that the thinner the copper foil is, the higher the flexibility is. However, if plated with a through hole, the copper foil becomes thicker. Therefore, a double-sided board having a through hole generally has low durability against repeated bending. When the copper foil is thick, it is difficult to form a circuit wiring with a narrow pitch. Therefore, the pitch of a double-sided board which must be plated through holes cannot be made as narrow as that of a single-sided board.

In order to make up for such a drawback, the following method has conventionally been adopted. As shown in FIG. 5, the wiring board has a structure in which a narrow pitch wiring portion 11, a flexible bending portion 12, and a double-sided wiring portion 13 having a through hole are divided by a relatively large area. Masking or shielding is applied to the pitch wiring portion so as not to cover the through-hole plating, and only the area having the through-hole is plated. In this method, since a portion that is not plated is thin, a circuit with a narrow pitch can be formed, and a thin underlying copper foil having good flexibility can be utilized. In order to prevent plating, a method of covering with a shielding plate, applying a plating-resistant resist, or masking with a tape has been performed.

As another method, a material of a thin copper foil is used, or a material obtained by thinning a copper foil by performing half etching is used. However, usually a thickness of 1
18μ for applying through-hole plating of about 5μm
m, and the effect of using a thin copper foil is not so large.

As another method, as shown in FIG. 6, copper foils 22 are adhered to both sides of a base material 21, and through holes 23 are provided in the copper foils 22.
A resist 24 having an opening one size larger than 3 is formed in advance, and then plating 25 of the exposed copper foil 22 and a through hole covering the surface of the through hole 23 is performed.
Next, the resist 24 was peeled off and a circuit was formed by etching.

[0006]

According to the first method,
The thickness differs between the portion plated with the through hole and the portion not plated. Further, in the method using a shielding plate, the plating slightly goes around the shielding portion, so that the thickness of the copper foil at the bent portion becomes uneven. Therefore, there is a problem that it is difficult to control a circuit width when a circuit is formed by etching, and that plating resistance is impaired if plating goes around a bent portion. Further, when a shielding plate is used, it is necessary to recreate the shielding plate for each design, which increases costs. When a plating resist is used, there is a problem that a step of forming the resist is required, and the cost is increased.

In the second method, a copper foil having a thickness of 35 μm or 18 μm can be obtained at a low cost and is often used for a flexible printed wiring board. In particular, 18 μm is used for a double-sided substrate. A copper foil thinner than this is expensive, and it is difficult to attach it to a base film from the viewpoint of handling, resulting in an increase in cost. Even when a thin copper foil is used or a material in which the copper foil is thinned by performing a half-etching in advance is used, the thickness is usually 15
18 μm to apply through-hole plating of about μm
However, there is a problem that the effect is not so great. 21 is a base material,
22 is a copper foil.

In the third method, the cost of forming a resist is high. In addition, as shown in FIG. 7, since plating is performed locally, current concentration occurs during plating, and it is difficult to control the plating thickness. When the thickness becomes too thick, a projection is formed only around the through hole. For this reason,
At the time of forming a circuit by etching, the resist is easily lifted or the resist is broken, so that an etching failure is likely to occur. Also,
Bubble 27 when laminating coverlay film in later process
Is easy to enter. Even when a liquid cover coat is applied instead of the coverlay film, the projections cannot be covered, and the projections are likely to protrude outside, resulting in impaired insulation.

[0009]

SUMMARY OF THE INVENTION The present invention has been made as a result of intensive studies in order to solve the various problems in the prior art, and the invention of claim 1 has a surface as shown in FIG. Are all formed with a narrow pitch wiring and are composed of a flexible bent portion 12 and a double-sided wiring portion 13. The double-sided wiring portion 13 is provided with a non-penetrating via hole (not shown). ing. There is no through-hole in the double-sided wiring portion as in the conventional case. Also, since the bent portion is a single-sided wiring, there is inevitably no via hole in the bent portion. The through-hole plating is performed only on the back surface, and the copper foil of the wiring on the front surface is not plated.

The first method of the present invention (invention of claim 2) is a method for manufacturing the wiring board of claim 1 as shown in the flow sheet of FIG. In the method, the outline is to prepare a double-sided copper-clad laminate, open a copper foil on the back surface at a predetermined position of the double-sided copper-clad laminate by etching, and perform a drilling process by laser or plasma or etching, After performing a residue treatment only at the time of laser processing, a through hole plating is performed only on the back surface, a circuit pattern is formed by etching, and a cover lay film is attached to form a printed wiring board.

The second method (the invention of claim 3) is as shown in FIG.
As shown in the flow sheet (b), the opening and drilling of the copper foil in the steps of the first method were performed using UV-
This is performed at a time with a YAG laser, and after that, if necessary, a residue treatment is performed. After only the back surface is plated with a through hole, a circuit pattern is formed by etching, and a coverlay film is attached to form a printed wiring board. In the plating after the residue treatment, the shielding plate is attached to the front surface of the substrate and only the rear surface is plated. Usually, it is plated to about 15 μm.
Thereafter, circuit wiring is formed by etching in the same manner as in a normal double-sided substrate.

As the base material, polyamide, polyimide, polyester, or the like is usually used. Many copper foils are 18 μm copper foils which are available at a low cost, and these are adhered to form a copper-clad laminate. The thickness of the copper foil is thin on the front surface and thick on the back surface, but since the thickness is uniform within the surface, the etching conditions are easy to control. In the case of a commonly used spray-type etching machine, if the spray pressure is controlled vertically (front and back) independently or the number of sprays operated vertically is adjusted,
Etching can be performed simultaneously on the front and back under optimal (suitable for copper foil thickness) conditions. After the etching, a post-process such as coverlay lamination is performed to complete the printed wiring board.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the embodiment of the present invention will be described. The entire surface is formed with a narrow pitch wiring, and is composed of a flexible bent portion 12 and a double-sided wiring portion 13. Provided with via holes (not shown) is the printed wiring board substrate of the present invention. A flexible plastic such as polyimide, polyamide or polyester is used as a base material, and a copper foil having a predetermined thickness is attached to the flexible plastic to form a substrate. Via holes are formed in the double-sided wiring portion 13 by the above-described method, and a circuit is finally formed by etching, and a surface thereof is pasted with a coverlay film to obtain a product.

FIG. 3 is a sectional view showing a specific embodiment of the present invention, and the same parts as those in FIG. 6 are denoted by the same reference numerals. That is, the copper foil 22 is provided on both surfaces of the base material 21 to form a double-sided copper-clad substrate. An opening is formed in a predetermined portion of the copper foil on the back surface of the double-sided copper-clad substrate by, for example, etching, and a laser beam having a slightly larger beam diameter is irradiated on the opening to perform drilling. Next, plating is performed on the copper foil on the back surface including the perforated portion to complete the via hole 14. Next, a circuit is formed by etching. It goes without saying that the etching for the opening may be stopped and the entire hole may be formed by a UV-YAG laser.

FIG. 4 is a cross-sectional view when components are mounted on a substrate having via holes, and the same parts as those in FIG. 3 are denoted by the same reference numerals. Reference numeral 28 denotes a mounting component such as a chip capacitor. In this figure, a via hole is provided on the back side of a component mounting pad. Usually, a land is provided around the via hole. However, since this land also serves as a mounting pad, high-density mounting is possible.

[0016]

Since the printed wiring board of the present invention has a structure in which only the back surface is plated with through holes, the copper foil thickness on the unplated surface (front surface) is small, and it is easy to form a narrow pitch circuit on this surface. . In addition, since the through-hole plating is performed only on the back surface, the copper foil thickness is uniform in the same plane, and therefore, the etching conditions are set higher than those in which the thickness varies in the same plane. easy. Furthermore, since the structure is such that only the rear surface is plated with through holes, the copper foil having high bending resistance as a starting material is left as it is on the non-plated surface, so that a double-sided substrate for bending applications can be realized. In addition, since etching and plating can use conventional equipment, no production cost is required.

[Brief description of the drawings]

FIG. 1 is a plan view of an embodiment of the present invention.

FIG. 2 is a flow sheet of the method of the present invention.

FIG. 3 is an explanatory sectional view of an example of a via hole forming step of the present invention.

FIG. 4 is a cross-sectional view of a via-hole setting unit in the case of component mounting according to the present invention.

FIG. 5 is a schematic top view of a target substrate.

FIG. 6 is an explanatory cross-sectional view of an example of a through-hole forming step of an example of a conventional method.

FIG. 7 is a cross-sectional view of a substrate in a state where protrusions are formed by a conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Narrow pitch wiring part 12 Flexible bending part 13 Double-sided wiring part 14 Via hole 21 Base material 22 Copper foil 23 Through hole 24 Resist 25 Through hole plating 26 Cover lay film 27 Bubbles 28 Mounting parts

Claims (5)

[Claims] 1. A flexible printed wiring board having a narrow pitch circuit on a surface, a flexible bent portion, and a double-sided wiring portion conducted by a via hole. 2. An etching method for preparing a flexible printed wiring board according to claim 1, wherein a double-sided copper-clad laminate is prepared, and an opening is formed in a copper foil to form a via hole at a predetermined position of a double-sided wiring portion from a back surface. A step of drilling from the opening by laser, plasma or etching, a step of performing a residue treatment accompanying the drilling as needed, and a step of plating a copper foil in the via hole and on the back surface with a through hole. And a step of forming a circuit pattern by etching, and a step of providing a coverlay film on the surface of the circuit pattern. 3. When forming a flexible printed wiring board according to claim 1, a double-sided copper-clad laminate is prepared, and a via hole is formed at a predetermined position of a double-sided wiring portion from the back surface, thereby providing UV.
A step of simultaneously drilling a copper foil and a base material with a YAG laser, a step of plating a copper foil inside and in a via hole, a step of forming a circuit pattern by etching, and a coverlay film on the surface of the circuit pattern Providing a flexible printed wiring board. 4. The flexible printed wiring board according to claim 1, wherein the base material is a polyamide film, a polyimide film or a polyester film. 5. The method for producing a flexible printed wiring board according to claim 2, wherein the base material is a polyamide film, a polyimide film, or a polyester film.