Wired circuit board

A wired circuit board can control characteristic impedance at connection points between wires of a suspension board with circuit and terminal portions of the wired circuit board connected thereto with a simple structure, to improve signal transmission efficiency even for fine pitch wiring or for high frequency signals. The wired circuit board includes a relay flexible wiring circuit board formed by a first wired circuit board including a first metal substrate, a first insulating base layer, a first conductor layer and a first insulating cover layer which is substantially identical in layer structure with the suspension board with circuit and a second wired circuit board connected with the first wired circuit board for connecting with a control circuit board. Since the suspension board with circuit and the first wired circuit board are rendered substantially identical in layer structure, both characteristic impedances at these connection points can be matched.

This application claims priority from Japanese Patent Application No. 2003-160130, filed Jun. 4, 2003, the entire contents of which are herein incorporated by reference to the extent allowed by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wired circuit board to connect with a suspension board with circuit equipped with a magnetic head of a hard disk drive.

2. Description of the Prior Art

In the hard disk drive, the suspension board with circuit equipped with the magnetic head usually has lines for transmitting read signals and write signals to the magnetic head. These lines are connected to terminal portions of the wired circuit board equipped with an electronic device such as a preamplifier IC. The signals from the lines are amplified by the electronic device such as the preamplifier IC placed on the wired circuit board and then transmitted from the wired circuit board to a control circuit board for controlling the magnetic head.

Meanwhile, along with improvement in recent years to fine pitch wiring and to high frequency signal, it is increasingly becoming necessary to control characteristic impedances at connection points between the lines of the suspension board with circuit and the terminal portions of the wired circuit board connected thereto.

If there is inconsistency in characteristic impedance at the connection points, transmission efficiency of the signal will be reduced. Particularly, the signal before input to the preamplifier IC (the signal before amplified) is so weak that it is easily influenced by the characteristic impedance at the connection points, so that the transmission efficiency of the signal is reduced easily.

For example, U.S. Pat. No. 5,712,749 proposes improvement of this suspension board with circuit, according to which an opening is formed in the stainless board at a location under the wires, to optimize the capacitance of the signal, so as to control the characteristic impedance.

This proposed construction can control the characteristic impedance of the suspension board with circuit itself, but it cannot control the characteristic impedances at the connection points between the lines of the suspension board with circuit and the terminal portions of the wired circuit board connected thereto. Accordingly, the problem of the reduction in transmission efficiency at the connection points remains unavoidable.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a wired circuit board that can control characteristic impedance at connection points between wires of a suspension board with circuit and terminal portions of the wired circuit board connected thereto with a simple structure, to improve signal transmission efficiency even for fine pitch wiring or for high frequency signal.

The present invention provides a novel wired circuit board to electrically connect with a suspension board with circuit having a metal substrate, an insulating base layer formed on the metal substrate, a conductor layer formed on the insulating base layer, and an insulating cover layer formed on the conductor layer, the wired circuit board comprising: a first wired circuit board electrically connected with the suspension board with circuit, and a second wired circuit board electrically connected with the first wired circuit board, for electrical connection with an external circuit, the first wired circuit board comprising a first metal substrate, a first insulating base layer formed on the first metal substrate, a first conductor layer formed on the first insulating base layer, and a first insulating cover layer formed on the first conductor layer.

In the wired circuit board of the present invention, it is preferable that the conductor layer of the suspension board with circuit and the first conductor layer of the first wired circuit board are formed by a semi-additive process.

In the wired circuit board of the present invention, it is preferable that the conductor layer of the suspension board with circuit and the first conductor layer of the first wired circuit board are substantially equal in thickness to each other.

Also, in the wired circuit board of the present invention, it is preferable that the insulating base layer of the suspension board with circuit and the first insulating base layer of the first wired circuit board are substantially equal in thickness to each other.

In addition, in the wired circuit board of the present invention, it is preferable that the insulating cover layer of the suspension board with circuit and the first insulating cover layer of the first wired circuit board are substantially equal in thickness to each other.

According to the wired circuit board of the present invention, since the first wired circuit board is substantially identical in layer structure with the suspension board with circuit, both characteristic impedances at a connection point between the suspension board with circuit and the first wired circuit board can be matched with each other, and as such can allow improvement in signal transmission efficiency in the wired circuit board having the first and second wired circuit boards even for fine pitch wiring of the suspension board with circuit or of the wired circuit board, or even for transmission of high-frequency signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a sectional view showing a use condition of a relay flexible wiring circuit board taken as an embodiment of a wired circuit board of the present invention.

InFIG. 1, a relay flexible wiring circuit board1is a flexible wired circuit board used for connecting between a long-tail type suspension board with circuit3equipped with a magnetic head2of a hard disk drive and a control circuit board4, which serves as an external circuit, for controlling the magnetic head2.

The magnetic head2is arranged at a front end portion of the suspension board with circuit3and is supported above a rapidly spinning magnet disk (not shown) with a minute space therefrom so that a flying state of the magnetic head2can be well held against an airflow generated in the minute gap between the magnetic head2and the magnetic disk.

The suspension board with circuit3comprises a metal substrate5, an insulating base layer6formed on the metal substrate5, a conductor layer7formed on the insulating base layer6, and an insulating cover layer8formed on the conductor layer7. In this suspension board with circuit3, the conductor layer7of a predetermined wired circuit pattern is formed on the flexible metal substrate5to be integral therewith. The suspension board with circuit3has a magnetic-head-side terminal portion12formed at a lengthwise front end portion thereof for connecting with the magnetic head2and a first-wired-circuit-board-side terminal portion11formed at a lengthwise rear end portion thereof for connecting with a first wired circuit board14of the relay flexible wiring circuit board1.

This suspension board with circuit3can be produced, for example, by a method shown inFIG. 2.

First, the metal substrate5is prepared in this method, as shown inFIG. 2(a). The metal substrate5is formed of a metal foil or a thin metal sheet. For example, stainless steel, copper, aluminum, copper-beryllium, phosphor bronze, and 42 alloy may be used for the metal substrate5. Stainless is preferably used for the metal substrate5, in view of its characteristic of spring and corrosion resistance. It is preferable that the metal substrate5usually has a thickness in the range of 10–50 μm, or preferably 18–25 μm and a width in the range of 100–500 mm, or preferably 250–300 mm.

Then, the insulating base layer6in the form of a predetermined pattern is formed on the metal substrate5as shown inFIG. 2(b). The insulating base layer6is formed of synthetic resin, such as polyimide, polyether nitrile, polyether sulfone, polyethylene terephthalate, polyethylene naphthalate, or polyvinyl chloride for example. Preferably, photosensitive synthetic resin is used for the insulating base layer6.

For forming the insulating base layer6of a predetermined pattern on the metal substrate5, the following method can be adopted, for example. First, solution of photosensitive resin, such as solution of polyamic acid resin, is applied to a surface of the metal substrate5. Then, the applied photosensitive resin is exposed to light and developed and thereafter it is cured by heating.

Then, a thin conductor film9, which serves as a ground layer, is formed on the insulating base layer6, as shown inFIG. 2(c). The vacuum film-forming method, sputtering in particular, is preferably used for forming the thin conductor film9. The conductive materials that may be used for the thin conductor film9include chromium and copper. Specifically, it is preferable that a thin chromium film and a thin copper film are sequentially formed on the entire area of the insulating base layer6by sputtering. It is preferable that the thin chromium film has a thickness of 50–1,000 Å and the thin copper film has a thickness of 200–3,000 Å.

Then, a plating resist25ais formed on a portion of the thin conductor film9to be a reversed portion of a conductor layer7forming portion of the same9, as shown inFIG. 2(d). The plating resist25amay be formed in a known method using dry film resist, for example.

Then, an electrolytic plated layer10is formed by electrolytic plating on the thin conductor film9exposed from the plating resist25a, as shown inFIG. 2(e). The electrolytic plated layer10may be formed in a proper method, without any particular limitation. For example, the electrolytic plating of copper, nickel, gold, solder or alloys thereof may be used for forming the electrolytic plated layer10. Preferably, the electrolytic copper plating is used. It is preferable that the electrolytic plated layer10usually has thickness in the range of 3–35 μm, or preferably 5–18 μm.

Thereafter, the plating resist25ais removed by a known etching method, such as chemical etching (wet etching), or by peeling, as shown inFIG. 2(f). Then, the thin conductor film9on which the plating resist25awas formed is also removed by a known etching method such as chemical etching (wet etching), as shown inFIG. 2(g). The conductor layer7comprising the thin conductor film9and the electrolytic plated layer10is formed by this semi-additive process.

The conductor layer7thus formed is in the form of a predetermined wired circuit pattern comprising e.g. write wires and read wires to the magnetic head2.

The conductor layer7thus formed may be coated with an electroless plated layer of nickel by electroless nickel plating, if necessary.

Then, the insulating cover layer8of a predetermined pattern opened at portions thereof corresponding to the magnetic-head-side terminal portion12and the first-wired-circuit-board-side terminal portion11is formed on the insulating base layer6including the conductor layer7, as shown inFIG. 2(h). For example, synthetic resin, such as polyimide, polyether nitrile, polyether sulfone, polyethylene terephthalate, polyethylene naphthalate, and polyvinyl chloride may be used for the insulating cover layer8, as is the case with the insulating base layer6. Preferably, photosensitive synthetic resin is used for the insulating cover layer8.

For forming the insulating cover layer8of a predetermined pattern on the insulating base layer6including the conductor layer7, the following method can be adopted, for example. First, solution of photosensitive resin, such as solution of polyamic acid resin, is applied to a surface of the insulating base layer6including the conductor layer7. Then, the applied photosensitive resin is exposed to light and developed and thereafter it is cured by heating. The insulating cover layer8of a predetermined pattern opened at portions thereof corresponding to the magnetic-head-side terminal portion12and the first-wired-circuit-board-side terminal portion11is formed in this manner.

Thereafter, the metal substrate5is etched in a predetermined outer shape, as shown inFIG. 2(i). A known etching method, such as chemical etching (wet etching), is used for the etching of the metal substrate5.

Then, a connecting pad13is formed in the magnetic-head-side terminal portion12by plating, for example, and also a connecting bump13′ is formed in the first-wired-circuit-board-side terminal portion11by solder printing, for example, as shown inFIG. 2(j). The suspension board with circuit3is produced in the manner mentioned above.

The relay flexible wiring circuit board1comprises the first wired circuit board14which is electrically connected with the suspension board with circuit3and a second wired circuit board15which is electrically connected with the first wired circuit board14and is to be electrically connected with the control circuit board4, as shown inFIG. 1.

The first wired circuit board14comprises a first metal substrate16, a first insulating base layer17formed on the first metal substrate16, a first conductor layer18formed on the first insulating base layer17, and a first insulating cover layer19formed on the first conductor layer18. The first wired circuit board14has a suspension-board-side terminal portion20, formed at a lengthwise front end portion thereof, for connecting with the suspension board with circuit3, a second-wired-circuit-board-side terminal portion21, formed at a lengthwise rear end portion thereof, for connecting with the second wired circuit board15, and an IC-side terminal portion23, formed at a lengthwise intermediate portion thereof, for equipment with a preamplifier IC22.

This first wired circuit board14can be produced, for example, by a method shown inFIG. 3.

First, the first metal substrate16is prepared in this method, as shown inFIG. 3(a). The first metal substrate16is formed of a metal foil or a thin metal sheet. For example, stainless steel, copper, aluminum, copper-beryllium, phosphor bronze, and 42 alloy may be used for the first metal substrate16. Stainless is preferably used for the first metal substrate16, in view of its characteristic of spring and corrosion resistance. It is preferable that the first metal substrate16usually has a thickness in the range of 10–50 μm, or preferably 18–25 μm and a width in the range of 100–500 mm, or preferably 250–300 mm.

It is preferable that the first metal substrate16is formed of the same metal as that of the metal substrate5of the suspension board with circuit3and also has substantially the same thickness and width as those of the metal substrate5. The use of the first metal substrate16formed of the same metal as that of the metal substrate5of the suspension board with circuit3and also having substantially the same thickness and width as those of the metal substrate5can provide reliable matching of characteristic impedances at the connection point, as mentioned later.

Then, the first insulating base layer17in the form of a predetermined pattern opened at a portion thereof corresponding to the second-wired-circuit-board-side terminal portion21is formed on the first metal substrate16, as shown inFIG. 3(b). The first insulating base layer17may be formed of synthetic resin, such as polyimide, polyether nitrile, polyether sulfonic, polyethylene terephthalate, polyethylene naphthalate, and polyvinyl chloride for example. Preferably, photosensitive synthetic resin is used for the first insulating base layer17.

For forming the first insulating base layer17of a predetermined pattern on the first metal substrate16, the following method can be adopted, for example. First, solution of photosensitive resin, such as solution of polyamic acid resin, is applied to a surface of the first metal substrate16. Then, the applied photosensitive resin is exposed to light and developed and thereafter it is cured by heating. The first insulating base layer17in the form of a predetermined pattern opened at a portion thereof corresponding to the second-wired-circuit-board-side terminal portion21is formed in this manner.

It is preferable that the first insulating base layer17is formed of the same material (synthetic resin) as that of the insulating base layer6of the suspension board with circuit3and also has substantially the same thickness as that of the insulating base layer6. The use of the first insulating base layer17formed of the same material (synthetic resin) as that of the insulating base layer6of the suspension board with circuit3and also having substantially the same thickness as that of the insulating base layer6can provide reliable matching of characteristic impedances at the connection point, as mentioned later.

Then, a first thin conductor film24, which serves as a ground layer, is formed on the first insulating base layer17, as shown inFIG. 3(c). The vacuum film-forming method, sputtering in particular, is preferably used for forming the first thin conductor film24. The conductive materials that may be used for the first thin conductor film24include chromium and copper. Specifically, it is preferable that a thin chromium film and a thin copper film are sequentially formed on the entire area of the first insulating base layer17by sputtering, as in the case of the suspension board with circuit3. It is preferable that the thin chromium film has a thickness of 50–1,000 Å and the thin copper film has a thickness of 200–3,000 Å.

Then, a first plating resist25bis formed on a portion of the first thin conductor film24to be a reversed portion to a first conductor layer18forming portion of the same24, as shown inFIG. 3(d). The first plating resist25bmay be formed in a known method using dry film resist, for example.

Then, a first electrolytic plated layer26is formed by electrolytic plating on the first thin conductor film24exposed from the first plating resist25b, as shown inFIG. 3(e). The first electrolytic plated layer26may be formed in a proper method, without any particular limitation. For example, the electrolytic plating of copper, nickel, gold, solder or alloys thereof may be used for forming the first electrolytic plated layer26. Preferably, the electrolytic copper plating is used. It is preferable that the first electrolytic plated layer26usually has thickness in the range of 3–35 μm, or preferably 5–18 μm.

Thereafter, the first plating resist25bis removed by a known etching method, such as chemical etching (wet etching), or by peeling, as shown inFIG. 3(f). Then, the first thin conductor film24on which the first plating resist25bwas formed is also removed by a known etching method such as chemical etching (wet etching) or by peeling, as shown inFIG. 3(g). The first conductor layer18comprising the first thin conductor film24and the first electrolytic plated layer26is formed by this semi-additive process.

The first conductor layer18thus formed is in the form of a predetermined wired circuit pattern comprising e.g. write wires and read wires corresponding to those of the conductor layer7of the suspension board with circuit3.

The first conductor layer18thus formed may be coated with an electroless plated layer of nickel by electroless nickel plating, if necessary.

It is preferable that the first conductor layer18is formed of the same metal as that of the conductor layer7of the suspension board with circuit3and also has substantially the same thickness and wiring as those of the conductor layer7of the suspension board with circuit3. The use of the first conductor layer18formed of the same metal as that of the conductor layer7of the suspension board with circuit3and also having substantially the same thickness and wiring as those of the conductor layer7of the suspension board with circuit3can provide reliable matching of characteristic impedances at the connection point, as mentioned later.

Then, the first insulating cover layer19of a predetermined pattern opened at portions thereof corresponding to the suspension-board-side terminal portion20and the IC-side terminal portion23is formed on the first insulating base layer17including the first conductor layer18, as shown inFIG. 3(h). For example, synthetic resin, such as polyimide, polyether nitrile, polyether sulfone, polyethylene terephthalate, polyethylene naphthalate, and polyvinyl chloride may be used for the first insulating cover layer19, as is the case with the first insulating base layer17. Preferably, photosensitive synthetic resin is used for the first insulating cover layer19.

For forming the first insulating cover layer19of a predetermined pattern on the first insulating base layer17including the first conductor layer18, the following method can be adopted, for example. First, solution of photosensitive resin, such as solution of polyamic acid resin, is applied to a surface of the first insulating base layer17including the first conductor layer18. Then, the applied photosensitive resin is exposed to light and developed and thereafter it is cured by heating. The first insulating cover layer19of a predetermined pattern opened at portions thereof corresponding to the suspension-board-side terminal portion20and the IC-side terminal portion23is formed in this manner. A portion of the first conductor layer18exposed from the opening corresponding to the suspension-board-side terminal portion20of the first insulating cover layer19serves as the suspension-board-side terminal portion20, and a portion of the first conductor layer18exposed from the opening corresponding to the IC-side terminal portion23of the first insulating cover layer19serves as the IC-side terminal portion23.

Thereafter, a portion of the first metal substrate16corresponding to the second-wired-circuit-board-side terminal portion21is etched, as shown inFIG. 3(i). A known etching method, such as chemical etching (wet etching), is used for the etching of the first metal substrate16. A portion of the first conductor layer18exposed from the opening of the first metal substrate16serves as the second-wired-circuit-board-side terminal portion21. The opening of the first metal substrate16is formed to be larger than a solder bump36mentioned later.

Then, a connecting pad41is formed in the IC-side terminal portion23by plating, for example, so that the preamplifier IC22is mounted on the IC-side terminal portion23through the connecting pad41, as shown inFIG. 3(j). The first wired circuit board14equipped with the preamplifier IC22is produced in the manner mentioned above.

The second wired circuit board15comprises a second insulating base layer27, a second conductor layer28formed on the second insulating base layer27, and a second insulating cover layer29formed on the second conductor layer28, as shown inFIG. 1. The second wired circuit board15has a first-wired-circuit-board-side terminal portion30, formed at a lengthwise front end portion thereof, for connecting with the second-wired-circuit-board-side terminal portion21of the first wired circuit board14and also has a control-circuit-board-side terminal portion31, formed at a lengthwise rear end portion thereof, for connecting with the control circuit board4. In order to acquire the bonding strength, the second wired circuit board15is provided with a stiffener board34bonded to the second insulating base layer27through an adhesive layer33to correspond in position to the first-wired-circuit-board-side terminal portion30and control-circuit-board-side terminal portion31.

This second wired circuit board15can be produced, for example, by a method shown inFIG. 4.

First, the second conductor layer28of a metal foil or a thin metal sheet is prepared in this method, as shown inFIG. 4(a). For example, a metal foil or a thin metal sheet of copper, nickel, gold, solder, or alloys thereof may be used for the second conductor layer28. Preferably, a copper foil is used for the second conductor layer28. It is preferable that the second conductor layer28usually has a thickness in the range of 5–50 μm, or preferably 9–35 μm.

Then, the second insulating base layer27of a predetermined pattern is formed on the second conductor layer28, as shown inFIG. 4(b). The second insulating base layer27is formed of synthetic resin, such as polyimide, polyether nitrile, polyether sulfone, polyethylene terephthalate, polyethylene naphthalate, and polyvinyl chloride may be used for the second insulating base layer27.

For forming the second insulating base layer27of a predetermined pattern on the second conductor layer28, the following method may be adopted, for example. The second insulating base layer27in the form of a film previously formed in a predetermined pattern is adhesively bonded to a surface of the second conductor layer28through the adhesive layer not shown. Alternatively, after solution of photosensitive resin, such as solution of polyamic acid resin, is applied to a surface of the second conductor layer28, the applied photosensitive resin is exposed to light, developed, and cured by heating, whereby the second insulating base layer27is formed directly on the second conductor layer28. The second insulating base layer27of a predetermined pattern can be formed in this manner.

Then, an etching resist32is formed on a portion of the second conductor layer28to be formed into a predetermined wired circuit pattern, as shown inFIG. 4(c). The etching resist32may be formed in a known method using dry film resist, for example.

Then, after the second conductor layer28exposed from the etching resist32is etched by a known etching method, such as chemical etching (wet etching), as shown inFIG. 4(d), the etching resist32is removed by a known etching method, such as chemical etching (wet etching), or by peeling, as shown inFIG. 4(e). The second conductor layer28is formed in a predetermined wired circuit pattern by the semi-additive process.

Then, the second insulating cover layer29of a predetermined pattern opened at its portions corresponding to the first-wired-circuit-board-side terminal portion30and the control-circuit-board-side terminal portion31is formed on the second insulating base layer27including the second conductor layer28, as shown inFIG. 4(f). For example, synthetic resin, such as polyimide, polyether nitrile, polyether sulfone, polyethylene terephthalate, polyethylene naphthalate, and polyvinyl chloride may be used for the second insulating cover layer29, as is the case with the second insulating base layer27.

For forming the second insulating cover layer29of a predetermined pattern on the second insulating base layer27including the second conductor layer28, the following method can be adopted, for example. The second insulating cover layer29in the form of a film previously formed in a predetermined pattern is adhesively bonded to a surface of the second conductor layer28through the adhesive layer not shown. Alternatively, after solution of photosensitive resin, such as solution of polyamic acid resin, is applied to a surface of the second conductor layer28, the applied photosensitive resin is exposed to light, developed, and cured by heating, whereby the second insulating cover layer29is formed directly on the second insulating base layer27. The second insulating cover layer29of a predetermined pattern opened at portions thereof corresponding to the first-wired-circuit-board-side terminal portion30and control-circuit-board-side terminal portion31is formed in this manner. A portion of the second conductor layer28exposed from the opening corresponding to the first-wired-circuit-board-side terminal portion30of the second insulating cover layer29serves as the first-wired-circuit-board-side terminal portion30, and a portion of the second conductor layer28exposed from the opening corresponding to the control-circuit-board-side terminal portion31of the second insulating cover layer29serves as the control-circuit-board-side terminal portion31.

Thereafter, the stiffener board34is adhesively bonded to the second insulating base layer27through the adhesive layer33to correspond in position to the first-wired-circuit-board-side terminal portion30and control-circuit-board-side terminal portion31, to thereby produce the second wired circuit board15, as shown inFIG. 4(g). The adhesive layer33is formed of epoxy adhesive for example and has a thickness in the range of 5–30 μm, for example. The stiffener board34is formed of a metal foil, such as an aluminum foil, a copper foil, or a stainless foil, and has a thickness in the range of 30–500 μm, for example.

The relay flexible wiring circuit board1is formed by connecting the first-wired-circuit-board-side terminal portion30of the second wired circuit board15and the second-wired-circuit-board-side terminal portion21of the first wired circuit board14electrically, as shown inFIG. 1.

For electrically connecting the first-wired-circuit-board-side terminal portion30of the second wired circuit board15and the second-wired-circuit-board-side terminal portion21of the first wired circuit board14, the method shown inFIG. 5can be adopted, for example.

First, an anisotropic conductive adhesive sheet35′ is adhesively bonded to the second insulating cover layer29of the second wired circuit board15at a front end portion thereof including its surrounding area around the opening corresponding to the first-wired-circuit-board-side terminal portion30, as shown inFIG. 5(a). The anisotropic conductive adhesive sheet35′ is formed of e.g. a thermosetting adhesive in which metallic particles are dispersed and has a thickness in the range of 10–50 μm, for example.

Then, the first metal substrate16of the first wired circuit board14is press-bonded to the anisotropic conductive adhesive sheet35′ in the condition that the second-wired-circuit-board-side terminal portion21is in alignment and to correspond with the first-wired-circuit-board-side terminal portion30, thus connecting the first wired circuit board14and the second wired circuit board15together, as shown inFIG. 5(b). After this manner, the relay flexible wiring circuit board1is produced wherein the first-wired-circuit-board-side terminal portion30of the second wired circuit board15and the second-wired-circuit-board-side terminal portion21of the first wired circuit board14are electrically connected with each other through the anisotropic conductive adhesive sheet35′.

In use, the relay flexible wiring circuit board1thus formed is for example in the condition in which the first-wired-circuit-board-side terminal portion11of the suspension board with circuit3and the suspension-board-side terminal portion20of the first wired circuit board14are electrically connected with each other through the connecting bump13′, as shown inFIG. 1. The control-circuit-board-side terminal portion31of the second wired circuit board15has a connecting pad42formed by plating for example. The control-circuit-board-side terminal portion31of the second wired circuit board15is connected with a connector40through the connecting pad42, and the second wired circuit board15is electrically connected with the control circuit board4through the connector40. The magnetic head2is mounted on the magnetic-head-side terminal portion12of the suspension board with circuit3through the connecting pad13.

In this relay flexible wiring circuit board1, read signals and write signals to the magnetic head2are amplified by the preamplifier IC22on the first wired circuit board14interposed between the suspension board with circuit3and the control circuit board4when transmitted from the second wired circuit board15to the control circuit board4.

In this relay flexible wiring circuit board1, the layer structure of the suspension board with circuit3is formed by the metal substrate5, the insulating base layer6, the conductor layer7and the insulating cover layer8, while also the layer structure of the first wired circuit board14is formed by the first metal substrate16, the first insulating base layer17, the first conductor layer18and the first insulating cover layer19. In other words, the suspension board with circuit3and the first wired circuit board14are substantially identical in layer structure with each other. This structure enables both characteristic impedances at the connection point between the first-wired-circuit-board-side terminal portion11of the suspension board with circuit3and the suspension-board-side terminal portion20of the first wired circuit board14to be matched with each other. This can allow improvement in signal transmission efficiency in the relay flexible wiring circuit board1having the first and second wired circuit boards14and15even the suspension board with circuit3and relay flexible wiring circuit board1are formed with fine pitch or high-frequency signals are transmitted.

In this relay flexible wiring circuit board1, the first conductor layer18of the first wired circuit board14is formed to have the same wired circuit pattern and thickness as the conductor layer7of the suspension board with circuit3by the semi-additive process. This can contribute to a reliable matching of the characteristic impedances.

Also, in this relay flexible wiring circuit board1, the second conductor layer28of the second wired circuit board15is formed in a predetermined wired circuit pattern by the subtractive process. This can prevent occurrence of cracking or breaking of wire in between the first-wired-circuit-board-side terminal portion30and the control-circuit-board-side terminal portion31of the second wired circuit board15(or between the stiffener boards34arranged at both lengthwise end portions of the relay flexible wiring circuit board1), ensuring good flexibility.

In addition, in this relay flexible wiring circuit board1, the metal substrate5of the suspension board with circuit3and the first metal substrate16of the first wired circuit board14are substantially equal in thickness to each other; the insulating base layer6of the suspension board with circuit3and the first insulating base layer17of the first wired circuit board14are substantially equal in thickness to each other; and the insulating cover layer8of the suspension board with circuit3and the first insulating cover layer19of the first wired circuit board14are substantially equal in thickness to each other. This can also contribute to the reliable matching of the characteristic impedances.

In this relay flexible wiring circuit board1, the first wired circuit board14may be formed in an alternative method shown inFIG. 6for example.

First, the first metal substrate16is prepared in this method, as is the case described above, as shown inFIG. 6(a). Then, the first insulating base layer17in the form of a predetermined pattern is formed on the first metal substrate16, as is the case described above, as shown inFIG. 6(b). In the method illustrated inFIG. 6, the opening need not be formed in the first insulating base layer17at a portion thereof corresponding to the second wired-circuit-board-side terminal portion21.

Then, after forming the first thin conductor film24, which serves as the ground layer, is formed on the first insulating base layer17in the same manner as in the case described above, as shown inFIG. 6(c), the first plating resist25bis formed on a portion of the first thin conductor film24corresponding to a reversed portion of the first conductor layer18forming portion of the same24, in the same manner as in the case described above, as shown inFIG. 6(d). Thereafter, the first electrolytic plated layer26is formed by electrolytic plating on the first thin conductor film24exposed from the first plating resist25b, in the same manner as in the case described above, as shown inFIG. 6(e). Then, the first plating resist25bis removed by a known etching method, such as chemical etching (wet etching), or by peeling, as is the case described above, as shown inFIG. 6(f). Thereafter, the first thin conductor film24on which the first plating resist25bwas formed is also removed by a known etching method such as chemical etching (wet etching), as shown inFIG. 6(g). The first conductor layer18comprising the first thin conductor film24and the first electrolytic plated layer26is formed by this semi-additive process.

Then, the first insulating cover layer19of a predetermined pattern opened at portions thereof corresponding to the suspension-board-side terminal portion20and the IC-side terminal portion23is formed on the first insulating base layer17including the first conductor layer18in the same manner described above, as shown inFIG. 6(h). Thereafter, a through hole38extending through the first insulating cover layer19, the first conductor layer18, the first insulating base layer17and the first metal substrate16is formed at a location corresponding to the second-wired-circuit-board-side terminal portion21, as shown inFIG. 6(i). It has a diameter of e.g. 0.2–0.5 mm φ. The through hole38can be formed by a known method, such as drilling, punching, or etching.

Preferably, the through hole38is formed in the following manner. The insulating cover layer19is previously formed to have a pattern having an opening of a larger diameter than the through hole38, first, and, then, the through hole38is formed to extend through the first conductor layer18, the first insulating base layer17and the first metal substrate16by the method mentioned above, as shown inFIG. 6(i).

Thereafter, a connecting pad41is formed in the IC-side terminal portion23by plating for example and then the preamplifier IC22is mounted on the IC-side terminal portion23through the connecting pad41, as shown inFIG. 6(j). The first wired circuit board14equipped with the preamplifier IC22is produced in this manner.

Then, the relay flexible wiring circuit board1is produced by connecting the first wired circuit board14thus formed with the second wired circuit board15by the method shown inFIG. 7for example.

First, an adhesive sheet35having an opening corresponding to the opening of the second insulating cover layer29is adhesively bonded to the second insulating cover layer29of the second wired circuit board15at a front end portion thereof including its surrounding area around the opening corresponding to the first-wired-circuit-board-side terminal portion30, as shown inFIG. 7(a).

Then, the first metal substrate16of the first wired circuit board14is press-bonded to the adhesive sheet35in the condition that the through hole38of the second-wired-circuit-board-side terminal portion21is in alignment and to correspond with the first-wired-circuit-board-side terminal portion30, thus connecting the first wired circuit board14and the second wired circuit board15together as shown inFIG. 7(b).

Thereafter, solder cream is filled in the through hole38and soldered by reflow soldering to form a solder connecting portion39, as shown inFIG. 7(c). After this manner, the relay flexible wiring circuit board1is produced wherein the first-wired-circuit-board-side terminal portion30of the second wired circuit board15and the second-wired-circuit-board-side terminal portion21of the first wired circuit board14are electrically connected with each other through the solder connecting portion39.

In this relay flexible wiring circuit board1as well, since the suspension board with circuit3and the first wired circuit board14are substantially identical in layer structure with each other, both characteristic impedances at the connection point between the first-wired-circuit-board-side terminal portion11of the suspension board with circuit3and the suspension-board-side terminal portion20of the first wired circuit board14can be matched with each other. This can allow improvement in signal transmission efficiency in the relay flexible wiring circuit board1having the first and second wired circuit boards14and15even for fine pitch of the suspension board with circuit3and relay flexible wiring circuit board1are formed with fine pitch or high-frequency signals are transmitted.

EXAMPLES

While in the following, the present invention will be described in further detail with reference to Examples and Comparative Example, the present invention is not limited to any Examples and Comparative Example.

1) Production of First Wired Circuit Board

A first metal substrate of stainless foil having thickness of 25 μm was prepared (Cf.FIG. 3(a)). Then, after solution of polyamic acid resin was applied to a surface of the first metal substrate, the applied resin was exposed to light and developed and thereafter cured by heating, thereby producing the first insulating base layer of polyimide having thickness of 10 μm of a predetermined pattern opened at a portion thereof corresponding to the second-wired-circuit-board-side terminal portion (Cf.FIG. 3(b)).

Then, a thin chromium film having thickness of 300 Å and a thin copper film having thickness of 800 Å were sequentially formed on the entire area of the first insulating base layer by sputtering, thereby forming a first thin conductor film which serves as a ground layer (Cf.FIG. 3(c)). Thereafter, a first plating resist was formed on a portion of the first thin conductor film corresponding to a reversed portion of a first conductor layer forming portion of the same (Cf.FIG. 3(d)). Then, a first electrolytic plated layer of copper having thickness of 10 μm was formed by electrolytic copper plating on the first thin conductor film exposed from the first plating resist (CF.FIG. 3(e)). Thereafter, the first plating resist was removed by the chemical etching (Cf.FIG. 3(f)). Then, the first thin conductor film on which the first plating resist had been formed was also removed by the chemical etching (Cf.FIG. 3(g)). The first conductor layer comprising the first thin conductor film and the first electrolytic plated layer was formed by this semi-additive process.

Then, after solution of polyamic acid resin was applied to a surface of the first insulating base1layer including the first conductor layer, the applied resin was exposed to light and developed and thereafter cured by heating, thereby producing the first insulating cover layer of polyimide having thickness of 3 μm of a predetermined pattern opened at portions thereof corresponding to the suspension-board-side terminal portion and the IC-side terminal portion (Cf.FIG. 3(h)).

Thereafter, a portion of the first metal substrate corresponding to the second-wired-circuit-board-side terminal portion was chemically etched (Cf.FIG. 3(i)). Then, a preamplifier IC was mounted on the IC-side terminal portion through the connecting pad, thereby producing the first wired circuit board equipped with the preamplifier IC (Cf.FIG. 3(j)).

2) Production of Second Wired Circuit Board

A second conductor layer of rolled copper foil having thickness of 18 μm was prepared (Cf.FIG. 4(a)). Then, a second insulating base layer of polyimide film having thickness of 25 μm was adhesively bonded to the second conductor layer through an epoxy adhesive layer having thickness of 15 μm (Cf.FIG. 4(b)). Thereafter, an etching resist of a dry film resist was formed on a portion of the second conductor layer to be formed into a predetermined wired circuit pattern (Cf.FIG. 4(c)). Then, after the second conductor layer exposed from the etching resist was etched by the chemical etching (Cf.FIG. 4(d)), the etching resist was removed by the chemical etching. The second conductor layer was formed in a predetermined wired circuit pattern by the semi-additive process (Cf.FIG. 4(e)).

Then, the second insulating cover layer of a polyimide film having thickness of 25 μm of a predetermined pattern opened at its portions corresponding to the first-wired-circuit-board-side terminal portion and the control-circuit-board-side terminal portion was adhesively bonded to the second conductor layer through an epoxy adhesive layer having thickness of 15 μm (Cf.FIG. 4(f)).

Thereafter, stiffener boards of aluminum having thickness of 100 μm were adhesively bonded to the second insulating base layer through the epoxy adhesive layer having thickness of 25 μm to correspond in position to the first-wired-circuit-board-side terminal portion and control-circuit-board-side terminal portion, respectively, thereby producing the second wired circuit board (Cf.FIG. 4(g)).

3) Production of Flexible Wired Junction Circuit Board

An anisotropic conductive adhesive sheet having thickness of 40 μm was adhesively bonded to the second insulating cover layer of the second wired circuit board produced in the manner described above at a front end portion thereof including its surrounding area around the opening corresponding to the first-wired-circuit-board-side terminal portion (Cf.FIG. 5(a)). Then, the first metal substrate of the first wired circuit board was press-bonded to the anisotropic conductive adhesive sheet in the condition that the second-wired-circuit-board-side terminal portion was in alignment and to correspond with the first-wired-circuit-board-side terminal portion, thereby producing the relay flexible wiring circuit board (Cf.FIG. 5(b)).

1) Production of First Wired Circuit Board

A first metal substrate of stainless foil having thickness of 25 μm was prepared (Cf.FIG. 6(a)). Then, after solution of polyamic acid resin was applied to a surface of the first metal substrate, the applied resin was exposed to light and developed and thereafter cured by heating, thereby producing the first insulating base layer of polyimide having thickness of 10 μm of a predetermined pattern (Cf.FIG. 6(b)).

Then, a thin chromium film having thickness of 300 Å and a thin copper film having thickness of 800 Å were sequentially formed on the entire area of the first insulating base layer by sputtering, thereby forming a first thin conductor film which serves as a ground layer (Cf.FIG. 6(c)). Thereafter, a first plating resist was formed on a portion of the first thin conductor film corresponding to a reversed portion of a first conductor layer forming portion of the same (Cf.FIG. 6(d)). Then, a first electrolytic plated layer of copper having thickness of 10 μm was formed by electrolytic copper plating on the first thin conductor film exposed from the first plating resist (CF.FIG. 6(e)). Thereafter, the first plating resist was removed by the chemical etching (Cf.FIG. 6(f)). Then, the first thin conductor film on which the first plating resist had been formed was also removed by the chemical etching (Cf.FIG. 6(g)). The first conductor layer comprising the first thin conductor film and the first electrolytic plated layer was formed by this semi-additive process.

Then, after solution of polyamic acid resin was applied to a surface of the first insulating base layer including the first conductor layer, the applied resin was exposed to light and developed and thereafter cured by heating, thereby producing the first insulating cover layer of polyimide having thickness of 3 μm of a predetermined pattern opened at portions thereof corresponding to the suspension-board-side terminal portion and the IC-side terminal portion (Cf.FIG. 6(h)).

Thereafter, a through hole having a diameter of 0.5 mm φ extending through the first insulating cover layer, the first conductor layer, the first insulating base layer and the first metal substrate was formed at a portion corresponding to the second-wired-circuit-board-side terminal portion (Cf.FIG. 6(i)). Thereafter, a preamplifier IC was mounted on the IC-side terminal portion through a connecting pad, thereby producing the first wired circuit board equipped with the preamplifier IC (CF.FIG. 6(j)).

2) Production of Second Wired Circuit Board

A second wired circuit board was produced in the same manner as in Example 1.

3) Production of Flexible Wired Junction Circuit Board

An adhesive sheet of acrylic adhesive having thickness of 25 μm was adhesively bonded to the second insulating cover layer of the second wired circuit board produced in the manner described above at a front end portion thereof including its surrounding area around the opening corresponding to the first-wired-circuit-board-side terminal portion (Cf.FIG. 7(a)). Then, the first metal substrate of the first wired circuit board was press-bonded to the adhesive sheet in the condition that the through hole of the second-wired-circuit-board-side terminal portion was in alignment and to correspond with the first-wired-circuit-board-side terminal portion, thereby connecting together the first wired circuit board and the second wired circuit board (Cf.FIG. 7(b)).

Thereafter, solder cream was filled in the through hole and soldered by reflow soldering to form a solder connecting portion, whereby the relay flexible wiring circuit board was produced wherein the first-wired-circuit-board-side terminal portion of the second wired circuit board and the second-wired-circuit-board-side terminal portion of the first wired circuit board were electrically connected with each other through the solder connecting portion.

Comparative Example 1

Only the second wired circuit board not connected with the first wired circuit board in Example 1 was prepared as a flexible wired junction circuit of Comparative Example (however, this type1was equipped with the preamplifier IC and provided with the first-wired-circuit-board-side terminal portion serving as the suspension-board-side terminal portion: any known one of this type may be used).

Evaluation

1) Production of Suspension Board with Circuit

A metal substrate of stainless foil having thickness of 25 μm was prepared (Cf.FIG. 2(a)). Then, after solution of polyamic acid resin was applied to a surface of the metal substrate, the applied resin was exposed to light and developed and thereafter cured by heating, thereby producing the insulating base layer of polyimide having thickness of 10 μm of a predetermined pattern opened at a portion thereof corresponding to the first-wired-circuit-board-side terminal portion (Cf.FIG. 2(b)).

Then, a thin chromium film having thickness of 300 Å and a thin copper film having thickness of 800 Å were sequentially formed on the entire area of the insulating base layer by sputtering, thereby forming a thin conductor film which serves as a ground layer (Cf.FIG. 2(c)). Thereafter, a plating resist was formed on a portion of the thin conductor film corresponding to a reversed portion of a conductor layer forming portion of the same (Cf.FIG. 2(d)). Then, an electrolytic plated layer of copper having thickness of 10 μm was formed by electrolytic copper plating on the thin conductor film exposed from the plating resist (CF.FIG. 2(e)). Thereafter, the plating resist was removed by the chemical etching (Cf.FIG. 2(f)). Then, the thin conductor film on which the plating resist had been formed was also removed by the chemical etching (Cf.FIG. 2(g)). The conductor layer comprising the thin conductor film and the electrolytic plated layer was formed by this semi-additive process.

Then, after solution of polyamic acid resin was applied to a surface of the insulating base layer including the conductor layer, the applied resin was exposed to light and developed and thereafter cured by heating, thereby producing the insulating cover layer of polyimide having thickness of 3 μm of a predetermined pattern opened at a portion thereof corresponding to the magnetic-head-side terminal portion (Cf.FIG. 2(h)).

Thereafter, a portion of the metal substrate corresponding to the first-wired-circuit-board-side terminal portion was chemically etched (Cf.FIG. 2(i)). Then, the magnetic head was mounted on the magnetic-head-side terminal portion through the connecting pad, thereby producing the suspension board with circuit equipped with the magnetic head (Cf.FIG. 2(j)).

Measurement of Characteristic Impedance

Solder bumps were formed on the suspension-board-side terminal portions in the flexible wired junction circuits of Examples and Comparative Example. Then, the flexible wired junction circuits of Examples and Comparative Example were electrically connected to the suspension boards with circuits thus produced via the solder bumps, respectively. Then, the characteristic impedance between the connection point and the magnetic head and the characteristic impedance between the preamplifier IC and the connection point were measured by the time domain reflectometer (TDR) method. The results are shown in TABLE 1.

It is clearly seen from TABLE 1 that in Examples, no difference was found between both characteristic impedances, so that the matched characteristic impedances were obtained, while on the other hand, in Comparative Example, a significant difference was found therebetween, so that the matched characteristic impedances were not obtained.