JP2001168480A - Flexible printed circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flexible printed circuit board(FPC) for improving flex resistance. SOLUTION: In the flexible printed circuit board where a base film, a base- film side adhesive layer, a meta foil layer for forming a pattern circuit, a cover lay side adhesive layer, and a cover lay film are laminated successively, the metal foil of the metal foil layer is made of highly pure copper foil or other low-recrystallization temperature metal foil, the metal foil has a low recrystallization temperature, recrystallization occurs due to distortion energy caused by deformation, and distortion energy is released appropriately, thus achieving improved flexibility, at the same time suppressing the generation of grain boundaries and cracks, and hence obtaining improved flex resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible printed circuit (FPC) having improved bending resistance.

[0002]

2. Description of the Related Art As shown in FIG.
A pattern circuit of a metal foil layer 3 is formed on a base film side adhesive layer 2 of a base film 1, and a cover lay film 5 on which a cover lay side adhesive layer 4 is applied is attached thereon.

The base film 1 is usually made of a flexible film such as a polyimide resin or a polyethylene terephthalate (PET) resin. Using a film with a metal foil (CCL = Copper Composite Laminate) to which the foil layer 3 is attached, a desired pattern circuit is formed on the metal foil layer portion by applying a technique such as lithography. In general,
As the metal foil, an electrolytic copper foil or rolled copper foil using ordinary tough pitch copper is used. Then, a coverlay (CL = Cover Layer) film 5 made of a flexible film such as a polyimide resin or a PET resin is used, and the coverlay-side adhesive layer 4 applied to one side of the coverlay film 5 is used. To the base film 1 as described above.

[0004] The FPC thus obtained often has an overall thickness of about 0.1 mm or less, is rich in flexibility, and has excellent bending characteristics. For this reason, in recent years, it is often used as a substrate for a movable part of various electronic devices, for example, a hard disk drive (HDD) which is a data recording device for a personal computer. Also, such an HDD is
In the future, it is expected to be used as an image recording unit of a home VTR, a data recording unit for a digital camera, or a data recording unit for car navigation. In fact, such HDD
The movable part requires high bending resistance of tens of millions to hundreds of millions of times.

[0005]

However, this type of F
In PCs, when bending, especially with a small bending radius, is repeated, if the bending reaches a condition exceeding the fatigue limit of the metal foil forming the pattern circuit, the conductor portion of the pattern circuit deteriorates due to fatigue and the flexibility decreases. Eventually, grain boundaries and cracks occurred in the circuit, sometimes leading to disconnection.

[0006] The deterioration of the conductor portion is mainly caused by lattice defects such as unavoidable dislocations that occur when the metal foil undergoes plastic deformation. These defects are saturated and aggregate as fatigue progresses. Grain boundaries and cracks are formed, which leads to disconnection. For this reason, in the case of FPCs used under particularly severe bending conditions, in order to improve the fatigue characteristics, a recrystallized rolled foil having few grain boundaries and internal strain is often used for the metal foil. Below, this recrystallized rolled foil is not sufficient, and higher reliability is required.

[0007] The present invention has been made in view of such conventional problems, and basically focuses on a metal foil to be used, and by using a metal foil having a specific performance, a high bending resistance is obtained. It is intended to provide an excellent FPC to be exhibited.

[0008]

According to the present invention, a base film, an adhesive layer on the base film side, a metal foil layer forming a pattern circuit, an adhesive layer on the coverlay side, and a coverlay film are laminated in this order. In the flexible printed circuit board, the metal foil of the metal foil layer is a high-purity copper foil.

According to a second aspect of the present invention, there is provided the flexible printed circuit board according to the first aspect, wherein the high-purity copper foil has a residual resistance ratio of 3000 or more.

According to a third aspect of the present invention, there is provided the flexible printed circuit board according to the first aspect, wherein the metal foil of the metal foil layer is a low recrystallization temperature metal foil.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic structure of an FPC according to the present invention is substantially the same as the structure shown in FIG. 1, and a metal foil layer is provided on the base film side adhesive layer of the base film. A pattern circuit is formed, and a coverlay film on which a coverlay-side adhesive layer is applied is attached thereon.

And, as a material for the metal foil layer,
A rolled copper foil obtained by rolling high-purity copper purified by electrolytic refining and induction zone melting, and preferably high-purity copper having a residual resistance ratio of 3000 or more is used. This high-purity copper foil has a characteristic that the recrystallization temperature is low and the recrystallization is caused by its strain energy when the material is slightly fatigued by deformation. For this reason, the strain energy is released each time,
Lattice defects such as dislocations are less likely to accumulate in the conductors of the pattern circuit. Therefore, work hardening due to accumulation of strain is unlikely to occur, high flexibility is obtained, and generation of grain boundaries and cracks is suppressed, and as a result, a highly reliable FPC having excellent bending resistance is obtained.

In the case of the above-mentioned high-purity copper foil, the conventional manufacturing process can be used as it is, and since the conductivity is large, the thickness of the conductor foil can be reduced, and the advantage that it can be made softer is obtained. It can be said. However, when the manufacturing process is changed, another low recrystallization temperature metal (a metal having a recrystallization temperature near room temperature or lower), or an alloy or a liquid metal having a liquid phase region in a use region, for example, various ultra-high purity metals Metals and liquid metals such as Hg, Ga, In and their alloys, P
Low melting point metals such as b, Sn, Zn, and Bi and foils of alloys thereof can also be used. These metal foils also have the same effects as described above because the recrystallization temperature is low.

[0014]

EXAMPLES Examples of the present invention will be more specifically described below in comparison with Comparative Examples which lack the conditions of the present invention. FIG. 2 is a plan view of a sample-produced FPC, FIG. 3 is a schematic view showing a method of a flexibility test, and FIG. 4 is a schematic view showing a method of a fatigue test.

In the sample FPC 10 shown in FIG.
The base film 11 and the cover lay film 15 are polyimide resin films, each having a length of 100 mm, a width of 10 mm, and a thickness of 25 μm each.
The adhesive of each adhesive layer was epoxy resin, the thickness was 10 μm, and the thickness of the metal foil layer 13 forming the pattern circuit was 35 μm.

In the present invention, high-purity copper having a residual resistance ratio of 3000 or more purified by electrolytic refining and induction zone melting is used as the metal foil layer 13, and the surface is removed by etching to prevent contamination. The copper foil was rolled to a thickness of 35 μm. On the other hand, in the comparative example, the conventional electrolytic copper foil and the conventional rolled recrystallized copper foil were used, and the thickness was 35 μm in the same manner.
Copper foil.

In the flexibility test shown in FIG. 3, a sample FPC 10 to be tested is placed on a lower jig 20 and an upper jig 30 which are arranged in parallel with each other and has a bending radius R of 2 m.
m. And the lower jig 20
Is fixed, and the upper jig 30 is pushed down to the stopper pin 32 on the other end side about the shaft 31 on the left end side in the drawing, thereby applying a bending force to the FPC 10 for 12 hours.
The sensor was set in the free state, and the repulsive force at this time was measured by the sensor (load cell) 33.

Table 1 shows an FPC (Example 1) which is an example of the present invention and an FPC which is a comparative example in the flexibility test.
9 shows the repulsive force (g) of Comparative Examples 1 and 2.
From Table 1, it can be seen that the FPC of the comparative example has a large repulsive force and poor flexibility. In contrast, the FPC of the present invention
It can be seen that the resilience is small and good flexibility is obtained.

[0019]

[Table 1]

In the fatigue test shown in FIG. 4, a sample FPC 10 to be tested is placed on a lower jig 20 and an upper jig 30 which are arranged in parallel with each other, and the bending radius R thereof is 2 mm.
It was pasted in such a way. Then, the lower jig 20 is fixed, and the upper jig 30 is
The FPC 10 was reciprocated in the horizontal direction at a speed of 0 times / minute, and the number of times of bending until the FPC 10 was broken was measured.

Table 2 shows the FPC (Example 1) which is an example of the present invention and the FPC which is a comparative example in the fatigue test.
It is the number of times of bending until breaking in Comparative Examples 1 and 2. From Table 2, it can be seen that the FPC of the comparative example has a small number of bending times and low bending resistance. On the other hand, it can be seen that the FPC of the present invention has a large number of bending times and high bending resistance.

[0022]

[Table 2]

[0023]

As is clear from the above description, according to the FPC according to the present invention, since the metal foil of the metal foil layer is a high-purity copper foil or another low recrystallization temperature metal foil, these metal foils In this case, the recrystallization temperature is low, the recrystallization occurs due to the strain energy due to deformation, and the strain energy is appropriately released, so that high flexibility can be obtained, and the generation of grain boundaries and cracks is suppressed, and the bending resistance is excellent. FPC of reliability
Is obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a schematic structure of an FPC.

FIG. 2 is a plan view of the FPC of FIG.

FIG. 3 is a schematic view showing a method of a flexibility test of an FPC.

FIG. 4 is a schematic view showing a method of a fatigue test of an FPC.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Base film 2 Base film side adhesive layer 3 Metal foil layer 4 Cover lay side adhesive layer 5 Cover lay film

Claims (3)

[Claims] 1. A flexible printed circuit board in which a base film, a base film side adhesive layer, a metal foil layer forming a pattern circuit, a coverlay side adhesive layer and a coverlay film are laminated in this order. Flexible printed circuit board characterized by using high-purity copper foil. 2. The high-purity copper foil has a residual resistance ratio of 3000.
The flexible printed circuit board according to claim 1, wherein: 3. The flexible printed circuit board according to claim 1, wherein the metal foil of the metal foil layer is a low recrystallization temperature metal foil.