Rolled copper foil and manufacturing method of rolled copper foil

The present application claims priority from Japanese patent application serial no. 2008-112476 filed on Apr. 23, 2008, which further claims priority from Japanese patent application serial no. 2008-001069 filed on Jan. 8, 2008, the contents of which are hereby incorporated by reference into this application.

1. Field of the Invention

The present invention concerns a rolled copper foil and, particularly, it relates to a rolled copper foil which has an excellent flexible fatigue property suitable for flexible wiring materials such as flexible printed circuits.

2. Description of Related Art

Flexible printed circuit (hereinafter simply referred to as FPC) boards have high freedom in a mounting form to electronic equipment due to their attractive features of small thickness and excellent flexibility. Accordingly, FPC boards have been used generally as, e.g., wirings for bending portions of foldable (clamshell type) cellular phones, movable portions of digital cameras, printer heads, etc., and movable portions of equipment relevant to disks such as HDDs (hard disk drives), DVDs (digital versatile disks) and CDs (compact disks).

As conductors for FPC board, pure copper foils or copper alloy foils (hereinafter collectively referred to as the copper foils) with various types of surface treatment applied have been generally used. The copper foils are classified into electrodeposited copper foils and rolled copper foils according to the manufacturing methods thereof. Since FPC boards are used as wiring materials for repetitively moving portions as described above, an excellent flexible fatigue property (e.g., flexible fatigue property of 1,000,000 cycles or more) has been required, and rolled copper foils are often selected as the copper foils.

Generally, the rolled copper foils are manufactured by applying a hot rolling step to a cast ingot made of tough pitch copper (JIS H3100 C1100) or oxygen-free copper (JIS H3100 C1020) as a raw material, and then by repeating a cold rolling step and a process annealing step until a predetermined thickness is proved. The thickness of rolled copper foils required for FPC boards is usually 50 μm or less, and in recent years thinner foils for FPC boards have been demanded to ten-odd μm or less.

A manufacturing for the FPC board generally comprises: “a step of bonding a copper foil for an FPC board and a base film (base material) comprising resin such as polyimide to form a CCL (copper clad laminate) (CCL step)”; “a step of forming a printed circuit by a method such as etching for the CCL”; “a step of applying surface treatment on the circuit for protection of the wirings”, etc. The CCL step includes two kinds of methods, i.e., a method of laminating a copper foil and a base material with an adhesive and then curing and adhering the adhesive by heat treatment (3-layered CCL), and a method of directly bonding a copper foil to which surface treatment has been applied to a base material without an adhesive and then integrating them by heating and pressing (2-layered CCL).

In the FPC board manufacturing step, cold rolled copper foils (in a hard state in which they are work hardened) have been often used from a viewpoint of easy handling. In a case where the copper foil is in an annealed (softened) state, the copper foil is easy to deform (e.g., elongation, creasing, flexing, etc.) upon cutting of the copper foil or lamination with the base material, resulting in a product failure.

On the other hand, the flexible fatigue property of the copper foil is improved remarkably by applying recrystallization annealing, as compared with the copper foil in the as-rolled state. Then, a manufacturing method has been generally selected in which the heat treatment for adhering the base material and the copper foil in the CCL step is also served for recrystallization annealing for the copper foil. The heat treatment condition in this case is usually at a temperature of 180° C. to 300° C. for 1 to 60 minutes (typically at 200° C. for 30 minutes) and the copper foil is in a state refined into a recrystallization texture.

For improving the flexible fatigue property of FPC boards, it is effective to improve the flexible fatigue property of the rolled copper foil as the material thereof. Further, it has been known that the flexible fatigue property of the copper foil after recrystallization annealing is more improved as a cubic texture is developed. In general, “development of the cubic texture” only means that the occupation ratio of a {200}_Cu plane is high at the rolled surface (e.g., 85% or more).

Heretofore, for rolled copper foils with an excellent flexible fatigue property and rolled copper foil manufacturing methods, there have been reported as follows. They are: e.g., a method of developing the cubic texture by increasing a total working ratio in a final cold rolling step (to, e.g., 90% or more); a copper foil defined for the degree of development of the cubic texture after recrystallization annealing (e.g., the intensity of a (200)_Cu plane determined by X-ray diffraction to the rolled surface is at least 20 times greater than that of the (200)_Cu plane determined by powder X-ray diffractometry); a method of further developing the cubic texture after recrystallization by developing the cubic texture during process annealing before the final cold rolling step to increase the total working ratio in the final cold rolling step to 93% or more; a copper foil for which the ratio of penetration crystal grains in the direction of thickness of the copper foil is defined (e.g., 40% or more as a cross sectional area ratio); a copper foil for which the softening temperature is controlled by the addition of a small amount of additive elements (e.g., controlled to a half-softening temperature of 120 to 150° C.); a copper foil for which the length of a twin boundary is defined (e.g., the total length of the twin boundary with a length exceeding 5 μm in an area of one square millimeter is 20 mm or less); a copper foil for which the recrystallization texture is controlled by the addition of a small amount of additive elements (e.g., the Sn is added by 0.01 to 0.2 mass % to control the average crystal grain size to 5 μm or less and the maximum crystal grain size to 15 μm or less), etc. (see, e.g., JP-A-2001-262296, JP-B-3009383, JP-A-2001-323354, JP-A-2006-117977, JP-A-2000-212661, JP-A-2000-256765, and JP-A-2005-68484).

As described above, it is reported that, in the prior art, as the total working ratio is increased, the cubic texture of the rolled copper foil is developed after recrystallization annealing and thereby the flexible fatigue property is improved. In cold rolling working, however, as the final rolling working ratio increases, the material (copper foil) becomes harder due to work hardening. It then becomes difficult to control the working ratio per rolling pass, lowering the efficiency of copper foil manufacturing. Specifically, when the total working ratio reaches about 90% (particularly, 93% or more), control of the working ratio per rolling pass and rolling itself become rapidly difficult.

In recent years, however, along with development in downsizing of electronic equipments, an increase in their integration degree (higher density mounting), and their higher performance, requirements for more improved flexible fatigue property than before have been increasingly made for FPC boards. Since the flexible fatigue property of the FPC board is determined substantially depending on that of the copper foil, it is essential to further improve the flexible fatigue property of the copper foil so as to satisfy the requirements. More and more demands for low costs of electronic components also rise.

Under these circumstances, it is an objective of the present invention to provide a rolled copper foil which is suitable to flexible wiring materials such as for flexible printed circuit (FPC) boards and has an excellent flexible fatigue property. Furthermore, it is another objective of the present invention to provide a method of stably manufacturing a rolled copper foil with an improved flexible fatigue property in an efficient manner (i.e., at a low cost) without carrying out working at a high degree, as is done in the prior art, in the final cold rolling step.

As the results of detailed metal crystallographic studies on a rolled copper foil by the inventors, it was found that there was a specific correlation among: the crystal grains orientation of a rolled copper foil after green sheet annealing and before the final cold rolling step; that of the rolled copper foil after the final cold rolling step and before the recrystallization annealing; that of the rolled copper foil after the recrystallization annealing; and the flexible fatigue property of the copper foil. It was also found that this correlation seems to be different from the principle that has been considered. Based on these findings, the present invention has been completed as described below. (Details will be described later.)

(1) According to one aspect of the present invention, a rolled copper foil obtained after a final cold rolling step but before recrystallization annealing includes a group of crystal grains which exhibits four-fold symmetry in results obtained by X-ray diffraction pole figure measurement with respect to a rolled surface, in which at least four peaks of a {200}_Cu plane diffraction of a copper crystal due to the group of crystal grains exhibiting the four-fold symmetry, which is obtained by β axis scanning with an α angle set to 45°, appear at intervals of 90°±5° along the β angle.

Besides, as a material for the rolled copper foil, is preferably used a copper metal with a purity of 99.9% or more (so-called three nines up) such as a tough pitch copper (e.g., JIS H3100 C1100), an oxygen-free copper (e.g., JIS H3100 C1020) and etc. Furthermore, a copper alloy with the copper purity of 99.9% or more may be also used.

In the above aspect (1), the following modifications and changes can be made.

(i) The diffraction peaks exhibiting the four-fold symmetry at intervals of 90°±5° along the β angle each have a diffraction intensity at least 1.5 times stronger than a minimum intensity of {200}_Cu plane diffractions of the copper crystal, which are obtained by the β axis scanning.