Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-12-04
2004-02-10
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S682000
Reexamination Certificate
active
06689235
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to flexible wiring members such as flexible printed circuits (hereinafter called “FPCs”) having excellent flex performance with ease of fabrication.
2. Prior art
Printed wiring boards based on organic substrates are roughly divided into two types; a rigid type having a rigid, copper-clad laminate consisting of glass-epoxy and paper-phenol substrates, and a flexible type having a flexible, copper-clad laminate consisting of polyimide and polyester substrates. Copper foil is mainly employed as a conductive material for the printed wiring boards. Foil products are classified into electrodeposited and rolled foils, depending on the manufacturing processes used.
Of the printed wiring boards, those used for flexible printed circuits (FPCs) are fabricated by laminating a copper foil to a resin substrate and joining the layers with adhesive or with the application of heat and pressure into an integral board. In recent years, multilayer boards known as “built-up” boards have come into extensive use as an effective means for high-density packaging or mounting. The copper foil that is used to form components for FPCs is, for the most part, rolled copper foil.
FPCs are largely used in printer heads, hard disk drives, and other components where wiring or conductive connections to movable parts are required. They are subjected to more than a million times of repetitive bending in service. With the recent tendency toward miniaturization and higher performance levels of devices, requirements for flex performance are becoming more severe than heretofore.
The material for copper foil to be used in FPCs is mostly tough-pitch copper (containing 100-500 ppm oxygen). The foil is manufactured by hot rolling an ingot of such material and then repeating cold rolling and annealing alternately until a predetermined thickness is achieved. The rolled copper foil is then plated for surface roughing for enhanced adhesion to a resin substrate. Following the roughing plating, the copper foil is cut into pieces and each piece is laminated to a resin substrate. To join the copper foil and resin together, an adhesive of thermosetting resin, e.g., epoxy, is used. The adhesive is hardened by heating at 130 to 170° C. for several hours to several days. Thereafter the copper foil is etched to form various wiring or conductive patterns.
The flex property of a copper foil is markedly improved by recrystallization annealing over that of the foil as rolled. Therefore, the foil is used in the annealed state as an FPC component. The annealing is done either by heat treatment after the roughing plating and cutting into a size or by utilizing the heating at the time of adhering to the resin substrate. The reason for which the annealing is performed during the course of fabrication rather than using an annealed copper foil from the beginning is that, when the copper foil is soft after annealing, it can be deformed or wrinkled upon cutting and laminating to the resin substrate, and a foil hard as rolled is preferred because of the ease of fabrication into an FPC.
For enhanced flex performance of an FPC, improving the flex fatigue property of a rolled copper foil as the starting material is beneficial. The flex fatigue property of an annealed copper foil is improved with the development of its cube texture. In order to help develop the cube texture, it is effective in the copper foil manufacturing process to increase the final rolling reduction ratio and decrease the grain diameter with the annealing immediately before the final rolling (Japanese Patent Application No. 10-101858). Actually, a copper foil manufactured by such a process shows a sharp drop of the softening temperature due to an increase in the plastic strain accumulated by rolling. In extreme cases the foil, even stored at room temperature, can soften after a long period of storage. As noted already, a softened copper foil, if used in the fabrication of an FPC, can cause troubles such as foil deformation and can seriously affect the ease of FPC fabrication. For these reasons it is necessary, when the above manufacturing process is adopted to obtain a copper foil with improved flex property, to heighten the softening temperature of the copper foil to a proper level.
The problem of rolled copper foil softening while being stored at room temperature is pointed out also by Japanese Patent Application Kokai No. 10-230303. The prior application, as a means of controlling the problem, proposes manufacturing a copper foil at a low rolling reduction ratio of 50-90%. In fact, however, a copper foil made at such a low rolling reduction ratio has a seriously deteriorated flex fatigue property, and the proposed means is not acceptable when a copper foil with an excellent flex property is to be obtained. Under the circumstances the present invention aims at raising the softening temperature of a copper foil to a proper level by adding trace amounts of alloying elements to an ordinary tough-pitch copper foil. Various copper alloy foils with additions of many different elements have hitherto been proposed for use in FPCs. However, none of them have proved helpful in solving the problem that the present invention faces; for example, (1) one of the purposes for which an alloying element is added is to inhibit the development of the (100) orientation of a cube texture, and the softening temperature of the resulting alloy foil is unusually high (Japanese Patent No. 2505480), and (2) a large proportion of an alloying element(s) is added to copper to improve its flex resistance (Japanese Patent Application Kokai No. 59-78592).
SUMMARY OF THE INVENTION
The object of the present invention is to provide a rolled copper foil for FPCs which combines an excellent flex property with an adequate softening property, by appropriately elevating the softening temperature of a high-flexing rolled copper foil. This eliminates the troubles that can otherwise result from foil softening during storage.
The present invention settles the problems of the prior art and concerns the following:
(1) A rolled copper foil for flexible printed circuits comprising, all by weight, from 0.0100 to 0.0400% of Ag, from 0.0100 to 0.0500% of oxygen, not more than 0.0030% in total of one or more elements selected from the group consisting of S, As, Sb, Bi, Se, Te, Pb, and Sn, and the balance copper, said foil having a thickness in the range of 5 to 50 microns, a half-softening temperature of 120 to 150° C., being capable of retaining a tensile strength of at least 300 N/mm
2
at 30° C., and possessing excellent flex property and adequate softening properties.
(2) A rolled copper foil for flexible printed circuits comprising, all by weight, from 0.0100 to 0.0400% of Ag, from 0.0100 to 0.0500% of oxygen, not more than 0.0030% in total of one or more elements selected from the group consisting of S, As, Sb, Bi, Se, Te, Pb, and Sn, and the balance copper, said foil having a thickness in the range of 5 to 50 microns, the intensity (I) of the (200) plane determined by X-ray diffraction of the rolled surf ace after annealing at 200° C. for 30 minutes being I/I
o
<20 with respect to the X-ray diffraction intensity (I
o
) of the (200) plane of fine copper powder, said foil having a half-softening temperature of 120 to 150° C., being capable of retaining a tensile strength of at least 300 N/mm
2
at 30° C., and possessing excellent flex property and adequate softening properties.
(3) A method of manufacturing the rolled copper foil according to (1) and (2) above by a process which comprises hot rolling an ingot, repeating cold rolling and annealing alternately, and finally cold rolling the work to a foil, the annealing immediately preceding the final cold rolling being performed under conditions that enable the annealed recrystallized grains to have a mean grain diameter of not greater than 20 microns, the reduction ratio of the final cold rolling being beyond 90.0%, whereby an excellent flex property and an adequate softening property are achieved.
Hatano Takaaki
Kurosawa Yoshio
Drinker Biddle & Reath LLP
Ip Sikyin
Nippon Mining & Metals Co., Ltd.
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