JP2008038170A - Rolled copper foil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide rolled copper foil not only having excellent bending resistance, but also free from deterioration in strength even if being stored at room temperature over a long period, and suitable for a flexible printed circuit (FPC). <P>SOLUTION: The rolled copper foil comprises 0.0003 to 0.0005 wt.% phosphorous. In the cross-sectional structure of the copper foil annealed after final rolling, the cross-sectional area ratio to the whole cross-sectional area of penetrated crystal grains 2 penetrated through the copper foil 1 to the sheet thickness direction is ≥40%. In the rolled copper foil, the number of bending exceeds 300,000 times sufficient as the bending resistance of copper foil for an FPC, and further, its tensile strength after being left at room temperature (30°C) for a year is ≥350 MPa. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a rolled copper foil having excellent bending resistance, which is suitable for a flexible wiring member such as a flexible printed circuit board (hereinafter also referred to as FPC).

  With the recent miniaturization of electrical equipment, FPCs that can be mounted in a narrow space are mounted as wiring materials in digital cameras, mobile phones, HDDs, printers, liquid crystal panels, and the like. Further, since the FPC can be bent and can be mounted in a narrow space, the FPC is often used for a movable part of a disk-related device such as an HDD, a DVD, and a CD-ROM, and a folding part of a foldable mobile phone.

  For such applications, the FPC is required to have high durability against repeated bending. It is said that the bending resistance of the FPC is determined by the copper foil material that is inferior in bending resistance compared to the base film and coverlay, and the bending resistance of the copper foil is the most important among the constituent materials. Because of such required characteristics, rolled copper foil having higher bending resistance than electrolytic copper foil tends to be used for FPC.

  As a general manufacturing process of FPC, for example, a copper foil is bonded to a base film made of polyimide or polyester via an adhesive, and the whole is heated to a temperature of 130 to 180 ° C. to cure the adhesive. After that, the wiring is patterned, and then a coverlay is applied to protect the wiring. There is also a method of integrating the base film and the copper foil by heating and pressing instead of bonding them with an adhesive.

  Generally, as a material of rolled copper foil used for FPC, there are many tough pitch coppers that are softened at a temperature (130 to 180 ° C.) exposed in the manufacturing process of FPC and improved in bending resistance. However, from the viewpoint of the durability of the equipment, a rolled copper foil with higher bending resistance is required, for example, strengthening the crystallographic orientation of the copper foil, increasing the rolling rate, crystal grains before final rolling Many attempts have been made to improve anti-bending properties such as reducing the diameter.

As such a rolled copper foil having excellent bending resistance, for example, Japanese Patent No. 3009383 has an elongation of 15% or more in a state where the recrystallized structure is tempered by heating at 200 ° C. for 30 minutes. The strength (I) of the (200) plane determined by X-ray diffraction of the rolled surface is I / I ₀ > with respect to the strength (I ₀ ) of the (200) plane determined by X-ray diffraction of fine powder copper. A rolled copper foil characterized by having a cubic texture of 20 has been reported.

  However, when the rolling rate is increased or when the crystal grain size before final rolling is reduced, the accumulation of strain in the copper foil increases, so even when stored at room temperature, recrystallization occurs partially in the material. Occurs and the strength of the copper foil is reduced. For this reason, in handling the copper foil alone before the base film is laminated, there is a problem that handling properties are very poor.

  Japanese Patent Application Laid-Open No. 1-319641 discloses a soft rolled copper foil excellent in folding fatigue strength and heat resistance, containing 0.0005 to 0.002% by weight of phosphorus and containing inevitable impurities such as oxygen. A copper foil having an amount of 0.005% by weight or less and a cold work degree before final rolling of 65% or more is described. However, the above publication does not mention the improvement of the bending resistance and room temperature strength reduction of the copper foil which is the object of the present invention.

Japanese Patent No. 3009383 JP-A-1-319641

  In view of the above-described conventional problems, the present invention provides a rolled copper foil that is not only excellent in bending resistance characteristics, but also does not decrease in strength even when stored at room temperature for a long period of time, and is suitable for a flexible printed circuit board. With the goal.

  In order to achieve the above object, the rolled copper foil provided by the present invention contains 0.0003 to 0.005% by weight of phosphorus, and in the cross-sectional structure of the copper foil annealed after the final rolling, The cross-sectional area ratio of the penetrating crystal grains is 40% or more, which is excellent in bending resistance and does not lower the room temperature strength.

  According to the present invention, it is possible to provide an excellent rolled copper foil which is excellent in bending resistance and does not decrease in strength even when stored at room temperature (30 ° C.) for 1 year or longer. Therefore, the rolled copper foil of the present invention not only has high durability against repeated bending, but also has excellent storage and handling properties of the copper foil alone before bonding the base film, and is suitable as a copper foil for a flexible printed circuit board. Is.

  The rolled copper foil of the present invention contains 0.0003 to 0.005% by weight of phosphorus and has a cross-sectional area ratio of crystal grains penetrating in the plate thickness (thickness) direction in the cross-sectional structure of the copper foil annealed after the final rolling. That is, the ratio of the cross-sectional area of crystal grains penetrating in the plate thickness direction to the total cross-sectional area is 40% or more. The rolled copper foil having such characteristics has excellent bending resistance, and at the same time, the decrease in strength at room temperature is extremely small by containing phosphorus in particular. The reason why the decrease in room temperature strength of rolled copper foil can be suppressed by the inclusion of phosphorus is that even if the accumulation of strain in the copper foil increases due to strong processing, the strain is released by the solid solution of phosphorus in the copper matrix. This is probably because recrystallization is less likely to occur.

  In the rolled copper foil of the present invention, when the phosphorus content is less than 0.0003% by weight, the effect of preventing the decrease in room temperature strength is not sufficient, and conversely, when the phosphorus content exceeds 0.005% by weight. This is because even if the rolling rate is increased or the crystal grain size before rolling is reduced, the crystal grains after the heat treatment are not coarsened and sufficient bending characteristics cannot be obtained. Further, when the phosphorus content exceeds 0.005% by weight, there is a problem that the conductivity of the copper foil is lowered.

  Moreover, in the rolled copper foil of this invention, in order to acquire sufficient bending-proof characteristic, it is necessary to make the cross-sectional area ratio of the crystal grain penetrated in the plate | board thickness direction of copper foil 40% or more. More preferably, by setting the cross-sectional area ratio of the crystal grains penetrating the copper foil in the plate thickness direction to 60% or more, the ratio of the crystal grains penetrating the copper foil in the plate thickness direction appearing on the copper foil surface rapidly increases. There is a tendency, and thus the bending resistance of the copper foil is further improved.

  The reason why the flexibility of the copper foil is improved as the number of crystal grains penetrating the copper foil in the plate thickness direction is considered as follows. That is, dislocations are usually generated from within the crystal grains due to deformation due to bending, and the dislocations accumulate at the grain boundary part, and breakage occurs at the grain boundary part. On the other hand, in the part where the crystal grain penetrates the thickness of the copper foil, deformation due to bending becomes deformation of the single crystal itself, and dislocations generated in the crystal grain escape to the surface, so dislocation accumulation does not occur and is repeated. This is considered to be because breakage is less likely to occur with respect to the deformation.

  As shown in FIG. 1, the cross-sectional area ratio of crystal grains penetrating the copper foil in the plate thickness direction is determined by observing a cross-sectional metal structure of the copper foil 1 between the surface 1 a and the surface 1 b of the copper foil 1. The cross-sectional area A of the penetrating crystal grain 2 penetrating in the direction of the thickness d is obtained, and the ratio to the cross-sectional area B of the entire copper foil 1 including the non-penetrating crystal grain 3 not penetrating, ie, cross-sectional area A / cross-sectional area B Calculated. The specific cross-sectional area was measured by embedding a copper foil in a resin, mechanically polishing the cross-section of the copper foil to a mirror finish, etching with ammonia-hydrogen peroxide, and then measuring the structure with an optical microscope.

  In addition, the thickness of the rolled copper foil used for a flexible printed wiring board (FPC) is generally 50 micrometers or less, and it is becoming still thinner recently. Therefore, when measuring the cross-sectional area of the copper foil, it is preferable that the copper foil is folded and laminated in several layers and embedded in the resin. In addition, the sampling of the copper foil used for the cross-sectional microscopic observation is preferably at least 100 times the plate thickness of the copper foil in order to reduce the influence of local coarse crystal grains and fine crystal grains.

  In the production of the rolled copper foil of the present invention, after hot rolling a copper material having a composition containing 0.0003 to 0.005% by weight of phosphorus, cold rolling and tempering are repeated until a predetermined thickness is reached. Final cold rolling is performed to finish to a predetermined plate thickness, preferably 50 μm or less. Thereafter, the final rolled copper foil is smelted. For the smelting, the heat treatment is performed after the surface treatment such as plating in the roughening process of the rolled copper foil, or the base film in the FPC manufacturing process. It is performed by a heat treatment at a temperature of 130 to 180 ° C. exposed during the integration.

  In the rolled copper foil annealed after the final rolling, the cross-sectional area ratio of the crystal grains penetrating in the plate thickness direction can be controlled by the following conditions. That is, (1) If the average grain size before final rolling is the same, the larger the rolling reduction (rolling rate) of the final rolling, the larger the cross-sectional area ratio of the penetrating crystal grains. (2) If the rolling reduction (rolling rate) of the final rolling is the same, the smaller the average crystal grain size before the final rolling, the larger the cross-sectional area ratio of the penetrating crystal grains.

  Phosphorus was added to a copper melt adjusted to an oxygen concentration of 0.001% by weight or less, and copper having a phosphorus concentration of 0.0002 to 0.0001% by weight was melted as shown in Table 1 below. Thereafter, ingots each having a thickness of 200 mm and a width of 650 mm were prepared from the molten copper of each sample.

  The ingots of these samples are hot-rolled to a plate thickness of 18 mm, the scale on the surface is removed by chamfering, thinned to a plate thickness of 2.0 mm by cold rolling, intermediate annealing and cleaning are performed, The edge portion was trimmed to a width of 600 mm. Thereafter, cold rolling and annealing / washing were further repeated, and a rolled copper foil having a thickness of 0.016 mm (16 μm) was obtained by final cold rolling.

  Each rolled copper foil having a plate thickness of 16 μm obtained by the final rolling was subjected to a heat treatment at 180 ° C. for 30 minutes, imitating the heat treatment in the FPC manufacturing process. At this time, by changing the plate thickness before the final rolling as shown in Table 1 below, the crystal structure in the state where the final rolled foil having a plate thickness of 16 μm was annealed was controlled. The average crystal grain size of the copper foil before final rolling was adjusted to 20 ± 3 μm.

  About the rolled copper foil of each sample in the tempered state after final rolling, the cross-sectional area ratio of crystal grains penetrating in the plate thickness direction was measured, and the results are shown in Table 1 below. In addition, each rolled copper foil is sampled to be 200 times longer than the plate thickness, and after copper foil is laminated and embedded in resin, the cross section of the copper foil is mechanically polished and finished to a mirror surface, After etching with ammonia-hydrogen peroxide solution, the cross-sectional metal structure was observed with an optical microscope. Specifically, the ratio of the area of crystal grains penetrating the copper foil in the plate thickness direction to the total cross-sectional area (through-grain cross-sectional area ratio) was determined from a micrograph obtained by taking a 400-fold micrograph.

  Moreover, the bending life of the rolled copper foil of each sample was measured by the apparatus shown in FIG. That is, the test copper foil piece 5 is fixed to the fixed plate 6 and the movable plate 7 of this apparatus, and the movable plate 7 is periodically vibrated so that the intermediate portion of the test copper foil piece 5 has a predetermined radius of curvature. It is bent into a hairpin shape and breaks when it reaches a certain number of times. The number of times until this breakage was defined as the bending life. The test copper foil piece 5 was sampled from the rolled copper foil so that the length direction was parallel to the rolling direction. The measurement conditions were as follows: test copper foil piece 5 having a width of 12.7 mm and a length of 200 mm, a radius of curvature of 2.5 mm, a vibration stroke of 25 mm, and a vibration speed of 500 times / minute. The obtained results are also shown in Table 1 below.

  Furthermore, about the rolled copper foil of each sample, the strength fall by storage at room temperature was investigated. Regarding the reduction in strength of the rolled copper foil at room temperature, it is necessary to maintain a tensile strength of 350 MPa or more after standing at 30 ° C. for one year. Therefore, the initial tensile strength of the rolled copper foil was measured, and the tensile strength after being stored in a thermostatic bath at 30 ° C. for one year was measured. The obtained results are also shown in Table 1 below.

  According to the result of the said Table 1, each rolled copper foil of the samples 1-5 of this invention exists in the range whose phosphorus content is 0.0003-0.005 weight%, and annealed rolled copper foil. When the cross-sectional area ratio of crystal grains (penetrating grains) penetrating in the plate thickness direction in the cross-sectional structure of the copper foil is 40% or more, and the number of bending exceeds 300,000 times sufficient as the bending resistance of the copper foil for FPC At the same time, it can be seen that the strength of 350 MPa can be maintained even after storage for one year.

  On the other hand, each of the rolled copper foils of Comparative Samples 6 to 8 and 10 has a phosphorus content of 0.0003% by weight or more, but the cross-sectional area ratio of the penetrating grains is less than 40%. It did not reach 300,000 times. Further, the rolled copper foil of Sample 9 as a comparative example has a phosphorus content of less than 0.0003% by weight, so that the bending resistance is sufficient, but the strength is 350 MPa or less when stored at room temperature for only 10 months. Declined.

It is a schematic diagram which shows the metal structure of a rolled copper foil cross section. It is the schematic of the bending test apparatus used for the measurement of the bending life of rolled copper foil.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Copper foil 1a, 1b Surface 2 Through crystal grain 3 Non-penetrating crystal grain 5 Copper foil piece for a test 6 Fixed plate 7 Movable plate

Claims (1)

In the cross-sectional structure of copper foil containing 0.0003 to 0.005% by weight of phosphorus and annealed after final rolling, the cross-sectional area ratio of the crystal grains penetrating the copper foil in the plate thickness direction is 40% or more, and is resistant to bending Rolled copper foil that is superior in strength and does not lower room temperature strength.

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