Softening-resistant copper alloy and method of forming sheet of the same

Abstract

A softening-resistant copper alloy contains Fe in an Fe content in the range of 0.01 to 4.0% by mass. The copper alloy has a cube orientation density of 50% or below and a mean grain size of 30 μm or below after being annealed at 500° C. for 1 min. A copper alloy sheet forming method of forming a copper alloy sheet comprises, in successive steps: a hot rolling process for hot-rolling a copper alloy sheet of the copper alloy according to any one of claims 1 to 4, at least two working cycles each of a cold rolling process and an annealing process, and a finish cold rolling process. Reduction ratio for each of the cold rolling processes of the working cycles is in the range of 50 to 80%, and reduction ratio for the finish cold rolling process is in the range of 30 to 85%.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a softening-resistant copper alloy having high softening resistance and a method of forming a sheet of the softening-resistant copper alloy. The softening-resistant copper alloy having high softening resistance can be effectively applied to various fields including the electric, the electronic and the mechanical field.

2. Description of the Related Art

The recent progressive advancement of device miniaturization, device thinning and weight reduction of various devices including electronic devices has urged the rapid progress of the miniaturization and weight reduction of copper alloy parts including lead frames, terminals and connectors for those small, lightweight devices.

For example, a copper alloy containing Fe in a small Fe content is sued widely for forming lead frames for semiconductor devices. A copper alloy designated by CDA194 excellent in strength, electric conductivity and thermal conductivity is used widely as an international standard copper alloy. The alloy CDA194 contains 2.1 to 2.8% by mass (hereinafter, referred to simply as “%”) Fe, 0.015 to 0.15% P, and 0.05 to 0.20% Zn.

Generally, lead frames having a plurality of leads are fabricated by subjecting a copper alloy sheet of the foregoing chemical composition to a stamping process. Recently, thickness reduction of copper alloy sheets and increase in the number of leads of each lead frame have progressively advanced to cope with device miniaturization, device thinning and weight reduction of electric and electronic devices. Residual stresses are liable to be induced in such thin lead frames having a large number of leads formed by the stamping process and the leads of such thin lead frames tend to be arranged irregularly. Therefore, the lead frame with many leads formed by subjecting the copper alloy sheet to the stamping process is subjected to a heat treatment, such as an annealing process, to remove residual stresses. Such a heat treatment often softens the workpiece, and the workpiece treated by the heat treatment is unable to maintain its initial mechanical strength. The heat treatment is desired to be carried out at a higher process temperature in a shorter time to improve productivity. Therefore, there is a strong demand for heat-resistant materials capable of maintaining its high strength after being heat-treated.

Alloy elements including Fe, P and Zn and additional trace elements including Sn, Mg and Ca are added to copper alloys or the contents of those alloy elements and additional trace elements are adjusted to meet such a demand. However, it is impossible to achieve the miniaturization, weight reduction and improvement of softening resistance of copper alloy parts satisfactorily simply through the adjustment of the chemical composition of the copper alloy. Therefore, studies have been made in recent years to develop techniques of controlling the texture of copper alloys.

A technique disclosed in JP-A No. 2003-96526 (Patent document 1) increases strength by controlling intensity ratio of diffraction after finish rolling and grain size before finish rolling. A technique disclosed in JP-A No. 2002-339028 (Patent document 2) improves workability by controlling cube orientation density in addition to controlling intensity ratio of diffraction.

The technique disclosed in Patent document 1 increases the strength of a copper alloy sheet of a copper alloy produced by adding a trace of Ag to oxygen-free copper by subjecting a copper alloy sheet to a hot rolling process, subjecting the hot-rolled copper alloy sheet to a plurality of working cycles each of a cold rolling process and a recrystallization annealing process, and subjecting the copper alloy sheet to finish rolling process, wherein the reduction ratio of the finish cold rolling is controlled, mean grain size after the recrystallization annealing process immediately before the finish cold rolling process and the reduction ratio of the cold rolling process subsequent to the last recrystallization annealing process are controlled to control intensity ratio of diffraction after finish cold rolling process and grain size before finish cold rolling process.

According to Patent document 1, x-ray diffraction strength must be properly controlled because strength decreases and an anisotropic etching character appears as cubic orientation density increases. Patent document 1 mentions about the high softening resistance of this copper alloy. However, high softening resistance intended by the present invention cannot be achieved by simply applying rolling and annealing conditions to processing the copper alloy sheet and hence further improvement is desired.

According to Patent document 2, a copper alloy suitable for forming electronic parts and having improved workability and formability can be obtained by properly controlling the intensity of diffraction of (200) and (220), and cube orientation density. However, high softening resistance intended by the present invention cannot be guaranteed by the technique disclosed in Patent document 2.

SUMMARY OF THE INVENTION

The present invention has been made in view of those problems in the prior art techniques an it is an object of the present invention to provide a copper alloy suitable for the miniaturization and weight reduction of copper alloy parts for electric and electronic devices and capable of maintaining high strength even if the copper alloy is processed by a heat treatment, such as an annealing process, and a method of forming a sheet of the copper alloy.

A copper alloy having high softening resistance in one aspect of the present invention contains Fe, has a cube orientation density of 50% or below after being annealed at 500° C. for 1 min, and, preferably, a mean grain size of 30 Km or below.

The copper alloy of the present invention contains inexpensive Fe as an essential alloy element in a small Fe content. Although there is not any particular restriction, a desirable Fe content is in the range of 0.01% to 4%. Other possible alloy elements are 0.03% or below P and about 1% Zn. Desirable respective contents of other elements are limited to those of unavoidable impurities.

A copper alloy sheet forming method in another aspect of the present invention includes, in successive steps, a hot rolling process for hot-rolling a copper alloy sheet of a copper alloy containing Fe, at least two working cycles each of a cold rolling process and an annealing process, and a finish cold rolling process; wherein reduction ratio for each of the cold rolling processes of the working cycles is in the range of 50 to 80%, and reduction ratio for the finish cold rolling process is in the range of 30 to 85%.

According to the present invention, a stable copper alloy having high softening resistance can be produced by controlling cube orientation density after annealing at 500° C. for 1 min to 50% or below. Reduction of strength that occurs in the sheet of the conventional material when the sheet is treated by a heat treatment for annealing or the like can be suppressed to the least unavoidable extent. Consequently, even in a case where decrease in dimensional accuracy of parts due to residual stress induced in the parts by a stamping process or the like is expected, strength reduction due to annealing can be suppressed, decrease in dimensional accuracy can be prevented, and copper alloy parts of stable quality can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a cube orientation mapping of a copper alloy sheet in a preferred embodiment according to the present invention produced by using “EBSP measuring and analyzing system OIM; and

FIG. 2 is a grain size histogram of the copper alloy sheet obtained by using “EBSP measuring and analyzing system OIM”.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A copper alloy having high softening resistance in a preferred embodiment according to the present invention contains Fe in a small Fe content. The copper alloy has a cube orientation density of 50% or below after annealing at 500° C. for 1 min and, preferably, the copper alloy has a mean grain size of 30 Km or below after annealing at 500° C. for 1 min for the following reasons.

The inventors of the present invention found through experiments that the higher the cube orientation density of a copper alloy containing Fe in a small Fe content after annealing, the higher the rate of reduction of the strength of the copper alloy due to heat treatment, that the lower the cube orientation density, the less is strength reduction and the higher is the softening resistance, that the orientation density can be quantitatively evaluated on the basis of cube orientation density after annealing under specific conditions of 500° C. and 1 min, that the rate of strength reduction due to heat treatment becomes obviously high when the cube orientation density is greater than 50%, and that copper alloys having orientation densities of 50% or below, preferably, 40% or below exhibit stable, high softening resistance.

Mean grain size after annealing at 500° C. for 1 min, as well as cube orientation density, is closely related with softening resistance. Copper alloys having mean grain sizes of 30 μm or below have particularly high softening resistance.

The term “Cube orientation” signifies a state where the <001> direction of crystals is parallel to rolling direction, a normal to a rolled surface and width. In a rolled surface, a (100) plane is oriented. The ratio of grains having the cube orientation increases as cube orientation develops. If cube orientation develops excessively, the strength of the copper alloy decreases. High softening resistance intended by the present invention can be secured if the cube orientation density is controlled to 50% or below.

The cube orientation density can be measured by an EBSP (electron back-scatter diffraction pattern) method. The EBSP method projects an electron beam on the surface of a specimen, and obtains a Kikuchi pattern (cube orientation mapping) as shown in FIG. 1 formed by reflected electrons. The crystal orientation in a part on which the electron beam falls can be known through the analysis of the Kikuchi pattern. A crystal orientation distribution is measured by two-dimensionally scanning the surface of the specimen with the electron beam and measuring crystal orientation at predetermined pitches.

However, if the specimen has many defects, such as strain fields and deformation bands formed by machining, such as stamping, and dislocation, it is difficult to obtain a Kikuchi pattern. A copper alloy sheet of the copper alloy of the present invention is finished by cold rolling by high reduction. Therefore, the cube orientation density of the copper alloy sheet as finished by cold rolling cannot be measured by the EBSP method. Therefore, the cube orientation density of the copper alloy sheet is measured after annealing the cold rolled copper alloy sheet at 500° C. for 1 min.

Crystal grains of the same orientation increase as cube orientation develops. Consequently, irregularity of atoms in grain boundaries decreases and grain obviously tend to grow. It was confirmed that the copper alloy sheet maintains high strength after annealing at 500° C. for 1 min when grain sizes are 30 Km or below, preferably 25 μm or below.

The copper alloy of the present invention contains Fe as an essential component. Although there are not particular restrictions on the Fe content and the composition of the copper alloy, it is desirable that the Fe content is between 0.01% and 4.0% to make the copper alloy exhibit its characteristics effectively. If the Fe content is less than 0.01%, the amount of Fe precipitates or Fe-base intermetallic compounds is small and the strength of the copper alloy sheet is insufficient for forming lead frames, terminals and connectors, and the softening resistance of the copper alloy sheet is insufficient. Strength does not increase and softening resistance does not improve even if the Fe content is increased beyond 4.0%, and a large amount of coarse dispersoids containing Fe adversely affecting the castability and workability of the copper alloy. Therefore, it is desirable that the Fe content of the copper alloy is 4.0% or below. Amore desirable Fe content is between 0.03% and 3.5%, more preferably, between 0.05% and 3.0% to provide a copper alloy satisfactory in strength, softening resistance, castability and hot-workability.

The copper alloy of the present invention may contain P and Zn in addition to Fe. A suitable P content is about 1% or below. Because when the P content is increased beyond 1%, a large amount of coarse dipersoids generate and they deteriorate castability. Zn is an element effective in preventing the separation of Sn and solder. The effect of Zn saturates at certain Zn content. Excessive Zn content deteriorates the wettability of molten Sn and molten solder. A desirable Zn content is about 1.0% or below. Other elements do not need to be added intentionally. The copper alloy may contain unavoidable impurities, such as Pb, Ni, Mn, Cr, Al, Mg, Ca, Be, Si, Zr and In, and some of those impurities may be intentionally added to the copper alloy without departing from the scope of the present invention.

A method of forming a copper alloy sheet of a copper alloy conforming to the foregoing cube orientation density and mean grain size and high in softening resistance will be described.

A method of forming a copper alloy sheet of the copper alloy of the present invention includes, in successive steps, a hot rolling process for hot-rolling a copper alloy sheet of a copper alloy, at least two working cycles each of a cold rolling process and an annealing process, and a finish cold rolling process. The copper alloy sheet is finished in a desired thickness by the finish cold rolling process. A generally used conventional copper alloy forming method includes the least necessary processes, such as a hot rolling process, a cold rolling process, an annealing process and a finish rolling process from the viewpoint of productivity and cost. The inventors of the present invention found that nuclei of cube orientation are formed if the reduction ratio in one cold rolling pass is excessively high and cube orientation is liable to develop during the annealing of the copper alloy sheet and that the development of rolling textures in B orientation ({011} and <211>) and S orientation ({123} and <634>) is suppressed when the reduction ration in one cold rolling pass is excessively low and cube orientation and many nuclei of cube orientation existed in the copper alloy sheet before the cold rolling process, i.e., after the hot rolling, remains in the copper alloy sheet.

When the copper alloy sheet is processed by at least two working cycles each of the cold rolling process and the annealing process, and reduction ratio for each cold rolling process is properly controlled, the development of cube orientation and the formation of nuclei can be effectively suppressed. If the reduction ratio for each cold rolling process is below 50% or greater than 80%, cube orientation grows easily when the copper alloy sheet is annealed and the cube orientation density increases beyond the foregoing desirable upper limit when the copper alloy sheet is annealed at 500° C. for 1 min. Grains grow abnormally as cube orientation develops, the mean grain size exceeds 30 μm and the softening resistance of the copper alloy sheet deteriorates. Thus the reduction ratio for each cold rolling process in the range of 50 to 80%, and the execution of at least two working cycles each of the cold rolling process and the annealing process are essential conditions of the present invention.

Although the repetition of the working cycle of the cold rolling process and the annealing process to suppress the development of cube orientation and the formation of nuclei effectively widens the allowable reduction ratio range for the finish cold rolling, it is desirable that reduction ratio for the finish cold rolling process is between 30% and 85%, more desirably, between 35% and 80%.

The present invention requires the cube orientation density of the copper alloy sheet controlled under predetermined conditions to be 50% or below. Thus the strength of the copper alloy sheet is decreased scarcely by annealing, and the copper alloy sheet has high softening resistance. A copper alloy sheet having high softening resistance can be surely manufactured by subjecting the copper alloy sheet to the working cycles each of the cold rolling process for rolling the copper alloy sheet at the predetermined reduction ratio and the annealing process, and properly controlling the reduction ratio for the finish cold rolling process.

The copper alloy sheet of the present invention thus manufactured has high softening resistance and strength that is scarcely decreased by heat treatment, such as annealing. The copper alloy sheet can be effectively used for forming copper alloy parts that are subjected to a heat treatment, such as annealing, after a final machining process, such as IC lead frames, terminals and connectors.

EXAMPLES

Examples of the present invention and comparative examples will be described.

Copper alloys of chemical compositions shown in Table 1 were melted in a coreless low-frequency induction furnace and copper alloy ingots of 50 mm in thickness, 200 mm in width and 500 mm in length were produced by a semicontinuous casting process. Each of the ingots was heated and the thickness was reduced to 12 mm by hot rolling and the sheet was machined by facing. Then the ingot was processed by a plurality of working cycles each of a cold rolling process and an annealing process, and the sheet was rolled in a copper alloy sheet of about 0.2 mm in thickness by finish cold rolling.

The copper alloy sheets were annealed at 500° C. fore 1 min in a salt bath. Specimens were sampled from the annealed copper alloy sheets. The specimens were ground and buffed. The surfaces of the specimens were finished by electrolytic polishing. A region of 500 μm×500 μm in the surface of each of the test specimens was measured at pitches of 1 μm by a scanning electron microscope (Model JEOL JSM 5410, Nippon Denshi) and an EBSP measuring and analyzing system OIM (orientation imaging macrograph) (TSL). Cube orientation densities (within 150 from an ideal orientation) and mean grain sizes were determined by using analyzing software “OIM Analysis” of the EBSP measuring and analyzing system.

FIG. 1 is a cube orientation mapping of the copper alloy sheet in Specimen 1 specified in Table 1 obtained by using the EBSP measuring and analyzing system OIM. In FIG. 1, black parts are cube orientation. Cube orientation density can be determined by analyzing the cube orientation mapping by the analyzing software. FIG. 2 is a grain size histogram of the copper alloy sheet in Specimen 1 obtained by the analyzing software. Mean grain size can be determined from the grain size histogram showing area fractions for grain sizes.

The softening resistance of each specimen was evaluated on the basis of the rate of reduction of hardness due to annealing. Test pieces of 0.2 mm in thickness, 10 mm in width and 10 mm in length were sampled from both a copper alloy sheet finished by the finish cold rolling and a copper alloy sheet obtained by annealing the copper alloy sheet finished by finish cold rolling at 500° C. for 1 min. The hardnesses of those test pieces were measured by a micro-Vickers hardness meter (“Bishyo Kodo-kei”, Matuzawa Seiki). The measuring load was 0.5 kg.

	TABLE 1
	Number of
	cold rolling	Maximum	Minimum	Finish		After annealing (500° C. × 1 min)
		cycles between	cold rolling	cold rolling	cold rolling				Cube	Mean
		hot rolling	reduction	reduction	reduction	Initial		Hardness	orientation	grain
Specimen	Chemical composition	and finish	ratio	ratio	ratio	hardness	Hardness	reduction	density	size
No.	Fe	P	Zn	cold rolling	(%)	(%)	(%)	(Hv)	(Hv)	(Hv)	(%)	(μm)
1	1.8	—	—	2	70	60	60	128	105	23	19	12
2	0.5	—	—	2	75	55	70	123	96	27	31	19
3	2.1	0.03	—	2	65	60	50	140	120	20	7	9
4	2.1	0.03	—	2	75	60	75	156	123	33	34	21
5	2.1	0.03	0.1	2	78	55	83	159	121	38	43	26
6	2.1	0.03	0.1	2	70	60	65	148	122	26	22	15
7	2.1	0.03	0.1	3	70	60	55	143	126	17	5	7
8	1.6	—	—	2	88	60	55	124	72	52	57	34
9	0.3	—	—	2	93	60	88	130	70	60	70	42
10	2.1	0.03	0.1	2	95	60	60	147	90	57	64	38
11	2.1	0.03	0.1	2	70	30	80	151	96	55	61	36
12	2.1	0.03	0.1	2	70	60	95	157	99	58	65	40
13	2.1	0.03	0.1	2	92	60	90	161	99	62	72	43
14	2.1	0.03	0.1	2	50	20	20	125	65	60	69	42
15	2.1	0.03	0.1	2	90	40	90	160	95	65	75	46
16	2.1	0.03	0.1	1	75	70	50	142	94	48	52	31

The copper alloy sheets in Specimens 1 to 7 are those meeting requirements specified by the present invention. Every one of the copper alloy sheets in Specimens 1 to 7 has a cube orientation density of 50% or below and a mean grain size of 30 Km or below. Hardness reductions in all the copper alloy sheets in Specimens 1 to 7 due to annealing were 40 Hv or below. It is known from data shown in Table 1 that the copper alloy sheets according to the present invention are high in softening resistance.

The copper alloy sheets in Specimens 8 to 16 are comparative examples not meeting all the requirements specified by the present invention. All the copper alloy sheets in comparative examples have cube orientation densities exceeding 50% and mean grain sizes greater than 30 μm. The strength of the copper alloy sheets in comparative examples was decreased greatly by annealing and copper alloy sheets in comparative examples were unsatisfactory in softening resistance.

Specimen 8: Maximum reduction ratios for the cold rolling processes between the hot rolling process and the finish cold rolling process are higher than 80%.

Specimen 9: Maximum reduction ratios for the cold rolling processes between the hot rolling process and the finish cold rolling process are higher than 80% and reduction ratio for the finish cold rolling is higher than 85%.

Specimen 10: Maximum reduction ratios for the cold rolling processes between the hot rolling process and the finish cold rolling process are higher than 80%.

Specimen 11: Minimum reduction ratios for the cold rolling processes between the hot rolling process and the finish cold rolling process are lower than 50%.

Specimen 12: Reduction ratio for the finish cold rolling is higher than 85%.

Specimen 13: Maximum reduction ratios for the cold rolling processes between the hot rolling process and the finish cold rolling process are higher than 80% and reduction ratio for the finish cold rolling is higher than 85%.

Specimen 14: Minimum reduction ratios for the cold rolling processes between the hot rolling process and the finish cold rolling process are lower than 50%. Reduction ratio for the finish cold rolling is lower than 30%.

Specimen 15: Minimum reduction ratios for the cold rolling processes between the hot rolling process and the finish cold rolling process are lower than 50%. Reduction ratio for the finish cold rolling is higher than 85%.

Specimen 16: The working cycle including the cold rolling process and the annealing process is performed only once between the hot rolling process and the finish cold rolling process.

Although the invention has been described in its preferred embodiments with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced other wise than as specifically described herein without departing from the scope and spirit thereof.

Claims

1. A softening-resistant copper alloy containing Fe, and having a cube orientation density of 50% or below after being annealed at 500° C. for 1 min.

2. A softening-resistant copper alloy containing Fe, and having a cube orientation density of 50% or below after being annealed at 500° C. for 1 min and a mean grain size of 30 μm or below.

3. The copper alloy according to claim 1, wherein the Fe content is in the range of 0.01 to 4.0% by mass.

4. The copper alloy according to claim 2, wherein the Fe content is in the range of 0.01 to 4.0% by mass.

5. A softening-resistant copper alloy sheet forming method of forming a copper alloy sheet comprising, in successive steps:

a hot rolling process for hot-rolling a copper alloy sheet of the copper alloy containing Fe;

at least two working cycles each of a cold rolling process and an annealing process; and

a finish cold rolling process;

wherein reduction ratio for each of the cold rolling processes of the working cycles is in the range of 50 to 80%, and reduction ratio for the finish cold rolling process is in the range of 30 to 85%.