BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to copper and to copper alloys having fine crystal
grains, and relates to a manufacturing method therefor, and more particularly, the
resent invention relates to a technology for enhancing the characteristics in bending
or other working when used for electronic devices such as terminals, connectors, and
lead frames for semiconductor integrated circuits.
Description of the Related Art
[0002] Recently, electronic devices such as terminals and connectors and their parts are
reduced in size and thickness, and copper and copper alloy used as materials thereof
are demanded to have high strength. In terminal and connector material, the contact
pressure must be increased in order to maintain electrical connections, and a high
strength material is essential for this purpose. In a lead frame, because the semiconductor
circuit is highly integrated, there is an increasing demand for multi-pin structures
and thin wall thicknesses. Accordingly, to prevent deformation while conveying or
handling the lead frame, the required strength level is progressively increasing.
[0003] Moreover, along with the trend in size-reduction of electronic devices and components,
a higher degree of freedom of forming performance is demanded, and workability of
connector materials is becoming important, and in particular, an excellent bending
properties are required. In the outer lead of the semiconductor lead frame, an excellent
bending properties are also needed in the case of gull-wing form bending processes.
[0004] In order to obtain an excellent bending properties not causing cracks in the bent
part when a material is bent and deformed, it is necessary to enhance material ductility
or to decrease grain size. Furthermore, for the copper alloy used for electronic device,
a function for allowing the heat generated during power feed to escape to the outside
is needed, aside from a function of transmitting an electric signal, and a high heat
conductivity is required in addition to electrical conductivity. In particular, to
cope with the recent trend of higher frequency electrical signals, the demand for
higher electrical conductivity is mounting.
[0005] Electrical conductivity of copper alloy is inversely related to strength, and when
an alloying element is added to enhance the strength, the electrical conductivity
is lowered, and therefore alloys which compromise strength and electrical conductivity
or price have been used, depending on the application. So far, alloys for enhancing
the strength and electrical conductivity have been intensively developed, and generally,
copper alloys of precipitation reinforced type containing second phase particles such
as Cu-Ni-Si alloy or Cu-Cr-Zr alloy have come to be used as high functional materials
which is superior in balance between both.
[0006] Thus, for mechanical characteristics of copper or copper alloys for electronic devices,
high strength and excellent workability are desired. However, first of all, strength
and ductility are inversely related to each other, and in each alloy system, when
rolling is processed in order to increase the strength by work hardening, the ductility
declines, and preferable workability is not obtained by rolling alone. On the other
hand, by reducing the grain size, increase in strength as indicated by the Hall-Petch
relation is expected, and it also leads to improvement of bending properties, and
hence it was generally controlled to reduce the grain size during annealing and recrystallization.
[0007] In this method, however, when the annealing temperature is lowered in order to reduce
the grain size, non-crystallized grains remain in part, and there is substantially
a limit to obtaining recrystallized grains of about 2 to 3 µm, and a technique for
further reducing the grain sizes has been demanded. Furthermore, by recrystallization
alone, the strength level is usually low, and it is not practical, and therefore a
certain rolling process is needed in a later step, which has led to reduction of ductility.
Accordingly, generally after rolling process, a process of stress relief annealing
was needed to recover the ductility. This process, however, causes lowered strength
once obtained in the rolling process, and sufficient ductility is not obtained after
stress relief annealing, and it was difficult to satisfy the recent extremely severe
demand for bending deformation performance.
[0008] More recently, instead of an annealing process, methods of obtaining fine crystal
grains and high ductility by working materials by strong shearing have been studied
and reported, for example, by Ito et al. (ARB (Accumulative Roll-Bonding, J. of Japan
Society of Metallurgy, 54 (2000), 429), and Hotta et al. (ECAP (Equal-Channel Angular
Press), Metallurgy seminar text: Approach to fine crystal grains (2000), Japan Society
of Metallurgy, 39). In these processing methods, however, a mass quantity sufficient
to be used as materials for electronic devices cannot be manufactured, and there are
not suited to industrial production.
SUMMARY OF THE INVENTION
[0009] The inventors have accumulated extensive research to solve these problems, and they
have discovered that fine crystal grains at a level not known thus far can be obtained
by controlling the conditions of the rolling process instead of the conditions of
the annealing. That is, in the structure of a material cold rolled with an ordinary
cold rolling reduction, when recrystallized by subsequent annealing, the decrease
in dislocation density occurs discontinuously when the recrystallized grain boundaries
pass a cell, and large crystal grains of uneven size are produced intermittently.
This is called static recrystallization. According to the research by the inventors,
by extremely increasing the reduction of cold rolling, dynamic recrystallization,
usually exhibited in high temperature regions, was also found to occur in cold rolling,
and dynamic continuous recrystallization is exhibited as the subgrains formed during
processing are transformed into high angle grain boundaries. By making use of this
mechanism, round and uniform crystal grains of grain size of 1 µm or less are obtained.
According to this method, fine crystal grains can be obtained without sacrificing
the strength in order to prevent reduction of ductility, and it is also found that
an elongation of 2% or more is obtained even immediately after final cold rolling,
and an allowable bending properties are obtained by cold rolling alone. Furthermore,
by adding stress relief annealing processing after final cold rolling, the elongation
is further enhanced, and thus is applicable also in the case exposed to extremely
severe bending. According to such a manufacturing method, moreover, materials for
electronic devices can be mass produced industrially. Continuous recrystallization
is explained in detail below.
[0010] The present invention is made on the basis of these findings, and provides copper
and copper alloy comprising: a structure having fine crystal grains with grain size
of 1 µm or less composed of crystal grain boundaries mainly formed of curved portions
after a final cold rolling, the structure obtained by dynamic continuous recrystallization
caused by the final cold rolling, and an elongation of 2% or more in a tensile test.
[0011] The present invention also provides a manufacturing method for copper and copper
alloy, the method comprising: a final cold rolling with a reduction (true stress)
η, wherein η is expressed in the following formula and satisfying η ≥ 3, thereby obtaining
a structure having fine crystal grains with grain size of 1 µm or less after the final
cold rolling, and
an elongation of 2% or more in a tensile test.

T
0: plate thickness before rolling, T
1: plate thickness after rolling.
[0012] The reasons for setting these numerical values are explained below together with
the functions of the invention.
A. Reduction of final cold rolling, elongation, and grain size
[0013] In order to obtain a favorable bending properties in a material subjected to final
cold rolling alone, a high ductility is essential. In order to obtain the favorable
bending properties not causing cracking in the bent portion, a fracture elongation
in a tensile test is required to be 2% or more at a gauge length of 50 mm. In order
to obtain a rupture elongation of 2% or more in the state of final cold rolling, the
grain size after final cold rolling must be 1 µm or less. Thus, sufficient elongation
is obtained in the cold rolled state by decreasing the grain size, which is because
dislocations are piled-up in the grain boundary when continuous recrystallized grains
are formed, and a grain boundary structure of a non-equilibrium state is formed and
a grain boundary sliding is expressed, thereby enhancing the ductility.
[0014] The grain size and elongation after final cold rolling vary depending on the cold
rolling reduction. The cold rolling reduction (true stress) η by final cold rolling
process until reaching the product plate thickness is expressed in the formula below.

T
0: plate thickness before rolling, T
1: plate thickness after rolling.
[0015] In this case, when the value of η is small, a rolled structure remains, and clear
fine crystal grains are not obtained, or if they are obtained, the grain size is large,
and the grain boundary sliding does not take place, and favorable ductility is not
obtained. According to the research by the inventors, it is known that the value of
η should be 3 or more in order to obtain a fine grain size of 1 µm or less.
[0016] The structure of a material cold rolled by a conventional ordinary cold rolling reduction
sometimes had a cell structure due to mutual entangling of dislocations introduced
in the crystal grains. In this case, however, since the misorientation among neighboring
cells is small, that is, 15° or less, properties as crystal grain boundary are not
realized. Accordingly, as shown in Fig. 1, when recrystallized by annealing after
cold rolling, as mentioned above, static crystallization takes place, that is, large
crystal grains of uneven size are formed intermittently.
[0017] In contrast, by setting the extremely high cold rolling reduction, fine crystal grains
are obtained. That is, at a very high cold rolling reduction, numerous regions locally
shearing deformed occur in the matrix in the entire material and thus subgrain structures
greatly grow. As a result, as shown in Fig. 1, dislocations are introduced in order
to compensate the large misorientation between the matrix and the subgrain, and they
are piled-up in the grain boundary. In this case, crystal grain boundaries having
a large misprientation of 15° or more (high angle grain boundary) are generated. That
is, the subgrain structure which has been initially a substructure of crystal grains
is directly formed as crystal grains. In this case, the crystal grain boundary is
largely different from the case of the static recrystallization, and there is no linearity
in the grain boundary, and it is a feature that a crystal grain boundary mainly composed
of curved portions is formed. This dynamic continuous recrystallization is mostly
formed in cold rolling. It is also known that a clearer high angle grain boundary
is grown by annealing at intentional low temperatures and bringing it into an ordinary
recovery regime. In this case, it is found that the ductility is further enhanced
as described below.
[0018] In this mechanism, if second phase particles such as precipitates and dispersoids
are present in the Cu matrix, dislocations introduced by plastic stress due to rolling
are accumurated around the second phase particles by forming dislocation loops or
the like, and the dislocation density is substantially increased. In this condition,
the particle size of the subgrains becomes much finer, and the strength becomes higher.
In the final cold rolling, unless recovered or recrystallized by annealing in an intermediate
processing, cold rolling may be performed by plural rolling machines by exchanging
rolling machines depending on the range of plate thickness, or pickling or polishing
may be performed in order to control the surface properties.
B. Stress relief annealing
[0019] When the material after final cold rolling is further annealed for stress relief,
the ductility is enhanced, and a further preferable bending properties are obtained.
As annealing conditions, it is necessary to set adequate annealing conditions to such
an extent that the product value will not be lost due to extreme decline of strength.
The annealing condition differs with the alloy system, but by selecting an appropriate
annealing condition in a temperature range of 80 to 500°C and in a range of 5 to 60
minutes, an elongation of 6% or more may be easily obtained, and it is applicable
to a severe bend forming.
[0020] Preferred examples of copper alloy of the invention include Cu-Ni-Si alloys having
precipitates of intermetallic compounds of Ni and Si such as Ni
2Si, and the copper alloys comprise Ni: 1.0 to 4.8 mass %, Si: 0.2 to 1.4 mass %, and
the balance of Cu. The invention also includes Cu-Cr-Zr alloys having precipitates
of pure Cr grains and intermetallic compounds of Cu and Zr, and the copper alloys
comprise Cr: 0.02 to 0.4 mass %, Zr: 0.1 to 0.25 mass %, and balance of Cu. These
copper alloy may be added with subsidiary components such as one or more of Sn, Fe,
Ti, P, Mn, Zn, In, Mg and Ag in a total amount of 0.005 to 2 mass %. Moreover, copper
alloys having second phase particles such as other kinds of precipitates and dispersed
particles may be used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
Fig. 1 is a schematic diagram for explaining the recrystallization process.
Fig. 2 is a transmission electron microscope photograph showing a structure of an
alloy in an example of the invention.
Fig. 3 is a transmission electron microscope photograph showing a structure of an
alloy in a comparative example of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(Embodiments)
[0023] These ingots were hot rolled at a temperature of 950°C into plates of 10 mm in thickness.
The oxide layer of the surface layer was removed by mechanical scalping, and the plates
were cold rolled to a thickness of 5 mm, and a solid solution treatment was applied
in the case of age precipitation type copper alloy, and recrystallization annealing
was applied once in the others. By further cold rolling, plates of an intermediate
thickness of 1.1 to 3.8 mm were obtained, and at this plate thickness, further, aging
treatment or second recrystallization annealing was performed. In the case of aging
treatment, the aging temperature was adjusted so that the product strength would be
highest in each alloy composition, or in the case of recrystallization, the temperature
condition was adjusted so that the grain size would be 5 to 15µm. By the final cold
rolling, plates of 0.15 mm in thickness were manufactured and obtained as experiment
samples for evaluation. The final cold rolling conditions are also shown in Tables
1 to 3.
[0024] Test pieces were sampled from the obtained plates, and the materials were tested
to evaluate "grain size", "strength", "elongation", "bending", and "electrical conductivity".
To evaluate the "grain size", the bright fields were observed by a transmission electron
microscope, and it was determined by the cut-off method of JIS H 0501 on the obtained
photograph. As for "strength" and "elongation", using No. 5 specimens conforming to
the tensile test specified in JIS Z 2241, the tensile strength and rupture elongation
were measured. As for "bending", by bend forming using a W-bend testing machine, the
bent part was observed by an optical microscope at a magnification of 50 times, and
presence or absence of cracking was observed. The mark "o" indicates that cracking
is absent, and the mark "x" indicates that cracking is present. The "electrical conductivity"
was determined by measuring the electrical conductivity according to a four-point
method.
[0025] Evaluation results are shown in Tables 1, 2, and 4. The alloys of the invention are
known to have excellent strength, elongation and bending properties. By contrast,
in comparative examples 6 to 8, 14 to 16, 33, and 34, since the reduction of final
rolling was low, the desired structure was not obtained, the ductility dropped, and
favorable bending properties were not achieved. Fig. 2 is a transmission electron
microscope photograph of sample No. 12 of the invention, in which the mean grain size
of the formed continuous recrystallization is 1 µm or less, and its crystal grain
boundary is mainly composed of curved portions and is round. By way of comparison,
a transmission electron microscope photograph of comparative example No. 6 is shown
in Fig. 3, in which the grain size is nearly linear.
[0026] The materials manufactured in embodiments 9, 22, 26, and 30 of the invention and
comparative examples 33, and 34 were further annealed for stress relief, and tensile
tests were conducted. Results are shown in Table 5. In the alloys of the invention,
by stress relief annealing, elongation is further enhanced as compared with that of
the alloys of the comparative examples. Hence, it is expected to be able to withstand
further more severe working.

1. Copper and copper alloy comprising:
a structure having fine crystal grains with grain size of 1 µm or less composed of
crystal grain boundaries mainly formed of curved portions after a final cold rolling,
the structure obtained by dynamic continuous recrystallization caused by the final
cold rolling, and
an elongation of 2% or more in a tensile test.
2. Copper and copper alloy comprising:
a structure having fine crystal grains with grain size of 1 µm or less after a final
cold rolling with a reduction η, wherein η is expressed in the following formula and
satisfying η ≥ 3; and
an elongation of 2% or more in a tensile test.

T
0: plate thickness before rolling, T
1: plate thickness after rolling.
3. A manufacturing method for copper and copper alloy, the method comprising:
a final cold rolling with a reduction η, wherein η is expressed in the following formula
and satisfying η ≥ 3,
thereby obtaining a structure having fine crystal grains with grain size of 1 µm or
less after the final cold rolling, and
an elongation of 2% or more in a tensile test.

T
0: plate thickness before rolling, T
1: plate thickness after rolling.
4. A manufacturing method for copper and copper alloy according to claim 3, wherein the
copper and copper alloy recited in claim 1 or 2 is processed by strain relieving annealing,
and elongation by a tensile test is improved to 6% or more.
5. Copper and copper alloy manufactured by the manufacturing method of claim 3 or 4.
6. A manufacturing method of copper and copper alloy according to claim 3 or 4, wherein
the copper alloy is Cu-Ni-Si alloy or Cu-Cr-Zr alloy.
7. Copper and copper alloy of claim 5, wherein the copper alloy is Cu-Ni-Si alloy or
Cu-Cr-Zr alloy.