Rolled copper foil for flexible printed circuit and method of manufacturing the same

A rolled copper foil for flexible printed circuits contains not more than 10 ppm by weight of oxygen and has a softening-temperature rise index T defined as T=0.60[Bi]+0.55[Pb]+0.60[Sb]+0.64 [Se]+1.36[S]+0.32[As]+0.09[Fe]+0.02[Ni]+0.76[Te]+0.48[Sn]+0.16[Ag]+1.24[P] (each symbol in the brackets representing the concentration in ppm by weight of the element) in the range of 4 to 34. The concentrations of the elements are in the ranges of[Bi]<5, [Pb]<10, [Sb]<5, [Se]<5, [S]<15, [As]<5, [Fe]<20, [Ni]<20, [Te]<5, [Sn]<20, [Ag]<50, and [P]<15 (each symbol in the brackets representing the concentration in ppm by weight of the element). The foil has a thickness in the range of 5 to 50 .mu.m and a half-softening temperature of 120 to 150.degree. C., is capable of continuously retaining a tensile strength of at least 300 N/mm.sup.2 at 30.degree. C., and possesses excellent flex property and adequate softening property.

BACKGROUND OF THE INVENTION
 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.
 Printed wiring boards based on organic substrate are roughly divided into
 two types; rigid type with a rigid, copper-clad laminate consisting of
 glass-epoxy and paper-phenol substrates and flexible type with a flexible,
 copper-clad laminate consisting of polyimide and polyester substrates.
 Copper foil is mainly employed as conductive material for the printed
 wiring boards. The foil products are classified into electrodeposited and
 rolled foils depending on the manufacturing processes used.
 Of the printed wiring boards, those 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 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, the requirement for the flex
 performance is becoming severer 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.degree. 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 joining 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, however, 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 (refer, e.g.,
 to Japanese Patent Application Kokai No. 10-230303).
 As noted already, a softened copper foil, if used in the fabrication of an
 FPC, can cause troubles such as foil deformation and 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.
 SUMMARY OF THE INVENTION
 The object of the present invention is to provide a rolled copper foil for
 FPCs which combines excellent flex property with adequate softening
 property, by appropriately elevating the softening temperature of a
 high-flexing rolled copper foil to eliminate the troubles that can
 otherwise result from its 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 characterized in
 that it contains not more than 10 parts per million by weight of oxygen,
 has a softening-temperature rise index T defined as
 T=0.60[Bi]+0.55[Pb]+0.60[Sb]+0.64[Se]+1.36[S]+0.09[Fe]+0.02[Ni]+0.
 76[Te]+0.48[Sn]+0.16[Ag]+1.24[P] (each symbol in the brackets representing
 the concentration in ppm by weight of the element) in the range of 4 to
 34, the concentrations of the elements being in the ranges of [Bi]&lt;5,
 [Pb]&lt;10, [Sb]&lt;5, [Se]&lt;5, [S]&lt;15, [As]&lt;5, [Fe]&lt;20, [Ni]&lt;20, [Te]&lt;5,
 [Sn]&lt;20, [Ag]&lt;50, and [P]&lt;15 (each symbol in the brackets representing the
 concentration in ppm by weight of the element), and the foil has a
 thickness in the range of 5 to 50 .mu.m, a half-softening temperature of
 120 to 150.degree. C., is capable of continuously retaining a tensile
 strength of at least 300 N/mm.sup.2 at 30.degree. C., and possesses
 excellent flex property and adequate softening property.
 (2) A rolled copper foil for flexible printed circuits according to (1)
 above, characterized in that the total amount of one or more of the
 components Ti, Zr, Hf, V, Ta, B, Ca, and Nb is not more than 20 ppm by
 weight.
 (3) A rolled copper foil for flexible printed circuits according to (1) or
 (2) above, characterized in that the intensity (I) of the (200) plane
 determined by X-ray diffraction of the rolled surface after annealing at
 200.degree. C. for 30 minutes, with respect to the X-ray diffraction
 intensity (I.sub.0) of the (200) plane of fine copper powder, is I/I.sub.0
 &gt;20.0.
 (4) A method of manufacturing the rolled copper foil for flexible printed
 circuits according to (1), (2), or (3) above characterized 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 .mu.m, the reduction ratio of the
 final cold rolling being beyond 90.0%, whereby excellent flex property and
 adequate softening property are achieved.
 DETAILED DESCRIPTION OF THE INVENTION
 When a copper foil is made by a process which involves a high reduction
 ratio or formation of fine grains to produce a developed cube texture, its
 flex fatigue property is improved but its softening temperature becomes
 too low. However, judicious control of the constituents in the material to
 raise the softening temperature will enable the resulting copper foil to
 have an adequate softening temperature.
 The expression "adequate softening temperature" as used herein may be
 defined by two conditions:
 (1) While the tensile strength of an as-rolled copper foil is in the range
 of 400-500 N/mm.sup.2, the foil should retain a tensile strength of not
 less than 300 N/mm.sup.2 after standing at 30.degree. C. for one year.
 (2) The copper foil should soften upon heat treatment either after roughing
 plating and cutting into a size or at the time of adhering to a resin
 substrate.
 The temperature corresponds to the range of 120-150.degree. C. in terms of
 the half-softening temperature obtained by annealing for 30 minutes (the
 annealing temperature at the point where the tensile strength is
 intermediate between that before the annealing and that after complete
 softening).
 As sheet stock for rolled copper foil for FPCs, tough-pitch copper has
 hitherto been used in preference to oxygen-free copper. The reason is that
 oxygen-free copper, with a softening temperature higher than that of
 tough-pitch copper by more than 30.degree. C., will not soften with the
 heat of treatment for its adhesion to a resin substrate. (Refer, e.g., to
 S. Sakai, Y. Nagai, K. Sugaya, and N. Otani: "Properties and Applications
 of Low-Temperature-Softening Oxygen-free Copper," Hitachi Cable, No.8
 (1989), pp.51-56; and T. Eguchi, S. Fujita, Y. Miyatake, S. Chigusa, A.
 Tokunaga, M. Satoh, and T. Inada: "Developments of
 Low-Temperature-Softening, High-Flex Performance Oxygen-free Copper,"
 Furukawa Electric Journal, No.86 (1990), pp.25-31.)
 It thus follows that if oxygen-free copper that has not been deemed
 suitable as a stock for rolled copper foil for FPCs is made into a foil by
 a process which involves a sufficiently high reduction ratio or fine grain
 formation to produce excellent flex property, it would be possible to
 obtain a copper foil with a low enough softening temperature for use in
 FPCs.
 Thus various proposals have been made to utilize oxygen-free copper as a
 material for rolled copper foils for FPCs. Most of them depend on trace
 additions of alloying elements for the drop of the softening temperature
 of oxygen-free copper to the level of tough-pitch copper. For example, one
 approach uses an oxygen-free copper whose softening temperature has been
 decreased by the addition of 10 to 600 ppm of boron (Japanese Patent No.
 1582981). Another method uses an oxygen-free copper with a softening
 temperature lowered by the addition of 10 to 300 ppm of one or more of Ca,
 Zr, and misch metals (Japanese Patent No. 1849316).
 Still another method adopts a special manufacturing process which lowers
 the softening temperature of oxygen-free copper to a level suitable for
 FPCs without resorting to the control of trace constituents. However, it
 requires final annealing at low temperature for a long period, at a
 sacrifice of copper foil production efficiency (Japanese Patent
 Application Kokai No. 1-212739). In any case the past attempts have been
 solely aimed at a decrease in the softening temperature of oxygen-free
 copper; none of them has ever positively taken the advantage of the high
 softening temperature inherent to oxygen-free copper.
 Impurity elements contained in relatively high concentrations in
 oxygen-free copper and which influence the softening temperature of the
 copper are Bi, Pb, Sb, Se, S, As, Fe, Ni, Te, Ag, Sn, and P. The softening
 temperature rises as the concentrations of these elements increase.
 However, the concentrations of the elements in oxygen-free copper vary
 depending on the chance of manufacture or fluctuations in manufacture
 conditions, and the usual level of the variations does not allow the
 half-softening temperature of the copper to be confined within the narrow
 range of 120 to 150.degree. C. In order to achieve it, the concentrations
 of the elements must be adjusted within proper ranges.
 The main material used in preparing oxygen-free copper is electrolytic
 copper. That is, the electrolytic copper is melted and then cast to the
 oxygen-free copper. The electrolytic copper includes, of the elements
 mentioned above, Bi, Pb, Sb, Se, S, As, Ni, Te, Ag, and Sn. For the
 reason, the oxygen-free copper also includes, as impurities, Bi, Pb, Sb,
 Se, S, As, Ni, Te, Ag, and Sn which are mostly carried in from the
 electrolytic copper. The concentrations of these impurities, in the
 oxygen-free copper, therefore, can be adjusted by choosing an electrolytic
 copper material to be used in consideration of its impurity contents.
 Following the melt-refining, the electrolytic copper is subjected to
 deoxidation by carbon, etc. for the adjustment of its oxygen concentration
 to be oxygen-free copper. During this stage Fe primarily finds its way as
 iron rust or the like into the copper. Controlling the Fe intrusion
 permits the adjustment of the Fe concentration in the product. Also, in
 the course of manufacture, a trace amount of P is sometimes added for
 deoxidation. The P concentration can then be adjusted by controlling the P
 addition.
 On the other hand, Ti, Zr, Hf, V, Ta, B, Nb and the like are known to lower
 the softening temperature of copper when they are added in low
 concentrations. The phenomenon occurs presumably because these elements
 combine with the above impurity elements to transform the impurities from
 the state of solid solution to precipitation or because they serve as
 nuclei for recrystallization to reduce the energy of recrystallization
 (cf. Japanese Patent Application Kokai No. 63-140052).
 These elements are contained in such slight amounts in electrolytic copper
 that, unless they are intentionally added during the course of manufacture
 of oxygen-free copper, they are seldom present in the copper in
 concentrations large enough to achieve an effect of lowering the softening
 temperature. Thus it is important not to use an electrolytic copper
 containing such elements, and it is particularly important to avoid the
 addition of such elements as a deoxidant or the like during the
 manufacture of oxygen-free copper.
 The present inventors have repeated experiments with the foregoing in view,
 and it has now been found possible through precise control of the trace
 impurities in oxygen-free copper and through strict control of the
 manufacturing process to obtain a product with excellent flex property and
 a half-softening temperature adjusted within the range of 120 to
 150.degree. C.
 The grounds on which various limitations are specified for the rolled
 copper foil according to the invention will be explained below.
 Under the invention the rolled copper foil is intended to retain a tensile
 strength of not less than 300 N/mm.sup.2 continuously at room temperature.
 More desirably the foil is to possess a tensile strength of not less than
 300 N/mm.sup.2 even after storage for one year at 30.degree. C.
 Here 30.degree. C. corresponds to a temperature above the average annual
 temperature in Japan. The expression "continuously" used for the storage
 period before the copper foil is fabricated into FPCs means that the foil
 is usually stored continuously, though for one year at the most. With a
 tensile strength of 300 N/mm.sup.2 or more, the copper foil will not
 wrinkle or have other trouble during fabrication. There is practically no
 problem, therefore, when the copper foil is capable of retaining a tensile
 strength of not less than 300 N/mm.sup.2 when allowed to stand at
 30.degree. C. for one year. Such softening property corresponds, in terms
 of the half-softening temperature obtained by annealing for 30 minutes, to
 a temperature of 120.degree. C. or upwards.
 However, when the half-softening temperature obtained by 30-minute
 annealing exceeds 150.degree. C., the copper foil is sometimes not
 softened by the heat treatment either after the roughing plating and
 cutting into a size orate the time of adhering to the resin substrate.
 That is why the half-softening temperature by 30-minute annealing is
 specified to be in the range of 120-150.degree. C.
 In order that the resulting FPC may have enhanced flex fatigue property,
 the copper foil must have enhanced flex fatigue property itself. The
 copper foil is incorporated in a recrystallized state in the FPC, and if
 the cube texture as a recrystallization texture of pure Cu is allowed to
 develop, the copper foil attains improved flex fatigue property.
 The degree of development of the cube texture that produces satisfactory
 flex fatigue property is specified to be such that the intensity of the
 (200) plane determined by X-ray diffraction of the rolled surface is to be
 I/I.sub.0 &gt;20, preferably I/I.sub.0 &gt;40.0, with respect to the X-ray
 diffraction intensity (I.sub.0) of the (200) plane of fine copper powder.
 Here the annealingat200.degree. C. for 30 minutes is conducted to
 recrystallize the copper foil for the measurement of its X-ray diffraction
 intensity.
 The reason for which the O content is specified to be not more than 10 ppm
 by weight is that it helps attain a softening temperature of 120.degree.
 C. or upwards. If the O concentration exceeds 10 wt.ppm, the process that
 otherwise produces a foil with high flex performance will not allow the
 foil to have a half-softening temperature above 120.degree. C., even
 though the amounts of trace constituents are adjusted within the ranges to
 be defined below. In contrast with this, decreasing the O concentration is
 accompanied with decreases in CuO inclusions, which bring an effect of
 improving the flex performance.
 Bi, Pb, Sb, Se, S, As, Fe, Ni, Te, Ag, Sn, and P are elements that
 determine the softening property of oxygen-free copper. By adjusting their
 concentrations the half-softening temperature of the copper can be
 controlled. Actually, these elements vary in their rates of contribution
 to the rise of softening temperature, and the contributions of the
 individual elements must be weighted accordingly. With this in view, the
 index of softening-temperature rise (T) was defined as follows.
 T=0.60[Bi]+0.55[Pb]+0.60[Sb]+0.64[Se]+1.36[S]+0.32[As]+0.09[Fe]+0.
 02[Ni]+0.76[Te]+0.48[Sn]+0.16[Ag]+1.24[P] (each symbol in the brackets
 representing the concentration in ppm by weight of the element).
 Here the coefficient of each element represents the linear inclination
 (.degree. C./wt.ppm) obtained by finding the relation between the
 concentrations of the element added alone in varied amounts to high-purity
 Cu and the half-softening temperature and by rearranging the relation in a
 linear function. It has been confirmed that the individual elements
 achieve the effects of raising the half-softening temperature
 additionally.
 When the concentrations of the elements are adjusted so that T comes in the
 range of 4 to 34, it is possible to confine the half-softening temperature
 of the copper foil made by a process that imparts high flex performance
 within the range of 120 to 150.degree. C. If T is less than 4 the
 half-softening temperature will be below 120.degree. C. and, conversely if
 T is more than 34, the half-softening temperature will be above
 150.degree. C.
 In order to bring T within the range of 4 to 34 it is only necessary as
 noted above to adjust the amounts of contaminating impurities. An
 alternative is to add those elements deliberately in the course of
 manufacture of oxygen-free copper. In either case adjustments are
 necessary to keep the concentrations of the individual elements within the
 specified ranges.
 The adjustments are needed for the following reasons:
 (1) Segregation of low-melting elements such as Bi, Pb, Se, S, and Sn along
 the grain boundaries of an oxygen-free copper ingot would induce cracking
 during hot rolling.
 (2) Nonmetallic elements such as S, Sb, Se, As, Te, and P would form
 nonmetallic inclusions between themselves and Cu and deteriorate
 mechanical properties including flex performance.
 (3) Ag and the like are so expensive that copious addition of such elements
 in adjusting the softening temperature is not justified on grounds of
 cost.
 (4) Increases in the concentrations of Fe, Ni, and the like tend to hamper
 the development of a recrystallization texture [decreasing the I/I.sub.0
 of the (200) plane], and thereby adversely affect the flex performance of
 the product.
 For these reasons the concentration ranges of these elements that will not
 bring unfavorable effects are specified, in accordance with the present
 invention, to be:
 [Bi]&lt;5, [Pb]&lt;10, [Sb]&lt;5, [Se]&lt;5, [S]&lt;15, [As]&lt;5, [Fe]&lt;20, [Ni]&lt;20, [Te]&lt;5,
 [Ag]&lt;50, [Sn]&lt;20, and [P]&lt;15 (each symbol in the brackets representing the
 concentration in ppm by weight of the element).
 Presence of Ti, Zr, Hf, V, Ta, B, Ca, and Nb in copper lowers the
 half-softening temperature, but if the combined proportion of such
 elements is not more than 20 wt.ppm, the phenomenon of softening
 temperature drop will not occur. Hence the total amount of the elements is
 confined within the limit of 20 wt.ppm.
 As for the thickness of a copper foil, the thinner the better the flex
 fatigue property because of the lower strains produced around the bend. If
 the foil is more than 50 .mu.m thick, desired flex fatigue property will
 no longer be attained even when the cube texture is developed. Conversely
 if the thickness is less than 5 .mu.m, the foil becomes difficult to
 handle since insufficient strength can lead to rupture or other failure.
 Hence the specified foil thickness range of 5-50 .mu.m.
 The copper foil according to the present invention is finished as such by
 cold rolling to a reduction ratio in excess of 90.0% following
 recrystallization annealing under conditions that produce a mean grain
 diameter of not greater than 20 .mu.m. If the mean diameter upon the
 annealing that precedes the rolling is more than 20 .mu.m or if the
 reduction ratio is less than 90.0%, then I/I.sub.0 &lt;20.0 and no favorable
 flex fatigue property will be achieved. Also, the half-softening
 temperature may sometimes exceed 150.degree. C. depending on the
 concentrations of trace constituents. The annealing before the final cold
 rolling may be combined with the hot rolling, in which case too the grain
 size as hot rolled is preferably adjusted to not greater than 20 .mu.m.