Copper foil for printed circuits

A copper foil for printed circuits including a thermal oxidation-resistant treated layer (e.g., Zn--Ni or Zn--Co alloy plating) formed on the shiny side of the foil and a Cr-base anticorrosive treated layer (e.g., chromate film, mixed film of chromium oxide and zinc and/or zinc oxide, or both) formed further thereon, characterized in that a copper plating layer or an etched surface is provided before forming the thermal oxidation resistant treated layer. The copper plating or etching permits the shiny side to be completely covered with fresh copper or purified to overcome any ununiformity of the chemical activity and/or any lack of smoothness of the entire surface of the shiny side. The evenness of the freshly formed copper surface enhances the homogeneity and completeness of the thermal oxidation-resistant plated layer to be formed subsequently by thermal oxidation resistance treatment, rendering it possible to take the full advantage of the thermal oxidation-resistant treated layer. Thus a surface treatment technique of imparting a thermal oxidation resistance upon heating at 300.degree. C. for 30 minutes to the shiny side of copper foil has now been successfully developed for the first time in the art.

INDUSTRIAL FIELD OF THE INVENTION 
This invention relates to a copper foil for printed circuits and a process 
for producing the foil, and more specifically to a copper foil for printed 
circuits characterized in that the shiny side of the foil is formed with a 
copper plating layer or is etched before being treated for thermal 
oxidation resistance, so as to improve its thermal oxidation resistance 
(resistance to tarnishing with oxidation due to the heat history during 
the fabrication of printed circuit boards or the like), and a process for 
producing the foil. 
BACKGROUND OF THE INVENTION 
Copper foils and copper alloy foils (hereinafter collectively called 
"copper foils") are significantly contributing to the progress of the 
electric, electronic and related industries. Notably, they are 
indispensable for the fabrication of printed circuits. Copper foil for 
printed circuits generally is laminated and bonded to a base of 
thermoplastic resin board, film or the like at high temperature and high 
pressure, with or without the aid of an adhesive, printed with necessary 
circuit patterns to form objective circuits, and then is etched to remove 
unnecessary portions. Finally, desired elements are soldered in place, and 
in this way various printed circuit boards for electronic devices are 
fabricated. Qualitative requirements for the copper foil for printed 
circuit boards differ with the sides, namely, the surface of the side to 
be bonded to the resin base (matt side) and the surface of the opposite 
side not to be bonded to the base (shiny side). 
Requirements for the matt side chiefly include: 
(1) No possibility of oxidative tarnishing during storage; 
(2) Adequate resistance to peeling from the base even after 
high-temperature heating, wet treatment, soldering, chemical treatment or 
the like; and 
(3) Freedom from so-called lamination spots that can result from lamination 
to the base and etching. 
Requirements for the shiny side include: 
(1) Good appearance and no oxidative tarnishing during storage; 
(2) Good solder wettability; 
(3) No oxidative tarnishing upon high-temperature heating; and 
(4) Good adhesion to resist. 
As for the thermal oxidation resistance of the shiny side, the requirement 
has become more and more stringent in recent years. For one thing, copper 
foils have come to be exposed to higher temperatures than heretofore 
because of the advent of novel, highly heat-resistant resins as well as a 
new manufacturing method which is known as the "double-layer flexible base 
process". This method comprises directly applying a polyimide varnish to a 
copper foil, thereby forming a double-layer structure of polyimide and 
copper foil layers. With conventional lamination techniques too, there is 
a tendency toward the replacement of the usual nitrogen atmosphere by air 
to reduce the cost of laminating and curing processes that involve heat 
treatment. This is another factor demanding an improvement in the thermal 
oxidation resistance of the shiny side of copper foil. The shiny side to 
be printed with minute electric circuits ought to be protected against 
tarnishing or other deleterious effects of oxidation. To be more concrete, 
the shiny surface today is required to undergo no tarnishing on holding at 
300.degree. C. for 30 minutes. 
For the surface treatment of the shiny side to meet all the foregoing 
requirements, a method combining zinc plating with chromate treatment has 
predominantly been used. However, the thermal oxidation resistance that 
the method imparts is limited, e.g., just enough for holding at 
200.degree. C. for about 30 minutes. The resistance increases in 
proportion to the amount of zinc plating used, but it is known to present 
problems of yellowing ("brassing") after bonding and of low printability 
due to decreased resistance to acid. 
Another method reportedly under development replaces the zinc plating with 
zinc-nickel plating, zinc-cobalt plating or the like for the formation of 
a zinc alloy layer as a thermal oxidation-resistant treated layer, 
followed by chromate treatment. Even the proposed method is unable to make 
the shiny side resistant to thermal oxidation under the conditions of 
300.degree. C. and 30 minutes; it imparts a resistance to tarnishing for 
at most 30 minutes at 240.degree. C. or 10 minutes at 270.degree. C. 
OBJECT OF THE INVENTION 
The present invention has for its object the development of a technique for 
the surface treatment of the shiny side of a copper foil to impart thermal 
oxidation resistance under the conditions of 300.degree. C. and 30 minutes 
so that the foil surface can be protected against yellowing as well as 
tarnishing with oxidation. 
SUMMARY OF THE INVENTION 
The present inventor has investigated on the possible causes of oxidative 
tarnishing and yellowing of the copper foil surface upon treatment for 
thermal oxidation resistance at 300.degree. C. for 30 minutes. The 
investigations have led to the conclusion that those defects are 
attributable to the ununiformity of chemical activity and/or the lack of 
smoothness of the shiny side of the copper foil before being treated for 
thermal oxidation resistance. Copper foils are commonly made by 
electrodepositing copper on a smooth cathode drum surface and then peeling 
off the electrodeposit. Since the side of the deposit on the cathode 
serves as the shiny side of the resulting foil, it has been believed that 
the shiny side is adequately smooth. Moreover, because the treatment for 
thermal oxidation resistance is preceded by acid pickling and water 
washing, the uniformity of chemical activity throughout has been taken for 
granted. These fixed ideas have hampered casting light on the facts to the 
contrary. Actually, observation under a scanning electron microscope of 
the copper foil so obtained reveals that its shiny side has numerous 
minute pits scattered throughout. When the copper foil thus manufactured 
is coated at its shiny side with a thin layer of Zn plating (0.001-0.01 
.mu.m thick) and its distribution condition is examined with the use of 
EPMA (Electron Probe Micro Analyzer) line analysis, there is an evidence 
of substantial segregation, which means that the chemical activity is not 
even too. If the shiny side that lacks in smoothness and/or uniformity of 
chemical activity is directly treated for thermal oxidation resistance, 
the thermal oxidation-resistant plating layer so formed inherits the 
deficiencies of the shiny side as peeled from the cathode drum, failing to 
take the full advantage of the thermal oxidation resistance treatment. The 
present inventor has now found for certain that thin copper plating onto 
or etching of the shiny side of copper foil is effective in overcoming the 
lack of smoothness and/or the ununiformity of chemical activity of the 
shiny side. 
On the basis of this discovery, the present invention provides: 
(A) a highly thermal oxidation-resistant copper foil for printed circuits 
including a thermal oxidation-resistant treated layer formed on the shiny 
side of the foil and a Cr-base anticorrosive treated layer formed further 
thereon, characterized in that a copper plating layer is provided between 
the shiny side of the foil and the thermal oxidation-resistant treated 
layer, said copper plating layer being preferably composed of fresh copper 
layer with a thickness of 1 to 20 mg/dm.sup.2 deposited over the entire 
surface, 
(B) a process for producing a copper foil for printed circuits which 
comprises treating the shiny side of the foil for thermal oxidation 
resistance and then subjecting it to a Cr-base anticorrosive treatment, 
characterized in that the shiny side of the copper foil is copper-plated 
before the thermal oxidation resistance treatment, 
(C) a highly thermal oxidation-resistant copper foil for printed circuits 
including a thermal oxidation-resistant treated layer formed on the shiny 
side of the foil and a Cr-base anticorrosive treated layer formed further 
thereon, characterized in that the shiny side of the foil is an etched 
surface, and 
(D) a process for producing a copper foil for printed circuits which 
comprises treating the shiny side-of the foil for thermal oxidation 
resistance and then subjecting it to a Cr-base anticorrosive treatment, 
characterized in that the shiny side of the copper foil is etched before 
the thermal oxidation resistance treatment, said etched surface preferably 
being formed using an aqueous solution of ammonium persulfate. 
DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
Deposition of a thin copper plating over the shiny side of a copper foil 
produces a complete covering of fresh copper on that side. The evenness of 
the copper surface enhances the homogeneity and completeness of the 
thermal oxidation-resistant plating layer to be subsequently formed by 
thermal oxidation-resistance treatment and permits those merits to be 
fully taken advantage of. Etching of the shiny side purifies the surface 
by the melting of the copper foil itself, to say nothing of copper oxide, 
and makes the chemical activity uniform throughout the surface. This 
uniformity, in turn, ensures the homogeneity and completeness of the 
thermal oxidation-resistant plating layer to be formed by thermal 
oxidation resistance treatment, rendering it possible to make the most of 
those merits. 
The copper foil to be used in the present invention may be either 
electrolytic or rolled copper foil. While the present invention itself is 
directed to the shiny side of the copper foil, the matt side of the foil 
is briefly described here for reference. Usually, the side of a copper 
foil to be bonded to a resin base, or the matt side, is matted by 
electrodeposition of copper knurls on a degreased copper foil surface for 
the purpose of increasing the peel strength of the foil after lamination. 
The electrodeposition of knurls is easily performed by forming so-called 
burnt electrodeposits. The matting is sometimes preceded or followed by 
ordinary copper plating or other pretreatment as pretreatment or finish 
treatment. Exemplary conditions for copper matting treatment are as 
follows: 
______________________________________ 
Copper matting treatment conditions 
______________________________________ 
Cu 10-25 g/l 
H.sub.2 SO.sub.4 20-100 g/l 
Temperature 20-40.degree. 
C. 
D.sub.k 30-70 A/dm.sup.2 
Time 1-5 second 
______________________________________ 
The matting desirably is followed by a treatment to form on the matted 
surface a metal layer or alloy layer of one or two or more metals selected 
from the group consisting of Cu, Cr, Ni, Fe, Co, and Zn. Combinations of 
Cu--Ni, Cu--Co, Cu--Ni--Co, and Cu--Zn may be cited as examples of alloy 
plating. (Refer to Patent Application Publication No. 9028/1981, Patent 
Application Public Disclosure Nos. 13971/1979, 292895/1990, and 
292894/1990, and Patent Application Publication Nos. 35711/1976 and 
6701/1979.) Such a treatment determines the ultimate properties of the 
copper foil and also provides a barrier for the foil. 
In accordance with the present invention, the shiny side of a copper foil, 
pretreated by acid pickling, water washing and the like in the usual 
manner with or without concomitant treatment of the matt side, is either 
(A) copper-plated or (B) etched. 
(A) In the case of copper plating, it is only necessary that fresh copper 
be deposited on the entire surface of the shiny side. There is no 
limitation, therefore, to the type of bath to be used. However, from the 
viewpoint of handling, an ordinary copper sulfate-sulfuric acid bath is 
the most convenient. The composition of the plating solution and the 
plating conditions are not exacting either. Desirable ranges for the 
plating solution composition are Cu.sup.2+ : 20-40 g/l and H.sub.2 
SO.sub.4 : 50-100 g/l. With regard to the plating conditions, the higher 
the bath temperature the better, but a temperature in the range of 
30.degree.-60.degree. C. gives satisfactory results. Proper current 
density cannot be unequivocally defined since it is dependent upon the 
quantity of electricity to be used. A generally effective range is 1-80 
A/dm.sup.2, preferably 20.+-.5 A/dm.sup.1. As for the plating time, a 
period of about 0.1-10 seconds is appropriate. The copper plating thus 
deposited improves the thermal oxidation resistance of the foil in 
proportion to the quantity of copper. However, excessive plating can 
deteriorate the mechanical properties of the copper foil itself or 
decelerate the treatment rate. A recommended range is 1-20 mg/dm.sup.2. 
The plated copper completely covers up the pits about 0.1-0.3 .mu.m in size 
that originally scatter on the shiny side of the copper foil, forming a 
copper coat of fresh copper formed over the entire surface by plating. 
This has experimentally been confirmed by observation under a scanning 
electron microscope (SEM) with a magnification of 30,000. 
(B) In the case of etching, the etching method need only produce a copper 
surface with a uniform chemical activity. The type of bath and the 
procedure of etching are not critical. Etching methods are roughly divided 
into two groups, dry (blast and ionizing radiation) and wet (chemical and 
electrochemical processes). Accuracy, economy, and other considerations 
make chemical etching the most expedient. While the type of etching bath 
is not specially restricted, an aqueous solution of ammonium persulfate 
well known as an etching solution for copper materials is a good choice. 
The amount of etching is dictated by the concentration of ammonium 
persulfate, bath temperature, time, agitating condition, and other 
factors. A desirable amount of etching is not specifically limited but, in 
terms of the theoretical mean thickness found from the amount of Cu in the 
etching bath, it ranges from 0.005 to 0.1 .mu.m. If it is less than 0.005 
.mu.m, it does not impart complete uniformity of chemical activity. The 
larger the amount of etching the better the thermal oxidation resistance 
the foil acquires. An amount in excess of 0.1 .mu.m is undesirable, 
however, because it can cause changes in the mechanical properties of the 
foil itself. More desirably, the amount of etching ranges between 0.01 and 
0.05 .mu.m. The ammonium persulfate concentration, bath temperature, time, 
and agitating conditions for that amount of etching cannot be 
unequivocally specified. Typically, the amount of etching is 0.03 .mu.m 
under the following set of conditions: 
ammonium persulfate concentration: 10 g/l, 
bath temperature: 25.degree. C., 
linear velocity of both agitation: 1 m/sec; and 
etching time: 5 seconds. It has been confirmed by examination with EPMA 
line analysis that Zn segregation is absent upon that etching when the 
shiny side of a copper foil is coated with a thin-layer Zn plating 
(0.001-0.01 .mu.m thick). 
Following this, procedure the copper-plated or etched shiny side is treated 
for thermal oxidation resistance and then subjected to a Cr-base 
anticorrosive treatment in the usual manner. 
The treatment for thermal oxidation resistance may be conducted in any 
known manner. For example, it may be carried out as a zinc alloy plating, 
such as with a Zn-Ni alloy or Zn-Co alloy. For the purposes of the 
invention, the "treatment as a thermal oxidation resistance" is defined as 
a treatment for preventing tarnishing as by oxidation under the conditions 
of 100.degree. C. or above for 30 minutes, preferably 200.degree. C. or 
above for 30 minutes, more preferably 240.degree. C. or above for 30 
minutes, in air. Practical examples include treatments with Zn or with an 
alloy of Zn and one or more metals chosen from among Ni, Co, V, W, Mo, Sn, 
Cr, etc. 
Taking the treatment with a Zn--Ni alloy for example, it is performed, 
preferably using a Zn--Ni electroplating bath, to form a very thin Zn--Ni 
alloy layer desirably comprising 50-97% by weight of Zn and 3-50% by 
weight of Ni at a deposition rate of 100-500 .mu.g/dm.sup.2. If the Ni 
content is less than 3% by weight, there is no improvement in thermal 
oxidation resistance as desired. If the Ni content exceeds 50% by weight, 
the solder wettability and thermal oxidation resistance are reduced. If 
the deposition rate of the Zn--Ni alloy layer is below 100 .mu.g/dm.sup.2, 
no improvement in thermal oxidation resistance results- If it is above 500 
.mu.g/dm.sup.2, diffusion of Zn, and of the alloying metal, deteriorates 
the electric conductivity. A proper Zn--Ni alloy layer enhances the 
thermal oxidation resistance of the shiny side of a copper foil while 
maintaining the solder wettability, resist adhesion, and other desirable 
properties of the foil unimpaired. The deposition rate is specified as 
above, for one thing, to form a layer thin enough to retain as much 
appearance of copper as possible. 
A, a Zn--Co alloy treatment is similarly conducted, preferably using a 
Zn-Co electroplating bath, to form a very thin Zn--Co alloy layer 
desirably comprising 50-97% by weight of Zn and 3-50% by weight of Co at a 
deposition rate of 100-500 .mu.g/dm.sup.2. If the Co content is less than 
3% by weight, there is no improvement in thermal oxidation resistance as 
desired. If the Co content is more than 50% by weight, the solder 
wettability and thermal oxidation resistance are reduced. If the 
deposition rate of the Zn--Co alloy layer is below 100 .mu.g/dm.sup.2, no 
improvement in thermal oxidation resistance results. If it is above 500 
.mu.g/dm.sup.2, diffusion of Zn, and of the alloying metal, deteriorates 
the electric conductivity. It also can affect the solder wettability 
adversely in processes where flux is not employed. The deposition rate is 
specified as above, for one thing, to form a layer thin enough to retain 
as much appearance of copper as possible. 
A typical composition of a Zn--Ni alloy or Zn--Co plating bath and typical 
plating conditions are as follows: 
______________________________________ 
Zn 5-50 g/l 
Ni (or Co) 1-50 g/l 
pH 2.5-4 
Temperature 30-60.degree. 
C. 
Current density 0.5-5 A/dm.sup.2 
Plating time 0.1-10 seconds 
______________________________________ 
After water washing, the thermal oxidation-resistant treated layer is 
anticorrosively treated with a Cr-base compound. The term "Cr-base 
anticorrosive treated layer" as used herein means an anticorrosive layer 
consisting mainly of chromium oxide formed by (1) coating with chromium 
oxide alone, (2) mixed coating with chromium oxide and zinc and/or zinc 
oxide, or (3) both. 
Coating with chromium oxide alone may be done by either dip chromate or 
electrolytic chromate treatment. Where weathering resistance is required, 
electrolytic chromate treatment is preferred. Whichever treatment is used, 
the conditions for the treatment conform to those conditions established 
in the art. Exemplary conditions for dip and electrolytic chromate 
treatments are given below, 
______________________________________ 
(A) Dip chromate treatment 
K.sub.2 Cr.sub.2 O.sub.7 
0.5-1.5 g/l 
pH 1.4-5.0 
Temperature 20-60.degree. 
C. 
Time 3-10 seconds 
(B) Electrolytic chromate treatment 
K.sub.2 Cr.sub.2 O.sub.7 (Na.sub.2 Cr.sub.2 O.sub.7 or CrO.sub.3) 
2-10 g/l 
NaOH or KOH 10-50 g/l 
pH 7-13 
Bath temperature 20-80.degree. 
C. 
Current density 0.05-5 A/dm.sup.2 
Time 5-30 seconds 
Anode Pt-Ti, stainless 
steel sheet, etc. 
______________________________________ 
The expression "mixed coating with chromium oxide and zinc/zinc oxide" as 
used herein means a treatment, known as electrolytic zinc-chromium 
treatment, whereby a surface is coated with an anticorrosive layer of zinc 
or a zinc-chromium-base mixture consisting of zinc oxide and chromium 
oxide by electric plating using a plating bath containing a zinc salt or 
zinc oxide and a chromic salt. As the plating bath, typically a mixed 
aqueous solution of at least one of bichromates, such as K.sub.2 Cr.sub.2 
O.sub.7 or Na.sub.2 Cr.sub.2 O.sub.7, at least one of zinc compounds, such 
as ZnO or ZnSO.sub.4.7H.sub.2 O, and an alkali hydroxide is used. A 
typical plating bath composition and electrolytic conditions are as 
follows. 
______________________________________ 
(C) Electrolytic zinc-chromium treatment 
K.sub.2 Cr.sub.2 O.sub.7 (Na.sub.2 Cr.sub.2 O.sub.7 or CrO.sub.3) 
2-10 g/l 
NaOH or KOH 10-50 g/l 
ZnO or ZnSO.sub.4.7H.sub.2 O 
0.05-10 g/l 
pH 7-13 
Bath temperature 20-80.degree. 
C. 
Current density 0.05-5 A/dm.sup.2 
Time 5-30 seconds 
Anode Pt-Ti, stainless 
steel sheet, etc. 
______________________________________ 
Coating amounts required of chromium oxide and zinc are at least 15 
.mu.g/dm.sup.2 as chromium and at least 30 .mu.g/dm.sup.2, respectively. 
The thickness of coating may differ for the shiny and matt sides of the 
foil. Anticorrosive processes of this character are disclosed in prior 
publications including Patent Application Publication Nos. 7077/1983, 
33908/1986, and 14040/1987. The combination of coating with chromium oxide 
alone and coating with a mixture of chromium oxide and zinc/zinc oxide is 
also effective. 
The copper foil obtained after subsequent water washing and drying in 
conformity with the present invention is capable of withstanding thermally 
oxidizing conditions of 300.degree. C. and 30 minutes. Moreover, it can 
retain its solder wettability, resist adhesion, and other properties 
unimpaired. In contrast to this, conventionally zinc-plated and 
chromate-treated copper foils are unable to resist less severely oxidizing 
environments of more than about 200.degree. C. for 30 minutes. Even those 
foils directly treated for thermal oxidation resistance without prior 
copper plating cannot resist tarnishing under high temperature conditions 
of more than 240.degree. C. for 30 minutes or 270.degree. C. for 10 
minutes. 
Finally, if needed, a silane treatment is performed whereby the 
anticorrosive layer is coated, on at least the matt side, with a silane 
coupling agent primarily for the purpose of increasing the adhesion 
between the copper foil and the resin base. The coating may be done in 
whatever manner desirable, e.g., by spraying, application by means of a 
coater, immersion, or casting of a solution of a silane coupling agent. 
For instance, Patent Application Publication No. 15654/1985 teaches 
improving the adhesion between the copper foil and the resin base by 
treating the matt side of the foil first with chromate and then with a 
silane coupling agent. For more details, refer to the cited publication. 
Additionally, when necessary, the copper foil may be annealed to enhance 
its ductility.

EXAMPLES 
By way of illustration, examples of the present invention and comparative 
examples are given below. Copper foils obtained were tested for the 
amounts of surface deposits, baking resistance, and solder wettability on 
the shiny side. For the surface analysis, each test specimen was dipped in 
acid, with its matt side masked by pressing with a base material, such as 
FR-4, to dissolve Zn and Ni or Co from only the shiny side, and the 
surface was analyzed by atomic-absorption spectroscopy. For the baking 
test, a test piece of copper foil, 600 mm wide and 100 mm long, was placed 
into an oven under heating conditions of 300.degree. C. and 30 minutes, 
taken out, and the shiny side was inspected for any tarnishing. As for the 
solder wettability, a copper foil pressed against a base was vertically 
dipped into a solder tank using a flux "JS64" made by San-ei Chemical Co. 
The angle of wetting with solder drawn up along the base surface was 
measured. The smaller the angle the better the solder wettability. 
Example 1 
The shiny side of a 35 .mu.m-thick electrolytic copper foil was 
acid-pickled and washed with water. It was then copper-plated, itself 
serving as a cathode and using a copper sheet as an anode, with a bath 
composition containing 35 g/l of Cu.sup.2+ and 100 g/l of H.sub.2 
SO.sub.4. The plating conditions were: bath temperature, 40.degree. C.; 
cathode current density, 20 A/dm.sup.2, and current supply time, 0.4 
second. Copper was electrodeposited at the rate of about 2.6 mg/dm.sup.2. 
The copper-plated shiny side, when observed under SEM, proved to have a 
smooth surface with a uniform layer of freshly electrodeposited copper. 
Immediately following this, Zn--Ni alloy plating was carried out using the 
shiny side thus obtained as a cathode and a zinc sheet as an anode, with a 
Zn--Ni bath of a composition containing 20 g/l Zn and 5 g/l Ni, at a pH of 
3.5, bath temperature of 40.degree. C., and cathode current density of 2.0 
A/dm.sup.2 for a current supply time of 0.6 second. The plated surface was 
chromate-treated by a dip into a chromate treating solution of a bath 
composition containing 3.5 g/l of CrO.sub.3, at a pH of 4.8 and a 
temperature of 50.degree. C. The treatment was immediately followed by 
water rinsing and drying, and a thermal oxidation-resistant, 
anticorrosively treated shiny side surface was obtained. The shiny surface 
contained 200 .mu.g/dm.sup.2 of Zn, 25 .mu.g/cm.sup.2 of Ni, and 15 
.mu.g/dm.sup.2 of Cr. Its angle of solder wetting was 42.7.degree. and the 
wettability was as good as 100%. 
The thermal oxidation-resistant copper foil so obtained showed no oxidative 
tarnishing or yellowing upon heating at 300.degree. C. for 30 minutes in a 
baking test. 
Comparative Example 1 
An electrolytic copper foil was Zn--Ni alloy-plated and chromate-treated in 
the same manner as that in Example 1 excepting the prior copper plating, 
and a thermal oxidation-resistant, anticorrosively treated shiny side was 
obtained. The shiny side contained 200 .mu.g/dm.sup.2 of Zn, 25 
.mu.g/dm.sup.2 of Ni, and 15 .mu.g/dm.sup.2 of Cr. The pits 0.1 to 0.3 
.mu.m in size that had originally scattered on the raw electrolytic copper 
foil remained almost unchanged. The resulting thermal oxidation-resistant 
copper foil showed oxidative tarnishing in the form of islets or speckles 
on heating at 300.degree. C. for 30 minutes. 
Example 2 
The shiny side of a 35 .mu.m-thick electrolytic copper foil was 
copper-plated in the manner described in Example 1. Next, Zn--Co alloy 
plating was carried out using the shiny side thus obtained as a cathode 
and a zinc sheet as an anode, with a Zn--Co bath of a composition 
containing 20 g/l Zn and 10 g/l Co, at a pH of 3, bath temperature of 
40.degree. C., and cathode current density of 2.0 A/dm.sup.2 for a current 
supply time of 0.7 second. The plated surface was chromate-treated by a 
dip into the chromate treating solution of Example 1. The treatment was 
immediately followed by water rinsing and drying, and a thermal 
oxidation-resistant, anticorrosively treated shiny side surface was 
obtained. The shiny surface contained 230 .mu.g/dm.sup.2 of Zn, 23 
.mu.g/dm.sup.2 of Co, and 30 .mu.g/dm.sup.2 of Cr. The thermal 
oxidation-resistant copper foil so obtained showed no oxidative tarnishing 
or yellowing upon heating at 300.degree. C. for 30 minutes. Its angle of 
solder wetting was 42.7.degree. and the wettability was as good as 100%. 
Comparative Example 2 
An electrolytic copper foil was treated in the manner described in Example 
2 except that it was not copper-plated. The resulting thermal 
oxidation-resistant copper foil showed oxidative tarnishing in the form of 
islets or speckles on heating at 300.degree. C. for 30 minutes. 
Example 3 
The shiny side of a 35 .mu.m-thick electrolytic copper foil was 
acid-pickled and water-washed and then etched by a dip in an aqueous 
ammonium persulfate solution at a concentration of 10 g/l, at a bath 
temperature of 30.degree. C. and an agitation linear velocity of 1 m/sec, 
for 5 seconds. The etching amount was 0.03 .mu.m. This treated surface 
exhibited uniformity of deposition when subjected to line analysis of Zn 
on the plated surface after thin-layer Zn plating as stated above. 
Next, Zn--Ni alloy plating was carried out using the shiny side thus 
obtained as a cathode and a zinc sheet as an anode, with a Zn--Ni bath of 
a composition containing 20 g/l Zn and 5 g/l Ni, at a pH of 3.5, bath 
temperature of 40.degree. C., and cathode current density of 2.0 
A/dm.sup.2 for a current supply time of 0.6 second. The plate surface was 
chromate-treated by a dip into a chromate treating solution of a bath 
composition containing 3.5 g/l of CrO.sub.3, at a pH of 4.8 and bath 
temperature of 50.degree. C. The treatment was immediately followed by 
water rinsing and drying, and a thermal oxidation-resistant, 
anticorrosively treated shiny side surface was obtained. The shiny surface 
contained 200 .mu.g/dm.sup.2 of Zn, 25 .mu.g/dm.sup.2 of Ni, and 15 
.mu.g/dm.sup.2 of Cr. Its angle of solder wetting was 42.7.degree. and the 
wettability was as good as 100%. 
The thermal oxidation-resistant copper foil so obtained showed no oxidative 
tarnishing or yellowing upon heating at 300.degree. C. for 30 minutes in a 
baking test. 
Comparative Example 3 
An electrolytic copper foil was Zn--Ni alloy-plated and chromate-treated in 
the same manner as according to Example 3 but without etching, and a 
thermal oxidation-resistant, anticorrosively treated shiny side was 
obtained. The shiny side contained 200 .mu.g/dm.sup.2 of Zn, 25 
.mu.g/dm.sup.2 of Ni, and 15 .mu.g/dm.sup.2 of Cr. The resulting thermal 
oxidation-resistant copper foil showed oxidative tarnishing in the form of 
islets or speckles on heating at 300.degree. C. for 30 minutes. 
Example 4 
The shiny side of a 35 .mu.m-thick electrolytic copper foil was etched in 
the manner described in Example 3. Next, Zn--Co alloy plating was carried 
out using the shiny side thus obtained as a cathode and a zinc sheet as an 
anode, with a Zn--Co bath of a composition containing 20 g/l Zn and 10 Co, 
at a pH of 3, bath temperature of 40.degree. C., and cathode current 
density of 2.0 A/dm.sup.2 for a current supply time of 0.7 second. The 
plated surface was chromate-treated by a dip into the same chromate 
treating solution as used in Example 3. The treatment was immediately 
followed by water rinsing and drying, and a thermal oxidation-resistant, 
anticorrosively treated shiny side surface was obtained. The shiny surface 
contained 230 .mu.g/dm.sup.2 of Zn, 23 .mu.g/dm.sup.2 of Co, and 30 
.mu.g/dm.sup.2 of Cr. The thermal oxidation-resistant copper foil so 
obtained showed no oxidative tarnishing or yellowing upon heating at 
300.degree. C. for 30 minutes. Its angle of solder wetting was 
41.6.degree. and the wettability was as good as 100%. 
Comparative Example 4 
An electrolytic copper foil was treated in the manner described in Example 
4 except that it was not etched. The resulting thermal oxidation-resistant 
copper foil showed oxidative tarnishing in the form of islets or speckles 
on heating at 300.degree. C. for 30 minutes. 
Advantages of the invention 
The present invention provides a copper foil having a highly thermal 
oxidation-resistant shiny side which undergoes no such oxidative 
tarnishing or yellowing upon heating at 300.degree. C. for 30 minutes that 
conventional products do. The shiny side is protected against tarnishing 
with the heat history during the fabrication of printed circuit boards, 
without concomitant impairment of its solder wettability, resist adhesion, 
and other desirable properties. Thus the product of the invention is 
capable of meeting future requirements for the copper foil for printed 
circuit boards.