1. Field of the Invention
The present invention relates to an electrolytic copper foil which is used as an electrode collector for a secondary battery, an electrolytic copper foil for printed circuit and a process for the production thereof.
2. Description of the Related Art
An electrolytic copper foil is produced on an industrial basis in the following manner. That is, an insoluble electrode of titanium or lead coated with a platinum group element is used as an anode while a rotary cylindrical metallic cathode made of stainless steel or titanium is used as an opposing cathode. The space between the electrodes is filled with an electrolytic solution comprising copper sulfate and sulfuric acid. When direct current is passed across the opposing electrodes, copper is deposited on the surface of the cathode. During this procedure, the rotary cylindrical metallic cathode is rotated so that copper thus deposited is peeled off the surface of the cathode. Accordingly, it is possible to continuously produce electrolytic copper.
In general, an electrolytic copper foil which has thus separated out from an electrolytic solution comprising two components, i.e., the matte side which is deposited from copper sulfate and sulfuric acid electrolytic solution, which is referred to as roughened surface, an abnormally deposited copper the thickness of which exceeds the normal foil thickness that causes a critical defect. The electrolytic copper foil thus produced has a matte side and has pinholes therein. The electrolytic copper thus obtained exhibits a prismatic structure which grows from the shiny side towards the matte side.
For the purpose of inhibiting the abnormal deposition and occurrence of pinholes on the matte side, it is a common practice to add glue or gelatin and a chloride to an sulfurically acidic copper sulfate electrolytic solution in an amount of from 0.1 to 10 mg/l and from 0.5 to 100 mg/l, respectively. However, an electrolytic copper foil having a high hardness cannot be obtained in this additive system. The resulting matte side has pyramidal crests and thus gives a high roughness.
In recent years, the copper-clad laminate as a main purpose of electrolytic copper foil is intended to rise in the fineness of circuit pattern and reduce the gap between insulation layers. Therefore, a copper foil having an even more thickness and a low roughness at the matte side has been desired.
During the production of a copper-clad laminate, the copper foil is subjected to stress when the resin expands or shrinks under heating, thereby causing breaking of circuit or warpage or twisting of printed circuit board. It is known that such a problem can be solved by the use of a copper foil having a high elongation at high temperatures.
During the lamination of copper-clad laminates, the surface of one copper foil rubs against another to cause damage on the copper-clad laminates. In some detail, when the copper foil is heated during press molding in the procedure of production of copper-clad laminate, copper is recrystallized to lower the hardness of the copper foil. When the copper foils rub against each other during the subsequent lamination step, they have scratches on the surface thereof. In an extreme case, the copper foil is peeled off from the surface of the resin substrate.
In recent years, as cured resin for multi-layer circuit board, there has been used a resin material having a low dielectric constant, a low dielectric loss and a high heat resistance. In general, the formation of such a resin into a multi-layer circuit board requires a high forming temperature. From the standpoint of heat resistance during passage through solder flow, too, the copper foil to be incorporated in the multi-layer circuit board preferably exhibits a small hardness drop due to recrystallization while showing a sufficient elongation at high temperatures.
On the other hand, as electrode collector for secondary battery there has already been used rolled copper foil in many cases. The proportion of electrolytic copper foil occupying in the art is small.
The reasons for this fact will be described below.
(1) An ordinary electrolytic copper foil essentially differs in roughness from one surface to another. Thus, the electrolytic copper foil differs in battery properties from one surface to another. Accordingly, this difference must be considered.
(2) An electrolytic copper foil exhibits a poorer elasticity than rolled copper foil and thus can be wrinkled easily when formed into a thin foil.
(3) An electrolytic copper foil exhibits a poor flexibility.
Thus, an electrolytic copper foil cannot be used as an electrode collector for secondary battery unless these difficulties are overcome.
However, while the maximum width of a rolled copper foil is limited to about 600 mm, an electrolytic copper foil can be formed into a form having a width of not less than twice that of the rolled copper foil. Further, unlike the rolled copper foil, which is liable to pinholing when formed into a thin foil, the electrolytic copper foil is insusceptible to such a defect and thus can be formed into a thin foil to advantage on an industrial basis.
With this advantage, the use of an electrolytic copper foil as an electrode collector makes a great contribution to the drastic rise in the efficiency of the step of application of an active battery material. Further, the use of an electrolytic copper foil having a reduced thickness makes a great contribution to the reduction of the battery weight and the battery production cost and the rise in the energy density of the battery.
Thus, it goes without saying that the use of an electrolytic copper foil as an electrode collector for secondary battery requires the foregoing difficulties to be overcome. Further, taking into account the requirement that the foil be roughened on both sides thereof to exert an anchoring effect that contributes to adhesion to active battery material and exhibit a high elongation at high temperatures in response to thermal expansion during heat generation by battery and the use in a secondary battery for mobile communications apparatus or electric car, a thin foil having a mechanical strength high enough to withstand vibration is desirable.
Thus, it has been required that the electrolytic copper foil to be used for the various purposes have a reduced thickness and a low roughness on the matte side. However, the electrolytic copper foil produced by the prior art method exhibits a low hardness and thus can be easily scratched. Further, such an electrolytic copper foil exhibits a low tensile strength and thus can be easily wrinkled when formed into a thin foil, making it very difficult to handle. Moreover, such an electrolytic copper foil has a high roughness on the matte side, which greatly differs from that of the shiny side.
In general, copper which has separated out from a sulfurically acidic copper sulfate electrolytic solution containing thiourea and a chloride exhibits a high hardness and tensile strength shortly after electrodeposition. However, copper thus electrodeposited shows a poor thermal stability of these mechanical properties and thus can easily undergo primary recrystallization at room temperature to show a hardness drop.
On the other hand, as a method for the production of an electrolytic copper having a high hardness and tensile strength there may be used, e.g., a method involving the production in a very low free chloride concentration (about 1.0 to about 4.5 ppm) as described in JP-A-7-188969 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, this method is disadvantageous in that chlorides contained in waste wire material used as a starting material or chlorine in tap water used can inevitably contaminate the electrolytic solution system. For the purpose of inhibiting the contamination by these chlorides, approaches such as (1) purification of starting material or use of ion-exchanged water and (2) use of soluble anode can be proposed. However, the purification of a starting material causes the increase in the number of required steps. Further, the use of a soluble anode finds trouble in controlling uniformly the foil thickness crosswise and longitudinally and difficulty in supplying starting material. The use of a soluble anode inevitably causes the production of anode slime that leads to abnormal deposition. As an approach for preventing this difficulty there may be proposed the use of an anode bag. However, this countermeasure causes a current efficiency drop.
Thus, the production of an electrolytic copper foil having a high tensile strength in a low free chloride concentration has many disadvantages. The solution to these problems requires great plant and equipment investment and leads to productivity drop, resulting in the rise in product cost.
As already mentioned, the use of conventional additives makes it impossible to provide the resulting electrolytic copper foil with an excellent thermal stability while maintaining a high Vickers' hardness. On the other hand, it is difficult to control the chloride concentration within an extremely low narrow range for the purpose of producing an electrolytic copper foil having these properties. Abnormal deposition can easily occur in such a low chloride concentration. The use of such a method requires great plant and equipment investment, thereby resulting in the rise in product cost.
Therefore, it is desired to produce an electrolytic copper foil exhibiting a high hardness as well as an excellent thermal stability by widening chloride concentration range in a sulfurically acidic copper sulfate.