The invention concerns a process for producing a self-supporting foil, in particular a copper foil, which by virtue of its constitution has a low shearing strength and can be structured in a sharp-edged configuration, wherein the metal foil is galvanically deposited on a rotating roller which forms a cathode and which is dipped into an electrolyte bath containing ions of the metal, an anode cage is provided at a constant spacing from the roller cathode and a defined adjustable dc voltage is applied during rotation of the roller cathode to the roller cathode and the anode cage so that a given adjustable high direct current flows between the roller cathode and the anode cage in the electrolyte bath, by which direct current a metal base layer is deposited on the roller cathode, at least one additional anode is provided between the roller cathode and the anode cage, a defined adjustable second dc voltage is applied between the roller cathode and the additional anode so that a given adjustable additional direct current flows between the roller cathode and the additional anode, whereby a cauliflower structure comprising the metal is deposited in firmly adhering relationship on the outside of the metal base layer, which is remote from the roller cathode, the metal foil comprising the metal base layer provided with the cauliflower structure is detached from the roller cathode, and the metal foil comprising the metal base layer provided with the cauliflower structure is rinsed and dried.
EP 0 063 347 discloses a metal foil which is used as a stamping foil and which is transferred on to a substrate by means of a suitable stamping member. In order to fix that known metal foil which can be structured in a sharp-edged configuration to the above-mentioned substrate, use is made of an adhesive which is preferably a hot melt adhesive. Such an adhesive represents an electrically insulating layer so that electrically conductive contacting of the metal foil which can be structured in a sharp-edged configuration with the substrate is not possible there. In regard to electrically insulating substrates, that is not a material consideration, but the circumstances are different in regard to ohmically conducting substrates.
EP 0 392 151 A2 proposes integrating solder powder particles into the adhesive layer between the metal foil and the substrate in order to make an electrically conducting connection between a substrate and such a metal foil which can be structured in a sharp-edged configuration.
Irrespective of whether solder powder particles are or are not integrated into the adhesive layer, the metal foil which can be structured in a sharp-edged configuration requires the adhesive layer to be applied to the side of the foil, which is towards the substrate. That represents a not inconsiderable production complication and expenditure.
U.S. Pat. No. 5,019,221 discloses an apparatus for the galvanic production of a metal foil such as a copper foil, on a roller cathode which dips into an electrolyte bath containing ions of the metal. An anode cage is disposed at a spacing from the roller cathode. The anode cage is in concentric relationship with the roller cathode. A defined dc voltage is applied to the roller cathode and the anode cage. The roller cathode is driven in rotation at the same time. As a result of the dc voltage a given high direct current flows between the roller cathode and the anode cage, with the metal foil thus being deposited on the roller cathode in the form of a metal base layer. On its outside which is remote from the roller cathode, that known metal foil involves a relatively low degree of surface roughness. The surface roughness of a copper foil produced in that way is at a maximum between 5 and 6 xcexcm. As a consequence of that degree of surface roughness, it has not been possible hitherto for such a known metal or copper foil to be applied directly to a substrate, while being structured in a sharp-edged configuration, without an adhesive layer.
U.S. Pat. Nos. 4,692,221 or 5,215,646 each disclose a process of the kind set forth in the opening part of this specification, for producing a metal foil, in particular a copper foil, which in comparison with a metal foil and in particular a copper foil without a cauliflower structure, as a consequence of its increased surface roughness, enjoys improved adhesion after having been embossed on to a substrate.
The object of the invention is so to develop a process of the kind set forth in the opening part of this specification that it is possible to produce a metal foil, in particular a copper foil, whose adhesive strength after embossing on to a substrate is still further improved.
In accordance with the invention in a process of the kind set forth in the opening part of this specification, that object is attained in that the dried metal foil comprising the metal base layer provided with the cauliflower structure is moved through a black oxide bath, and the metal foil comprising the metal base layer provided with the cauliflower structure, subsequently to the black oxide bath, is rinsed and dried.
The process steps according to the invention provide that the shearing strength of the metal foil when embossed on to a substrate is further improved.
The anode cage can be for example at a constant spacing from the roller cathode, which for example can be of the order of magnitude of around 50 mm. The at least one additional anode between the anode cage and the roller cathode can for example have an anode bar which is at a constant spacing from the roller cathode, which spacing can be of the order of magnitude of between 2 and 3 mm. The metal base layer, in particular copper, is deposited on the rotating roller cathode by means of the anode cage. Depending on the dc voltage applied to the roller cathode and the anode cage, and the direct current flowing therebetween, that base layer can be of the order of magnitude of between 10 and 100 xcexcm. The at least one additional anode between the roller cathode and the anode cage, depending on the second dc voltage applied between them and the additional direct current produced thereby, produces the cauliflower structure consisting of the metal or the copper, on the metal, in particular copper, base layer. That cauliflower structure involves a surface roughness of between 10 and 25 xcexcm
The metal foil, in particular the copper foil, consisting of the base layer and the cauliflower structure therefore advantageously entails such a level of surface roughness that the cauliflower structure is hookingly engaged to the substrate, thus affording a high level of shearing strength for the foil on the substrate virtually independently of the material of the substrate. The substrate may be for example an electrically or ohmically conducting body. If such a body is embossed with the foil produced in accordance with the invention, in a condition of being structured in a sharp-edged configuration, it can be used for example as a resistance heating element. The foil produced in accordance with the invention can also be applied to an electrically insulating substrate in a condition of being structured in a sharp-edged configuration, in order for example to produce a circuitry structure with a high power consumption. It is likewise possible for the foil which is produced in accordance with the invention and which can be structured in a sharp-edged configuration to be embossed on to a substrate having an adhesive layer, or for the foil to be provided with an adhesive layer in order for it to be embossed on to any substrate in a condition of being structured in a sharp-edged configuration.
It is desirable if the speed of rotation of the roller cathode and the dc voltage as between the roller cathode and the anode cage are adapted to each other in order to produce the metal and in particular copper base layer in a given thickness. That means that the thickness of the metal and in particular copper base layer is directly proportional to the dc voltage between the roller cathode and the anode cage and consequently to the direct current between the roller cathode and the anode cage and it is inversely proportional to the speed of rotation of the roller cathode.
In order to provide a given thickness or height of the metal cauliflower structure on the metal base layer, the additional dc voltage between the additional anode and the roller cathode and the speed of rotation of the roller cathode can be suitably adapted to each other. The consideration in regard to the thickness or height of the metal cauliflower structure on the metal base layer is similar to the thickness of the metal base layer, in that the thickness or height of the metal cauliflower structure is directly proportional to the additional dc voltage between the additional anode and the roller cathode and the additional direct current flowing therebetween and it is inversely proportional to the speed of rotation. Consequently, by suitably adjusting the additional dc voltage and the speed of rotation of the roller cathode, it is specifically and targetedly possible to adjust the thickness or height of the metal cauliflower structure as desired at between 10 and 25 xcexcm. A greater thickness or height for the metal cauliflower structure is less meaningful because then particles of the cauliflower structure become detached therefrom, which is unwanted. A smaller thickness or height of the metal cauliflower structure than 10 xcexcm is also not appropriate because then the adhesive strength of the metal foil produced in accordance with the invention, which can be structured in a sharp-edged configuration, does not enjoy the desired shearing strength after being embossed on to a substrate.
In accordance with the invention the metal foil comprising the metal base layer provided with the cauliflower structure is moved after the rinsing and drying operations through a black oxide bath. Subsequently to the black oxide bath, in accordance with the invention a rinsing operation and a drying operation are carried out. The metal foil, in particular a copper foil, which is produced in accordance with the invention in that way and which, by virtue of its constitution, has a low level of shearing strength and consequently can be structured in a sharp-edged configuration by embossing not only has a cauliflower structure by virtue of the treatment in the black oxide bath, but a fibrous hairy cauliflower structure by which the shearing strength of the metal foil embossed to a substrate is advantageously improved.