1. Field of the Invention
The present invention relates to a method of forming an electrically conductive (hereinafter simply referred to "conductive") circuit by fixing a thin metal belt having a predetermined pattern onto a surface of an insulating substrate such as a metal substrate, an enameled substrate, or the like, and particularly to a method of forming a conductive circuit or a plate heating unit circuit which is required to conduct a large current.
2. Description of the Prior Art
Conventionally, typical conductive circuit boards have been formed in such a manner that copper foil having about 5 to 15 .mu.m of thickness bonded onto the surface of an insulating substrate made of such as phenolic resin, epoxy resin containing glass fibers, or the like, was etched to form a predetermined conductive circuit. The circuit substrate of this kind is small in size, light in weight, inexpensive in cost, and has been used in various kinds of electric apparatuses.
However, it becomes necessary to conduct a micro-current not larger than 1 .mu.A in the case where an IC, an LSI circuit, or the like, is employed on the circuit board, while it is required to conduct an instantaneous large current if an IC, an LSI circuit, or the like, is used in an automatic focusing circuit, a motor driving circuit, a stroboscopic lamp circuit, or the like, in an electronic camera, a VTR camera, or the like. That is, it is necessary to form a given one of various circuits which are different from each other in level of current over eight digits, that is, over a range form about 0.5 .mu.A to 10A, on the same circuit board, so that it is impossible to ensure formation of a highly accurate conductive circuit on such a circuit board as described above.
Although making the thickness of the copper foil thicker to be within a range from about 50 to 150 .mu.m has been tried in order to overcome the above disadvantage, a problem occurs in that it takes a long time for etching the copper foil having such thickness so that protrusions are remarkably caused particularly at the corner portions of a circuit due to a phenomenon of overhanging, under-cutting and therefore it is impossible to maintain an accurate circuit resistance. Accordingly, in order to obtain a circuit pattern which can endure various levels of currents, from a micro one to a large one, it is necessary to employ a method of punching correctly a thin copper belt having a relatively large thickness by using a press.
Further WO No. 84/00275 discloses a plate heating unit in which a heating element consisting of conductive foil having a predetermined pattern is fixed and covered with an enamel layer on a surface of an enameled substrate. It is convenient to produce the heating element to be used in the heating unit of this kind by punching metal foil by a press.
In the case of forming, by punching, conductive foil having a predetermined pattern for constituting a conductive circuit for such an electronic apparatus or for heating units as described above, it is required to provide means for maintaining the conductive foil in its predetermined shape in order to prevent the conductive foil from being locally transformed or injured in handling the conductive foil to store the same, to displace the same, positioning the same on the substrate, etc., during a period until the conductive foil has been fixed on the substrate.