Coil inductor with metal film on wire
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coil inductor, and more particularly to a coil inductor employed in an electric circuit such as a filter circuit or an oscillation circuit. 
     2. Description of Related Art 
     A copper wire which is covered with an insulating coat such as polyurethane has been used as a coil of a coil inductor. After the wire is coiled, the insulating coat of the wire is partly removed at an end, and the bare end of the copper wire is soldered to a terminal (or an electrode). 
     The coil, particularly the copper wire is thinned, when the inductance is wanted to be increased, or the inductor is wanted to be more compact. However, since copper is diffusible into solder, using an excessively thinned wire causes a problem that the copper partly diffuses into the solder to breaking of the wire when the wire is connected to the terminal (or the electrode) or when the inductor is mounted on a printed-wiring board by soldering. Therefore, the diameter of the copper wire had to be kept approximately 50 μm at the minimum, and could not be thinned more. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a compact inductor which has large inductance. 
     We attempted to attain the object by solving the problem that the diffusion of copper into solder prevents the thinning of the coil. This is realized by using a metal wire comprising a copper wire; a first metal film covering the copper wire, the first metal film being made of a metal which is not diffusible into solder; and a second metal film covering the first metal film, the second metal film being made of a metal which has fine solderability. 
     In a coil inductor employing the above wire, the first metal film prevents diffusion of copper into solder, and the second metal film ensures fine solderability. Moreover, since a core material of the wire is copper as in a conventional coil inductor, an electrical performance such as direct current resistance will not be damaged. For example, Ni, Fe, Pd, NiCu, etc. are used as the material of the first metal film, and Cu, Sn, Ag, Au, Cd solder, etc. are used as the material of the second metal film. 
     According to the present invention, a metal wire comprising a copper wire which is 10 to 40 μm in diameter can be used as a coil of a coil inductor which has larger inductance than a conventional coil inductor of the same size because the number of coiling can be increased within the same space. Also, it is possible to obtain a coil inductor which is more compact than a conventional coil inductor having the same inductance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     This and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiments in reference to the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a wire which is an essential component of a coil inductor according to the present invention; 
     FIG. 2 is an elevation view of the coil inductor whose wire is coiled around a ferrite core, showing the coil part in a sectional view; and 
     FIG. 3 is an elevation view of another coil inductor according to the present invention, showing the coil part in a sectional view. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The description of preferred embodiments according to the present invention is given below, referring to the drawings. 
     FIG. 1 shows a cross-sectional view of a wire 1. A copper wire 2 is used as a core of the wire 1, and the wire 1 is covered with a first metal film 3 of, for example, 0.1 to 2 μm thickness. The first metal film 3 is formed by plating or cladding a metal which is not diffusible into solder, such as Fe, Ni, Pd, Pt and NiCu (monel). Further, the first metal film 3 is covered with a second metal film 4 of, for example, 0.1 to 2 μm thickness. A metal which has fine solderability, such as Cu, Sn, Ag, Au, Cd and solder, is used as the second metal film 4. The copper wire 2 covered with the metal films 3 and 4 is finished as the wire 1 by covering the second metal film 4 with an insulating coat 5 made of polyurethane resin and the like. 
     The wire 1 of the above structure is coiled around a ferrite core 10 as shown in FIG. 2. The ferrite core 10 is composed of a body 11 and flanges 12 and 13 installed at upper and lower ends of the body 11. The coil 1&#39; is coiled around the body 11 of the core 10, and the insulating coat 5 of the coil 1&#39; is removed both ends 1a and 1b, so that the metal film 4 is exposed at the ends 1a and 1b. The both ends 1a, 1b are bound around terminals 14a and 14b which are in stalled at the lower flange 13 of the core 10, and soldered by solders 15a and 15b. 
     Since the coil inductor of the above structure has the copper wire 2 covered with the first metal film 3 and the second metal film 4, when the ends 1a and 1b of the coil 1&#39; are soldered to the terminals 14a and 14b, or when the inductor is soldered to a printed-wiring board, diffusion of copper into solder is avoided. More specifically, the first metal film 3 which is formed on the surface of the copper wire 2 prevents the copper from diffusing into the molten solder, and the second metal film 4 compensates for bad solderability of the first metal film 3. 
     Therefore, a thin copper wire (for example, a copper wire of 10 to 40 μm diameter) can be used as a component of a coil inductor, and it becomes possible to increase the inductance and decrease the dimensions. 
     FIG. 3 shows another embodiment of the coil inductor according to the present invention. This coil inductor is a surface mounting type. 
     The ferrite core 20 is composed of a body 21 and flanges 22 and 23 which are provided at upper and lower ends of the body 21. The wire 1 shown in FIG. 1 is used as the coil 1&#39;, and coiled around the body 21 of the core 20. The both ends 1a and 1b are soldered to electrodes 24a and 24b which are installed at the lower flange 23 of the core 20. 
     Since the wire 1 shown in FIG. 1 is also used for this coil inductor, the coil inductor has the same effect as shown in FIG. 2. 
     Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are apparent to a person skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention defined by the appended claims. 
     For example, an air-core inductor, a mutual inductor and the like can be composed using the wire which is shown in FIG. 1.