Abstract:
This electric power fuse has a fuse element that is formed continuously and integrally by a plurality of heat dissipating parts formed from a conductive film by the conductive film being formed on a ceramic substrate and a plurality of isolating parts. The conductive film is constituted of printed layers formed by printing one or more times on the surface of the ceramic substrate, and the number of laminations of printed layers formed in the heat dissipating parts is greater than or equal to the number of laminations of the printed layers constituting the isolating parts.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an electric power fuse, which has an electrically conductive film disposed on a substrate, and includes heat radiation zones and current-interruption grids that are provided integrally in succession. 
       BACKGROUND ART 
       [0002]    Heretofore, a main requirement for electric power fuses for protecting semiconductor switching devices such as GTO (Gate Turn Off) thyristors and IGBTs (Insulated Gate Bipolar Transistors) is to have a quick cutoff performance. 
         [0003]    Such electric power fuses have a fuse element embedded in an arc-extinguishing material, which is housed in a fuse tube. Known types of fuse elements include fuse elements produced by a pressing process and fuse elements produced by an etching process (see Japanese Laid-Open Patent Publication No. 2006-073331 and Japanese Laid-Open Patent Publication No. 2009-193723). A pressed fuse element includes an array of several narrow cutoff canals, each having a small cross-sectional area, which are punched out of a ribbon of metal, e.g., silver (Ag), by a pressing die. An etched fuse element has an electrically conductive thin film of copper, silver, or the like disposed on the upper surface of a ceramic substrate. An electrically conductive thin film is etched and patterned into the array of several narrow cutoff canals each having a small cross-sectional area. The pressed fuse element includes an electrically conductive thin film that is limited in both thickness and line width to 150 μm, which poses limitations on efforts to lower the I 2 t value and to reduce the size of the electric power fuse. On the other hand, the electrically conductive thin film that is made up of the etched fuse element can have a smaller thickness and line width, thus allowing the etched fuse element to have a lower I 2 t value and a smaller size than the pressed fuse element. However, the etched fuse element leaves much to be improved in relation to cost and manufacturing variations, which tend to occur when the etched fuse element is mass-produced. The I 2 t value refers to a representative value indicative of a cutoff performance, which is calculated by integrating the square of a cutoff current I (I 2 dt) over a cutoff time from 0 to t (t: total cutoff time). 
       SUMMARY OF THE INVENTION 
       [0004]    If a fuse element is fabricated by etching, a liquid etchant, which exhibits a property to corrode and dissolve a target metal, is applied in order to remove portions of an electrically conductive thin film disposed on a ceramic substrate, thereby producing a desired conductive pattern. The conductive pattern required on the fuse element is a pattern having a high aspect ratio, such that heat radiation zones have a thickness of about 100 μm, and current-interruption grids have a width ranging from 65 to 100 μm and a thickness of about 25 μm. 
         [0005]    Fabrication of a fuse element by etching suffers from the following problems: 
         [0006]    (a) If the electrically conductive thin film is etched deeply, then the electrically conductive film is susceptible to corrosion beneath the etching mask, creating undercuts. Therefore, it is difficult to micro-fabricate the electrically conductive thin film with high precision. 
         [0007]    (b) Since the etching rate changes depending on the temperature of the etchant and the stirring speed at which the etchant is stirred, repeatability of the etching process, i.e., repeatability of the conductive pattern, is poor. 
         [0008]    As a result, the conductive pattern of the current-interruption grids varies at different positions on the substrate, or varies among ceramic substrates. 
         [0009]    Consequently, the amount of pattern conductor in each of the current-interruption grids and the overall resistance value of the fuse element are likely to vary, leading to variations in the I 2 t value and variations in the rated current. 
         [0010]    Minimizing variations between the current-interruption grids and variations between fuse elements poses limitations on efforts to reduce the width of the current-interruption grids. Therefore, the I 2 t value, the cost, and the size of the electric power fuse cannot be reduced sufficiently. 
         [0011]    The present invention has been made in view of the above problems. It is an object of the present invention to provide an electric power fuse, which makes it possible to reduce the I 2 t value, the cost, and the size of the electric power fuse, while at the same time minimizing variations between the current-interruption grids and variations between fuse elements. 
         [0012]    [1] An electric power fuse according to the present invention includes a fuse element having an electrically conductive film, which is disposed on a substrate and includes a plurality of heat radiation zones and a plurality of current-interruption grids that are provided integrally in succession, wherein the electrically conductive film comprises a printed layer disposed on a surface of the substrate by one or more printing processes, and a number of laminae of the printed layer of the heat radiation zones is equal to or greater than a number of laminae of the printed layer of the current-interruption grids. 
         [0013]    With the above arrangement, variations in the film thickness of the narrow cutoff canals between the current-interruption grids and variations between fuse elements can be minimized, thereby minimizing variations in the I 2 t value. Since the heat radiation zones and the current-interruption grids are printed, the heat radiation zones and the current-interruption grids can be formed separately from each other, so that the thickness of the narrow cutoff canals of the current-interruption grids can be controlled as desired independently of the thickness of the heat radiation zones. By controlling the thickness of the narrow cutoff canals in this manner, a reduction in the I 2 t value can be achieved. Consequently, the electric power fuse can be reduced in cost and size. 
         [0014]    [2] According to the present invention, each of the current-interruption grids may have a plurality of narrow cutoff canals arrayed in parallel, and the current-interruption grids may be arranged in series, thereby providing the fuse element. 
         [0015]    [3] The current-interruption grids, each having the narrow cutoff canals arrayed in parallel, and which are shaped identical to each other, may serve as first current-interruption grids. The first current-interruption grids may be arranged in series, thereby making up a first fuse section, and the first fuse section and a second fuse section, which has current vs. fusing time characteristics that differ from the first fuse section, may be connected in succession on the same substrate. An electric power fuse constructed in this manner exhibits characteristics in which the gradient of time with respect to current in a higher current range is greater than the gradient of time with respect to current in a lower current range. 
         [0016]    [4] The second fuse section may comprise a plurality of second current-interruption grids arranged in series, and the second current-interruption grids may differ from the first current-interruption grids of the first fuse section in relation to at least one of a shape of the narrow cutoff canals, a width of the narrow cutoff canals, and the number of laminae of the printed layer. 
         [0017]    [5] A metal material of the printed layer of the first current-interruption grids of the first fuse section and a metal material of the printed layer of the second current-interruption grids of the second fuse section may be different from each other. 
         [0018]    [6] In the electric power fuse of the present invention, an antioxidizing film may be disposed on surfaces of at least the current-interruption grids. The antioxidizing film is effective to prevent at least the current-interruption grids from becoming oxidized, thereby enabling the fuse element to operate reliably over a long period of time. 
         [0019]    [7] According to the present invention, an arc-extinguishing material paste may be printed on at least the current-interruption grids. In this manner, the internal space that houses the arc-extinguishing material therein is reduced. The printed arc-extinguishing material paste is effective to significantly reduce the size of the electric power fuse. 
         [0020]    As described above, the electric power fuse according to the present invention offers the following advantages: 
         [0021]    (1) Variations in the film thickness of the narrow cutoff canals between the current-interruption grids and variations between fuse elements can be minimized, thereby minimizing variations in the I 2 t value. 
         [0022]    (2) Since the heat radiation zones and the current-interruption grids are printed, the heat radiation zones and the current-interruption grids can be formed separately from each other, so that the thickness of the narrow cutoff canals of the current-interruption grids can be controlled as desired independently of the thickness of the heat radiation zones. By controlling the thickness of the narrow cutoff canals in this manner, a reduction in the I 2 t value can be achieved. 
         [0023]    (3) On account of advantages (1) and (2), the electric power fuse can be reduced in cost and size. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  is a cross-sectional view of an electric power fuse according to an embodiment of the present invention; 
           [0025]      FIG. 2  is a plan view, partially omitted from illustration, showing by way of example a conductive pattern of a fuse element of the electric power fuse; 
           [0026]      FIG. 3  is a cross-sectional view, partially omitted from illustration, of the fuse element; 
           [0027]      FIG. 4A  is a cross-sectional view, partially omitted from illustration, showing an arc-extinguishing material, which is made into a paste with a solvent (hereinafter referred to as an “arc-extinguishing material paste”), and is printed on current-interruption grids; 
           [0028]      FIG. 4B  is a cross-sectional view, partially omitted from illustration, showing the arc-extinguishing material paste printed on current-interruption grids and heat radiation zones; 
           [0029]      FIG. 5  is a plan view showing a general structure of a fuse element according to first (first fuse element) through sixth (sixth fuse element) modifications; 
           [0030]      FIG. 6A  is a plan view, partially omitted from illustration, showing a conductive pattern in a first fuse section of the first fuse element; 
           [0031]      FIG. 6B  is a plan view, partially omitted from illustration, showing a conductive pattern in a second fuse section of the first fuse element; 
           [0032]      FIG. 7A  is a cross-sectional view, partially omitted from illustration, showing a first fuse section of the second fuse element; 
           [0033]      FIG. 7B  is a cross-sectional view, partially omitted from illustration, showing a second fuse section of the second fuse element; 
           [0034]      FIG. 8A  is a plan view, partially omitted from illustration, showing a conductive pattern in a first fuse section of the third fuse element; 
           [0035]      FIG. 8B  is a plan view, partially omitted from illustration, showing a conductive pattern in a second fuse section of the third fuse element; 
           [0036]      FIG. 9A  is a plan view, partially omitted from illustration, showing a conductive pattern in a first fuse section of the fourth fuse element; 
           [0037]      FIG. 9B  is a plan view, partially omitted from illustration, showing a conductive pattern in a second fuse section of the fourth fuse element; 
           [0038]      FIG. 10A  is a plan view, partially omitted from illustration, showing a conductive pattern in a first fuse section of the fifth fuse element; 
           [0039]      FIG. 10B  is a plan view, partially omitted from illustration, showing a conductive pattern in a second fuse section of the fifth fuse element; 
           [0040]      FIG. 11A  is a cross-sectional view, partially omitted from illustration, showing a first fuse section of the sixth fuse element; 
           [0041]      FIG. 11B  is a cross-sectional view, partially omitted from illustration, showing a second fuse section of the sixth fuse element; 
           [0042]      FIG. 12  is a graph showing by way of example fusing characteristics of an electric power fuse that incorporates the sixth fuse element; and 
           [0043]      FIG. 13  is a graph showing operating characteristics (rated current vs. operating I 2 t value characteristics) of Inventive Example 1 (see  FIGS. 2 and 3 ) and Comparative Examples 1 and 2. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0044]    Electric power fuses according to embodiments of the present invention will be described below with reference to  FIGS. 1 through 13 . In the following description, the terms “from” and “to” in numerical ranges should be interpreted as inclusive of numerical values that follow these terms as lower and upper limit values of the numerical ranges. 
         [0045]    As shown in  FIG. 1 , an electric power fuse  10  according to an embodiment of the present invention includes a casing  12  made of resin and having a round tubular shape, a rectangular tubular shape, or the like, a first terminal  14   a  and a second terminal  14   b  made of metal and mounted respectively on both sides of the casing  12 , and an arc-extinguishing material  16  such as silica sand or the like and a fuse element  18 , which are housed in the casing  12 . 
         [0046]    As shown in  FIGS. 2 and 3 , the fuse element  18  includes a ceramic substrate  20  made of alumina or the like having a thickness of 1 mm, for example, and an electrically conductive film  22  disposed on the ceramic substrate  20 . More specifically, the fuse element  18  comprises the electrically conductive film  22  disposed on the ceramic substrate  20 , and which includes a plurality of heat radiation zones  24  and a plurality of current-interruption grids  26  that are provided integrally in succession. Among the heat radiation zones  24 , the heat radiation zones  24  that are positioned on both sides are electrically connected to corresponding terminals (the first terminal  14   a  and the second terminal  14   b  shown in  FIG. 1 ) by metal connecting plates  28  (see  FIG. 1 ). The heat radiation zones  24  that are positioned on both sides may also be referred to as a first terminal connector  24   a  and a second terminal connector  24   b . A direction from the first terminal connector  24   a  to the second terminal connector  24   b  (or a direction from the second terminal connector  24   b  to the first terminal connector  24   a ) is referred to as a lengthwise direction (x direction), whereas a direction perpendicular to the lengthwise direction on the electrically conductive film  22  is referred to as a widthwise direction (y direction). 
         [0047]    As shown in  FIG. 2 , each of the current-interruption grids  26  has a plurality of narrow cutoff canals  30  arrayed in parallel along the y direction. The current-interruption grids  26  also are arranged in series along the x direction, thereby providing the fuse element  18 . In  FIG. 2 , each of the current-interruption grids  26  has thirty-two narrow cutoff canals  30 , which are arrayed in parallel along the y direction, whereas the current-interruption grids  26  are arranged in series along the x direction, with each heat radiation zone  24  being sandwiched between two adjacent current-interruption grids  26 . The narrow cutoff canals  30 , particularly the side walls thereof as viewed in plan, are substantially straight in shape. 
         [0048]    As shown in  FIG. 2 , the electrically conductive film  22  comprises a printed layer  32 , which is arranged on the surface of the ceramic substrate  20  by one or more printing processes. The number of laminae of the printed layer  32  of the heat radiation zones  24  is equal to or greater than the number of laminae of the printed layer  32  of the current-interruption grids  26 . The printed layer  32  may be fabricated from an ink such as copper paste, silver paste, or the like, for example. In  FIG. 2 , the number of laminae of the printed layer  32  of the heat radiation zones  24  is 2, whereas the number of laminae of the printed layer  32  of the current-interruption grids  26  is 1. The numbers of the laminae may be in any combination, insofar as the number of laminae of the printed layer  32  of the heat radiation zones  24  is equal to or greater than the number of laminae of the printed layer  32  of the current-interruption grids  26 . A printed layer  32   a , which is provided as the first lamina, and a printed layer  32   b , which is provided as the second lamina, may have the same thickness or different thicknesses. In  FIG. 3 , the printed layer  32   a , which serves as the first lamina, is deposited to a thickness ranging from 20 to 30 μm, for example, on the ceramic substrate  20  by a first screen printing process, whereas the printed layer  32   b , which serves as the second lamina, is deposited to a thickness ranging from 75 to 100 μm, for example, on the printed layer  32   a  by a second screen printing process. When the printed layer  32   a  that serves as the first lamina is printed, the narrow cutoff canals  30  of the current-interruption grids  26  are produced simultaneously therewith. According to a conventional etching process, a plated layer is deposited to a thickness corresponding to the thickness of the current-interruption grids, and then is etched selectively in order to produce the current-interruption grids, after which an additional plated layer is deposited to produce the heat radiation zones while the current-interruption grids are in a masked state. The conventional etching process is complex and poor in accuracy, since different processes need to be repeated including the plating process and the etching process. 
         [0049]    According to the present embodiment, since the electrically conductive film  22 , which includes the heat radiation zones  24  and the current-interruption grids  26 , is formed on the ceramic substrate  20  by a screen printing process, the electrically conductive film  22  can be produced more easily than by the etching process described above. Further, since upper portions of the narrow cutoff canals  30  and the heat radiation zones  24  are not subject to corrosion, any variations in the pattern shape (thickness, etc.) between the current-interruption grids  26  or between the heat radiation zones  24 , and any variations in the pattern shape (thickness, etc.) between fuse elements  18  are minimized when the patterned electrically conductive film  22  is formed. Accordingly, a conductive pattern made up of the electrically conductive film  22  can be fabricated with high precision. 
         [0050]    More specifically, variations in the film thickness of the narrow cutoff canals  30  between the current-interruption grids  26  and variations between fuse elements  18  can be minimized, thereby minimizing variations in the I 2 t value. Moreover, since the heat radiation zones  24  and the current-interruption grids  26  are printed, they can be formed separately from each other, so that the thickness of the narrow cutoff canals  30  of the current-interruption grids  26  can be controlled as desired independently of the thickness of the heat radiation zones  24 . By controlling the thickness of the narrow cutoff canals  30  in this manner, a reduction in the I 2 t value can be achieved. Consequently, the electric power fuse  10  can be reduced in cost and size. 
         [0051]    According to a preferred feature of the fuse element  18 , an antioxidizing film of CuO or the like is disposed on surfaces of at least the current-interruption grids  26 . Preferably, a CuO paste or the like is deposited only on upper surfaces of the current-interruption grids  26 , for example, by a screen printing process to thereby form an antioxidizing film having a thickness of about several μm. The antioxidizing film, which is printed in this manner, is effective to prevent at least the current-interruption grids  26  from becoming oxidized, thereby enabling the fuse element  18  to operate reliably over a long period of time. 
         [0052]    According to another preferred feature of the fuse element  18 , the arc-extinguishing material  16  is made into a paste and is printed on the surface of the fuse element  18 . More specifically, as shown in  FIG. 4A , the arc-extinguishing material  16  is made into a paste (of SiO 2  or the like), i.e., an arc-extinguishing material paste  34 , with a solvent, and the arc-extinguishing material paste  34  is printed on the current-interruption grids  26 . Alternatively, as shown in  FIG. 4B , the arc-extinguishing material paste  34  may be printed respectively on the current-interruption grids  26  and the heat radiation zones  24 . Generally, the majority of the internal space of the electric power fuse  10  is filled with the arc-extinguishing material  16 . Since the region that actually is required to quench arcs in the electric power fuse  10  merely comprises a region that lies close to surfaces of the current-interruption grids  26 , the arc-extinguishing material paste  34  is printed on at least the current-interruption grids  26 . In this manner, the internal space in which the arc-extinguishing material  16  is accommodated can be reduced. Further, the printed arc-extinguishing material paste  34  is effective to significantly reduce the size of the electric power fuse  10 . 
         [0053]    Various modifications of the fuse element  18  will be described below with reference to  FIGS. 5 through 11B . 
         [0054]    As shown in  FIG. 5 , a fuse element according to a first modification (hereinafter referred to as a “first fuse element  18   a ”) includes a first fuse section  36 A and a second fuse section  36 B, which are disposed between the first terminal connector  24   a  and the second terminal connector  24   b , and are connected in succession (in series) with a central heat radiation zone  24   c  being interposed therebetween. 
         [0055]    As shown with partial omission in  FIG. 6A , the first fuse section  36 A includes a plurality of first current-interruption grids  26 A, each having thirty-two parallel narrow cutoff canals  30 , for example, arranged in series along the x direction. As shown with partial omission in  FIG. 6B , the second fuse section  36 B includes a plurality of second current-interruption grids  26 B, each having thirty-two parallel narrow cutoff canals  30 , for example, arranged in series along the x direction. The narrow cutoff canals  30  of the first current-interruption grids  26 A have a width (a length in the y direction) da, and the narrow cutoff canals  30  of the second current-interruption grids  26 B have a width (a length in the y direction) db, which differs from the width da. More specifically, as shown in  FIGS. 6A and 6B , the width db of the narrow cutoff canals  30  of the second current-interruption grids  26 B is greater than the width da of the narrow cutoff canals  30  of the first current-interruption grids  26 A. 
         [0056]    A fuse element according to a second modification (hereinafter referred to as a “second fuse element  18   b ”) essentially is the same in structure as the first fuse element  18   a  described above, but differs therefrom as described below. 
         [0057]    As shown in  FIGS. 7A and 7B , the number of laminae of the printed layer  32  of the first current-interruption grids  26 A and the number of laminae of the printed layer  32  of the second current-interruption grids  26 B differ from each other. In  FIGS. 7A and 7B , the number of laminae of the printed layer  32  of the first current-interruption grids  26 A is 1, whereas the number of laminae of the printed layer  32  of the second current-interruption grids  26 B is 2. 
         [0058]    A fuse element according to a third modification (hereinafter referred to as a “third fuse element  18   c ”) essentially is the same in structure as the first fuse element  18   a  described above, but differs therefrom as described below. 
         [0059]    As shown in  FIGS. 8A and 8B , the narrow cutoff canals  30  of the first current-interruption grids  26 A have a width da and an array pitch Pa, and the narrow cutoff canals  30  of the second current-interruption grids  26 B have a width db and an array pitch Pb. The respective widths and array pitches are related as follows: 
         [0000]      da=db 
         [0000]      Pa&lt;Pb 
         [0060]    A fuse element according to a fourth modification (hereinafter referred to as a “fourth fuse element  18   d ”) essentially is the same in structure as the first fuse element  18   a  described above, but differs therefrom as described below. 
         [0061]    As shown in  FIGS. 9A and 9B , the width db and the array pitch of the narrow cutoff canals  30  of the second current-interruption grids  26 B are greater than the width da and the array pitch of the narrow cutoff canals  30  of the first current-interruption grids  26 A. 
         [0062]    A fuse element according to a fifth modification (hereinafter referred to as a “fifth fuse element  18   e ”) essentially is the same in structure as the first fuse element  18   a  described above, but differs therefrom as described below. 
         [0063]    The narrow cutoff canals  30  of the first current-interruption grids  26 A and the narrow cutoff canals  30  of the second current-interruption grids  26 B differ in shape. In  FIGS. 10A and 10B , the side walls of the narrow cutoff canals  30  of the first current-interruption grids  26 A are substantially straight in shape as viewed in plan, whereas the side walls of the narrow cutoff canals  30  of the second current-interruption grids  26 B are of a curved shape. The width da (the length in the y direction) of the narrow cutoff canals  30  of the first current-interruption grids  26 A may be different from or identical to a smallest width db of the narrow cutoff canals  30  of the second current-interruption grids  26 B. 
         [0064]    A fuse element according to a sixth modification (hereinafter referred to as a “sixth fuse element  18   f ”) essentially is the same in structure as the first fuse element  18   a  described above, but differs therefrom as described below. 
         [0065]    As shown in  FIGS. 11A and 11B , the number of laminae of the printed layer  32  of the first current-interruption grids  26 A and the number of laminae of the printed layer  32  of the second current-interruption grids  26 B are the same as each other. On the other hand, the metal material of the printed layer  32  of the first current-interruption grids  26 A differs from the metal material of the printed layer  32  of the second current-interruption grids  26 B. For example, the first current-interruption grids  26 A have a printed layer  32  made of silver paste, whereas the second current-interruption grids  26 B have a printed layer  32  made of copper paste. Insofar as the metal material of the printed layer  32  of the first current-interruption grids  26 A and the metal material of the printed layer  32  of the second current-interruption grids  26 B differ from each other, metal materials having low melting points, which generally are used as fuses, may be used in combination. 
         [0066]    The first current-interruption grids  26 A and the second current-interruption grids  26 B of the first through sixth fuse elements  18   a  through  18   f  may be combined as desired to fabricate a new fuse element. 
         [0067]    With respect to the first through sixth fuse elements  18   a  through  18   f , the fusing characteristics (current vs. fusing time characteristics) of the first fuse section  36 A and the second fuse section  36 B may be changed. In particular, as shown in  FIG. 12 , according to current vs. fusing time characteristics from a first current value A1 to a second current value A2, in the sixth fuse element  18   f , the second fuse section  36 B exhibits a sharper change in fusing time with respect to current than the first fuse section  36 A. 
         [0068]    As a consequence, as indicated by the solid line in  FIG. 12 , the electric power fuse  10  exhibits, as an overall current vs. fusing time characteristic curve of the sixth fuse element  18   f , characteristics such that a change in the time with respect to current in a higher current range is sharper than a change in the time with respect to current in a lower current range. 
       EXAMPLES 
       [0069]    Operating characteristics (rated current vs. operating I 2 t value characteristics) associated with Comparative Examples 1 and 2 and Inventive Example 1 were confirmed.  FIG. 13  shows the operating characteristics (rated current vs. operating I 2 t value characteristics) of Inventive Example 1 together with those of Comparative Examples 1 and 2. In  FIG. 13 , the characteristic curve plotted with  pertains to Inventive Example 1, the characteristic curve plotted with ▴ pertains to Comparative Example 1, and the characteristic curve plotted with ◯ pertains to Comparative Example 2. 
         [0070]    The characteristic curve of Comparative Example 1 shown in  FIG. 13  is plotted based on data of a commercially available product, which was etched to produce a pattern equivalent to the pattern shown in FIG. 3 of Japanese Laid-Open Patent Publication No. 2006-073331. 
         [0071]    The characteristic curve of Comparative Example 2 shown in  FIG. 13  is plotted based on data of a commercially available product, which was fabricated by pressing a silver ribbon. 
         [0072]    The characteristic curve of Inventive Example 1 is plotted based on data of an electric power fuse, which is similar in structure to the electric power fuse  10  according to the present embodiment. The fuse element  18  was fabricated in the following manner. First, as shown in  FIG. 3 , an alumina substrate having a thickness of 1 mm was used as the ceramic substrate  20 , and a printed layer  32   a  (printed layer of copper paste) having a thickness of 25 μm was formed as the first lamina on the alumina substrate by a screen printing process. At this time, the printed layer  32   a  was printed in the pattern shown in  FIG. 2 . Thereafter, another printed layer  32   b  (printed layer of copper paste) having a thickness of 75 μm was formed as the second lamina on the printed layer  32   a  by a second screen printing process. At this time, the printed layer  32   b  was printed only in areas that were intended to become the respective heat radiation zones  24 . 
         [0073]    As can be understood from the results shown in  FIG. 13 , Inventive Example 1 exhibits better operating characteristics than Comparative Examples 1 and 2. More specifically, the electric power fuse according to Inventive Example 1 is capable of reducing the I 2 t value, is both low in cost and small in size, and at the same time, is capable of minimizing variations between the current-interruption grids  26  and variations between the fuse elements  18 . 
         [0074]    The electric power fuse according to the present invention is not limited to the above embodiment, but may incorporate various additional or alternative arrangements without departing from the scope of the invention.