Patent Publication Number: US-9852868-B2

Title: Chip fuse and manufacturing method therefor

Description:
TECHNICAL FIELD 
     The present invention relates to a chip fuse and a manufacturing method for the same. 
     BACKGROUND ART 
     Heretofore, a chip fuse which is a miniature fuse has been known as one component surface-mounted on a printed circuit board of an electronic device. Such a chip fuse prevents overcurrent damage to an electronic circuit on the printed circuit board. 
       FIG. 12  shows a cross-sectional view of a conventional chip fuse  1 . As shown in this figure, a heat-storing layer (adhesion layer)  3  made of an epoxy-based resin is formed on a surface  2   a  of an insulated substrate  2  that is an alumina substrate. A fuse film  4  made of copper is formed on the heat-storing layer  3 . Specifically, interposing the heat-storing layer  3  between the insulated substrate  2  and the fuse film  4  prevents the fuse film  4  from coming into contact with the insulated substrate  2 . Hence, when a current passes through the chip fuse  1 , heat generated at a fuse element section  4   b  is stored in the heat-storing layer  3  without dissipating to the insulated substrate  2 . 
     The fuse film  4  includes surface electrode sections  4   a  on both ends in a length direction of the chip fuse  1  (a right-left direction in  FIG. 12 : hereinafter, this is simply referred to as chip fuse length direction), and the fuse element section (fuse element)  4   b  between the surface electrode sections  4   a . The fuse element section  4   b  is a melting section configured to be melted by heat generated at the fuse element section  4   b  when an overcurrent flows through the chip fuse  1 . The section has a width narrower than the surface electrode sections  4   a . The fuse element section  4   b  is provided with a plating film  5  for preventing diffusion, and a plating film  6  for facilitating the melting. The plating film  5  is a nickel film formed on the copper fuse film  4  by an electroplating method. The plating film  6  is a tin film formed on the nickel film  5  by an electroplating method. 
     Moreover, a first protective layer  7  made of an epoxy-based resin is formed on the fuse element section  4   b  (tin film  6 ), the first protective layer  7  serving as an undercoat. Further, a second protective layer  8  made of an epoxy-based resin is formed on the first protective layer  7 , the second protective layer  8  serving as a first overcoat. A third protective layer  9  made of an epoxy-based resin is formed on the second protective layer  8 , the third protective layer  9  serving as a second overcoat. A mark  10  is formed on a surface  9   a  of the third protective layer  9  by laser marking. The mark  10  indicates the rated current and the like of the chip fuse  1 . 
     Backside electrodes  11  made of a silver-containing resin are formed on sections  2   b - 1  on both ends in the chip fuse length direction of a back surface  2   b  of the insulated substrate  2 . End surface electrodes  12  made of a silver-containing resin are formed on end surfaces  2   c  on both ends in the chip fuse length direction of the insulated substrate  2 . Each of the end surface electrodes  12  is formed across from the surface electrode section  4   a  to the backside electrode  11 , electrically connecting the surface electrode section  4   a  and the backside electrode  11  to each other. 
     Moreover, the end surface electrode  12  is provided with plating films  13 ,  14 ,  15 . The plating film  13  is a copper film formed on the end surface electrode  12  by an electroplating method. The plating film  14  is a nickel film formed on the copper film  13  by an electroplating method. The plating film  15  is a tin film formed on the nickel film  14  by an electroplating method. These plating films  13 ,  14 ,  15  are formed across from the surface electrode section  4   a  to the back surface  2   b  of the insulated substrate  2 , covering the end surface electrode  12  and the backside electrode  11  entirely. 
     Incidentally, the following Patent Documents 1 to 3 are examples of prior art documents disclosing chip fuses. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent Application Publication No. Hei 10-308160 
         Patent Document 2: Japanese Patent Application Publication No. Hei 10-308161 
         Patent Document 3: Japanese Patent Application Publication No. Sho 63-141233 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Recent demands for further reducing the size of electronic devices and for improving the reliability, and other demands have made demands for further improvements in interruption performances of chip fuses. The interruption performances of a chip fuse include a change in appearance before and after an interruption, a sustained arc during an interruption, and so forth. A chip fuse having high interruption performances is capable of retaining an appearance as before an interruption while inhibiting a matter from scattering even after the interruption, and has a short sustained arc period during the interruption. 
     In order to check such interruption performances, Interruption Tests A, B have been conducted on the above-described conventional chip fuse  1  under various test conditions as follows. 
     Interruption Test A is an interruption test with 32 V and 50 A. The chip fuse  1  subjected to this Interruption Test A had a resistance value of 0.029Ω before the interruption test. Although the illustration is omitted, the interruption period was 0.38 ms as a result of conducting Interruption Test A. Moreover, a sustained arc was observed only to a lesser degree. In addition, portions of the protective layers  7 ,  8 ,  9  appeared to be damaged and scattered by the impact (pressure) in the melting of the fuse element section  4   b , while a melt  4   b - 1  of the fuse element section  4   b  attached around the damaged portions of the protective layers  7 ,  8 ,  9 . Since the protective layers  7 ,  8 ,  9  formed of the epoxy-based resin are relatively hard, the layers are likely to be damaged by the impact. 
     Interruption Test B is an interruption test with 76 V and 50 A. The chip fuse  1  subjected to this Interruption Test B had a resistance value of 0.029Ω before the interruption test. As a result of conducting Interruption Test B, the interruption period was 0.55 ms, and a sustained arc was observed as long as approximately 0.2 ms, as shown in  FIG. 10( a ) . Moreover, as shown in  FIG. 11 , portions of the protective layers  7 ,  8 ,  9  appeared to be damaged and scattered by the impact (pressure) in the melting of the fuse element section  4   b , while a melt  4   b - 1  of the fuse element section  4   b  attached around damaged portions  16  of the protective layers  7 ,  8 ,  9 . 
     Therefore, in view of the above-described circumstances, an object of the present invention is to provide a chip fuse and a manufacturing method for the same, the chip fuse being capable of improving interruption performances such as retention of appearance and reduction of sustained arc. 
     Means for Solving the Problems 
     A chip fuse according to a first aspect of the invention for attaining the above object is a chip fuse comprising: 
     an insulated substrate; 
     a heat-storing layer formed on the insulated substrate: 
     a fuse film formed on the heat-storing layer, the fuse film comprising surface electrode sections on both ends in a chip fuse length direction and a fuse element section between the surface electrode sections; and 
     a protective layer formed on the fuse element section, characterized in that 
     a rectangular bank section is formed on the heat-storing layer and the surface electrode sections in such a manner as to surround the fuse element section, and 
     the protective layer is formed on an inner side of the bank section. 
     Further, a chip fuse according to a second aspect of the invention is the chip fuse according to the first aspect of the invention, characterized in that the bank section includes sections on both ends in the chip fuse length direction, the sections being formed outwardly in the chip fuse length direction of ends at inner sides in the chip fuse length direction of the surface electrode sections. 
     Further, a chip fuse according to a third aspect of the invention is the chip fuse according to any one of the first and the second aspects of the invention, characterized in that 
     the surface electrode sections each include a first electrode section at an outer side in the chip fuse length direction, and a second electrode section at an inner side in the chip fuse length direction, 
     the second electrode section has a width narrower than a width of the first electrode section, 
     the bank section includes sections on both ends in a chip fuse width direction, the sections being provided on the heat-storing layer and formed outwardly in the chip fuse width direction of ends on both ends in the chip fuse width direction of the second electrode section, and 
     the heat-storing layer and the bank section are formed of the same material. 
     Further, a chip fuse according to a fourth aspect of the invention is the chip fuse according to the third aspect of the invention, characterized in that the heat-storing layer and the bank section are formed of the same photosensitive-group-containing material. 
     Further, a chip fuse according to a fifth aspect of the invention is the chip fuse according to any one of the first to the fourth aspects of the invention, characterized in that the protective layer is formed of an epoxy-group-containing silicone-based resin. 
     Further, a chip fuse according to a sixth aspect of the invention is the chip fuse according to the fifth aspect of the invention, characterized in that another protective layer made of an inorganic-filler-containing silicone-based resin is formed on the protective layer. 
     Further, a chip fuse according to a seventh aspect of the invention is the chip fuse according to the sixth aspect of the invention, characterized in that the other protective layer is formed to have a thickness smaller than the protective layer. 
     Further, a chip fuse according to an eighth aspect of the invention is the chip fuse according to any one of the sixth and the seventh aspects of the invention, characterized in that 
     the other protective layer is transparent, and 
     a mark made of a silicone-based resin is formed on the protective layer and provided between the protective layer and the other protective layer. 
     Further, a manufacturing method for a chip fuse according to a ninth aspect of the invention is a manufacturing method for the chip fuse according to any one of the first to the eighth aspects of the invention, characterized in that the manufacturing method comprises: 
     a first step of forming the rectangular bank section on the heat-storing layer and the surface electrode sections; and 
     a second step of forming the protective layer on the inner side of the bank section. 
     Further, a manufacturing method for a chip fuse according to a tenth aspect of the invention is the manufacturing method for a chip fuse according to the ninth aspect of the invention, characterized in that 
     the first step includes:
         laminating a sheet-shaped photosensitive-group-containing material on the fuse element section, the surface electrode sections, and the heat-storing layer; and   exposing the sheet-shaped photosensitive-group-containing material to ultraviolet light for development (photo-etching) to thereby form the rectangular bank section.       

     Effects of the Invention 
     In the chip fuse according to the first aspect of the invention, the heat-storing layer is formed on the insulated substrate; the fuse film is formed on the heat-storing layer, the fuse film including the surface electrode sections on both ends in the chip fuse length direction and the fuse element section between the surface electrode sections; and the protective layer is formed on the fuse element section. The chip fuse is characterized in that: the rectangular bank section is formed on the heat-storing layer and the surface electrode sections in such a manner as to surround the fuse element section; and the protective layer is formed on the inner side of the bank section. Accordingly, when the protective layer is formed, the rectangular bank section makes it possible to block the material for forming the protective layer (for example, epoxy-group-containing silicone-based resin) from flowing and spreading therearound. Thus, the protective layer is sufficiently ensured. Further, the thickness of the protective layer even at end sections thereof is not reduced, and a sufficiently large thickness is ensured. Hence, it is possible to prevent the protective layer from being damaged by an impact (pressure) in the melting of the fuse element section. 
     According to the chip fuse according to the second aspect of the invention, the chip fuse according to the first aspect of the invention is characterized in that the sections on both ends in the chip fuse length direction of the bank section are formed outwardly in the chip fuse length direction of the ends at the inner sides in the chip fuse length direction of the surface electrode sections. Accordingly, the sections on both ends in the chip fuse length direction of the bank section do not cover end sections of the fuse element section. Hence, the bank section will not be damaged by the impact (pressure) in the melting of the fuse element section. 
     According to the chip fuse according to the third aspect of the invention, the chip fuse according to any one of the first and the second aspects of the invention is characterized in that: the surface electrode sections each include the first electrode section at the outer side in the chip fuse length direction, and the second electrode section at the inner side in the chip fuse length direction; the width of the second electrode section is narrower than the width of the first electrode section; the sections on both ends in the chip fuse width direction of the bank section are provided on the heat-storing layer and formed outwardly in the chip fuse width direction of the ends on both ends in the chip fuse width direction of the second electrode section; and the heat-storing layer and the bank section are formed of the same material. Accordingly, the sections on both ends in the chip fuse width direction of the bank section are provided on the heat-storing layer and adhere to the heat-storing layer as a whole. This increases the adhesion of the bank section and surely prevents the detachment. 
     According to the chip fuse according to the fourth aspect of the invention, the chip fuse according to the third aspect of the invention is characterized in that the heat-storing layer and the bank section are formed of the same photosensitive-group-containing material. Accordingly, the bank section formed of a photosensitive-group-containing material surely adheres to the heat-storing layer formed of the same photosensitive-group-containing material. 
     According to the chip fuse according to the fifth aspect of the invention, the chip fuse according to any one of the first to the fourth aspects of the invention is characterized in that the protective layer is formed of an epoxy-group-containing silicone-based resin. The protective layer formed of an epoxy-group-containing silicone-based resin is soft and elastic in comparison with a conventional protective layer formed of an epoxy-based resin. Accordingly, the protective layer is capable of absorbing an impact (pressure) in the melting of the fuse element section and thus is not likely to be damaged by the impact. 
     According to the chip fuse according to the sixth aspect of the invention, the chip fuse according to the fifth aspect of the invention is characterized in that another protective layer made of an inorganic-filler-containing silicone-based resin is formed on the protective layer. The other protective layer formed of an inorganic-filler-containing silicone-based resin is hard and excellent in friction resistance and blocking resistance in comparison with the protective layer formed of of an epoxy-group-containing silicone-based resin, and is hardly caught by manufacturing equipment and hardly detached. Hence, the productivity of the chip fuse is improved. Moreover, the other protective layer formed of the silicone-based resin closely adheres to the protective layer formed of a similar silicone-based resin, so that the other protective layer is less likely to be detached therefrom. Furthermore, since the other protective layer is formed of such an inorganic-filler-containing silicone-based resin, this makes it possible to improve the strength as a product. 
     According to the chip fuse according to the seventh aspect of the invention, the chip fuse according to the sixth aspect of the invention is characterized in that the other protective layer is formed to have a thickness smaller than the protective layer. The other protective layer is not only hard as being formed of an inorganic-filler-containing silicone-based resin and also has a thickness smaller than the protective layer. Accordingly, the elasticity of the protective layer is ensured. Hence, an impact (pressure) in the melting of the fuse element section is absorbed, making it possible to prevent damage by the impact, as well. 
     According to the chip fuse according to the eighth aspect of the invention, the chip fuse according to any one of the sixth and the seventh aspects of the invention is characterized in that: the other protective layer is transparent; and the mark made of a silicone-based resin is formed on the protective layer and provided between the protective layer and the other protective layer. Since the protective layer, the mark, and the other protective layer are formed of a silicone-based resin as a whole, they closely adhere to and are hardly detached from one another, and the absorbability of an impact (pressure) in the melting of the fuse element section is so high that the damage is little. This makes it possible to retain the mark and the protective layers. 
     According to the manufacturing method for a chip fuse according to the ninth aspect of the invention, the manufacturing method for the chip fuse according to any one of the first to the eighth aspects of the invention is characterized in that the manufacturing method includes: the first step of forming the rectangular bank section on the heat-storing layer and the surface electrode sections; and the second step of forming the protective layer on the inner side of the bank section. Accordingly, when the protective layer is formed in the second step, the rectangular bank section formed in the first step makes it possible to block the material for forming the protective layer (for example, epoxy-group-containing silicone-based resin) from flowing and spreading therearound. Thus, the thickness of the protective layer even at the end sections is not reduced, and a sufficiently large thickness is ensured. Hence, it is possible to prevent the protective layer from being damaged by the impact (pressure) in the melting of the fuse element section. 
     According to the manufacturing method for a chip fuse according to the tenth aspect of the invention, the manufacturing method for a chip fuse according to the ninth aspect of the invention is characterized in that the first step includes: laminating the sheet-shaped photosensitive-group-containing material on the fuse element section, the surface electrode sections, and the heat-storing layer; and exposing the sheet-shaped photosensitive-group-containing material to ultraviolet light for development (photo-etching) to thereby form the rectangular bank section. Accordingly, the bank section has a uniform thickness in comparison with a case where the bank section is formed by screen printing or the like, and an inner side surface that serves as a surface configured to block the flow of the material for forming the protective layer is perpendicular to a surface of the insulated substrate. Thus, it is possible to more surely ensure the thicknesses of the end sections of the protective layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a chip fuse according to an embodiment of the present invention (the cross section taken along the line B-B indicated by arrows in  FIG. 2 ). 
         FIG. 2  is a top view of the chip fuse according to the embodiment of the present invention (the view seen in the direction of the arrow A in  FIG. 1 ). 
         FIG. 3  is a top view of the chip fuse according to the embodiment of the present invention, the view showing a state where a first protective layer, a second protective layer, a mark, end surface electrodes, and copper films, nickel films, and tin films on the end surface electrodes are omitted. 
         FIGS. 4( a ) to ( d )  are views for illustrating an insulated substrate scribing step, a heat-storing layer forming step, and a fuse film forming step in a manufacturing process for a chip fuse according to an embodiment of the present invention. 
         FIGS. 5( a ) to ( d )  are views for illustrating the fuse film forming step in the manufacturing process for a chip fuse according to the embodiment of the present invention. 
         FIGS. 6( a ) to ( c )  are views for illustrating a fuse element section forming step in the manufacturing process for a chip fuse according to the embodiment of the present invention. 
         FIGS. 7( a ) and ( b )  are views for illustrating a bank section forming step in the manufacturing process for a chip fuse according to the embodiment of the present invention. 
         FIGS. 8( a ) to ( d )  are views for illustrating a first protective layer forming step, a mark forming step, a second protective layer forming step, and other steps in the manufacturing process for a chip fuse according to the embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of the chip fuse in a case where no bank section is formed. 
         FIG. 10( a )  is a graph showing the interruption period (including the sustained arc period) of a conventional chip fuse in conducting Interruption Test B on the chip fuse, and  FIG. 10( b )  is a graph showing the interruption period (no sustained arc was observed) of the chip fuse of the present invention in conducting Interruption Test C on the chip fuse. 
         FIG. 11  is a drawing showing an appearance of the conventional chip fuse in conducting Interruption Test B on the chip fuse. 
         FIG. 12  is a cross-sectional view of the conventional chip fuse. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail on the basis of the drawings. 
     First of all, a structure of a chip fuse  21  according to an embodiment of the present invention will be described based on  FIGS. 1 to 3 . 
     Note that  FIG. 3  shows a state where a first protective layer  28 , a second protective layer  29 , a mark  30 , end surface electrodes  32 , and copper films  33 , nickel films  34 , and tin films  35  on the end surface electrodes  32 , which are shown in  FIG. 1 , are omitted. Moreover,  FIG. 3  shows that portions of a nickel film  25  and a tin film  26  on a fuse element section (fuse element)  24   b  and a surface electrode section  24   a  (second electrode section  24   a - 2 ) are cut away.  FIG. 4( d )  shows that a portion of a copper foil  52  is cut away.  FIG. 5( a )  shows that portions of the copper foil  52  and a photosensitive film  53  are cut away. 
     As shown in  FIGS. 1 to 3 , a heat-storing layer (adhesion layer)  23  made of a photosensitive-group-containing epoxy-based resin is formed on a surface  22   a  of an insulated substrate  22  that is an alumina substrate. A fuse film  24  made of copper is formed on the heat-storing layer  23 . Specifically, interposing the heat-storing layer  23  between the insulated substrate  22  and the fuse film  24  prevents the fuse film  24  from coming into contact with the insulated substrate  22 . Hence, when a current passes through the chip fuse  21 , heat generated at a fuse element section  24   b  is stored in the heat-storing layer  23  without dissipating to the insulated substrate  22 . 
     The fuse film  24  includes surface electrode sections  24   a  on both ends in a length direction of the chip fuse  21  (a right-left direction in  FIGS. 1 to 3 : hereinafter, this is simply referred to as chip fuse length direction), and the fuse element section  24   b  between the surface electrode sections  24   a . The fuse element section  24   b  is a melting section configured to be melted by heat generated at the fuse element section  24   b  when an overcurrent flows through the chip fuse  21 . The section has a width, that is, a width in a width direction of the chip fuse  21  (an up-down direction in  FIGS. 2, 3 : hereinafter, this is simply referred to as chip fuse width direction), narrower than the surface electrode sections  24   a . Note that the fuse element section  24   b , in the illustrated example, has a shape extending straight in the chip fuse length direction, but is not limited to this. The fuse element section  24   b  can be formed into an appropriate shape (for example, zigzag shape or the like) according to desired melting properties and so forth. 
     Moreover, the fuse element section  24   b  is provided with a plating film  25  for preventing diffusion and a plating film  26  for facilitating the melting. The plating film  25  is a nickel film formed on the copper fuse film  24  by an electroplating method. The plating film  26  is a tin film formed on the nickel film  25  by an electroplating method. 
     Each of the surface electrode sections  24   a  includes a first electrode section  24   a - 1  at an outer side in the chip fuse length direction, and a second electrode section  24   a - 2  at an inner side in the chip fuse length direction. The second electrode section  24   a - 2  has a width (width in the chip fuse width direction) W 2  ( FIG. 3 ) narrower than a width (width in the chip fuse width direction) W 1  ( FIG. 3 ) of the first electrode section  24   a - 1 . Note that the plated nickel film  25  and tin film  26  are provided not only to the fuse element section  24   b  but also to the second electrode section  24   a - 2  of the surface electrode section  24   a  so as to adjust a variation in resistance value. Since the width W 2  of the second electrode section  24   a - 2  is narrower than the width W 1  of the first electrode section  24   a - 1 , it is possible to adjust variations in thicknesses of the nickel film  25  and the tin film  26  after the plating, and a variation in resistance value. 
     Further, in the chip fuse  21  of the present embodiment, a bank section (dam)  27  made of a photosensitive-group-containing epoxy-based resin is formed. The bank section  27  has a rectangular shape (i.e., rectangular when seen from the above as shown in  FIGS. 2 and 3 ), and is formed on the heat-storing layer  23  and the surface electrode sections  24   a  in such a manner as to surround the fuse element section  24   b.    
     To be more specific, the rectangular bank section  27  includes sections  27   a  on both ends in the chip fuse length direction, and sections  27   b  on both ends in the chip fuse width direction. 
     The sections  27   b  on both ends in the chip fuse width direction extend straight in the chip fuse length direction, and are formed outwardly in the chip fuse width direction of ends  24   a - 3  on both ends in the chip fuse width direction of the surface electrode sections  24   a  (the second electrode sections  24   a - 2 ). Accordingly, the sections  27   b  on both ends in the chip fuse width direction each have a central section  27   b - 1  in the chip fuse length direction and end sections  27   b - 2  on both ends in the chip fuse length direction in such a manner that not only is the central section  27   b - 1  formed on the heat-storing layer  23 , but the end sections  27   b - 2  are also formed on the heat-storing layer  23 . Thus, the sections  27   b  adhere to the heat-storing layer  23  as a whole. 
     The sections  27   a  on both ends in the chip fuse length direction extend straight in the chip fuse width direction, have inner edges  27   a - 1  curved on both ends in the chip fuse width direction, and are formed on the surface electrode sections  24   a . Moreover, the sections  27   a  on both ends in the chip fuse length direction are formed outwardly in the chip fuse length direction of ends  24   a - 4  at inner sides in the chip fuse length direction of the surface electrode sections  24   a  (the second electrode section  24   a - 2 ) (i.e., the ends  24   a - 4  are at boundary positions between the surface electrode sections  24   a  and the fuse element section  24   b ). It should be noted that, in the illustrated example, the sections  27   a  on both ends in the chip fuse length direction are formed on both of the first electrode section  24   a - 1  and the second electrode section  24   a - 2  of the surface electrode sections  24   a.    
     A black first protective layer  28  made of an epoxy-group-containing silicone-based resin is formed on an inner side of the rectangular bank section  27 , the first protective layer  28  serving as an undercoat. In other words, the first protective layer  28  is formed along an inner side surface  27   c  of the rectangular bank section  27  ( FIG. 1 ). The first protective layer  28  is formed on the fuse element section  24   b  (the tin film  26 ) and also formed on the surface electrode sections  24   a  (the second electrode section  24   a - 2 ) and the heat-storing layer  23 , covering the entire fuse element section  24   b  (the tin film  26 ), portions of the surface electrode sections  24   a  (the second electrode sections  24   a - 2 ), and a portion of the heat-storing layer  23 . The first protective layer  28  formed of the epoxy-group-containing silicone-based resin is soft and elastic in comparison with a conventional protective layer formed of an epoxy-based resin. 
     On the first protective layer  28 , a second protective layer  29  is formed which is made of a silicone-based resin containing an inorganic filler (for example, containing a silica powder and an alumina powder) and serves as a transparent overcoat. The second protective layer  29  formed of the inorganic-filler-containing silicone-based resin is hard in comparison with the first protective layer  28  formed of the epoxy-group-containing silicone-based resin. Hence, the second protective layer  29  is made to have a thickness smaller than the first protective layer  28  to thereby have an elasticity. 
     Further, using a silicone-based resin, a milky-white mark  30  is formed between the first protective layer  28  and the second protective layer  29 . Specifically, after the mark  30  is formed on the first protective layer  28 , the second protective layer  29  is formed on the first protective layer  28  in such a manner as to cover the mark  30 . Since the second protective layer  29  is transparent, the mark  30  is seen through the second protective layer  29  from the above. The mark  30  indicates the rated current and the like of the chip fuse  21 . 
     Backside electrodes  31  made of a silver-containing resin are formed on sections  22   b - 1  on both ends in the chip fuse length direction of a back surface  22   b  of the insulated substrate  22 . 
     End surface electrodes  32  made of a silver-containing resin are formed on end surfaces  22   c  on both ends in the chip fuse length direction of the insulated substrate  22 . Each of the end surface electrodes  32  is formed across from the surface electrode section  24   a  to the backside electrode  31 , electrically connecting the surface electrode section  24   a  to the backside electrode  31 . 
     In addition, the end surface electrode  32  is provided with the plating films  33 ,  34 ,  35 . The plating film  33  is a copper film formed on the end surface electrode  32  by an electroplating method. The plating film  34  is a nickel film formed on the copper film  33  by an electroplating method. The plating film  35  is a tin film formed on the nickel film  34  by an electroplating method. These plating films  33 ,  34 ,  35  are formed across from the bank section  27  to the back surface  22   b  of the insulated substrate  22 , covering the end surface electrode  32  and the backside electrode  31  entirely. 
     Next, a manufacturing process for the chip fuse  21  according to an embodiment of the present invention will be described based on  FIGS. 1 to 9 . 
     First of all, in an insulated substrate scribing step, multiple parallel first slits  41  and multiple parallel second slits  42  are formed in a surface  22   a  of a sheet-shaped insulated substrate (alumina substrate)  22  by a laser scribing method in such a manner that the first slits  41  and the second slits  42  are orthogonal to each other as shown in  FIG. 4( a ) . As a result, multiple individual regions  43  are aligned consecutively in lengthwise and widthwise directions, one individual region  43  corresponding to one chip fuse  21 . The first slits  41  and the second slits  42  cut the sheet-shaped insulated substrate  22  into strip shapes and further divide the insulated substrate  22  into the individual regions  43 . 
     Subsequently, steps are performed on the multiple individual regions  43  until the insulated substrate  22  is divided by the first slits  41  and the second slits  42 .  FIGS. 4( b ) to ( d ) ,  FIGS. 5( a ) to ( d ) ,  FIGS. 6( a ) to ( d ) ,  FIGS. 7( a ) and ( b ) , and  FIGS. 8( a ) to ( d )  show only a portion corresponding to one individual region  43 . 
     In the subsequent heat-storing layer forming step, first, a sheet-shaped photosensitive-group-containing material  51  in a B stage state is laminated on the insulated substrate  22  as shown in  FIG. 4( b ) . The material is for forming a heat-storing layer  23 . 
     Note that the method for laminating the sheet-shaped photosensitive-group-containing material  51  includes: a method in which a sheet-shaped photosensitive-group-containing material  51  formed to a size corresponding to that of one or multiple insulated substrates  22  in advance is laminated on the one or multiple insulated substrate  22 ; a method in which a large sheet-shaped photosensitive-group-containing material  51  is cut into a size corresponding to that of one or multiple insulated substrates  22  and laminated on the one or multiple insulated substrates  22 ; a method in which a photosensitive-group-containing material  51  wound into the shape of a roll is pulled out, formed into the shape of a sheet, and laminated on one or multiple insulated substrates  22 ; and the like. 
     Then, the sheet-shaped photosensitive-group-containing material  51  laminated on the insulated substrate  22  is exposed to ultraviolet light (UV) through a mask (the illustration is omitted) for development (photo-etching) to thereby form a heat-storing layer  23  with a pattern as shown in  FIG. 4( c ) . 
     Here, as the sheet-shaped photosensitive-group-containing material  51  for forming the heat-storing layer  23 , used is a photosensitive-group-containing epoxy-based resin formed in the shape of a sheet or roll. Note that, other than this, polyimides, silicone-based resins, polyesters, acrylic polymers, and the like, which contain a photosensitive group, and which are formed in the shape of a sheet or roll, can also be used as the sheet-shaped photosensitive-group-containing material  51  for forming the heat-storing layer  23 . 
     In the subsequent fuse film forming step, first, a copper foil  52  is, laminated on the heat-storing layer  23  as shown in  FIG. 4( d ) . The copper foil  52  is a material for forming a fuse film  24 . 
     Then, as shown in  FIG. 5( a ) , a photosensitive film (resist)  53  to serve as a mask is laminated on the copper foil  52 . The photosensitive film  53  is exposed to ultraviolet light for development (photo-etching) to thereby create a pattern as shown in  FIG. 5( b ) . 
     Subsequently, as shown in  FIG. 5( c ) , the copper foil  52  is etched (patterned), and thereafter the photosensitive film  53  is detached. Thus, a fuse film  24  having a pattern as shown in  FIG. 5( d )  is formed. Specifically, the fuse film  24  has a structure having, as described above: surface electrode sections  24   a , on both ends, each including a first electrode section  24   a - 1  having a wide width and a second electrode section  24   a - 2  having a narrow width; and a fuse element section  24   b  between the surface electrode sections  24   a.    
     In the subsequent fuse element section forming step, as shown in  FIG. 6( a ) , a resist  54  to serve as a mask is screen printed on the first electrode sections  24   a - 1  of the surface electrode sections  24   a.    
     In this state, nickel plating and tin plating are sequentially performed by an electroplating method. Thereby, as shown in  FIG. 6( b ) , plating films, a nickel film  25  and a tin film  26 , are formed on the entire fuse element section  24   b  and the second electrode sections  24   a - 2  of the surface electrode sections  24   a , on which the resist  54  is not formed. 
     Then, as shown in  FIG. 6( c ) , the resist  54  is detached to expose the first electrode sections  24   a - 1  of the surface electrode sections  24   a , on which the plating films  25 ,  26  are not formed. 
     Thereafter, in the subsequent bank section forming step, first, a photosensitive-group-containing material  55  in a B stage state formed in the shape of a sheet is laminated on the fuse element section  24   b , the surface electrode sections  24   a , and the heat-storing layer  23  as shown in  FIG. 7( a ) . The material is for forming a bank section  27 . 
     Note that the method for laminating the sheet-shaped photosensitive-group-containing material  55  includes: a method in which a sheet-shaped photosensitive-group-containing material  55  formed to a size corresponding to that of one or multiple insulated substrates  22  in advance is laminated on the fuse element section  24   b , the surface electrode sections  24   a , and the heat-storing layer  23  on the one or multiple insulated substrates  22 ; a method in which a large sheet-shaped photosensitive-group-containing material  55  is cut into a size corresponding to that of one or multiple insulated substrates  22  and laminated on the fuse element section  24   b , the surface electrode sections  24   a , and the heat-storing layer  23  on the one or multiple insulated substrates  22 ; a method in which a photosensitive-group-containing material  55  wound into the shape of a roll is pulled out, formed into the shape of a sheet, and laminated on the fuse element section  24   b , the surface electrode sections  24   a , and the heat-storing layer  23  on one or multiple insulated substrates  22 ; and the like. 
     Then, the sheet-shaped photosensitive-group-containing material  55  laminated on the fuse element section  24   b , the surface electrode sections  24   a , and the heat-storing layer  23  is exposed to ultraviolet light (UV) through a mask (the illustration is omitted) for development (photo-etching) to thereby form a bank section  27  with a pattern as shown in  FIG. 7( b ) . Specifically, the bank section  27  formed has a rectangular structure including sections  27   a  on both ends in the chip fuse length direction and sections  27   b  on both ends in the chip fuse width direction as described above in such a manner that the sections  27   b  on both ends in the chip fuse width direction are formed on the heat-storing layer  23 , and the sections  27   a  on both ends in the chip fuse length direction are formed on the surface electrode sections  24   a.    
     Here, as the sheet-shaped photosensitive-group-containing material  55  for forming the bank section  27 , used is a photosensitive-group-containing epoxy-based resin formed in the shape of a sheet or roll. Note that, other than this, polyimides, silicone-based resins, polyesters, acrylic polymers, and the like, which contain a photosensitive group, and which are formed in the shape of a sheet or roll, can also be used as the sheet-shaped photosensitive-group-containing material  55  for forming the bank section  27 . 
     The bank section  27  formed of the photosensitive-group-containing material  55  is cured by irradiation with ultraviolet light. In this event, the bank section  27  contracts and the thickness thereof is reduced. For this reason, in the present embodiment, multiple sheet-shaped photosensitive-group-containing materials  55  each having a thickness of 20 to 60 μm are laminated on each other and then exposed to ultraviolet light for development (photo-etching) to form the bank section  27 , and the bank section  27  is cured by irradiation with ultraviolet light. This ensures that the bank section  27  has a thickness of 5 to 100 μm in the product. 
     In the subsequent first protective layer forming step, using an epoxy-group-containing silicone-based resin, a black first protective layer  28  is formed on an inner side of the rectangular bank section  27  by screen printing as shown in  FIG. 8( a ) . 
     Since the epoxy-group-containing silicone-based resin has a high flowability, if the bank section  27  is not formed, the silicone-based resin flows and spreads therearound after the screen printing. Consequently, the thicknesses of end sections  28   a  of the first protective layer  28  are reduced as shown in  FIG. 9 . 
     In contrast, in the present embodiment, the bank section  27  is formed. The bank section  27  (inner side surface  27   c ) makes it possible to block the silicone-based resin from flowing and spreading therearound after the screen printing. Consequently, as shown in  FIG. 1 , the thickness of the first protective layer  28  even at end sections  28   a  is not reduced, and a sufficiently large thickness is ensured. 
     The first protective layer  28  formed of the epoxy-group-containing silicone-based resin is soft and elastic in comparison with a conventional protective layer formed of an epoxy-based resin. Accordingly, the first protective layer  28  is capable of absorbing an impact (pressure) in the melting of the fuse element section  24   b  and thus is not likely to be damaged by the impact. 
     Moreover, since the bank section  27  ensures the thicknesses of the end sections  28   a  of the first protective layer  28  formed of the epoxy-group-containing silicone-based resin, the end sections  28   a  will not be damaged by the impact, either. 
     In addition, the first protective layer  28  formed of the epoxy-group-containing silicone-based resin is viscous in comparison with a protective layer formed of a silicone-based resin containing no epoxy group. This viscosity makes perforation by the impact less likely. 
     In the subsequent mark forming step, using a silicone-based resin, a milky-white mark  30  is formed on the first protective layer  28  by screen printing as shown in  FIG. 8( b ) . As the silicone-based resin for forming the milky-white mark  30 , for example, ones containing aluminium oxide, silica, carbon black, dimethylcyclosiloxane, or the like can be used. 
     In the subsequent second protective layer forming step, using a silicone-based resin containing an inorganic filler (for example, containing a silica powder and an alumina powder), a transparent second protective layer  29  is formed on the first protective layer  28  by screen printing in such a manner as to cover the mark  30  as shown in  FIG. 8( c ) . 
     If the second protective layer  29  is as soft as the first protective layer  28 , the second protective layer  29  is likely to be caught by manufacturing equipment and also likely to be detached, so that the chip fuse productivity is reduced. In contrast, in the present embodiment, the second protective layer  29  is formed of an inorganic-filler-containing silicone-based resin and made relatively hard. This makes it hard for the second protective layer  29  to be caught by the manufacturing equipment and detached, so that the chip fuse productivity is improved. Moreover, the second protective layer  29  formed of the silicone-based resin closely adheres to the first protective layer  28  formed of a similar silicone-based resin, so that the second protective layer  29  is less likely to be detached therefrom. 
     In addition, the second protective layer  29  is formed of such an inorganic-filler-containing silicone-based resin and hard and has a thickness smaller than the first protective layer  28 . Thereby, the elasticity of the first protective layer  28  is ensured, and the impact (pressure) in the melting of the fuse element section  24   b  is absorbed, making it possible to prevent damage by the impact, as well. 
     Subsequently, steps such as a backside electrode forming step, a first dividing step, an end surface electrode forming step, a second dividing step, and an end surface electrode plating step are sequentially performed. 
     In the backside electrode forming step, using a silver-containing resin, backside electrodes  31  are formed on a back surface  22   b  of the insulated substrate  22  by screen printing ( FIG. 1 ). 
     In the subsequent first dividing step, the sheet-shaped insulated substrate  22  is divided into strip shapes along the first slits  41  ( FIG. 4( a ) ). 
     In the subsequent end surface electrode forming step, using a resin containing silver, nickel-chromium, titanium, or gold, end surface electrodes  32  are formed on end surfaces  22   c  of the insulated substrate  22  across from the surface electrode sections  24   a  to the backside electrodes  31 , respectively, by printing, dipping, or sputtering ( FIG. 1 ). 
     In the subsequent second dividing step, the insulated substrates  22  in the strip shapes are divided into the individual regions  43  along the second slits  42  ( FIG. 4( a ) ). 
     In the subsequent end surface electrode plating step, copper plating, nickel plating, and tin plating are sequentially performed by an electroplating method. Thereby, a copper film  33 , a nickel film  34 , and a tin film  35  are formed across from the bank section  27  to the back surface  22   b  of the insulated substrate  22 . These plating films  33 ,  34 ,  35  cover the end surface electrodes  32  and the backside electrodes  31  entirely. 
     Thus, the chip fuse  21  as shown in  FIGS. 1 and 8 ( d ) is manufactured. 
     As described above, in the chip fuse  21  of the present embodiment, the heat-storing layer  23  is formed on the insulated substrate  22 ; the fuse film is formed on the heat-storing layer  23 , the fuse film including the surface electrode sections  24   a  on both ends in the chip fuse length direction and the fuse element section  24   b  between the surface electrode sections  24   a ; and the protective layer is formed on the fuse element section  24   b  (in the illustrated example, on the tin film  26  over the fuse element section  24   b ). The chip fuse  21  is characterized in that: the rectangular bank section  27  is formed on the heat-storing layer  23  and the surface electrode sections  24   a  in such a manner as to surround the fuse element section  24   b ; and the first protective layer  28  is formed on the inner side of the bank section  27 . Accordingly, when the first protective layer  28  is formed, the rectangular bank section  27  makes it possible to block the epoxy-group-containing silicone-based resin, which is a material for forming the first protective layer  28 , from flowing and spreading therearound. Thus, the thickness of the first protective layer  28  even at the end sections  28   a  is not reduced, and a sufficiently large thickness is ensured. Hence, it is possible to prevent the first protective layer  28  including the end sections  28   a  from being damaged by the impact (pressure) in the melting of the fuse element section  24   b.    
     In contrast, if the bank section  27  is not formed, the thicknesses of the end sections  28   a  of the first protective layer  28  are reduced as shown in  FIG. 9 . Consequently, the end sections  28   a  are likely to be damaged particularly by the impact (pressure) in the melting of the fuse element section  24   b.    
     Further, the chip fuse  21  of the present embodiment is characterized in that the sections  27   a  on both ends in the chip fuse length direction of the bank section  27  are formed outwardly in the chip fuse length direction of the ends  24   a - 4  at the inner sides in the chip fuse length direction of the surface electrode sections  24   a . Accordingly, the sections  27   a  on both ends in the chip fuse length direction of the bank section  27  do not cover end sections of the fuse element section  24   b . Hence, the bank section  27  will not be damaged by the impact (pressure) in the melting of the fuse element section. 
     Further, the chip fuse  21  of the present embodiment is characterized in that: the surface electrode sections  24   a  each include the first electrode section  24   a - 1  at the outer side in the chip fuse length direction, and the second electrode section  24   a - 2  at the inner side in the chip fuse length direction; the width W 2  of the second electrode section  24   a - 2  is narrower than the width W 1  of the first electrode section  24   a - 1 ; the sections  27   b  on both ends in the chip fuse width direction of the bank section  27  are provided on the heat-storing layer  23  and formed outwardly in the chip fuse width direction of the ends  24   a - 3  on both ends in the chip fuse width direction of the second electrode section  24   a - 2 ; and the heat-storing layer  23  and the bank section are formed of the same material (photosensitive-group-containing epoxy-based resin). Accordingly, the sections  27   b  on both ends in the chip fuse width direction of the bank section  27  are provided on the heat-storing layer  23  and adhere to the heat-storing layer  23  as a whole. This increases the adhesion of the bank section  27  and surely prevents the detachment. 
     Further, the chip fuse  21  of the present embodiment is characterized in that the first protective layer  28  is formed of an epoxy-group-containing silicone-based resin. The first protective layer  28  formed of an epoxy-group-containing silicone-based resin is soft and elastic in comparison with a conventional protective layer formed of an epoxy-based resin. Accordingly, the first protective layer  28  is capable of absorbing an impact (pressure) in the melting of the fuse element section  24   b  and thus is not likely to be damaged by the impact. 
     Further, the chip fuse  21  of the present embodiment is characterized in that the second protective layer  29  made of an inorganic-filler-containing silicone-based resin is formed on the first protective layer  28 . The second protective layer  29  formed of an inorganic-filler-containing silicone-based resin is hard and excellent in friction resistance and blocking resistance in comparison with the first protective layer  28  formed of an epoxy-group-containing silicone-based resin, and is hardly caught by manufacturing equipment and hardly detached. Hence, the productivity of the chip fuse  21  is improved. Moreover, the second protective layer  29  formed of the silicone-based resin closely adheres to the first protective layer  28  formed of a similar silicone-based resin, so that the second protective layer  29  is less likely to be detached therefrom. Furthermore, since the second protective layer  29  is formed of such an inorganic-filler-containing silicone-based resin, this makes it possible to improve the strength as a product. 
     Further, the chip fuse  21  of the present embodiment is characterized in that the second protective layer  29  is formed to have a thickness smaller than the first protective layer  28 . The second protective layer  29  formed of an inorganic-filler-containing silicone-based resin and having a thickness smaller than the first protective layer  28  ensures the elasticity of the first protective layer  28 . Accordingly, an impact (pressure) in the melting of the fuse element section  24   b  is absorbed, making it possible to prevent damage by the impact, as well. 
     Further, the chip fuse  21  of the present embodiment is characterized in that: the second protective layer  29  is transparent; and the mark  30  made of a silicone-based resin is formed on the first protective layer  28  and provided between the first protective layer  28  and the second protective layer  29 . Since the first protective layer  28 , the mark  30 , and the second protective layer  29  are formed of a silicone-based resin as a whole, they closely adhere to and are hardly detached from one another, and the absorbability of an impact (pressure) in the melting of the fuse element section  24   b  is so high that the damage is little. 
     Note that if the mark  30  is formed of an epoxy-based resin, the mark  30  poorly adheres to the first protective layer  28  formed of the silicone-based resin, and the mark  30  may be detached and fall off. Moreover, the mark  30  formed of an epoxy-based resin is hard and likely to be damaged by the impact (pressure) in the melting of the fuse element section  24   b . Consequently, the effect of increasing an impact absorbability owing to the first protective layer  28  formed of an epoxy-group-containing silicone-based resin is reduced. 
     Further, the manufacturing method for the chip fuse  21  of the present embodiment is characterized in that the manufacturing method includes: the bank section forming step (first step) of forming the rectangular bank section  27  on the heat-storing layer  23  and the surface electrode sections  24   a ; and the first protective layer forming step (second step) of forming the first protective layer  28  on the inner side of the bank section  27 . Accordingly, when the first protective layer  28  is formed in the first protective layer forming step (second step), the rectangular bank section  27  formed in the bank section forming step (first step) makes it possible to block the material for forming the first protective layer  28  (epoxy-group-containing silicone-based resin) from flowing and spreading therearound. Thus, the thickness of the first protective layer  28  even at the end sections  28   a  is not reduced, and a sufficiently large thickness is ensured. Hence, it is possible to prevent the first protective layer  28  including the end sections  28   a  from being damaged by the impact (pressure) in the melting of the fuse element section  24   b.    
     Further, the manufacturing method for the chip fuse  21  of the present embodiment is characterized in that the bank section forming step (first step) includes: laminating the sheet-shaped photosensitive-group-containing material  55  on the fuse element section  24   b , the surface electrode sections  24   a , and the heat-storing layer  23 ; and exposing the sheet-shaped photosensitive-group-containing material  55  to ultraviolet light for development (photo-etching) to thereby form the rectangular bank section  27 . Accordingly, the bank section  27  has a uniform thickness in comparison with a case where the bank section is formed by screen printing or the like, and the inner side surface  27   c  that serves as a surface configured to block the flow of the epoxy-group-containing silicone-based resin forming the first protective layer  28  is perpendicular to the surface  22   a  of the insulated substrate  22 . Thus, it is possible to more surely ensure the thicknesses of the end sections  28   a  of the first protective layer  28 . 
     Now, description will be given of the result of Interruption Test C conducted on the chip fuse  21  of the present embodiment. 
     Interruption Test C is an interruption test with 76 V and 50 A. The chip fuse  21  subjected to this Interruption Test C had a resistance value of 0.032Ω before the interruption test. As a result of conducting Interruption Test C, the interruption period was 0.14 ms, and no sustained arc was observed, as shown in  FIG. 10( b ) . Moreover, the protective layers  28 ,  29  appeared to be not damaged even after the interruption (after the fuse element section  24   b  was melted). The chip fuse  21  retained the appearance as before the interruption (the state as shown in  FIG. 2 ). 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to the chip fuse and the manufacturing method for the same, and is usefully applied in cases of improving interruption performances of chip fuses, such as retention of appearance and reduction of sustained arc. 
     REFERENCE SIGNS LIST 
     
         
           21  CHIP FUSE 
           22  INSULATED SUBSTRATE (ALUMINA SUBSTRATE) 
           22   a  SURFACE 
           22   b  BACK SURFACE 
           22   b - 1  SECTIONS ON BOTH ENDS OF BACK SURFACE IN CHIP FUSE LENGTH DIRECTION 
           22   c  END SURFACE 
           23  HEAT-STORING LAYER (ADHESION LAYER) 
           24  FUSE FILM 
           24   a  SURFACE ELECTRODE SECTION 
           24   a - 1  FIRST ELECTRODE SECTION 
           24   a - 2  SECOND ELECTRODE SECTION 
           24   a - 3  ENDS ON BOTH ENDS IN CHIP FUSE WIDTH DIRECTION 
           24   a - 4  END AT INNER SIDE IN CHIP FUSE LENGTH DIRECTION 
           24   b  FUSE ELEMENT SECTION 
           25  PLATING FILM (NICKEL FILM) 
           26  PLATING FILM (TIN FILM) 
           27  BANK SECTION (DAM) 
           27   a  SECTIONS ON BOTH ENDS IN CHIP FUSE LENGTH DIRECTION 
           27   a - 1  INNER EDGES ON BOTH ENDS IN CHIP FUSE WIDTH DIRECTION 
           27   b  SECTIONS ON BOTH ENDS IN CHIP FUSE WIDTH DIRECTION 
           27   b - 1  CENTRAL SECTION IN CHIP FUSE LENGTH DIRECTION 
           27   b - 2  END SECTIONS ON BOTH ENDS IN CHIP FUSE LENGTH DIRECTION 
           27   c  INNER SIDE SURFACE 
           28  FIRST PROTECTIVE LAYER 
           28   a  END SECTION 
           29  SECOND PROTECTIVE LAYER 
           30  MARK 
           31  BACKSIDE ELECTRODE 
           32  END SURFACE ELECTRODE 
           33  PLATING FILM (COPPER FILM) 
           34  PLATING FILM (NICKEL FILM) 
           35  PLATING FILM (TIN FILM) 
           41  FIRST SLIT 
           42  SECOND SLIT 
           43  INDIVIDUAL REGION 
           51  SHEET-SHAPED PHOTOSENSITIVE-GROUP-CONTAINING MATERIAL 
           52  COPPER FOIL 
           53  PHOTOSENSITIVE FILM 
           43  RESIST 
           55  SHEET-SHAPED PHOTOSENSITIVE-GROUP-CONTAINING MATERIAL