Patent Publication Number: US-2022230830-A1

Title: Fuse element, fuse device and protection device

Description:
TECHNICAL FIELD 
     The present invention relates to a fuse element, and a fuse device and a protection device using the fuse element. 
     Priority is claimed on Japanese Patent Application No. 2019-113530 filed in Japan on Jun. 19, 2019, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     As a current cut-off device which cuts off a current path when an overcurrent which exceeds a rated current is applied to a circuit board, a fuse device is known that cuts off the current path using a fuse element which generates heat and fuse itself. As a fuse element for a fuse device, for example, Patent Document 1 describes a fuse element which includes a low-melting-point metal layer and a high-melting-point metal layer laminated on the low-melting-point metal layer, and having a configuration in which the low-melting-point metal layer is melted when a current exceeding a rated current is applied and a molten material thereof melts the high-melting-point metal layer to fuse the fuse element. In Patent Document 1, solder, tin, and a tin alloy are exemplified as a material of the low-melting-point metal layer, and silver, copper, and an alloy containing silver or copper as a main component are exemplified as a material of the high-melting-point metal layer. 
     Also, as a current cut-off device which cuts off a current path when an abnormality other than occurrence of an overcurrent occurs in a circuit board, a protection device using a heating element (heater) is known. The protection device is configured to cause a heating element to generate heat by applying a current to the heating element in the event of an abnormality other than occurrence of an overcurrent and use the generated heat to fuse the fuse element. As a fuse element (meltable conductor) which is used for the protection device using a heating element, for example, Patent Document 2 describes a fuse element which is formed of a laminate including a high-melting-point metal layer and a low-melting-point metal layer and having a configuration in which the low-melting-point metal layer is melted by heat generated by a heating element and then melts the high-melting-point metal layer to fuse the fuse element. In Patent Document 2, Pb-free solder, tin, and a tin alloy are exemplified as a material of the low-melting-point metal layer, and silver, copper, and an alloy containing silver or copper as a main component is exemplified as a material of the high-melting-point metal layer. 
     CITATION LIST 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent No. 6420053 
         Patent Document 2: Japanese Patent No. 6249600 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     It is preferable that a fuse element is fused such that a low-melting-point metal layer is rapidly melted and a molten material thereof melts a high-melting-point metal layer in the event of an abnormality such as occurrence of an overcurrent. For this purpose, the low-melting-point metal layer and the high-melting-point metal layer are necessarily in close contact with each other. However, when a high-melting-point metal layer having a lower ionization tendency than a low-melting-point metal layer is formed on a surface of this low-melting-point metal layer by, for example, a plating method, a special pretreatment process is required to ensure adhesion at an interface between the low-melting-point metal layer and the high-melting-point metal layer, resulting in high costs. 
     The present invention has been made in view of the above circumstances, and an objective thereof is to provide a fuse element in which adhesion between a low-melting-point metal layer and a high-melting-point metal layer is high to allow rapid fusing in the event of an abnormality such as occurrence of an overcurrent and a production cost is low, and provides a fuse device and a protection device using the fuse element. 
     Solution to Problem 
     The present invention provides the following means to solve the above-described problems. 
     (1) A fuse element according to a first aspect of the present invention includes a low-melting-point metal layer, a high-melting-point metal layer laminated on at least one surface of the low-melting-point metal layer, and an intermediate layer disposed between the low-melting-point metal layer and the high-melting-point metal layer, in which the high-melting-point metal layer and the intermediate layer are layers formed of a metal which is melted by a molten material of the low-melting-point metal layer, and the intermediate layer has a higher ionization tendency than an ionization tendency of the high-melting-point metal layer. 
     (2) A fuse element according to a second aspect of the present invention includes a low-melting-point metal layer, a high-melting-point metal layer laminated on at least one surface of the low-melting-point metal layer, and an intermediate layer disposed between the low-melting-point metal layer and the high-melting-point metal layer, in which the high-melting-point metal layer and the intermediate layer are layers formed of a metal which is melted by a molten material of the low-melting-point metal layer, and the intermediate layer has a higher melting point than a melting point of the high-melting-point metal layer. 
     (3) In the aspect according to the above-described (1) or (2), the low-melting-point metal layer may be a layer formed of tin or a tin alloy which contains tin as a main component. 
     (4) In the aspect according to any one of the above-described (1) to (3), the high-melting-point metal layer may be a layer formed of silver or a silver alloy which contains silver as a main component. 
     (5) In the aspect according to any one of the above-described (1) to (4), the intermediate layer may be a layer formed of at least one type of a metal selected from the group consisting of copper, iron, and nickel, or an alloy which contains these metals as a main component. 
     (6) In the aspect according to any one of the above-described (1) to (5), the intermediate layer may have a lower ionization tendency than that of the low-melting-point metal layer. 
     (7) In the aspect according to any one of the above-described (1) to (6), a film thickness of the low-melting-point metal layer may be 30 μm or more, a film thickness of the high-melting-point metal layer may be 1 μm or more, and a film thickness of the intermediate layer may be within a range of 0.01 μm or more and 1 μm or less. 
     (8) A fuse device according to one aspect of the present invention includes an insulating substrate, and the fuse element according to any one of the above-described (1) to (7) disposed on a surface of the insulating substrate. 
     (9) A protection device according to one aspect of the present invention includes an insulating substrate, the fuse element according to any one of the above-described (1) to (7) disposed on a surface of the insulating substrate, and a heating element disposed on a surface of the insulating substrate and configured to heat the fuse element. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a fuse element with high adhesion between a low-melting-point metal layer and a high-melting-point metal layer and low production cost, and a fuse device and a protection device using the fuse element. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic perspective view illustrating an example of a fuse element according to a first embodiment of the present invention. 
         FIG. 2  is a schematic perspective view illustrating another example of the fuse element according to the first embodiment of the present invention. 
         FIG. 3  is a schematic perspective view illustrating still another example of the fuse element according to the first embodiment of the present invention. 
         FIG. 4  is a schematic plan view illustrating an example of a fuse device according to a second embodiment of the present invention. 
         FIG. 5  is a cross-sectional view along line V-V′ of  FIG. 4 . 
         FIG. 6  is a schematic plan view illustrating an example of a protection device according to a third embodiment of the present invention. 
         FIG. 7  is a cross-sectional view along line VII-VII′ of  FIG. 6 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, preferred examples of embodiments of a fuse element according to the present invention, and a fuse device and a protection device using the fuse element will be described in detail with reference to the drawings as appropriate. In the drawings used in the following description, there are cases in which characteristic portions are enlarged for convenience of illustration so that characteristics can be easily understood, and dimensional proportions or the like of respective constituent elements may be different from actual ones. Materials, dimensions, and the like illustrated in the following description are merely examples, and the present invention is not limited thereto and can be implemented with appropriate modifications within a range in which the effects of the present invention are achieved. Changes, omissions, additions, substitutions, and other modifications can be made to positions, numbers, ratios, types, sizes, shapes, or the like within a range not departing from the gist of the present invention. Unless there is a particular problem, preferable characteristics and conditions in the examples may be shared with each other. 
     Fuse Element (First Embodiment) 
       FIG. 1  is a schematic perspective view of a fuse element according to a first embodiment of the present invention. 
     As illustrated in  FIG. 1 , a fuse element  10  includes a low-melting-point metal layer  11 , a high-melting-point metal layer  12  laminated on a surface of the low-melting-point metal layer  11 , and an intermediate layer  13  disposed between the low-melting-point metal layer  11  and the high-melting-point metal layer  12 . A shape of the fuse element  10  in a plan view and a cross-sectional shape thereof can be arbitrarily selected. 
     A melting point of the low-melting-point metal layer  11  is preferably equal to or lower than a heating temperature during reflow performed when a fuse device or a protection device is manufactured. When the reflow temperature is 240° C. to 260° C., a melting point of a material constituting the low-melting-point metal layer  1 I is preferably in a range of 200° C. or higher and 235° C. or lower. The above-described melting point may be in a range of 200° C. or higher and 218° C. or lower or 218° C. or higher and 235° C. or lower as necessary. 
     A material of the low-melting-point metal layer  11  is preferably tin or a tin alloy containing tin as a main component. Containing “as a main component” means that the component is contained in an amount exceeding 50% by mass. A tin content of the tin alloy is preferably 40% by mass or more, and more preferably 60% by mass or more. The content mentioned above may also be 70% by mass or more or 80% by mass or more. An upper limit value of the content can be arbitrarily selected but may be, for example, 100% by mass or less, 99% by mass or less, or 97% by mass or less. As an example of the tin alloy, a Sn—Bi alloy, an In—Sn alloy, or a Sn—Ag—Cu alloy can be exemplified. 
     The high-melting-point metal layer  12  is a layer formed of a metal material that is melted by a molten material of the low-melting-point metal layer  11 . When a material which constitutes the low-melting-point metal layer  11  is tin or a tin alloy, a material which constitutes the high-melting-point metal layer  12  is preferably silver or an alloy containing silver as a main component. A silver content of the silver alloy is preferably 40% by mass or more, and more preferably 60% by mass or more. The content described above may also be 70% by mass or more or 80% by mass or more. An upper limit value of the content can be arbitrarily selected but may be, for example, 100% by mass or less, 99% by mass or less, or 97% by mass or less. As an example of the silver alloy, a silver-palladium alloy may be exemplified. Also, silver is a noble metal, has a low ionization tendency, basically barely oxidizes in the atmosphere, and is easily melted by a molten material of tin which constitutes the low-melting-point metal layer  11 . Therefore, silver can be suitably used as a material of the high-melting-point metal layer  12  which is an outermost layer of the fuse element. Further, an ionization tendency of each of these metal is well known. Also, a higher ionization tendency means a greater likelihood of emitting electrons to produce cations, that is, it is a greater likelihood of oxidation. Also, an ionization tendency of each layer may mean an ionization tendency of a metal serving as a main component of the material which forms each layer. 
     It is preferable that a melting point of a material which constitutes the high-melting-point metal layer  12  is within a range of +100° C. or higher and +800° C. or lower with respect to the melting point of the low-melting-point metal layer  11 . That is, the melting point of the high-melting-point metal layer  12  is preferably higher than that of the low-melting-point metal layer  11  by 100 to 800° C. The melting point of the high-melting-point metal layer  12  is preferably in a range of 300° C. or higher and 1000° C. or lower. The melting point of the high-melting-point metal layer  12  may be in a range of 300° C. or higher and 500° C. or lower, 500° C. or higher and 700° C. or lower, or 700° C. or higher and 1000° C. or lower as necessary. 
     The intermediate layer  13  is a layer formed of a metal material that is melted by the molten material of the low-melting-point metal layer  11 . When the material which constitutes the low-melting-point metal layer  11  is tin or a tin alloy, a material which constitutes the intermediate laver  13  is preferably at least one type of a metal selected from the group consisting of copper, iron, and nickel, or a metal alloy containing the metal as a main component. An amount of copper, iron, and nickel in the metal alloy is preferably 40% by mass or more, and more preferably 60% by mass or more. The content described above may also be 70% by mass or more or 80% by mass or more. An upper limit value of the content can be arbitrarily selected but may be, for example, 100% by mass or less, 99% by mass or less, or 90% by mass or less. As an example of the copper alloy, phosphor bronze can be exemplified. As an example of the iron alloy, nickel iron can be exemplified. As an example of the nickel alloy, nickel-cobalt can be exemplified. Among metals that can be used for the intermediate layer  13 , copper, iron, nickel, and alloys thereof have high rigidity and are preferable because the fuse element  10  using such a metal does not readily deform during reflow when the fuse device or the protection device is manufactured. 
     The intermediate layer  13  preferably has a higher ionization tendency than the high-melting-point metal layer  12 . Due to the high ionization tendency of the intermediate layer  13 , adhesion at an interface between the intermediate layer  13  and the high-melting-point metal layer  12  is improved when the high-melting-point metal layer  12  is formed by a plating method. 
     It is more preferable that the ionization tendency of the intermediate layer  13  be lower than that of the low-melting-point metal layer  11 . That is, the ionization tendency of the intermediate layer  13  is more preferably between ionization tendencies of the low-melting-point metal layer  11  and the high-melting-point metal layer  12 . When the ionization tendency of the intermediate layer  13  is between the ionization tendencies of the low-melting-point metal layer  11  and the high-melting-point metal layer  12 , a difference in ionization tendency at the time of each plating can be reduced by interposing the intermediate layer  13  therebetween compared to a case in which the high-melting-point metal layer  12  is formed directly on the low-melting-point metal layer  11  by a plating method. As a result, it is possible to improve stability in plating, improve the quality, and reduce processing costs. Also, it is possible to obtain the intermediate layer  13  having a uniform film thickness and in which melting by the molten material of the low-melting-point metal layer  11  proceeds easily. 
     With regard to the intermediate layer  13 , it is preferable that a melting point of the material constituting the layer be higher than the melting point of the high-melting-point metal layer  12 . Thus if a thickness of the high-melting-point metal layer  12  is reduced, the fuse element  10  does not readily deform during reflow when the fuse device or the protection device is manufactured. The melting point of the intermediate layer  13  is preferably in a range of +50° C. or higher and +500° C. or lower with respect to the melting point of the high-melting-point metal layer  12 . When the melting point of the intermediate layer  13  is too low, the above-described effects due to the intermediate layer  13  may not be obtained. On the other hand, when the melting point of the intermediate layer  13  is too high, there is a likelihood that it will become difficult for melting of the intermediate layer  13  by the molten material of the low-melting-point metal layer  11  to proceed, and a fusing speed of the fuse element  10  will decrease. The melting point of the intermediate layer  13  is preferably in a range of 950° C. or higher and 1600° C. or lower. The melting point of the intermediate layer  13  may be in a range of 950° C. or higher and 1200° C. or lower, 1200° C. or higher and 1400° C. or lower, or 1400° C. or higher and 1600° C. or lower as necessary. 
     In the event of an abnormality such as occurrence of an overcurrent, the low-melting-point metal layer  11  is melted, a produced molten material thereof melts the intermediate layer  13  and the high-melting-point metal layer  12 , and thereby the fuse element  10  is fused. In the fuse element  10 , the low-melting-point metal layer  11  is contained in an amount necessary for melting the intermediate layer  13  and the high-melting-point metal layer  12  to fuse the fuse element  10 . The intermediate layer  13  and the high-melting-point metal layer  12  are contained in an amount necessary for maintaining a shape of the fuse element  10  during reflow when the fuse device or the protection device is manufactured. 
     From the above-described viewpoint, a film thickness of the low-melting-point metal layer  11  can be arbitrarily selected but is preferably 30 μm or more. The film thickness of the low-melting-point metal layer  11  may also be 60 μm or more, 100 μm or more, or 500 μm or more. An upper limit value of the film thickness of the low-melting-point metal layer  11  can be arbitrarily selected but may be, for example, 3000 μm or less. It may be 2000 μm or less, 1500 μm or less, or the like as necessary. 
     Also, a film thickness of the high-melting-point metal layer  12  can be arbitrarily selected but is preferably 1 μm or more. The film thickness of the high-melting-point metal layer  12  may be 5 μm or more or 10 μm or more. An upper limit value of the film thickness of the high-melting-point metal layer  11  can be arbitrarily selected but may be, for example, 100 μm or less or 50 μm or less. 
     Further, a film thickness of the intermediate layer  13  can be arbitrarily selected but is preferably in a range of 0.01 μm or more and 1 μm or less. It may be in a range of 0.01 μm or more and 0.1 μm or less, 0.05 μm or more and 0.5 μm or less, or 0.5 μm or more and 1.0 μm or less as necessary. 
     Also, a film thickness ratio of a total film thickness of the high-melting-point metal layer  12  and the intermediate layer  13  to the film thickness of the low-melting-point metal layer  11  (the former: the latter) can be arbitrarily selected but is preferably in a range of 1:2 to 1:100. It may be in a range of, for example, 1:2 to 1:10, 1:10 to 1:30, 1:30 to 1:100, or the like as necessary. If the total film thickness of the high-melting-point metal layer  12  and the intermediate layer  13  becomes too large, there is a likelihood that a time until the intermediate layer  13  and the high-melting-point metal layer  12  are melted will become long in the event of an abnormality, and the fusing speed of the fuse element  10  will decrease. On the other hand, when the film thickness of the low-melting-point metal layer  11  becomes too large, it may be difficult to maintain the shape of the fuse element  10  during reflow when the fuse device or a protection device is manufactured. 
     The fuse element  10  can be manufactured by, for example, using a plating method. Specifically, the fuse element  10  can be manufactured by preparing a metal foil as the low-melting-point metal layer  11 , forming the intermediate layer  13  on a surface of the metal foil by a plating method, and then forming the high-melting-point metal layer  12  on a surface of the intermediate layer  13  by a plating method. When tin or a tin alloy is used as the low-melting-point metal layer  11 , the low-melting-point metal layer  11  readily oxidizes, and a passive film may be formed on a surface thereof. In this case, it is preferable to use a method of electrolytic plating (strike plating method) in a short time by applying a high current when the intermediate layer  13  is formed. 
     The fuse element  10  illustrated in  FIG. 1  has a configuration in which the intermediate layer  13  and the high-melting-point metal layer  12  are laminated on a surface of the low-melting-point metal layer  11 , but a configuration of the fuse element is not limited thereto. Examples of other configurations of the fuse element  10  are illustrated in  FIGS. 2 and 3 . 
       FIG. 2  is a schematic perspective view illustrating another example of the fuse element according to the first embodiment of the present invention. A fuse element  20  illustrated in  FIG. 2  includes a low-melting-point metal layer  21  having a rectangular cross section, a high-melting-point metal layer  22  laminated around the low-melting-point metal layer  21 , and an intermediate layer  23  disposed between the low-melting-point metal layer  21  and the high-melting-point metal layer  22 . In the fuse element  20 , both a main surface and a side surface of the low-melting-point metal layer  21  are covered with the intermediate layer  23  and the high-melting-point metal layer  22 . Therefore, rigidity of an outer shell formed of the high-melting-point metal layer  22  and the intermediate layer  23  is increased, and a shape of the fuse element  10  is easily maintained during reflow. 
       FIG. 3  is a schematic perspective view illustrating still another example of the fuse element according to the first embodiment of the present invention. A fuse element  30  illustrated in  FIG. 3  includes a low-melting-point metal layer  31  having a circular cross section, a high-melting-point metal layer  32  laminated around the low-melting-point metal layer  31 , and an intermediate layer  33  disposed between the low-melting-point metal layer  31  and the high-melting-point metal layer  32 . In the fuse element  30 , since a side surface of the low-melting-point metal layer  31  is concentrically covered with the intermediate layer  33  and the high-melting-point metal layer  32 , the low-melting-point metal layer  31  barely oxidizes. Also, thicknesses of the intermediate layer  33  and the high-melting-point metal layer  32  are easily made uniform, and melting of the intermediate layer  33  and the high-melting-point metal layer  32  easily proceed uniformly. Therefore, a fusing speed of the fuse element  30  further increases. 
     In the fuse elements  10 ,  20 , and  30  according to the first embodiment of the present invention having the above-described configuration, when ionization tendencies of the intermediate layers  13 ,  23 , and  33  are higher than ionization tendencies of the high-melting-point metal layers  12 ,  22 , and  32 , the high-melting-point metal layers  12 ,  22 , and  32  having excellent interfacial adhesion with the intermediate layers  13 ,  23 , and  33  and high stability can be formed at low cost by using a plating method. Particularly, in the fuse elements  10 ,  20 , and  30  in which the intermediate layers  13 ,  23 , and  33  are formed by a strike plating method, interfacial adhesion between the low-melting-point metal layers  11 ,  21 , and  31 , the intermediate layers  13 ,  23 , and  33 , and the high-melting-point metal layer  12  and  22  is excellent, and therefore fusing can be made more rapidly in the event of an abnormality such as occurrence of an overcurrent. Also, in the fuse elements  10 ,  20 , and  30  in which melting points of the intermediate layers  13 ,  23 , and  33  are higher than melting points of the high-melting-point metal layers  12 ,  22  and  32 , since adhesion of each layer at a high temperature is not easily decreased and each layer is not easily peeled off, fusing can be made more rapidly even when a temperature becomes high due to occurrence of an overcurrent or the like. 
     The fuse elements  10 ,  20 , and  30  according to the first embodiment of the present invention may further include a layer made of a metal having a lower melting point than the intermediate layers  13 ,  23 , and  33  and a higher melting point than the high-melting-point metal layers  12 ,  22 , and  32 , and that is melted by molten materials of the low-melting-point metal layers  11 ,  21 , and  31  between the intermediate layers  13 ,  23 , and  33  and the high-melting-point metal layers  12 ,  22 , and  32 . Also, an antioxidant layer may be provided on surfaces of the high-melting-point metal layers  12 ,  22 , and  32 . 
     Next, an embodiment of the fuse device and the protection device according to the present invention will be described by taking a case in which the fuse element  10  illustrated in  FIG. 1  is used as a fuse element as an example. 
     Fuse Device (Second Embodiment) 
       FIG. 4  is a schematic plan view of a fuse device according to a second embodiment of the present invention.  FIG. 5  is a cross-sectional view along line V-V′ of  FIG. 4 . Further,  FIG. 4  is in a state in which a cover member of the fuse device is removed. 
     As illustrated in  FIGS. 4 and 5 , a fuse device  40  includes an insulating substrate  41 , a first electrode  42  and a second electrode  43  disposed on a surface  41   a  of the insulating substrate  41 , and a fuse element  10  electrically connecting the first electrode  42  and the second electrode  43 . 
     The insulating substrate  41  is not particularly limited as long as it has electrical insulating properties, and a known insulating substrate used as a circuit board such as a resin substrate, a ceramics substrate, or a composite substrate of a resin and a ceramic may be used. As an example of the resin substrate, an epoxy resin substrate, a phenolic resin substrate, or a polyimide substrate can be exemplified. As an example of the ceramic substrate, an alumina substrate, a glass ceramic substrate, a mullite substrate, or a zirconia substrate can be exemplified. As an example of the composite substrate, a glass epoxy substrate can be exemplified. 
     The first electrode  42  and the second electrode  43  are disposed at a pair of opposite end portions of the insulating substrate  41  facing each other. The first electrode  42  and the second electrode  43  are each formed by a conductive pattern such as silver wiring, copper wiring, or the like. Surfaces of the first electrode  42  and the second electrode  43  are each covered with an electrode protective layer  44  for suppressing change of properties in electrode characteristics which may be caused due to oxidation or the like. As a material which constitutes the electrode protective layer  44 , for example, a Sn plating film, a Ni/Au plating film, a Ni/Pd plating film, a Ni/Pd/Au plating film, or the like can be used. Also, the first electrode  42  and the second electrode  43  are electrically connected to a first external connection electrode  42   a  and a second external connection electrode  43   a  formed on a back surface  41   b  of the insulating substrate  41  via castellation, respectively. Connections of the first electrode  42  and the second electrode  43  to the first external connection electrode  42   a  and the second external connection electrode  43   a  are not limited to castellation and may be performed by a through hole. 
     The fuse element  10  is electrically connected to the first electrode  42  and the second electrode  43  via a connecting material  45  such as solder. 
     A flux  46  is applied to a surface of the fuse element  10 . When the flux  46  is applied, oxidation of the fuse element  10  is prevented, and wettability of the connecting material  45  when the fuse element  10  is connected to the first electrode  42  and the second electrode  43  via the connecting material  45  is improved. Also, when the flux  46  is applied, a molten metal adhering to the insulating substrate  41  due to arc discharge can be suppressed, and insulating properties after the fuse element  10  is fused can be improved. 
     As illustrated in  FIG. 5 , the fuse device  40  preferably includes a cover member  50  attached via an adhesive. When the cover member  50  is attached, the inside of the fuse device  40  can be protected and a molten material produced when the fuse element  10  is fused can be prevented from scattering. As a material of the cover member  50 , various engineering plastics and ceramics can be used. 
     The fuse device  40  is mounted on a current path of a circuit board via the first external connection electrode  42   a  and the second external connection electrode  43   a . While a rated current flows through the current path of the circuit board, the low-melting-point metal layer  11  of the fuse element  10  provided in the fuse device  40  does not melt. On the other hand, when an overcurrent which exceeds the rated current is applied to the current path of the circuit board, the low-melting-point metal layer  11  of the fuse element  10  generates heat and melts, a produced molten metal melts the intermediate layer  13  and the high-melting-point metal layer  12 , and thereby the fuse element  10  is fused. Then, due to the fusing of the fuse element  10 , the first electrode  42  and the second electrode  43  are disconnected, and the current path of the circuit board is cut off. 
     The fuse device  40  according to the second embodiment of the present invention having the above-described configuration uses the fuse element  10  according to the first embodiment of the present invention. Therefore, the fuse element  10  is rapidly fused in the event of occurrence of an overcurrent. Therefore, the current path of the circuit board can be cut off at an early stage. 
     Protection Device (Third Embodiment) 
       FIG. 6  is a schematic plan view of a protection device according to a third embodiment of the present invention.  FIG. 7  is a cross-sectional view along line VII-VII′ of  FIG. 6 . Further, in  FIG. 6 , the protection device is in a state in which a cover member is removed. 
     As illustrated in  FIGS. 6 and 7 , a protection device  60  includes an insulating substrate  61 , a first electrode  62  and a second electrode  63  disposed on a surface  61   a  of the insulating substrate  61 , a heating element  70  disposed between the first electrode  62  and the second electrode  63 , a first heating element electrode  64  and a second heating element electrode  65  connected to the heating element  70 , a heating element lead-out electrode  66  connected to the second heating element electrode  65  and positioned at a place overlapping the heating element  70  in a plan view, and a fuse element  10  disposed on a surface of the heating element lead-out electrode  66 . 
     The insulating substrate  61  is not particularly limited as long as it has electrical insulating properties. As the insulating substrate  61 , a known insulating substrate used as a circuit board can be used as in the case of the fuse device  40  of the second embodiment. In the present example, the insulating substrate  61  is rectangular in a plan view but is not limited to the shape and may have an arbitrarily selected shape. 
     The first electrode  62  and the second electrode  63  are disposed at a pair of opposite end portions of the insulating substrate  61  facing each other. The first heating element electrode  64  and the second heating element electrode  65  are disposed at another pair of opposite end portions of the insulating substrate  61  facing each other. The first electrode  62 , the second electrode  63 , the first heating element electrode  64 , the second heating element electrode  65 , and the heating element lead-out electrode  66  are each formed by a conductive pattern such as silver wiring, copper wiring, or the like. Also, the first electrode  62 , the second electrode  63 , the first heating element electrode  64 , the second heating element electrode  65 , and the heating element lead-out electrode  66  are preferably covered with an electrode protective layer  67  for suppressing change of properties in electrode characteristics which may be caused due to oxidation or the like. A material of the electrode protective layer  67  is the same as that in the case of the fuse device  40  of the second embodiment. Further, the first electrode  62 , the second electrode  63 , and the first heating element electrode  64  are electrically connected to a first external connection electrode  62   a , a second external connection electrode  63   a , and a heating element feeding electrode  64   a  formed on a back surface  61   b  of the insulating substrate  61  via castellation, respectively. Further, respective connections of the first electrode  62 , the second electrode  63 , and the first heating element electrode  64  to the first external connection electrode  62   a , the second external connection electrode  63   a , and the heating element feeding electrode  64   a  are not limited to castellation and may be performed by a through hole. 
     The heating element  70  is formed of a high resistance conductive material that has relatively high resistance and generates heat due to energization. The heating element  70  is formed of, for example, nichrome, W, Mo, Ru, or the like or a material containing these. The heating element  70  can be preferably formed by a calcination method or the like after a paste form is prepared by mixing powder substances of an alloy, a composition or a compound which contains the above-described elements with a resin binder or the like, and the paste form is formed into a pattern on a surface of the insulating substrate  61  using a screen-printing technology. 
     The heating element  70  is covered with an insulating member  71 . As a material of the insulating member  71 , for example, glass can be used. The heating element lead-out electrode  66  is disposed to face the heating element  70  via the insulating member  71 . With this disposition, the heating element  70  is superposed on the fuse element  10  via the insulating member  71  and the heating element lead-out electrode  66 . With such a superposed structure, heat generated by the heating element  70  can be efficiently transferred to the fuse element  10  in a narrow range. 
     Both ends of the fuse element  10  are electrically connected to the first electrode  62  and the second electrode  63 , and a central portion thereof is connected to the heating element lead-out electrode  66 . The fuse element  10  is electrically connected to the first electrode  62 , the second electrode  63 , and the heating element lead-out electrode  66  via a connecting material  68  such as solder. With such a configuration, in the protection device  60 , a first energization path is formed through the heating element feeding electrode  64   a , the first heating element electrode  64 , the heating element  70 , the second heating element electrode  65 , the heating element lead-out electrode  66 , and the fuse element  10 , and a second energization path is formed through the first external connection electrode  62   a , the first electrode  62 , the fuse element  10 , the second electrode  63 , and the second external connection electrode  63   a . Also, a flux  69  is applied to a surface of the fuse element  10 . 
     As illustrated in  FIG. 7 , in the protection device  60 , a cover member  80  is preferably attached via an adhesive. A material of the cover member  80  is the same as that of the fuse device  40  of the second embodiment. 
     The protection device  60  is mounted on a current path of a circuit board via the first external connection electrode  62   a , the second external connection electrode  63   a , and the heating element feeding electrode  64   a . Thereby, the fuse element  10  of the protection device  60  is connected in series on a current path of an external circuit board via the first external connection electrode  62   a  and the second external connection electrode  63   a . The heating element  70  is connected to a current control device provided on the circuit board via the heating element feeding electrode  64   a.    
     The protection device  60  is configured such that, when an abnormality occurs in the circuit board, the heating element  70  is energized via the heating element feeding electrode  64   a  by the current control device provided on the circuit board. This energization causes the heating element  70  to generate heat. Then, the heat is transferred to the fuse element  10  via the insulating member  71  and the heating element lead-out electrode  66 . Due to the heat, the low-melting-point metal layer  11  of the fuse element  10  is melted, and a produced molten material melts the intermediate layer  13  and the high-melting-point metal layer  12 . As a result, the fuse element  10  is fused. Then, due to the fusing of the fuse element  10 , the first electrode  62  and the second electrode  63  are disconnected, and the current path of the circuit board is cut off. 
     The protection device  60  according to the third embodiment of the present invention having the above-described configuration uses the fuse element  10  according to the first embodiment of the present invention. As a result, the fuse element  10  is rapidly fused in the event of an abnormality. Therefore, the current path of the circuit board can be cut off at an early stage. 
     INDUSTRIAL APPLICABILITY 
     A fuse element with high adhesion between a low-melting-point metal layer and a high-melting-point metal layer and low production cost, and a fuse device and a protection device using the fuse element are provided. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 ,  20 ,  30  Fuse element 
               11 ,  21 ,  31  Low-melting-point metal layer 
               12 ,  22 ,  32  High-melting-point metal layer 
               13 ,  23 ,  33  Intermediate layer 
               40  Fuse device 
               41  Insulating substrate 
               41   a  Surface 
               41   b  Back surface 
               42  First electrode 
               42   a  First external connection electrode 
               43  Second electrode 
               43   a  Second external connection electrode 
               44  Electrode protective layer 
               45  Connecting material 
               46  Flux 
               50  Cover member 
               60  Protection device 
               61  Insulating substrate 
               61   a  Surface 
               61   b  Back surface 
               62  First electrode 
               62   a  First external connection electrode 
               63  Second electrode 
               63   a  Second external connection electrode 
               64  First heating element electrode 
               64   a  Heating element feeding electrode 
               65  Second heating element electrode 
               66  Heating element lead-out electrode 
               67  Electrode protective layer 
               68  Connecting material 
               69  Flux 
               70  Heating element 
               71  Insulating member 
               80  Cover member