Patent Publication Number: US-2011068889-A1

Title: Thermal fuse element, thermal fuse and battery using the thermal fuse

Description:
This is a continuation of U.S. application Ser. No. 10/558,373, filed Nov. 29, 2005, which is the National Stage of International Application No. PCT/JP2004/007657, filed May 27, 2004. 
    
    
     TECHNICAL FIELD  
     The present invention relates to a thermal fuse element, a thermal fuse and a battery using such a thermal fuse. 
     BACKGROUND ART  
     In recent years, it has become problematic that Cd and Pb used in electronic devices leach into the natural environment and, accordingly, there has been an increasing demand for making electronic devices Cd-free and Pb-free. Therefore, thermal fuses used to protect the electronic devices are desired to contain neither Pb nor Cd. 
     Particularly, in a packaged battery used as a power supply of a mobile phone, a battery and a thermal fuse are connected by spot welding, and Pb-free solder has been used even in a protecting circuit for controlling the charging and discharging of the battery. Thus, there has been a strong demand for thermal fuses containing neither Pb nor Cd. 
     Since the heat capacity of the above packaged battery becomes smaller with the miniaturization thereof, there has been an increasing tendency to speed up a temperature rising rate during the heat generation. Thus, the thermal fuse is required to have a low operating temperature of 85 to 108° C. in order to quickly shut off a current in the event of an abnormality. 
     In the case of using the packaged battery, for example, under the scorching sun, the packaged battery is, in some cases, used while the surface temperature thereof is about 55° C. due to the ambient temperature and the heat generation by the battery. Therefore, the thermal fuse is required to secure its function even in the case of being used for a long time in a temperature state of about 55° C. 
       FIG. 7  is a section of a prior art thermal fuse. As shown in  FIG. 7 , the prior art thermal fuse comprises: a cylindrical insulating casing  1  having openings at opposite ends; a fusible alloy  2  in the form of a substantially round column or a substantially rectangular column arranged in the insulating casing  1 ; a pair of lead conductors  3  each having one end portion connected to the corresponding end of the fusible alloy  2 , and the having other end portion protruded through the corresponding opening of the insulating casing  1  to the outside of the insulating casing  1 ; flux (not shown) coated on the fusible alloy  2 ; and sealing elements  4  sealing the openings at the opposite end portions of the insulating casing  1 . 
     For example, in a thermal fuse operable at 85 to 108° C., the fusible alloy  2  has been made of an Sn—Cd—In eutectic alloy (melting point of 93° C.) or an Sn—Bi—Pb eutectic alloy (melting point of 95° C.). An alloy-type thermal fuse disclosed in Japanese Unexamined Patent Publication No. 2000-90792 is known as such a prior art thermal fuse. 
     However, since the prior art thermal fuse comprises the fusible alloy  2  containing Pb or Cd, there is a possibility of disturbing the natural environment due to the leached Pb or Cd if electronic devices using this thermal fuse are discarded. 
     SUMMARY OF THE INVENTION  
     An object of the present invention is to provide a thermal fuse element, a thermal fuse and a battery using such a thermal fuse, which are designed to suppress harm to the natural environment. 
     One aspect of the present invention is directed to a thermal fuse element which is cut off by melting at a specified temperature, and includes either an Sn—Bi—In—Zn alloy or an Sn—In—Zn alloy. 
     Since this thermal fuse element includes either an Sn—Bi—In—Zn alloy or an Sn—In—Zn alloy, neither Pb nor Cd is contained in the thermal fuse element unlike the prior art. Thus, it is possible to provide a thermal fuse element capable of suppressing harm to the natural environment. 
     The thermal fuse element preferably includes an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5 weight % of Zn; with the balance being Sn. In this case, it is possible to provide a thermal fuse element having an arbitrary operating temperature of 85° C. to 107° C. 
     The thermal fuse element more preferably includes an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 48 to 55 weight % of In; 1.3 to 5 weight % of Zn; with the balance being Sn. 
     In this case, the thermal fuse element can be used for a long time at a temperature exceeding the melting point of flux, and the fluctuation of the melting point caused by the composition variation can be reduced. Thus, it is possible to provide a thermal fuse element having a highly precise cut-off temperature by melting. 
     The thermal fuse element may include an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5 weight % of Zn; with the balance being Sn. 
     In this case, the melting point of the Sn—In—Zn alloy is about 107° C. and a difference between a solidus temperature and a liquidus temperature becomes smaller so as to reduce a temperature range of a solid-liquid mixture. Thus, it is possible to provide a thermal fuse element having a small operating temperature variation at about 107° C. 
     The thermal fuse element more preferably includes an Sn—In—Zn alloy containing: 48 to 55 weight % of In; and 1.3 to 5 weight % of Zn; with the balance being Sn. 
     In this case, the thermal fuse element can be used for a long time at a temperature exceeding the melting point of the flux, and the fluctuation of the melting point caused by the composition variation can be reduced. Thus, it is possible to provide a thermal fuse element having a highly precise cut-off temperature by melting. 
     Another aspect of the present invention is directed to a thermal fuse comprising a fusible alloy cut off by melting at a specified temperature, wherein the fusible alloy includes either an Sn—Bi—In—Zn alloy or an Sn—In—Zn alloy. 
     Since the fusible alloy includes either an Sn—Bi—In—Zn alloy or an Sn—In—Zn alloy in this thermal fuse, neither Pb nor Cd is contained in the fusible alloy unlike the prior art. Thus, it is possible to provide a thermal fuse capable of suppressing harm to the natural environment. 
     The fusible alloy preferably includes an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5 weight % of Zn; with the balance being Sn. In this case, it is possible to provide a thermal fuse element having an arbitrary operating temperature of 85° C. to 107° C. 
     The fusible alloy more preferably includes an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 48 to 55 weight % of In; and 1.3 to 5 weight % of Zn; with the balance being Sn. 
     In this case, the thermal fuse can be used for a long time at a temperature exceeding the melting point of the flux, and the fluctuation of the melting point caused by the composition variation can be reduced. Thus, it is possible to provide a thermal fuse having a highly precise cut-off temperature by melting. 
     The fusible alloy may include an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5 weight % of Zn; with the balance being Sn. 
     In this case, the melting point of the Sn—In—Zn alloy is about 107° C. and a difference between a solidus temperature and a liquidus temperature becomes smaller so as to reduce a temperature range of a solid-liquid mixture. Thus, it is possible to provide a thermal fuse element having a small operating temperature variation at about 107° C. 
     The fusible alloy may include an Sn—In—Zn alloy containing: 48 to 55 weight % of In; 1.3 to 5 weight % of Zn; with the balance being Sn. 
     In this case, the thermal fuse can be used for a long time at a temperature exceeding the melting point of the flux, and the fluctuation of the melting point caused by the composition variation can be reduced. Thus, it is possible to provide a thermal fuse having a highly precise cut-off temperature by melting. 
     Preferably, the fusible alloy is coated by flux and the melting point of the flux is 60° C. or higher. 
     In this case, even if the thermal fuse is used for a long time at about 55° C., a decrease of an activator contained in the flux can be made smaller. As a result, it is possible to provide a thermal fuse whose quick cut-off ability by melting can be ensured for a long time. 
     Preferably, the fusible alloy is coated by flux and a bromide activator is added in the flux. 
     In this case, even if the thermal fuse is used for a long time at a temperature exceeding the melting point of the flux, a decrease of the activator contained in the flux can be reduced. As a result, it is possible to provide a thermal fuse whose quick cut-off ability by melting can be ensured for a long time. 
     The thermal fuse preferably further comprises: a pair of metal terminals; a first insulating film to which the pair of metal terminals are attached; and a second insulating film attached to the first insulating film so as to define a space with the first insulating films; wherein the fusible alloy is arranged between the first and second insulating films and connected between the leading end portions of the pair of metal terminals. 
     In this case, it is possible to cover the fusible alloy by the first and second insulating films, and to seal the fusible alloy by sealably fixing an outer peripheral portion of the insulating film and that of the second insulating film while leaving a portion where the fusible alloy is provided unfixed. Thus, the deterioration of the fusible alloy can be prevented. 
     La is preferably set to lie within a range of 2.0 mm to 7.5 mm, wherein La is a length of a thermal-fuse main portion comprising the first insulating film, the second insulating film and the fusible alloy. In this case, the thermal fuse can be miniaturized. 
     Lb is preferably set to lie within a range of 0.4 mm to 1.5 mm, wherein Lb is a thickness between the outer surface of the first insulating film and the outer surface of the second insulating film. In this case, the thermal fuse can be thinned. 
     Preferably, a projection is formed at an end portion of each metal terminal, the pair of metal terminals are attached to the first insulating film such that the projections project from the first insulating film side toward the second insulating film side, and the fusible alloy is connected to the projections. In this case, the pair of metal terminals can be precisely attached to the first insulating film, and the thin-type thermal fuse can be produced with high precision. 
     A projection is preferably formed at an end portion of each metal terminal which extends outwardly from the first insulating film and from the second insulating film. In this case, if the metal terminals and external wiring are connected at the projections by electric welding, a welding current can be concentrated. Thus, welding strength and welding positions can be stabilized, thereby improving productivity. 
     The thermal fuse may further comprise: an insulating casing in a form of a tube having a bottom and having an opening; a pair of lead conductors each having a first end portion protruded in the same direction through the opening of the insulation to the casing outside the insulating casing; and a sealing element for sealing the opening of the insulating casing; wherein the fusible alloy is arranged in the insulating casing and connected to second end portions of the pair of lead conductors. 
     In this case, the pair of lead conductors whose second end portions are connected to the fusible alloy protrudes the first end portions in the same direction through the opening of the insulating casing to the outside of the insulating casing. Thus, a degree of freedom in mounting this thermal fuse on a battery or the like can be improved. 
     The thermal fuse may further comprise: an insulating casing in a form of a cylindrical tube having openings at opposite ends; a pair of lead conductors each having a first end portion protruded through a corresponding one of the openings at the opposite ends of the insulating casing; and sealing elements for sealing the openings at the opposite ends of the insulating casing; wherein the fusible alloy is arranged in the insulating casing and connected to second end portions of the pair of lead conductors. 
     In this case, since the insulating casing having the fusible alloy placed therein is in a form of a cylindrical tube having openings at the opposite ends, there is no directionality upon mounting the thermal fuse on a battery or the like, wherefore the thermal fuse can be easily handled at the time of production. 
     Still another aspect of the present invention is directed to a battery, comprising: a battery main body; and a thermal fuse electrically connected to the battery main body so as to shut off a current upon an abnormal heat generation of the battery main body, wherein the thermal fuse includes a fusible alloy which is cut off by melting at a specified temperature, and the fusible alloy includes either an Sn—Bi—In—Zn alloy or an Sn—In—Zn alloy. 
     In this battery, neither Pb nor Cd is contained in the fusible alloy of the thermal fuse unlike the prior art. Thus, it is possible to provide a battery capable of suppressing harm to the natural environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1A  is a top view of a thin-type thermal fuse according to a first embodiment of the invention, 
         FIG. 1B  is a section along I-I of  FIG. 1A , and 
         FIG. 1C  is a top view partly in section corresponding to  FIG. 1A . 
         FIG. 2A  is a top view of a thin-type thermal fuse according a second embodiment of the invention, and 
         FIG. 2B  is a section along II-II of  FIG. 2A . 
         FIG. 3A  is a top view of a thin-type thermal fuse according to a third embodiment of the invention, 
         FIG. 3B  is a section along III-III of  FIG. 3A , and 
         FIG. 3C  is a top view partly in section corresponding to  FIG. 3A . 
         FIG. 4  is a perspective view of a battery using the inventive thin-type thermal fuse. 
         FIG. 5  is a section of a radial-type thermal fuse according to a fourth embodiment of the invention. 
         FIG. 6  is a section of an axial-type thermal fuse according to a fifth embodiment of the invention. 
         FIG. 7  is a section of a prior art thermal fuse. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     First Embodiment  
       FIGS. 1A to 1C  are views showing the construction of a thin-type thermal fuse according to a first embodiment of the invention, wherein  FIG. 1A  is a top view,  FIG. 1B  is a section along I-I of  FIG. 1A , and  FIG. 1C  is a top view partly in section corresponding to  FIG. 1A . 
     As shown in  FIGS. 1A and 1B , the thin-type thermal fuse includes a pair of metal terminals  12 , a first insulating film  11  to which the pair of metal terminals  12  are attached, a second insulating film  14  attached to the first insulating film  11  in such a manner as to define a space with the first insulating film  11 , and a fusible alloy  13  arranged between the first and second insulating films  11 ,  14  and connected between the leading end portions of the pair of metal terminals  12 . 
     The insulating film  11  is a sheet-like insulating film having a monolayer structure, and the pair of metal terminals  12  narrower than the first insulating film  11  are attached to the first insulating film  11 . The fusible alloy  13  forms a thermal fuse element and bridges the leading end portions of the metal terminals  12  in the middle of the upper surface of the first insulating film  11 , thereby being located above the first insulating film  11  to connect the leading end portions of the metal terminals  12 . 
     Flux (not shown) including a resin obtained by adding a wax component containing amide stearate and the like and by adding an activator to rosin is coated around the fusible alloy  13 . The second insulating film  14  is a sheet-like insulating film having a monolayer structure, is located above the fusible alloy  13 , and is so sealed to the first insulating film  11  as to define an inner space with the first insulating film  11 . 
     In this way, the first and second insulating films  11 ,  14  enclose the fusible alloy  13 , and the outer peripheral portion of the first insulating film  11  and that of the second insulating film  14  are sealed and fixed to each other while leaving a portion where the fusible alloy  13  is provided unfixed, thereby sealing the fusible alloy  13  to prevent the deterioration of the fusible alloy  13 . 
     The thicknesses of the first and second insulating films  11 ,  14  are preferably 0.15 mm or smaller. If the thickness exceeds 0.15 mm, the thickness of the thermal fuse itself becomes thick, which is unsuitable for the thin-type thermal fuse. 
     Specific materials for the first and second insulating films  11 ,  14  may be a resin (preferably thermoplastic resin) containing any one of PET (polyethylene terephthalates), PEN (polyethylene naphthalates), ABS resin, SAN resin, polysulfone resin, polycarbonate resin, noryl, vinyl chloride resin, polyethylene resin, polyester resin, polypropylene resin, polyamide resin, PPS resin, polyacetal, fluororesin and polyesters as a main component. 
     Although the first and second insulating films  11 ,  14  have monolayer structures in this embodiment, the present invention is not particularly limited thereto. Sheets made of different materials may be laminated. For example, if the first and second insulating films  11 ,  14  are made of a film obtained by laminating a PET (polyethylene terephthalate) film and a PEN (polyethylene naphthalate) film, the strengths thereof can be increased, whereby mechanical strength can be improved. Further, in the case of fabricating the first and second insulating films  14  to have multilayer structures, a combination of a material having a low heat resistance and the one having a high heat resistance can be used in addition to the above combination of the materials. 
     If La denotes the length of a thermal-fuse main portion including the first and second insulating films  11 ,  14  and the fusible alloy  13 , (i.e. the length of the longer sides of the first and second insulating films  11 ,  14 ), a sufficient insulation distance cannot be ensured after having cut off by melting in the case that La is below 2.0 mm. On the other hand, in the case that La exceeds 7.5 mm, a necessary installation area increases if the thin-type thermal fuse is installed in a small-size battery. Thus, this is not practical. Accordingly, the length La of the thermal-fuse main portion is preferably 2.0 mm to 7.5 mm. 
     If Lb denotes the thickness from the outer surface of the first insulating film  11  to the outer surface of the second insulating film  14 , (i.e. the thickness from the lower surface of the first insulating film  11  to the upper surface of the second insulating film  14 ), a sufficient space for accommodating the fusible alloy  13  cannot be ensured in the case that Lb is below 0.4 mm. On the other hand, in the case that Lb exceeds 1.5 mm, the thickness of the thin-type thermal fuse becomes too thick in relation to projections, (e.g. those of the electrodes of the battery in which the thin-type thermal fuse is used), and this hinders the miniaturization of the battery. Accordingly, the thickness Lb from the lower surface of the first insulating film  11  to the upper surface of the second insulating film  14  is preferably 0.4 mm to 1.5 mm. 
     The pair of metal terminals  12  are strip-shaped or wire-shaped and mainly include for example nickel metal, nickel alloy like a copper nickel alloy, nickel or nickel alloy added with the other element(s), and the like. For instance, if the metal terminals  12  include a material whose nickel content is 98% or higher, its electrical resistivity is as low as 6.8×10 −8  Ω·m to 12×10 −8 Ω·m. Thus, reliability such as corrosion resistance can be astonishingly improved. 
     The thickness of the metal terminals  12  themselves is preferably 0.15 mm or smaller. This is because the thermal fuse becomes too thick and unsuitable for a thin-type thermal fuse if the thickness exceeds 0.15 mm. 
     The metal terminals  12  are preferably made of a material having Young&#39;s modulus of 3×10 10  Pa to 8×10 10  Pa and a tensile strength of 4×10 8  Pa to 6×10 8  Pa. In this case, there is no likelihood of inadvertently bending the metal terminals  12  during handling or transportation, it is easy to bend the terminals and occurrences of breakage and other undesirable event can be prevented during bending. 
     More specifically, if Young&#39;s modulus of the metal terminals  12  is below 3×10 10  Pa, it is too easy to bend the metal terminals  12  and, accordingly, portions (e.g. electrically connecting portions at the end portions of the metal terminals  12 ) of the metal terminals  12  which should not be bent are likely to become uneven, thereby causing a problem of making it difficult to electrically connect the metal terminals  12  with the fusible alloy  13  by welding. On the other hand, if Young&#39;s modulus exceeds 8×10 10  Pa, portions of the metal terminals desired to be bent are difficult to bend, or broken. Further, if the tensile strength of the metal terminals  12  is below 4×10 8  Pa, it is too easy to bend the metal terminals  12 . On the other hand, if the tensile strength exceeds 6×10 8  Pa, portions of the metal terminals  12  desired to be bent are difficult to bend, or broken. 
     The fusible alloy  13  can be produced by squeezing the wire-shaped fusible alloy having a circular cross section into a wire having a thickness of 0.4 mm or smaller and a rectangular or elliptical cross section and cutting the resulting wire to a suitable length. Die drawing, die extrusion or the like can be used as a method for producing the fusible alloy  13  into the wire. Further, the metal terminals  12  and the fusible alloy  13  can be connected by laser welding, heat welding, ultrasonic welding or the like. Particularly in the case of using laser welding, the fusible alloy  13  can be connected without damaging its unwelded portions since a heat-generating portion can be made smaller. 
     The fusible alloy  13  includes an Sn—In—Zn alloy or an Sn—Bi—In—Zn alloy. In this case, a thin-type thermal fuse having an operating temperature of 108° C. or lower can be provided. 
     The fusible alloy  13  preferably includes an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. In such a case, the melting point of the Sn—In—Zn alloy is about 107° C., and a difference between a solidus temperature and a liquidus temperature becomes smaller to reduce a temperature range of a solid-liquid mixture. Thus, it is possible to provide a thin-type thermal fuse having a small operating temperature variation at about 107° C. 
     Since the melting point can be decreased by adding Bi to the above Sn—In—Zn alloy and reducing the content of Sn, the fusible alloy  13  may include an Sn—Bi–In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. In such a case, it is possible to provide a thin-type thermal fuse having an arbitrary operating temperature of 85° C. to 107° C. Specifically, if the content of Bi exceeds 15 weight %, the melting point falls below 85° C. and the difference between the solidus temperature and the liquidus temperature becomes larger to increase an operating temperature variation, wherefore a practical thin-type thermal fuse cannot be provided. 
     If the content of Zn exceeds 5.0 weight % in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the viscosity of the melted fusible alloy  13  increases. Thus, a time required for the fusible alloy  13  to be cut off after being melted is likely to vary. Therefore, the composition ratio of Zn is preferably 5.0 weight % or smaller. 
     In the case that the flux coated on the fusible alloy  13  is melted, the composition ratios of In and Zn may decrease since, out of the metals constituting the fusible alloy  13 , In and Zn have a higher reactivity with the flux than other metals. Thus, in the case of using the thermal fuse at a temperature exceeding the melting point of the flux for a long time, it is necessary to set high composition ratios for these metals. Therefore, in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, it is preferable to set the content of In to 48 to 55 weight % and that of Zn to 1.3 to 5.0 weight %. 
     Further, if the composition ratio of Zn is 1.3 to 5.0 weight % in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the melting point becomes quite stable for the composition ratio of Zn. Thus, if the composition ratio of Zn is 1.3 to 5.0 weight %, a melting point variation caused by a composition variation becomes smaller when the fusible alloy  13  is produced. Therefore, it is possible to provide a thin-type thermal fuse having a highly precise cut-off temperature by melting. 
     If the thermal fuse is used at a temperature exceeding the melting point of the flux for a long time, it is necessary to set a high composition ratio for Zn as described above. In such a case, the composition ratio of Zn is preferably 2.0 to 5.0 weight %. 
     The flux to be coated on the fusible alloy  13  includes a resin obtained by adding a wax component containing amide stearate and the like and by adding an activator to rosin. This flux promotes the surface tension of the fusible alloy  13  by the action of rosin in the flux to rapidly fuse the fusible alloy  13 . 
     The melting point of the flux can be adjusted between about 50° C. and about 120° C. by adjusting the added amount of the wax component. Since the flux cannot fulfil its function of promoting the surface tension of the fusible alloy  13  unless being melted, the melting point of the flux is set to be lower than that of the fusible alloy  13  by adjusting the added amount of the wax component. It should be noted that the melting point of the flux is measured as a peak of a measurement result in a measurement by a differential scanning calorimeter (DSC). 
     As the activator to be added to the flux may be preferably used: a chloride activator for example, aniline hydrochloride, hydrazine hydrochloride, phenylhydrazine hydrochloride, tetrachloronaphthalene, methylhydrazine hydrochloride, methylamine hydrochloride, dimethylamine hydrochloride, ethylamine hydrochloride, diethylamine hydrochloride, butylamine hydrochloride, cyclohexylamine hydrochloride, diethylethanolamine hydrochloride, etc.; or a bromine activator for example, aniline bromide, hydrazine bromide, phenylhydrazine bromide, cetylpyridine bromide, methylhydrazine bromide, methylamine bromide, dimethylamine bromide, ethylamine bromide, diethylamine bromide, butylamine bromide, cyclohexylamine bromide, diethylethanolamine bromide, etc. 
     The chloride activator has a higher reactivity with In and Zn than the bromine activator and quickly decreases in quantity. Thus, in the case that the fusible alloy  13  includes the Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the effect of the activator can be kept for a long time even when the thermal fuse is used at a temperature exceeding the melting point of the flux if the bromine activator is used. 
     It is preferable to add about 0.1 to 3% of the activator in order to enhance the activity of rosin. In such a case, if the thin-type thermal fuse is cut off by melting, for example, on a condition of increasing the temperature by 1° C. per minute, the cut-off temperature by melting is reduced by about 1° C. to 2° C. Thus, it is possible to provide a thin-type thermal fuse having an excellent quick cut-off ability by melting. 
     If the flux is melted, the activator reacts with the metals in the fusible alloy  13  to gradually decrease its quantity. Accordingly, if the melting point of the flux is set at 60° C. or higher, the activator in the flux hardly decreases in quantity even if the thin-type thermal fuse is, for example, used in a packaged battery or the like at about 55° C. for a long time. Thus, it is possible to provide a thin-type thermal fuse having an excellent quick cut-off ability by melting. 
     Further, as shown in  FIG. 1C , metal layers  16  including Sn, Cu or a like metal having a good wettability to the fusible alloy  13  are provided on the upper surfaces of the leading end portions of the metal terminals  12  and connected with the fusible alloy  13 . In this case, transportations of the fusible alloy  13  toward the metal layers  16  after cut off by melting are accelerated since the wettability of Sn or Cu constituting the metal layer  16  to the fusible alloy  13  is better than that of nickel constituting the metal terminals  12 . As a result, the fusible alloy  13  can be quickly cut off after fusing. 
     Here, if S 1 , S 2  denote an area of each metal layer  16  at the opposite sides along a direction perpendicular to the longitudinal direction of the fusible alloy  13  and an area thereof at the outer side of the end of the fusible alloy  13 , the fusible alloy  13  is more quickly cut off as an amount of the melted fusible alloy  13  toward the areas S 2  increases. Therefore, a relationship of S 1 , S 2  is set preferably to be S 1 &lt;S 2 , and more preferably to be S 1 &lt;2S 2 . 
     A single metal of Cu, Sn, Bi or In or an alloy of these metals may be used as a material for the metal layers  16 . It is also preferable to use an alloy having the same composition as the fusible alloy  13  as a material for the metal layers  16 . In such a case, even if the metal constituting the metal layers  16  diffuses into the fusible alloy  13 , the melting point of the fusible alloy  13  does not change since this diffusion amount is tiny. 
     The thickness of the metal layers  16  is preferably 15 μm or smaller. If the thickness exceeds 15 μm, a larger amount of the metal constituting the metal layers  16  diffuses into the fusible alloy  13 , thereby changing the melting point of the fusible alloy  13  to cause an operating temperature variation of the thermal fuse. 
     As described above, in this embodiment, the fusible alloy  13  forming the thermal fuse element located above the first insulating film  11  and connected between the leading end portions of the pair of metal terminals  12  includes the Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn or the Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. Thus, neither Pb nor Cd is contained in the fusible alloy unlike the prior art, with the result that a thin-type thermal fuse having no harm to the natural environment can be provided. 
     Second Embodiment  
       FIGS. 2A and 2B  show the construction of a thin-type thermal fuse according to a second embodiment of the present invention, wherein  FIG. 2A  is a top view and  FIG. 2B  is a section along II-II of  FIG. 2A . The thin-type thermal fuse shown in  FIG. 2  differs from the one shown in  FIG. 1  in that a pair of metal terminals  12   a  are formed to have a strip shape and projections  15  are provided at parts of the metal terminals  12   a.  Since other points are the same as in the thin-type thermal fuse shown in  FIG. 1 , no detailed description is given thereon. 
     As shown in  FIGS. 2A and 2B , the round projections  15  are formed at ends of the metal terminals  12   a  extending outwardly from a first insulating film  11  and a second insulating film  14 . In this embodiment, effects similar to those of the first embodiment can be obtained. In addition, welding strength and welding positions can be stabilized to improve productivity since a welding current can be concentrated if a connection is made at the projections  15  in the case that the pair of metal terminals  12   a  and external wiring (not shown) are connected by electric welding. 
     Third Embodiment  
       FIGS. 3A to 3C  are views showing the construction of a thin-type thermal fuse according to a third embodiment of the invention, wherein  FIG. 3A  is a top view,  FIG. 3B  is a section along III-III of  FIG. 3A , and  FIG. 3C  is a top view partly in section corresponding to  FIG. 3A . 
     The thin-type thermal fuse shown in  FIG. 3  differs from the one shown in  FIG. 1  in that a pair of metal terminals  12   b  are attached to a first insulating film  11   a  such that end portions thereof partly project from the lower surface of the first insulating film  11  toward the upper surface thereof as shown in  FIGS. 3B and 3C . Since other points are the same as in the thin-type thermal fuse shown in  FIG. 1 , no detailed description is given thereon. 
     As shown in  FIGS. 3B and 3C , an inner end portion of each metal terminal  12   b  is bent to have a substantially wavelike shape, thereby forming a projection  15   a  at a part of the end portion of the metal terminal  12   b,  and the first insulating film  11   a  is formed with notches  17  where the respective projections  15   a  are to be mounted. By inserting the projections  15   a  into the notches  17 , the pair of metal terminals  12   b  are attached to the first insulating film  11   a  such that the projections  15   a  thereof project from the first insulating film  11   a  toward a second insulating film  14 . A fusible alloy  13  is connected to the upper surfaces of the projections  15   a.  In this way, the fusible alloy  13  forming a thermal fuse element is arranged between the first and second insulating films  11   a,    14  to be connected between the leading end portions of the pair of metal terminals  12   b.    
     As described above, in this embodiment, the positional relationship of the pair of metal terminals  12   b  and the first insulating film  11   a  can be determined by engaging the projections  15   a  with the notches  17 . Thus, the pair of metal terminals  12   b  can be precisely attached to the first insulating film  11   a  and the thin-type thermal fuse can be produced with high precision. 
     Further, in the thin-type thermal fuse of this embodiment as well, the fusible alloy  13  includes an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn or an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn similar to the thin-type thermal fuse of the first embodiment. Thus, neither Pb nor Cd is contained in the fusible alloy unlike the prior art. As a result, it is possible to provide a thin-type thermal fuse having no harm to the natural environment. 
     In this embodiment as well, the melting point of the flux applied to the fusible alloy  13  is set at 60° C. or higher similar to the first embodiment. Thus, even if the thermal fuse is used at about 55° C. for a long time, for example, by being used in a packaged battery, the activator in the flux hardly decreases in quantity, wherefore a thin-type thermal fuse having an excellent quick cut-off ability by melting can be provided. 
     Similar to the first embodiment, metal layers  16  comprising Sn, Cu or the like having a good wettability to the fusible alloy  13  are also provided on the upper surfaces of the leading end portions of the metal terminals  12   b  and are connected to the fusible alloy  13  as shown in  FIG. 3C  in this embodiment. In this case, since the wettability of Sn or Cu constituting the metal layers  16  to the fusible alloy  13  is better than that of nickel constituting the metal terminals  12   b,  the transportation of the fusible alloy  13  after cut off by melting to the metal layers  16  is accelerated, with the result that the fusible alloy  13  can be quickly cut off. 
     Similar to the first embodiment, the fusible alloy  13  is more quickly cut off as an amount of the melted fusible alloy  13  toward areas S 2  increases in this embodiment if S 1 , S 2  denote an area of each metal layer  16  at the opposite sides along a direction perpendicular to the longitudinal direction of the fusible alloy  13  and an area thereof at the outer side of the end of the fusible alloy  13 . Therefore, it is preferable to set a relationship of S 1 , S 2  to be S 1 &lt;S 2 , more preferably to be S 1 &lt;2S 2 . 
       FIG. 4  is a perspective view of a battery using an inventive thermal fuse. As shown in  FIG. 4 , a thermal fuse  22  is mounted on one side surface at a longer side of a battery main body  21 . The thermal fuse  22  is electrically connected so as to shut off a current upon abnormal heat generation of the battery main body  21 , so that the current is shut off if the heat generated from the battery main body  21  reaches a specified level or higher. Any of the above thin-type thermal fuses can be used as the thermal fuse  22 . Either a radial-type thermal fuse or an axial-type thermal fuse to be described later may also be used. An external electrode  23  of the battery main body  21  is provided on one side surface at a shorter side of the battery main body  21 , and one terminal  25  of the thermal fuse  22  and the external electrode  23  are electrically connected at a connecting portion  26  by means of spot welding or the like. 
     Further, a protecting circuit  24  and the battery main body  21  are electrically connected, and the protecting circuit  24  and an other electrode  27  of the thermal fuse  22  are electrically connected at a connecting portion  28  by means of spot welding or the like. Parts of the protecting circuit  24  are mounted on the protecting circuit  24  by Pb-free solder such as Sn—Ag based solder or Sn—Cu based solder. 
     In the above battery, any of the thin-type thermal fuses according to the first to third embodiments is used as the thermal fuse  22  and, in this thin-type thermal fuse, the fusible alloy  13  constituting the thermal fuse element includes an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn or an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; and a remainder of Sn similar to the thin-type thermal fuse of the first embodiment. Thus, neither Pb nor Cd is contained in the fusible alloy unlike the prior art. As a result, it is possible to provide a thin-type thermal fuse which is not harmful to the natural environment. 
     Further, in the above battery, any of the thin-type thermal fuses according to the first to third embodiments is used as the thermal fuse  22  and, in these thin-type thermal fuses, the melting point of the flux coated on the fusible alloy  13  is set at 60° C. or higher. Thus, even if the thermal fuse is used at about 55° C. for a long time, for example, by being used in a packaged battery, the activator in the flux hardly decreases in quantity, wherefore a thin-type thermal fuse having an excellent quick cut-off ability by melting can be provided. 
     Fourth Embodiment  
       FIG. 5  is a section of a radial-type thermal fuse according to a fourth embodiment of the present invention. In  FIG. 5 , an insulating casing  31  is in a form of: a cylindrical tube having a bottom and having an opening; or a rectangular tube having a bottom and having an opening, and comprising any of PBT (polybutylene terephthalate), PPS (polyphenyl sulphide), PET (polyethylene terephthalate), phenol resin, ceramic, glass and like materials. 
     The fusible alloy  32  is substantially in a form of a cylinder or a rectangular column arranged in the insulating casing  31  and including an Sn—In—Zn alloy or an Sn—Bi—In—Zn alloy. Thus, a radial-type thermal fuse free from Pb and Cd and having an operating temperature of 108° C. or lower can be provided. 
     The fusible alloy  32  preferably includes an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. In such a case, the melting point of the Sn—In—Zn alloy is about 107° C., and a difference between a solidus temperature and a liquidus temperature becomes smaller so as to reduce a temperature range of a solid-liquid mixture. Thus, it is possible to provide a radial-type thermal fuse having a small operating temperature variation at about 107° C. 
     Since the melting point can be decreased by adding Bi to the above Sn—In—Zn alloy and reducing the content of Sn, the fusible alloy  32  may include an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. In such a case, it is possible to provide a radial-type thermal fuse having an arbitrary operating temperature of 85° C. to 107° C. Specifically, if the content of Bi exceeds 15 weight %, the melting point falls below 85° C. and the difference between the solidus temperature and the liquidus temperature becomes larger so as to increase an operating temperature variation, wherefore a practical radial-type thermal fuse cannot be provided. 
     If the content of Zn exceeds 5.0 weight % in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the viscosity of the melted fusible alloy  32  increases. Thus, a time required for the fusible alloy  32  to be cut off after being melted is likely to vary. Therefore, the composition ratio of Zn is preferably 5.0 weight % or smaller. 
     Flux (not shown) including a resin obtained by adding a wax component containing amide stearate and the like and by adding an activator to rosin is coated around the fusible alloy  32 . This flux acts to remove an oxide film of the fusible alloy  32  by being melted when the ambient temperature increases. The flux also acts to promote the surface tension of the fusible alloy  32  by the action of rosin in the flux to rapidly be cut off the fusible alloy  13 . 
     The melting point of the flux can be adjusted between about 50° C. and about 120° C. by adjusting the added amount of the wax component. Since the flux cannot fulfil its function of promoting the surface tension of the fusible alloy  32  unless being melted, the melting point of the flux is set to be lower than that of the fusible alloy  32  by adjusting the added amount of the wax component. It should be noted that the melting point of the flux is measured as a peak of a measurement result in a measurement by a differential scanning calorimeter (DSC). 
     As the activator to be added to the flux may be preferably used: a chloride activator for example, aniline hydrochloride, hydrazine hydrochloride, phenylhydrazine hydrochloride, tetrachloronaphthalene, methylhydrazine hydrochloride, methylamine hydrochloride, dimethylamine hydrochloride, ethylamine hydrochloride, diethylamine hydrochloride, butylamine hydrochloride, cyclohexylamine hydrochloride, diethylethanolamine hydrochloride, etc.; or a bromine activator for example, aniline bromide, hydrazine bromide, phenylhydrazine bromide, cetylpyridine bromide, methylhydrazine bromide, methylamine bromide, dimethylamine bromide, ethylamine bromide, diethylamine bromide, butylamine bromide, cyclohexylamine bromide, diethylethanolamine bromide, etc. 
     The chloride activator has a higher reactivity with In and Zn than the bromine activator and quickly decreases in quantity. Thus, in the case that the fusible alloy  32  includes the Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the effect of the activator can be kept for a long time even when the radial-type thermal fuse is used at a temperature exceeding the melting point of the flux if the bromine activator is used. 
     It is preferable to add about 0.1 to 3% of the activator in order to enhance the active force of rosin. In such a case, if the radial-type thermal fuse is cut off by melting, for example, on a condition of increasing the temperature by 1° C. per minute, the cut-off temperature by melting is reduced by about 1° C. to 2° C. Thus, it is possible to provide a radial-type thermal fuse having an excellent quick cut-off ability by melting. 
     If the flux is melted, the activator reacts with the metals in the fusible alloy  32  to gradually decrease its quantity. Accordingly, if the melting point of the flux is set at 60° C. or higher, the activator in the flux hardly decreases in quantity even if the radial-type thermal fuse is, for example, used in a packaged battery or the like at about 55° C. for a long time. Thus, it is possible to provide a radial-type thermal fuse having an excellent quick cut-off ability by melting. 
     If the flux is melted, the composition ratios of In and Zn may decrease since, out of the metals constituting the fusible alloy  32 , In and Zn have a higher reactivity with the flux than other metals. Thus, in the case of using the thermal fuse at a temperature exceeding the melting point of the flux for a long time, it is necessary to set high composition ratios for these metals. Therefore, in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, it is preferable to set the content of In to 48 to 55 weight % and that of Zn to 1.3 to 5.0 weight %. 
     Further, in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the melting point becomes quite stable for the composition ratio of Zn if the composition ratio of Zn is 1.3 to 5.0 weight %. Thus, if the composition ratio of Zn is 1.3 to 5.0 weight %, a melting point variation caused by a composition variation becomes smaller when the fusible alloy  32  is produced. Therefore, it is possible to provide a radial-type thermal fuse having a highly precise cut-off temperature by melting. 
     If the thermal fuse is used at a temperature exceeding the melting point of the flux for a long time, it is necessary to set a high composition ratio for Zn as described above. In such a case, the composition ratio of Zn is preferably 2.0 to 5.0 weight %. 
     One end portion of each lead conductor  33  is connected to a corresponding end portion of the fusible alloy  32 , whereas the other end portion of each lead conductor  33  protrudes through the opening of the insulating casing  31  to the outside of the insulating casing  31 . The lead conductors  33  are wires comprising a single metal such as Cu, Fe or Ni or an alloy of these metals, and metal plating of any one of Sn, Zn, Bi, In, Ag and Cu or an alloy containing these metals is applied to the outer surfaces of the lead conductors  33 . 
     A sealing element  34  comprises a hard resin such as an epoxy or a silicone for sealing the opening of the insulating casing  31 . The fusible alloy  32  and a pair of lead conductors  33  can be connected by welding or ultrasonic welding or by applying a power to the lead conductors  33  and the fusible alloy  32  to melt the fusible alloy  32 . 
     As described above, in the radial-type thermal fuse of this embodiment, the fusible alloy  32  constituting the thermal fuse element includes an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn or an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. Thus, neither Pb nor Cd is contained in the fusible alloy unlike the prior art. Therefore, it is possible to provide a radial-type thermal fuse having no harm to the natural environment. Further, since a pair of lead conductors  33  connected to the fusible alloy  32  at one end portion are protruded through the opening of the insulating casing  31  outside the insulating casing  31  so as to extend in parallel with each other, a degree of freedom in mounting this radial-type thermal fuse on a battery or the like can be improved. 
     Fifth Embodiment  
       FIG. 6  is a section of an axial-type thermal fuse according to a fifth embodiment of the present invention. In  FIG. 6 , an insulating casing  41  is in a form of a cylindrical tube having openings at opposite ends, and comprises any of PBT (polybutylene terephthalate), PPS (polyphenyl sulphide), PET (polyethylene terephthalate), phenol resin, ceramic, glass and like materials. 
     The fusible alloy  42  is substantially in a form of a cylinder or a rectangular column arranged in the insulating casing  41  and includes an Sn—In—Zn alloy or an Sn—Bi—In—Zn alloy. Thus, an axial-type thermal fuse free from Pb and Cd and having an operating temperature of 108° C. or lower can be provided. 
     The fusible alloy  42  preferably includes an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. In such a case, the melting point of the Sn—In—Zn alloy is about 107° C., and a difference between a solidus temperature and a liquidus temperature becomes smaller to reduce a temperature range of a solid-liquid mixture. Thus, it is possible to provide an axial-type thermal fuse having a small operating temperature variation at about 107° C. 
     Since the melting point can be decreased by adding Bi to the above Sn—In—Zn alloy and reducing the content of Sn, the fusible alloy  42  may include an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. In such a case, it is possible to provide an axial-type thermal fuse having an arbitrary operating temperature of 85° C. to 107° C. Specifically, if the content of Bi exceeds 15 weight %, the melting point falls below 85° C. and the difference between the solidus temperature and the liquidus temperature becomes larger to increase an operating temperature variation, wherefore a practical axial-type thermal fuse cannot be provided. 
     If the content of Zn exceeds 5.0 weight % in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the viscosity of the melted fusible alloy  42  increases. Thus, a time required for the fusible alloy  42  to be cut off after being melted is likely to vary. Therefore, the composition ratio of Zn is preferably 5.0 weight % or smaller. 
     Flux (not shown) including a resin obtained by adding a wax component containing amide stearate and the like and by adding an activator to rosin is coated around the fusible alloy  42 . This flux acts to remove an oxide film of the fusible alloy  42  by being melted when the ambient temperature increases. The flux also acts to promote the surface tension of the fusible alloy  42  by the action of rosin in the flux to rapidly fuse the fusible alloy  13 . 
     The melting point of the flux can be adjusted between about 50° C. and about 120° C. by adjusting the added amount of the wax component. Since the flux cannot fulfil its function of promoting the surface tension of the fusible alloy  42  unless being melted, the melting point of the flux is set to be lower than that of the fusible alloy  42  by adjusting the added amount of the wax component. It should be noted that the melting point of the flux is measured as a peak of a measurement result in a measurement by a differential scanning calorimeter (DSC). 
     As the activator to be added to the flux may be preferably used: a chloride activator for example, aniline hydrochloride, hydrazine hydrochloride, phenylhydrazine hydrochloride, tetrachloronaphthalene, methylhydrazine hydrochloride, methylamine hydrochloride, dimethylamine hydrochloride, ethylamine hydrochloride, diethylamine hydrochloride, butylamine hydrochloride, cyclohexylamine hydrochloride, diethylethanolamine hydrochloride, etc.; or a bromine activator for example, aniline bromide, hydrazine bromide, phenylhydrazine bromide, cetylpyridine bromide, methylhydrazine bromide, methylamine bromide, dimethylamine bromide, ethylamine bromide, diethylamine bromide, butylamine bromide, cyclohexylamine bromide, diethylethanolamine bromide, etc. 
     The chloride activator has a higher reactivity with In and Zn than the bromine activator and quickly decreases in quantity. Thus, in the case that the fusible alloy  42  includes the Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the effect of the activator can be kept for a long time even when the axial-type thermal fuse is used at a temperature exceeding the melting point of the flux if the bromine activator is used. 
     It is preferable to add about 0.1 to 3% of the activator in order to enhance the activity of rosin. In such a case, if the axial-type thermal fuse is cut off by melting, for example, on a condition of increasing the temperature by 1° C. per minute, the cut-off temperature by melting is reduced by about 1° C. to 2° C. Thus, it is possible to provide an axial-type thermal fuse having an excellent quick cut-off ability by melting. 
     If the flux is melted, the activator reacts with the metals in the fusible alloy  42  to gradually decrease its quantity. Accordingly, if the melting point of the flux is set at 60° C. or higher, the activator in the flux hardly decreases in quantity even if the axial-type thermal fuse is, for example, used in a packaged battery or the like at about 55° C. for a long time. Thus, it is possible to provide a axial-type thermal fuse having an excellent quick cut-off ability by melting. 
     If the flux is melted, the composition ratios of In and Zn may decrease since, out of the metals constituting the fusible alloy  42 , In and Zn have a higher reactivity with the flux than other metals. Thus, in the case of using the thermal fuse at a temperature exceeding the melting point of the flux for a long time, it is necessary to set high composition ratios for these metals. Therefore, in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, it is preferable to set the content of In to 48 to 55 weight % and that of Zn to 1.3 to 5.0 weight %. 
     Further, in the above Sn—In—Zn alloy or Sn—Bi—In—Zn alloy, the melting point becomes quite stable for the composition ratio of Zn if the composition ratio of Zn is 1.3 to 5.0 weight %. Thus, if the composition ratio of Zn is 1.3 to 5.0 weight %, a melting point variation caused by a composition variation becomes smaller when the fusible alloy  42  is produced. Therefore, it is possible to provide an axial-type thermal fuse having a highly precise cut-off temperature by melting. 
     If the thermal fuse is used at a temperature exceeding the melting point of the flux for a long time, it is necessary to set a high composition ratio for Zn as described above. In such a case, the composition ratio of Zn is preferably 2.0 to 5.0 weight %. 
     One end portion of each lead conductor  43  is connected with a corresponding end portion of the fusible alloy  42 , whereas the other end portion thereof is protruded through the corresponding opening of the insulating casing  41  to the outside of the insulating casing  41 . The lead conductors  43  are wires comprising a single metal such as Cu, Fe or Ni or an alloy of these metals, and metal plating of any one of Sn, Zn, Bi, In, Ag and Cu or an alloy containing these metals is applied to the outer surfaces of the lead conductors  43 . 
     Sealing elements  44  comprise a hard resin such as an epoxy or a silicone for sealing the openings at the opposite ends of the insulating casing  41 . The fusible alloy  42  and a pair of lead conductors  43  can be connected by welding or ultrasonic welding or by applying a power to the lead conductors  43  and the fusible alloy  42  to fuse the fusible alloy  42 . 
     As described above, in the axial-type thermal fuse of this embodiment, the fusible alloy  42  constituting the thermal fuse element includes an Sn—In—Zn alloy containing: 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn or an Sn—Bi—In—Zn alloy containing: 0.5 to 15 weight % of Bi; 45 to 55 weight % of In; and 0.5 to 5.0 weight % of Zn; with the balance being Sn. Thus, neither Pb nor Cd is contained in the fusible alloy unlike the prior art. Therefore, it is possible to provide an axial-type thermal fuse which is not harmful to the natural environment. Further, since the insulating casing  41  in which the fusible alloy  42  is provided is in the form of a cylindrical tube having the openings at the opposite ends, there is no directionality upon mounting this axial-type thermal fuse on a battery or the like, wherefore the axial-type thermal fuse can be easily handled at the time of production. 
     An electrical device to which the inventive thermal fuses are applied is not particularly limited to the above battery, and the inventive thermal fuses are similarly applicable to other electrical devices to obtain similar effects. Further, characterizing portions of the respective embodiments can be arbitrarily combined. In such a case, the functions and effects of the characterizing portions can be fulfilled. 
     INDUSTRIAL APPLICABILITY  
     As described above, since the thermal fuse element includes an Sn—Bi—In—Zn alloy or an Sn—In—Zn alloy according to the present invention, neither Pb nor Cd is contained in the thermal fuse element unlike the prior art. Thus, harm to the natural environment can be suppressed and the present invention can be suitably applied to a thermal fuse element, a thermal fuse, a battery using such a thermal fuse, etc.