Thermal fuse

Abstract

To provide a thermosensitive pellet type thermal fuse having a low risk of a short circuit or the like of a circuit caused by tin plating applied to a lead wire even when placed in a high temperature environment.SOLUTION: A case having an internal space; two lead wires provided at respective ends of said case and connected to a circuit; and a thermal pellet and an electrical contact provided in said internal space, said thermal pellet melting at a predetermined operating temperature or higher; A temperature fuse configured such that the electric contact is disconnected to interrupt a circuit, wherein at least one of the two lead wires includes a core material formed of copper or a copper alloy and a covering portion covering the core material, and the covering portion includes a tin layer formed as an outermost layer and a copper-tin alloy layer formed between the tin layer and the core material.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a thermal fuse using a thermal pellet. [Background technology]

[0002] Conventionally, a known type of fuse is mounted in home appliances and the like and detects abnormal overheating of the appliance due to a malfunction, a short circuit in the internal circuit, etc., and cuts off the circuit (see, for example, Patent Document 1). This thermal fuse contains a thermal pellet in a case, and is configured so that when the thermal pellet melts above a predetermined operating temperature, the internal electrical contacts are separated and the circuit is cut off.

[0003] In such thermal fuses, tin-plated copper wire is used as the lead wire connected to the circuit, and the surface of the copper wire is coated with tin, which improves solderability, crimping bondability, oxidation resistance, and corrosion resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-168258 Summary of the Invention [Problem to be solved by the invention]

[0005] However, if a thermal fuse using tin-plated copper wire as the lead wire is used in a high-temperature environment, for example, above the melting point of tin, the tin plating on the lead wire may melt and flow onto the circuit during use (including while the fuse is waiting to operate and during operation) or after operation, causing an accident such as a short circuit.

[0006] The present invention has been made to solve the above problems, and its main object is to provide a thermal pellet type thermal fuse that has a low risk of circuit shorts and the like caused by melting of the tin plating on the lead wires, even when placed in a high-temperature environment. [Means for solving the problem]

[0007] In other words, the thermal fuse of the present invention comprises a case having an internal space, two lead wires, one at each end of the case and connected to a circuit, and a thermal pellet and electrical contacts provided in the internal space, and is configured so that the thermal pellet melts at a predetermined operating temperature or higher, causing the electrical contacts to disconnect and interrupt the circuit, and at least one of the two lead wires comprises a core made of copper or a copper alloy and a coating portion covering the core material, and the coating portion has a tin layer formed as the outermost layer and a copper-tin alloy layer formed between the tin layer and the core material.

[0008] In a thermal fuse configured in this manner, the coating is made up of a tin layer and a copper-tin alloy layer, so the thickness of the tin layer can be made thinner than in a lead wire whose coating is made up of only a tin layer. This reduces the amount of tin that melts and flows when placed in an environment with a temperature higher than the melting point of tin, thereby reducing the risk of short circuits and the like. Furthermore, since a tin layer is provided on the outermost layer of the lead wire, the contact resistance can be made smaller than when the outermost layer of the lead wire is made of a copper-tin alloy layer.

[0009] In a specific embodiment of the present invention, the melting point of the copper-tin alloy layer is preferably higher than the melting point of tin alone (232°C or higher). From the viewpoint of manufacturing, the melting point is preferably 676°C or lower.

[0010] Furthermore, in order to suppress the melt flow of the copper-tin alloy layer at least at the melting point of tin, the copper-tin alloy layer preferably contains an intermetallic compound of copper and tin. More preferably, this intermetallic compound is formed by combining the copper component derived from the core material with the tin component derived from the tin layer. For example, the copper-tin alloy layer contains one or more intermetallic compounds selected from Cu6Sn5 and Cu3Sn. Furthermore, the copper-tin alloy layer preferably contains a total of 50 mass% or more of Cu6Sn5 and Cu3Sn, preferably a total of 80 mass% or more. In this case, since the copper-tin alloy layer contains intermetallic compounds such as Cu6Sn5 (melting point 415°C) and Cu3Sn (melting point 676°C), which have higher melting points than tin alone, it is possible to prevent the copper-tin alloy layer from melting and flowing together with the tin layer at the melting point of tin. Furthermore, if the copper-tin alloy layer is formed by combining the copper component of the core material (e.g., copper wire) and the tin component of the tin layer (e.g., tin plating), it can be easily formed, for example, by subjecting the tin-plated copper wire to heat treatment to diffuse the copper and tin at the interface between the copper wire and the tin plating.

[0011] In order to suppress the occurrence of pinholes and to exhibit sufficient oxidation resistance and corrosion resistance, the total thickness of the tin layer and the copper-tin alloy layer is preferably 3 μm or more. If the volume of the tin layer contained in the lead wire is large, the amount of tin that melts and flows in a high-temperature environment increases, so the thickness of the coating is preferably 10 μm or less, and more preferably 6 μm or less.

[0012] If the tin layer is thin, copper from the copper-tin alloy layer will disperse into the tin layer, forming an alloy such as Cu3Sn in the outermost layer of the lead wire (at the surface of the tin layer), and if the tin layer is thin, the alloy layer will extend to the surface, resulting in poor solderability. Therefore, the thickness of the copper-tin alloy layer is set to 50% or less, preferably 30% or less, of the thickness of the coating portion. In this case, the tin layer is thick enough to prevent copper oxide, which has high contact resistance, from forming on the outermost layer of the lead wire, and it is possible to maintain solderability and crimping processability.

[0013] The covering portion may further include a nickel layer formed between the core material and the copper-tin alloy layer. This limits the amount of copper that diffuses from the base material, the core material, or the copper-tin alloy layer into the tin layer, preventing the formation of a copper-tin alloy with high contact resistance in the outermost layer of the lead wire.

[0014] The lead wire may be: a first lead wire that is inserted into a through hole formed at one end of the case and has a fixed contact at its tip located in the internal space; and a second lead wire that is electrically connected to the other end of the case, the thermal pellet is disposed on the second lead wire side in the internal space, The one lead wire is preferably the second lead wire. While the first lead wire, which has a fixed contact, could be plated with silver, which is resistant to oxidation, this is not necessary for the second lead wire, which has no contact, and therefore can be plated with less expensive tin. In this case, the operating temperature and heat resistance temperature of the thermal fuse would be limited to the melting point of tin (which is lower than the melting point of silver). However, with the present invention, by providing the second lead wire with a copper-tin alloy layer, the operating temperature and heat resistance temperature after operation can be raised above the melting point of tin. This also improves solderability or crimpability when connecting to the other end of the case. [Effects of the Invention]

[0015] According to the present invention configured in this manner, it is possible to provide a thermal pellet type thermal fuse that has a low risk of circuit shorts and the like caused by the tin plating applied to the lead wire, even when placed in a high-temperature environment. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a perspective view schematically showing a thermal fuse according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view schematically showing the internal structure of the thermal fuse of the embodiment. [Figure 3]3A to 3C are diagrams illustrating the operation of the thermal fuse of the embodiment. [Figure 4] FIG. 4 is an enlarged view of a second lead wire according to the embodiment. [Figure 5] FIG. 4 is an enlarged cross-sectional view of a second lead wire according to the embodiment. [Figure 6] FIG. 2 is a diagram showing the manufacturing conditions of each sample in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0017] A thermal fuse 100 according to one embodiment of the present invention will be described below with reference to the drawings.

[0018] <Summary> The thermal fuse 100 of this embodiment is mounted in home appliances, automobiles, office equipment, etc., and detects abnormal overheating due to equipment malfunction, a short circuit in the internal circuit, etc., and cuts off the circuit. The thermal fuse 100 of this embodiment is a so-called thermal pellet type that includes a case with an internal space, two lead wires, one on each end of the case and connected to the circuit, a thermal pellet and electrical contacts provided in the internal space, and is configured so that the thermal pellet melts at a predetermined operating temperature or higher, cutting off the electrical contacts and cutting off the circuit.

[0019] <Configuration of Thermal Fuse 100> Specifically, as shown in FIGS. 1 and 2, the thermal fuse 100 comprises a substantially cylindrical case 1 having an internal space 1s, a first lead wire 2 inserted into a through hole 11o formed in one end 11 of the case 1 and having a fixed contact 21 at its tip located in the internal space 1s, a second lead wire 3 electrically connected to the other end 12 of the case 1, a movable contact 41 facing the fixed contact 21 of the first lead wire 2 and a contact end 42 in contact with the inner wall surface of the case 1, and the contact end 42 is in contact with the inner wall surface of the case 1 when the contact end 42 is in contact with the inner wall surface of the case 1. The contactor comprises a movable electrode 4 arranged in the internal space 1s so as to be slidable along the axial direction, a thermal pellet 5 arranged in the internal space 1s closer to the second lead wire 3 than the movable electrode 4, a first spring element 6 arranged in a compressed state between the thermal pellet 5 and the movable electrode 4 and biasing the movable electrode 4 in a direction to bring the movable contact 41 closer to the fixed contact 21, and a second spring element 7 arranged in a compressed state on the opposite side of the first spring element 6 across the movable electrode 4 and biasing the movable electrode 4 in a direction to move the movable contact 41 away from the fixed contact 21. The fixed contact 21 and the movable contact 41 correspond to electrical contacts in the claims and are used to energize (or cut off) the first lead wire 2 and the second lead wire 3.

[0020] 3(a), in a normal state where the temperature of the thermal pellet 5 is below the operating temperature (a state where the thermal pellet 5 is not melted), the thermal fuse 100 is configured such that the extension force of the first spring element 6 is greater than the extension force of the second spring element 7, and the movable contact 41 is pressed against the fixed contact 21. In this normal state, current flows through the first lead wire 2, movable electrode 4, case 1, and second lead wire 3 in this order (or in the reverse order).

[0021] On the other hand, when the ambient temperature rises and the temperature of the thermal pellet 5 reaches the operating temperature, the thermal pellet 5 melts, causing the first spring element 6 and the second spring element 7 to extend (operating state), as shown in Fig. 3(b). The extension force of the second spring element 7 gradually exceeds the extension force of the first spring element 6, causing the movable contact 41 to separate from the fixed contact 21 and breaking the circuit. Fig. 3(c) shows the state in which the thermal pellet 5 is completely melted (completely melted state).

[0022] The configuration of each member will be described below. Case 1 is made of a metal material such as pure copper or a copper alloy containing zinc, and has good electrical and thermal conductivity, and its surface may be coated with a highly conductive coating such as silver plating. Through holes 11o and 12o are formed at both ends 11 and 12 of case 1 that intersect with the axial direction, and through which first lead wire 2 and second lead wire 3 are inserted, respectively.

[0023] An insulating tube 9 is provided near one end 11 in the internal space 1s of the case 1 to fix and hold the inserted first lead wire 2. This insulating tube 9 is made of, for example, ceramics and is also called a porcelain tube. This insulating tube 9 is housed in the internal space 1s so that its tube axis coincides with the central axis of the case 1.

[0024] The first lead wire 2 is a copper wire with a silver coating formed on its surface by plating. By applying silver plating, which is resistant to oxidation, the electrical conductivity of the fixed contact 21 is ensured.

[0025] The first lead wire 2 is inserted through a through hole 11o formed in one end 11 of the case 1, and is also inserted through an insulating tube 9 in the internal space 1s, with its outer surface being held by the wall of the insulating tube 9. The aforementioned fixed contact 21 is provided at the tip of the first lead wire 2 that is exposed from inside the insulating tube 9. With the first lead wire 2 inserted, the through hole 11o formed in the one end 11 of the case 1 is airtightly sealed from the outside of the internal space 1s by a sealant 8 made of, for example, an organic adhesive.

[0026] 4, the second lead wire 3 is a copper wire having a tin coating formed on its surface by plating. The second lead wire 3 is inserted into a through hole 12o in the other end 12 of the case 1, and an end of the second lead wire 3 is fixed to and electrically connected to the other end 12.

[0027] In a normal state, the movable electrode 4 is in contact with both the first lead wire 2 and the case 1, electrically connecting them. The movable electrode 4 is made of a metal such as a silver alloy and is roughly bowl-shaped. Specifically, the movable electrode 4 has a disk portion 4a that faces the tip of the first lead wire 2 and functions as the aforementioned movable contact 41, and a curved portion 4b that extends from the outer edge of the disk portion 4a while curving toward the axial direction of the case 1 and comes into contact with the inner wall surface of the case 1.

[0028] The thermal pellet 5 is made of a known organic material that melts at a predetermined operating temperature or higher. For example, a 97Bi-3Ag alloy (melting temperature 268°C) that melts at a higher temperature than the melting point of tin (232°C) may be used as the thermal pellet. For example, a columnar pellet made of an organic material such as a thermoplastic resin, or an organic chemical tablet, can be suitably used as the thermosensitive pellet 5. The thermosensitive pellet 5 is not limited to an organic material, and may be made of an insulating material that melts above a predetermined operating temperature. The thermosensitive pellet 5 has a substantially cylindrical shape and is disposed at the end of the internal space 1s on the second lead wire 3 side, with its central axis coinciding with the central axis of the case 1.

[0029] The first spring element 6 and the second spring element 7 are both compression coil springs. These spring elements are arranged with the movable electrode 4 in between, with their central axes aligned and coinciding with the central axis of the case 1. The first spring element 6 is sandwiched between the movable electrode 4 and the thermal pellet 5 via disk-shaped retaining plates 61 and 62.

[0030] The thermal fuse 100 of this embodiment is configured as follows so as to reduce the amount of tin that melts and flows from the second lead wire 3 when placed in a temperature environment above the melting point of tin. 5, the second lead wire 3 of this embodiment includes a core material X that forms the core portion of the second lead wire 3, and a coating Y that tightly covers the outer periphery of the core material X, with the coating Y having a tin layer TL formed as the outermost layer and a copper-tin alloy layer AL formed between the tin layer TL and the core material X. Fig. 5 is an enlarged view of the area indicated by the symbol A in Fig. 4.

[0031] The core material X is linear and functions as a conductor of the second lead wire. The core material X here is pure copper containing copper as the main component (containing 99.90 mass% or more of copper), such as oxygen-free copper (C1020), tough pitch copper (C1100), or phosphorus-deoxidized copper (C1201).

[0032] The coating Y is a protective coating that protects the core X from oxidation and corrosion, and in order to fully exert this function, the thickness of the coating Y is preferably 3 μm or more. Also, the thickness is preferably 15 μm or less, more preferably 10 μm or less or 6 μm or less.

[0033] The tin layer TL is a film-like layer that forms the surface of the second lead wire 3. The tin layer TL is mainly composed of tin, and more specifically, contains more than 50 mass% tin, such as pure tin. The tin layer TL here is tin plating formed by plating the copper wire described above. The surface portion of the tin layer TL preferably contains 90 mass% or more tin, more preferably 99 mass% or more tin.

[0034] The copper-tin alloy layer AL is in the form of a film that covers the outer periphery of the core material X, and the outer periphery of the film is further covered with a tin layer TL.

[0035] The copper-tin alloy layer AL contains an alloy consisting of the copper component derived from the copper wire (core material) and the tin component derived from the tin plating (tin layer), and here, the alloy includes an intermetallic compound of copper and tin. The intermetallic compound has a crystal structure based on the atomic ratio of the bonded metal elements. For example, the copper-tin alloy layer contains one or more intermetallic compounds selected from Cu6Sn5 and Cu3Sn. The copper-tin alloy layer AL here contains 50 mass% or more, preferably 80 mass% or more, of Cu6Sn5 and Cu3Sn combined. The content of Cu3Sn is preferably greater than the content of Cu6Sn5. The content of each metal here is measured by cross-sectional observation using SEM-EDX or quantitative elemental analysis in the depth direction using XPS. The ratio of the intermetallic compounds may be determined based on XPS (X-ray photoelectron spectroscopy).

[0036] The thickness of the copper-tin alloy layer AL is 50% or less, preferably 30% or less, of the thickness of the covering portion Y. Here, the thickness is measured by, for example, cross-sectional observation using an SEM. The thickness of the copper-tin alloy layer AL is at least 0.5 μm or more, preferably 1 μm or more.

[0037] The melting point of the copper-tin alloy layer is preferably higher than the melting point of tin alone (232° C. or higher).

[0038] <Method of manufacturing the thermal fuse 100 and the lead wire 3> A method for manufacturing the thermal fuse 100 and the second lead wire 3 will now be described.

[0039] (1) Tin plating First, the copper wire is tin-plated to deposit tin on the surface of the copper wire to form a tin coating. Here, a tin coating with a thickness of about 10 μm is formed using a so-called electroplating method. Tin plating can also be performed using other conventional methods, such as hot-dip plating.

[0040] (2) Heat treatment Next, the tin-plated copper wire is heated (hereinafter simply referred to as heat treatment) in, for example, an electric furnace. This causes diffusion of copper and tin at the interface between the copper wire substrate and the tin coating, and a copper-tin alloy layer AL containing a copper-tin alloy such as an intermetallic compound of copper and tin is formed at the interface. The thickness of the copper-tin alloy layer AL formed is adjusted, for example, by the heating time and heating temperature. Note that such heat treatment may be performed using various heating means other than an electric furnace.

[0041] The specific heat treatment conditions are as follows: (2-1) Heat treatment after electroplating Heat treatment may be performed at a temperature above the melting point of tin (232°C) for a short period of time (for example, several minutes or several tens of seconds), or at a temperature below the melting point of tin (for example, about 180°C to 231°C) for about 1 hour to 10 hours. (2-2) Heat treatment after hot dip plating The above-mentioned tin plating and heat treatment may be performed simultaneously by immersing the copper wire in molten tin heated to a temperature equal to or higher than the melting point of tin (for example, 232°C to 450°C). In this case, the thickness of the tin layer TL and the copper-tin alloy layer AL can be adjusted by adjusting the temperature of the molten tin and the immersion time in the molten tin. Specifically, the wire is immersed in molten tin at 230°C to 450°C for 10 to 60 seconds, and more preferably in molten tin at 230°C to 300°C for 10 to 60 seconds. The heating temperature and time can be varied as desired depending on the thickness of the alloy layer, and the temperature in the tin plating bath does not need to be uniform. Heat treatment may be performed after hot-dip plating.

[0042] (3) Assembly By subjecting the tin-plated copper wire to heat treatment in this manner, a copper-tin alloy layer AL is formed, and the above-mentioned second lead wire 3 is produced. The second lead wire 3 is fixed to the case 1 by inserting its end portion into the internal space 1s of the case 1 through the through-hole 12o of the other end portion 12 and crimping the other end portion 12. The second lead wire 3 may also be fixed to the case 1 by soldering or the like.

[0043] <Example> The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and modifications can be made within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present invention.

[0044] In this embodiment, a coating portion Y having a tin layer TL and a copper-tin alloy layer AL is formed on the surface of a core material X using a copper plate instead of a copper wire by the above-mentioned tin plating process and heat treatment process.

[0045] Specifically, five 0.3 mm thick oxygen-free copper (C1020) plates were electroplated to form a 10 μm thick tin film, creating five tin-plated copper plates. Each plate was then subjected to a heat treatment under different conditions. The heat treatment conditions are shown in Figure 6.

[0046] Figure 6 also shows the thickness of the coating portion Y of five samples (No. 1 to 5) in which a coating (coating portion Y) was formed on a copper plate under these conditions, the thickness of the copper-tin alloy layer AL that constitutes the coating portion Y, and the weight loss ratio when each sample was heated.

[0047] The thicknesses of the coating portion Y and the copper-tin alloy layer AL shown in Fig. 6 were measured by observing the polished cross-section of the sample piece with an SEM. The results show that in Samples Nos. 2 to 5, a copper-tin alloy layer AL having a thickness of 50% or less of the thickness of the coating portion Y was formed.

[0048] The weight loss ratios in Figure 6 were obtained by holding each sample upright in an electric furnace set to 300°C for 10 minutes, recording the weight loss, and expressing the weight loss of each sample as a relative ratio when the weight loss of sample No. 1 was set at 100. It was found that the higher the temperature at which the heat treatment was performed, i.e., the thicker the alloy layer, the less coating melted and fell off.

[0049] <Effects> In the thermal fuse 100 of this embodiment configured as described above, the coating Y is composed of a tin layer TL and a copper-tin alloy layer AL, so the thickness of the tin layer can be made thinner than in a lead wire whose coating is composed only of a tin layer. This reduces the amount of tin that melts and flows when placed in an environment with a temperature higher than the melting point of tin, thereby reducing the risk of short circuits and the like.

[0050] Furthermore, since the copper-tin alloy layer AL contains Cu6Sn5 and Cu3Sn, which have melting points higher than that of tin alone, the copper-tin alloy layer AL can be prevented from melting and flowing together with the tin layer TL at the melting point of tin. In this way, even if the tin layer TL melts and flows out, the copper-tin alloy layer AL can continue to protect the core material X from oxidation and corrosion.

[0051] Furthermore, since a tin layer TL is provided on the outermost layer of the second lead wire 3, the contact resistance can be made smaller than when the outermost layer of the second lead wire 3 is composed of a copper-tin alloy layer AL, and since the thickness of the tin layer TL is sufficiently large, the formation of a copper-tin alloy, which has high contact resistance, on the outermost layer of the second lead wire 3 is suppressed, thereby preventing an increase in the contact resistance of the second lead wire 3.

[0052] Furthermore, since the copper wire is reflow treated after tin plating, residual stress in the tin coating is removed, thereby suppressing the occurrence of whiskers.

[0053] The first lead wire may also be manufactured by plating a copper wire with nickel and then performing a reflow process, instead of silver plating, and may have the core material, a tin layer, and a copper-tin alloy layer.

[0054] The copper wire used for each lead wire may be made of a copper alloy. In this case, the core material X preferably contains a copper alloy as a main component (containing 90 mass% or more of copper alloy) and has a conductivity (%IACS) of 80 or more.

[0055] The coating may further include a nickel layer formed between the copper-tin alloy layer and the core material, which acts as a barrier to restrict copper diffusion from the core material to the coating, suppressing the formation of a copper-tin alloy with high contact resistance in the outermost layer of the second lead wire 3 and preventing an increase in the contact resistance of the second lead wire 3. The nickel layer is a coating of nickel, and is formed, for example, by plating a copper wire with nickel before the tin plating process.

[0056] The coating may have a nickel layer between the core material and the copper-tin alloy layer. The nickel layer directly coats the core material and contains, for example, 90 mass % or more of nickel. In this case, the coating portion is formed, for example, by first applying nickel plating and copper plating in sequence to form a nickel coating on a copper core material, and then applying the above-mentioned tin plating process and heat treatment to the copper wire on which a copper coating is formed on the nickel coating. In this case, the nickel layer coating the core material functions as a barrier to prevent the copper contained in the core material from diffusing into the coating, preventing the formation of an excessive copper-tin alloy layer. Also, by adjusting the thickness of the coating formed by copper plating, it becomes easier to adjust the thickness of the copper-tin alloy layer to the desired thickness.

[0057] Furthermore, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. [Explanation of symbols]

[0058] 100...Thermal fuse 1 Case 1s...inner space 11...One end 11о...Through hole 12...Other end 12o...Through hole 2. First lead wire 21...Fixed contact 3 Second lead wire 4. Movable electrode 41...Movable contact 42...Contact end 4a...Disc section 4b Curved section 5. Temperature-sensitive pellets 6. First spring element 7 Second spring element 8. Encapsulating material 9. Insulating tube X Core material Y: Covering part TL...Tin layer AL...Copper-tin alloy layer

Claims

1. a case having an interior space; two lead wires, one at each end of the case and connected to a circuit; a thermal pellet and an electrical contact provided in the internal space, A thermal fuse configured such that the thermal pellet melts at a predetermined operating temperature or higher, disconnecting the electrical contacts and breaking the circuit, At least one of the two lead wires includes a core material made of copper or a copper alloy and a coating portion that covers the core material, The thermal fuse has a covering portion having a tin layer formed as an outermost layer and a copper-tin alloy layer formed between the tin layer and the core material.

2. 2. The thermal fuse according to claim 1, wherein the melting point of the copper-tin alloy layer is 232 degrees Celsius or higher.

3. 2. The thermal fuse according to claim 1, wherein the copper-tin alloy layer contains an intermetallic compound of copper and tin.

4. 4. The thermal fuse according to claim 3, wherein the intermetallic compound is formed by combining a copper component derived from the core material with a tin component derived from the tin layer.

5. The intermetallic compound is Cu ６ Sn ５ or Cu ３ 4. The thermal fuse according to claim 3, wherein the metal is Sn.

6. The copper-tin alloy layer is Cu ６ Sn ５ or Cu ３ 6. The thermal fuse according to claim 5, containing 50 mass % or more, preferably 80 mass % or more, of Sn.

7. 2. The thermal fuse according to claim 1, wherein the thickness of the covering portion is 3 μm or more and 10 μm or less.

8. 2. The thermal fuse according to claim 1, wherein the thickness of the copper-tin alloy layer is 50% or less, preferably 30% or less, of the thickness of the covering portion.

9. 2. The thermal fuse according to claim 1, wherein the covering further comprises a nickel layer formed between the core material and the copper-tin alloy layer.

10. The lead wire may be: a first lead wire that is inserted into a through hole formed at one end of the case and has a fixed contact at its tip located in the internal space; and a second lead wire that is electrically connected to the other end of the case, the thermal pellet is disposed on the second lead wire side in the internal space, 2. The thermal fuse according to claim 1, wherein said one lead wire is said second lead wire.

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