Fusible link and protective element

Abstract

A fusible link element (10; 15; 25; 35) comprising a metal composite material in which a first fusible metal (11; 21; 31) and a second fusible metal (12; 22; 32) are layered on top of each other, wherein the second fusible metal (12; 22; 32) has a lower melting temperature than the first fusible metal (11; 21; 31), characterized by the fact that the first fusible metal (11; 21; 31) is a Sn-Ag alloy and the Sn-Ag alloy contains at least 20 wt% and at most 30 wt% Ag, and the second fusible metal (12; 22; 32) is a Sn-Bi alloy and the Sn-Bi alloy contains at least 40 wt% and at most 70 wt% Bi, wherein at a common working temperature at least part of a component of the second fusible metal (12; 22; 32) is melted and part of a component of the first fusible metal (11; 21; 31) is melted, and where, at the common working temperature, the difference between a liquidus temperature and a solidus temperature of the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32) is reduced by a shift in the equilibrium of Sn between the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32) and by a mutual diffusion of Ag and Bi between the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32).

Description

TECHNICAL AREA

[0001] The present disclosure relates to a fuse element and a protective element containing the fuse element. STATE OF THE ART

[0002] With the rapid proliferation of small electronic devices, such as mobile devices, in recent years, a smaller size and thickness of protective element is being mounted on a power supply protection circuit. For example, a surface-mount device (SMD) chip protection element is conveniently used for a secondary battery pack protection circuit. The chip protection element includes a one-shot protection element that detects excessive heat generation due to overcurrent in a protected device and trips a fuse to interrupt a circuit under a predefined condition. Other examples of chip protection elements include a one-shot protection element that trips a fuse to interrupt a circuit under a predefined condition in response to an abnormal rise in ambient temperature.

[0003] When the protection circuit detects an abnormal condition occurring in a device, the protection element generates heat using a resistance element and a signal current. The protection element ensures the safety of the device by interrupting the circuit either by melting a fuse element made of an alloy material that melts due to the generated heat, or by melting the fuse element with an overcurrent.

[0004] For example, JP 2013-239405A discloses a protective element produced by laminating at least one layer of high-melting-point base material, consisting of a fusible metal with a first melting point that does not melt at the reflow temperature, and at least one layer of low-melting-point coating material, consisting of a fusible metal with a second melting point that does melt at the reflow temperature. This protective element thus comprises a resistive element that generates heat when an abnormal condition occurs and is provided on an insulating substrate such as a ceramic substrate.

[0005] The alloy used for the fuse element of the protective device described above has recently tended to be lead-free to comply with stricter regulations on chemical substances under a revised RoHS directive or similar legislation. For example, a fuse element made of a lead-free metal composite, as described in JP 2015-079608A (PTL 2), is available. This fuse element consists of a low-melting-point metal material that melts at the soldering temperature used for surface mounting the fuse element on a circuit substrate, and a high-melting-point metal material in a solid phase that dissolves into the low-melting-point metal material in a liquid phase at the soldering temperature.The low-melting-point and high-melting-point metal components of the fuse element are formed as a single piece. Within the fuse element, the low-melting-point metal, which has molten, is held in the solid phase by the high-melting-point metal until the soldering process is complete.

[0006] The low-melting-point and high-melting-point metals of the fuse element are bonded together. While the low-melting-point metal, which is molten during soldering, is held in place without being melted by the high-melting-point metal in its solid phase at operating temperature, the fuse element can be connected to the molten metal with an electrode pattern of the protective element. Furthermore, this prevents the fuse element from melting at the soldering temperature when the protective element is surface-mounted on the circuit substrate.The protective element carries out a melting process by generating heat through an embedded resistance element to diffuse or dissolve the high-melting-point metal material of the fuse element into the low-melting-point metal material that serves as the medium.

[0007] The electrical resistance of a fuse element, which serves to interrupt current through a power line, is preferably as low as possible to minimize electrical energy loss. In this respect, a fuse element containing a high-melting-point metal, such as silver, which has low electrical resistance, is particularly advantageous.

[0008] The high-melting-point metal material, which consists of silver, does not melt at the operating temperature of the protective element. Therefore, the high-melting-point metal material either does not dissolve sufficiently or diffuses into the low-melting-point metal material, potentially remaining as a thick film. In this case, a conventional fuse element requires an excessively long melting time, and in extreme cases, the melting process may be faulty. For these reasons, the thickness of the high-melting-point metal material is insufficient to reduce the electrical resistance.

[0009] By reducing the size and thickness of a fuse element, electrode, and substrate, a thinner fuse element can be used, thus limiting the thickness of the high-melting-point metal. Therefore, when connecting a fuse element to an electrode, excessive amounts of the high-melting-point metal diffuse or dissolve into the low-melting-point metal in the liquid phase, resulting in a thinner component. This can cause the fuse element to deform, or the surface of the high-melting-point metal may become wavy, compromising the integrity of the fuse. CITATION LIST PATENT LITERATURE PTL 1: JP 2013 - 239 405 A PTL 2: JP 2015 - 079 608 A SUMMARY OF THE INVENTIONAL PROBLEM

[0010] One objective of the present disclosure is to provide a fuse element that complies with the regulations against chemical substances, has a low electrical resistance and achieves a reduction in the size and thickness of a protective element, wherein the fuse element is able to interrupt the current flow more reliably at the time of actuation of a fuse and a protective element containing it. SOLUTION TO THE PROBLEM

[0011] According to the present disclosure, a fusible link element is provided which contains a metal composite material in which a first fusible metal and a second fusible metal are layered on top of each other, wherein the first fusible metal is a Sn-Ag alloy and the Sn-Ag alloy contains at least 20 wt% and at most 30 wt% Ag, the second fusible metal is a Sn-Bi alloy and the Sn-Bi alloy contains at least 40 wt% and at most 70 wt% Bi, wherein at a common operating temperature at least a part of a component of the second fusible metal is molten and a part of a component of the first fusible metal is molten.and wherein, at the common working temperature, the difference between a liquidus temperature and a solidus temperature of the first fusible metal and the second fusible metal is reduced by a shift in the equilibrium of Sn between the first fusible metal and the second fusible metal and by a mutual diffusion of Ag and Bi between the first fusible metal and the second fusible metal.

[0012] A portion of a component of the first fusible metal is dissolved at a common working temperature, and a portion or all of a component of the second fusible metal is melted at the same working temperature. A prescribed component of both the first and second fusible metals is mixed in between, so that the first and second fusible metals are closer to prescribed respective liquidus temperatures. With the fusible link according to this disclosure, the fuse element can be joined by a reflow process without the use of a joining material such as solder paste. Since the fuse element consists of fusible metals, it is free of any residue left behind after melting. Furthermore, the electrical resistance of the fuse element can be reduced without using a high-melting-point metal such as silver.Deformation or waviness of the surface of the fusible link is not a concern, which contributes to more economical production.

[0013] According to another aspect of the present disclosure, a protective element including the fuse element described above is provided.

[0014] In particular, the protective element according to the present disclosure further comprises an insulating substrate, a plurality of electrodes provided on the insulating substrate, a fuse element electrically connected to one of the plurality of electrodes, and a heat-generating element provided on the insulating substrate, wherein the heat-generating element is configured to heat and melt the fuse element. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0015] With the fuse element and the protective element according to the present disclosure, the current line can be interrupted more reliably at the time a fuse is actuated. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a perspective view showing a fusible link according to an embodiment of the present disclosure. Fig. Figure 2 is a perspective exploded view showing a protective element according to the embodiment of the present disclosure. Fig. Figure 3 shows the protective element according to the embodiment of the present disclosure, wherein (a) shows a cross-sectional view along Illa-Illa in (b), (b) shows a cross-sectional view along IIIb-IIIb in (a) and (c) shows a view from below. Fig. Figure 4 shows the protective element according to the embodiment of the present disclosure, wherein (a) shows a cross-sectional view along IVa-IVa in (b), (b) shows a cross-sectional view along IVb-IVb in (a) and (c) shows a view from below. DESCRIPTION OF THE EXECUTION FORMS

[0016] A fusible link element 10 according to the present disclosure consists of a metallic composite material in which a first fusible metal 11 and a second fusible metal 12 are arranged as in Fig. 1 shown are layered on top of each other, wherein part of a component of the first fusible metal 11 is dissolved at a common working temperature, the second fusible metal 12 has a lower melting temperature than the first fusible metal 11 and at least part of a component of the second fusible metal 12 is melted at the common working temperature.

[0017] A portion of the component of the first fusible metal 11 dissolves into the second fusible metal 12 at their common working temperature. The first fusible metal 11 and the second fusible metal 12, with part or all of the component of the second fusible metal 12 having melted at their common working temperature, diffuse into each other or mix together. As a result of the diffusion or mixing, the first fusible metal 11 and the second fusible metal approach their respective prescribed liquidus temperatures.

[0018] Examples of the first melting metal 11 of the fuse element include an 80Sn-20Ag alloy (with a liquidus temperature of 370°C and a solidus temperature of 221°C). Examples of the second melting metal 12 include a 60Sn-40Bi alloy (with a liquidus temperature of 175°C and a solidus temperature of 139°C). The fuse element 10 consists of a metal composite material in which the second melting metal 12 is layered onto a surface of the first melting metal 11. The solidus and liquidus temperatures are derived from differential scanning calorimetry (DSC).

[0019] Although the first fusible metal is not particularly restricted, for example, it should only be a lead-free tin-based solder, wherein part of a component of the lead-free tin-based solder is dissolved at a prescribed common working temperature which is higher than the solidus temperature of the second fusible metal and lower than the liquidus temperature of the first fusible metal (preferably a peak temperature which is lower than about 300°C with regard to the heat resistance of a peripheral component).

[0020] The second melting metal should consist solely of tin or a lead-free, tin-based solder, with part or all of the second melting metal being meltable at the prescribed common working temperature. Alternatively, the second fusible metal 12 may consist of a single-melting-point metal, a eutectic alloy, or a single-melting-range alloy. In addition to the examples described above, a Sn-Cu alloy, an Sn-Sb alloy, an Sn-Zn alloy, and an Sn-Al alloy are available as the first fusible metal. Likewise, Sn, an Sn-Ag alloy, an Sn-Ag-Cu alloy, an Sn-Ag-Cu-Bi alloy, an Sn-Cu alloy, an Sn-In alloy, an Sn-Ag-In alloy, an Sn-Bi-Ag alloy, an Sn-Ag-Bi-In alloy, an Sn-Sb alloy, an Sn-Zn alloy, an Sn-Zn-Bi alloy and an Sn-Al alloy are available as further second fusible metals.

[0021] The first and second fusible metals are each lead-free metals with a high tin content and are therefore more susceptible to oxidation than conventional lead-containing metals. Consequently, at least one of phosphorus, gallium, or germanium can be added as a trace element to one or both of the first and second fusible metals at a concentration of more than 3 ppm and less than 300 ppm to prevent oxidation.

[0022] If the first fusible metal 11 has the form of a flat plate, a shape like the one in Fig. 1 (a) Fusible element 10 shown is applicable, in which the second fusible metal 12 is layered onto one of the plate surfaces of the first fusible metal 11. Alternatively, a shape such as that shown in 1 (a) can be used. Fig. 1 (b) the fusible link element 15 shown, in which the second melt metal 12 is layered on both plate surfaces of the first melt metal 11.

[0023] The means for layering the second melt metal 12 onto the first melt metal 11 are not particularly limited, and the second melt metal 12 should only be layered onto the first melt metal 11. For example, methods such as cladding (pressure bonding), plating, and hot melt coating are available.

[0024] The fuse element according to the present disclosure can be placed directly onto an electrode without the use of solder paste and connected to the electrode by a reflow process. Since the fuse element is made of a fusible metal, it is free of residues that remain unmelted. More precisely, a high-melting-point metal material consisting of silver was commonly used; however, this high-melting-point silver material did not melt at the operating temperature of the fuse element. In the fuse element of the present embodiment, both the first melting metal 11 and the second melting metal 12 are fusible at the operating temperature of the fuse element. Therefore, such faulty operation, in which a portion of the fuse element remains unmelted, as seen in a conventional fuse element, can be prevented.A numerical value preceding a chemical symbol in the designation of the composition of an alloy, such as an 80 Sn-20 Ag alloy, represents the mass percent of the element.

[0025] The melting element according to the present disclosure is connected by melting to an electrode 24a, which consists of an electrically conductive element provided on a heat-resistant insulating substrate 23 and serves as a fusible link 25 of the protective element, as shown in Fig. Figure 2 shows that the common working temperature is preferably set such that it exceeds the solidus temperature of the second molten metal and is lower than the liquidus temperature of the first molten metal.

[0026] The fuse element 25 and the electrode 24a are joined together in the following steps. Flux for joining is applied to at least one surface of the electrode 24a, to which the fuse element 25 is to be joined, and to at least one surface of a second molten metal 22 of the fuse element 25. The fuse element 25 is positioned so that the second molten metal 22 is in contact with the electrode 24a. The fuse element 25 and the insulating substrate 23 are heated to their common operating temperature in order to melt part of the first molten metal 21 and part or all of the second molten metal 22, thus joining the fuse element 25 to the electrode 24a.

[0027] Afterwards, the connecting flux for operation is applied at least to the fuse element 25, and the fuse element 25 provided with connecting flux is covered and packaged together with the insulating substrate 23 with a cap-shaped lid 26 to assemble a protective element 20.

[0028] Examples of the first melting metal 21 of the fusible link 25 include an 80Sn-20Ag alloy (with a liquidus temperature of 370°C and a solidus temperature of 221°C). Examples of the second melting metal 22 include a 60Sn-40Bi alloy (with a liquidus temperature of 175°C and a solidus temperature of 139°C). In this example, as a result of the joining process at the common operating temperature, some of a component of the first fusible metal 21 diffuses into or mixes with the second fusible metal 22, with some or all of a component of the second fusible metal 22 being molten at the common operating temperature. As a result of the diffusion or mixing, the first fusible metal 21 and the second fusible metal 22 are closer to the required liquidus temperatures.

[0029] In the first fusible metal 21, tin (Sn) from the second fusible metal 22 diffuses into the liquid phase, approaching an equilibrium state, and thus the tin content increases. Ag also diffuses into the second fusible metal 22, and the Ag concentration decreases. Consequently, the Ag concentration in the first fusible metal 21 decreases relatively, and the liquidus temperature drops from 370°C in the initial state towards the solidus temperature of 221°C.

[0030] In the second fusible metal 22, as a result of the dissolution and diffusion of the first fusible metal 21, Sn migrates into the first fusible metal 21, is closer to equilibrium, and thus the Sn content decreases. Bi also diffuses into the first fusible metal 21, and the Bi concentration decreases. Consequently, the Bi concentration in the second fusible metal 22 increases relatively, and the liquidus temperature begins to decrease towards the solidus temperature of 139°C.The fusible link element is connected and a difference between the liquidus temperature and the solidus temperature in the first melting metal and the second melting metal is reduced by utilizing the equilibrium shift of the Sn component, which is a common element of the first melting metal 21 and the second melting metal 22, and the mutual diffusion of Ag and Bi, which are different elements, so that a range of operating temperature of the fuse is self-regulating.

[0031] The fuse element can be fused more quickly than a conventional fuse element, which is fused by erosion of a solid pure silver coating by lead-free, tin-based solder. Since no silver coating is used, there are no concerns about faulty melting due to corrosion from sulfurization, silver migration, or silver coating residue.

[0032] The protective element 20 according to the present disclosure comprises the fuse element, and as in Fig. As shown in Figure 2, it comprises an insulating substrate 23, a plurality of electrodes 24a and 24b provided on the insulating substrate 23, a fusible link element 25 electrically connected to a prescribed electrode (24a in Fig. 2) the electrodes 24a and 24b are connected, and a heat-generating element is provided on the insulating substrate 23 to heat and melt the fusible link 25, and is electrically connected to a prescribed electrode (in Fig. 2 arranged on the back of the insulating substrate 23). The melting element 25 consists of a metallic composite material in which a first melting metal 21 and a second melting metal 22 are layered on top of each other, wherein a part of a component of the first melting metal 21 is dissolved at the common operating temperature, the second melting metal 22 has a lower melting temperature range than the first melting metal 21, and at least a part of a component of the second melting metal 22 is melted at the common operating temperature.

[0033] The first fusible metal 21 of the fusible link 25 preferably has a liquidus temperature that is lower than a peak temperature of the heat-generating element (a highest temperature during heat generation by the heat-generating element). Thus, for example, even if some of the first molten metal 21 remains without being dissolved into the second molten metal 22, the heat-generating element can still melt the first molten metal 21 to fuse the fusible link 25. Examples

[0034] The fusible link element 10 in Example 1 according to the present disclosure consists of a metal composite material in which a first melt metal 11, which is formed as an alloy plate of a 70 Sn-30 Ag alloy with a thickness of 80 µm (with a liquidus temperature of 415°C and a solidus temperature of 221°C), and a second melt metal 12, which is formed as an alloy plate of a 60 Sn-40 Bi alloy with a thickness of 10 µm (with a liquidus temperature of 175°C and a solidus temperature of 139°C), are layered on top of each other by cladding, as shown in Fig. 1 (a) is shown.

[0035] Furthermore, for the in Fig. 1 (a) The fusible link element 10 shown also uses a composite metal material in which a first melt metal 11, which is formed as an alloy plate of a 67 Sn-33 Ag alloy with a thickness of 65 µm (with a liquidus temperature of 416°C and a solidus temperature of 220°C), and a second melt metal 12, which is formed as an alloy plate of a 30 Sn-70 Bi alloy with a thickness of 25 µm (with a liquidus temperature of 173°C and a solidus temperature of 139°C), are layered on top of each other by cladding.

[0036] The fusible link element 15 in Example 2 according to the present disclosure consists of a three-layer composite metal material, in which the second melt metal 12, which is formed as an alloy plate of a 60 Sn-40 Bi alloy with a thickness of 5 µm (with a liquidus temperature of 175°C and a solidus temperature of 139°C), is layered over the upper and lower surfaces of the first melt metal 11, which is formed as an alloy plate of an 80 Sn-20 Ag alloy with a thickness of 80 µm (with a liquidus temperature of 370°C and a solidus temperature of 221°C), as shown in Fig. 1 (b) is shown. By applying the second melting metal 12 to the top and bottom surfaces of the first melting metal 11, the front and back surfaces of the fusible link 15 do not need to be distinguished from each other.

[0037] Therefore, incorrect placement of a fuse element plate in a process for assembling the protective element can be prevented.

[0038] The fusible link element in Example 1 or 2 is connected to the electrode 24a, which consists of an Ag alloy and is provided on a surface of the insulating substrate 23, which consists of aluminum oxide ceramic, as shown in Fig. 2 shown, in order to form the protective element shown below in example 3 or 4.

[0039] In the protective element, the electrode 24a is placed on the insulating substrate, onto which a bonding flux has been previously applied, and the second melt metal 22 of the fuse element 25 is positioned so that they are in contact with each other. The protective element is placed in a reflow oven under conditions such as a temperature profile with a residual heat temperature of 110 to 130°C and a residence time of 70 seconds, whereby the residence time is reduced to 30 seconds at a temperature equal to or higher than 150°C and a peak temperature of 170°C. In this way, a portion of the first fusible metal 21 dissolves and a portion or all of the second fusible metal 22 melts, so that a tin phase, where tin is an element common to both the first and second fusible metals, diffuses mutually and is close to equilibrium.At the same time, the molten second metal 22 connects the fuse element 25 to the electrode 24a. After the fuse element 25 is connected to the electrode 24a, the joining flux is applied to the fuse element 25. The fuse element 25, together with the insulating substrate 23, is covered with a cap-shaped lid 26 made of a heat-resistant plastic, and the cap-shaped lid 26 and the insulating substrate 23 are fixed with an epoxy-based resin to form the protective element 20.

[0040] The protective element in Example 3 according to the present disclosure comprises a protective element 30 containing the fuse element in Example 1 or 2, an insulating substrate 33 made of aluminum oxide ceramic, a plurality of pattern electrodes 34 made of an Ag alloy and provided on the upper and lower surfaces of the insulating substrate 33, a resistance heat-generating element 38 electrically connected to the pattern electrode 34 and provided on the lower surface of the insulating substrate 33, a fuse element 35 electrically connected to the pattern electrode 34 on the upper surface of the insulating substrate 33, and a cap-shaped cover 36, which covers the fuse element 35 from above, is attached to the insulating substrate and is made of a liquid crystal polymer, as shown in Fig. Figure 3 shows the fusible link element 35. The fusible link element consists of a metal composite material in which a first fusible metal 31, formed as an alloy plate of a 70Sn-30Ag alloy with a thickness of 80 µm (with a liquidus temperature of 415°C and a solidus temperature of 221°C), and a second fusible metal 32, formed as an alloy plate of a 60Sn-40Bi alloy with a thickness of 10 µm (with a liquidus temperature of 175°C and a solidus temperature of 139°C), are layered on top of each other by cladding. The template electrode 34 contains a half-through hole 37 made of an Ag alloy, the half-through hole enabling an electrical connection between the template electrodes 34 on the upper and lower surfaces of the substrate.

[0041] Although not explicitly shown, a glass overglaze is provided on one surface of the resistance heat-generating element in Example 3. The heat-generating element 38 of the protective element in Example 3 is provided on a substrate surface (underside) of the insulating substrate 33, which differs from a substrate surface (topside) on which the fusible link element 35 is provided.

[0042] The heat-generating element 38 used in Example 3 has a peak temperature of, for example, 430°C, which is higher than 415°C, the liquidus temperature of the first fusible metal 31, and 175°C, the liquidus temperature of the second fusible metal 32. Thus, although the first fusible metal 31 does not dissolve in the second fusible metal 32, the heat from the heat-generating element 38 melts the first fusible metal 31. Consequently, a malfunction of the protective element can be avoided.

[0043] A protective element 40 in Example 4 according to the present disclosure is a modification of the protective element in Example 3 and it is a protective element that includes the fuse element in Example 1 or 2. As in Fig.As shown in Figure 4, the protective element comprises an insulating substrate 43 made of aluminum oxide ceramic, a plurality of pattern electrodes 44 made of an Ag alloy and provided on the upper and lower surfaces of the insulating substrate 43, a resistance heat-generating element 48 electrically connected to the pattern electrode 44 and provided on the upper surface of the insulating substrate 43, a fusible link element 45 abutting the resistance heat-generating element 48 and electrically connected to the pattern electrode 44 on the upper surface of the insulating substrate 43, and a cap-shaped cover 46 that covers the fusible link element 45 from above, is attached to the insulating substrate 43 and is made of a liquid crystal polymer.

[0044] The fusible link element 45 consists of a metal composite material in which a first melting metal 41, formed as an alloy plate of an 80Sn-20Ag alloy with a thickness of 80 µm (with a liquidus temperature of 370°C and a solidus temperature of 221°C), and a second melting metal 42, formed as an alloy plate of a 60Sn-40Bi alloy with a thickness of 10 µm (with a liquidus temperature of 175°C and a solidus temperature of 139°C), are layered on top of each other by cladding. The pattern electrode 44 contains a half-through hole 47 made of an Ag alloy, the half-through hole enabling an electrical connection between the pattern electrodes 44 on the upper and lower surfaces of the substrate.

[0045] The heat-generating element 48 used in Example 4 has a peak temperature of, for example, 400°C, which is higher than 370°C, the liquidus temperature of the first fusible metal 41, and 175°C, the liquidus temperature of the second fusible metal 42. Thus, although the first fusible metal 41 does not dissolve in the second fusible metal 42, the heat from the heat-generating element 48 melts the first fusible metal 41. This prevents a malfunction of the protective element.

[0046] Although not explicitly shown, in Example 4 an overglaze made of a glass material is provided on a surface of the resistance heat-generating element 48. The heat-generating element 48 of the protective element in Example 4 is provided on a substrate surface (top side) of the insulating substrate 43, which corresponds to the substrate surface (top side) on which the fusible link element 45 is provided.

[0047] In the protective element in Examples 3 and 4, the wiring means that electrically connect the pattern electrodes on the top and bottom of the insulating substrate can be changed by a conductor through-hole passing through the substrate or by a surface wire formed by a planar electrode pattern instead of the half through-hole.

[0048] To improve the wettability of an electrode, the Sn-Bi alloy, which constitutes the second fusible metal in Examples 1 to 4, can be changed to an Sn-Bi-Ag alloy, obtained by further addition of Ag to the Sn-Bi alloy.

[0049] It should be understood that the embodiment disclosed herein is in every respect illustrative and not limiting. The scope of the present invention is defined more by the terms of the claims than by the above description and is intended to include all modifications within the scope and meaning that correspond to the terms of the claims. INDUSTRIAL APPLICABILITY

[0050] A fuse element made of a metallic composite material according to the present invention can be incorporated into and mounted on a protective element by heating and melting the entire assembly, e.g., by reflow soldering. The protective element, including the fuse element, can be used for a protective device for a secondary battery, such as a battery pack, by soldering and mounting it, along with other surface-mount components, onto an electrical circuit substrate, again by reflow soldering. LIST OF REFERENCE MARKS

[0051] 10, 15, 25, 35, 45 Fusible element; 11, 21, 31, 41 First fusible metal; 12, 22, 32, 42 Second fusible metal; 20, 30, 40 Protective element; 23, 33, 43 Insulating substrate; 24a, 24b, 34, 44 Electrode; 26, 36, 46 Cover; 37, 47 Half through hole; 38, 48 Heat generating element

Claims

A fusible link element (10; 15; 25; 35) comprising a metal composite material in which a first fusible metal (11; 21; 31) and a second fusible metal (12; 22; 32) are layered on top of each other, wherein the second fusible metal (12; 22; 32) has a lower melting point than the first fusible metal (11; 21; 31), characterized in that the first fusible metal (11; 21; 31) is a Sn-Ag alloy and the Sn-Ag alloy contains at least 20 wt% and at most 30 wt% Ag, and the second fusible metal (12; 22; 32) is a Sn-Bi alloy and the Sn-Bi alloy contains at least 40 wt% and at most 70 wt% Bi, wherein in a common At the operating temperature, at least part of a component of the second fusible metal (12; 22; 32) has melted and part of a component of the first fusible metal (11; 21;31) is melted, and wherein at the common working temperature a difference between a liquidus temperature and a solidus temperature of the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32) is reduced by a shift in the equilibrium of Sn between the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32) and a mutual diffusion of Ag and Bi between the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32). The fusible link element (10; 15; 25; 35) according to claim 1, wherein the first fusible metal (11; 21; 31) is an 80 Sn-20 Ag alloy or a 70 Sn-30 Ag alloy. The fusible link element (10; 15; 25; 35) according to one of claims 1 to 2, wherein the second fusible metal (12; 22; 32) is a 60 Sn-40 Bi alloy or a 30 Sn-70 Bi alloy. A protective element (20; 30; 40) comprising: an insulating substrate (23; 33; 43); a plurality of electrodes (24a, 24b; 34; 44) provided on the insulating substrate (23; 33; 43); a fuse element (25; 35; 45) electrically connected to one of the plurality of electrodes (24a, 24b; 34; 44); and a heat-generating element (38; 48) provided on the insulating substrate (23; 33; 43), wherein the heat-generating element (38; 48) is configured to heat and melt the fusible link element (25; 35; 45), wherein the fusible link element (25; 35; 45) comprises a metal composite material in which a first fusible metal (21; 31; 41) and a second fusible metal (22; 32; 42) are layered on top of each other, wherein the second fusible metal (22; 32; 42) has a lower melting temperature than the first fusible metal (21; 31; 42), characterized in that the first fusible metal (21; 31;31) is a Sn-Ag alloy and the Sn-Ag alloy contains at least 20 wt% and at most 30 wt% Ag, and the second fusible metal (12; 22; 32) is a Sn-Bi alloy and the Sn-Bi alloy contains at least 40 wt% and at most 70 wt% Bi, wherein at a common working temperature at least a part of a component of the second fusible metal (22; 32; 42) is molten and a part of a component of the first fusible metal (21; 31; 41) is molten, and wherein at the common working temperature a difference between a liquidus temperature and a solidus temperature of the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32) is caused by an equilibrium shift of Sn between the first fusible metal (11; 21; 31) and the second fusible metal (12; 22; 32) and a mutual diffusion of Ag and Bi between the first fusible metal (11; 21;31) and the second fusible metal (12; 22; 32) is reduced.; The protective element (20; 30; 40) according to claim 4, wherein the heat-generating element (38; 48) is provided on a substrate surface of the insulating substrate (23; 33; 43) which differs from a substrate surface on which the fusible link element (25; 35; 45) is provided. The protective element (20; 30; 40) according to claim 4, wherein the heat-generating element (38; 48) is provided on a substrate surface of the insulating substrate (23; 33; 43) which is identical to a substrate surface on which the fusible link element (25; 35; 45) is provided. The protective element (20; 30; 40) according to one of claims 4 to 6, wherein the first fusible metal (21; 31; 41) is an 80 Sn-20 Ag alloy or a 70 Sn-30 Ag alloy. The protective element (20; 30; 40) according to one of claims 4 to 7, wherein the second fusible metal (22; 32; 42) is a 60 Sn-40 Bi alloy or a 30 Sn-70 Bi alloy. The protective element (20; 30; 40) according to one of claims 4 to 8, wherein the liquidus temperature of the first fusible metal (21; 31; 41) is lower than a peak temperature of the heat-generating element (38; 48).

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