Fuse, battery pack using the fuse, and method of manufacturing the fuse

A fuse has a pair of lead terminals disposed on a substrate, an intermediate layers for welding formed on the surface of at least one of the lead terminals, and a fuse element. The fuse element is welded to the pair of lead terminals through the intermediate layer so as to span the same. Further, the intermediate layer is formed on at least one of the lead terminals except a face thereof opposing each other. Owing to this configuration, after the fuse has melted down, the melted fuse element is prevented from being spread out into the space between the opposing faces of the lead terminals and insulation therebetween is secured.

TECHNICAL FIELD

The present invention relates to a fuse used for preventing failure of electronic equipment and the like due to abnormal heating and overcurrent, a battery pack using the fuse, and a method of manufacturing the fuse.

BACKGROUND ART

In order to prevent occurrence of a failure of electronic equipment due to abnormal heating of a pack battery used in electronic equipment such as mobile telephones, it is required that a temperature fuse be mounted on the pack battery. It is also required that a current fuse be mounted on electronic equipment to prevent occurrence of a failure due to abnormal current.

In a conventional temperature fuse, a fuse element which melts down when it reaches a predetermined temperature is provided at its both ends with terminal portions and the terminal portions are connected to a circuit such as a power supplying circuit. When abnormal heating is produced in a component constituting a power supply or the like (such as a battery), the fuse element serves its function at a high temperature caused by the abnormal heating. Thus, the temperature fuse is adapted to break the circuit to prevent the internal components and the like from being damaged.

In the conventional fuse, a fuse element made of a fusible member is welded to a pair of lead terminals so as to span the same, and thereby, it is electrically connected to the lead terminals. Fuses of the described type are disclosed in, for example, Japanese Unexamined Patent Publication No. H11-273520 and Japanese Unexamined Patent Publication No. 2002-33035. In those cases, an intermediate layer for welding is provided between the fuse element and the lead terminal for ensuring good welding of the fuse element to the lead terminal.

FIG. 11is a lateral sectional view of a prior art fuse.FIG. 12is a lateral sectional view of fuse element104as have been melted down. The fuse is constructed by disposing a pair of lead terminals100on substrate103and by welding fuse element104to lead terminals100through plated layer101provided on the surface of lead terminals100. Though it is not shown, some flux or a cover may be provided on fuse element104. A fuse is manufactured in the following way. First, a pair of lead terminals100are disposed on substrate103. Then, plated layers101are formed on the pair of lead terminals100arranged as above. At this time, plated layers101are formed so far as they are even spread on opposing faces102of lead terminals100. Fuse element104is placed on plated layers101and pressed and heated from its upper side to be welded to plated layers101. A fuse is manufactured through the above described steps.

However, in the prior art fuse, plated layers101are formed to be extended over opposing faces102. Therefore, when fuse element104is welded, spread-out portions105thereof are formed on the opposing portions so that the insulation distance between lead terminals100opposing each other is shortened. Furthermore, when fuse element104is melted down, melted fuse element104is spread out over opposing faces102, whereby large spread-out portions105are formed as shown inFIG. 12. Then, the insulation distance between opposing lead terminals100becomes still shorter after the melting down of the fuse element. Hence, insulating capability becomes insufficient though the fuse element is melted down.

In a case of a large sized fuse, it is no problem if there exist spread-out portions105because lead terminals100originally have a large face-to-face distance. However, in these days, electronic equipment is becoming increasingly smaller in size and lower in profile. Also with respect to batteries having fuses mounted thereon, advances are being made in making them lower profiled and smaller sized. This makes it indispensable to make the fuse smaller sized and lower profiled and it is naturally required to narrower the face-to-face distance of lead terminals100. However, there is a limit in making the face-to-face distance narrower due to the deterioration of insulating capability caused by existence of spread-out portions105. Hence, it is required to previously provide a sufficiently large face-to-face distance. As a result, it becomes impossible to make the fuse smaller sized and lower profiled.

SUMMARY OF THE INVENTION

A fuse of the present invention has a substrate, a pair of lead terminals disposed on the substrate, an intermediate layer for welding formed on the surface of at least one of the lead terminals, and a fuse element made of a fusible member. The fuse element is welded to the pair of lead terminals through the intermediate layer such that is spans the pair of lead terminals. The intermediate layer is formed on at least one of the lead terminals except a face thereof opposing each other. Further, a method of manufacturing a fuse of the present invention includes a step of connecting a pair of lead terminals onto a substrate, a step of forming an intermediate layer for welding on the surface of at least one of the lead terminals except a face of thereof opposing each other, and a step of welding the fuse element to the intermediate layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An exemplary embodiment of the present invention will be described with reference to the accompanying drawings. A case where a single structure of a plated layer is used for the intermediate layer for welding, or a case where a combined structure of a plated layer and a meltable layer for welding is used therefor, will be taken as an example in the following description.

FIG. 1is a lateral sectional view of a fuse in the first exemplary embodiment of the present embodiment.FIG. 2AandFIG. 2Bare lateral sectional views of lead terminals.FIG. 3toFIG. 7are drawings showing steps in manufacturing the fuse andFIG. 8AtoFIG. 8Care drawings showing steps in welding the fuse element.FIG. 9is a perspective view of a fuse melted down.

Substrate1is made of an insulating material, such as a plastic, glass, or ceramic material, or it may be made of metal with an insulating coating formed thereon. In the present embodiment, an alumina ceramic plate is used as substrate1. Substrate1may be formed not only of a rectangular plate but also of a disk, oval, triangular, pentagonal, or a polygonal plate in polygonal number of five or larger. A pair of lead terminals2are bonded onto substrate1and arranged to oppose each other. Lead terminals2are made of an electrically conductive material and preferably made of metal. To be concrete, a single metallic material, at least, selected from iron, nickel, copper, aluminum, gold, silver, and tin, or an alloy made of the metallic materials may be used. Otherwise, a metallic material and the like obtained by adding an element or elements not belonging to the above material group to a single material, at least, selected from the above material group or an alloy may be used.

As the material for bonding lead terminal2onto substrate1, a plastic resin, glass, or a metallic film containing a plastic resin or glass may be used. When a metallic film is used as a bonding agent, the metallic film is formed on substrate1by printing or the like, lead terminals2are placed on the metallic film, and lead terminals2are bonded to substrate1by such a method as ultrasonic welding. When the material of substrate1is a thermoplastic resin, such a method may also be used as to place lead terminals2on substrate1and then apply quick heating, followed by quick cooling, to them to have the surface of substrate1directly melted. In the present embodiment, a nickel plate with its surface plated with tin is used as lead terminal2and an epoxy resin containing filler formed of alumina and silica is used as the bonding agent.

A pair of lead terminals2are arranged to have their opposing faces5opposing each other across face-to-face distance31. When a fuse is made small in size, distance31must be reduced correspondingly. It is an important factor how to reduce distance31in realizing a small-sized fuse. When distance31is extremely small, a sufficient insulating capability cannot be provided between lead terminals2after melting down of fuse element4. Accordingly, distance31needs to be large enough to secure the insulating capability.

Plated layers3are formed on the surfaces of lead terminals2. At this time, plated layers3are formed on the surfaces except, at least, opposing faces5. Further, there are provided non-plated regions6in the ranges of a predetermined distance from the end faces of lead terminals2opposing each other. In other words, plated layers3are formed at the portions a predetermined distance set back from the end faces of lead terminals2opposing each other. By virtue of the existence of non-plated regions6, formation of spread-out portions of fuse element4is prevented more positively as discussed in the following. As the material of plated layer3, is used such a metallic material as tin, copper, silver, gold, nickel, and zinc. If formation of plated layers3is confined to the surfaces of the end portions of lead terminals2except, at least, opposing faces5opposing each other, it may be unnecessary to provide non-plated regions6. Further, non-plated regions6may be provided by small amounts of edge burrs12produced at the cut faces of end portions51of lead terminals2. Namely, as shown inFIG. 2A, a small amount of non-plated region6with width32may be secured by edge burr12. Further, edge burrs12are protruded portions provided at end portions51of lead terminals2opposing each other and they are raised portions having curved surfaces121. Edge burr12also serves for preventing plated layer3from being formed on opposing face5.

Instead of providing plated layer3, a metal paste such as a silver paste may be applied. Otherwise, metal depositing or sputtering may be applicable. Plating may be either electroplating or electroless plating, or an electroplated layer may be formed on an electroless plated layer. Further, plated layer3may be formed of a single layer or plural, two or more, layers. By forming plated layer3in plural layers, a merit can be obtained that weldability with fuse element4is enhanced and the strength of weld is secured. Further, it is preferable to select materials having good welding compatibility with the material of fuse element4.

Fuse element4is a fusible member, which is welded onto plated layers3and electrically connected with lead terminals2through plated layers3. Fuse element4is welded to lead terminals2at end portions thereof opposing each other and so connected as to span the pair of lead terminals2. Fuse element4is a material to produce a fusing function. As current fuses, such material may be used as gold, copper, silver, nickel, aluminum, tin, and a compound or alloy of such materials. As temperature fuses, such low-melting point metal as tin, bismuth, indium, lead, and cadmium may be used as a single metal or an alloy having such metals mixed therein. In the present embodiment, a fuse element of a temperature fuse formed of a ternary alloy of tin, bismuth, and lead and having a melting point of 96° C. is used as fuse element4. Although fuse element4used in the present embodiment is substantially rectangular parallelepiped, that in a disk shape, cylindrical shape, or linear shape may be used.

Although it is not shown inFIG. 1andFIG. 2A, fuse element4may be welded onto the surfaces of plated layers3after applying a welding flux to the surfaces. The welding flux is an auxiliary agent for electrically connecting fuse element4with lead terminals2and it is selected after making it sure that its compatibility with the material of lead terminals2and the material of fuse element4is good. For example, a liquid flux of the rosin group may be selected as the welding flux.

In addition to the above mentioned components, cover film27may be provided to cover the fuse portion including fuse element4as shown inFIG. 1. This arrangement is meritorious because melted fuse element4is thereby prevented from spattering around. Further, cover film27may be formed of an upper and a lower film pasted together or may be formed of a single sheet of film rolled up. Further, flux28containing rosin and the like to facilitate melting down of fuse element4may be filled in the interior of cover film27. This also prevents melted fuse element4from spattering around.

Further, fuse element4may be welded onto plated layer3after disposing meltable layer13for welding thereon as shown inFIG. 2B. Namely, the intermediate layer for welding may be formed of two layers of plated layer3and meltable layer13. Thereby, such a merit can be obtained that the strength of connection between fuse element4and lead terminals2is enhanced. As meltable layer13, a meltable metal such as a solder and silver paste is suitable.

First, at step1, substrate1is prepared. Then, at step2, a pair of lead terminals2are bonded onto substrate1. They are bonded via thermo-compression bonding, or a solvent including adhesive components is used for bonding. At this time, face-to-face distance31of lead terminals2is set so large that insulating capability between lead terminals2after melting down of fuse element4as described above can be secured. A metal ribbon or the like are cut to produce lead terminals2. Then, edge burrs (protrusions)12are sometimes produced at end portions51of lead terminals2opposing each other as shown inFIG. 2A. Edge burrs12, as described in the foregoing, serve as non-plated regions at step3that follows.

At subsequent step3, plated layers3are formed on portions of the surfaces of lead terminals2. At this time, plated layers3are formed on the portions of lead terminals2except opposing faces5. When required, plated layers3are formed except non-plated regions6provided in ranges a predetermined distance set back from the opposing end faces of lead terminals2. To achieve this, plating may be performed on lead terminals2after, for example, masking non-plated regions6and opposing faces5with a resin film or the like, and the resin film may be peeled off after plating. Otherwise, edge burrs12unavoidably formed by cutting work as described above may be positively utilized for preventing plated layers3from being formed on opposing faces5. Further, when necessary, meltable layers13may be formed at least at a portion of the top face of plated layers3as shown inFIG. 2B. In that case, fuse element4is welded to meltable layers13at below discussed step5. Further, when it is needed, a flux mainly composed of rosin may be placed on the surface of plated layers3.

Then, at step4, fuse element4formed of a fusible member is placed on lead terminals2. Fuse element4here is in a parallelepiped shape. Due to placing fuse element4such that its end face comes within the range of the top face of plated layers3, the welding to be described at step5becomes easy. Fuse element4may otherwise be placed beyond the range of plated layer3.

Then, at step5, fuse element4is welded onto plated layers3. Due to being welded to plated layers3, fuse element4is electrically connected to lead terminals2and enabled to function as a fuse. Weld portion7ensures connection of fuse element4to plated layer3. According to need, fuse element4may be filled in cover film27or a case, or flux28may be applied to it. Fuse element4may be in an oval or linear form other than the rectangular parallelepiped form.

At steps1and2, such processes are described in which plated layers3are formed on lead terminals2after lead terminals2have been bonded to substrate1. Other than that, lead terminals2having been provided with plated layer3may be bonded to substrate1. Otherwise, fuse element4may be welded to lead terminals2having been provided with plated layers3and, thereafter, lead terminals2may be bonded to substrate1. Such processes may be suitably changed according to incurring cost and ease of processing.

Welding of fuse element4will now be described in detail with reference to FIG.8A–FIG. 8C.

First, cooling plate8is placed, as shown inFIG. 8B, on the top face of fuse element4mounted on lead terminals2so as to span the same as shown inFIG. 8A. Cooling plate8has an effect to temporarily fix fuse element4onto lead terminals2mechanically and another effect to cool fuse element4. As the material of cooling plate8to obtain a cooling effect, a material having a thermal conductivity of at least 20W·m−1K−1may be used. To be concrete, aluminum, magnesium, copper, titan, gold, silver, nickel, iron, carbon (graphite), and silicon may be used as a single metal, as an alloy of such metals, as an oxide or nitride, or a composite of them. In the case of the present embodiment, an aluminum alloy including at least 90% of aluminum is used.

There is glass as a material having thermal conductivity lower than 20W·m−1K−1, namely 1.1 W·m−1K−1. If such a material is used for cooling plate8, the effect to temporarily fix fuse element4to lead terminals2mechanically can be obtained. However, at the time of welding of fuse element4, fuse element4is spheroidized near the weld portion by the melting heat at the weld portion. Otherwise, there is formed a narrowed or broken portion between the center of fuse element4and the weld portion or an unwelded portion when the welding heat is low. Therefore, limits are imposed on welding conditions and the production yield is lowered in the case of mass production.

Then, end portions4A,4B of fuse element4are heated to be welded to plated layers3. There are various ways for heating end portions4A,4B of fuse element4. Such methods are possible as to heat lead terminals2with heaters, to pass a current through lead terminal2alone so that lead terminal2proper is heated to a high temperature, and to heat lead terminal2, end portion4A and end portion4B directly with infrared rays. In the present embodiment, laser beams9of near-infrared regions are radiated in the direction indicated by the arrows. Thereby, plated layers3and fuse element4are welded together.

Then, cooling plate8is removed from the fuse element4after the welding as shown inFIG. 8C. Since fuse element4is melted only at end portions4A,4B, and scarcely melted at the portion kept in touch with cooling plate8during the welding, the cross-sectional configuration of fuse element4is kept. Therefore, the welding conditions are not so much limited and variations in resistance value are kept low even if the fuses are mass-produced and, hence, production yield is greatly improved.

In order to enhance the cooling effect of cooling plate8, it is effective to place a liquid having a boiling point close to the melting point of fuse element4between cooling plate8and fuse element4. Then, the cooling effect is enhanced by vaporization heat of the liquid. Further, due to the liquid filled in a small gap between cooling plate8and fuse element4, the effect of heat conduction to the cooling plate8is enhanced. Further, by the liquid washing out dirt on cooling plate8due to repeated use of it, foreign substances are prevented from attaching to cooling plate8. To be concrete, pure water, methanol, ethanol, propanol, and butanol may be used. A solvent used in a liquid flux is preferably used.

Through the above steps of processing, fuse element4and plated layers3are welded together and thereby lead terminals2are electrically connected with fuse element4.

Since, at this time, plated layers3are formed at the portions excluding opposing faces5, fuse element4as a fusible member is prevented from forming spread-out portions over opposing faces5. Further, when plated layers3are formed at the portions excepting, further, non-plated regions6, which are set back from the opposing faces of lead terminals2, practically no spread-out portions are formed. Therefore, formation of spread-out portions to shorten the insulation distance of lead terminals2, as is in the case with the prior art where the plated layers were formed on the faces including the opposing faces, can be prevented from occurring at the time of welding of fuse element4. Thus, the insulation distance is prevented from being narrowed and the preset distance31for securing insulating capability can be maintained.

Below will be given description about melting down of fuse element4. Fuse element4is a fusible member and made of metal or the like as a conductor. Therefore, a current flows between lead terminals2through fuse element4. When the flow of current exceeds a predetermined value and becomes an overcurrent, fuse element4generates heat greatly and exceeds its melt-down temperature to be melted down. Likewise, when electronic equipment or a battery pack incorporating a fuse generates heat abnormally to raise its temperature, the fuse exceeds its melt-down temperature to be melted down. The former is a case where the fuse is used as a current fuse and the latter is a case where the fuse is used as a temperature fuse.

FIG. 9shows a fuse as have been melted down. Incidentally, there are not shown cover film27and flux28inFIG. 9. When fuse element4melts down upon exceeding its melt-down temperature, fuse element4is torn apart at melted portion10toward the sides of both lead terminals2so as to collect on the upper surfaces of plated layers3. As a result, melted-down fusible member11coagulates on each of the top faces of two plated layers3. Hence, melted-down fusible members11produced by melting down of fuse element4are prevented from being spread out into the space between the opposing sides of lead terminals2. Namely, melted-down fusible members11, even after being melted down, are not spread out into the space between lead terminals2and, hence, the insulation distance is not narrowed. Therefore, the insulation distance is maintained as preset face-to-face distance31after the melting down, and hence insulating capability can be fully maintained. In the case of the prior art where plated layers101are disposed as far as they reach opposing faces102, melted fuse element104is spread out over plated layers101and, thereby, the insulation distance is decreased corresponding to the spread-out amount. In this case, although the face-to-face distance is determined at the stage of designing to secure required insulating capability, the insulation distance after the melting down becomes shorter than the designed face-to-face distance and there arises the possibility that the designed insulating capability becomes unobtainable. If, allowing for it, the face-to-face distance is set larger at the time of designing, reduction in size of the fuse cannot be attained. In the case of the fuse of the present embodiment, in contrast with prior art fuses, spread-out portions are not formed on opposing faces after the melting down and, hence, insulating capability can be secured.

Therefore, in contrast to the case of melting down in the prior art fuses, the face-to-face distance of lead terminals after melting down, i.e., the insulation distance, can be maintained the same as face-to-face distance31set at the time of designing. Hence, a fuse can be constructed with a smaller terminal-to-terminal distance. As a result, a fuse smaller in size than that in the prior art can be realized and, also, by the use of the fuse, it becomes possible to configure a temperature fuse and current fuse very small in size.

Now will be described an example in which the fuse configured as above is applied to a battery pack, hereinafter.FIG. 10A,FIG. 10Bare perspective views with a portion broken away of a battery pack according to the present embodiment of the invention.

Fuse22is a fuse of the above described configuration and it is used as a temperature fuse. In fuse22, a pair of lead terminals24,25are bonded to the top of a substrate and a fuse element made of a fusible member is welded to lead terminals24,25so as to span the same. The fuse element is welded to lead terminals24,25through the plated layers which are formed thereon except opposing faces of the lead terminals. Hence, the fuse element even after it has been melted down is prevented from being spread out over the opposing faces of the lead terminals. Further, a center of fuse22is covered with a cover film made of PEN (polyethylene naphthalate) and a flux containing rosin and the like as the main components is sealed therein.

Wiring23A is led out from either positive or negative terminal26of battery20. One lead terminal24of fuse22is connected with wiring23A led out from battery20, while the other lead terminal25, is connected with wiring23B across fuse22. Wiring23B is led out of the outside of casing21, constituting the main body of pack battery19, as a battery terminal of the same polarity as that connected with wiring23A (namely, a positive or negative terminal of battery20) and connected to another electronic component (not shown) to realize power supply. The terminal of opposite polarity of battery20is led out separately and connected to the electronic component. Thereby, power is supplied from pack battery19to the electronic component. Namely, fuse22is attached to either positive or negative terminal26of pack battery19and disposed between wirings23A,23B.

When abnormal heat is produced in pack battery19, the fuse element made of a fusible member is melted down and fuse22is rendered non-conductive. Hence, power supply from pack battery19is cut off. By the cutoff of the power supply, heat production thereafter is suppressed and occurrence of damage to the user due to the abnormal heat can be prevented. Further, even when the current value becomes abnormally large, the fuse element melts down due to heat produced by the overcurrent, whereby fuse22is rendered non-conductive. Thereby, influence to the user and failure of the electronic equipment caused by overcurrent thereafter can be prevented.

At this time, even after the melting down, the melted fuse element is prevented from being spread out over the opposing faces of lead terminals24,25as described above. Accordingly, such a problem does not arise that the insulation distance becomes shorter after the melting down and a proper insulating capability is made unobtainable. With prior art fuses, there are cases where objects to be attained by the fuses, i.e., protection of electronic equipment and users from abnormal heating and overcurrent, are unattainable, though the fuse has been melted down, because of the formation of the spread-out portions and, hence, sufficient insulation is not secured. In contrast therewith, in the case of fuses of the present embodiment, the insulation distance is not shortened and maintained as the preset face-to-face distance after the melting down, and hence the insulating capability is maintained as originally designed. Therefore, electronic equipment and users are protected by the melting down from abnormal temperature or over current. Further, since the insulation distance is not shortened by the melting down, it is possible to design originally the distance between the lead terminals at the minimum value capable of securing sufficient insulation. Thus, it is made possible to produce fuse22in small size. Therefore, even if pack battery19is made smaller sized and lower profiled, a fuse can be mounted thereon and, hence, it is made possible to provide a fuse capable of keeping pace with the development of miniaturization of pack battery19.

An example where fuse22is mounted on pack battery19is shown inFIG. 10A. Otherwise, the fuse can function as a current fuse even if it is mounted on an electronic substrate or around an IC.

Further, pack battery19is mounted on a mobile telephone, personal digital assistance, notebook-size computer, and the like. The same may be said, not only of the battery mounted on such electronic apparatuses as mentioned above, but also of the battery mounted on a desktop personal computer and precision electronic instrument. In order that such electronic apparatuses are protected from abnormal heat and over-current and that users are prevented from suffering damage before it occurs, mounting of a fuse on them is important. In this connection, while a large number of pieces of electronic equipment are being made smaller sized, lower profiled, and more densely packed, miniaturization of fuses mounted on them becomes important. Under these circumstances, since the insulation distance of the lead terminals after the fuse has melted down and the face-to-face distance of the lead terminals designed for securing insulation can be made identical, very small fuses can be realized. Therefore, it becomes possible to meet requirements for the electronic equipment incorporating the fuses.

Although fuse22is disposed so as to contact with casing21forming the body of pack battery19inFIG. 10A, it may be disposed in contact with battery20as shown inFIG. 10Bto operate more accurately as a temperature fuse.

Although the intermediate layers including plated layers3or meltable layers13are provided on a pair of lead terminals2in the present embodiment, the intermediate layer may be provided on only one of lead terminals2. This structure has an effect like that including intermediate layers on both lead terminals2as described above. Edge burrs (protrusions)12or protruded portions having curved surfaces121may be provided on only one of lead terminals2.

In the present invention, intermediate layers for welding used in the welding of a fuse element to lead terminals are formed on the lead terminals except opposing faces of the lead terminals. Thereby, formation of spread-out portions over the opposing faces of the lead terminals is prevented from occurring when the fuse element is welded, and hence the insulation distance is prevented from being narrowed. Further, when the fuse element has been melted down, the melted fuse element is prevented from being spread out over the opposing faces. Hence even after the melting down, the insulation distance remains intact and the insulating capability is secured. Therefore, the face-to-face distance of the lead terminals set at designing can be made equal to the distance required for securing the insulating capability. Accordingly, the face-to-face distance can be originally set to a minimum possible value and it is made possible to realize small sized and low profiled fuses. Further, it is made possible to realize miniaturization of pack batteries with a fuse mounted thereon and electronic equipment incorporating such a battery.