EP0935809B1

EP0935809B1 - Electrical fuse

Info

Publication number: EP0935809B1
Application number: EP97950058A
Authority: EP
Inventors: André Jöllenbeck; Bernd FRÖCHTE; Kirsten Thume
Original assignee: Wickmann Werke GmbH
Current assignee: Wickmann Werke GmbH
Priority date: 1996-10-31
Filing date: 1997-10-31
Publication date: 2001-03-28
Anticipated expiration: 2017-10-31
Also published as: ATE200164T1; DE69704434D1; EP0935809A1; DE69704434T2; WO1998019322A1; AU5315398A; JP2001503558A; DE19644026A1

Abstract

The present invention relates to an electrical fuse element (1) having a substantially ceramic housing and a fusible conductor (3, 4) and to a process for its production. All known fuses of the said type have problems in mass production, in particular in the fixing and fastening of their fusible conductors. The object is therefore to provide a fuse and a process for its production which simplifies the fixing and fastening of fusible conductors and altogether brings about a reduction in cost in comparison with known processes. The object is achieved according to the invention in that a fusible conductor (3, 4) is placed between two green ceramic layers (2, 5) in such a way that it runs at least between mutually opposite end faces of the ceramic layers (2, 5) and this arrangement is pressed and subsequently sintered.

Description

The present invention relates to an electrical fuse element having a substantially ceramic housing and a fusible conductor according to the preamble of claim 1.

Fuse elements of the said type are installed in large numbers in electric circuits as protection against excessive currents. Their field of application extends from the light-current range into the range of relatively high voltage levels. In light-current circuits, such fuses are used in particular as SMD-type chip fuses of very small dimensions. When used at relatively high voltage levels, these fuses are distinguished in particular by the resistance of their ceramic housing to high temperatures and great mechanical stresses.

WO A 96/08831 discloses an electrical fuse element having a substantially ceramic housing with the fuse element been produced in a multiple repeat, where in the fuse element a fusible conductor runs between two ceramic layers, bonded to each other in the green state, from one end face to the other end face and after consolidating the arrangement, contacts are arranged on the end faces at which the fusible conductor emerges. Such electrical fuse elements have the disadvantage that the switching characteristics of the electrical fuse element can not be selectively influenced.

The object is therefore to provide a fuse, in which the arc-quenching characteristics are increased. According to the invention the object is achieved in that the fuse element contains at a least cavity in the center region of its ceramic housing.

On the surface of at least one green ceramic layer a hollow is impressed, for example for receiving a solvent or for producing a cavity within the laminate. The fusible conductor may be free in such a half-sided cavity and consequently run, for example in its warmest region, the hot spot, without substrate contact. By such a measure, the switching characteristics of a fuse element can be selectively influenced. A hollow may, however, also be arranged on both sides around the hot spot, with the result that the fusible conductor runs in an unsupported manner through, for example, a gas-filled cavity. A filling of the cavity, and of the hollow, with a porous or powdered solvent is possible, with the result that the invention allows entirely new approaches to be adopted in designing the properties of the fuse.

At least one of the green ceramic layers may be perforated in the region of the centre axis before the fusible conductor is placed in. After the lamination of the fuse element, consequently at least one open region has been created at the centre axis of the arrangement. This open region advantageously has, for example opposite a hollow, a much greater free diameter, with the result that marginal influences originating from the walls of the hollow can become negligible. Thus, taking into consideration the dimensions in the open region, a half-space with approximately constant properties can be created. The parameters essential for the fuse, or the tripping behaviour of the fuse, can then be defined in a suitable way by the user in this free half-space, which is openly accessible from the outside. In this case, the perforation may be performed as a simple round or in any way shaped punching, or else as a slot or as a succession of a plurality of holes. Precisely in the case of an embodiment with a plurality of holes over the length of the fusible conductor is the selective use of a plurality of arcing chambers possible.

In this case, the perforations do not necessarily have to be arranged above the fusible conductor, they may also be arranged in the form of many small perforations in the ceramic material alongside the fusible conductor and lend the ceramic material a predetermined porosity. Thus, the heat flow from the hot spot towards the external contacts can be influenced, just as the dissipation of the pressure energy produced when the fusible conductor melts through can be influenced by breaking up of material bridges in the region surrounding the fusible conductor.

By one of the measures mentioned above, it is advantageously possible to produce different switching characteristics and nominal fuse currents from the same fusible conductor material and with the same fusible conductor cross-section, just by the described selective variations of the direct surroundings of the hot spot. In terms of production technology, punching devices are used for the perforations. However, lasers have also been successfully used for this purpose in the case of green or fired ceramic layers.

In an alternative embodiment, between the outer ceramic layers there is arranged at least one ceramic layer which has, in particular in the green state, a different material composition, for example for producing a specific porosity, preferably by a predetermined content of small polystyrene beads in the green ceramic. During the sintering step, the small polystyrene beads create cavities within the material of the ceramic layer, without carbon residues being left behind at the end in the pores to any great extent. This rules out a formation of carbon bridges. In this way, pressure equalization chambers are created with a large surface for taking up electrically conductive vapours in the event of tripping of the fuse. At the same time, the pores advantageously impair the heat flow within the fuse, in that they worsen the carrying away of heat from the hot spot to the terminal pads.

An open, air-filled half-space may have a ceramic half-space lying opposite it on the other side of the fusible conductor. If the fusible conductor then runs at the boundary between these two half-spaces, the fuse element can be influenced unilaterally in its disconnection properties by the shape of the free, here only air-filled half-space. According to the invention, the perforation, through which the fusible conductor in any event runs, is arranged in the central region of the fuse element. In the case of operation, the highest temperatures occur in the chosen region of the fuse element, whereby every variation in the surrounding properties induced by a change in material can have a maximum effect on the disconnection properties of the fuse element.

Perforations are advantageously arranged one above the other in both ceramic layers surrounding the fusible conductor. In the region of the perforation there are consequently two half-spaces available, in the boundary plane of which the fusible conductor runs. Depending on the choice of filling materials, the half-spaces may have different material parameters, which can be isolated from one another. However, both half-spaces may be filled in the same way with the same material, with the result that the fusible conductor runs through a homogeneous space in the region of the perforation as well. The filling of the half-spaces may take place by the use of solid media in one-piece form, for example by prefabricated mouldings, which may also partially enclose the fusible conductor. To be mentioned here as a technical configuration is a tablet-shaped, green sintered body with a groove for receiving the fusible conductor.

Preferred, however, is a filling by flowable media, for example by pastes of a sinterable glass ceramic material. The fusible conductor is mechanically supported by the filling, with the result that the fuse element as a whole is insusceptible to vibrations and mechanical shock. Furthermore, a paste-like filling composition and an otherwise flowable medium can bond optimally with the inner wall of the perforation, with the result that no further bonding measures have to be taken or forms of fastening provided.

In a development, a perforation is covered by an outer, further green ceramic layer. After lamination, there is thus produced a space of defined size with predetermined geometry, which is closed in a gastight manner at the ends. Consequently, the fusible conductor is securely protected mechanically and against ambient influences. This closed arrangement also has, however, the advantage that now even flowable or pourable filling materials can be used, and they do not have to be made to set or protected from the surroundings to retain their material properties. For example, in an arrangement perforated on one side or else on two sides, covering with an additional ceramic layer allows the cavity produced in a defined way to be filled with fine quartz sand or aluminium-oxide powder.

In the process for producing the electrical fuse element according to the invention, geometrically identically shaped ceramic layers may be advantageously used for constructing and producing a fuse element. This restricts the complexity of the steps, in terms of production technology, before the assembly of the fuse element and the lamination. However, the process according to the invention also allows the use of differently shaped ceramic layers for the construction of specially shaped ceramic bodies. A lower and a covering, upper ceramic layer with a fusible conductor arranged in between may be dimensioned such that the upper ceramic layer is, for example, of a narrower design than the lower ceramic layer, with the result that the upper ceramic layer is set back from the lower ceramic layer at the end faces. This geometry can be used to produce, for example, larger contact areas on the end faces, or to change them decisively in their form. Thus, for example, terminals are exposed for the installation of the fuse element as a composite fusible conductor in a fuse housing, as is still to be explained with reference to an exemplary embodiment.

At least on the surface of one green ceramic layer which comes into contact with the fusible conductor, conductive paste may be applied in the mutually opposite regions. These regions lie at a distance from one another which is determined by the dimensions of the component. The application of the conductive paste preferably takes place in the marginal regions at the locations at which, for example after the production of strips, the external contacts are applied as a finishing step. Without this measure, the external contact goes over directly into the fusible conductor with its comparatively small diameter. Thus, during operation, very high current densities can occur already in the region of the external contacts of the fuse element.

The paste is preferably applied in strip form, in order effectively to enlarge the contactable cross-section of the fusible conductor towards the outside and consequently lower the current density at the terminal. Furthermore, in this form, the application of the paste can also be carried out particularly simply, for example in the form of a material-saving screen- printing process. Thus, it can be ensured overall that the melting through during tripping takes place in the centre of the fuse. The closed ceramic housing absorbs the pressure produced during the tripping of the fuse element, with the result that no metallic vapours can escape to the outside. Also, an arc which always occurs during tripping consequently never leaves the fuse element into the surroundings.

In a development, the ceramic layers may, furthermore, also have plated-through holes at any location. These plated-through holes may be filled or else unfilled and consequently open up possibilities of a different type of fastening, contacting or use of the fuse element.

A fuse element according to the invention can be used directly as a surface-mounted device. In addition, it may also be inserted as a combination fusible conductor into existing fuse housings, in order in this way to form fuse types with new switching properties. Fuse elements may be advantageously used as combination fusible conductors of a special internal design in known fuse housings. For example, the fastening and contacting of a fuse element on the external contact legs arranged appropriately in the fixed IEC standard pattern of the respective voltage level may take place in analogy with the insertion of surface-mounted devices in a circuit board.

The additional housing serves for further encapsulation of the arrangement, with the result that, even when the fuse element bursts during tripping of the fuse, no fragments can escape to the outside. Moreover, a further thermal insulation of the fuse element occurs. Thus, compliance with further-reaching test regulations can be achieved more easily on the way to an international certification of such a fuse. For explanation, at this point reference should also be made to the description of preferred embodiments of the invention.

A fuse element according to the invention can be produced in a multiple repeat with all the previously described process steps in a mass-production process. Used for this purpose are green ceramic layers of standardized sizes, which correspond to a multiple of the size or the edge lengths of a single fuse element. At the same time, a plurality of fusible conductors are arranged and at least partially covered by at least one upper ceramic layer, as described above. After lamination, pressing the arrangement under an increased ambient temperature, an individual separation of the fuse elements can then already take place by cutting, breaking or sawing, since the arrangement is adequately fixed already in this state.

In the case of arrangements with a thickness of more than two ceramic layers, it is preferred to perform the individual separation after the lamination, for example by cutting. It is also possible, however, to separate the finally consolidated arrangement, with a finished, ceramic outer body, after sintering, by breaking or sawing.

In an essential development of the process for producing fuse elements according to the invention in a multiple repeat, at least two layers are divided up into strips along lines while in the green state and are sintered. These lines are preferably essentially perpendicular to the fusible conductors, with the result that the fusible conductors can also be simultaneously cut into pieces of predetermined length in this production step of dividing up the laminate. The strips thus obtained comprise a multiplicity of individual fuse elements, which lie parallel to one another and, after the sintering step, are available in a form which is optimally protected for the further processing. In this case, the strips are of course much larger than the individual fuse elements and can correspondingly be handled more easily.

Alternatively, by selecting green ceramic in the form of long strips or tapes, the production may also be set up directly for producing the laminate strips described above, with the subsequent sintering step. In a preferred production process, however, sintered strips are in any event used at least as intermediate products.

The sintered strips are advantageously metallized on the end faces by electroplating and are then individually separated by breaking or sawing, in particular along grooves. Consequently, in a particularly inexpensive and efficient process, fuse elements according to the invention can also be provided with external contacts in the multiple repeat. The preferred process of metallization by electroplating additionally has the advantage over other pro cesses that the fusible conductor does not have to run through any additional thermal process step. The selective weakening of the ceramic body by the grooves is likewise not influenced by the electroplated contacts. Moreover, the electroplated contacts can also be broken in the region of the grooves in the ceramic housing of a strip, with the overall result that fuse elements of adequately high accuracy, including with respect: to the dimensions of the housing, are obtained.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawings, in which:

Figure 1a: shows a perspective representation of a number of assembly steps for the production of an embodiment of a fuse element;
Figure 1b: shows a perspective representation of the embodiment from Figure 1a in an intermediate step;
Figures 1c, 1d: show perspective representations of the fuse element from figure 1b with filling and as a finished fuse element;
Figures 2a - 2c: show perspective representations of the assembly of a further embodiment of a fuse element
Figures 3a, 3b and 4: show perspective representations of further embodiments of three-layered construction and
Figures 5a, 5b: show sketches of the installation of a fuse element of the embodiment from Figure 2c or Figure 3a as a surface-mounted device into standardized or popular housings.

Figure 1a contains a perspective representation of a number of assembly steps for the production of an embodiment of a fuse element, in this embodiment a perforation 20 being located in the centre of the upper green ceramic layer 5. The perforation 20 has the effect that the wire 4 running along the centre axis 12 is then not sealed in by the, later closed, outer ceramic body in a region which is essential for determining the disconnection behaviour of the fuse element 1. In the open region, the wire 4 becomes hottest during operation, and this is where it is also severed during tripping of the fuse element. The opening in the ceramic body 8 provides the possibility of decisively varying the disconnection behaviour of the fuse element 1, by the choice of a suitable covering or a composition 22 with which the perforation 20 is closed over the wire 4. Furthermore, the ageing behaviour of the fuse can also be changed. It is preferred for viscous sealing compositions to be used for this purpose, in other words, for example, ceramic pastes which, as the production process progresses, are made to set as the ceramic body is consolidated. However, self-setting substances may also be used, as long as they positively influence the disconnection behaviour, for example by the irreversible absorption of hot gases from an arc at the instant of disconnection. The inclusion of small cavities in the region of the perforation 20 may also have the effect of creating near the wire 4 pressure equalization spaces which can, at the same time, increase the thermal insulation of the wire 4.

Figure 1b shows a perspective representation of the embodiment from Figure 1a in an intermediate step, in which the wire 4, exposed by the perforation 20 of the upper ceramic layer 5 in the arrangement, can be seen well. In the region of the line 9, drawn in to mark the former boundary layer between the

ceramic layers

2, 5, one of the terminal areas 19 can now also be seen on the end face 7.

Figure 1c shows a perspective representation of the fuse element from Figure 1b after the filling of the perforation 20 by a composition 22, which terminates planar with the surface of the upper ceramic layer 5. For reasons of pressure resistance of the covering, the thickness of the composition 22 over the wire 4 should not be chosen to be too small. Thus, in the case of the illustrations described, it must always be borne in mind that the

ceramic layers

2, 5 represented themselves have very small layer thicknesses. It must also be borne in mind when selecting the composition 22 that arc quenching properties of the composition are desired. However, the composition must not under any circumstances assist burning under the influence of an arc, for example by forming conducting carbon bridges.

Figure 1d shows in a perspective representation the fuse element from Figure 1b with filling, as a finished fuse element with electroplated contacts 10. The fuse element 1 represented could be produced as a surface-mounted device of very small dimensions, so the composition 22 may be used to identify the properties of the component. This may be carried out by the outer form of the covering over the upper ceramic layer 5 or by a colouring of the composition 22, to mention just a few examples.

The process represented in Figures 1a - 1d can also be used in the production of fuse elements in which perforations 20 have been made in the centre of both

ceramic layers

2, 5, whereby the switching capacity and the disconnection behaviour of the fuse element 1 can be influenced to an even greater extent, since the wire 4 is located in its most intensely heated region, in this case in a homogeneous material.

In the production of electrical fuses, even of extremely small geometrical dimensions, there is often the need to allow the fusible conductor to run through media which, although pourable, are not curable. An example of this is fine quartz sand. Furthermore, there are also examples of filling materials which have to be protected against surrounding influences. Figures 2a - 2c show in perspective representation a development of the production process represented in Figures 1a - 1d for providing a further embodiment of a fuse element for the applications mentioned. The wire 4 is enclosed by an upper and a lower green

ceramic layer

2, 5, which respectively have in their central region a perforation 20, which in the assembled state come, to lie one over the other in a way analogous to Figure 1a. This arrangement is supplemented by two outer green ceramic layers 23, which cover the perforations 20, with the result that a cavity of defined dimensions is produced in the interior of the arrangement 6.

Before applying the outer green ceramic layer 23, arranged on top, it is possible in an intermediate step (not represented here) to fill this cavity, with wire 4 passing through, with a composition 22. The closing off and lamination then follow. With or without filling of the cavity, after lamination an arrangement 6 is obtained such as that represented in Figure 2b. The lines 9 represent the former boundary surfaces, no longer visible in reality, between the layered one on top of the other, adhesively bonded with one another and pressed green

ceramic layers

23, 2, 5 and 23. Between the

layers

2 and 5 there can be seen on one end face 7 a terminal area 19 with the end of the wire 4.

The final application of external contacts 10 to the arrangement 6 takes place in Figure 2c.

Figures 3a, 3b and Figure 4 show perspective representations of further embodiments very much similar to the construction from Figure 2a, in each case only a three-layered construction is represented, but this can be developed into a construction of four or more layers without any problems. In Figure 3a, the form of the perforation 20 has been changed in comparison with Figure 2a to that of a slot 20a, in order to obtain, for example, a particularly large hot-spot region.

In Figure 3b, on the other hand, in the ceramic layer 5 there have been made three perforations 20, through which the wire 4 runs. These perforations 20b are thus arranged in series in the manner of arcing chambers and thus increase the voltage drop during disconnection of the fuse, in that the voltage drops of each arc are cumulative in a series arrangement.

Figure 4 shows a perspective representation of a further embodiment in a three-layered construction, the porosity of the ceramic layer 5 being changed in a predetermined way by a large number of small perforations 20c. This allows the heat flow from the hot spot towards the external contacts to be impaired and consequently the nominal current of a fuse element to be set. Given the same parameters of the wire 4, in this way alone the nominal currents can be set in a wide range by varying the porosity of the layer 5, without any further changes to the construction.

For this purpose, the perforations 20c do not necessarily have to lie above the wire 4 in the hot spot, as already diagrammatically sketched in Figure 4. They may be made in a green ceramic layer by punching or laser cutting. As an alternative to the measures described above, a green ceramic layer 5 is also advantageously used with a varied material composition, strongly influencing the porosity of the ceramic layer 5 in the sintered state. For example, a green ceramic layer 5 is prepared by admixing with a specific amount of small polystyrene beads. During sintering, the small polystyrene beads disintegrate and leave behind in the ceramic layer 5 corresponding cavities, which are even free from carbon to the extent that arcing back by the building up of carbon bridges can be ruled out.

Figures 5a, 5b show examples of the use and installation of a fuse element 1 as a combination fusible conductor 24 in standardized or known and popular types of housing 25 from the instrument fuse sector. Figure 5a shows a type of installation of a finished fuse element 1, to which there has been added a perforation 20 with filling by a composition 22, into a small glass tube 26. The small glass tube 26 is closed at the ends by metal caps 27. During the assembly of the arrangement, an electrically conducting connection is established between the contacts 10 of the fuse element 1 and the associated metal cap 27. This completes the adaptation of the fuse element 1 to circuits and customary connection devices and fuse holders from the field of 125V and 250V voltage levels.

The installation of a fuse element 1 as a surface-mounted device in a standardized or popular fuse housing 25 is represented in Figure 5b. Here, the housing of the known TR 5® fuse has been chosen by way of example as fuse housing 25. The housing comprises a cap 28, which is locked together with a housing base 29, through which pins 30 emerge as electric contacts. The fuse element 1 is turned over, with the result that the terminal areas 19 of sintered conductive paste, integrated herein to the production process, respectively come into contact with one end 31 of a pin 30. In the present example, the ends 31 are punched flat, with the result that the fuse element 1 rests on the ends 31 in the region of the terminal areas 19. The ends 31 may also be punched to form steps and/or be bent inwards or outwards, to allow the fuse element 1 to be held better or to clamp it in securely before soldering. The centred fuse element 1 is then soldered at the terminal areas 19 to the ends 27 in a known way.

Alternatively, a part 32 of the ceramic outer body may also serve as a base 33, at which the fuse element 1 can be adhesively attached on the housing base 25. In this production process, the length of the pins 30 from the housing base 29 to the ends 31 must be matched to the height of the base 29. The contacting takes place here via the terminal areas 19 at the pins 30 by soldering on in a customary SMD process.

Claims

Fuse element (1) having a substantially ceramic housing with the fuse element (1) been produced in a multiple repeat, where in the fuse element (1)

a fusible conductor (3)

runs between two ceramic layers (2,5), bonded to each other in the green state,

from one end face (7) to the other end face (7), and

after consolidating the arrangement (6),

contacts (10) are arranged

on the end faces (7)

at which the fusible conductor (3) emerges,

characterised in that
the fuse element (1) contains at least one cavity in the centre region of its ceramic housing.
Fuse element according to Claim 1, characterised in that the arrangement (6) is consolidated by a sintering process.
Fuse element according to one or both of the preceding Claims, characterised in that the fusible conductor (3) is designed as a wire (4).
Fuse element according to any of the preceding Claims 1 - 3, characterised in that a filling material is laminated in between the ceramic layers (2, 5), preferably an arc-quenching filling material.
Fuse element according to one or more of the preceding Claims 1 - 4, characterised in that the surface (2b) of at least one ceramic layer (2, 5) has in the region of the centre axis (12) at least one perforation (20) or a recess.
Fuse element according to one or more of the preceding Claims 1 - 5, characterised in that the fusible conductor (3) runs along the centre axis (12) between two ceramic layers (2, 5), both ceramic layers (2, 5) having in the region of the centre axis (12) a perforation (20) or a recess, over which the fusible conducfor (3) runs in a self-supporting manner.
Fuse element according to one or more of the preceding Claims 1 - 6, characterised in that the recess or the perforation (20) is filled with a composition (22).
Fuse element according to one or more of the preceding Claims 1 - 7, characterised in that the perforations (20) are covered already in the green state of the ceramic layers (2, 5) in each case by a further ceramic layer (23).
Fuse element according to one or more of the preceding Claims 1 - 8, characterised in that the upper ceramic layer (5) and the lower ceramic layer (2) have the same geometrical form and size.
Fuse element according to one or more of the preceding Claims 1 - 9, characterised in that at least one of the ceramic layers (2, 5, 23) is set back from at least one neighbouring layer (2, 5, 23) at the two end faces (7) over which the fusible conductor (3) runs.
Fuse element according to one or more of the preceding Claims 1 - 10, characterised in that the ceramic layers (2, 5, 23) have plated-through holes.
Fuse element according to one or more of the preceding Claims 1 - 13, characterised in that between the outer ceramic layers (23) there is arranged at least one ceramic layer (2, 5) which has a different material composition, in particular for producing a specific porosity, preferably by a predetermined content of small polystyrene beads in the green ceramic material.
Fuse element according to one or more of the preceding Claims 1 - 12, characterised in that electroplated contacts (10) are arranged on the fuse element (1) in electrically conducting connection with the fusible conductor (3).
Fuse element according to one or more of the preceding Claims 1 - 13, characterised by

at least one surface containing a part of a groove (16) and an edge

where the fuse element (1) was separated from the arrangement (6a) or a strip (17), especially by breaking,

after consolidating or sintering the arrangement (6a) or the strip (17) with

the arrangement (6a) or the strip (17) consisting of a plurality of fusible conductors (3) being arranged in parallel to each other and being laminated between ceramic layers (2, 5, 5a).