ELECTRICAL FUSE

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: Electrical Fuse The present invention relates to an electrical fuse ele- ment having a substantially ceramic housing and a fusible conductor and to a process for its production. Fuse elements of the said type are installed in large numbers in electric circuits as protection against exces- sive 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 . All fuses of the said type have problems in mass produc- tion, 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 faste- ning 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 - is placed between two green ceramic layers in such a way that it runs at least between mutually opposite end faces of the ceramic layers and this arrangement is pressed and - subsequently sintered. Within the scope of this invention, unfired, flexible SUBSTITUTE SHEET(RULE 25) ceramic sheets, also known as green tape or low-temperature cofiring ceramic, are referred to alike as green ceramic layers. In a process according to the invention, the property of green ceramic layers to bond adhesively with one another under the influence of pressure is utilized in order to enclose a fusible conductor placed between two green ceramic layers. For this purpose, the fusible conductor is simply placed over one green ceramic layer and covered by a second green ceramic layer. These green ceramic layers are deforτnable within limits and therefore enclose the fusible conductor during the pressing together of the arrangement, in that they deform appropriately in the contact region with the fusible conductor. The fusible conductor is thus fixed and securely held in its position. Furthermore, the adhesive bonding of the layers produces a hermetic and, after sintering, permanent sealing of the fusible conductor from the surroundings. At the same time, the green ceramic layers are consolidated in the sintering step as a one-piece ceramic housing without the aid of further auxiliaries, such as for example adhesive. This homogeneous outer body is at least as good as known types of housing in its properties with respect to strength and compactness. In an essential development, in an intermediate step, the arrangement is pressed in temperature-controlled surroundings, for example in a heating press or an isostatic press. Heating has the effect of further softening the green layers. Under the influence of pressure and temperature, the adhesive bonding, described above, of green layers with one another in the contact region is continued to give an irreversible bond between neighbouring layers, any type of transitional or boundary layer of the ceramic layers disappearing completely. By contrast with pressing under conditions which are not temperature-controlled, SUBSTITUTE SHEET (RULE 25) instead of a surface adhesion or bonding there is then produced a homogeneous body with the previous surface boundaries completely eliminated. This body is also referred to as a laminate. Furthermore, this process step may be used for drying the solvent-containing green layers as a preliminary stage before sintering. The arrangement is advantageously metallized in the region of the end faces on which the fusible conductors are loca- ted. This creates external contacts for the electrical access to the fusible conductor, for example by applying, drying and subsequently sintering a paste. Preferably, however, external contacts are applied by electroplating, with the result that the fusible conductor does not have to be subjected to any further thermal process step in the course of fuse production. Consequently, external contacts have already been applied and the fuse element is ready for installation, for example in an SMD circuit. In a development, the fusible conductor is provided in the form of a wire. A wire is generally defined precisely enough, by its material composition, its cross-sectional area and its length, for describing the behaviour in an electric circuit. According to the invention, the wire is used over a specific path in the described fuse element assembled in a sandwich-like manner. Thus, elements whose tripping properties lie in a narrowly limited interval can be produced in production, since the same, defined ambient influences for the fusible conductor always prevail inside each and every fuse element. In terms of production technology, it has proven successful to draw the wire in a straight line between the green ceramic layers during the construction of the fuse element, or to tension it between the layers during assembly. After the laminating step described under Claim 2, the wire is then carried by the laminate itself, comprising at least two ceramic layers, and is fixed in its position. SUBSTITUTE SHEET (RULE 25) A filling material, for example aluminium-oxide powder, may also be advantageously placed or introduced between the green ceramic sheets and laminated in, for example for influencing the disconnection behaviour of the fuse ele- ment, in that the filling material preferably encloses the fusible conductor at least partially. For this purpose, the filling material may be arranged on or over the fusible conductor and be covered by the upper green ceramic sheet. In the centre of the arrangement, a thickening may be produced by the insertion of the filling material. Allowance can be made for this situation during lamination by using an isostatic press. Furthermore, a conventional press may have a press plate profiled in a way corresponding to the construction of the fuse element to be lamina- ted. In both cases, a laminate having the properties mentioned above can be produced. Furthermore, before the placing together and laminating of the green ceramic layers, there may be impressed on the surface of at least one green ceramic layer a hollow, 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. In an essential development, at least one of the green ceramic layers is perforated in the region of the centre SUBSTITUTE SHEET(RULE 25) 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 . According to Claim 7 , this open region advan- tageously 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 con- stant 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 SUBSTITUTE SHEET(RULE 25) 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 poro- sity, 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 mate- rial can have a maximum effect on the disconnection properties of the fuse element. SUBSTITUTE SHEET (RULE 25) 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 otherwi- se 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 SUBSTITUTE SHEET(RULE 25) 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 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 . According to Claim 14, at least on the surface of one green ceramic layer which comes into contact with the fusible conductor, conductive paste is 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 SUBSTITUTE SHEET(RULE 25) 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 elemen . 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 SUBSTITUTE SHEET(RULE25) - in 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 arrange- ment 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 SUBSTITUTE SHEET(RULE 25) 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 elec- troplating additionally has the advantage over other processes 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 SUBSTITUTE SHEET(RULE 25) 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 la shows a perspective representation of a number of assembly steps for the production of a fuse element in a first embodiment; Figure lb shows a perspective representation of the assembled fuse element from Figure la; Figure lc shows a perspective representation of the fuse element of Fig. la provided with contacts; Figure 2a shows a perspective representation of a number of assembly steps for the production of a fuse element from Figure la in a multiple repeat; Figure 2b shows a perspective representation of a number of production steps of a further embodiment; Figure 2c shows a perspective representation of the finished laminate from Figure 2b; Figure 3a shows a perspective representation of a strip as an intermediate product for the embodiment from Figure 2c; Figure 3b shows a perspective representation of a strip as an intermediate product for the embodiment from Figure 2a; Figure 4 shows a sketch of production steps of an alternative embodiment for Figure 2a in a multiple repeat ,- Figure 5a shows a perspective representation of a number of assembly steps for the production of a further embodiment of a fuse element ; Figure 5b shows a perspective representation of the embodiment from Figure 5a in an intermediate step; Figures 5c, 5d show perspective representations of the fuse SUBSTITUTE SHEET(RULE25) element from Figure 5b with filling and as a finished fuse element; Figures 6a - 6c show perspective representations of the assem- bly of a further embodiment of a fuse element in a sequence analogous to that in Figures la-lc; Figures 7a, 7b and 8 show perspective representations of further embodiments of three-layered construction and Figures 9a, 9b 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 1 shows a perspective representation of the assembly of a fuse element 1 in a basic form. In a first step, in Figure la, a fusible conductor 3 in the form of a wire 4 is drawn and placed over a lower planar, green ceramic layer 2. The wire 4 is then covered by an upper green ceramic layer 5. The arrangement 6 thus produced is heated in a subsequent process step to about 70°C and consequently softens, whereby the upper green ceramic layer 5 adhe- sively bonds inseparably with the lower green ceramic layer 2. This bonding is supported by a pressing operation, proceeding in parallel with the heating, in which the arrangement 6 is pressed together. This step is also referred to as lamination. At the same time, the arrange- ment 6 may be predried, since at the temperatures mentioned at least part of the solvent contained escapes from the green layers 2 , 5. Thus, a precompacted body is obtained, as is shown by Figure lb in a perspective representation. Here, the fuse element assembled to the stage reached in Figure la is shown after the wire 4 has been detached at the end faces SUBSTITUTE SHEET (RULE 25) 7 of the arrangement 6. At the end faces 7 there can be seen the cross-section of the fusible conductor 3, closely surrounded in the region of the end faces 7. In this state, the pressed and predried arrangement 6 has a certain strength, even if the ceramic layers 2, 5 bonded with each other by their own adhesion have not yet been sintered to form a solid outer ceramic body 8. However, this body 8 already exists as a unit in this unsintered state. The line 9 is no longer visible, however, in the arrangement 6, it has been included in the sketch only for the sake of overall clarity and as a guide for the viewer, to mark the former boundary between the two green layers 2, 5. Figure lc shows a sketch of the fuse element 1 from Figure lb, provided with contacts 10 and already sintered. For producing the contacts 10, the arrangement has been immersed by the end faces 7 in a conductive paste 11. The arrangement must now be heated once again in order to cure the conductive paste 11. It is preferred, however, for contacts 10 to be applied by means of electroplated metallization, as still to be described below. An electroplating step dispenses with the need for a sintering step, whereby the fusible conductor then only has to be subjected to high temperatures once during the production of the fuse. In both cases, a contact 10 which can be electrically accessed easily from the outside, can moreover also be soldered and is conductingly connected to the wire 4 is produced by the process step described. The actual path of the wire 4 between the ceramic layers 2 , 5 can generally be freely selected by the chosen type of contacting. In the case of the present example, a defined path along a centre axis 12 of the arrangement 6 has been chosen. It is also conceivable, however, to depart from the part represented in Figure la and, for example, take the wire 4 diagonally over the surface 2a of the ceramic layers 2, in order, for example given a predetermined geometry of the outer ceramic body 8, to increase the length of the wire SUBSTITUTE SHEET RULE 25) 4, as can be seen from the illustration of Figure la. Altogether, the process described provides an arrangement 6 in the electrically conducting part of which there is established a temperature profile which only reaches its maximum value in the pressure-sealed, protected interior of the ceramic body 8, and is accordingly also burned through only at one location in the central region of the wire 4, in the hot spot. At this location, all the materi- al parameters are definable in wide ranges by design measures, such as for example by the insertion of a moulding which covers the fusible conductor 3 and is itself subsequently covered and sealed by the upper ceramic layer 5. Further preferred possibilities of designing individual fuse elements are explained below with reference to further exemplary embodiments. However, these embodiments always represent modifications or expansions of a fundamental production process, so individual steps of a production process are described in advance with reference to the illustrations of Figure 2a to Figure 3b. Figure 2a is a perspective representation of a number of assembly steps for the production of the fuse element 1 from Figure la in a multiple repeat. Here, a plurality of wires 4 running parallel to one another are drawn over a lower green ceramic layer 13 of standard dimensions and are covered by an upper green ceramic layer 14 in a way corresponding to the arrows drawn in. The upper green ceramic layer 14 and the lower green ceramic layer 13 have in this case the same dimensions, which are respectively a multiple of the edge lengths of an upper and lower green ceramic layer 2, 5 of an individual fuse element 1 according to Figure la. To illustrate these differences in size, in Figure 2a there has been respectively drawn in on the ceramic layers 13 and 14 a grid network which is formed by parallel running lines 15 and parallel grooves 16 SUBSTITUTE SHEET RULE 25 which, perpendicular to one another, form the limitations respectively of an upper and lower green ceramic layer 2, 5 of an individual fuse element 1. The grooves 16 are advantageously only impressed into the ceramic layers 13, 14 during lamination itself, in other words in the production step which immediately follows the steps shown in Figure 2a. It is preferred for a correspondingly profiled and heated press to be used for this purpose, with the result that at the same time an optimum, one-piece bond between the upper and lower green ceramic layers 13, 14 is established, while grooves 16 are also impressed at the same time. In the present exemplary embodiment, the lines 15 serve merely as a guide, they are not actually visible in the production steps represented. The significance of the lines 15 within the production process is discussed below with respect to the first exemplary embodiment on the basis of Figure 3b. Sketched in Figure 2b, in a representation analogous to Figure 2a, is a process adapted to the production of a second embodiment of a fuse element. In this case, the steps for preparing the lower green ceramic layer 13 with wires 4 and the arrangement of grooves 16 and lines 15 are unchanged in comparison with the representation from Figure 2a. However, the covering is now carried out by green ceramic strips 14a instead of by a single green ceramic layer 14 , only a central region being covered in the case of each and every fuse element. Figure 2c shows in a perspective representation the finished laminate produced by the process according to Figure 2b. Individual strips 17 of identically constructed, semi- finished fuse elements can be seen from the lines 15 drawn in. The fusible conductors of the parallel lying fuse elements, as pieces of the wires 4, are respectively covered only in a central region of each fuse element by an upper ceramic layer 5a. These strips 17 are subsequently further processed to form individual fuse elements. The exposed parts of the wires 4 can be pushed into the soft lower ceramic layer 13 , for fixing in the region of the lines 15, by a press during lamination. After lamination, the overhanging ends of the wires 4 are cut off. Figure 3a shows a perspective representation of a strip 17 as an intermediate product for the embodiment from Figure 2c, the end faces 7 here having been produced in each case by the severing of the laminate of the construction from Figure 2c along the lines 15. The wires 4 are shown resting on the lower ceramic 2, but they may be fixed in this position, for example by applying a protective film, before severing, as a precaution. However, even without this measure, the wires 4 can be reliably separated along the lines 15 with the green laminate without any damage. The inner construction of a fuse element is represented in Figure 3a on the basis of a section along the centre axis 12 of an element perpendicular to the ceramic layers 2, 5a, to illustrate the electrical connection of an external contact 10 via the wire 4 to the opposite end face 7. Here, for the sake of overall clarity, the electroplated contact 10 has been partially cut away. The separation of the strip 17, provided with contacts 10, takes place by breaking along the grooves 16, which act as notches and ensure that the predetermined outer dimensions are maintained during the individual separation of the fuse elements, which is to be carried out in a very quick and simple way. Figure 3b reproduces a perspective representation of a strip 17 as an intermediate product for the embodiment SUBSTITUTE SHEET RULE 25) from Figure 2a in analogy with Figure 3a. Here, the strip 17 is a single homogeneous block, in which the line 9 is no longer actually visible as the former separating line between the two ceramic layers . The final outer contours of a fuse element are reproduced by the grooves 16, which have been supplemented by breaking lines 16a, as are produced during individual separation after sintering and application of the contacts 10. The metallizations of the contacts 10 are exaggerated in the drawing of Figure 3b. In reality, although the contacts 10 have good soldering properties and resist alloy separation, they can be applied with mostly no discernible increase in the outer dimensions of the individual fuse element and, if necessary, also be reinforced. This can be compensated, in particular, by parts of the contact 10 which reach around the end face 7 running in an edge bevelled appropriately with respect to the outer surfaces as a groove . An alternative embodiment to that from Figure 2a is represented in Figure 4 in the case of production in a multiple repeat. In a first step, layers 18 of a paste in tape form are applied to the lower planar, green ceramic layer 2 and, after sintering and individual separation along the lines 15, are adjacent to the end faces 7 of the lower ceramic layer 2. Perpendicularly to these layers 18, the fusible conductors 3 in the form of wires 4 are drawn and placed onto the surfaces. The layers 18 thus increase the contact area of the comparatively thin wires 4 to the outside, whereby the current density is lowered at these later terminal areas 19 with respect to each wire 4. The hottest spot in each and every fuse element is consequently shifted optimally into the centre. Consequently, mel- ting through of the wire 4 also always takes place in the centre of the fuse element. The production according to Figure 2a and Figure 3b is not impaired by this expansion of the construction by including a further production step. In particular, the dividing up of the laminate into strips is not made any more difficult. Figure 5a contains a perspective representation of a number of assembly steps for the production of a further embodiment of a fuse element in a representation analogous to Figures la - lc, 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 composi- tion 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. The assembly of the embodiment from Figure 5a takes place in the way described on the basis of Figures la - lc. Thus, Figure 5b shows a perspective representation of the SUBSTITUTE SHEET RULE 25) embodiment from Figure 5a 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 5c shows a perspective representation of the fuse element from Figure 5b 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 5d shows in a perspective representation the fuse element from Figure 5b with filling, as a finished fuse element with electroplated contacts 10. The different construction of the fuse element 1 can be seen from Figure 5d in comparison with Figure lc, by the perforation 20 in the centre of the element, closed by the composition 22. 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 . E The process represented in Figures 5a - 5d 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 dis- connection 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 6a - 6c show in perspective representation a development of the production process represented in Figures 5a - 5d for providing a further embodiment of a fuse element for the applications mentio- ned. 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 5a. The construction and assembly take place in turn in the way represented in Figures la -lc. 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 lami- nation then follow. With or without filling of the cavity, after lamination an arrangement 6 is obtained such as that represented in Figure 6b. 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 6c in a way analogous to the way described with respect to Figures 3a, 3b. Like the embodiment from Figure la, this embodiment may also be produced in a process corresponding to the representations of Figures 2a and 3b. Figures 7a, 7b and Figure 8 show perspective representations of further embodiments. Very much similar to the construction from Figure 6a, in each case only a three-layered construction is represented, but this can be developed into a construction of four or more layers wit- hout any problems. In Figure 7a, the form of the perforation 20 has been changed in comparison with Figure 6a to that of a slot 20a, in order to obtain, for example, a particularly large hot-spot region. In Figure 7b, 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 8 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 exter- nal contacts to be impaired and consequently the nominal current of a f se 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 8. 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 9a, 9b 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 9a shows a type of installation of a finished fuse element 1 from Figure 3a, 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 of the enhanced embodiment from Figure 3a as a surface-mounted device in a standardized or popular fuse housing 25 is represented in Figure 9b. 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: Claims 1. Process for producing an electrical fuse element having a substantially ceramic housing and a fusible conductor, characterized in that a fusible conductor (3) is placed between two green ceramic layers (2, 5) in such a way that it runs at least between mutually opposite end faces (7) of the ceramic layers (2, 5) and this arrangement (6) is pressed and subsequently sintered. 2. Process according to Claim 1, characterized in that the arrangement (6) is pressed in temperature-co - trolled surroundings. 3. Process according to one or both of the preceding claims, characterized in that the arrangement (6) is metallized in the region of the end faces (7) on which the fusible conductor (3) is located, for example by applying and subsequently sintering a conductive paste (11) and preferably by electroplating for forming conducting contacts (10) . 4. Process according to one or more of the preceding claims, characterized in that the fusible conductor (3) as a wire (4) is taken between the green ceramic layers (2, 5) in a specific form, in particular is drawn or tensioned in a straight line. 5. Process according to one or more of the preceding claims, characterized in that a filling material is placed between the green ceramic layers (2, 5) and laminated in, the filling material preferably enclosing the fusible conductor (3) at least partially. 6. Process according to one or more of the preceding claims, characterized in that, before the placing together and laminating of the green ceramic layers (2, 5, 23), there may be impressed at least on the surface (2a) of at least one green ceramic layer (2, 5, 23) a hollow, for producing a cavity within the laminate . 7. Process according to one or more of the preceding claims, characterized in that at least one of the green ceramic layers (2. 5) is perforated in the region of the centre axis (12) before the fusible conductor (3) is placed in. 8. Process according to Claim 7, characterized in that the fusible conductor (3) is arranged between two green ceramic layers (2, 5) perforated in the region of the centre axis (12) . 9. Process according to one or both of the preceding Claims 7 and 8, characterized in that the perforations (20) of the green ceramic layers (2, 5) are arranged one above the other. 10. Process according to one or more of the preceding Claims 7 - 9, characterized in that the perforations (20) are filled with a composition (22) . 11. Process according to one or more of the preceding Claims 7 - 10, characterized in that the perfor- ations (20) of the arrangement (6) with the fusible conductor (3) lying in between are covered by at least one outer green ceramic layer (23) and, in particular, both perforations (20) are respectively closed off by at least one outer green ceramic layer (23) . 12. Process according to one or more of the preceding claims, characterized in that green ceramic layers (2, 5, 23) of the same form are joined together in the arrangement (6) . 13. Process according to one or more of the preceding Claims 1 - 11, characterized in that in the production position, over the fusible conductor (3) there is applied at least one green ceramic layer (5, 23), which is set back from the lower ceramic layer (2) on the two sides over which the fusible conduc or (3 ) runs . 14. Process according to one or more of the preceding claims, characterized in that on the surface (2b) of at least one green ceramic layer (2) which comes into contact with the fusible σonduc- tor (3) there are applied in regions opposite one another at a distance, preferably the regions of the end faces (7) , terminal areas (19) composed of a paste, preferably in strip form. 15. Process according to one or more of the preceding claims, characterized in that the fuse element (1) is produced in a multiple repeat. 16. Process according to Claim 15, characterized in that the arrangement (6) is individually separated after sintering, preferably by breaking. 17. Process according to one or more of the preceding claims, characterized in that, in the step of pressing the arrangement (6) , at the same time grooves (16) or other depressions are also impressed into the green layers (2, 5, 23) of the arrangement (6) . 18. Process according to one or more of the preceding claims, characterized in that arrangements (6) comprising at least two layers (2, 5) are individually separated after pressing and before sintering, preferably by cutting, sawing or else by breaking. 19. Process according to one or more of the preceding Claims 15 - 18, characterized in that, for producing fuse elements according to one of Claims 1 - 15 in a multiple repeat, at least two layers (2, 5) in the green state - are divided up along lines (15) into strips (17) and are sintered. 20. Process according to Claim 19, characterized in that the strips (17) are electroplated on the end faces (7) and - then individually separated by breaking or sawing, in particular along grooves (16) . 21. Fuse element having a substantially ceramic housing, characterized in that a fusible conductor (3) - runs between two ceramic layers (2, 5), bonding 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. 22. Fuse element according to Claim 21, characterized in that the arrangement (6) is consolidated by a sintering process. 23. Fuse element according to one or both of the preceding Claims 21 and 22, characterized in that the fusible conductor (3) is designed as a wire (4) . 24. Fuse element according to one or more of the preceding Claims 21 - 23, characterized in that a filling material is laminated in between the ceramic layers (2, 5). 25. Fuse element according to one or more of the preceding Claims 21 - 24, characterized 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. 26. Fuse element according to one or more of the preceding Claims 21 - 25, characterized 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 conductor (3) runs in a self-supporting manner. 27. Fuse element according to one or more of the preceding Claims 21 - 26, characterized in that the recess or the perforation (20) is filled with a composition (22) . 28. Fuse element according to one or more of the preceding Claims 21 - 27, characterized 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) . 29. Fuse element according to one or more of the preceding Claims 21 - 28, characterized in that the upper ceramic layer (5) and the lower ceramic layer (2) have the same geometrical form and size. 30. Fuse element according to one or more of the preceding Claims 21 - 29, characterized 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. 31. Fuse element according to one or more of the preced- ing Claims 21 - 30, characterized in that the ceramic layers (2, 5, 23) have plated-through holes . 32. Fuse element according to one or more of the preceding Claims 21 - 31, characterized 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. 33. Fuse element according to one or more of the preceding Claims 21 - 32, characterized in that electroplated contacts (10) are arranged on the fuse element (1) in electrically conducting connection with the fusible conductor (3) .

Inventors: FROECHTE BERND, JOELLENBECK ANDRE, THUME KIRSTEN, FROECHTE, BERND, JOELLENBECK, ANDRE, THUME, KIRSTEN
Applicants: FROECHTE BERND, JOELLENBECK ANDRE, THUME KIRSTEN, WICKMANN WERKE GMBH, FROECHTE, BERND, JOELLENBECK, ANDRE, THUME, KIRSTEN, WICKMANN-WERKE GMBH
Application year: 1997
Main classification: H01H69
IPC codes: H01H69, H01H85