ELECTRICAL FUSE ELEMENT

Abstract: In an electrical fuse element (1), comprising a substrate (2) with contacts (11) arranged on two opposite sides of the same, between which contacts there runs a fusible conductor (3) which is produced in a laminating process and is connected electrically conductively to the contacts (11), the fusible conductor (3) is covered at least in the region in which the current-breaking operation proceeds by a pasty composition solidifying in a thermal process to form a mechanically stable layer (8), vitrifying in particular at least on one surface. Alternatively the covering of the fusible conductor (3) in the above-mentioned region is formed by a pasty composition which solidifies to a porous state and which for its part is covered by at least one further, mechanically stable layer and is closed from the outside.

Description: Electrical Fuse Element The present invention relates to an electrical fuse element comprising a substrate with contacts arranged on two oppo- site sides of the same, between which contacts there runs a fusible conductor which is produced in a laminating process and is connected electrically conductively to the contacts. An electrical fuse element of the said type is known for example from DE 44 44 599 in the form of a surface-mountable subminiature fuse. On an insulating substrate, a fusible conductor in laminated form is applied between two contacts attached to opposite end faces of the substrate. An electrically insulating cover made of glass completely covers the fuse element and the terminal areas running to the external contacts and thus uniformly covers over the entire upper side of the substrate. The external contacts are additionally coated with electrically conductive material . In the case of this structure, the material of the outer covering of the multi-layer arrangement is quite brittle. Consequently, in the individually separating step following the process of producing the elements in a multiple repeat, the only possible way of separating them is by sawing. This is a costly process. It is the object of the present invention to improve an electrical fuse element of the said type to the extent that it can be produced in a multiple repeat in an inexpensive process. This object is achieved according to the invention by the fusible conductor being covered, at least in the region in which the current-breaking operation proceeds, by a pasty composition solidifying in a thermal process to form a me- chanically stable layer, vitrifying in particular at least on one surface . According to the invention, the covering of the electrical fuse element on the surface of the substrate with the exclusion of the external contacts is restricted to the region in which a current-breaking operation can proceed. The covering is performed by a pasty composition which reliably encloses the fusible conductor on account of its flowabili- ty. The pasty composition additionally solidifies in a thermal process to form a mechanically stable layer. It thereby vitrifies, at least on its surface, and then adheres well on the covered region of the substrate surf ce. This describes a protective layer which can be subjected to mechanical loading in a precisely defined region of the fuse element. In comparison with the fuse elements according to the prior art, the novel arrangement of the covering requires significantly less material to be used for the production of the electrical fuse elements according to the present invention. Furthermore, a fuse element according to the invention is outstandingly suitable for production in a multiple repeat, for example on a rectangular standard substrate. On a sub- strate of a large surface area there can be produced, for example, a multiplicity of fuse elements next to one another in a two-dimensional arrangement in just a few process steps . An individual fuse element according to the prior art always has a very brittle material for the outer covering. As already mentioned, this has the effect that, in the individually separating step following the process of producing the elements in a multiple repeat, the only possible way of separating them is by sawing. This process is comparatively time-consuming and cost-intensive. In the case of the arran- gement according to the invention, on the other hand, the subsequent separating faces do not run through the covering. Thus, when using a ceramic as the substrate material, individual separation by breaking is possible once the covering layer has hardened. In this case, the edges where breaking occurs can be prepared by lasering or scoring in the fired state or else by grooving in the unfired state. The vitrified covering, which is mechanically stable after the heat treatment, does not hinder individual separation of the fuse elements, since the separating lines between the individual fuse elements are not coated with covering material. Moreover, damage to the covering is consequently ruled out, even if the individual separation of the fuse elements is performed quickly and efficiently by breaking the substrate, which leads to considerable cost reductions in comparison with any sawing process. In an alternative form of the invention, the covering of the "hot spot" is formed, according to Claim 2 , by a pasty composition which solidifies to a porous state. This layer is for its part covered by at least one further, mechanically stable layer and is closed from the outside. In a preferred embodiment, the porous inner layer is formed by a high-melting paste, which is dried at temperatures below its own melting point, to be enclosed subsequently by a pressure- stable outer layer. The outer layer hardens at a comparatively lower temperature in a warming process, while the inner layer does not undergo any transformation at the process temperatures and consequently remains porous. In the case of fuses with layer-type fusible conductors, the amounts of material of the fusible conductor in the "hot spot" which have to vaporize during current breaking are very small even in comparison with the thinnest wire-type fusible conductors. Consequently, current breaking takes place essentially without an arc. Investigations have shown that, when tripping a fuse according to the invention, a very high pressure is built up in the region in which the current-breaking operation proceeds. The form of a solidified and, in particular, vitrifying covering proposed in Claim 1 is distinguished by its good electrical and thermal insulating properties as well as high mechanical load- bearing capacity. It is therefore well suited for this application. The multi-layer arrangement proposed according to Claim 2 as an alternative to a single-layer covering forms an increased or additional pressure buffer in comparison with an arrangement according to Claim 1 by means of the inner, porous layer over the fusible conductor. The pressure buffer assumes the function of additionally damping the brief, but strong pressure loading with respect to the outer layer. Thus, the porous, second layer can effectively contribute to preventing the covering from rupturing in the region of the "hot spot" during current breaking. Furthermore, the porous layer can be given further properties, such as for example those of a quenching agent by analogy with the sand filling in glass-enclosed wire-type fuses for arc cooling . A glass paste suitable for printing is advantageously used as the material for the mechanically stable layer according to Claim 1 and also Claim 2. It is preferably applied in a screen printing process which, for low process costs, produces an outer layer with adequate positioning accuracy on the substrate and concentration on the "hot spot". Furthermore, the outer covering produced according to the foregoing features is gas-tight by virtue of the mechanically stable layer. Thus, no substances from the area surrounding the fuse element can penetrate to the fusible conductor and influence the switching characteristics in an unforesee- able way. Consequently, of course, no plasma or vaporized metal of the fusible conductor can escape from the fuse and be deposited in the area surrounding the fuse element during current breaking either. Process technology can be used for checking the gas-tight vitrification of the layer by measu- ring the surface roughness. The high reliability of this procedure has been demonstrated in tests. In a development of a fuse element according to the invention as claimed in one of Claims 2 to 4 , the porous covering is formed by a dielectric coating, in particular by a glass paste suitable for printing in a thick-film process. In this case, the material of the outer covering vitrifies at a lower temperature than the material of the inner covering. Glasses which are suitable for printing are available with a wide variety of properties, such as their melting or sintering temperature for example. They can be used in thick-film circuits as a dielectric or insulation with vitrification temperatures adapted to the respective process conditions. With suitable process control, however, it is also possible to vitrify only the outer surface of a paste which vitrifies at a relatively low temperature, while the interior of the paste layer remains unvitrified. This behaviour can be adjusted on the basis of a temperature gradient which always occurs within a firing or sintering process, the paste re- spectively used and its material properties having to be taken into consideration for the process control. In an advantageous development according to Claim 6, the fusible conductor is coated with a layer of tin, or some other material which diffuses into the material of the fusible conductor, at least in the region in which the current-breaking operation proceeds. It has been found in tests that the behaviour of a fusible conductor known as the M effect can also be advantageously used in a fuse according to the invention for producing slow-blowing characteristics. In the porous layer of the covering there is advantageously incorporated a flux, in particular in the form of a powder which is added to the covering paste. Even in a small amount, the flux is in this case also capable of supporting the current-breaking operation in the region of the hot spot, since it brings about a lasting improvement in the flow behaviour of the layer of tin on the fusible conductor. According to Claim 8, the fusible conductor is covered, and in particular is printed over, with a flux, at least in the region of the hot spot. By selective use of the flux, the amount of material used can be further reduced while enhancing its effect. A gas-tight outer covering layer in this case further prevents vapours from forcing their way out during current breaking of the fuse . The gas-tight outer covering layer is advantageously formed from a plastic which also extends a vitrified outer layer by adding an additional outer enclosure. This outermost layer of the structure preferably consists of a plastic which is suitable for printing. The function of this layer is to enclose the entire covering, in order to give it additional mechanical stability. The good thermal insulating properties of the glass layer lower the outside temperature of the fuse body to such an extent that many commercially available plastics can be used for the material of the outer enclosure or outer covering. The plastic outer layer can also easily be used, by its shape, colour or by bearing printed text for example, for identifying the characteristics of the component. What is more, the plastic layer may serve as an effective protection against chemicals, such as are produced in a galvanic application of the external contacts for example. The installation of a fuse element according to the inven- tion by SMD technology is particularly advantageous, the covering layers being turned towards the surface of the circuit carrier. This produces a direct contact between the uppermost covering of the "hot spot" and the material of the board, or only a very thin gap forms between them. During current breaking of the fuse, such an arrangement with a supporting capability may serve for the further mechanical strengthening of the covering, since it can then reliably withstand even higher internal pressures . On account of the good thermal insulation of the covering, a disadvantageous effect on the material of the board is ruled out even if the fuse element is subjected to persistent strong loading. For carrying out the present invention, it is possible to rely on materials and processes known from the area of thick-film circuitry for mass production, it being possible to use respectively known processes, materials and production machines for the individual steps involved in the production of fuse elements. Together with a fusible conductor covering according to the invention, a variety of inexpensive processes for high-quality, reliable products is thus obtained. The invention can be advantageously varied from a production technology viewpoint and adapted to existing processes, as explained on the basis of the exemplary embodiments of the invention described in more detail below with respect to the drawing. In the illustrations: Figure 1 shows a first embodiment of a fuse element in a perspective representation; Figure 2 shows a second embodiment of a fuse element in a perspective representation; Figure 3 shows a sectional representation in a plane A-A of the fuse element from Figure 2; Figure 4 shows a third embodiment of a fuse element in a perspective representation and Figure 5 shows a representation of individual steps of a process for producing the fuse element from Figure 2. A first embodiment of a fuse element 1 is perspectively represented in Figure 1. It comprises a substrate 2 made of a fired glass ceramic, to which a fusible conductor 3 has been applied, for example in a thick-film and/or thin-film process, as part of a layer 4. With two leads 5, the layer 4 at both ends thereby covers over two terminal areas 6, which in this exemplary embodiment have been printed onto the substrate 2 in a thick-film process, in a process step of their own as conducting layers of their own. In the process step represented in Figure 1, a region 7 depicted by dashed lines around the fusible conductor 3 is covered by a layer 8 of a pasty composition. The application of the composition as a uniform layer likewise takes place in a thick-film process. The area to be covered in this case comprises at least the region of the fusible conductor 3 and the adjoining parts nearby of the leads 5, to reliably prevent any escape of plasma during the current breaking of the fuse 1. A covering of the surrounding, free surface 9 of the substrate 2 serves for increasing the adhering area and stabilizes the covering by the layer 8 after hardening of the composition. For the electrically reliable SMD connection of the fuse element 1 to an external circuit (not shown any further here) on a circuit board, end faces 10 of the arrangement described are provided with contacts 11 engaging partially around them. The contacts 11 are applied in a galvanic pro- cess, it being possible for a thin metallization to have been applied and fired beforehand by using a resinate paste, which is not represented in Figure 1, however. By using commercially available pastes which are suitable for printing, such as are used for example for dielectrics or insulating layers in thick-film circuitry, the hardening of the composition can be controlled in the case of the present embodiment in such a way that the composition is either vitrified completely or else forms only a layer 8 which is vitrified on its surface 8a, while the material lying underneath retains its granular or powder-like form. In the first case, an airtight covering is created, in the second case a pressure buffer is formed in addition to the air-tight covering by virtue of the pore content of the in- ternal powder material. Figure 2 shows a second embodiment of a fuse element, analogous to the perspective representation chosen in Figure 1. As a difference with respect to Figure 1, here, however, a porous layer 12 is arranged as a direct covering over the region of the fusible conductor 3. It is covered over and closed off from the outside in a gas-tight manner by the layer 8. A glass paste which is suitable for printing, but the melting point of which is higher than that of the layer 8 lying on the outside, is used as the material for producing the layer 12. In the production process, the paste of the layer 12 is printed onto the arrangement comprising the substrate 2 and the conductive layer 5 with contact areas 6 and is dried, so that all the readily volatile solvents can escape from the paste. Subsequently, as a further paste, the layer 8 is printed over this arrangement in a covering manner. Following a subsequent drying step, this arrangement is subjected to a thermal process, in which only the layer 8 is vitrified, at least on its upper side 8a. A mechanically stable and gas-tight termination is thus created. The layer 12, lying further inward, is below its sintering or vitrification temperature during this process, so that it retains its granular consistency or pore content to the full extent. Consequently, the layer 12, as an intermediate layer, forms a glass buffer which can damp the increase in pressure occurring abruptly at the moment at which the fuse element is tripped, in order in this way to additionally protect the entire covering from rupturing. As a sectional representation in a plane A-A, Figure 3 shows the basic structure of the fuse element from Figure 2 once again in detail. In the illustration, the covering by the layer 8 has been drawn up to the external contacts. This is not necessary in principle, since even with less coverage for the substrate surface, the layer 8 is an adequate, gas- tight and mechanically stable protection for the circuit lying thereunder, in particular for the fusible conductor 3, by virtue of good surface adhesion. What is more, the fuse 1 does not break the current in the region of the leads 5 but at the fusible conductor 3, as the warmest point of the arrangement . In addition, the covering from Figure 3 has been extended in comparison with the representation from Figure 2 by adding a layer of plastic 13, which extends over the layer 8. The layer of plastic 13 may be regarded as a further additional measure for the mechanical stabilization of the covering or for strengthening the layer 8, and consequently as additional protection against rupturing of the covering during the current breaking of the fuse 1. Virtually every commercially available plastic with adequate adhesion on the vitrified surface 8a comes into consideration as the material for the layer of plastic 13. Further selection criteria are the processability of the plastic and its mechanical stabi- lity. The temperature stability of the plastic may be regar- ded as uncritical, since the thermal insulation by the layer 8, or the layers 8 and 12, is so great that, even in continuous operation, inadmissibly high temperatures are not reached in the region of the layer of plastic 13. What is more, the layer of plastic should be resistant to chemicals customarily used in the field of electrical engineering. The measure described of an additional covering with the layer of plastic 13 may, of course, also be taken in the case of a structure according to Figure 1. Figure 4 represents a further alternative embodiment, in which the region of the hot spot 3a in the embodiment from Figure 2 is covered by a layer of tin 14. By means of thermally induced diffusion processes, an altogether slow-blo- wing current-breaking behaviour of the fuse element 1 is brought about by this structure. As a difference from the embodiment from Figure 2, here the porous layer 12 is covered directly by the layer of plastic 13. In the production of this fuse, there is therefore no longer any necessity for a further thermal process step, which in the case of glasses suitable for printing at customary sintering temperatures could trigger diffusion processes in the region of the hot spot 3a and lead to an undesired premature ageing of the fuse 1. To improve the flow behaviour of the tin covering 14, powdered flux 15 has been added in the porous layer 12. On account of the internal temperature distribution during the drying step, only a very small part of the flux 15 is sepa- rated out from the layer 12. There is consequently still an adequate flux content in the region 7 around the hot spot 3a on the fusible conductor, which greatly improves the flowing apart of the layer of tin 14 during the current breaking of the fuse 1. Alternatively, the flux 15 may also be applied directly to the layer of tin 14 or be printed on in a custo- mary screen printing process. By this direct contact, the flux 15 acts particularly strongly during the separation of the layer of tin 14 during the current breaking of the fuse 1, while at the same time allowing less to be used. Figure 5 represents individual steps of a process for producing the fuse element from Figure 2 in a multiple repeat. The construction of the electrically conductive structure comprising the terminal areas 6, leads 5 and fusible conduc- tor 3 on the surface of the substrate 2 is advantageously performed on a substrate material in the form of a board. In one of the forms described above, the covering is also applied to the substrate board and fired, so that subsequently the entire arrangement is mechanically fixed and already protected against external influences. The substrate material in the form of a board is then broken into individual bars 16, which correspond in their width to the width of an individual fuse element 1. Each bar 16 com- prises a multiplicity of individual fuse elements 1, to the uniform end faces 10 of which the external contacts 11 are advantageously applied in a galvanic process only after applying and hardening the layer 8, which is a departure from the procedure in Figures 1 and 2. The covering by the outer layer 8 or 13 thereby assumes the important function of a protective layer for the thin and correspondingly sensitive electrically conductive structure of each fuse element against chemically aggressive substances of the galvanic process. Consequently, no additional protective devices or layers or substances have to be provided in the case of this process step either. Seen over a bar 16, the covering by the outer layer 8 or 13 is not continuous. Rather, the regions of the subsequent breaking edges 17 are exposed. They may have been prepared by embossing, channelling or lasering. Even if there is a variation or displacement of the printing masks or breaking devices, as indicated by the arrows, the covering does not impede individual separation by breaking. After the brea- king of the bars 16, finished fuse elements 1 are immediately available and, owing to their simple-to-produce structure and minimal use of materials, can altogether be produced inexpensively and economically. Alternatively, instead of being enclosed by the layer of plastic 13, the individually separated finished fuse elements 1 may also be enclosed by an outer enclosure, which consists for example of a shrinkable plastic. Further possibilities for forming an outer enclosure, such as fusing in glass or encapsulating in a plastic, for example, are possible but not generally required. The installation of the fuse elements 1 preferably takes place in such a way that the surface bearing the fusible conductor 3 and the layers of the covering has direct contact with the surface of the board after the soldering of the fuse. By means of this contact, an additional dissipation of force towards the material of the board can take place during the tripping of the fuse, as a result of which the pressure resistance of the covering layers 8, 12, 13, and consequently also the breaking capacity of the fuse, can be further increased by this installation measure alone.

Claims: Claims 1. Electrical fuse element comprising a substrate with contacts arranged on two opposite sides of the same, between which contacts there runs a fusible conductor which is produced in a laminating process and is connected electrically conductively to the contacts, characterized in that the fusible conductor (3) is covered, at least in a hot-spot region (3a) in which the current-breaking operation proceeds, by a pasty composition solidifying in a thermal process to form a mechanically stable layer (8) , vitrifying in particular at least on one surface (8a) . 2. Electrical fuse element comprising a substrate with contacts arranged on two opposite sides of the same, between which contacts there runs a fusible conductor which is produced in a laminating process - and is connected electrically conductively to the contacts, characterized in that the fusible conductor (3) is covered, at least in a hot-spot region (3a) in which the current-breaking operation proceeds, by a porous layer (12) , which is covered by at least one further, mechanically stable layer (8) . 3. Fuse element according to either of Claims 1 or 2 , characterized in that the mechanically stable layer (8) comprises a thermally solidifiable, pasty composition, preferably a dielectric coating, and in particular a glass paste suitable for printing in a thick-film process . 4. Fuse element according to Claim 3, characterized in that the mechanically stable layer (8) forms a gas-tight covering. 5. Fuse element according to one or more of Claims 2 to 4, characterized in that the porous layer (12) is a dielectric coating, in particular a glass paste suitable for printing in a thick- film process, the material of the outer, mechanically stable layer (8) vitrifying at a lower temperature than the material of the inner layer (12) . 6. Fuse element according to Claim 2, characterized in that the fusible conductor (3) is coated with a layer of tin (14) , or some other material which diffuses into the material of the fusible conductor, at least in the hot-spot region (3a) in which the current-breaking operation proceeds. 7. Fuse element according to one or more of Claims 2 and 6, characterized in that a flux (15) is incorporated in the porous layer (12) . Fuse element according to one or more of Claims 2, 6 and 7, characterized in that the fusible conductor (3) is covered, and in particular is printed over, with a flux (15) , at least in the hot-spot region (3a) . 9. Fuse element according to one or more of the preceding claims, characterized in that the mechanically stable layer (8) is encapsulated by a layer of plastic (13) or an outer covering, which in particular consists of a plastic which is suitable for printing or can be shrunk on. 10. Fuse element according to one or more of the preceding claims, characterized in that the fuse element (1) is installed with the mechanically stable layer (8) or the layer of plastic (13) turned towards the board, in particular such that it is in contact with it.

Inventors: BAUS ANDREAS, FROECHTE BERND, RUPALLA MANFRED, BAUS, ANDREAS, FROECHTE, BERND, RUPALLA, MANFRED
Applicants: BAUS ANDREAS, FROECHTE BERND, RUPALLA MANFRED, WICKMANN WERKE GMBH, BAUS, ANDREAS, FROECHTE, BERND, RUPALLA, MANFRED, WICKMANN-WERKE GMBH
Application year: 1998
Main classification: H01H85
IPC codes: H01H85