Fuse elements are used in large numbers for protecting elec- trical and electronic circuits from excessive currents. In such cases, they have to be adapted to the current ranges occurring in an application, by the tripping characteristics respectively required. The generally perceivable and ever increasing tendency for circuit components to be made smaller while having the same or even enhanced capabilities leads to considerable problems in the area of electrical fuse elements.

It is therefore the object of the present invention to provide a fuse element for all known tripping characteristics by a cost-effective production technique for the medium and low current range. Furthermore, by having a smaller outer geome- try, the fuse element is to be adaptable to modern methods of insertion.

According to the invention, this object is achieved by the carrier consisting of a material of poor thermal conduction, in particular of a glass ceramic.

In the past there have been numerous known attempts to make the outer dimensions of electrical fuse elements considerably smaller while retaining their operational current range, their switching capacity and their specific tripping characteri- stics. However, these attempts resulted in failure because either the internal heating of the fuse element became too great and/or the desired tripping characteristic could not be achieved, or the fuse element became unsoldered at its contact points owing to increased self-heating.

By the use according to the invention of a carrier made of a material of poor thermal conduction, the present invention overcomes a widespread prejudice to the use of such materials in fuse construction. By the use of such a carrier material, the hot zone (hot spot) of the fuse element can be advantage- ously restricted to the core region of the carrier or of the housing, since the heat dissipation is very low. Thus, the heat removal by conduction via the external contacts is signi- ficantly less. Consequently, unsoldering of its own accord or inadmissible heating is no longer possible for a fuse element according to the invention. Furthermore, by concentrating the "hot spot"in a certain region, the entire power consumption of a fuse element according to the invention is lowered. Thus, a minimal power consumption also results in less of a retroac- tive effect on the surrounding electric circuit.

Among suitable materials of poor thermal conduction are cera- mics, glass ceramics or glass. Glass ceramics are preferred, however.

For a cost-effective mass production of fuse elements accor- ding to the invention with their small geometrical sizes, formation of the carrier in sheet form is advantageous, prefe- rably in the form of a sheet-like substrate. Thus, fuse ele- ments according to the invention can be produced in a cost- saving manner in multiple repeats, for example in the size of customary service-mounted devices (SMDs) on a planar substra- te.

In a fuse element according to the invention, the fusible conductor may act as a single heat source. However, to set different nominal currents and switching characteristics, an indirect heating of the fusible conductor is preferred.

At least one additional heating element serves for this purpo- se. In the case of some embodiments, two heating elements are used with preference, for example, as is shown below with

reference to illustrations of a number of exemplary embodi- ments according to the invention. Cases with more than two heating elements are also conceivable. When reference is made below to a heating element, these possibilities are also in- tended to be included.

In developments of the invention, the heating element is ar- ranged together with the fusible conductor jointly on the substrate. In this case, the degree of thermal coupling bet- ween heating element and fusible conductor is influenced in each case by the distance from each other. The consequently achievable effects of shifting the characteristic curve of the fusible element are explained in more detail below with refe- rence to exemplary embodiments.

There are, in principle, several possibilities that are con- ceivable for the electrical wiring to supply the heating ele- ment and the fusible conductor with power, for example a par- allel connection. However, it is preferred for the heating element to be electrically connected in series with the fusi- ble conductor on the substrate. Consequently, with the in some cases very small outer dimensions, only two external contacts are required on a fuse element according to the invention.

In a major development of the invention, the heating element itself is also designed as a fusible conductor. This provides a fuse element according to the invention as an electrical connection of two fuse elements, which are in their design primarily assigned the tasks of heating element and fusible conductor by the selection of material and geometry. This type of construction advantageously opens up the possibility of designing the heating element for a different, preferably much higher nominal current IN than the fusible conductor.

By designing the characteristics of the fusible conductor and heating element in the way according to the invention, these curves intersect at a commutation point. From this point, the fusible conductor characteristic of the heating element re-

sponds faster than the actual fusible conductor, as will be shown with reference to a diagram. For the following electric circuit, this produces additional protection in the case of extremely high short-circuit currents.

In a development, the distance produced between the heating element and the fusible conductor is kept variable, in order to set the degree of thermal coupling and consequently the tripping characteristic of the fusible conductor and the nomi- nal current while otherwise retaining the same materials and the same geometry of the circuit. With a fixed circuit geome- try, setting of the characteristic is possible by simply shif- ting the individual production masks in relation to one anot- her in a predetermined way and fixed amount.

The distance between the heating element and the fusible con- ductor assumes a minimal value when the heating element and the fusible conductor are arranged lying one over the other.

This minimal value is in this case determined by the layer thickness of an electrical insulation, which may consist of a dielectric such as glass, but also a ceramic or a curable paste. The good thermal contact may take place over the entire base area of the fusible conductor. Preferably, the fusible conductor is arranged over the heating element, so that there is adequate space available for receiving the gases and parti- cles released in the event of the fusible conductor tripping, as well as for pressure equalization.

According to the invention, the properties of the fusible conductor can be significantly influenced directly by the thermal coupling with the heating element. The thermal coup- ling is intensified in a simple way by the actual fusible conductor being applied to a thin layer, which preferably consists of silver and effects adhesive bonding with good conduction on the substrate surface. As a result, the charac- teristic can be reproduced even more exactly.

In the case of a fusible conductor formed as a multilayer arrangement, for example in the case of a material combination of a layer of silver and a covering layer of tin, an addi- tional influencing of the tripping characteristic can be achieved by diffusion processes. Other material combinations with mutual solubility are also possible.

Furthermore, the fusible conductor may have a constriction or tapering in its central region. This reduction in cross-sec- tion increases the intrinsic resistance. What is more, the material of the fusible conductor is weakened at this notable point and correspondingly less material has to be melted du- ring tripping. The constriction is advantageously in the"hot spot"of the fuse element.

Alternatively, the fusible conductor may, however, also be a wire, which has, for example, as described above, a silver-tin layering on its surface and/or itself a constriction. To im- prove the thermal coupling, the wire may be pressed onto or fused onto the substrate.

A further advantage is obtained by a covering, preferably of each fusible conductor, by means of a low-melting substance.

In the event of tripping of the fuse, the covering prevents molten parts coming into contact with the surroundings. It may be realized in the form of a two-layer structure, a drop of hot-melt adhesive as the core, for example, being covered for its part on the outside and sealed by a thermally stable sub- stance, such as for example a curing embedding compound or a resin. At operating temperature, the core already melts and creates a cavity for receiving gases etc., which is stabilized by the outer shell.

Advantageously, an electrical fuse element according to the invention can be easily adapted in its outer form and dimen- sions to the requirements of modern insertion methods. A cu- boidal form is preferred. The external contacting takes place

in adaptation to customary SMD soldering methods by external contacts arranged on two opposite end edges. They are then preferably applied in a galvanic process, if fusible elements with diffusion processes are contained in the fuse element.

A number of exemplary embodiments of the invention are ex- plained in more detail below with reference to the drawing, in which: Figure la shows a basic representation of a first embodiment of a fuse element in a plan view; Figure lb shows a representation of an alternative embodi- ment of the fuse element from Figure la; Figure lc shows a representation of a further alternative embodiment of the fuse element from Figure la; Figure 2 hows a plan view of a further embodiment of a fuse element with a fusible conductor arranged over the heating element; Figure 3 shows a perspective view of a fuse element in an exploded representation and Figure 4 shows a sketched family of characteristic curves with the switching characteristics achievable in principle of the fuse elements from Figure lc and Figure 2.

In Figure la, a first embodiment of a fuse element 1 is repre- sented in its basic structure in a plan view. A fusible con- ductor 3 is arranged together with two heating elements 4 in an S-shaped series connection on a substrate 2 of poor thermal conduction. The individual elements are electrically connected to one another by conducting tracks 5. There is thus obtained here overall a series connection of three elements, which may in each case be designed as a fusible conductor with specific properties. The two heating elements 4 are arranged here sym- metrically with respect to the fusible conductor 3 at a di- stance d, which in both cases is equal. Thus, they heat up the fusible conductor 3 by thermal conduction via the substrate 2

equally in a symmetrically shaped"hot spot".

Among the materials used for substrate 2 of poor thermal con- duction is a glass ceramic. Measurements have produced the following, surprising values for the thermal conductivity of such a material in comparison with the Ale03 ceramic otherwise preferred in fuse construction: Substrate StaticthermalThermalimpedance resistance Glassceramic 190 K/W 6K/W Al203 ceramic 26 K/W 5.4K/W It is evident from the values in this table that an Al203 cera- mic dissipates the heat per watt of heating output between the ends of a substrate better by a factor of approximately 7 than the glass ceramic measured here. These values relate to the consideration of the case of steady-state heat removal, which in the case of Al203 ceramic substrates leads to the undesired unsoldering of the external contacts.

If, however, the investigation is restricted to the dynamic thermal conduction behaviour and if, correspondingly, a very small space is considered, also referred to as a segment, only a relatively insignificant difference in heat removal of about 10W is established between Alz03 ceramic and the glass ceramic.

The thermal coupling between fusible conductor and heating element is thus almost as good with the use of a glass ceramic substrate as in the case of an Ale03 ceramic substrate. Accor- dingly, significant differences occur only in the considera- tion of the thermal conduction at the ends of common substrate sizes, where an Al203 ceramic effects an undesired heating of the external contacts on account of its much better thermal conduction.

The degree of thermal coupling between the heating element and

the fusible conductor can be set over a wide range by the distance d. The influence of the thermal coupling on the swit- ching characteristics of the fuse element is shown and des- cribed later with reference to a family of characteristic curves.

Arranged adjacent to two opposite end edges 7 of the substrate 2 are conducting faces 8. To complete the production process, the end edges 7 are metallized, so that they form the external contacts 9, which are electrically connected to the faces 8.

Use of the substrate 2 of poor thermal conduction has the effect that there is little heating up of the external con- tacts 9. There is consequently also a reduction in the power loss of the fuse element required as heating power, so that this fuse element 1 has little influence on the remaining electric circuit.

The fuse element 1 from Figure la has been realized in its essential parts by a screen-printing process. In the case of very small structure sizes, a photolithographic process is more suitable. In the present case, the fusible conductor 3 is produced as a thick film, which has a tapering 6 in its cen- tral region. The tapering 6 is a further measure for influen- cing the tripping characteristic. Depending on the desired characteristic, it may also be omitted. As a further produc- tion possibility, the fusible conductor 3 may also be used in the production process in the form of a piece of wire. In the present case, the fusible conductor 3 is applied to the sub- strate 2 as a thin layer of silver, onto which subsequently a layer of tin is applied as the actual, low-impedance conduc- tor.

The central region of the fuse element 1, in which the heating elements 4 and, in particular, the fusible conductor 3 are located, is provided with a covering 10. The covering 10 is indicated in Figure la as a dashed line and protects the sen- sitive part of the circuit on the substrate 2 from external

influences. Furthermore, gases or metal particles emitted during tripping of the fuse element 1 are kept away from the surrounding electric circuit.

Figure 1b represents an alternative form of the fuse element 1 from Figure la, which contains only a heating element 4 and a fusible conductor 3 without constriction 6. The thermal coup- ling entered in the form of arrows, is less than in the arran- gement from Figure la on account of the appreciably increased distance d between heating element 4 and fusible conductor 3.

The basic representation of Figure lb is primarily intended to demonstrate the freedom of design, with several possibilities for the arrangement, although no change has been made to the basic geometry of the circuit, comprising conductive faces 8, external contacts 9 and conductive tracks 5.

Figure lc represents a further developed form of the fuse element 1 from Figures la and lb, in which the heating element 4 and the fusible conductor 3 are again moved closer together, reducing the distance d, to increase the thermal coupling. It is intended by the different type of representation in Figure lc to point out that the regions of the faces 8 and conductive tracks 5 of good electrical conductance can also be produced in two or more mask steps. Setting the thermal coupling by variation of the distance d is advisable, however, when using two masks for building up the conductive tracks 5 and 5a, since in this way the distance d can easily be changed by shifting the masks in relation to each other, without the production of a new mask being required.

Figure 2 represents a plan view of an alternative embodiment of a fuse element 1, the fusible conductor 3 here being arran- ged over the heating element 4 on the substrate 2. Arranged between the fusible conductor 3 and the heating element 4 is an electrical insulation 11, which is formed here by way of example by a thin layer of glass. The thermal coupling in the embodiment represented takes place over the entire surface

area of the fusible conductor 3 and therefore, and because of the minimal distance dmin, increases to a maximum value.

Depending on the selection of materials, the circuit from Figure 2 may also be produced in two process steps, which are in each case completed by a sintering operation. In a first step, the conducting faces 8, the conductive tracks 5, the heating element 4 and the insulation 11 over the heating ele- ment are applied in one mask. In a subsequent production step, the second level is applied, which essentially contains the fusible conductor 3 and two conductive tracks 5, which electrically connect a conducting face 8 to the fusible conductor and establish a conducting connection with the lower level of the circuit via a contacting assembly 12.

Subsequently, the circuit may be covered, at least in the region of the fusible conductor 3, by a curing embedding com- pound. This covering is applied in two steps, with a low-mel- ting substance being applied first of all. This is, for ex- ample, a hot-melt adhesive, which covers only the fusible conductor. It is covered by a thermally stable substance.

During the operation of the fuse element, the melting drop of adhesive creates directly above the fusible conductor, in the "hot spot", a stable cavity for receiving plasma during the tripping of the fuse element 1.

The direct comparison of Figures lc and 2 shows that, in prin- ciple, the same masks are used here for producing fuse ele- ments with very different switching characteristics and/or nominal currents IN. Introducing the insulation only necessita- tes one further mask step in the production sequence according to Figure 2. The mask of the upper conductive track 5a requi- res a small modification. Essentially, however, these structu- res are the same as one another. Consequently, only one set of masks is required for producing a wide variety of SMD-inser- table fuse elements and a standard, adapted range of pastes or the like can be used in cost-effective mass processes.

Figure 3 perspectively shows in an explosive representation a design for a fuse element 1 with all the individual elements listed above. The solid lines and arrows in this case repre- sent conducting connections. The line 13 shows the outline of the bearing face for the insulation 11. The elements represen- ted in planes may be produced here as layers, in each case by a process mask. The arrangement of the elements with respect to one another and the forming of the conductive tracks 5 opens up the possibility here that the fusible conductor 3 and the heating element 4 can be varied in relation to each other by shifting the process masks in terms of the distance d bet- ween them. The variation in distance is not shown in this illustration. However, the arrangement represented in Figure 3 can be used correspondingly to realize, as limiting cases, either fuse elements according to Figure 2 or fuse elements according to Figure lc. In this case, the fuse element 1 ac- cording to Figure 2 contains only one heating element 4, so that, although the thermal coupling can be set here by varia- tion of the distance d, the"hot spot"is not fully symmetri- cally formed in the region of the fusible conductor 3. Howe- ver, this influence can be minimized by appropriate design of the circuit. As soon as the distance between the tapering 6 of the fusible conductor 3 and the heating element 4 is large enough that there is no overlap between fusible conductor 3 and heating element 4 and an adequate insulation between the conductors is obtained, the insulation 11 may be omitted, thus dispensing with one substep in the process.

Figure 4 represents a sketched general family of characteri- stic curves to represent switching characteristics of diffe- rent fuses. The curves are plotted with a logarithmic scale on both axes. It can be seen that, in the present case, the hea- ting element alone is designed for a lower nominal current IN than the fusible conductor. The fusible conductor is, for example, built up as a multilayer conductor by using a silver- tin diffusion and accordingly has only a quick-acting swit- ching characteristic, while the heating element alone trips

with a very quick action. With this design of the individual elements, the series connection with thermal coupling allows an increase in the inertia in the overall fuse element to be achieved. In the converse case, a greater tripping capacity can be produced.

The characteristic of the individual elements in any event differs distinctly from that of the overall circuit. It shows here a distinctly slow-acting characteristic, which until now could not be realized by components of small dimensions. The influence of the thermal coupling between the heating element and the fusible conductor can be seen in the shift to the left, into the range of lower nominal currents IN, of the curve for the switching characteristic of the fusible conductor. The curve in itself changes its shape only insignificantly. By variation of the distance d, the shifting of the fusible con- ductor characteristic can be influenced. With a minimal di- stance dmin, the nominal current IN assumes a minimal value if the material and the geometry of the fusible conductor remain the same, see curve B. Consequently, by a construction accor- ding to Figure 3, the wide range between the curves A and B represented in Figure 4 can be freely set during production by variation of the distance d. Consequently, with the geometry and material selection remaining the same, a large range of nominal currents can be covered with the same tripping charac- teristic.

In the lower third, the shifted curves intersect with the characteristic of the heating element at a so-called commuta- tion point K. This point is in practice to correspond to a current of slightly more than 10 x IN. For higher currents, the curve of the heating element then determines the tripping characteristic of the respective fuse element, no longer the characteristic of the indirectly heated fusible conductor.

Thus, faster tripping times are realized for higher short- circuit currents.

In tests, fuse elements were constructed with substrate dimen- sions of 6.5 x 2. mm and 4.6 x 3.2 mm. These are common di- mensions in SMD technology. At ten times the nominal current IN, switching times of 10-15 ms were measured for nominal currents of about 0.4 A. Consequently, efficient fuse elements with slow-acting tripping characteristics were realized for the first time in the size of SMD components. With a fuse element corresponding to Figure lc, the heating resistance was 0.6 Q. The fusible conductor resistance was in this case 0.03 Q. Thus, for the series connection, altogether only a resi- stance of about 0.63 Q is obtained.

In the case of the variant according to Figure 2, a heating resistance of 0.1 Q and a fusible conductor resistance of 0.03 Q were realized for a nominal current IN of about 0.315 A, a layer of glass of the thickness din of about 20 Am being used as the dielectric. Both circuit variants were produced by thick-film technology on a glass ceramic substrate, using paste materials common in hybrid technology. In thick-film technology production processes, currently line widths of up to 0.1 mm can be reliably produced in the case of layer thick- nesses of between 6 and 20 ym.

It can be seen from these actually realized exemplary embodi- ments that, in the case of the variant according to Figure 2, the heating resistance of the heating element 4 may turn out to be relatively low on account of the much improved thermal coupling.