DE29616063U1 - Electrical fuse - Google Patents

Description

(04207.9)

Description

The present invention relates to an electrical fuse.

Known fuses are manufactured in different shapes and with varying components and are used in large numbers in electrical and electronic circuits. There they protect individual components or complete assemblies against excessive current loads caused by sustained overcurrents or short circuits.

The design of a fuse must protect the fuse element from environmental influences. On the other hand, the environment must also be shielded and protected from metal vapors and the like when the fuse is switched off. In addition, a fuse must contain appropriate connections for the electrical connection to the surrounding circuit. In order to meet the requirements mentioned, known fuses have a complex design that requires relatively high production costs and/or sizes that take up a lot of space.

The present invention is based on the object of creating an electrical fuse which has a simple design, can be manufactured with little effort and also allows for the smallest dimensions.

This object is achieved according to the invention by a fuse with 0 the feature combination of claim 1.

The fuse according to the invention therefore comprises a simple substrate made of an electrically insulating material, which has at least two electrical contacts that are arranged at a distance from one another on the substrate. A fuse element connects the two contacts to one another in an electrically conductive manner. The fuse element is at least partially arranged between the contacts.

covered with an electrically insulating casting compound that is connected to the substrate. The cover of the fusible element performs the following very important tasks: protecting the fusible element from mechanical stresses and environmental influences, protecting the environment from the arc and evaporating parts of the fusible element and "cooling" the arc when the fuse is switched off, in order to bring about a safe end to the flow of current by extinguishing the arc.

The above-mentioned tasks are fulfilled in the fuse according to the invention in a simple form by applying a casting compound or by enclosing the fusible element with the casting compound. Depending on the choice of casting compound, the switch-off characteristics of the fuse can also be advantageously influenced.

In comparison with known fuses, at least one process step is eliminated in the manufacture of the fuse according to the invention due to this structure, since the cover does not have to be prefabricated as a discrete part and/or attached to the substrate using auxiliary materials. Furthermore, the tightness of the closure can easily be ensured by the potting compound. 25

Advantageously, a rectangular plate is chosen as the substrate for a fuse according to the invention, so that fuses of the type described can preferably be manufactured in multiple use and separated in a final step, for example by breaking or other separation processes. However, there are also a number of common components that are suitable for use as a fuse substrate due to their material properties.

In a further development, a ceramic substrate is used to build a fuse. Ceramic materials are due to their material properties for use as a safety

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They are particularly suitable for use as a substrate because they are thermally stable and cannot form conductive bridges, such as carbon bridges, even in the presence of arc gaps.

There are great advantages to using a hardenable paste as a casting compound. As a flowable medium, a paste can completely and tightly enclose the fusible conductor in one production step, and then be solidified in a subsequent step. The hardening or solidification of the paste has the advantage that the paste reliably encloses the fusible conductor and can also contribute to the mechanical stability and connection of the arrangement, as it adheres securely to the substrate. This point is particularly important if the fusible conductor according to claim 9 is a wire. Fusible conductor wires are usually very thin and therefore have little mechanical strength. The hardened paste serves as important mechanical protection here.

Further advantages in terms of switching behavior arise when the casting compound is porous. This includes the possibility that the casting compound itself is porous and/or becomes porous when it hardens by solidifying and forming cavities. A substance with this consistency can be very useful for extinguishing an arc when the fuse is switched off, as the pores give the gaseous, hot components of the arc enough space to expand or spread and surfaces to settle. They therefore serve as pressure equalization chambers and reduce the internal pressure of a fuse when it is switched off. A high pore content in the casting compound also has thermal advantages, as the gases contained in the cavities provide ideal thermal insulation for the fusible element.

Furthermore, the cured casting compound is thermally stable. The requirements listed above can be met, for example, by a non-conductive ceramic paste or by other insulating fillers in natural or

Additives in a modified form. The characteristics of the fuse can be strongly influenced by the type of cover and the properties of the casting compound.

The electrical contacts required to connect the fusible conductor are advantageously formed by metallizing two edges and/or front sides on the substrate, for example by applying an electrically conductive paste to these places and burning it in.

In the fuse according to the invention, the most important part, the fusible conductor, is used as a finished element. It is integrated into the structure in the form of a wire with defined properties. This is done with minimal effort by soldering the wire stretched over the arrangement to the contacts. It is conceivable to apply a solder paste to the contacts after metallization and to solder them using infrared or laser treatment. The fusible conductor wire attached in this way has better thermal properties than, for example, an electrically conductive layer, since even a very thin wire does not necessarily need a carrier layer. A layer formed from a paste is, for example, dependent on at least one carrier film during production, which in turn influences the electrical and thermal properties of the fuse.

According to claim 10, the substrate of the fuse is rounded at the edges or front sides to be metallized. This makes it easier to metallize the edges of the substrate. Furthermore, in a metallization process according to claim 7 and/or 8, the amount of paste to be absorbed is minimized, since no sharp edges need to be coated. Finally, this measure means that the increase in width in this fuse structure in the area of the contacts is smaller overall.

In a further development, the substrate has the metal to be

tapered or flattened edges or front sides. This measure helps to reduce the height of the connections significantly, which prevents the contacts from protruding laterally over the component's substrate. In the case of SMD assembly, the component rests with the middle part of its substrate on the board of the circuit to be assembled. The fuse can be held securely by gluing it until the contacts are soldered, for example. The middle part of the substrate can thus serve as a support and adhesive surface on the one hand, but can also safely separate the closely spaced soldering surfaces of the connections on the other. This advantageously prevents short-circuit bridges from forming when the solder paste is applied to the board or when soldering.

The triggering characteristics of fuses can be set more easily and reliably if the fuse element runs through a space with thermally approximately homogeneous properties. In a significant development, a depression or an elongated recess is therefore arranged in the middle of the substrate, over which the fuse wire runs without contact with the substrate. For the reasons listed above, the fuse element in the fully assembled fuse is preferably not self-supporting in this area, but rather is homogeneously enclosed by the casting compound over its entire free length.

Advantageously, in a fuse according to the invention, at least one cover can be arranged between the fusible conductor and the casting compound, on which the casting compound is arranged. This can be used, for example, to form a closed cavity through which the fusible conductor runs in a self-supporting manner. The cavity can also contain porous or granular media, such as sand. Layered structures can also be implemented in this way to influence the triggering characteristics of the fuse, as is described below using an exemplary embodiment.

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In the following, embodiments of the invention are explained in more detail with reference to the drawings. The figures show:

Fig. la - lc

a plan view of a first embodiment of a fuse and two side views;

Fig. 2a - 2d

perspective representations of various

Forms of implementation of the substrate of the fuse of Fig. la;

Fig. 3a - 3b

perspective representations of another

guide shape of the substrate of the fuse of Fig. la and the assembled fuse before potting;

Fig. 4a - 4b

perspective views of further embodiments of a fuse and

Fig. 5a - 5b

perspective representations of alternative contacts

tations to the embodiments of Fig. 4a.

Fig. la shows a plan view of a fuse 1 having a fusible conductor 2 which runs on a substrate 3 between two contacts 4 on opposite edges 5 of the substrate 3 under a cover 6 made of a potting compound 7.

To produce the fuse 1, in a first step 5 the contacts 4 are applied to the edges 5 of the ceramic substrate 3 in the form of electrically conductive pastes 8 in a "dip & blot" process and burned or sintered so that

then contacts 4 are created on the substrate 3, which are accessible from the outside and partially enclose the front sides 9 and can be soldered. A soldering paste 10 is then applied to the contacts 4, into which the fusible conductor 2, designed as a wire 11, is pressed. The arrangement is then heated to such an extent that the soldering paste 10 liquefies and connects the wire 11 to the contacts 4 in a mechanically permanent and electrically conductive manner. The melting point of the soldering paste 10 is above the melting point of SMD soldering processes, so that the fuse 1 can be used in SMD processes.

Before or after the wire 11 is separated into individual fuse elements for each fuse 1, a pasty, hardenable casting compound 7 is applied as a cover 6 of the arrangement, which covers the fuse element 2 between the contacts 4 in a gas-tight and pressure-tight manner. The casting compound 7 is portioned out, for example, using a dispenser, in a screen printing or stencil printing process. In addition to the pasty initial form and the ability to harden, the essential properties of the casting compound 7 are mechanical and thermal stability and good electrical insulation. In addition, the casting compound 7 should also have a high pore content in order to be able to offer gaseous components pressure equalization spaces when the fuse is switched off and metal vapors a large area to settle.

By using a different color for the hardened casting compound 7, it is possible to identify the fuse in principle to differentiate, for example, the triggering characteristics as "quick" or "slow" or to identify the nominal current during production, since these properties are already determined by the dimensioning and the choice of material. Due to their small size, a general basic identification of these components using color coding is sensible.

Fig. 1b shows a side view of the fuse 1, seen from a front side 9 to illustrate the simple and flat design. The fuse element 2 has been electrically secured to the contact 4 using a solder paste 10. The cover 6 extends on the top of the fuse 1 almost to the edges 5.

Fig. Ic shows a view of the long side of the fuse 1 of Fig. la. The distance between the contacts 4 has been selected to match the hole or pad distance of the voltage level of the fuse according to the IEC standard. This distance can therefore be very small.

Furthermore, the distance of the fusible conductor 2 from the surface of the substrate 3 is also very small. The fusible conductor 2 therefore runs along the boundary between two half-spaces, one of which is formed by the casting compound 7 and the other by the substrate 3, as can also be seen from Fig. 1b. The thermal properties of these half-spaces should be as similar to one another as possible at low cost.

The measures or properties mentioned are realized by a form of the substrate modified from the basic form shown in Fig. Ia-Ic, as shown in Fig. 2b - 2d.

Fig. 2a - 2d show ceramic plates 12 with a progressive number of design measures, the advantages of which are described using this selection of sketches. The different shapes of the substrates 3 result from a progressive implementation of further design measures:

In Fig. 2b, compared to the plate 12 shown in Fig. 2a, curves 13 are provided on the edges 5 of the end faces 9, whereby the edges 5 can be metallized more easily in a "dip & blot" process.

The elongated recess 14 or depression 15 in the central surface 16 of the plate 12 of Fig. 2c serves to improve the thermal decoupling of the fusible conductor, since the fusible conductor is now embedded in the casting compound over its free length.

The tapering or flattening 17 shown in Fig. 2d in an enhanced representation at the end of the substrate 3 or at the edges 5 fulfills the task of not allowing the edge metallization of the contacts 4 to protrude, i.e. the fuse rests only with the middle part 18 of the ceramic substrate 3.

Fig. 3a shows an alternative to Fig. 2b in perspective. Here, the recess 15 has been extended even further than in Fig. 2b, so that the side walls 19 of the recess 15 can also serve as mechanical protection. The wire fuse element 11 can be inserted into the middle of the recess 15 in slots 20 on the end faces 9 of the substrate 3, as shown in Fig. 3b. The wire 11 is thus located through a cover 6 in the thermally homogeneous environment of the casting compound 7, which is not shown here for reasons of clarity.

Since in Fig. 3a an arc can be extinguished within the casting compound when the fuse is switched off by the complete and extensive enclosure of the fusible conductor, it is possible here to manufacture the "tub"-shaped substrate 3 from a plastic instead of a ceramic, which must be stable at the soldering temperatures of approx. 24 0 ⁰ C.

As can be seen from Fig. 3b, in the present case, as an alternative to external contacting by means of paste-metallized end faces 9, contact plates 21 with a slot 22 are arranged on the end faces 9, to which the fusible conductor 2 is electrically connected, for example by soldering, clamping or other known methods.

Fig. 4a shows perspective views of further embodiments of a fuse using a design of the housing or substrate 3 alternative to those of Fig. 2c and Fig. 3a. The substrate 3 here is U-shaped in cross section, with the fusible conductor 2 resting on the bottom 23 of the recess 15. This form of substrate 3 can be produced very inexpensively from a profile strand in which a fusible conductor 3 is tensioned and cast. After covering and the casting compound has hardened, the arrangement can then be divided into sections, e.g. by sawing. One such section is shown in Fig. 5a. Finally, measures must be taken at the front edges to ensure electrically secure external contact.

Fig. 4b shows the possibility of a multi-layer structure using a cover 24 which is placed over the fusible element 2 before casting, in order to form after casting for example a cavity with or without a special filling, which is sealed pressure-tight and gas-tight by the casting compound 7 on the end faces or front faces 9 of the fuse 1.

Fig. 5a - 5b show perspective views of another alternative contact on the fully covered embodiments of Fig. 4a and 4b. Fig. 5a shows a front side 9 of a fuse 1. The fuse element 3 is embedded in the front side 9, which is shown here in an enlarged view due to the small diameter of the fuse element 3.

0 By means of a "dip & blot" contact and/or electro-chemical growth of metals, an SMD-solderable external contact 25 is connected in Fig. 5b, which is L-shaped in this embodiment. The contact 25 can, however, also cover only the front side 9 alone.

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List of reference symbols

File 04207.9

1 fuse 33

2 Fusible conductors 34

3 Substrate 35

4 Contact 36 5 Edge 37

6 Cover 38

7 Casting compound 3 9

8 (Solder) Paste 40

9 Front side 41 10 Solder paste 42

11 Wire 43

12 tiles 44

13 Rounding 45

14 Recess 46 15 Recess 47

16 Middle area 48

17 Tapering/Flattening 49

18 middle part of substr 50

19 side walls 51 20 slot in the "tub" 52

21 Contact plate 53

22 slot in (21) 54

23 Bottom of the depression 55

24 Cover of the S.M. 56 25 L-shape, external contact 57

26 58

27 59

28 60 29

Claims (13)

(04207.9) Claims 1. Electrical fuse (1) , characterized by the following combination of features: a substrate (3) made of an electrically insulating material, - at least two electrical contacts (4), which are arranged at a distance from one another on the substrate (3),
a fusible conductor (2) ,
which connects the contacts (4) in an electrically conductive manner, and - an electrically insulating casting compound (7) on the substrate (3), with which the fusible conductor (2) is at least partially covered between the contacts (4). 2. Fuse according to claim 1, characterized in that the substrate (3) is a rectangular plate (12). . Fuse according to one or both of the preceding claims, characterized in that the substrate (3) consists of a ceramic material.
25 4. Fuse according to one or more of the preceding claims, characterized in that the casting compound (7) consists of a hardenable paste which, after hardening, adheres to the substrate (3).
30 5. Fuse according to one or more of the preceding claims, characterized in that the casting compound (7) is porous. 6. Fuse according to one or more of the preceding claims, characterized in that the casting compound (7) is thermally stable. 7. Fuse according to one or more of the preceding claims, characterized in that the contacts (4) are formed by metallization of opposite edges (5) and/or end faces (9) of the substrate (3). 8. Fuse according to one or more of the preceding claims, characterized in that the contacts (4) are made by burnable pastes (8). 9. Fuse according to one or more of the preceding claims, characterized in that the fusible conductor (2) is designed as a wire (11). 15 10. Fuse according to one or more of the preceding claims, characterized in that the substrate (3) is rounded at the edges (5) or end faces (9) to be metallized. 11. Fuse according to one or more of the preceding claims, characterized in that the substrate (3) has tapering or flattening portions (17) on the edges (5) or end faces (9) to be metallized. 12. Fuse according to one or more of the preceding claims, characterized in that a depression (15) or a recess (14) is arranged in the middle of the substrate (3). 13. Fuse according to one or more of the preceding claims, characterized in that between the fusible conductor (2) and the casting compound (7) at least one cover (24) is arranged, on which the casting compound (7) is arranged.