Thin film fusible element - Patent Review 5091712

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BACKGROUND OF THE INVENTION

The invention relates to thin film fusible elements that are supported on substrates and electrical fuses employing them.

It is known to provide fusible elements from thin films of conductive material supported on insulating substrates. This permits an element thickness that is less than that achievable by stamping (i.e., 0.002") in order to provide low-current capacity and ease of handling at low-current capacity. Examples of patents describing fusible elements having thin films of conductive material on substrates provided by various deposition techniques are: U.S. Pat. Nos. 3,271,544; 4,140,988; 4,208,645; 4,376,927; 4,494,104; 4,520,338; 4,749,980; and 4,873,506.

SUMMARY OF THE INVENTION

In general, the invention features a thin film fusible element that is supported on an insulating substrate and has a body portion for conducting electricity therethrough and cooling arms that extend laterally from a side of the body portion. The body portion has back-up sections and a fusible portion of narrower conductive area than back-up sections. The cooling arms conduct and dissipate heat but do not conduct electricity. The cooling arms facilitate the removal of heat from a fusible portion of the body portion, thus regulating the fusible portion temperature and melting characteristics.

In preferred embodiments, the body portion has a plurality of alternating back-up sections and fusible portions. In some embodiments, the cooling arms extend generally perpendicular to a longitudinal body axis along the body portion; in some other embodiments, the cooling arms make acute angles with the body axis, and in some other embodiments the cooling arms have segments that extend first in one direction generally parallel to the body axis and then the other direction generally parallel to the body axis.

The fusible element is preferably used in a fuse in a fuse casing having arc-quenching fill material therein. The cooling arms better distribute the heat throughout the fill material because the arms project into more regions of the fill material than the body portion alone. A thermally conductive paste can be placed on the arms to enhance the removal of the heat from the arms to the arc-quenching fill material.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will now be described.

DRAWINGS

FIG. 1 is perspective view, partially broken away, of a fuse including a thin film fusible element according to the invention.

FIGS. 2-4 are a partial plan views showing different geometries that can be employed for the fusible element used in the FIG. 1 fuse.

STRUCTURE

Referring to FIG. 1, there is shown electrical fuse 10 having fuse casing 12, end cap terminals 14, 16, and fusible element 18 supported on ceramic substrate 20 within casing 12. Metal strip 22 is soldered to fusible element 18 and metal end cap terminal 16 and makes electrical contact between them.

Referring to FIG. 2, it is seen that fusible element 18 has body portion 24 that extends along longitudinal body axis 26. Body portion 24 has fusible portions 28 (also referred to as notch regions) and back-up sections 30 of larger width therebetween. Extending laterally from back-up sections 30 are cooling arms 32. The body portions and cooling arms are preferably made of copper deposited thereon by D.C. magnetron sputtering. The thickness of copper depends upon the fuse rating; a one amp fuse would have copper approximately 70 microinches thick. In the FIG. 2 embodiment, cooling arms 32 extend perpendicular to body axis 26. In the embodiment shown in FIG. 3, cooling arms 34 make an acute angle .theta. with body axis 26. In the embodiment shown in FIG. 4, cooling arms 36 follow a zig-zag pattern and have segments that extend generally parallel to axis 26 first in one direction, then the other. Conductive paste is placed on arms 32, 34, 36 to conduct heat to arc-quenching fill material 38 (e.g., 50/70 quartz). The paste substantially fills all voids adjacent to the cooling fins. (The fill material, shown only partially filling casing 12 in FIG. 1, in fact fills the entire casing.) Conductive paste generally is not placed on body portion 24 so as to not interfere with circuit breaking characteristics of body portion 24 during overload conditions.

OPERATION

In operation, electrical current is conducted from and to an external electrical circuit via end cap terminals 14, 16 and metal strips 22 to fusible element 18. During normal current load conditions, current flows through body portion 24; current density is not significantly affected by the existence of cooling arms 32, 34, or 36, because there are no electrical paths through them. The temperature of the cooling arms is less than that of back-up sections 30, which are at lower temperature than fusible portions 28. Heat flows to the cooling arms and is dissipated to the arc-quenching fill material, the arms permitting good distribution of heat. The removal of heat influences the melt time characteristics of the particular fusible portions 28, allowing for a thinner fuse element material and reduced Joule heating. At overload circuit conditions, fusible portions 28 melt, creating an open circuit.

OTHER EMBODIMENTS

Other embodiments of the invention are within the scope of the claims.

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