Abstract:
A subminiature circuit protector includes an electrically insulating substrate, a layer of electrically conducting material on the top surface of the substrate, the electrically conducting layer having a narrow central part forming a fuse element, opposing end parts of the electrically conducting layer extending to end and lateral edges of the substrate, an electrically insulation cover over the fuse element and end parts, and end terminations formed over the end and a portion of the lateral surfaces of the substrate electrically connecting with the end parts.

Description:
This is a divisional of commonly-owned U.S. patent application Ser. No. 08/166,882, filed Dec. 15, 1993, now U.S. Pat. No. 5,432,378. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a circuit protector. More particularly, the present invention relates to a subminiature surface mounted circuit protector. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Subminiature circuit protectors are useful in applications in which size and space limitations are important, for example, on circuit boards for electronic equipment, for denser packing and miniaturization of electronic circuits. Ceramic chip type fuses are known, but current structures are limited in size reduction by the structure of the fusing elements and encapsulation and sealing. 
     A problem in miniaturizing circuit protectors is that the small size of the fuse element and of the circuit protector itself results in a small contact area between the fuse element and the electrical terminations. The small contact area results in unnecessarily high resistance at the contact, and reduces the reliability and operation of the unit. 
     The present invention, generally, provides a subminiature surface mountable circuit protector that is simple and relatively inexpensive to manufacture. The present invention also provides a subminiature board mountable circuit protector that has improved reliability and operation. 
     The subminiature circuit protector of the present invention can be easily manufactured for a variety of voltage and current ratings. 
     The circuit protector according to the invention includes a substrate of electrically insulating material, such as ceramic or glass. The substrate has a flat top surface, opposing end edges and opposing lateral edges. Termination pads of electrically conductive material are deposited on the top surface at each end, and extend to the end edge and along a portion of the lateral edges. A fuse element of predetermined fusible response positioned across a space between the termination pads connects the termination pads to form a conductive path from end to end of the substrate. A cover of electrically insulating material suffuses over the termination pads and the fuse element to contact and envelop all of the underlying elements. 
     According to the invention, end terminations are formed by a coating of at least one layer of electrically conductive material that contacts the termination pads along the end edge and lateral edges of the substrate. The end terminations provide a greater contact area than previously known in the art for improving the electrical connection of the terminations to the fuse. In a preferred embodiment, the end coating comprises an inner layer of a silver alloy, a middle layer of nickel, and an outer layer of a tin/lead alloy. The end coating also extends along the lateral edges of the substrate as far as permitted by industry standards. 
     According to the invention, a cover may comprise glass or ceramic deposited on the top of the circuit protector over the previously deposited components or a polymer material applied to the top and cured by suitable means. Alternatively, the cover may comprise a plate of electrically insulating material, such as glass or ceramic, that is bonded to the top surface by suitable means, such as glass frit or adhesive. The cover may also comprise an uncured ceramic plate mechanically pressed on the top surface and cured to harden and bond it to the substrate. 
     The fuse element may be a deposited or printed film of gold or silver or another conductive material. Alternatively, the fuse element may comprise a conductive wire. Electrically conductive element pads may be provided to connect the fuse element with the termination pads and improve the electrical connection therethrough. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Preferred embodiments of the present invention are illustrated in the appended drawings, wherein like elements are provided with the same reference numerals. In the drawings: 
     FIG. 1 is a side cross-sectional view of a circuit protector in accordance with the present invention; and, 
     FIG. 2 is a top view of the circuit protector of FIG. 1 with a cover element removed. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring first to FIG. 1 and FIG. 2, a circuit protector 10 in accordance with this invention is shown. It is understood that the figures are not to scale, and that the thickness of the various components has been exaggerated for the purposes of clarity of illustration. Further, the invention is not limited to the particular illustrated configurations; the drawing figures illustrate combinations of aspects of the invention that may be selectively incorporated in a circuit protector according to the invention. 
     The circuit protector 10 comprises a substrate 20 of electrically insulating material, and at least one layer of electrically conductive deposited material on the substrate, including a fuse element 40, and electrical termination pads 60, 62 electrically connecting the fuse element to opposing end portion edges of the substrate. End terminations 70, 80 covering the end portions electrically connect with the termination pads 60, 62 to form external electrical terminals for connecting the circuit protector 10 in a circuit. 
     As an illustrative, but not limiting, example of its size, the circuit protector 10 of the present invention may be made in the range of about 0.050 to 0.400 inches long, 0.020 to 0.300 inches wide, and about 0.020 to 0.250 inches thick. 
     The substrate 20 is formed of a material such as ceramic or glass in a substantially rectangular shape. The substrate 20 has a planar top surface 22, a bottom surface 24, opposing end edges 26, 28, and opposing lateral edges 30, 32. 
     The fuse element 40 is a film made of an electrically conductive material such as gold, silver or another suitable material, and is deposited on the top surface by suitable means. The fuse element 40 is shaped with a predetermined cross sectional area to provide a desired fuse response, as is known in the art. For example, the fuse element 40 may be formed in the range of 0.0002 to 0.015 inches wide, 0.010 to 0.400 inches long, and 2KÅ to 0.003 inches thick. It may be necessary to deposit the fuse element material in more than one step to obtain a desired thickness, or to etch the deposited material to obtain a desired width, as needed for the particular electrical application. 
     The termination pads 60 and 62 are confined to opposing end portions 64, 66 of the top surface 22, so that a middle portion 68 of the top surface between the termination pads carries only the fuse element 40. The termination pads 60, 62 are formed of electrically conductive material and, as best seen in FIG. 2, may be deposited on the substrate as is known in the art. The termination pads 60, 62 extend to the end edges 26, 28, and to both of the lateral edges 30, 32 of the top surface 22. The termination pads 60, 62 are deposited over the fuse element 40 to form electrical connections at opposing ends of the fuse element. The termination pads 60, 62 are formed with a predetermined thickness that is at least as thick as the fuse element 40. Referring to FIG. 2, the thickness of the termination pads 60, 62 shown is greater than the thickness of the fuse element 40. This provides good electrical conductivity from the fuse element 40 through the termination pads 60, 62. In addition, the thickness of the termination pads 60, 62 is sufficient to provide a good contact area on the end edges 26, 28 and the lateral edges 30, 32 of the substrate 20 for connecting with the end terminations 70, 80. As an illustrative example, the termination pads 60, 62 may be in the range of 0.0002 to 0.002 inches thick. 
     A cover 90 of electrically insulating material is placed directly on the termination pads 60, 62 and the fuse element 40 on the top surface 22. The cover 90 may be formed of glass or ceramic or another suitable material. The cover suffuses the top surface 22 and deposited components, that is, contacts all exposed surfaces and of the termination pads 60, 62, the fuse element 40, and the top surface 22, and fills any voids around and between them. 
     In FIG. 1, the cover 90 is printed glass material applied directly on the top surface 22. The cover 90 may also alternatively comprise a plate of electrically insulating material that is bonded by a layer of bonding material to the top surface 22 over the assembled components. The bonding material is applied to the top surface 22 to suffuse the top surface and the assembled components as described above, and the cover placed on the bonding material. The bonded cover may comprise a glass plate bonded by a glass frit layer. Alternatively, the bonded cover may comprise a plate of cured ceramic bonded by a ceramic adhesive. 
     The end terminations 70, 80 comprise electrically conductive material coated over the end portions 64, 66 of the circuit protector subassembly after the cover has been put in place. The end terminations 70, 80 may be coated on the circuit protector subassembly as is known in the art, for example, by dipping an end portion of the subassembly in a suitable coating bath followed by firing. The end terminations 70, 80 contact the termination pads 60, 62 at the end edges 26, 28 and on the lateral edges 30, 32. The end terminations 70, 80 extend along the lateral edges 30, 32 of the substrate as far allowed by industry standards, and so that the lateral edges of the termination pads 60, 62 are at least partially enclosed in the end terminations. The end terminations 70, 80 also correspondingly extend over a portion of the cover 90 and the bottom surface 24 of the substrate. 
     According to a preferred embodiment of the invention, as illustrated in FIGS. 1 and 2, the end terminations 70, 80 comprise an inner layer 72, 82 of an electrically conductive material, such as silver, a silver alloy or a silver containing composition such as palladium-silver. A middle barrier layer coating 74, 84 of a material such as nickel is applied over the inner layer, and an outer layer 76, 86 of a solderable material, such as a lead/tin composition is applied over the middle layer. The outer layers 76, 86 facilitate attachment by soldering of the circuit protector in an electrical circuit. 
     The foregoing has described the preferred principles, embodiments and modes of operation of the present invention; however, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations, changes and equivalents may be made by others without departing from the scope of the present invention as defined by the following claims.