Abstract:
Subminiature surface mount chip fuses include two part housings enclosing a fuse element and prefabricated end caps. The housing ends are shaped to restrict freedom of movement of the fuse element ends as the end caps are assembled to the housing. The end caps may include features to positively secure them in place and restrict relative movement of the end caps relative to the housing. Holes may be provided in the end caps that allow solder flow from a location exterior to the end caps to flow interior to the end caps to establish electrical connection with the fuse element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The field of the invention relates generally to electrical fuses, and more specifically to surface mount fuses for circuit board applications. 
         [0002]    Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Typically, fuse terminals or contacts form a current path and electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals or contacts, so that when electrical current through the fuse exceeds a predetermined threshold, the fusible elements melt, disintegrate, sever, or otherwise open the current path through the fuse element, and hence the circuit associated with the fuse to prevent electrical component damage. 
         [0003]    A proliferation of electronic devices in recent times has resulted in increased demands on fusing technology. Particularly for miniaturized fuses designed to be surface mounted to circuit boards, manufacturing improvements and performance improvements are especially desired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Non-limiting and non-exhaustive embodiments are described with reference to the following Figures, wherein like reference numerals refer to like parts throughout the various drawings unless otherwise specified. 
           [0005]      FIG. 1  is a perspective view of a first exemplary embodiment of a surface mount fuse in accordance with an aspect of the present invention. 
           [0006]      FIG. 2  illustrates the surface mount fuse shown in  FIG. 1  partly broken away. 
           [0007]      FIG. 3  is an exploded view of the fuse shown in  FIGS. 1 and 2 . 
           [0008]      FIG. 4  is a side elevational view of a portion of the fuse assembly shown in  FIGS. 2 and 3 . 
           [0009]      FIG. 5  is an end view of the portion of the fuse assembly shown in  FIG. 4 . 
           [0010]      FIG. 6  is a bottom plan view of the fuse assembly shown in  FIG. 5 . 
           [0011]      FIG. 7  is a cross sectional view of the assembly shown in  FIG. 6  taken along line  7 - 7 . 
           [0012]      FIG. 8  is a perspective view of the fuse assembly shown in  FIGS. 4-7 . 
           [0013]      FIG. 9  is a partial exploded view of the fuse shown in  FIG. 1 . 
           [0014]      FIG. 10  is a perspective view of a first alternative end cap construction in accordance with an aspect of the present invention. 
           [0015]      FIG. 11  is a perspective view of a second exemplary embodiment of a surface mount fuse including end caps as shown in  FIG. 10 . 
           [0016]      FIG. 12  is a perspective view of a second alternative end cap construction in accordance with an aspect of the present invention. 
           [0017]      FIG. 13  is a perspective view of a second exemplary embodiment of a fuse sub-assembly for use with the end cap shown in  FIG. 12 . 
           [0018]      FIG. 14  is a perspective view of a second exemplary embodiment of a fuse including end caps as shown in  FIG. 12 . 
           [0019]      FIG. 15  is a perspective view of a third alternative end cap construction in accordance with an aspect of the present invention. 
           [0020]      FIG. 16  is a perspective view of a third exemplary embodiment of a fuse sub-assembly for use with the end cap shown in  FIG. 15 . 
           [0021]      FIG. 17  is a perspective view of a third exemplary embodiment of a fuse including end caps as shown in  FIG. 15 . 
           [0022]      FIG. 18  is a perspective view of a fourth alternative end cap construction in accordance with an aspect of the present invention. 
           [0023]      FIG. 19  is a perspective view of a fourth exemplary embodiment of a fuse sub-assembly for use with the end cap shown in  FIG. 18 . 
           [0024]      FIG. 20  is a perspective view of a fourth exemplary embodiment of a fuse including end caps as shown in  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Exemplary embodiments of surface mount fuse constructions for circuit board applications and electronic devices are described hereinbelow that overcome numerous problems in the art. In order to understand the invention to its fullest extent, the following disclosure is presented in different Parts or segments, wherein Part I introduces the art and problems associated therewith, and Part II discloses advantageous embodiments of fuse constructions and methods overcoming the issues discussed in Part I. 
         [0026]    Part I: Introduction 
         [0027]    Conventionally, fuses for electronic applications included a wire fuse element (or alternatively a stamped and/or shaped metal fuse element) encased in a glass cylinder or tube and suspended in air within the tube. The fuse element is extended between conductive end caps attached to the tube for connection to an electrical circuit. However, when used with printed circuit boards in electronic applications, the fuses typically must be quite small, and tend to require leads which may be soldered to a circuit board having through-holes therein for receiving the leads. Miniature electronic fuses of this type are known and can be effective in protecting electronic circuitry. However, such fuses can be fragile, and through-hole mounting of such fuses can be tedious and difficult to install to circuit boards, especially as the physical size of the fuse is decreased. 
         [0028]    At least in part to avoid manufacturing and installation difficulties of through-hole mounted miniature electronic fuses, so-called chip fuses have been developed which may be surface mounted to circuit boards. Chip fuses may be manufactured in layers, eliminating a need for separately provided, fragile fuse tubes and the lead assemblies of the devices described above, while at the same time providing better fusing characteristics (e.g., faster acting fuses) for some electronic circuits. Such chip fuses may include, for example, a substrate layer, a fuse element layer, one or more insulative or protective layers overlying the fuse element layer, and end terminations formed over the substrate and the fuse element layer for surface mounting to a circuit board. While such chip fuses provide low cost fuse products that are rather easily surface mounted to circuit boards, they can be relatively expensive to manufacture and are limited in their performance capabilities. 
         [0029]    Still more recently, chip-type fuses have been constructed having a prefabricated body and cover that collectively house a fuse element, and prefabricated end caps that are assembled to the body and electrically connected to the fuse element. Typically, the fuse element is soldered to the end caps. The fuse elements and the end caps can be quite small, however, and practical difficulties exist in making the soldered connections. 
         [0030]    Of particular concern is incomplete bonding between the fuse element, the solder used, and the conductive end caps. Such bonding issues may result in what is sometimes referred to as “cold soldered” joints that are known to be unreliable, and hence undesirable, in establishing the electrical connection. Cold solder joints may result for different reasons, including, but not limited to a failure to expose the solder to its reflow temperature during soldering processes, and relative movement of the parts being soldered (e.g., the fuse element and the end caps) during soldering processes. Instances of cold soldered joints can be difficult to control or detect when especially small parts are being soldered, such as in modern chip fuse devices. Cold solder joints result in performance variations, and sometimes inoperative fuses, that are unacceptable to electronic device manufacturers. 
         [0031]    Recent emphasis on lead-free soldering processes for chip fuses and other electronic components has introduced further challenges to the industry. Known lead-free solders require a higher reflow temperature, typically 30° to 40° C., than conventional soldering materials including lead (e.g., tin/lead solder). As such, because of the higher reflow temperature for preferred soldering materials, undesirable cold soldered joints may be somewhat more likely than before. 
         [0032]    Also, exposing ever smaller parts such as those used in known chip fuses to higher soldering temperatures required by lead free solder materials presents still other issues for heat sensitive components of the fuses. Specifically, one or more components of the fuse may distort or become permanently damaged in higher temperature soldering processes, particular when the desired reflow temperature is exceeded, which can sometimes be difficult to control. Plastic materials, for example, used to fabricate the electrically insulating portions of the fuses are susceptible to degrading or melting at higher soldering temperatures, which can negatively impact fuse performance and reliability. 
         [0033]    Part II. Exemplary Surface Mount Fuses and Methods 
         [0034]    Embodiments of surface mount fuses are described hereinbelow that avoid, if not eliminate, instances of defective cold solder joints as well as provide manufacturing advantages including but not limited to lower costs and improved reliability and performance characteristics. Manufacturing and installation methods associated with the surface mount fuses described will be in part apparent and in part specifically pointed out in the discussion below. Like reference numerals refer to like parts throughout the various drawings unless otherwise specified 
         [0035]      FIG. 1-9  illustrate a first exemplary embodiment of a surface mount fuse  100  for surface mount connection to a circuit board  102  (shown in phantom in  FIG. 1 ). As shown in  FIG. 1 , the fuse  100  includes an electrically insulating or nonconductive body or housing  104 , and conductive end caps  106 ,  108  coupled to opposing ends of the housing  104 . The conductive end caps  106 ,  108  define respective surface mount areas for connection to circuit pads or traces  110 ,  112  (shown in phantom in  FIG. 1 ) according to known techniques, including but not limited to soldering processes. The pad or trace  110  may be connected to a power supply or line side circuitry  114  associated with the board  110 , and the pad or trace  112  may be connected to load side components or circuitry  116  associated with the board  102 . 
         [0036]    As shown in  FIG. 2 , wherein the housing  104  of the fuse  100  is partly broken away, a fuse element  120  extends through the housing  104  between the end caps  106  and  108 , and the end caps  106  and  108  are electrically connected to the fuse element  120  such that a conductive current path is established through the fuse  100 . When the end caps  106  and  108  are, in turn, electrically connected to the circuit traces or pads  110 ,  112  ( FIG. 1 ), an electrical circuit is completed through the fuse element  120  between the line side circuitry  114  and the load side circuitry  116 . The fuse element  124  is constructed to melt, disintegrate, sever, or otherwise open the current path through the fuse element  120  established between the line side circuitry  114  and the load side circuitry  114  ( FIG. 1 ) when electrical current through the fuse exceeds a predetermined threshold. The load side circuitry  116  may therefore be electrically isolated from the line side circuitry  114  and potentially damaging current conditions when they occur. Sensitive and costly components in the load side circuitry  116  may therefore be protected by the fuse  100 . 
         [0037]    As also seen in  FIG. 2 , the housing  104  includes a first piece, sometimes referred to as a base  122 , and a second piece, sometimes referred to as a cover  124 . The base  122  and cover  124  may be assembled as explained below, and collectively surround and protect the fuse element  120  for reliable operation thereof. 
         [0038]      FIG. 3  illustrates the components of the fuse  100  in exploded view. The housing base  122  is fabricated from an electrically insulating or nonconductive material and includes opposed longitudinal side walls  126 ,  128  and opposing end walls  130 ,  132  interconnecting the longitudinal side walls  126 ,  128 . In one embodiment the housing base  122  is fabricated from a ceramic material having sufficient temperature resistance to capably withstand high temperature soldering operations, although other nonconductive materials may likewise be utilized in other embodiments if desired. 
         [0039]    The housing base  122  defines a longitudinal axis  134  ( FIG. 4 ) and the longitudinal side walls  126 ,  128  extend generally parallel to one another and to the longitudinal axis  134 . The end walls  130  and  132  extend generally perpendicular to the longitudinal axis  134  and the longitudinal side walls  126 ,  128 . As such, the housing base  122  is generally rectangular and box-like, and a fuse element cavity  136  is defined interior to the side walls  126  and  128  and the end walls  130  and  132 . The fuse element cavity  136  is generally open on one side  138  of the base  122  (the upper side as shown in  FIGS. 3 and 4 ) for assembly of the fuse element  120  as explained below, and is closed on the opposing side  140  of the base  122  (i.e., the lower side as shown in  FIGS. 3 and 4 ). 
         [0040]    As shown in  FIGS. 3 ,  4  and  6 , the longitudinal side walls  126  and  128  include a stepped outer or exterior surface having a center surface  142  and end surfaces  144  extending on either side of the center surface  142  and opposing one another. The center surface  142  and the end surfaces  144  are generally flat and planar in the illustrated embodiment. The end surfaces  144  are depressed or recessed relative to the center surface  142 , such that the end surfaces  144  provide thinner end portions of the side walls  126 ,  128  leading to the end walls  130 ,  132 . As best seen in  FIG. 6 , in the exemplary embodiment illustrated the center surfaces  142  of each of the side walls  126 ,  128  extends in a plane spaced from but parallel to the longitudinal axis  134  at a first distance D 1 , and the end surfaces  144  of each of the side walls  126 ,  128  extends in another plane spaced from but parallel to the longitudinal axis  134  at a second distance D 2  that is less than the first distance D 1 . The difference between these dimensions D 1  and D 2  is approximately equal to a thickness of the end caps  106  and  108  such that when the end caps  106 ,  108  are installed over the respective end walls  130 ,  132  the end caps are approximately flush with the exterior surfaces of the housing base  122  that are not covered by the end caps  106 ,  108  as best shown in  FIG. 1 . 
         [0041]    As shown in  FIGS. 2-6 , the end walls  130 ,  132  each include a stepped outer surface including a center surface  146  and end surface  148  extending on either side of the center surface  146  and opposing one another. The center surface  146  in each end wall  130 ,  132  is depressed or recessed relative to the end surfaces  148  as best seen in  FIG. 6 , while the end surfaces  148  outwardly project from the center surface  146 . An elongated fuse receiving slot  150  is formed in each end wall  130 ,  132 , and as best shown in  FIG. 5  extends from a first edge  152  of the end wall toward an approximate midpoint of the center wall. The slot  150  is also approximately centered between the projecting end surfaces  148 , and is aligned with the end surfaces  148  such that the end surfaces  148  and the slot  150  extend generally parallel to one another in an axial direction (e.g., a vertical direction in the plane of  FIG. 5 .) 
         [0042]    The fuse element  120 , as shown in  FIGS. 3 ,  5 , and  6  is received in the slots  150  in each end wall. As shown in  FIG. 3 , the fuse element  120  may be loaded in the fuse element cavity  136  of the housing base  122  via the slots  150 , and as shown in  FIG. 5 , a bonding agent  154  may be provided to secure the fuse element  120  in position toward the midpoint of each end wall  130 ,  132 . In various embodiments, the bonding agent may be an epoxy based material, a non-epoxy based material, a UV curable glue or other adhesive familiar to those in the art. In other embodiments the bonding agent may be considered optional. When used, the bonding agent  154  secures and maintains the fuse element  120  in a desired position relative to the housing base  122 , thereby ensuring a greater reliability in the electrical connections and performance of the fuse element  120 . 
         [0043]    As illustrated in  FIG. 3 , the fuse element  120  is a generally straight wire fuse element extending across the housing base  122  between the end walls  130  and  132 . The fuse element  120  may be fabricated from a conductive material known in the art, including but not limited to silver, copper, nickel, tin, zinc and alloys thereof, or still other materials if desired. The current capacity of the fuse element is determined by the conductive material utilized and diameter of the wire. Ideally, the fuse element has a high electrical resistance such that relatively small currents actually flow through the fuse element in use. Metcalf techniques and the like may be utilized to vary the fusible action of the element and achieve different performance objectives. 
         [0044]    While one particular type of fuse element  120  is shown, it is understood that other types of fuse elements may likewise be used, including but not limited to stamped metal elements having one or more areas of reduced cross section. Additionally, wire fuse elements having other configurations than that shown in  FIG. 3  may likewise be utilized. For example, a wire fuse element that is helically wound about a core element may be utilized instead of the substantially straight wire shown in  FIG. 3 . Still other fuse element types and configurations are possible, and more than one fuse element may be utilized in combination in further and/or alternative embodiments. 
         [0045]    The housing cover  124  is, for example, a generally planar cover having a uniform thickness throughout, and is generally rectangular in shape as shown in  FIG. 3 . The cover  124  is fitted into a complementary-shaped opening  156  in the base  122 . As seen in  FIGS. 2 ,  5 ,  6  and  7 , the cover  124  substantially closes the fuse element cavity  136  of the housing base  122 . As seen in  FIG. 6 , however, the ends of the cover  124  are longitudinally spaced from the fuse element receiving slots  150  in the housing base end walls  130 ,  132 . That is, the cover  124  does not extend over the fuse element receiving slots  150 , and the fuse element receiving slots are accessible after the cover  124  is installed. 
         [0046]    The cover  124 , like the housing base  122 , may be fabricated from an electrically insulating or nonconductive material. In one embodiment the cover  124  is fabricated from a ceramic material having sufficient temperature resistance to capably withstand high temperature soldering operations, although other nonconductive materials may likewise be utilized in other embodiments if desired. The cover  124  need not be fabricated from the same material as the base in all contemplated embodiments. That is, the housing base  122  and cover  124  may be fabricated from different non-conductive materials having different properties. The cover  124  may be mechanically fitted with the housing base  122  with a slight interference fit, via frictional engagement, via other mechanical engagement techniques, or with bonding agents or adhesives in various exemplary embodiments. 
         [0047]    Referring again to the exemplary embodiment shown in  FIGS. 3 and 9 , the end caps  106  and  108  are each independently fabricated from the remainder of the assembly and are provided as separate component parts for later assembly. The end caps  106 ,  108  are sometimes referred to as prefabricated parts, and are distinguishable from termination structure formed on the surfaces of the housing itself using metallization techniques, dipping techniques and the like. The end caps  106  are formed according to known methods, and each generally include end walls  158  and four generally orthogonal side walls  160  extending from the end walls  158  and defining a generally rectangular receptacle  162  that may be fitted over and assembled with the respective end walls  130 ,  132  of the housing base  122 . An interior surface of the end wall  158  in each end cap  106 ,  108  may be provided with an electrical connection media  164  such as solder or conductive ink that may be reflowed and solidified to establish an electrical connection with the fuse element  120 . In some embodiments, the electrical connection media  164  may be considered optional and may be omitted. 
         [0048]    As partly shown in  FIG. 7 , the fuse element cavity  136  in the housing base  122  may further be filled with an arc quenching media  166  in some embodiments. In various embodiments, the arc quenching media  166  may be sand or silica materials familiar to those in the art, or glass materials and the like having arc quenching properties to extinguish electrical arcs when the fuse element operates. In other embodiments, the arc quenching media  166  may be considered optional and may be omitted and the fuse element  120  may be surrounded by air inside the housing base  122 . 
         [0049]    As shown in  FIGS. 8 and 9 , the free ends  168  of the fuse element  120  extending exterior to the fuse element cavity  136  of the hosing base  122  are bent at approximately right angles (about 90°) in the example shown, such the fuse element ends  168  extend generally parallel to the end surfaces  148  of the end walls and generally parallel to the center surface  146  of the end walls. Also, the bent ends  168  of the fuse element  120  are generally axially aligned with the fuse element receiving slots  150  in the end walls. The fuse element ends  168  are generally protected by the projecting end surfaces  148  extending alongside the fuse element as the end caps  106 ,  108  are assembled to the housing base  122 . Inadvertent damage to the fuse element ends  168 , which can present reliability issues for the completed fuse, is therefore substantially avoided as the end caps  106 ,  108  are fitted to the housing base  122 . 
         [0050]    The assembly may also more capably withstand higher soldering temperatures when lead free soldering materials are provided as the connection media  164  ( FIG. 9 ) in the end caps  106 ,  108 . 
         [0051]    Also, the projecting end surfaces  148  generally limit movement of the fuse element ends  168  relative to the end caps  106  and  108  as the end caps  106 ,  108  are assembled/installed over the ends of the housing base  122  and as electrical connections are completed between the fuse element ends  168  and the end caps  106 ,  108 . A confined contact area is consistently established by locating the fuse element ends  168  alongside the depressed center surface  146  in the end walls  130 ,  132  between the projecting end surfaces  148 . Limiting the freedom of movement of the fuse element ends  168 , as well as providing a consistent contact area in a predetermined location offers further improvement in the reliability of the electrical connection between the end caps  106 ,  108  and the fuse element ends  168 . As such, cold solder joints and other reliability issues believed to result from movement of the fuse element relative to the end caps  106 ,  108  as the electrical connections with the fuse element ends  168  are established are substantially avoided. 
         [0052]    The assembly is also capable of being implemented on a miniaturized level. Fuses may be provided in miniaturized package sizes for use as chip fuses having a similar scale to other components mounted on a circuit board for an electronic device. Dimensions of such chip fuses are typically measured in millimeters. In one example, completed fuses  100  may be about 6 mm in length measured along the longitudinal axis  134  ( FIGS. 4 and 6 ) and about 3 mm or less in width measured in a direction perpendicular to the longitudinal axis  134  (i.e. a width of the end walls  130 ,  132  extending between the longitudinal side walls  126 ,  128  of the housing base  122 ). Greater or lesser dimensions are possible. 
         [0053]    In still another embodiment, when a conductive ink is used in lieu of solder materials, high temperatures associated with soldering techniques, whether lead free solder or otherwise, may be avoided altogether, leading to cost savings in the manufacturing process. 
         [0054]      FIG. 10  is a perspective view of a first alternative end cap structure  200  that may be utilized in lieu of the end caps  106 ,  108  as described above with even further benefits. Like the end caps  106 ,  108 , the end cap  200  includes an end wall  158  and four generally orthogonal side walls  160  extending from the end wall  158  and defining a generally rectangular receptacle  162  that may be fitted over and assembled with the respective end walls  130 ,  132  of the housing base  122  as described above. Unlike the end caps  106 ,  108  shown in  FIG. 9 , the end cap  200  does not include an electrical connecting media (e.g., solder or conductive ink). 
         [0055]    As shown in  FIG. 10 , the end cap includes an aperture or hole  202  formed completely through the thickness of the end cap. The hole  202  is formed in a known manner using stamping or punching techniques, for example, and is located adjacent the side wall  160  that is surface mounted to a circuit board, such as the board  102  shown in  FIG. 11 . End caps  200  may be assembled to the remainder of the assembly described above in a substantially similar manner to that described above to form a completed fuse  210  as shown in  FIG. 11 . While in the embodiment shown, the hole  202  is formed in a generally square or rectangular shape, it may alternatively be elliptical, rounded, or otherwise shaped in various alternative embodiments. 
         [0056]    The hole  202  in the end caps  200  is advantageous because it eliminates any need for a connection media such as solder or conductive ink to be provided in the end caps  200  to make an effective electrical connection with the ends  168  of the fuse element  120 . Rather, the electrical connection between the fuse element ends  168  and the end caps  200  is established when the fuse  210  is soldered to the circuit board  102 . A portion of the solder used to connect the end caps  200  to the board  102 , initially provided external to the fuse  210 , will wick inside the holes  202  that are positioned proximate the board  102  and will directly make contact with the fuse element ends  168  interior to the end caps  200 . The depressed center surface  146  in the housing end walls  130 ,  132  ( FIG. 9 ) defines a channel for the solder to flow within the interior of the end cap and it can be practically ensured that the solder will contact the fuse element ends  168 . 
         [0057]    This direct path connection and simultaneous connection of the end caps  200  to the board  102  as well as the fuse elements ends  168 , made possible by the holes  202  in the end caps  200 , will result in a lower electrical resistance compared to a fuse including the end caps  106 ,  108  including internal solder connections without the holes  202  being present. Electrical current need not flow through the end cap  200  itself, but because of the hole  202  allowing the external solder to flow interior to the end caps  200  as the fuse  210  is installed, current may flow through the solder only from a location exterior to the end caps  200  to locations interior to the end caps  200  where the fuse elements ends  168  reside. Material costs associated with solder materials in the construction of the fuse  210  and also labor costs of making separate solder connections internal to the fuse  210 , prior to mounting of the fuse  210  on the board  102 , are therefore avoided. 
         [0058]      FIG. 12  is a perspective view of a second alternative end cap structure  220  that may be utilized in lieu of the end caps  200  as described above with still other benefits. Like the end caps  200 , the end cap  220  includes an end wall  158  and four generally orthogonal side walls  160  extending from the end wall  158  and defining a generally rectangular receptacle  162  that may be fitted over and assembled with the respective end walls  130 ,  132  of the housing base  122  as described above. The end cap  200  includes the hole  202  providing the advantages described above, and in one of the walls  160  opposing the hole  202 , a retention dimple  222  is also provided in the end cap  200 . The dimple  222  may be formed, for example, via a stamping process or other known technique such that an indentation is provided in the exterior surface of the end cap  220 , and a projection is provided in the interior of the end cap  220  at the location of the dimple  222 . 
         [0059]    As shown in  FIG. 13  the housing base  122  is provided with outward facing retention cavities  224  that are shaped in a complementary manner to the retention dimples  222  in the end caps  220 . When the end caps  200  are fitted over the end walls  130 ,  132  of the housing base  122  to form a completed fuse  230  ( FIG. 14 ), the retention dimples  222  projecting interior to the end caps  220  are interlocked with the retention cavities  224  of the housing base  122 , positively securing the end caps  220  to the housing base  122 . Relative movement of the end caps  220  relative to the housing base  122  is impeded by the interlocking housing base  122  and end caps  220 . Cold solder joints and other undesirable effects attributable to movement of the end caps  220  as the internal electrical connections are established with the fuse element ends  168  are therefore substantially avoided, if not eliminated. 
         [0060]      FIG. 15  is a perspective view of a third alternative end cap structure  240  that may be utilized in lieu of the end caps  220  as described above. Like the end cap  220 , the retention dimple  222  is provided, but the hole  202  is not. Because the hole  202  is not provided, the electrical connection media  164  (e.g., solder or conductive ink) is provided in the end cap  240  and the media  164  may be reflowed to establish the electrical connection between the end cap  240  and the fuse element end  168  ( FIG. 16 ). 
         [0061]    As shown in  FIG. 16 , the cover  124  is provided with retention openings  242  that are shaped in a complementary manner to the retention dimples  222  in the end caps  240 . When the end caps  240  are fitted over the end walls  130 ,  132  of the housing base  122  to form a completed fuse  250  ( FIG. 17 ), the retention dimples  222  are interlocked with the retention openings  242  and the end caps  240  are positively secured to the housing base  122 . Relative movement of the end caps  240  relative to the housing base  122 , which can lead to cold solder joints and other undesirable effects, are therefore substantially avoided, if not eliminated. 
         [0062]      FIG. 18  is a perspective view of a fourth alternative end cap end cap structure  260  that may be utilized in lieu of the end caps  240  as described above. The end cap  260  includes two retention dimples  222  situated on opposing walls  160  of the end cap  260 . 
         [0063]    As shown in  FIG. 19 , the housing base  122  is provided with the retention cavities  224  and the cover  124  is provided with retention openings (not visible in  FIG. 19  but similar to the openings  242  shown in  FIG. 16 ) that are shaped in a complementary manner to the retention dimples  222  in the end caps  260 . When the end caps  260  are fitted over the end walls  130 ,  132  of the housing base  122  to form a completed fuse  270  ( FIG. 20 ), one of the retention dimples  222  in each end cap  260  is interlocked with one of the retention openings  242  in the housing base  122 , and the one of the retention dimples  222  in each end cap  260  is interlocked with the retention openings  242  in the cover  124 . As such the end caps  260  are positively secured on more than one side of the fuse housing, resulting in even greater stability of the relative position of the end caps  260  relative to the fuse element ends  168  during assembly, installation, and completion of the electrical connections between the fuse element ends  168  and the end caps  260 . Relative movement of the end caps  260  relative to the fuse element ends  168 , which can lead to cold solder joints and other undesirable effects and reliability issues, are therefore substantially avoided, if not eliminated. 
         [0064]    III. Conclusion 
         [0065]    The benefits and advantages of the exemplary embodiments are now believed to be apparent. 
         [0066]    An embodiment of an electrical fuse is disclosed that includes a nonconductive housing comprising a base and a separately provided cover fitted to the base. The base comprises opposing longitudinal side walls and opposing end walls interconnecting the longitudinal side walls. The longitudinal side walls extend parallel to a longitudinal axis, the end walls extend perpendicular to the longitudinal axis. The longitudinal side walls and the end walls define an interior fuse element cavity therebetween, and at least one of the end walls comprises a fuse element receiving slot in communication with the interior fuse element cavity. 
         [0067]    The cover substantially closes the interior fuse element cavity when the cover is fitted to the base, and the cover is longitudinally separated from the fuse element receiving slot when the cover is fitted to the base. A fuse element is received in the base. The fuse element extends through the fuse element receiving slot and extends across the fuse element cavity between the opposing end walls of the base. 
         [0068]    First and second conductive end caps are fitted over the respective opposing end walls of the base adjacent respective ends of the fuse element, the first and second end caps defining a surface mount area for connection to a circuit board. 
         [0069]    Optionally, the fuse element includes a bend at a location adjacent to the fuse element receiving slot, whereby a portion of the fuse element end extending exterior to the fuse element receiving cavity extends generally parallel to the end wall. The end wall may include a generally planar surface, and the fuse element receiving slot may be elongated in the plane of the planar surface. The portion of the fuse element extending exterior to the fuse element receiving cavity may be axially aligned with the elongated fuse element receiving slot. 
         [0070]    The longitudinal side walls may optionally include a stepped outer surface. The stepped outer surface may include opposing end surfaces and a center surface between the end surfaces, with the end surfaces being depressed relative to the center surface. 
         [0071]    The at least one end wall may optionally include a stepped outer surface. The stepped surface may include opposing end surfaces and a center surface between the end surfaces, with the center surface being depressed relative to the end surfaces. The fuse element receiving slot may be formed through the center surface and may be substantially equally spaced from the end surfaces. 
         [0072]    The fuse element may optionally extend straight across the fuse element cavity between the opposing end walls. 
         [0073]    At least one of the first and second end caps may optionally be provided with solder to establish electrical connection between the at least one end cap and one of the fuse element ends. Alternatively neither of the first and second end caps may be soldered to the fuse element. In one embodiment, one of the first and second end caps may be provided with conductive ink to establish electrical connection between the at least one end cap and one of the fuse element ends. 
         [0074]    At least one of the end caps may optionally include at least one retention dimple for securing the end cap to the base. The base may be formed with an exterior end cap receiving cavity adjacent at least one of the end walls, and the retention dimple may be interlocked with the receiving cavity when the at least one end cap is fitted to the base. The cover may be formed with an end cap receiving opening, the end cap receiving opening located adjacent at least one of the end walls when the cover is fitted to the base, and the retention dimple being interlocked with the receiving opening when the at least one end cap is fitted to the cover. The at least one end cap may include an end wall, a first side wall and a second side wall, and the at least one retention dimple may include a first retention dimple formed in the first side wall and a second retention dimple formed in the second side wall. The retention dimple may optionally be substantially rectangular in shape. 
         [0075]    At least one of the end caps may optionally include an aperture extending completely through a thickness of the end cap, with the aperture located proximate the surface mount area. The end cap may further include a retention dimple for positively securing the end cap to one of the base and the cover. 
         [0076]    At least one of the base and the cover may optionally be fabricated from a ceramic material. The fuse element receiving cavity may optionally be filled with an arc quenching media. The fuse element may optionally be bonded to the fuse element receiving slot. The cover may be a generally planar cover having a uniform thickness. 
         [0077]    An embodiment of an electrical fuse is also disclosed including a nonconductive housing comprising a base and a cover. The base comprises opposing longitudinal side walls and opposing end walls interconnecting the longitudinal side walls, with the lateral side walls and the end walls defining a fuse element cavity therebetween. The cover is fitted to the base and substantially closes the fuse element cavity. A fuse element received in the fuse element receiving slot and extends across the fuse element cavity between the end walls of the base. First and second terminal elements include conductive end caps fitted over the respective end walls of the base proximate a respective end of the fuse element. The first and second end caps each define a surface mount area for connection to a circuit board. One of the end caps comprises at least one of a retention dimple and an opening formed completely through a thickness of the end cap proximate the surface mount area. 
         [0078]    Optionally, the base may include an exterior end cap retention cavity that receives the retention dimple. The cover may optionally include an end cap retention opening that receives the retention dimple. At least one of the end walls may include a fuse element receiving slot. The cover may be longitudinally spaced from the fuse element receiving slot when the cover is fitted to the base. 
         [0079]    One of the end caps may optionally be provided with solder to establish electrical connection between the one end cap and one of the ends of the fuse element. Alternatively, neither of the end caps is internally provided with solder to establish the electrical connection between the one end cap and one of the ends of the fuse element. One of the end caps may be provided with conductive ink to establish the electrical connection between the one end cap and one of the ends of the fuse element. 
         [0080]    At least one of the base and the cover may be fabricated from a ceramic material. The fuse element receiving cavity may be filled with an arc quenching media. The fuse element may be bonded to the fuse element receiving slot. The cover may include a generally planar element of uniform thickness. 
         [0081]    An embodiment of an electrical fuse is disclosed including a nonconductive housing comprising a base and a separately provided cover. The base includes opposing longitudinal side walls and opposing end walls interconnecting the longitudinal side walls, with the lateral side walls and the end walls defining an interior fuse element cavity therebetween. The cover is fitted to the base and substantially closing the fuse element cavity. A fuse element is received in the fuse element receiving slot and extends across the fuse element cavity between the end walls of the base. First and second terminal elements comprising conductive end caps fitted over the respective end walls of the base. The first and second end caps define a surface mount area for connection to a circuit board. One of the end caps includes an opening formed completely through a thickness of the end cap proximate the surface mount area, whereby when the end cap is soldered to a circuit board solder may flow through the opening from an exterior of the end cap to the interior of the end cap and establish a direct electrical connection to the fuse element. 
         [0082]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.