Patent Publication Number: US-10319551-B2

Title: Sealed fuse

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority to U.S. patent application Ser. No. 15/291,164, filed Oct. 12, 2016, entitled Sealed Fuse, and incorporated by reference herein in its entirety. 
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to the field of circuit protection devices, and relates more particularly to a sealed fuse adapted to prevent the ingress of solder during installation of the fuse on a circuit board. 
     FIELD OF THE DISCLOSURE 
     Fuses are commonly used as circuit protection devices and are typically installed between a source of electrical power and a component in a circuit that is to be protected. One type of fuse, commonly referred to as a “cartridge fuse” or “tube fuse,” includes a tubular, electrically insulating fuse body containing a fusible element that extends between electrically conductive, metallic endcaps that cover opposing longitudinal ends of the fuse body. Upon the occurrence of a specified fault condition, such as an overcurrent condition, the fusible element melts or otherwise separates to interrupt the flow of electrical current between the electrical power source and the protected component. 
     The endcaps of a fuse are commonly fastened to the ends of a fuse body using solder or electrically conductive adhesive, which also connects the fusible element of the fuse to the endcaps and provides an electrically conductive pathway therebetween. When the fuse is operatively installed, such as on a printed circuit board (PCB), the endcaps may be soldered to respective terminals on the PCB, placing the fuse in electrical communication with various other circuit components (e.g., a source of electrical power and a protected load). 
     A shortcoming associated with traditional cartridge fuses is that when such a fuse is soldered to a PCB, heat from the soldering process can cause the endcaps of the fuse, as well as solder that fastens the endcaps to the fuse body of the fuse (hereinafter “the endcap solder”), to undergo thermal expansion at a rate greater than that of the fuse body. This is due to a mismatch between the coefficient of thermal expansion of the insulative fuse body and the coefficients of thermal expansion of the conductive endcaps and endcap solder. Thus, the heated endcaps and endcap solder may expand away from the fuse body, resulting in the formation of gaps therebetween. Solder that is being applied to the endcaps during installation of the fuse on a PCB may, in its fluid state, migrate through these gaps and may infiltrate the interior of the fuse body. It has been observed that such infiltration can have deleterious effects on the performance of fuses. 
     It is with respect to these and other considerations that the present improvements may be useful. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter. 
     An exemplary embodiment of a sealed fuse in accordance with the present disclosure may include a tubular fuse body, a trench formed in an exterior of the fuse body, and an electrically conductive endcap that fits over an end of the fuse body and is fastened to the fuse body by an electrically conductive material having a lip portion that extends into the trench to provide a barrier that extends between the fuse body and the endcap. In an embodiment, the trench may be formed in an end face of the fuse body and may extend entirely around an opening in the end of the fuse body. In another embodiment, the trench may be formed in an outwardly-facing surface of a sidewall of the fuse body and may extend entirely around the fuse body. 
     An exemplary embodiment of a method for manufacturing a sealed fuse in accordance with the present disclosure, may include providing a tubular fuse body having a trench formed in an exterior of the fuse body, and fastening an electrically conductive endcap to an end of the fuse body by an electrically conductive material that forms a lip portion that extends into the trench to provide a barrier that extends between the fuse body and the endcap. In an embodiment, the trench may be formed in an end face of the fuse body and may extend entirely around an opening in the end of the fuse body. In another embodiment, the trench may be formed in an outwardly-facing surface of a sidewall of the fuse body and may extend entirely around the fuse body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    is a cross sectional view illustrating an exemplary sealed fuse in accordance with the present disclosure; 
         FIG. 1 b    is an isometric view illustrating a fuse body of the sealed fuse shown in  FIG. 1   a;    
         FIG. 1 c    is a cross sectional view illustrating the sealed fuse shown in  FIG. 1 a    installed on a printed circuit board; 
         FIG. 2  is a flow diagram illustrating an exemplary method of manufacturing the sealed fuse shown in  FIGS. 1 a -1 c    in accordance with the present disclosure; 
         FIG. 3 a    is a cross sectional view illustrating an exemplary sealed fuse in accordance with the present disclosure; 
         FIG. 3 b    is an isometric view illustrating a fuse body of the sealed fuse shown in  FIG. 3   a;    
         FIG. 3 c    is a cross sectional view illustrating the sealed fuse shown in  FIG. 3 a    installed on a printed circuit board; 
         FIG. 4  is a flow diagram illustrating an exemplary method of manufacturing the sealed fuse shown in  FIGS. 3 a -3 c    in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a sealed fuse and a method for manufacturing the same in accordance with the present disclosure will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the present disclosure are presented. The sealed fuse and the accompanying method of the present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the sealed fuse and the accompanying method to those skilled in the art. In the drawings, like numbers refer to like elements throughout unless otherwise noted. 
     Referring to  FIG. 1 a   , a cross-sectional view of a sealed fuse  100  (hereinafter “the fuse  100 ”) in accordance with an exemplary embodiment of the present disclosure is shown. The fuse  100  may include a tubular fuse body  112  having opposing open ends  114 ,  116 . The fuse body  112  may be a square cylinder (as shown in  FIG. 1 b   ), but this is not critical. Alternative embodiments of the fuse  100  may have a fuse body that is a round cylinder, an oval cylinder, a triangular cylinder, etc. 
     A pair of conductive endcaps  118 ,  120  may fit over the open ends  114 ,  116  of the fuse body  112 , respectively, and may be fastened thereto by solder fillets  130 ,  132 . Alternatively, and as will become apparent below, any type of electrically conductive adhesive that may be applied in a fluid or semi-fluid state and subsequently cured or hardened may be substituted for the solder fillets  130 ,  132 . A fusible element  124  (e.g., a fuse wire) may extend through the hollow interior  125  of the fuse body  112  and may be secured to the endcaps  118 ,  120  in electrical communication therewith by the solder fillets  130 ,  132 . Alternatively, one or both ends of the fusible element  124  may extend through respective holes in the endcaps  118 ,  120  and may be soldered to exterior faces of the endcaps  118 ,  120 . 
     The fuse body  112  of the fuse  100  may be formed of an electrically insulating and preferably heat resistant material, including, but not limited to, ceramic or glass. The endcaps  118 ,  120  may be formed of an electrically conductive material, including, but not limited to, copper or one of its alloys, and may be plated with nickel or other conductive, corrosion resistant coatings. The fusible element  124  may be formed of an electrically conductive material, including, but not limited to, tin or copper, and may be configured to melt and separate upon the occurrence of a predetermined fault condition, such as an overcurrent condition in which an amount of current exceeding a predefined maximum current flows through the fusible element  124 . The fusible element  124  may be any type of fusible element suitable for a desired application, including, but not limited to, a fuse wire, a corrugated strip, a fuse wire wound about an insulating core, etc. In some embodiments, the fusible element  124  may extend diagonally through the hollow interior  25  of the fuse body  112 . In some embodiments, the hollow interior  125  of the fuse body  112  may be partially or entirely filled with an arc-quenching material, including, but not limited to, sand, silica, etc. 
     Referring to  FIGS. 1 a  and 1 b   , the fuse body  112  may include channels or trenches  134 ,  136  formed in longitudinal end faces  138 ,  140  thereof, respectively. The trenches  134 ,  136  may be continuous (i.e., without termini) and may entirely surround openings  142 ,  144  in the respective open ends  114 ,  116  of the fuse body  112 . In some exemplary, non-limiting embodiments, the trenches  134 ,  136  may have widths in a range of 0.15 millimeters-0.20 millimeters and may have depths in a range of 0.10 millimeters-0.15 millimeters. The trenches  134 ,  136  may have a semi-circular or rounded cross-sectional shape as shown in  FIG. 1 a   , but this is not critical. The cross-sectional shape of one or both of the trenches  134 ,  136  may alternatively be rectangular, V-shaped, etc. 
     As shown in  FIG. 1 a   , the solder fillets  130 ,  132  may include respective lip portions  146 ,  148  that extend into, and substantially fill, the trenches  134 ,  136 , respectively. The lip portions  146 ,  148  may be formed during assembly of the fuse  100  when the solder fillets  130 ,  132  are in a fluid or semi-fluid state (e.g., before cooling/curing) and are compressed between the endcaps  118 ,  120  and the end faces  138 ,  140  of the fuse body  112 , whereby the fluid or semi-fluid solder may flow into, and may conform to the shapes of, the trenches  134 ,  136 . 
     Referring now to  FIG. 1 c   , a cross-sectional view of the fuse  100  soldered to a printed circuit board (PCB)  150  by quantities of solder  152  (hereinafter “the board solder  152 ”) is shown. Heat from the application of the board solder  152  may cause the endcaps  118 ,  120  and the solder fillets  130 ,  132  to undergo thermal expansion at a rate greater than that of the fuse body  112 . This occurs due to a mismatch between the coefficient of thermal expansion of the insulative fuse body  112  and the coefficients of thermal expansion of the conductive endcaps  118 ,  120  and solder fillets  130 ,  132 . Thus, the heated endcaps  118 ,  120  and the solder fillets  130 ,  132  may expand away from the fuse body  112 , resulting in the formation of gaps  154 ,  156  therebetween. 
     During application of the board solder  152  (i.e., while the board solder is in an uncured, fluid state), the board solder  152  may migrate through the gaps  154 ,  156  toward the end faces  138 ,  140  of the fuse body  112 . Advantageously, the lip portions  146 ,  148  of the solder fillets  130 ,  132 , which may also expand relative to the fuse body  112  as a result of heating from application of the board solder  152 , remain disposed within the respective trenches  134 ,  136  in the fuse body  112  and provide barriers that firmly seal the gaps  154 ,  156  between the heated endcaps  118 ,  120  and the fuse body  112 . Since these barriers entirely surround the openings  142 ,  144  of the fuse body  112 , they (the barriers) may effectively prevent the ingress of the fluid or semi-fluid board solder  152  into the openings  142 ,  144  and hollow interior  125  of the fuse body  112  during installation of the fuse  100  on the PCB  150 . Thus, degradation in the performance of the fuse  100  that might otherwise result from the migration of the board solder  152  into the fuse body  112  is mitigated or entirely prevented. 
     Referring to  FIG. 2 , a flow diagram illustrating an exemplary method for manufacturing the above-described fuse  100  in accordance with the present disclosure is shown. The method will now be described in conjunction with the illustrations of the fuse  100  shown in  FIGS. 1 a   - 1   c.    
     At step  200  of the exemplary method, the tubular fuse body  112  having a hollow interior  125  and open ends  114 ,  116  may be provided. The fuse body  112  may have channels or trenches  134 ,  136  formed in longitudinal end faces  138 ,  140  thereof, respectively. The trenches  134 ,  136  may be continuous (i.e., without termini) and may entirely surround the openings  142 ,  144  in the respective open ends  114 ,  116  of the fuse body  112 . 
     At step  210  of the exemplary method, the conductive endcap  118  may be fastened to the open end  114  of the fuse body  112  with the solder fillet  130  or, alternatively, by an electrically conductive adhesive that may be applied in a fluid or semi-fluid state. When the endcap  118  is pressed onto the open end  114  of the fuse body  112 , the solder fillet  130 , which may be in a fluid or semi-fluid state prior to curing or hardening, may be compressed between the endcap  118  and the end face  138  of the fuse body  112 , whereby the fluid or semi-fluid solder may flow into, and may conform to the shape of, the trench  134 , thereby forming a lip portion  146  that substantially fills the trench  134 . 
     At step  220  of the exemplary method, the fusible element  124  may be inserted into the hollow interior  125  of the fuse body  112  and may be secured to the solder fillet  130  while the solder fillet  130  is still in a fluid or semi-fluid state, thereby placing the fusible element  124  in electrical communication with the endcap  118 . 
     At step  230  of the exemplary method, the conductive endcap  120  may be fastened to the open end  116  of the fuse body  112  with the solder fillet  132  or, alternatively, by an electrically conductive adhesive that may be applied in a fluid or semi-fluid state. When the endcap  120  is pressed onto the open end  116  of the fuse body  112 , the solder fillet  132 , which may be in a fluid or semi-fluid state prior to curing or hardening, may be compressed between the endcap  120  and the end face  140  of the fuse body  112 , whereby the fluid or semi-fluid solder may flow into, and may conform to the shape of, the trench  136 , thereby forming a lip portion  148  that substantially fills the trench  136 . The solder fillet  132  may also engage and form a connection with the free end of the fusible element  124 , thereby placing the fusible element  124  in electrical communication with the endcap  120 . 
     Referring to  FIG. 3 a   , a cross-sectional view of a sealed fuse  300  (hereinafter “the fuse  300 ”) in accordance with another exemplary embodiment of the present disclosure is shown. The fuse  300  may be similar to the fuse  100  described above and may include a tubular fuse body  312  having opposing open ends  314 ,  316 . The fuse body  312  may be a square cylinder (as shown in  FIG. 2 b   ), but this is not critical. Alternative embodiments of the fuse  300  may have a fuse body that is a round cylinder, an oval cylinder, a triangular cylinder, etc. 
     A pair of conductive endcaps  318 ,  320  may fit over the open ends  314 ,  316  of the fuse body  312 , respectively, and may be fastened thereto by solder fillets  330 ,  332 . Alternatively, and as will become apparent below, any type of electrically conductive adhesive that may be applied in a fluid or semi-fluid state and subsequently cured or hardened may be substituted for the solder fillets  330 ,  332 . A fusible element  324  (e.g., a fuse wire) may extend through the hollow interior  325  of the fuse body  312  and may be secured to the endcaps  318 ,  320  in electrical communication therewith by the solder fillets  330 ,  332 . Alternatively, one or both ends of the fusible element  324  may extend through respective holes in the endcaps  318 ,  320  and may be soldered to exterior faces of the endcaps  318 ,  320 . 
     The fuse body  312  of the fuse  300  may be formed of an electrically insulating and preferably heat resistant material, including, but not limited to, ceramic or glass. The endcaps  318 ,  320  may be formed of an electrically conductive material, including, but not limited to, copper or one of its alloys, and may be plated with nickel or other conductive, corrosion resistant coatings. The fusible element  324  may be formed of an electrically conductive material, including, but not limited to, tin or copper, and may be configured to melt and separate upon the occurrence of a predetermined fault condition, such as an overcurrent condition in which an amount of current exceeding a predefined maximum current flows through the fusible element  324 . The fusible element  324  may be any type of fusible element suitable for a desired application, including, but not limited to, a fuse wire, a corrugated strip, a fuse wire wound about an insulating core, etc. In some embodiments, the fusible element  324  may extend diagonally through the hollow interior  325  of the fuse body  312 . In some embodiments the hollow interior  325  of the fuse body  312  may be partially or entirely filled with an arc-quenching material, including, but not limited to, sand, silica, etc. 
     Referring to  FIGS. 3 a  and 3 b   , the fuse body  312  may include channels or trenches  334 ,  336  formed in the outwardly-facing surface  337  of the sidewall  339  thereof (i.e., wherein the outwardly-facing surface  337  is parallel to a longitudinal axis of the fuse body  312 ) adjacent the opposing longitudinal ends of the fuse body  312 , respectively, spaced longitudinally inward from the end faces  338 ,  340  but covered by the endcaps  318 ,  320 . The trenches  334 ,  336  may be continuous (i.e., without termini), and may extend entirely around the fuse body  312 . In some exemplary, non-limiting embodiments, the trenches  334 ,  336  may have widths in a range of 0.15 millimeters-0.20 millimeters and may have depths in a range of 0.10 millimeters-0.15 millimeters. The trenches  334 ,  336  may have a semi-circular or rounded cross-sectional shape as shown in  FIG. 3 a   , but this is not critical. The cross-sectional shape of one or both of the trenches  334 ,  336  may alternatively be rectangular, V-shaped, etc. 
     As shown in  FIG. 3 a   , the solder fillets  330 ,  332  may include respective lip portions  346 ,  348  that extend into, and substantially fill, the trenches  334 ,  336 , respectively. The lip portions  346 ,  348  may be formed during assembly of the fuse  300  when the solder fillets  330 ,  332  are in a fluid or semi-fluid state (e.g., before cooling/curing) and are compressed between the endcaps  318 ,  320  and the outwardly-facing surface  337  of the sidewall  339 , whereby the fluid or semi-fluid solder may flow into, and may conform to the shapes of, the trenches  334 ,  336 . 
     Referring now to  FIG. 3 c   , a cross-sectional view of the fuse  300  soldered to a printed circuit board (PCB)  350  by quantities of solder  352  (hereinafter “the board solder  352 ”) is shown. Heat from the application of the board solder  352  may cause the endcaps  318 ,  320  and the solder fillets  330 ,  332  to undergo thermal expansion at a rate greater than that of the fuse body  312 . This occurs due to a mismatch between the coefficient of thermal expansion of the insulative fuse body  312  and the coefficients of thermal expansion of the conductive endcaps  318 ,  320  and solder fillets  330 ,  332 . Thus, the heated endcaps  318 ,  320  and the solder fillets  330 ,  332  may expand away from the fuse body  312 , resulting in the formation of gaps  354 ,  356  therebetween. 
     During application of the board solder  352  (i.e., while the board solder is in an uncured, fluid state), the board solder  352  may migrate through the gaps  354 ,  356  toward the end faces  338 ,  340  of the fuse body  312 . Advantageously, the lip portions  346 ,  348  of the solder fillets  330 ,  332 , which may also expand relative to the fuse body  312  as a result of heating from application of the board solder  352 , remain disposed within the respective trenches  334 ,  336  in the fuse body  312  and provide barriers that firmly seal the gaps  354 ,  356  between the heated endcaps  318 ,  320  and the fuse body  312 . Since these barriers extend entirely around the fuse body  312 , they (the barriers) may effectively prevent the ingress of the fluid or semi-fluid board solder  352  into the open ends  314 ,  316  and hollow interior  325  of the fuse body  312  during installation of the fuse  300  on the PCB  350 . Thus, degradation in the performance of the fuse  300  that might otherwise result from the migration of the board solder  352  into the fuse body  312  is mitigated or entirely prevented. 
     Referring to  FIG. 4 , a flow diagram illustrating an exemplary method for manufacturing the above-described fuse  300  in accordance with the present disclosure is shown. The method will now be described in conjunction with the illustrations of the fuse  300  shown in  FIGS. 3 a   - 3   c.    
     At step  400  of the exemplary method, the tubular fuse body  312  having a hollow interior  325  and open ends  314 ,  316  may be provided. The fuse body  312  may have channels or trenches  334 ,  336  formed in the outwardly-facing surface  337  of the sidewall  339  thereof adjacent the opposing longitudinal ends of the fuse body  312 , respectively, spaced longitudinally inward from the end faces  338 ,  340  of the fuse body  312 . The trenches  334 ,  336  may be continuous (i.e., without termini) and may entirely surround the fuse body  312 . 
     At step  410  of the exemplary method, the conductive endcap  318  may be fastened to the open end  314  of the fuse body  312  with the solder fillet  330  or, alternatively, by an electrically conductive adhesive that may be applied in a fluid or semi-fluid state. When the endcap  318  is pressed onto the open end  314  of the fuse body  312 , the solder fillet  330 , which may be in a fluid or semi-fluid state prior to curing or hardening, may be compressed between the endcap  318  and the outwardly-facing surface  337  of the sidewall  339 , whereby the fluid or semi-fluid solder may flow into, and may conform to the shape of, the trench  334 , thereby forming a lip portion  346  that substantially fills the trench  334 . 
     At step  420  of the exemplary method, the fusible element  324  may be inserted into the hollow interior  325  of the fuse body  312  and may be secured to the solder fillet  330  while the solder fillet  330  is still in a fluid or semi-fluid state, thereby placing the fusible element  324  in electrical communication with the endcap  318 . 
     At step  430  of the exemplary method, the conductive endcap  320  may be fastened to the open end  316  of the fuse body  312  with the solder fillet  332  or, alternatively, by an electrically conductive adhesive that may be applied in a fluid or semi-fluid state. When the endcap  320  is pressed onto the open end  316  of the fuse body  312 , the solder fillet  332 , which may be in a fluid or semi-fluid state prior to curing or hardening, may be compressed between the endcap  320  and the outwardly-facing surface  337  of the sidewall  339 , whereby the fluid or semi-fluid solder may flow into, and may conform to the shape of, the trench  136 , thereby forming a lip portion  348  that substantially fills the trench  336 . The solder fillet  332  may also engage and form a connection with the free end of the fusible element  324 , thereby placing the fusible element  324  in electrical communication with the endcap  320 . 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     While the present disclosure makes reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present disclosure, as defined in the appended claim(s). Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.