Abstract:
The invention relates to a method of manufacturing a circuit protector and to a circuit protector. The method comprises the steps of providing a substrate having opposing end portions, coupling an element layer to the top surface of the substrate, and laser machining the element layer to shape the element layer into a predetermined geometry. The circuit protector comprises a substrate having opposing end portions, termination pads coupled to the top surface at opposing end portions of the substrate, a fuse element disposed across a space between the termination pads and electrically connecting the termination pads, the fuse element having a predetermined geometry; the predetermined geometry having the narrowest width of about 0.025 to about 0.050 millimeters, a cover coupling the top surface and suffusing the substrate, the fuse element and the termination pads, and end terminations in electrical contact with the termination pads at the opposing end portions.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to a circuit protector and, more particularly, to SMD and through-hole fuses and methods of manufacturing SMD and through-hole fuses. In particular, the present invention may be used in connection with all standard sizes of surface mountable devices and through-hole fuses including, but not limited to, 1206, 0805, 0603 and 0402 fuses, as well as with all non-standard fuse sizes. U.S. application Ser. No. 11/091,665, entitled, “Hybrid Chip Fuse Assembly Having Wire Leads And Fabrication Method”, which was published on Sep. 28, 2006 as U.S. Publication No. 20060214259, relates to through-hole fuses and is incorporated by reference herein. 
         [0002]    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. 
         [0003]    Ceramic chip type fuses are typically manufactured by depositing an element layer on a ceramic or glass substrate plate, screen printing the element layer, printing the element layer to a predetermined thickness and width to obtain a certain resistance, attaching an insulating cover over the element layer, and cutting, or dicing, individual fuses from the finished structure. The element layer loses definition when the screen printing operation is performed. The screen printing operation is not very accurate and the edge acuity of the resulting element layer is not very good. Photolithography etching may be used as an alternative to the screen printing operation, but this process is relatively expensive due to additional required processing steps and the longer lead times. 
         [0004]    There is a need for a method of manufacturing a subminiature circuit protector that is simple and relatively inexpensive. Additionally, there is also a need for a method of manufacturing a subminiature circuit protector, wherein the element layer may be designed to a certain geometry and also has a fine edge acuity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The foregoing and other features and aspects of the invention will be best understood with reference to the following description of certain exemplary embodiments of the invention, when read in conjunction with the accompanying drawings, wherein: 
           [0006]      FIG. 1  illustrates a perspective view of a circuit protector in accordance with certain exemplary embodiments of the present invention; 
           [0007]      FIG. 2  illustrates a side cross-sectional view of the circuit protector of  FIG. 1 , taken along line  2 - 2  in accordance with certain exemplary embodiments of the present invention; 
           [0008]      FIG. 3  is a flowchart depicting an exemplary method of manufacturing a circuit protector; 
           [0009]      FIGS. 4A-4J  illustrate a circuit protector during various stages of manufacture in accordance with certain exemplary embodiments of the present invention; 
           [0010]      FIG. 5  is a flowchart depicting another exemplary method of manufacturing a plurality of circuit protectors; 
           [0011]      FIG. 6  illustrates a top view of a plurality of spaced, substantially parallel columns of the element layer coupled to a substrate, from which a plurality of circuit protectors may be formed, in accordance with exemplary embodiments of the present invention. 
           [0012]      FIGS. 7A-7C  illustrate top views of exemplary circuit protectors having fuse elements of various geometries, in accordance with certain exemplary embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  illustrates a perspective view of a circuit protector  100  in accordance with an exemplary embodiment. It is understood that the figures are not to scale, and that the thickness of the various components has been exaggerated for the purpose of clarity. 
         [0014]    The circuit protector  100  comprises a substrate  110  of electrically insulating material, an element layer  120  of electrically conductive material coupled to the top surface  112  of the substrate  110 , a cover  130  coupled to at least a portion of the element layer  120 , and electrically conductive termination ends  140 ,  142  coupled to opposing end portions  116 ,  117  of the substrate  110 . The termination ends  140 ,  142  are electrically coupled to the element layer  120 , so as to form a circuit pathway through the circuit protector  100 . Additionally, a marking  150  may be coupled to the surface of the cover  130 . Marking  150  may include symbols or colors for identifying certain characteristics of the fuse. These characteristics may include, but is not limited to, the technology used to make the fuse, the footprint of the fuse, electrical characteristics of the fuse and ampere rating of the fuse. In an alternative embodiment, the cover  130  may be coupled to at least a portion of the element layer  120  and to at least a portion of the substrate  110 . 
         [0015]      FIG. 2  illustrates a side cross-sectional view of the circuit protector  100  of  FIG. 1  taken along line  2 - 2  in accordance with an exemplary embodiment. It may be seen that the circuit protector  100  further comprises electrical termination pads  160 ,  162  coupled to the element layer  120  (e.g., on the top surface thereof). Termination ends  140 ,  142  cover the opposing end portions  116 ,  117  of the substrate  110  and are electrically coupled to the termination pads  160 ,  162 . The termination ends  140 ,  142  thus form the external electrical terminals for connecting the circuit protector  100  in a circuit (not shown). 
         [0016]    In certain embodiments, the element layer  120  may comprise termination pads  160 ,  162  and a fuse element  122  disposed between and electrically connecting the termination pads  160 ,  162 . The termination pads  160 ,  162  and the fuse element  122  may be a monolithic structure that is formed from the element layer  120 . Additionally, the fuse element  122  and the termination pads  160 ,  162  may each have a predetermined thickness. For example, the thickness of the termination pads  160 ,  162  may be at least the thickness of the fuse element  122 . 
         [0017]    In other embodiments, termination pads  160 ,  162  may be formed separately from and electrically coupled to the element layer  120 . 
         [0018]    Having briefly described the structure of the circuit protector  100  in accordance with certain exemplary embodiments, an exemplary method for manufacturing a circuit protector in accordance with the present invention will now be described with respect to  FIG. 3  and  FIGS. 4A-4J .  FIG. 3  is a flowchart depicting an exemplary method  300  of manufacturing a circuit protector  100 .  FIGS. 4A-4J  illustrate a single exemplary circuit protector  100  during various stages of manufacture, such as in accordance with the exemplary method  300  described with reference to  FIG. 3 . 
         [0019]    The exemplary method  300  begins at step  301  and advances to step  310 , where a substrate  110  having opposing end portions  116 ,  117  is provided. In certain embodiments, the provided substrate  110  may be roughly the size of one circuit protector. The top view and the side view of the substrate  110 , which forms the basis for a single circuit protector  100  are illustrated in  FIG. 4A  and  FIG. 4B , respectively. The substrate  110  may be formed of any suitable electrically insulative material, including, but not limited to, ceramic, glass, polymer materials such as polyimide, FR4, alumina, steatite, forsterite, or a mixture thereof. In the illustrated embodiment, the substrate is formed in a substantially rectangular cross-sectional shape. However, in alternative embodiments, the substrate  110  may be formed in other sizes and shapes without departing from the scope and spirit of the invention. The substrate  110  has a top surface  112 , a bottom surface  114 , opposing end portions  116 ,  117 , and opposing lateral edges  118 ,  119 . In some embodiments, the top surface  112  of the substrate  110  is substantially planar. 
         [0020]    Next at step  320 , an element layer  120  is coupled to the top surface  112  of the substrate  110  by suitable means, as is known in the art. The top view and the side view of the substrate  110  and element layer  120  are illustrated in  FIG. 4C  and  FIG. 4D , respectively. The element layer  120  may be made of any suitable electrically conductive material, which may include, but is not limited to, silver, gold, palladium silver, copper, nickel or any alloys thereof. 
         [0021]    In certain embodiments, glass frit is typically included in the element layer  120  and is used as an adhesive to couple the element layer  120  to the substrate  110 . In such embodiments, the element layer  120  may be applied onto the top surface  112  of the substrate  110  in liquid form, which would result in the glass frit settling to the bottom of the element layer  120 . As described above, the termination pads  160 ,  162  may be formed as part of the element layer  120 . Alternatively, the termination pads  160 ,  162  may be formed separately from the element layer  120 . Other known methods for applying the element layer  120  to the substrate  110 , including, but not limited to, thick film methods, thin film methods, sputtering methods, and laminating film methods, may be employed at step  320  without departing from the scope and spirit of the present invention. 
         [0022]    The chosen thickness of the element layer  120  may vary greatly depending upon the desired characteristics (e.g., resistance) of the circuit protector  100 , which are typically dictated by application requirements. For example, when applying the element layer  120  as a thin film, the thickness may be about 0.2 microns. However, when applying the element layer  120  as a thick film, the thickness may be about 12 microns to about 15 microns. 
         [0023]    At step  330 , the element layer  120  is laser machined to a predetermined geometry. This predetermined geometry defines the time current characteristics of the resulting fuse element  122 . The top view and the side view of the substrate  110  and the element layer  120  laser machined to a predetermined geometry are illustrated in  FIG. 4E  and  FIG. 4F , respectively.  FIG. 4E  shows the geometry of the element layer  120  to be substantially serpentine. The termination pads  160 ,  162  may also be formed from the element layer  120  by way of laser machining. 
         [0024]    Laser machining allows the element layer  120  to be formed into various complex geometries while maintaining fine edge acuity and allowing for sharp right angles or curves along the sidewalls of the geometry. Thus, the sidewalls have a 90° cut when the element layer  120  is laser machined. Accordingly, laser machining allows for the fuse element  122  to be thicker in depth and narrower in width, when compared to SMD fuses of the prior art. The fuse element manufactured via laser machining may have a reduced number of pin holes, when compared to current manufacturing processes. Pin holes are approximately 0.05 mm-0.2 mm diameter holes which result from air bubbles in the ink during printing and firing processes. This reduced number of pin holes results in reducing the nuisance blows. Additionally, laser machining may enhance the circuit protector performance due to better localized heating of the fuse element  122 , which reduces the heat dissipation into the substrate  110 . 
         [0025]    By way of example (and not by way of limitation), laser machining technology can be used to produce a fuse element geometry in which the width of the narrowest portion of the fuse element  122  may be as small as about 0.025 mm, while still maintaining a fine edge acuity. Additionally, the narrowest vaporized width surrounding the narrowest portion of the fuse element  122  may be as small as about 0.019 mm and still maintain a fine edge acuity. Those skilled in the art will appreciate that laser machining can also be used to produce fuse element geometries having larger or smaller widths, which choice of which will typically depend upon application requirements for the circuit protector  100 , without departing from the scope and spirit of the present invention. 
         [0026]    In certain embodiments of the present invention, a YLP Series Laser, manufactured by IPG Photonics Corporation, is used to perform the laser machining. One suitable model in the YLP Series is the YLP-0.5/80/20 model. The wavelength, power, beam quality and spot size are some of the parameters that determine the laser machining dynamics. This model is a ytterbium fiber laser that utilizes a pulsed mode of operation and delivers 0.5 millijoules per pulse. The pulse width is about 80 nanoseconds. These lasers deliver a high power 1060 to 1070 nanometer wavelength laser beam, which is not within the visible spectrum, directly to the worksite via a flexible metal-sheathed fiber cable. The laser provides low heat so that the element layer  120  may be laser machined without damaging the substrate  110  during the laser machining process. Additionally, the laser beam is collimated and is typically focused to a spot size of a few microns or less. Furthermore, the output fiber delivery length is about  3 - 8  meters. The pulse repetition rate for this laser ranges from 20-100 kHz. Additionally, the nominal average output power of this laser is about 10 W, while the maximum power consumption is about 160 W. 
         [0027]    Fiber lasers have wide dynamic operating power range and the beam focus and its position remain constant, even when the laser power is changed, allowing for consistent processing results every time. A wide range of spot sizes may also be achieved by changing the optics configuration. These features enable the user to choose an appropriate power density for cutting various materials and wall thicknesses. 
         [0028]    The high mode quality and small spot size of the fiber laser with optimized pulses facilitate laser machining of intricate features and geometries in thin material. This pulsed mode-cutting results in minimal slag and HAZ, which are very critical to many micro-machining applications. High power density associated with small spot sizes of the fiber laser also translates into faster cutting with superior edge quality. 
         [0029]    These fiber lasers allow the undesired metallization of the element layer  120  to be vaporized and still maintain the fine geometry that is required for optimum performance of the fuse element  122 . When such a fiber laser is used on gold, the focal point is about 15 micrometers. However, when the laser is used on silver, the focal point is about 20-25 micrometers. Since gold is not as reflective as silver, it is easier to cut. Depending upon the properties of the element layer, the fiber laser may have a focal point that is about 10 micrometers. A smaller focal point may be achieved by limiting the light emitting area. In alternative embodiments, another type of fiber laser or another type of laser may be used without departing from the scope and spirit of the present invention, so long that the laser produces fine resolution on the element layer  120  without damaging substrate  110 . 
         [0030]    After the element layer  120  is laser machined in step  330 , a cover  130  is coupled to at least a portion of the element layer  120  in step  340 . The top view and the side view of the substrate  110 , element layer  120  and cover  130  are illustrated in  FIG. 4G  and  FIG. 4H , respectively. The cover  130  may be formed of glass or ceramic or other electrically insulating suitable material. The cover  130  suffuses at least a portion of the top surface  112  of the substrate  110 , the fuse element  122 , and at least a portion of the termination pads  160 ,  162 , and fills any voids around and between them. In an alternative embodiment, the cover  130  is coupled to at least a portion of the element layer  120  and to at least a portion of the substrate  110 . 
         [0031]    In certain embodiments, the cover  130  may be printed glass or a high temperature stable polymer material applied directly on the top surface  112  of the substrate  110  and the surfaces of the element layer  120  (including the fuse element  122  and the termination pads  160 ,  162 ). In one embodiment, the glass has no metals and may be applied as a thick film. The glass film is dried, then fired, and then cooled. Alternatively, the cover  130  may comprise a layer of ceramic material that is mechanically pressed over the top surface  112  of the substrate  110  to suffuse the underlying components (i.e., the fuse element  122  and the termination pads  160 ,  162 ), and the assembly is then fired to cure the cover  130 . In yet other embodiments, the cover  130  may comprise a plate of electrically insulating material that is bonded by a layer of bonding material to the top surface  112  over the assembled components. The bonding material may be applied to the top surface  112  to suffuse the top surface  112  and the assembled components as described above, and the cover  130  placed on the bonding material. The cover  130  may act as a passivation layer which has arc quenching characteristics. 
         [0032]    Next at step  350 , the circuit protector  100  is terminated. The top view and the side view of the terminated circuit protector  100  are illustrated in  FIG. 41  and  FIG. 4J , respectively. The termination ends  140 ,  142  may comprise electrically conductive material coated over the end portions of the circuit protector subassembly after the cover  130  has been coupled thereto. The termination ends  140 ,  142  may be coated on the circuit protector subassembly in any suitable manner known in the art. By way of example, but not by way of limitation, termination ends  140 ,  142  may be applied by dipping the end portions of the subassembly in a suitable coating bath followed by firing. The termination ends  140 ,  142  contact the termination pads  160 ,  162  at the end portions  116 ,  117  of the substrate  110 . The termination ends  140 ,  142  preferably extend along the lateral edges  118 ,  119  of the substrate  110  as far as allowed by industry standards, so that the lateral edges of the termination pads  160 ,  162  are at least partially enclosed in the termination ends  140 ,  142 . The termination ends  140 ,  142  also correspondingly extend over a portion of the cover  130  and the bottom surface  114  of the substrate  110 . In certain embodiments, the termination ends  140 ,  142  may be made from silver ink that is then plated with silver tin. Other conducting materials may be used for the termination ends  140 ,  142  without departing from the scope and spirit of the present invention. Following termination of the circuit protector  100 , the method  300  ends at step  360 . 
         [0033]    An alternative method for manufacturing a plurality of circuit protectors  100  is described with respect to  FIG. 5  and  FIG. 6 .  FIG. 5  is a flowchart depicting another exemplary method  500  of manufacturing a plurality of circuit protectors  100 .  FIG. 6  a top view of a plurality of spaced, substantially parallel columns of the element layer  120  coupled to a substrate  110 , from which a plurality of circuit protectors  100  can be formed, such as in accordance with the exemplary method  500 . 
         [0034]    The exemplary method  500  of  FIG. 5  begins at start step  501  and proceeds to step  510 , where a plurality of spaced, substantially parallel columns of an element layer  120  are coupled to the top surface  112  of a substrate  110 .  FIG. 7  illustrates the plurality of spaced, substantially parallel columns of the element layer  120  coupled to the top surface  112  of the substrate  110 . The illustrated substrate  110  has a substantially rectangular cross-section. By way of example, the substrate  110  may be about 2½″ to about 3″ square, which may be suitable for forming a plurality of circuit protectors  100 . Depending on the dimensions of the circuit protectors  100 , a single substrate of about 2½″ to about 3″ square may accommodate approximately  798  circuit protectors. Other sizes and shapes of substrates  110  may alternatively be utilized without departing from the scope and spirit of the present invention. 
         [0035]    Exemplary methods for application of the element layer  120  to the substrate  110  have been described above. In certain embodiments, the element layer  120  may be coupled to the top surface  112  of the substrate  110  by forming metallization lines  170  spaced apart on the substrate  110  by areas  172 . After the element layer  120  is applied, the element layer  120  is laser machined to shape it into a predetermined geometry at step  520 . As described previously, laser machining allows the element layer  120  to be formed into various complex geometries while maintaining edge acuity. The sidewalls of the complex geometry may have a 90° cut. 
         [0036]    Next at step  530 , the cover  130  is coupled to the top surface  112  of the substrate  110 , wherein the cover  130  covers at least a portion of the element layer  120 . That is, the cover  130  suffuses at least a portion of the top surface  112  of the substrate  110 , the fuse element  122 , and at least a portion of the termination pads  160 ,  162  of each circuit protector  100 , and fills any voids around and between them. In an alternative embodiment, the cover  130  suffuses at least a portion of the fuse element  122 . Exemplary methods for application of the cover  130  have been described above. 
         [0037]    At step  540 , the substrate  110  is singularized to form a plurality individual circuit protectors  100 , wherein each circuit protector  100  comprises a substrate  110  with opposing end portions  116 ,  117 . For example the plurality of circuit protectors  100  may be singularized from the substrate  110  by dicing horizontally across the substrate  110  along the areas  172  and vertically across the metallization lines  170 . According to certain embodiments, such dicing may be performed via a diamond dicing saw. In alternative embodiments, other known methods may be used for singularizing the plurality of circuit protectors  100  from the substrate  110  without departing from the scope and spirit of the present invention. 
         [0038]    After the plurality of circuit protectors  100  are singularized from the substrate  110 , the opposing end portions  116 ,  117  of each circuit protector  100  are terminated at step  550 . Exemplary methods for terminating the circuit protectors  100  have been described above. After termination of the circuit protectors  100 , the exemplary method  500  ends at step  560 . 
         [0039]      FIGS. 7A-7C  illustrate top views of exemplary circuit protectors  100  having fuse elements  122  of various geometries, in accordance with certain exemplary embodiments of the invention. As shown in  FIG. 7A , the element layer  120  of the exemplary circuit protector  100  has been laser machined to form a fuse element  122  having a narrow straight line geometry extending from a first termination pad  160  to the second termination pad  162 . As shown in  FIG. 7B , the element layer  120  of the exemplary circuit protector  100  has been laser machined to form a fuse element  122  having a narrow serpentine geometry extending from a first termination pad  160  to the second termination pad  162 . As shown in  FIG. 6C , the element layer  120  of the exemplary circuit protector  100  has been laser machined to form a fuse element  122  having a relatively narrow straight line geometry extending from a first termination pad  160  to the second termination pad  162 , wherein the relatively narrow straight line geometry further comprises larger rectangular sections therein. Thus, it may be seen that laser machining allows a fuse element  122  to be formed into various complex geometries while maintaining the fine edge acuity. 
         [0040]    Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. It is, therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the scope of the invention.