Abstract:
A method for fabricating an inductor which includes a core, a shield and a length of epoxy tape is provided which includes the steps of winding the wire into a coil onto the core, wrapping the epoxy tape around a perimeter of the core, installing the core including the coil and epoxy tape into the shield, and heating the inductor causing the epoxy tape to bond to the shield. An inductor incorporating the method is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to manufacture of electronic components, and more specifically to manufacturing of inductors.  
           [0002]    At least one type of Inductor includes a conductive wire wrapped around a core, sometimes referred to as a drum. The wrapped wire is commonly referred to as a coil, with each end of the coil being referred to as a lead for coupling the inductor to an electronic circuit. A shield is disposed around the coil, and consequently around the core, for isolation of the coil from electromagnetic fields which could induce undesirable voltages in the coil, as well as to mechanically protect the coil from unintentional contact and environmental conditions during manufacture, assembly, and installation of inductors to printed circuit boards and circuitry. As spacing between the coil and the shield can affect open circuit inductance and bias (an open circuit inductance with DC current) of an inductor, centering of the coil to maintain a consistent spacing between the coil, wound on the core, and the shield is important to the consistent manufacture of reliable, high quality inductors. Use of mechanical tooling to center the coil, and subsequently the core, within the shield is difficult and expensive to implement.  
           [0003]    Manufacturing processes for inductors, like other components, have been scrutinized as a way to reduce costs in the highly competitive electronics manufacturing business. Reduction of manufacturing costs are particularly desirable when the components being manufactured are low cost, high volume components. In a high volume component, any reduction in manufacturing costs is, of course, significant. Manufacturing costs as used herein, refers to material cost and labor costs. It is possible that one material used in manufacturing a component, may have a higher cost than another material, but the labor savings more than makes up for the increase in material costs. It is also possible that the opposite is true in other component manufacturing circumstances.  
           [0004]    Conventionally, to avoid mechanical tooling costs in inductor fabrication, an adhesive tape has been used as a spacer between the core and the shield. A liquid epoxy adhesive is then externally applied to the inductor to mechanically bond the core to the shield. Application of the external adhesive adds a manufacturing step and associated expense to the inductor fabrication process. Additionally, a smooth and polished surface of the spacing tape can undesirably compromise the bonding between the tape and the shield, and because it is difficult to externally apply adhesive to an entire surface area of the core within the shield, only a portion of the core surface area is bonded to the shield. Poor bonding of the core to the shield can undesirably affect performance of the inductors.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In an exemplary embodiment, a method for fabricating an inductor includes the step of wrapping an epoxy tape around a perimeter of an inductor core, positioning the wrapped core into a shield, and reflowing the epoxy tape to form a uniform bond between the core an the shield.  
           [0006]    More specifically, the epoxy tape includes a layer of structural adhesive film laminated to an adhesive layer. The structural adhesive film is affixed to the perimeter of the core, and the core is bonded to the shield by heating the adhesive layer of the epoxy tape to a transition temperature to melt the adhesive layer, and curing the adhesive layer to a solid state bonded to the shield.  
           [0007]    The epoxy tape ensures centering of the coil and core within the shield and further ensures a complete bonding between the core and the shield, thereby improving inductor performance and reliability while avoiding conventional manufacturing steps. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a top plan assembly view of an inductor.  
         [0009]    [0009]FIG. 2 is a top plan view of an epoxy tape for the inductor shown in FIG. 1.  
         [0010]    [0010]FIG. 3 is cross sectional view of the epoxy tape shown along line  3 - 3  in FIG. 2.  
         [0011]    [0011]FIG. 4 is a side view of a portion of the inductor shown in FIG. 1 at a first stage of manufacture.  
         [0012]    [0012]FIG. 5 is a top plan view of the portion of the inductor shown in FIG. 4.  
         [0013]    [0013]FIG. 6 is a top plan view of the inductor shown in FIG. 1 at a second stage of manufacture. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIG. 1 is a top plan view of an illustrative embodiment of an inductor  10  in which the benefits of the invention are demonstrated. It is recognized, however, that inductor  10  is but one type of electrical component in which the benefits of the invention may be appreciated. Thus, the description set forth below is for illustrative purposes only, and it is contemplated that benefits of the invention accrue to other sizes and types of inductors as well as other passive electronic components. Therefore, there is no intention to limit practice of the inventive concepts herein solely to the illustrative embodiment described, that is inductor  10 .  
         [0015]    Inductor  10  includes a core  12 , sometimes referred to as a drum, and a shield  14 . A coil of conductive wire (not shown) is wound onto core  12 , and the coil and core  12  are disposed within a protective shield  14 . The coil includes a number of turns of conductive wire in order to achieve a desired inductance value for a selected end application of inductor  10 . As those in the art will recognize, an inductance value of inductor  10 , in part, depends upon wire type, a number of turns of wire in the coil, and wire diameter. As such, inductance ratings of inductor  10  may be varied considerably for different applications  
         [0016]    Shield  14 , in one embodiment, is fabricated from a magnetic material to provide both a magnetic path and mechanical protection for the coil of inductor  10  both mechanically and electrically. Shield  14  includes a bore for receiving core  12  therein, and serves to provide a path for concentrating the magnetic field between ends of coil  10 , thus containing the magnetic field to strengthen the field around the coil and reduce the effect of the field on the ambient environment. In the embodiment illustrated in FIG. 1, shield  14  includes an eight sided polygonal outer perimeter, but in alternative embodiments it is recognized that greater or fewer perimeter sides, including one or more curved sides, could likewise be used in alternative embodiments without departing from the scope of the present invention.  
         [0017]    Core  12  in an illustrative embodiment is fabricated from a low loss powdered iron or other iron based ceramic material, although in other embodiments other known suitable materials may be employed. In a further embodiment, core  12  is spool shaped and includes a generally cylindrically, elongated inner circumference section (not shown) of a first diameter disposed between two generally flat disk-like outer circumference sections  16  (only one of which is shown in FIG. 1) of a larger diameter than the inner circumference section first diameter. Outer circumference sections extend from opposing ends of the inner circumference section, and as shown in the FIG. 1, outer circumference sections  16  each include a plurality of indentations or guides  18  which are configured for guiding and retaining leads (not shown) of a conductive wire coil wound about the inner circumference section of core  12  as the leads extend from the inner circumference section of core  12 .  
         [0018]    Centering of core  12  and the associated coil within shield  14  maintains a desired open circuit inductance and a selected inductor bias (open circuit inductance with DC current). Coil leads extend through guides  18  for attachment to a circuit (typically a circuit board), or, in an alternative embodiment, the leads are connected to insulated posts  20  located on and extending from opposing sides of the outer perimeter of shield  14  for surface mounting of inductor  10  on a printed circuit board (not shown) according to known techniques When core  12  is properly centered within shield  14 , a uniform gap or clearance  22  is maintained about the circumference of the coil and core  12 . In one embodiment, clearance  22  is approximately 0.004 inches to about 0.005 inches wide, although in alternative embodiments greater or lesser clearances may be employed.  
         [0019]    [0019]FIGS. 2 and 3 are a top plan view and cross sectional view, respectively, of one embodiment of an epoxy tape  40  for use in constructing inductor  10  in an exemplary embodiment of the present invention. Epoxy tape  40  includes a first layer for affixing to the core, and a second layer for forming a bond with shield  14 , More specifically, tape  40  includes a structural adhesive film  42  and a laminating adhesive  44 .  
         [0020]    In one exemplary embodiment, structural adhesive film  42  includes an epoxy base resin, such as an “AF42” bonding film available from Minnesota Mining and Manufacturing Company (3M™) of St. Paul, Minn., and laminating adhesive  44  is a solvent-free acrylic adhesive, such as “467MP” roll laminating adhesive, also available from Minnesota Mining and Manufacturing Company (3M™) of St. Paul, Minn. As such, structural adhesive film  42  has adequate heat resistance and structural bond properties for the operating environment of inductor  10 , and laminating adhesive  44  exhibits sufficient humidity resistance, U.V. resistance, water resistance, chemical resistance and shear strength to withstand manufacturing, assembly, and operating environments of inductor  10 .  
         [0021]    In alternative embodiments, other known materials having similar properties and characteristics may be employed to fabricate tape  40  fur use in inductor  10  as described below.  
         [0022]    In one exemplary embodiment for fabrication of an inductor, such as inductor  10 , tape  40  has a length L of approximately 12 millimeters and a width W of about 1.6 millimeters. Further, structural adhesive film  42  has a thickness T 1  of about 3 mils and laminating adhesive  44  has a thickness T 2  of about 2 mils. It is recognized that this is but one exemplary embodiment with exemplary dimensions, and that other dimensions both smaller and larger may be used in alternative embodiments within the scope of the present invention.  
         [0023]    A bottom surface  46  of structural adhesive film  42  is gummy or tacky and is affixed to the perimeter of core  12  after the conductive wire coil is wound therein, such that epoxy tape  40  substantially occupies clearance  22  (shown in FIG. 1) when core  12  (shown in FIG. 1) is inserted into shield  14 . Once located in clearance  22  after structural adhesive film  42  is bonded to the outer circumference of core  14 , epoxy tape  40 , and more specifically, laminating adhesive  44 , is bonded to an inner circumference of shield  14  using a heating and curing process. The heating and curing process is sometimes referred to as a reflow process via heating of laminating adhesive  44  to a transition temperature that causes the adhesive to melt and “flow” within clearance  22 , and then curing laminating adhesive back to a solid state. As such, laminating adhesive  44  uniformly forms a mechanical bond between core  12  and shield  14 , and more specifically between shield  14  and structural adhesive film  42 . It is believed that those in the art could accomplish this type of heating and curing process without further description or explanation.  
         [0024]    In one embodiment, both structural adhesive film  42  and laminating adhesive  44  are translucent so that a proper positioning of core  12  within shield  14  may be optically confirmed. In an alternative embodiment, epoxy tape  40  is fabricated from opaque materials. It is contemplated, however, that visual or optic assurance of proper positioning of shield  14  with respect to core  12  could be accomplished with opaque materials as well, including but not limited to selection of appropriate color combinations of tape  40 , shield  14  and core  12  to facilitate visual confirmation of spacing between core  12  and shield  14 .  
         [0025]    [0025]FIG. 4 is a side view of inductor core  12  at a first stage of manufacture wherein the conductive coil (not shown) is wrapped around the inner circumference of core  12  and epoxy tape  40  is wrapped around an outer circumference of core  12 . Tape bottom surface  46  (shown in FIG. 3) is affixed to outer circumference sections  16  (also shown in FIG. 1) of the outer perimeter of core  12 , or in other words, tape bottom surface  46  is adhered to core  12  such that laminating adhesive  44  is “face up” on the external surface of core  12  when tape  40  is attached to core. As shown in FIG. 4, laminating adhesive  44  of epoxy tape  40  is exposed when tape  40  has been affixed to outer circumference sections  16  of core  12 .  
         [0026]    [0026]FIG. 5 illustrates core  12  with tape  40  affixed thereto and circumscribing core  12  in a substantially uniform fashion. In an illustrative embodiment, tape  40  retains leads (not shown) of the conductive coil wound into core  12  and extending from the coil through guides  18 . In various embodiment, tape  40  is wrapped around the outer perimeter of the core one or more times to form a wrapping thickness T 3  sufficient to fill clearance  22  (shown in FIG. 1) when tape  40  is reflowed to bond core  12  to shield  14 .  
         [0027]    [0027]FIG. 6 illustrates inductor  10  at a second stage of manufacture after tape  40  is reflowed and cured to solid form to form a strong bond between core  12  and shield  14 . Unlike conventional manufacturing methods including application of external epoxy glue to bond core  12  to shield  14 , reflowed tape  40  provides optimal uniform spacing and bonding between core  12  and shield  14  about substantially an entire outer surface of wrapped core  12 . Coil leads (not shown) are extend through guides  18  for attachment to insulated posts  20  extending from shield  14  for electrical connection to a circuit or a circuit board according to known methods and techniques.  
         [0028]    Use of reflowing epoxy tape  40  removes conventional liquid adhesive dispensing process and associated costs, as well as eliminates potential quality issues from associated incomplete or inadequate bonds. Further, elimination of the dispensing process allows improvements in the consistency of the bond between core  12  and shield  14 , thereby allowing for reductions in physical size of inductor  10  while maintaining comparable power ratings in comparison to conventionally manufactured inductors.  
         [0029]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.