Abstract:
A wound capacitor has a lead configuration that enables enhanced control over equivalent series resistance (ESR) and equivalent series inductance (ESL). The capacitor has a case and a wound foil disposed within the case. A ball-and-lead configuration is coupled to the foil and extends from the case.

Description:
BACKGROUND OF THE INVENTION  
       [0001]      1).  Field of the Invention  
         [0002]     The present invention generally relates to capacitors. More particularly, the invention relates to a capacitor having a reduced equivalent series resistance (ESR) and equivalent series inductance (ESL).  
         [0003]     2). Discussion of Related Art  
         [0004]     In the highly competitive personal computer (PC) industry, the difficult trade-off between cost and performance has been well documented. For example, in an effort to provide more features to the consumer at a lower cost, significant focus has been placed on streamlining motherboard systems and components. Most motherboards employ one or more voltage regulation circuits to ensure that the proper reference voltages are supplied to the motherboard processors, and it has been determined that such a circuit has a direct effect on the switching performance of the processor. The voltage regulation circuit typically has a voltage regulation module and a capacitor arrangement (among other components). The capacitor arrangement is critical because it provides decoupling between the voltage regulation module (VRM) and the processor. Conventional approaches to the capacitor arrangement involve the use of multiple high performance bulk-type capacitors, which can have very low temperature stability, wide frequency range, and long life. Low ESR reduces the unwanted parasitic impedance and heating effects that degrade capacitors, and is highly desirable in the computing industry. The cost of these performance capacitors, however, is relatively high with respect to other capacitors. It is therefore desirable to provide an alternative to conventional. performance capacitors in voltage regulation circuits.  
         [0005]     The alternative approach to the above design would be to use a combination of ceramic and conventional electrolytic capacitors, which would reduce the costs of the capacitor arrangement considerably. The trade-off to reduced cost, however, is reduced performance under conventional approaches.  
         [0006]     Thus, the conventional capacitor arrangement provides a significant cost-saving opportunity, but certain difficulties remain. For example, the typical wound aluminum electrolytic capacitor is unable to meet the exacting ESR and ESL requirements of modem day high-speed processors.  FIGS. 1-3  illustrate that the conventional wound capacitor  10  has a case  12 , a winding  14  disposed within the case  12  and a dual lead configuration  16 . The dual lead configuration  16  is coupled to the winding  14  and extends from the case  12  for connection to an external printed wiring board (PWB, not shown). It can be seen that the dual lead configuration  16  has a cathode terminal assembly  18  and anode terminal assembly  20 . Each terminal assembly  18  and  20  includes a termination  22  ( 22   a,    22   b ), a lead  24  ( 24   a,    24   b ), and a weld  26  ( 26   a,    26   b ) coupling the termination  22  to the lead  24 .  
         [0007]     It has been determined that the design of the dual lead configuration  16  can be quite challenging with regard to ESR and ESL. For example, limiting the lead configuration to two terminal assemblies has been found to contribute to a relatively high ESR in a capacitor of this type. The distance between terminal assemblies  18  and  20  has a significant impact on ESR and ESL.  
         [0008]     It is also important to note that the weld  26  contributes to loop ESL and that the relatively long two-piece termination/lead design adds to both loop ESL and ESR. It is therefore desirable to provide a lead configuration that enables ESR to be reduced without sacrificing with respect to ESL.  
         [0009]     It will also be appreciated that certain manufacturing and placement difficulties can also result from the conventional capacitor  10 . For example, the typical solder connection that occurs at the leads  24  requires wave soldering, which prohibits the use of the capacitor  10  in dual-sided reflow PWBs. Furthermore, the traditional case  12  can be relatively tall, which limits the use of the capacitor  10  to applications having a great deal of clearance space. Tall body and long leads contribute to higher ESR/ESL as well. Redesign of the height could bring the ESR/ESL down.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The invention is described by way of example with reference to the accompanying drawings, wherein:  
         [0011]      FIG. 1  is a perspective view of a conventional wound capacitor, useful in understanding the invention;  
         [0012]      FIG. 2  is a perspective view of a first surface of a conventional foil before winding;  
         [0013]      FIG. 3  is a top plan view of a conventional capacitor;  
         [0014]      FIG. 4  is a perspective view of an electronic assembly including a printed wiring board and a capacitor in accordance with one embodiment of the present invention;  
         [0015]      FIG. 5  is a side view of the assembly shown in  FIG. 4 ;  
         [0016]      FIG. 6  is a bottom view of an example of a capacitor in accordance with one embodiment of the present invention;  
         [0017]      FIG. 7  is a perspective view of an example of a foil and lead configuration before winding in accordance with one embodiment of the present invention;  
         [0018]      FIG. 8  is a flowchart of an example of a method of fabricating a capacitor in accordance with one embodiment of the present invention; and  
         [0019]      FIG. 9  is a flowchart of an example of a process of providing a case in accordance with one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     A capacitor and method of fabricating a capacitor in accordance with the principles of the present invention provide enhanced performance. The capacitor has a case and a wound foil disposed within the case. A ball-and-lead configuration is coupled to the foil and extends from the case. The use of ball-and-lead technology for the lead configuration provides substantial benefits with respect to ESR, ESL, manufacturability, and placement.  
         [0021]     In another aspect of the invention, a ball-and-lead configuration of a capacitor includes a cathode termination having a first end coupled to a first surface of a foil. An anode termination has a first end coupled to a second surface of the foil. The ball-and-lead configuration further includes two conductive solder balls, of the kind used in ball grid array technology, coupled to second ends of the terminations, where the solder balls enable electrical connection of the lead configuration to a printed wiring board (PWB).  
         [0022]     In another aspect of the invention, a method of fabricating a capacitor is provided. A case is provided, and a ball-and-lead configuration is coupled to a foil. The method further provides for disposing the foil within the case such that the ball-and-lead configuration extends from the case.  
         [0023]     It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.  
         [0024]      FIG. 4  shows a wound capacitor  30  having significantly improved ESL as well as enhanced manufacturability and placement flexibility. While the capacitor  30  will be primarily described with respect to a motherboard voltage regulation circuit, it is important to note that the invention is not so limited. In fact, the capacitor  30  can be beneficial to any application in which ESR, ESL, manufacturability, or placement are of concern. Notwithstanding, motherboard voltage regulation circuits have a number of aspects for which the capacitor  30  is uniquely suited.  
         [0025]     It can generally be seen that the capacitor  30  has a case  32  and a wound foil  34  disposed within the case  32 . A ball-and-lead configuration  36  is coupled to the foil  34  and extends from the case  32 . The ball-and-lead configuration  36  includes one elongate flat cathode termination  38  having a first end coupled to a first surface of the foil  34 . The ball-and-lead configuration  36  includes one elongate flat anode termination  40  having a first end coupled to a second surface of the foil  34 . In another embodiment, the ball-and-lead configuration  36  may have a total of three, four, or five terminations. The ball-and-lead configuration  36  may have an equal number of cathode and anode terminations or may have one more of one than the other.  
         [0026]     As will be discussed in greater detail below, the foil  34  can be an aluminum foil of the type that is commercially available and widely used in the industry. U.S. Pat. No. 6,110,233 to O&#39;Phelan, et al. describes one such foil, but other techniques may be used without parting from the nature and scope of the invention. It can further be seen that the ball-and-lead configuration  36  includes two solder balls  42  coupled to second ends of the terminations  38 ,  40 , where the solder balls  42  enable electrical connection of the lead configuration to a printed wiring board  28  as best shown in  FIG. 5 .  
         [0027]     Turning now to  FIG. 7 , it can be seen that the foil  34  is made up of a first surface  33 , a second surface (not shown), and a dielectric layer  35  “sandwiched” between two foil surfaces. The surfaces of the foil  34  may be electrically conductive layers that are deposited on, grown on, or otherwise attached to the dielectric layer  35 . Although the terminations  38 ,  40  are shown as being attached to the sides of the electrically conductive layers that face outward with respect to the dielectric layer  34 , the inward facing sides may also be used. Furthermore, it should be noted that the terminations  38 ,  40  may be coupled to either side of the foil  34 , and that coupling may be accomplished by welding, soldering, swagging, electrically conductive adhesive, or any other suitable technique.  
         [0028]     With continuing reference to  FIGS. 1-7 , it will be appreciated that eliminating the conventional weld further reduces loop ESL, and the shorter terminations  38 , 40  reduce both loop ESL and ESR.  
         [0029]     Additionally, by using solder balls  42 , the physical height of the capacitor is reduced, which enables the component to fit into height-constrained locations. Furthermore, the solder balls  42  enable surface mount connection of the lead configuration  36  to the PWB  28 . As best shown in  FIGS. 5 and 6 , the case  32  includes a plurality of orientation pins  46  ( 46   a - 46   c ) extending from the case  32 , where the orientation pins  46  facilitate assembly of the capacitor  30  to the PWB  28 . Specifically, the orientation pins  46  prevent mis-orientation of the capacitor  30  during board assembly and provide improved stability as the board is soldered. In this regard, it has been determined that conventional wound capacitors can be inserted backwards, causing dramatic failures, and may require a hold-down feature to prevent wobbling as the board goes through wave soldering.  
         [0030]     Turning now to  FIG. 8 , it will be appreciated that the present invention further provides a method  50  of fabricating a capacitor. Generally, a case is provided at processing block  52  and a ball-and-lead configuration is coupled to a foil at block  54 . The foil is then disposed within the case at block  56  (after being wound) such that the lead configuration extends from the case. A first end of the cathode termination is coupled to a first surface of the foil at block  60 . Block  62  provides for coupling of the first end of the anode termination to a second surface of the foil. A film with an etched surface and oxide layer is provided for at block  63  in order to achieve an appropriate dielectric layer. It should be noted that the resulting foil can be achieved through standard etching, oxidation, and alignment processes. It can further be seen that a plurality of solder balls are coupled to second ends of the terminations at block  64 , where the solder balls enable electrical connection of the lead configuration to the PWB.  
         [0031]     It will be appreciated that the order in which the processing blocks are performed may vary without departing from the nature and scope of the invention. For example, the solder balls may be coupled to the terminations before the terminations are coupled to the foil. It should also be noted that the solder balls enable surface mount connection of the lead configuration to the PWB.  
         [0032]     Turning now to  FIG. 9 , one approach to providing a case for the capacitor is shown. Specifically, a can is provided at step  66 , which enables the overall height of the capacitor to be significantly reduced. Block  68  provides for extending a plurality of orientation pins from the case, where the orientation pins facilitate assembly of the capacitor to a PWB. The orientation pins may be of any shape and number, and can be coupled to the can by any acceptable technique. For example, in the illustrated embodiment ( FIG. 6 ), three orientation pins  46   a,    46   b,    46   c,  have a rectangular shape and are welded to an outer surface of the case  32 .  FIG. 5  shows that corresponding apertures are provided in the PWB  28  to facilitate alignment of the solder balls  42  with the corresponding set of pads on the upper surface of the PWB  28 .  
         [0033]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.  
         [0034]     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art.