Patent Abstract:
The surface mount MELF capacitor of the present invention includes a wire and a conductive powder element electrically connected to the wire. The surface mount MELF capacitor has insulative material surrounding at least a portion of the conductive powder element and the wire extending from the conductive powder element. A first terminal is formed on the surface mount chip capacitor at the first end surface of the wire and a second terminal is formed by being electrically connected to the conductive powder element. The surface mount MELF capacitor of the present invention is created by methods which include the steps of providing a wire and placing conductive powder upon the wire. An embodiment of the present invention feeds the wire in a reel to reel system and electrophoretically deposits the conductive powder element upon the wire.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to capacitors. More particularly, though not exclusively, the present invention relates to improved surface mount MELF capacitors and methods for manufacturing the same.  
       BRIEF SUMMARY OF THE INVENTION  
       [0002]     Capacitors exist in the art which are made from a capacitive element such as a tantalum slug or pellet. To create a conventional tantalum slug, tantalum powder is pressed with a binder and then exposed to a process for forming a polarized capacitor having a positive end and a negative end. A typical tantalum slug will have an anode comprised of a wire extending from the slug and a cathode comprised of a conductive surface formed at the opposite side of the tantalum slug.  
         [0003]     The usual method for making tantalum pellets for use in tantalum capacitors includes steps wherein tantalum powder is first pressed or compacted into a pellet. The resulting pressed pellets then undergo a sintering process wherein the pellets are heated in a vacuum. The heating allows the tantalum particles to stick together so they can hold a lead wire, which functions as the anode.  
         [0004]     Following the sintering process, the tantalum pellet is dipped in an acid solution to form a dielectric film on the outer surface of the pellet and the particles within the pellet which is typically tantalum pentoxide. The pellet and the particles within the pellet are then subsequently coated with various other metal-containing materials which form the cathode.  
         [0005]     These capacitors have the anode and the cathode attached to a circuit board by connection wires.  
         [0006]     Modern methods of mounting components use the possibility of soldering the components directly to conductor tracks of printed circuit boards without the use of connection wires. This technology is used to an ever increasing extent under the indication “Surface Mounted Device” (SMD).  
         [0007]     Capacitors suitable for the SMD technique may be manufactured as a chip component and as a MELF component. Chip components generally have supporting members in the form of rectangular parallelepipeds which have end faces suitable for soldering or in the form of flipchips which have a face with both cathode and anode terminals suitable for soldering. MELF (Metal Electrode Face Bonding) components typically start from cylindrical supporting members having connection caps in which the connection wires are omitted and the caps themselves are made suitable for soldering at their surfaces by an electroplating treatment and are soldered directly with said connection caps to conductor tracks of printed circuit boards.  
         [0008]     The great advantage of the SMD technology is that extremely high packing densities of components on the printed circuit boards are possible. For realizing ever increasing densities, smaller and smaller components suitable for the SMD technique become necessary.  
         [0009]     However, SMD technology encounters problems with producing devices with productivity and uniformity. It can therefore be seen that there is a need for an improved surface mount MELF capacitor and method for making the same.  
         [0010]     In addition, current SMD technology may require the manipulation of individual capacitors as opposed to using techniques for mass manipulation of capacitors. One particularly useful technique of mass manipulation is through the use of a reel to reel process. Therefore, a further feature of the present invention is the provision of a capacitor that is efficiently manufactured using a reel to reel process.  
         [0011]     Also, current SMD technology may be improved by the use of electrophoretic deposition. Some of the advantages of electrophoretic deposition include a high coating rate of charged particles upon the substrate, a resulting film of charged particles upon the substrate that is dense and uniform, a thickness of film that is able to be controlled by depositing condition, and a simple process that is easy to scale up. Accordingly, a still further feature of the present invention is the provision of a method that uses electrophoretic deposition to increase the capacitor uniformity, tolerance, capacitance and the density per volume.  
         [0012]     It is still a further feature of the present invention to provide a surface mount MELF that is easy to make and economical to manufacture.  
         [0013]     The device and method of accomplishing these and other features will become apparent from the following description of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a side cross sectional view of a surface mount MELF capacitor of the present invention.  
         [0015]      FIGS. 2-9  are cross sectional side views of the surface mount MELF capacitor shown in  FIG. 1  at various manufacturing stages.  
         [0016]      FIG. 10  is a schematic drawing of a prior art capacitor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     The present invention will be described as it applies to the preferred embodiment. It is not intended that the present invention be limited to the described embodiment. It is intended that the invention cover all alternatives, modifications, and equivalencies which may be included within the spirit and scope of the invention.  
         [0018]      FIG. 10  shows a typical prior art capacitor  10 . Capacitors are used in many types of electronic devices. The more popular uses for capacitors are in personal computers, disk drives, cellular phones, printers, hand held pagers, automobiles and in military equipment.  
         [0019]     The capacitor  10 , as shown, has two conductors, namely, the tantalum pellet  12  and the manganese dioxide (MnO 2 )  16 , which is actually a semiconductor. The dielectric film  14  is tantalum pentoxide (Ta 2 O 5 ). When the capacitor  10  is in use, the tantalum pellet  12  is positively charged and acts as the anode, and the manganese dioxide  16  is negatively charged and acts as the cathode. The capacitor also includes a tantalum anode lead wire  18 , a metallized outer electrode or silver  20  and a layer of carbon  22  inside the outer electrode  20 .  
         [0020]     The prior art capacitor  10  is usually made by taking tantalum powder and compressing or compacting into a pellet. The resulting pressed pellet  12  then undergoes a sintering process wherein the pellet  12  is heated in a vacuum. The heating allows the tantalum particles to stick together so they can hold the lead wire  18 .  
         [0021]     After the sintering process, the pellet  12  is typically dipped in an acid solution to form a dielectric film  14  on the outer surface of the pellet  12 . The pellet  12  is then subsequently coated with various other metal-containing materials which form the cathode. Typically, MnO 2    16  is placed around the dielectric film  14  which may be followed by the layer of carbon graphite  22  which is painted with silver print  20 . Other conductive polymers such as polypirrolle can also be used in place of manganese oxide. The cathode portion ends in a cathode termination.  
         [0022]     The lead wire  18  is usually coated with an insulating substance such as Teflon™ (not shown). The lead wire  18  is typically the anode termination. These terminations can be connected to a circuit board for mounting the capacitor  10  in an electrical circuit.  
         [0023]      FIG. 1  shows a surface mount MELF capacitor  30  of the present invention. Note that in the figures, for clarity, the various portions of the capacitors are shown with straight and sharply cornered edges. The actual capacitors may have slightly rounded corners, etc. In addition, the capacitors have been shown in a standard shape and size; however, the shape and size may vary to include different lengths, widths, heights, size proportions of components, etc.  
         [0024]     The capacitor  30  includes a wire  32 . The wire  32  is typically made of tantalum. Alternatively, the wire may be made of another valve metal (i.e., Niobium (Nb), Hafnium (Hf), Zirconium (Zr), Titanium (Ti), Vanadium (V), Tungsten (W), Beryllium (Be), or Aluminum (Al)). Alternatively, the wire may be made of a substrate containing a valve metal (i.e., Ta, Nb, Hf, Zr, Ti, V, W, Be, or Al). The wire is preferably between 50-100 μm thick. The wire is typically cylindrical with a circular cross section; however, the wire  32  can be in any shape and cross section.  
         [0025]     A conductive powder element  34  is upon the wire  32 . The conductive powder element may be a valve metal. Alternatively, the conductive powder element may be a valve metal substrate. The conductive powder element  34  may have a low capacitor-voltage (CV) (i.e. 10 CV) up to 100-150 KCV. The conductive powder element  34  before being placed upon the wire  32  may be in a form of a powder that is regularly agglomerated, sieved, and/or crushed. The conductive powder element  34  has a density in the range of 3-8 g/cc when attached to the wire  32  in a layer.  
         [0026]     A dielectric film  36  is over the surface of the conductive powder element  34  and the anode wire  32 . The dielectric film  36  is typically tantalum pentoxide (Ta 2 O 5 ).  
         [0027]     A solid electrolyte, i.e. manganese dioxide (MnO 2 ) or a conductive polymer is a dielectric film  40 . The solid electrolyte impregnates spaces within the dielectric film  36  coated conductive powder element  34  to form the cathode of the capacitor.  
         [0028]     A conductive counterelectrode layer overlies the manganese dioxide layer  40  and is in electrical continuity with the manganese dioxide layer  40  of the capacitor  30 . The counterelectrode layer is preferably comprised of a first sublayer  42  of graphite carbon and an overlayer of metal particles  44 , preferably silver, in a binder or organic resin. The counterelectrode layer extends around the cathode end  46  of the conductive powder element  34  as well as helps seal the manganese dioxide layer  40 . The counterelectrode layer overlies substantially all of the side surfaces of the conductive powder element  34  to obtain a capacitor having a minimum dissipation factor and ESR, but is maintained separate from, and out of electrical continuity with the anode wire  32 .  
         [0029]     An organic coating or passivation coating  48  is formed over the counterelectrode layer on the outer perimeter of the conductive powder element  34  and over the conductive powder element  34  at each end. A cathode ring  54  is bonded in contact with the cathode end  46  of the counterelectrode layer, thus forming a cathode terminal  56 . An anode end cap  58  is bonded to the wire  32  which is in contact with the anode end  50  of the conductive powder element  34 , thus forming an anode terminal  60 .  
         [0030]     The cathode terminal  56  and the anode terminal  60  are connections that can be connected to a circuit board for mounting the capacitor  30  in an electrical circuit. While the method described below and shown in  FIGS. 2-9  below is applied to a capacitor, it is also possible to utilize the present method for any type of chip component.  
         [0031]      FIG. 2  is a side view of a wire  32 . The wire is preferably 50-100 μm thick.  
         [0032]     As seen in  FIG. 3 , the conductive powder element  34  is placed upon the wire by electrophoretic deposition that comprises essentially two steps: first, charged particles of powder (0.2-40 μm) in suspension are moved to the wire  32  by applied voltage and second, the particles of powder are deposited (discharged and flocculated) on the wire  32 . The resulting film of charged particles is the conductive powder element  34  which is dense and uniform.  
         [0033]     The next step is to place the wire  32  with conductive powder element  34  through a sintering process to heat the conductive powder element  34  in a vacuum. The temperature for this process is between 600-1400° C. for tantalum and niobium. The conductive powder element  34  is held in a vacuum at the specified temperature for between about 2-20 minutes and then cooled in accordance with conventional cooling procedures that are well known in the art.  
         [0034]     As seen in  FIG. 4 , after the sintering process the conductive powder element  34  is placed in an oxygen-forming solution such that a thin dielectric film  36  is formed. As an example, when using tantalum or niobium powder the thin dielectric film  36  will be tantalum pentoxide or niobium pentoxide.  
         [0035]     Next, the cathode portion of the capacitor is formed. Typically, manganese dioxide  40  is placed around the dielectric film  36  which may be followed by a layer of carbon graphite  42  which is printed with silver  44 . The silver print  44  is comprised of an organic resin heavily filled with silver flakes, making it conductive. The first sublayer  42  of graphite carbon  42  and the overlayer of metal particles are collectively called a conductive counterelectrode layer.  
         [0036]     As seen in  FIG. 6 , an insulation or passivation material  46  is placed surrounding the conductive powder element  34 , first and second ends and outside perimeter, and the exposed portion of the wire  32  side surfaces.  
         [0037]     As seen in  FIG. 7 , the openings for the anode terminal  60  and the cathode terminal  56  are laser opened to expose the wire  32  and the conduction counterelectrode layer, respectively. While laser opening is the preferred method to expose the conductive surface of the wire  32  and counterelectode layer, other techniques could be used. Once the wire is exposed, a silver print  58 ,  56  can be applied as seen in  FIG. 7 .  
         [0038]     The next step is to cut the surface mount MELF capacitor  30  from the series into single components. The surface mount MELF capacitor  30  may be removed from the series a number of ways well known in the art.  
         [0039]     While the present invention can be accomplished using the methods described above, it us understood that various other methods could be used within the spirit and scope of the present invention.  
         [0040]     The preferred embodiment of the present invention has been set forth in the drawings and specification, and although specific terms are employed, these are used in a generic or descriptive sense only and are not used for purposes of limitation. Changes in the form and proportion of parts as well as in the substitution of equivalents are contemplated as circumstances may suggest or render expedient without departing from the spirit and scope of the invention as further defined in the following claims.

Technology Classification (CPC): 8