Abstract:
The surface mount flipchip capacitor of the present invention includes a wire and a conductive powder element electrically connected to the wire. The surface mount flipchip capacitor has insulative material surrounding at least a portion of the conductive powder element and the wire extending below the conductive powder element. A first terminal is formed on the surface mount flipchip 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 flipchip capacitor of the present invention is created by methods which include the steps of providing a wire and placing conductive powder upon the wire. One embodiment of the present invention creates multiple wires from a foil sheet and electrophoretically deposits conductive powder element upon the wire.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to capacitors. More particularly, though not exclusively, the present invention relates to improved surface mount flipchip capacitors and methods for manufacturing the same. 
     BRIEF SUMMARY OF THE INVENTION 
     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. 
     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. 
     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. 
     These capacitors have the anode and the cathode attached to a circuit board by connection wires. 
     Modem 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). 
     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 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. 
     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. 
     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 flipchip capacitor and method for making the same. 
     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. 
     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. 
     It is still a further feature of the present invention to provide a surface mount flipchip device that is easy to make and economical to manufacture. 
     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 
         FIG. 1  is a side cross sectional view of a surface mount flipchip capacitor of the present invention. 
         FIGS. 2-10  are cross sectional views of the surface mount flipchip capacitor shown in  FIG. 1  at various manufacturing stages. 
         FIG. 11  is a schematic drawing of a prior art capacitor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
       FIG. 11  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. 
     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 . 
     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 . 
     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. 
     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. 
       FIG. 1  shows a surface mount flipchip 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. 
     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. 
     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. 
     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 ). An insulative coating  38  such as Teflon™ coats a portion of the wire  32 , the sides of the sintered tantalum layer  34 , and a portion of the top of the sintered tantalum layer  34 . 
     A solid electrolyte, i.e. manganese dioxide (MnO 2 ) or a conductive polymer is a dielectric cap  40 . The solid electrolyte impregnates spaces within the dielectric film  36  coated conductive powder element  34  to form the cathode of the capacitor. 
     A conductive counterelectrode layer overlies the dielectric cap  40  and is in electrical continuity with the dielectric cap  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 over the cathode end  46  of the tantalum layer  34  as well as helps seal the manganese dioxide layer  40 . The counterelectrode layer overlies substantially all of the cathode end  46  of the tantalum layer  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 . 
     An organic coating or passivation coating  48  is formed over the counterelectrode layer and over the insulative coating  38 . A cathode end cap  54  is bonded in contact with counterelectrode layer through an opening in the passivation coating  48 , thus forming a cathode terminal  56 . An anode end cap  58  is bonded to the wire  32  which is in electrical continuity with the anode end  50  of the tantalum layer  34  and built up in height to make a side of the capacitor  30 , thus forming an anode terminal  60  the same height as the cathode terminal  56 . 
     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. 2-10  below is applied to a capacitor, it is also possible to utilize the present method for any type of chip component requiring termination at the same end. 
       FIG. 2  is a side view of a foil  70 . The foil  70  is preferably 50-100 μm thick. The foil  70  may vary in length and width to accommodate any multiple of capacitors  30  in both the length or width or combination of the two. The foil  70  may be isolated into a wire  32  sized to accommodate a row of capacitors  30 . 
       FIGS. 3 ,  4  and  5  are side views of the foil  70  being masked to limit an area for electrophoretic deposition. As seen in  FIG. 3 , first a mask layer  72  is placed on the foil  70 . The mask layer  72  is patterned, using conventional photolithographic methods and may be a photoresist layer. As seen in  FIG. 4 , the foil is then oxidized creating a dielectric film  36 . The mask layer  72  is then removed to expose an area  74  for electrophoretic deposition. 
     As seen in  FIG. 6 , the conductive powder element  34  is placed upon the foil  70  at area  74  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 foil  70 . The resulting film of charged particles is the conductive powder element  34  which is dense and uniform. 
     The next step is to place the foil  70  with conductive powder element 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. 
     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 over the conductive powder element  34 . As an example, when using tantalum or niobium powder the thin dielectric film  36  will be tantalum pentoxide or niobium pentoxide. 
     The next step in the process, as seen in  FIG. 7 , is the addition of a Teflon™ print or other insulating substance  38 . 
     Next, as seen in  FIG. 8 , the cathode portion of the capacitor  30  is formed. Typically, manganese oxide  40  is placed upon the dielectric film  36  in the inner area defined by the insulating substance  38  which may be followed by a layer of carbon graphite  42 , and a layer of silver print  44 . The silver print  44  is comprised of an organic resin heavily filled with silver flakes, making it conductive. 
     As further seen in  FIG. 8 , an insulation or passivation material  46  is placed surrounding the silver print layer  44  and the insulating substance  38 . 
     As seen in  FIG. 9 , the anode is laser opened to remove the passivation material  46  and the dielectric film  36  to expose the wire  32 . While laser opening is the preferred method to expose the conductive surface of the wire  32 , other techniques could be used. 
     With any process, the conductive surface of the wire  32  should be exposed. Once the wire  32  is exposed, anode end cap  52  can be applied through silver printing to become the anode terminal  60 . In addition, the cathode is laser opened through the insulating substance and a cathode end cap applied through silver printing. 
     The next step is to cut from the surface mount chip capacitor  30  from the foil  70 , as seen in FIG.  10 . The surface mount chip capacitor  30  may be removed from the foil  70  a number of ways well known in the art to then possess individual wire  32 . 
     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. 
     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.