Abstract:
An aluminum electrolytic capacitor includes an aluminum foil substrate, a porous aluminum layer, an insulating layer, an electrically conductive polymer material, an electrically conductive material, and at least two terminal electrodes. The porous aluminum layer is attached to the aluminum foil substrate. The insulating layer is formed on the porous aluminum layer. The electrically conductive polymer material overlays the insulating layer. The terminal electrodes respectively connect to the aluminum foil and the electrically conductive material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an aluminum electrolytic capacitor and a method of manufacturing the same, and relates more particularly to an SMD-type aluminum electrolytic capacitor and a method of manufacturing the same. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, electronic mobile apparatuses, designed for smaller size and greater speed, need to have capacitors exhibiting better high frequency performance, smaller size, larger capacitance and lower resistance. Current multi-layer ceramic capacitors (MLCC), the most common type of capacitors used in mobile apparatuses, appear unable to continue meeting the increasingly-challenging above requirements. Therefore, the development of new capacitor designs must be accelerated to keep pace with the requirements of newly-developed electronic mobile apparatuses. 
         [0005]    Conventional solid electrolytic capacitors mainly include metal such as aluminum, tantalum, niobium, or titanium, of which aluminum and tantalum are most commonly used to manufacture solid aluminum electrolytic capacitors or solid tantalum electrolytic capacitors. 
         [0006]    The dielectric layer in an aluminum electrolytic capacitor is normally a metal oxide (aluminum oxide) layer on a surface of a porous aluminum plate. However, such a configuration has a limited ability to increase the area of the dielectric layer. U.S. Pat. No. 6,775,127 discloses using a tantalum or niobium foil as the substrate of a solid capacitor. Tantalum or niobium powder is coated on the foil, and is then sintered to obtain a porous dielectric structure, which can provide a larger area for forming a metal oxide film, the dielectric layer. However, the process requires temperatures as high as 1600 degrees Celsius. If the processing temperature can be lowered and a structure combining the porous metal foil and the porous metal block is adopted, the capacitor can be manufactured to have a higher capacitance. 
         [0007]    Current SMD-type solid capacitors use lead frames to connect external printed circuit boards, as disclosed in U.S. Pat. No. 6,249,424 and Taiwan Utility Model Patent No. M320738. However, the contacts between the lead frame and the solid capacitor in the disclosed solid capacitor introduce interface resistances, which combine with the resistance of the lead frame, increasing equivalent series resistance (ESR) of the solid capacitor. If the solid capacitor can be improved by removing the lead frame, the ESR can be minimized and its manufacturing cost can be reduced. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an aluminum electrolytic capacitor that can be easily manufactured with high capacitance and low ESR, and a method of manufacturing the same. The manufacturing method comprises sintering aluminum powder, securely attaching the sintered aluminum powder to an aluminum foil substrate, and forming a large area of oxide dielectric layer on the aluminum powder particles and a surface of the aluminum foil substrate, thereby increasing the capacitance. 
         [0009]    The present invention provides an aluminum electrolytic capacitor without using a lead frame. Such aluminum electrolytic capacitor can have low interface resistance, low transmission impedance and low ESR, and offers superior high-frequency performance. 
         [0010]    The present invention provides an aluminum electrolytic capacitor, which includes capacitor units that can easily stack on each other and connect in parallel with each other. Such aluminum electrolytic capacitor can have improved capacitance and low ESR. 
         [0011]    In summary, the present invention discloses an aluminum electrolytic capacitor including an aluminum foil substrate, a porous aluminum layer, an insulating layer, an electrically conductive polymer material, an electrically conductive material, and at least two terminal electrodes. The porous aluminum layer is attached to the aluminum foil substrate. The insulating layer is formed on the porous aluminum layer. The electrically conductive polymer material overlays the insulating layer. The electrically conductive material overlays the electrically conductive polymer material. The at least two terminal electrodes electrically connect, in a respective manner, the aluminum foil substrate and the electrically conductive material. 
         [0012]    The invention further discloses an aluminum electrolytic capacitor including an insulating substrate, a first aluminum layer, a porous second aluminum layer, an insulating layer, an electrically conductive polymer material, an electrically conductive material, and at least two terminal electrodes. The first aluminum layer is attached to the insulating substrate. The second aluminum layer is formed on the first aluminum layer. The insulating layer is formed on the first and second aluminum layers. The electrically conductive polymer material overlays the insulating layer. The electrically conductive material overlays the electrically conductive polymer material. The at least two terminal electrodes electrically connect, in a respective manner, the first aluminum layer and the electrically conductive material. 
         [0013]    The present invention discloses a method of manufacturing an aluminum electrolytic capacitor. The method comprises providing an aluminum foil substrate, forming a porous aluminum layer on the aluminum foil substrate, forming an oxide layer on the aluminum layer, overlaying the oxide layer with an electrically conductive polymer material, overlaying the electrically conductive polymer material with an electrically conductive material, and forming at least two terminal electrodes respectively electrically connecting the aluminum foil substrate and the electrically conductive material. 
         [0014]    The present invention discloses another method of manufacturing an aluminum electrolytic capacitor. The method comprises providing an insulating substrate, forming a first aluminum layer on the insulating substrate, forming a porous second aluminum layer on the first aluminum layer, forming an oxide layer on the first and second aluminum layers, overlaying the insulating layer with an electrically conductive polymer material, overlaying the electrically conductive polymer material with an electrically conductive material, and forming at least two terminal electrodes respectively electrically connecting the aluminum foil substrate and the electrically conductive material. 
         [0015]    To better understand the above-described objectives, characteristics and advantages of the present invention, embodiments, with reference to the drawings, are provided for detailed explanations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention will be described according to the appended drawings in which: 
           [0017]      FIG. 1A through 1E  are cross-sectional views demonstrating the steps of a method of manufacturing an aluminum electrolytic capacitor according to one embodiment of the present invention; 
           [0018]      FIG. 2  is a cross-sectional view showing an aluminum electrolytic capacitor including a stack of capacitor units according to one embodiment of the present invention; 
           [0019]      FIG. 3  is an enlarged view of an aluminum electrolytic capacitor according to one embodiment of the present invention; 
           [0020]      FIG. 4A through 4E  are cross-sectional views demonstrating the steps of a method of manufacturing an aluminum electrolytic capacitor according to one embodiment of the present invention; 
           [0021]      FIG. 5  is a cross-sectional view showing an aluminum electrolytic capacitor including a stack of capacitor units according to one embodiment of the present invention; and 
           [0022]      FIG. 6  is an enlarged view of an aluminum electrolytic capacitor according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]      FIG. 1A through 1E  are cross-sectional views demonstrating the steps of a method of manufacturing an aluminum electrolytic capacitor according to one embodiment of the present invention. As illustrated in  FIG. 1A , an aluminum foil substrate  100  is provided. The aluminum foil substrate  100  can, preferably, be an etched aluminum foil having a rough surface on which a plurality of cavities or dents are formed. A layer of aluminum powder is coated on a surface of the aluminum foil substrate  100  by a printing method, and is then sintered at a temperature in a range of from 550 to 650 degrees Celsius to form a porous aluminum layer  101  securely combined and electrically connected with the aluminum foil substrate  100 . 
         [0024]    As shown in  FIG. 1B , an insulating layer or a dielectric layer  102  is formed on a surface of the porous aluminum layer  101  and a surface of the aluminum foil substrate  100 . In one embodiment, aluminum foil substrate  100  coated with the porous aluminum layer  101  is, preferably, placed in a solution containing phosphoric acid. An electrical current is then applied to form an aluminum oxide layer on the surface of the porous aluminum layer  101  and the surface of the aluminum foil substrate  100 . Alternatively, a thermal oxidation process or the like can be employed to oxidize the aluminum on the surface to form aluminum oxide (Al 2 O 3 ). Because the aluminum layer  101  is constituted by fine aluminum particles, pores exist between adjacent sintered-together aluminum particles. Namely, the coverage of the insulating layer  102  is not only on the surfaces of the porous aluminum layer  101  and the aluminum foil substrate  100  as shown in  FIG. 1B , but also on walls defining the pores. 
         [0025]    As shown in  FIG. 1C , an electrically conductive polymer material  103 , a first electrically conductive material  104 , and a second electrically conductive material  105  are sequentially formed on the insulating layer  102 . The electrically conductive polymer material  103  may include polyaniline, polypyrrole, or polythiophene, wherein polyaniline is preferred. For example, polyaniline can be obtained by polymerization of monomer aniline using an oxidant and a medium. The first electrically conductive material  104  can be carbon epoxy, carbon paste, or carbon ink. The second electrically conductive material  105  can be a silver paste. It can be noted that if a suitable material is chosen as the electrically conductive polymer material  103 , the capacitor may not necessarily include the first electrically conductive material  104  and a second electrically conductive material  105 . 
         [0026]    As shown in  FIG. 1D , a dielectric polymer material  106  is formed to overlay the surface of the second electrically conductive material  105  while a side surface of the second electrically conductive material  105  is exposed outside the dielectric polymer material  106 ; meanwhile, one side portion of the aluminum foil substrate  100  is also exposed outside the dielectric polymer material  106 . 
         [0027]    On the exposed side portion of the aluminum foil substrate  100  and on the exposed side surface of the second electrically conductive material  105 , two terminal electrodes  107  are respectively formed. Thereafter, a solder layer  108  is coated on each of the two terminal electrodes  107  as shown in  FIG. 1E . The solder layer  108  may comprise tin or tin lead alloy.  FIG. 1E  shows a cross section of the aluminum electrolytic capacitor  10  in accordance with one embodiment of the present invention. The capacitor  10  can have terminal electrodes  107  without the assistance of a lead frame. Therefore, it has low ESR, reduced interface resistance, and low transmission impedance, and further offers superior high-frequency performance. 
         [0028]      FIG. 2  is a cross-sectional view showing an aluminum electrolytic capacitor  20  including a stack of capacitor units according to one embodiment of the present invention. The aluminum electrolytic capacitor  20  is formed by vertically stacking three similar capacitor units, as shown in  FIG. 1C . In the three vertically stacked capacitor units, the second electrically conductive material  105  of the lower capacitor unit supportively contacts the second electrically conductive material  105  of the upper capacitor unit. Similarly, a dielectric polymer material  106 ′ is formed to overlay the surfaces of three second electrically conductive materials  105  while the right side surfaces of the second electrically conductive materials  105  are exposed outside the dielectric polymer material  106 ′; meanwhile, the left side portions of the aluminum foil substrates  100  are also exposed outside the dielectric polymer material  106 ′. On the exposed side portions of the aluminum foil substrates  100  and on the exposed side surfaces of the second electrically conductive materials  105 , terminal electrodes  107 ′ are respectively formed. Thereafter, a solder layer  108  is coated on each of the two terminal electrodes  107 ′. Consequently, such multi-capacitor units, which are stacked on each other and connected in parallel, can have greater capacitance and lower serial resistance. 
         [0029]      FIG. 3  is an enlarged view of an aluminum electrolytic capacitor according to one embodiment of the present invention. An insulating layer  102  of aluminum oxide is formed on the surface of the porous aluminum layer  101  and the surface of the aluminum foil substrate  100 . Because the aluminum layer  101  is constituted by fine aluminum particles, pores exist between adjacent sintered-together aluminum particles. Thus, the insulating layer  102  may exist in the pores, on the surfaces of the aluminum particles. In addition, the electrically conductive polymer material  103  may also be formed on the surface of the insulating layer  102  in the pores between the aluminum particles. 
         [0030]      FIGS. 4A through 4E  are cross-sectional views demonstrating the steps of a method of manufacturing an aluminum electrolytic capacitor according to one embodiment of the present invention. As shown in  FIG. 4A , an insulating substrate  400  is provided. In one embodiment, the insulating substrate  400  is, preferably, an aluminum oxide, aluminum nitride, or glass substrate. A layer of aluminum powder is coated on a surface of the insulating substrate  400  by a printing method, and is then sintered at a temperature in a range of from 650 to 750 degrees Celsius to form a first aluminum layer  4011  with a dense structure. The first aluminum layer  4011  can be securely combined and electrically connected with the insulating substrate  400 . Alternatively, instead of aluminum, other electrically conductive materials or other types of electrically conductive layers can be used as a replacement for the first aluminum layer  4011 . 
         [0031]    Another layer of aluminum powder is thereafter coated on a surface of the first aluminum layer  4011  by a printing method, and a sintering process is performed at a temperature in a range of from 550 to 650 degrees Celsius so as to form a porous second aluminum layer  4012 , which is securely combined and electrically connected with the first aluminum layer  4011 . 
         [0032]    An insulating layer or dielectric layer  402  is next formed on a surface of the first aluminum layer  4011  and a surface of the second aluminum layer  4012 , as shown in  FIG. 4B . The insulating layer  402  on the left side surface of the first aluminum layer  4011  is then removed by, for example, a sandblasting method to expose a portion of the first aluminum layer  4011 . Because the second aluminum layer  4012  is constituted by fine aluminum particles, pores exist between adjacent sintered-together aluminum particles. Thus, the insulating layer  402  also exists on the surfaces of the aluminum particles in the pores. Specifically, the coverage of the insulating layer  402  is not only as shown in  FIG. 4B , but also extends on the walls defining the pores. 
         [0033]    As shown in  FIG. 4C , an electrically conductive polymer material  403 , a first electrically conductive material  404 , and a second electrically conductive material  405  are sequentially formed on the insulating layer  102 . The electrically conductive polymer material  403  may include polyaniline, polypyrrole, or polythiophene, wherein polyaniline is preferred. For example, polyaniline can be obtained by polymerization of monomer aniline using an oxidant and a medium. The first electrically conductive material  404  can be carbon epoxy, carbon paste, or carbon ink. The second electrically conductive material  405  can be a silver paste. It should be noted that a suitable material is chosen as the electrically conductive polymer material  403 ; the capacitor may not necessarily include the first electrically conductive material  404  and a second electrically conductive material  405 . 
         [0034]    As illustrated in  FIG. 4D , a dielectric polymer material  406  is formed to overlay the surface of the second electrically conductive material  405  while a side surface of the second electrically conductive material  405  is exposed outside the dielectric polymer material  406 ; meanwhile, the left side portion of the first aluminum layer  4011  is also exposed outside the dielectric polymer material  406 . 
         [0035]    Terminal electrodes  407  are formed respectively on the exposed left side portion of the first aluminum layer  4011  and the exposed side surface of the second electrically conductive material  405 . A solder layer  408  is coated on each of the two terminal electrodes  407  as shown in  FIG. 4E .  FIG. 4E  shows a cross section of the aluminum electrolytic capacitor  40  in accordance with one embodiment of the present invention. The capacitor  40  can have terminal electrodes  407  without the assistance of a lead frame. Therefore, it has low ESR, reduced interface resistance, and low transmission impedance, and further offers superior high-frequency performance. 
         [0036]      FIG. 5  is a cross-sectional view showing an aluminum electrolytic capacitor  50  including a stack of capacitor units according to one embodiment of the present invention. The aluminum electrolytic capacitor  50  is formed by vertically stacking three similar capacitor units, as shown in  FIG. 4C . In the three vertically stacked capacitor units, the second electrically conductive material  405  of the lower capacitor unit supportively contacts the second electrically conductive material  105  of the upper capacitor unit. Similarly, a dielectric polymer material  406 ′ is formed to overlay the surfaces of three second electrically conductive materials  405  while the right side surfaces of the second electrically conductive materials  405  are exposed outside the dielectric polymer material  406 ′; meanwhile, the left side portions of the first aluminum layer  4011  are also exposed outside the dielectric polymer material  406 ′. On the exposed side portions of the insulating substrate  400  and on the exposed side surfaces of the second electrically conductive materials  405 , terminal electrodes  407 ′ are respectively formed. Thereafter, a solder layer  408 ′ is coated on each of the two terminal electrodes  407 ′. Consequently, such multi-capacitor units, which are stacked on each other and connected in parallel, can have greater capacitance and lower serial resistance. 
         [0037]      FIG. 6  is an enlarged view of an aluminum electrolytic capacitor according to one embodiment of the present invention. An insulating layer  402  of aluminum oxide is formed on the surface of the first aluminum layer  4011  and the surface of the porous second aluminum layer  4012 . Because the second aluminum layer  4012  is constituted by fine aluminum particles, pores exist between adjacent sintered-together aluminum particles. The insulating layer  402  may exist in the pores, on the surfaces of the aluminum particles. In addition, the electrically conductive polymer material  403  may also be formed on the surface of the insulating layer  402  in the pores between the aluminum particles. The first aluminum layer  4011  is also constituted by fine aluminum particles. Since the first aluminum layer  4011  is sintered at a higher temperature, it has a denser structure. 
         [0038]    The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by person&#39;s skilled in the art without departing from the scope of the following claims.