Patent Publication Number: US-7715173-B2

Title: High voltage capacitors

Description:
PRIORITY STATEMENT 
   This application is a U.S. Continuation application filed under 35 U.S.C. §111(a) and claiming the benefit under 35 U.S.C. §120 of PCT Application No. PCT/US06/23338, filed Jun. 15, 2006, which claims priority to. Ser. No. 11/359,711, now U.S. Pat. No. 7,336,475, the foregoing applications are herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   Multilayer ceramic capacitors generally have alternating layers of ceramic dielectric material and conductive electrodes. Various types of dielectric materials can be used and various types of physical configurations have been used. Capacitors for high voltage performance have been produced for many years using a “series design”. In the series design the charge is stored between the floating electrode and electrodes connected to the terminals on either side as shown for a single floating electrode designs in  FIG. 1 . This compares to a standard capacitor design shown in  FIG. 2  in which the electrodes alternatively connect to different terminals and the charge is stored between these electrodes. The capacitance for these designs is given by:
 
C=   o     rAN/T  
     Where C=Capacitance in F     o=Permittivity of Free Space=8.854×10 −12 Fm −1        r=Permittivity of the Ceramic Material, a material dependent dimensionless constant   A=Effective Overlap Area of Electrodes m 2      N=Number of electrodes−1   T=Fired Active Thickness of Ceramic Separating the Layers   

   However, in the case of the series design the effective overlap area is significantly reduced. The advantage of the series design is that the internal voltage acting on the electrodes is halved for the single floating electrode. It is possible to further separate the floating electrode to give more than one floating electrode per layer to reduce the internal voltage but this also lowers the effective overlap area reducing capacitance. The average voltage breakdowns (n=50) for 27 lots of case size 1812 MLCCs, 47nF±10% standard designs and the same number of case size 1812, 22nF±10% single floating electrode series designs are shown in  FIG. 3 . In all these cases the fired active thickness separating the electrodes was 0.0023″, 58 microns with an overall thickness of 0.051±0.003″ (1.30±0.08 mm) for the standard design and 0.068±0.003″ (1.73±0.08 mm) for the series capacitors. The length and width dimensions were 0.177±0.010″ (4.50±0.25 mm) and 0.126±0.008″ (3.20±0.20 mm) respectively for all these 1812 case size capacitors. Cross-sections of the 1812 standard design and the single electrode series design are shown in  FIGS. 4 and 5  respectively. 
   In addition to the internal voltage withstanding capability of these MLCCs it is also critical that these parts are resistant to arc-over from the capacitor terminals. U.S. Pat. No. 4,731,697, to McLarney discloses a surface electrode with portions of the margin covered by a further dielectric layer to prevent arc over that requires laser trimming. However, it is important to note that exposed electrodes are subject to corrosion. Also the properties of exposed electrodes are significantly impacted by the environment factors, such as humidity, limiting the applications in which these capacitors can be used. 
   U.S. Pat. No. 6,627,509 to Duva discloses a method for producing surface flashover resistant capacitors by applying a para-poly-xylylene coating to the surface of multilayer ceramic capacitors followed by trimming the excess material from the terminals. In this case significant costs are associated with coating of the capacitors. Furthermore, the coating may not be compatible with the circuit board assembly processes and the presence of organic coatings in some electronic application such as satellites is limited because of out gassing concerns. 
   Thus, despite various efforts to reduce produce capacitors with high voltage breakdown and which minimize occurrence of arc over, problems remain. What is needed is an improved high voltage capacitor. 
   BRIEF SUMMARY OF THE INVENTION 
   Therefore, it is a primary object, feature, or advantage of the present invention to improve upon the state of the art. 
   It is a further object, feature, or advantage of the present invention to provide a multilayer ceramic capacitor which is resistant to arc-over. 
   It is a still further object, feature, or advantage of the present invention to provide a multilayer ceramic capacitor with high voltage breakdown in air. 
   A still further object, feature, or advantage of the present invention is to provide a multilayer ceramic capacitor with a design which retains high capacitance. 
   Another object, feature, or advantage of the present invention is to minimize the occurrence of unwanted disruptions due to arc-over when the capacitor is incorporated into an electronic circuit. 
   Yet another object, feature, or advantage of the present invention is to provide a capacitor with high voltage withstanding capability with a smaller case size allowing for miniaturization of circuits. 
   A further object, feature, or advantage of the present invention is to provide an improved capacitor which can be manufactured conveniently and economically. 
   One or more of these and/or other objects, features, or advantages of the present invention will become apparent from the specification and claims that follow. 
   According to one aspect of the present invention, a multilayer ceramic capacitor component is provided. The capacitor component includes a ceramic capacitor body having opposite ends and comprised of a plurality of electrode layers and dielectric layers. The capacitor component further includes first and second external terminals attached to the ceramic capacitor body. The capacitor component also includes a plurality of internal active electrodes within the ceramic capacitor body configured in an alternating manner such that a first of the plurality of internal active electrodes extends from one end of the ceramic capacitor body inwardly and a next internal active electrode extends from an opposite end of the ceramic capacitor body inwardly. There is also a plurality of internal electrode shields within the ceramic capacitor body to thereby assist in providing resistance to arc-over. The plurality of internal electrode shields include a top internal electrode shield and an opposite bottom internal electrode shield wherein the top internal electrode shield and the opposite bottom internal electrode shield are on opposite sides of the plurality of internal active electrodes and each internal electrode shield extends inwardly to or beyond a corresponding external terminal to thereby provide shielding. There are also side shields. Each side shield extends inwardly from one end of the capacitor body and the side shields are configured to further shield an active electrode to thereby further resist arc over between active electrodes and terminals. 
   According to another aspect of the present invention, a multilayer ceramic capacitor component for providing improved high voltage characteristics is provided. The capacitor includes a ceramic capacitor body having opposite ends and comprised of a plurality of electrode layers and dielectric layers. First and second external terminals are attached to the ceramic capacitor body. The plurality of electrode layers include a top layer having an electrode shield extending inwardly to or beyond the first terminal, a bottom layer having an electrode shield extending inwardly to or beyond the second terminal, and a plurality of alternating layers of active electrodes extending inwardly from alternating ends of the ceramic capacitor body. Each of the alternating layers of active electrodes also includes side shields. 
   According to another aspect of the present invention a method of manufacturing a multilayer ceramic component is provided. The method includes forming a ceramic capacitor body from a plurality of electrode layers and dielectric layers and attaching first and second external terminals on opposite ends of the ceramic capacitor body. The plurality of electrode layers comprises layers of active electrodes and layers of shielding electrodes and wherein the layers of active electrodes are configured in an alternating manner such that a first of the plurality of active electrodes extends from one end of the ceramic capacitor body inwardly and a next internal active electrode extends from an opposite end of the ceramic capacitor body inwardly. The layers of shielding electrodes include a top internal electrode shield and an opposite bottom internal electrode shield wherein the top internal electrode shield and the opposite bottom internal electrode shield are on opposite sides of the plurality of active electrodes and each electrode shield extends inwardly to or beyond a corresponding external terminal to thereby provide shielding. The layers of active electrodes also include layers of side shields on opposite sides of the active electrodes to thereby provide additional shielding. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a cross-section through a series capacitor design with a single floating electrode. 
       FIG. 2  is a diagram of a cross-section through a standard capacitor sign. 
       FIG. 3  shows an average voltage breakdown of series and standard capacitor designs. 
       FIG. 4A  shows a cross-section photograph of 1812 MLCC standard design. 
       FIG. 4B  shows an end view photograph of an 1812 MLCC standard design. 
       FIG. 5A  is a cross-section photograph of 1812 MLCC single floating electrode series design. 
       FIG. 5B  shows an end view photograph of an 1812 MLCC single floating electrode series design. 
       FIG. 6  is a diagram of capacitor designs according to several embodiments of the present invention. 
       FIG. 7  is a table showing the average capacitance and dimensions for the capacitor designs of  FIG. 6 . 
       FIG. 8A  is a side view cross-section drawing of Example 1. 
       FIG. 8B  is an end view cross-section drawing of Example 1. 
       FIG. 9A  is a side view cross-section drawing of Example 2. 
       FIG. 9B  is an end view cross-section drawing of Example 2. 
       FIG. 10A  is a side view cross-section drawing of Example 3. 
       FIG. 10B  is an end view cross-section drawing of Example 3. 
       FIG. 11  shows a voltage breakdown of Examples 1, 2 and 3. 
       FIG. 12A  is a photograph of a cross-section of Example 1. 
       FIG. 12B  is a photograph of an end view of the cross-section of Example 1. 
       FIG. 13A  is a photograph of a cross-section of Example 2. 
       FIG. 13B  is a photograph of an end view of the cross-section of Example 2. 
       FIG. 14A  is a photograph of a cross-section of Example 3. 
       FIG. 14B  is a photograph of an end view of the cross-section of Example 3. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   This invention describes a novel arrangement of internal electrodes that results in an arc resistant multilayer ceramic capacitor with very high voltage breakdown in air. Furthermore these designs retain a high capacitance. To assist in describing the present invention, each of three designs and MLCC performance is described and then a more detailed description of each example is provided with reference to the drawings. The designs and MLCC performance is described in the following examples. 
   EXAMPLE 1 
   A standard case size 1206 capacitor design was manufactured using a production MLCC X7R materials system C-153. 
   EXAMPLE 2 
   A case size 1206 capacitor design was manufactured using a production MLCC X7R materials system C-153 with shield electrodes on top and bottom. The purpose of these shield electrodes is to prevent an arc-over between the terminal and the internal electrode of opposite polarity or across the top or bottom surface of the capacitor between terminals of opposite polarity. For this reason it is only necessary to have one shield electrode present in the case where the active below is of opposite polarity. However, during the course of manufacturing capacitors of different values by shielding both terminal areas at the top and bottom of the capacitor there is no need to change the screens for different numbers of electrodes improving manufacturability. 
   EXAMPLE 3 
   A case size 1206 capacitor design was manufactured using a production MLCC X7R materials system C-153 with side shield electrodes on either side of the active in additions to shield electrodes on top and bottom. The purpose of the side shield electrode is to prevent an arc-over between the terminal and different internal electrode layers of opposite polarity or across the sides of the capacitor between terminals of opposite polarity. As for the top and bottom side shield electrodes, two side shield electrodes on each side were used but it is only necessary to have one side shield electrode at the side of each layer with terminal of opposing polarity. The two side shield electrodes on each side allow to accurately check alignment of the electrode stack. 
   The design and electrode pattern for all three examples is shown in  FIG. 6 . Terminals were applied to these examples consisting of a thick film fired silver paste and these were then over plated with nickel followed by tin. The parts were screened through a 1000V Hi-Pot and IR verified. The average capacitances (n=100) and dimensions (n=5) were measured as shown in  FIG. 7 . 
   It can be seen that the Number of Electrodes-1 (N) are almost the same for all these examples, 27±1. The Fired Active Thickness of Ceramic Separating the Layers (T) is also the same for all three examples and since the same ceramic material system was used to manufacture all the capacitors the Permittivity ( r) is the same. The only variable affecting capacitance is therefore the Effective Overlap Area of Electrodes (A). This is lower for Example 3 because of the presence of the side-shields. The actual cross-sections of Examples 1, 2 and 3 are shown in  FIGS. 12A and 12B  (Example 1),  FIGS. 13A and 13B  (Example 2) and  FIGS. 14A and 14B  (Example 3) 
   A sample of 50 capacitors for examples 1, 2 and 3 were tested to failure by applying voltage at a ramp rate of 500V/s per method 103 of EIA 198-2-E. The results are shown in  FIG. 11 . The instrument used for testing was the Associated Research 7512DT HiPot. Data of  FIG. 11  represents dielectric breakdown voltage levels, which include arc-over and or physical destruction. Post IR testing of Example 1 parts had 13/50 Insulation Resistance (IR) failures, Examples 2 and 3 had 48/50 and 50/50 IR failures respectively indicating that failures due to arc-over were not observed in Example 3. It is also important to note that repeated arc-over occurrences on applying voltage would eventually cause IR failure. 
   It can clearly be seen that Example 3 has the highest average voltage breakdown &gt;2.5 kV of the examples cited. The voltage breakdown and capacitance of the 1206 case size capacitor in Example 3 are similar to the 1812 1000V rated single floating electrode serial capacitors described in the prior art. The capacitors described in Example 3 therefore allow the circuits required to handle high voltages to be significantly miniaturized. 
     FIG. 1  illustrates a prior art capacitor design. In  FIG. 1 , a capacitor  10  is shown with a first terminal  12  and an opposite second terminal  14  on the opposite end of the capacitor body  16 . Floating electrodes  18  are shown.  FIG. 2  illustrates another prior art capacitor design. In  FIG. 2 , instead of floating electrodes, the electrodes alternate.  FIG. 3  compares the series and standard designs. In particular,  FIG. 3  shows the average voltage breakdowns (n=50) for 27 lots of case size 1812 MLCCs, 47nF±10 percent standard designs and the same number of case size 1812, 22nF±10 percent single floating electrode series designs. In all these cases the fired active thickness separating the electrodes was 0.0023″, 58 microns with an overall thickness of 0.051±0.003″ (1.30±0.08 mm) for the standard design and 0.068±0.003″ (1.73±0.08 mm) for the series capacitors. The length and width dimensions were 0.177±0.010″ (4.50±0.25 mm) and 0.126±0.008″ (3.20±0.20 mm) respectively for all these 1812 case size capacitors. Cross-sections of the 1812 standard design and the single electrode series design are shown in  FIGS. 4A-4B  and  5 A- 5 B, respectively. 
     FIG. 6  is a table which shows three different capacitor design examples. The first example is a standard design used for comparison purposes. The second example is one embodiment of the present invention where top and bottom shields are used. The third example is another embodiment of the present invention where both top and bottom shields as well as side shields are used. 
   As shown in  FIG. 6 , in the standard design, the fired active thickness of the capacitor is 0.0020 inches or 51 microns. The standard design includes 26 active electrodes. In the top/bottom shield design, the fired active thickness of the capacitor is also 0.0020 inches or 51 microns. The top/bottom shield design includes 27 active electrodes. In the top/bottom and side shield design, the fired active thickness is 0.0020 inches or 51 microns. In the top/bottom side shield design there are 28 active electrodes. 
     FIG. 6  also shows the electrode layout plans for the various types of design. According to the standard design there is a first electrode  20  and a staggered second electrode  22 . A third electrode  24  is aligned with the first electrode  20 . A fourth electrode  26  is aligned with the second electrode  22 . This alternating pattern continues, with additional alternating electrodes until the second to last electrode, N−1, and the last electrode  30 . 
   In the top/bottom shield design the first electrode layer includes a first top shield  32  and a second top shield  34  as well as a first bottom shield  36  and a second bottom shield  38 . It is of particular note that only the first top shield  32  and the second bottom shield  38  are active—the other shields need not even be present. The first top shield  32  and second bottom shield  38  are necessary to prevent arc-over from terminations of opposed polarity and shields  34  and  26  are present for manufacturing convenience. 
   In the top/bottom and side shields embodiment, there is a first top shield  32  and a second top shield  34  as well as a first bottom shield  36  and a second bottom shield  38 . For each active electrode there are also side shields  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 , and  70 . The side shields  40 ,  42 ,  52 ,  54 ,  56 ,  58 ,  68 , and  70  are required to protect the inner active electrodes from arc over from the termination of opposed polarity whereas the other side shields were included to test the electrode alignment within the parts. 
   The designs shown in  FIG. 6  are further illustrated in  FIGS. 8A to 10B .  FIG. 8A  is a cross-section of Example 1 (standard design) while  FIG. 8B  is an end view of the cross-section of Example 1. In  FIG. 8A , a multilayer ceramic capacitor component  48  is shown with a first terminal  12  and a second terminal  14  on opposite ends of a multilayer ceramic capacitor component  16 . The internal active electrodes of the ceramic capacitor body are configured in alternating manners such that a first internal active electrode  20  extends from one end of the ceramic capacitor body inwardly toward the terminal on the opposite end of the ceramic capacitor body. The next internal active electrode  22  extends from the opposite end of the ceramic capacitor body inwardly toward the terminal on the opposite end of the ceramic body. The end view cross-section of  FIG. 8B  illustrates the electrodes. 
     FIG. 9A  is a side view cross-section of Example 2 (top/bottom shields) while  FIG. 9B  is an end view of the cross-section of Example 2. In  FIG. 9A , a multilayer ceramic capacitor component  50  is shown. Note the presence of the internal electrode shields within the ceramic capacitor body which assist in providing resistance to arc-over between the terminals and internal electrodes. The internal electrode shields shown include a top internal electrode shield  32  and an opposite bottom internal electrode shield  38 . The top internal electrode shield  32  and the opposite bottom internal electrode shield  38  are on opposite sides of the multilayer ceramic capacitor body  16 . Each internal electrode shield  32 ,  38  extends inwardly to or beyond a corresponding terminal  12 ,  14  to thereby provide shielding. As previously mentioned, additional structures  34 , and  36  are provided but are not required as they do not provide actual shielding due to the polarity of the terminals. They are included for convenience in the manufacturing process. 
     FIG. 10A  is a side view cross-section of Example 3 (top/bottom shields and side shield) while  FIG. 10B  is an end view of the cross-section of Example 3. The multilayer ceramic capacitor  60  of  FIG. 10A  includes not only the top shield  32  and opposite bottom shield  38 , but also side shields. The side shields are best shown in  FIG. 10B  that depicts a cross-section through the capacitor. The side shield in question depends on the depth of the cross-section hence the side shields shown are  40 ,  42 ,  48 , and  50 . 
     FIG. 7  provides a table for comparing the standard design to two designs according to the present invention. The table shows the average capacitance and dimensions for the capacitor designs of  FIG. 6 . 
     FIG. 11  shows a voltage breakdown of Examples 1, 2 and 3. Note that in  FIG. 11 , the top/bottom shield embodiment (Example 2) provides increased voltage break down relative to the standard design (Example 1). The top/bottom and side shield embodiment (Example 3) provides further increased break down voltage. Thus, the present invention can be used to create multi-layer ceramic capacitors having voltage breakdowns above 1000 V, 1500 V, 2000V, 2500 V, or even 3000V. 
   Therefore an improved high voltage capacitor has been disclosed. The present invention is not to be limited to the specific embodiments shown in here. For example, the present invention contemplates numerous variations in the types of dielectric used, types of conductors used, sizes, dimensions, packaging, and other variations.