Patent Publication Number: US-9431176-B2

Title: Multilayer ceramic capacitor having multi-layered active layers and board having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2013-0133963 filed on Nov. 6, 2013, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a multilayer ceramic electronic component and a board having the same. 
     Electronic components using a ceramic material include a capacitor, an inductor, a piezoelectric element, a varistor, a thermistor, and the like. 
     Among ceramic electronic components, a multilayer ceramic capacitor (MLCC) has advantages such as compactness, guaranteed high capacitance, and ease of mounting. 
     Such an MLCC is a chip-type condenser mounted on circuit boards of various electronic products including display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs) and the like, as well as including computers, personal digital assistants (PDA), mobile phones, and the like, to serve to charge and discharge electricity. 
     An MLCC may include a plurality of stacked dielectric layers, internal electrodes having different polarities and opposed to each other with the dielectric layers interposed therebetween, and external electrodes electrically connected to the internal electrodes. 
     Since the dielectric layer has piezoelectric and electrostrictive properties, a piezoelectric phenomenon occurs between the internal electrodes when a direct current (DC) or alternating current (AC) voltage is applied to the multilayer ceramic capacitor, such that vibrations may be generated. 
     These vibrations may be transferred to a board on which the multilayer ceramic capacitor is mounted through external electrodes of the multilayer ceramic capacitor, such that the entirety of the board is a sound radiating surface to generate a vibration sound, noise. 
     The vibration sound may be within an audio frequency range of 20 to 20,000 Hz, which may cause listener discomfort and is referred to as an acoustic noise. 
     SUMMARY 
     An aspect of the present disclosure may provide a multilayer ceramic electronic component allowing for an effective reduction in noise caused by vibrations due to a piezoelectric phenomenon. 
     According to an aspect of the present disclosure, a multilayer ceramic electronic component may include: a ceramic body having a plurality of dielectric layers stacked therein; active layers including a plurality of first and second internal electrodes disposed to be alternately exposed to end surfaces of the ceramic body with the dielectric layers interposed between the first and second internal electrodes; and first and second external electrodes disposed on the end surfaces of the ceramic body and electrically connected to the first and second internal electrodes, respectively, wherein the active layers include a first active layer including a ferroelectric layer and a second active layer including a paraelectric layer, the first and second active layers being alternately stacked. 
     Each of the first and second active layers may include a plurality of first internal electrodes and a plurality of second internal electrodes. 
     Each of the first and second active layers may only include a single first internal electrode and a single second internal electrode. 
     The ferroelectric layer may be formed of at least one of a solid solution of (Ba 1-x Ca x ) m  (Ti 1-y Zr y )O 3 , BaTiO 3 , PbTiO 3 , and SrTiO 3 . 
     The paraelectric layer may be formed of at least one of (Ca,Sr)(Ti,Zr) O3 , BaO—Ti O2 —N d2O3 , and CaTi O3 —MgTi O3 . 
     A dielectric constant of the first active layer may be 10 to 1,000 times that of the second active layer. 
     Upper and lower cover layers may be disposed on upper and lower portions of the active layers. 
     Each of the upper and lower cover layers may include a ferroelectric layer. 
     According to another aspect of the present disclosure, a board having a multilayer ceramic electronic component may include: a substrate having first and second electrode pads disposed on the substrate; and the multilayer ceramic electronic component installed on the substrate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a partially cut-away perspective view schematically illustrating a multilayer ceramic capacitor (MLCC) according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view illustrating the MLCC according to the exemplary embodiment of the present disclosure, taken in length and thickness directions thereof; 
         FIG. 3  is a cross-sectional view illustrating an MLCC according to another exemplary embodiment of the present disclosure, taken in length and thickness directions thereof; 
         FIG. 4  is a partially cut-away perspective view schematically illustrating a state in which the multilayer ceramic capacitor (MLCC) according to the exemplary embodiment of the present disclosure is mounted on a board; 
         FIG. 5  is a cross-sectional view illustrating the board of  FIG. 4  taken in length and thickness directions thereof; 
         FIG. 6  is a cross-sectional view schematically illustrating a form in which ferroelectric layers of first active layers of the MLCC illustrated in  FIG. 4  contract when a voltage is applied to the MLCC mounted on a board; and 
         FIG. 7  is a cross-sectional view schematically illustrating a form in which the ferroelectric layers of the first active layers of the MLCC illustrated in  FIG. 4  expand when a voltage is applied to the MLCC mounted on the board. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. 
     The disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. 
     In the drawings, the shapes and dimensions of elements may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like elements. 
     Hereinafter, a multilayer ceramic electronic component according to an exemplary embodiment of the present disclosure will be described as a multilayer ceramic capacitor, by way of example, but the present disclosure is not limited thereto. 
     Multilayer Ceramic Capacitor (MLCC) 
       FIG. 1  is a partially cut-away perspective view schematically illustrating a multilayer ceramic capacitor (MLCC) according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 1 , a multilayer ceramic capacitor (MLCC)  100  according to the exemplary embodiment may include a ceramic body  110 , active layers including a plurality of first and second internal electrodes  121  and  122 , and first and second external electrodes  131  and  132 . 
     The ceramic body  110  is formed by stacking a plurality of dielectric layers in a thickness direction and subsequently sintering the same, and a shape and dimensions of the ceramic body  110  and the number of stacked dielectric layers may not be limited to those illustrated in the exemplary embodiment. 
     Also, the plurality of dielectric layers forming the ceramic body  110  may be in a sintered state may integrated such that boundaries therebetween may not be readily apparent without using a scanning electron microscope (SEM). 
     The ceramic body  110  may have, for example, a hexahedral shape, but the shape of the ceramic body  110  is not particularly limited. 
     In the exemplary embodiment, surfaces of the ceramic body  110  opposing each other in the thickness direction will be defined as main surfaces, surfaces of the ceramic body  110  connecting the both main surfaces and opposing each other in a length direction will be defined as end surfaces, and surfaces of the ceramic body  110  connected to both end surfaces thereof to be perpendicular thereof and opposing each other in a width direction will be defined as side surfaces. 
     Directions of the ceramic body  110  will be defined in order to clearly describe exemplary embodiments of the present disclosure. L, W and T shown in the accompanying drawings refer to a length direction, a width direction, and a thickness direction of the ceramic body  110 , respectively. 
     Here, the thickness direction may be used to have the same concept as that of a direction in which the dielectric layers are stacked. 
       FIG. 2  is a cross-sectional view illustrating the MLCC according to the exemplary embodiment of the present disclosure, taken in length and thickness directions thereof. 
     Referring to  FIG. 2 , the ceramic body  110  may include the active layers as parts contributing to capacitance formation of the capacitor, and upper and lower cover layers  113  and  114  formed on an upper portion and a lower portion of the active layers, respectively, as upper and lower margin portions. 
     The active layers may have a structure in which a plurality of dielectric layers  111  and  112  are provided, that is, first active layers D11 and D12 including ferroelectric layers  111  and second active layers D21 and D22 including paraelectric layers  112  are alternately stacked. 
     Also, each of the first active layers D11 and D12 and the second active layers D21 and D22 may be formed by alternately stacking the plurality of internal electrodes  121  and  122  with the ferroelectric layers  111  or the paraelectric layers  112  interposed therebetween. 
     In this case, the ferroelectric layers  111  and the paraelectric layers  112  may contain a ceramic powder and further contain various types of ceramic additive such as a transition metal oxide or carbide, a rare earth element, magnesium (Mg), aluminum (Al), and the like, an organic solvent, a plasticizer, a binder, a dispersing agent, or the like, added therein, as needed, in addition to the ceramic powder. 
     Also, thicknesses of the ferroelectric layers  111  and the paraelectric layers  112  may be arbitrarily changed according to a capacity design of the MLCC  100 . Preferably, a thickness of a single dialectic layer may range from 0.01 μm to 1.00 μm, but the present disclosure is not limited thereto. 
     Also, the ferroelectric layers  111  may be formed of at least one of a solid solution of (Ba 1-x Ca x ) m (Ti 1-y Zr y )O 3 , BaTiO 3 , PbTiO 3 , and SrTiO 3 . 
     Also, the paraelectric layers  112  may be formed of at least one of (Ca,Sr)(Ti,Zr)O 3 , BaO—TiO 2 —Nd 2 O 3 , and CaTiO 3 —MgTiO 3 . 
     A dielectric constant of the ferroelectric layers  111  of the first active layers D11 and D12 may be 10 to 1,000 times that of the paraelectric layers  112  of the second active layers D21 and D22. 
     For example, the dielectric constant of the ferroelectric layers  111  may range from 2,000 to 4,000, while the dielectric constant of the paraelectric layers  112  may range from 20 to 150. 
     The upper and lower cover layers  113  and  114  may be formed of the same material and have the same configuration as those of the active layers, except that the upper and lower cover layers  113  and  114  do not include the internal electrodes. Preferably, the upper and lower cover layers  113  and  114  may be formed of the same material and have the same configuration as those of the ferroelectric layers  111  of the first active layers D11 and D12. 
     Also, the upper and lower cover layers  113  and  114  may be formed by stacking a single dielectric layer or two or more dielectric layers on upper and lower surfaces of the active layers in the thickness direction. Basically, the upper and lower cover layers  113  and  114  may serve to prevent damage to the first and second internal electrodes  121  and  122  due to physical or chemical stress. 
     The first and second internal electrodes  121  and  122  having different polarities may be formed by printing a conductive paste including a conductive metal on the respective dielectric layers  111  and  112  at a predetermined thickness. 
     In this case, the first and second internal electrodes  121  and  122  may be formed to be alternately exposed to both end surfaces of the ceramic body  110  in the stacking direction of the dielectric layers, while having the dielectric layers interposed therebetween. In this case, the first and second internal electrodes  121  and  122  may be electrically insulated from each other by the dielectric layers disposed therebetween. 
     Also, the first and second internal electrodes  121  and  122  may be electrically connected to the first and second external electrodes  131  and  132  through portions thereof alternately exposed to the both end surfaces of the ceramic body  110 . 
     Thus, when a voltage is applied to the first and second external electrodes  131  and  132 , electrical charges are accumulated between the first and second internal electrodes  121  and  122  opposed to each other, and in this case, a capacitance of the MLCC  100  may be proportional to an area of regions in which the first and second internal electrodes  121  and  122  overlap each other in the respective active layers  111 . 
     A thickness of the first and second internal electrodes  121  and  122  may be determined according to an intended usage thereof. For example, a thickness of the first and second internal electrodes  121  and  122  may be determined within a range of 0.2 μm to 1.0 μm in consideration of a size of the ceramic body  110 , for example, but the present disclosure is not limited thereto. 
     Also, a conductive metal contained in a conductive paste for forming the first and second internal electrodes  121  and  122  may be formed of one of silver (Ag), palladium (Pd), platinum (Pt), nickel (Ni), and copper (Cu), or an alloy thereof, or the like, but the present disclosure is not limited thereto. 
     Also, as a printing method of the conductive paste, a screen printing method, a gravure printing method, or the like, may be used, but the present disclosure is not limited thereto. 
     The first and second external electrodes  131  and  132  may be formed on the both end surfaces of the ceramic body  110  and may be in contact with the exposed portions of the first and second internal electrodes  121  and  122  to be electrically connected thereto. 
     The first and second external electrodes  131  and  132  may be formed of a conductive paste including a conductive metal, and the conductive metal may be Ag, Ni, Cu, or an alloy thereof, but the present disclosure is not limited thereto. 
     Meanwhile, first and second plating layers (not shown) may be formed on the first and second external electrodes  131  and  132  as needed. 
     The first and second plating layers may be provided to increase bonding strength between the MLCC  100  and a board when the MLCC  100  is mounted on the board by soldering. 
     For example, the first and second plating layers may have a structure including nickel (Ni) plating layers formed on the first and second external electrodes  131  and  132  and tin (Sn) plating layers formed on the nickel plating layers, but the present disclosure is not limited thereto. 
     In the present exemplary embodiment, each of the first and second active layers may be configured to include the plurality of first internal electrodes  121  and the plurality of second internal electrodes  122 , but the present disclosure is not limited thereto. 
     For example, as illustrated in  FIG. 3 , according to another exemplary embodiment, only a single first internal electrode  121  and a single second internal electrode  122  may be included in each of first and second active layers D1 and D2, and the ceramic body  110  may be configured by alternately stacking the first and second active layers D1 and D2. 
     Here, since a formation structure of the first and second external electrodes  131  and  132  and the upper and lower cover layers  113  and  114  may be identical to that of the foregoing exemplary embodiment as described above, a detailed description thereof will be omitted to avoid redundancy. 
     Board Having MLCC 
       FIG. 4  is a partially cut-away perspective view schematically illustrating a state in which the multilayer ceramic capacitor (MLCC) according to the exemplary embodiment of the present disclosure is mounted on a board.  FIG. 5  is a cross-sectional view illustrating the board of  FIG. 4  taken in length and thickness directions thereof. 
     Referring to  FIGS. 4 and 5 , a board  200  having the MLCC  100  according to the exemplary embodiment may include a substrate  210  on which the MLCC  100  is horizontally mounted and first and second electrode pads  221  and  222  formed on an upper surface of the substrate  210  to be spaced apart from each other. 
     Here, the MLCC  100  may be electrically connected to the substrate  210  by soldering parts  230  in a state in which the lower cover layer  114  of the MLCC  100  is disposed downwardly and the first and second external electrodes  131  and  132  are positioned on the first and second electrode pads  221  and  222  to be in contact therewith. 
     In the state in which the MLCC  100  is mounted on the substrate  210 , when a voltage is applied to the MLCC  100 , acoustic noise may be generated. 
     Here, sizes of the first and second electrode pads  221  and  222  may be used as an indicator for determining an amount of the soldering parts  230  connecting the first and second external electrodes  131  and  132  of the MLCC  100  and the first and second electrode pads  221  and  222 , and a magnitude of acoustic noise may be adjusted according to the amount of the soldering parts  230 . 
       FIG. 6  is a cross-sectional view schematically illustrating a form in which the ferroelectric layers of the first active layers of the MLCC illustrated in  FIG. 4  contract when a voltage is applied to the MLCC mounted on a board, and  FIG. 7  is a cross-sectional view schematically illustrating a form in which the ferroelectric layers of the first active layers of the MLCC illustrated in  FIG. 4  expand when a voltage is applied to the MLCC mounted on the board. 
     Referring to  FIGS. 6 and 7 , in a state in which the MLCC  100  is mounted on the substrate  210 , when voltages having different polarities are applied to the first and second external electrodes  131  and  132  formed on the both end surfaces of the MLCC  100 , the ceramic body  110  may expand and contract in the thickness direction due to the inverse piezoelectric effect of the dielectric layers  111 , while the both end surfaces of the ceramic body  110  on which the first and second external electrodes  131  and  132  are formed may contract and expand due to the Poisson effect, in a manner contrary to that of the expansion and contraction of the ceramic body  110  in the thickness direction. 
     Referring to  FIG. 6 , when the first active layers of the MLCC  100  have expansive force {circle around (3)} in the length direction, the ferroelectric layers  111  of the first active layers receive contractive force {circle around (1)} in the thickness direction, and on the other hand, the second active layers have contractive force {circle around (4)} in the length direction due to a difference in dielectric constants between the first active layers and the second active layers and accordingly, the paraelectric layers  112  of the second active layers receive expansive force {circle around (2)} in the thickness direction. 
     Referring to  FIG. 7 , when the first active layers of the MLCC  100  have contractive force {circle around (6)} in the length direction, the ferroelectric layers  111  of the first active layers receive expansive force {circle around (5)} in the thickness direction, and on the other hand, the second active layers have expansive force {circle around (7)} in the length direction due to the difference in dielectric constants between the first active layers and the second active layers and accordingly, and the paraelectric layers  112  of the second active layers receive contractive force in the thickness direction. 
     Thus, in the exemplary embodiment, when voltages are applied to the MLCC  100 , the first active layers and the second active layers of the MLCC  100  expand and contract in a reverse manner, cancelling out mutual stress between the active layers, thus reducing acoustic noise. 
     As set forth above, according to exemplary embodiments of the present disclosure, a ceramic body may be formed such that first active layers including ferroelectric layers and second active layers including paraelectric layers are alternately stacked and thus, expansion and contraction stress caused by a piezoelectric behavior generated in the ferroelectric layers of the first active layers may be cancelled out by stress generated in the paraelectric layers of the second active layers in a reverse manner to that of that first active layers, whereby vibrations generated in an MLCC may be reduced, and thus, acoustic noise generated due to the vibrations transferred from the MLCC to a board may be reduced. 
     While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the spirit and scope of the present disclosure as defined by the appended claims.