Abstract:
A layout structure and a method for reducing audible noise generated by a flexible printed circuit are provided. The layout structure includes a flexible printed circuit and a piezoelectric element. The flexible printed circuit is disposed with respect to an axis. When an alternating voltage/current is applied across the piezoelectric element, the piezoelectric element expands and contracts alternately. The piezoelectric element is located on the flexible printed circuit, and an angle between the axis and a side of the piezoelectric element is less than 90 degrees.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Taiwan Patent Application No. 97133736 entitled “LAYOUT STRUCTURE AND METHOD FOR REDUCING AUDIBLE NOISE GENERATED BY FLEXIBLE PRINTED CIRCUIT”, filed on Sep. 3, 2008, which is incorporated herein by reference and assigned to the assignee herein. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a structure and a method for reducing audible noise generated by vibration of a flexible printed circuit, and more particular, to a structure and a method for reducing audible noise generated by vibration of a flexible printed circuit caused by piezoelectric effect. 
       BACKGROUND 
       [0003]    The printed circuit board (PCB) can be used to provide circuit connections between various electronic components. With the development of microminiature and folding design of the electronic equipments, the requirement of the flexible printed circuit (FPC) is growing rapidly due to its properties of ductility, lightness, soft, thinness, and 3D routing capability. Typically, the flexible printed circuit is made of flexible substrate and conductive material, which can be bent or rolled according to the application requirement. 
         [0004]    The multi-layer ceramic capacitor (MLCC) is one of common devices used in the flexible printed circuit.  FIG. 1  illustrates a prior art structure of a multi-layer ceramic capacitor  120  disposed on a flexible printed circuit  110 . As shown in  FIG. 1 , the multi-layer ceramic capacitor  120 , with a lateral length of L, mainly includes two electrodes  140  at both sides and a plurality of ceramic plates  130 , wherein the two electrodes  140  are electrically connected to the flexible printed circuit  110  by bonding pads  150 . When applying an alternating voltage, the ceramic plates  130  will expand and contract alternately due to piezoelectric effect. Therefore, a periodical deformation in the lateral length of the multi-layer ceramic capacitor  120  will be introduced, for example, the lateral length can vary between L 1  and L 2  as shown in  FIGS. 2A and 2B . 
         [0005]    Since the multi-layer ceramic capacitor  120  is fixed on the flexible printed circuit  110  (through the bonding pads  150  in this example), the periodical deformation in the length of the multi-layer ceramic capacitor  120  will drive the flexible printed circuit  110  to vibrate up and down. Regarding to  FIGS. 2A and 2B , the vibration amplitude of the flexible printed circuit  110  will vary between v and −v. Typically, the loudness of the sound wave generated by the vibration of the flexible printed circuit increases as the vibration amplitude of the flexible printed circuit increases, such that the audible noise that can be heard by the human ear may be generated. 
         [0006]    Therefore, it is desired to have a structure and a method capable of reducing audible noise caused by vibration of the flexible printed circuit. 
       SUMMARY 
       [0007]    According to one embodiment of the present invention, a layout structure including a flexible printed circuit and a piezoelectric element is provided. The flexible printed circuit is disposed with respect to an axis. The piezoelectric element expands and contracts alternately along a side thereof when being applied with an alternating voltage. The piezoelectric element is located on the flexible printed circuit, and an angle between the side of the piezoelectric element and the axis is less than 90 degrees. 
         [0008]    According to another embodiment of the present invention, a method for reducing audible noise generated by vibration of a flexible printed circuit is provided. The flexible printed circuit is disposed with respect to an axis. The method includes the following steps: determining that vibration of the flexible printed circuit is caused by a piezoelectric element located on the flexible printed circuit, the piezoelectric element expanding and contracting alternately along a side of the piezoelectric element when an alternating voltage is applied; and adjusting an angle between the side and the axis to reduce vibration of the flexible printed circuit. 
         [0009]    According to still another embodiment of the present invention, a method for reducing audible noise generated by vibration of a flexible printed circuit is provided. The flexible printed circuit is disposed with respect to an axis. The method includes the following steps: determining that vibration of the flexible printed circuit is caused by a piezoelectric element located on the flexible printed circuit, wherein a longitudinal deformation of the piezoelectric element in a side of the piezoelectric element is caused when an alternating voltage is applied on the piezoelectric element; and adjusting a physical parameter of the piezoelectric element to reduce a component of the longitudinal deformation along a direction perpendicular to the axis. 
         [0010]    The other aspects of the present invention, part of them will be described in the following description, part of them will be apparent from description, or can be known from the execution of the present invention. The aspects of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will now be further described by way of example only with reference to the accompany drawings in which: 
           [0012]      FIG. 1  illustrates exemplary structure of a multi-layer ceramic capacitor disposed on a flexible printed circuit; 
           [0013]      FIGS. 2A and 2B  are diagrams showing the vibration of the flexible printed circuit caused by the multi-layer ceramic capacitor; 
           [0014]      FIGS. 3A and 3B  are respectively a top-view and a side-view of a multi-layer ceramic capacitor located on a flexile printed circuit in accordance with one embodiment of the present invention; 
           [0015]      FIG. 4  shows a method for reducing vibration of the flexible printed circuit in accordance with one embodiment of the present invention; 
           [0016]      FIG. 5  shows a method for reducing vibration of the flexible printed circuit in accordance with another embodiment of the present invention; 
           [0017]      FIG. 6  shows a method for reducing vibration of the flexible printed circuit in accordance with still another embodiment of the present invention; and 
           [0018]      FIG. 7  is a flowchart of an illustrative method for reducing audible noise generated by vibration of the flexible printed circuit in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0019]    The present invention discloses a layout structure and a method for reducing audible noise generated by a flexible printed circuit by means of adjusting physical characteristics of piezoelectric elements disposed on the flexible printed circuit. The present invention will be described more fully hereinafter with reference to the  FIGS. 3A-7 . However, the devices, elements, and methods in the following description are configured to illustrate the present invention, and should not be construed in a limiting sense. 
         [0020]      FIGS. 3A and 3B  are respectively a top-view and a side-view of a multi-layer ceramic capacitor  320  located on a flexile printed circuit  310  according to one embodiment of the present invention. Referring to  FIGS. 3A and 3B  simultaneously, the flexible printed circuit  310  is disposed with respect to an axis  360 , and is bent downward about the axis  360  when being in use (as shown in  FIG. 3B ). Typically, the magnitude and orientation of curvature of the flexible printed circuit  310  may vary depending on the application requirement. In this embodiment, since the axis  360  is parallel to Y-direction, the flexible printed circuit  310  has higher vibration resistance in Y-direction. In other words, when the flexible printed circuit  310  is bent with respect to the axis  360 , it generally becomes difficult to vibrate or be bent with respect to any axis parallel to X-direction. Because the multi-layer ceramic capacitor  320  is made of piezoelectric material, a stress F will be generated when an alternating voltage is applied to the multi-layer ceramic capacitor  320 . The stress F makes the multi-layer ceramic capacitor  320  expand and contract periodically according to the frequency of the alternative voltage, such that the length of the side  322  of the multi-layer ceramic capacitor  320  will be accordingly lengthened and shortened periodically. In this embodiment, because the multi-layer ceramic capacitor  320  is fixed on the flexible printed circuit  310  by the bonding pads  350  at a position where the axis  360  passes through, the length variation of the multi-layer ceramic capacitor  320  will drive the flexible printed circuit  310  to vibrate up and down with respect to the axis  360 . 
         [0021]    In this embodiment, only the force perpendicular to the axis  360  can drive the flexible printed circuit  310  to vibrate with respect to the axis  360 , while the force parallel to the axis  360  can be ignored due to the high vibration resistance in Y-direction of the flexible printed circuit  310 . In other words, the larger the component of the stress F in X-direction is, the higher the vibration amplitude of the flexible printed circuit  310  is. Therefore, in one embodiment of the present invention, the angle θ between the side  322  of the multi-layer ceramic capacitor  320  and the axis  360  is adjusted to reduce the component of the stress F in X-direction. The component of the stress F along X-direction (F·sin θ) is getting smaller with decreasing angle θ, and accordingly, the force which can drive the flexible printed circuit  310  to vibrate is becoming smaller. Furthermore, the smaller the angle θ is, the smaller the length variation of the side  322  of the multi-layer ceramic capacitor  320  along X-direction is, such that the vibration with respect to the axis  360  caused by the length variation will be reduced. The magnitude of angle θ can vary depending upon requirement, and typically the angle θ is adjusted to make the amplitude of vibration small enough so that there is no perceivable noise. In one embodiment, the multi-layer ceramic capacitor  320  is arranged to make the side  322  parallel to Y-axis (i.e. θ=0) to minimize the vibration of the flexible printed circuit  310 . 
         [0022]      FIG. 4  shows a method for reducing vibration of the flexible printed circuit in accordance with one embodiment of the present invention. In this embodiment, when being subjected to piezoelectric effect, a length of the side  422  of the multi-layer ceramic capacitor  420  will change by a stress F caused by piezoelectric effect. The length change of the side  422  is in proportion to a pressure caused by the stress F, wherein the pressure is the stress F divided by area A 1  of the cross-section  425  which is perpendicular to the side  422 . As described above, the larger the length variation of the side  422  is, the higher the vibration amplitude of the flexible printed circuit is. Therefore, this embodiment replaces the multi-layer ceramic capacitor  420  with a multi-layer ceramic capacitor  420 ′, wherein the multi-layer ceramic capacitor  420 ′ has a cross-section  425 ′ with a larger area A 2  and the same capacitance compared with the multi-layer ceramic capacitor  420  (for example, the material of the multi-layer ceramic capacitor  420 ′ is different from that of the multi-layer ceramic capacitor  420 ). Since A 2 &gt;A 1  and (F/A 1 )&gt;(F/A 2 ), the length variation of the side  422 ′ is smaller than that of the side  422  under the influence of the stress F. In short, this embodiment reduces the pressure applied on the multi-layer ceramic capacitor by increasing area of the cross-section, which leads to decrease in both length change of the side and the vibration amplitude of the flexible printed circuit. 
         [0023]      FIG. 5  shows a method for reducing vibration of the flexible printed circuit in accordance with another embodiment of the present invention. In this embodiment, the multi-layer ceramic capacitor  520 , which is fixed on the flexible printed circuit  510  by the bonding pads  550 , has a side of length L which includes two electrodes  540  of length l 1  and a ceramic plate  530  of length l 2 . According to the physical properties of piezoelectric effect, the length change of the ceramic plate  530  caused by the piezoelectric effect is proportional to the length l 2  of the ceramic plate  530 . Therefore, this embodiment reduces the vibration of the flexible printed circuit  510  by reducing the length of the ceramic plate  530  on the condition that the capacitance of the multi-layer ceramic capacitor  520  keeps unchanging. The manner of reducing the length of the ceramic plate  530  is not limited by the present invention, for example, the length of the electrode  540  can be increased without changing the side length of the capacitor  520 . For example, as the multi-layer ceramic capacitor  520 ′ shown in  FIG. 5 , the length of the ceramic plate  530 ′ can be reduced to l 2 ′ (&lt;l 2 ) by increasing the length of the electrode  540 ′ to l 1 ′ (&gt;l 1 ) and keeping the side length of the multi-layer ceramic capacitor  520 ′ the same. In another embodiment, the size of the whole capacitor can be reduced in an equal proportion, as the multi-layer ceramic capacitor  520 ″ shown in  FIG. 5 . The lengths of the electrode  540 ″ and the ceramic plate  530 ″ are reduced to l 1 ″ (&lt;l 1 ) and l 2 ″ (&lt;l 2 ) respectively in equal proportion, so the side length of the multi-layer ceramic capacitor  520 ″ is reduced to L″(&lt;L) accordingly. In a word, this embodiment lessens longitudinal deformation of the ceramic plate by reducing the length of the ceramic plate, and then further lowers the vibration amplitude of the flexible printed circuit. 
         [0024]      FIG. 6  shows a method for reducing vibration of the flexible printed circuit in accordance with still another embodiment of the present invention. In this embodiment, a buffer pad  660  is disposed between the flexible printed circuit  610  and the multi-layer ceramic capacitor  620  for absorbing vibration. The buffer pad  660  can absorb energy of vibration so that the vibration amplitude of the flexible printed circuit  610  can be reduced. Furthermore, by adding the buffer pad  660 , an additional contact point between the flexible printed circuit  610  and the multi-layer ceramic capacitor  620  is introduced. Therefore, referring to  FIG. 6 , the vibration of the flexible printed circuit  610  is now caused by a longitudinal deformation in a length L/2. Because the magnitude of the deformation is approximately proportional to the length, the deformation in length L/2 is half of that in length L and the vibration amplitude of the flexible printed circuit  610  is decreased accordingly. 
         [0025]    In some embodiments, not only the vibration of the flexible printed circuit caused by the multi-layer ceramic capacitor but also any other vibrations of the flexible printed circuit caused by piezoelectric effect can be reduced. 
         [0026]      FIG. 7  is a flowchart of an illustrative method for reducing audible noise generated by vibration of a flexible printed circuit in accordance with one embodiment of the present invention. First, in step S 700 , vibration of the flexible printed circuit is determined to be caused by a piezoelectric element located on the flexible printed circuit. A longitudinal deformation in a side of the piezoelectric element is generated when an alternating voltage is applied to the piezoelectric element. Next, in step S 710 , an angle between the side of the piezoelectric element and an axis is adjusted to reduce a component of the longitudinal deformation along a direction perpendicular to the axis, wherein the flexible printed circuit is bent with respect to the axis. In step S 720 , an area of a cross-section which is perpendicular to the side is increased for lowering the longitudinal deformation of the piezoelectric element. In step S 730 , a length of the side of the piezoelectric element is reduced for lowering the longitudinal deformation of the piezoelectric element. In step S 740 , a buffer pad is disposed between the flexible printed circuit and the piezoelectric element to absorb vibration for lowering the longitudinal deformation of the piezoelectric element. It should be understood and appreciated that the present invention is not limited by the order of step S 710  to step S 740 , and some steps can even be skipped if not required. For example, if the audio noise generated by the flexible printed circuit is eliminated after performing the step S 720 , then the steps S 710 , S 730 , and S 740  can be skipped. 
         [0027]    While this invention has been described with reference to the illustrative embodiments, these descriptions should not be construed in a limiting sense. Various modifications of the illustrative embodiment, as well as other embodiments of the invention, will be apparent upon reference to these descriptions. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as falling within the true scope of the invention and its legal equivalents.