Patent Publication Number: US-11657944-B2

Title: Flexible printed circuit board and image stabilization module

Description:
TECHNICAL FIELD 
     The disclosures herein relate to flexible printed circuit boards and image stabilization modules. 
     The present application is based on and claims priority to Japanese patent application No. 2020-059088 filed on Mar. 27, 2020, and the entire contents of the Japanese patent application are hereby incorporated by reference. 
     BACKGROUND ART 
     Printed circuit boards are widely used to form circuits for various electronic devices. Patent Document 1, for example, discloses a camera image stabilization apparatus utilizing a printed circuit board on which two planar coils (X-direction drive coil and Y direction drive coil) having an oblong shape in a plan view are arranged perpendicularly to each other (see Japanese Laid-open Patent Publication No. 2006-67562). 
     RELATED-ART DOCUMENTS 
     Patent Document 
     
         
         [Patent Document 1] Japanese Laid-open Patent Publication No. 2006-67562 
       
    
     SUMMARY OF THE INVENTION 
     A flexible printed circuit board according to one aspect of the present disclosures includes a base film having an insulating property and a planar coil disposed on a surface of the base film, wherein a number of turns on an inside of a center point of a coil width of the planar coil in a plan view is greater than a number of turns on an outside of the center point. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic cross-sectional view illustrating an example of a camera image stabilization mechanism provided with an image stabilization module according to one embodiment of the present disclosures. 
         FIG.  2    is a schematic axonometric view illustrating the image stabilization module according to the embodiment of the present disclosures. 
         FIG.  3    is a schematic axonometric view illustrating a flexible printed circuit board according to the embodiment of the present disclosures. 
         FIG.  4    is a cross-sectional view taken along the line A-A in the flexible printed circuit board of  FIG.  3   . 
         FIG.  5    is a cross-sectional view of a flexible printed circuit board according to a different embodiment. 
         FIG.  6    is a cross-sectional view of a flexible printed circuit board according to a different embodiment. 
         FIG.  7    is a cross-sectional view of a flexible printed circuit board according to a different embodiment. 
         FIG.  8    is a cross-sectional view of a flexible printed circuit board according to a different embodiment. 
         FIG.  9    is a cross-sectional view of a flexible printed circuit board according to a different embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Problem to be Solved by the Present Disclosures 
     The related-art technology described above uses a method of obtaining information about the position of a coil plate by detecting a change in magnetic flux in the proximity of a border between the N pole and the S pole of a permanent magnet. For use in a camera image stabilization module, another method has also been studied that performs image stabilization by using a magnetic force exerted by a coil to drive an imaging device or a correction lens in the direction in which camera shake is canceled. In the case of the method noted above, however, the inventors of the present invention have found that when a coil winding density is constant in the coil, the magnetic force (Lorentz force) exerted by the coil varies depending on the positional relationship between a magnet and the coil upon a movement of the magnet. Improving the accuracy of image stabilization by a camera image stabilization module requires reducing the variation of Lorentz force occurring upon movement of a magnet. 
     The present disclosures are made in consideration of the above-noted circumstances, and are directed to providing a flexible printed circuit board that can reduce the variation of Lorentz force occurring upon movement of a magnet of an image stabilization module. 
     Advantage of the Present Disclosures 
     According to the flexible printed circuit board of the present disclosures, the variation of Lorentz force occurring upon movement of a magnet of an image stabilization module is reduced. 
     Description of Embodiments of the Present Disclosures 
     Embodiments of the present disclosures will be listed and described first. 
     A flexible printed circuit board according to one aspect of the present disclosures includes a base film having an insulating property, and a planar coil disposed on a surface of the base film, wherein a number of turns on an inside of a center point of a coil width of the planar coil in a plan view is greater than a number of turns on an outside of the center point. 
     In the case of a coil winding density being constant, the Lorentz force observed upon displacement of a magnet will be less than the Lorentz force observed when the magnet is positioned at the center of the coil. This is because the Lorentz force exerted by the coil is generated according to interaction between the coil winding density and the density of magnetic flux generated by the magnet. Accordingly, the density of turns is set relatively high in the peripheral area of the center portion of the coil where Lorentz force by the coil is easy to occur, while the density of turns is set relatively low in the outer side of the coil. This arrangement is found, by the inventors, to improve the Lorentz force generated by the coil and to reduce the variation of Lorentz force occurring upon displacement of the magnet, as long as in the peripheral area of the center portion of the coil. In the noted flexible printed circuit board, the planar coil disposed on the surface of the base film is configured such that the number of turns inside the center point of a coil width of the planar coil in a plan view is greater than the number of turns outside the center point. As a result, the noted flexible printed circuit board can reduce the variation of Lorentz force occurring upon movement of a magnet in the peripheral area of the center portion of the coil provided in an image stabilization module. The flexible printed circuit board thus provides highly accurate image stabilization when the range of shake stays within the predetermined peripheral area of the center portion because a stable Lorentz force is obtained regardless of a displacement of the magnet. Further, the flexible printed circuit board can improve its maximum Lorentz force, and can thus be used for a high-performance weighty optical configuration having a plurality of lenses and the like. Moreover, Lorentz force can be maintained by a relatively small electric current, compared with a conventional optical configuration having the same weight, which enables the reduction of power consumption. In the present application, the term “Lorentz force” means a drive force exerted in a straight-line direction based on a magnetic force. 
     The noted flexible printed circuit board is preferably such that the proportion of the number of turns on the inside in a total number of turns is greater than or equal to 55%. The fact that the proportion of the number of turns on the inside in a total number of turns is greater than or equal to 55% makes it possible to reduce the variation of Lorentz force occurring upon a movement of the magnet in the image stabilization module. 
     The average line width of the turns is preferably greater than or equal to 10 μm and less than or equal to 125 μm. The fact that the average line width of each turn is within the above-noted range allows the turns to be easily formed, and, also, makes it easy to set the thickness and density of turns within a desired range. 
     The ratio of the average line width of the turns on the outside to the average line width of the turns on the inside is preferably greater than or equal to 1 and less than or equal to 5. The fact that the ratio of the average line width of the turns on the outside to the average line width of the turns on the inside is within the above-noted range allows the turns to be easily formed, and, also, makes it easy to set the thickness and density of turns within a desired range. 
     The average intervals between adjacent turns among the turns on the inside are preferably equal. The fact that the average intervals between adjacent turns among the turns on the inside are equal makes it easy to form the planar coil. 
     The average intervals between adjacent turns among the turns on the inside preferably are different. The fact that the average intervals between adjacent turns among the turns on the inside are different makes it easy to adjust the density of the turns on the inside. 
     An image stabilization module according to another aspect of the present disclosures includes the noted flexible printed circuit board and a permanent magnet. 
     The image stabilization module is provided with the noted flexible printed circuit board, so that the variation of Lorentz force occurring upon a movement of a magnet of the image stabilization module is reduced. The image stabilization module can thus improve the accuracy of camera image stabilization. 
     Details of Embodiments of the Present Disclosures 
     In the following, embodiments of the flexible printed circuit board and the image stabilization module according to the present disclosures will be described with reference to the drawings. With respect to the “front/back” side of the flexible printed circuit board of the present embodiment, the side of the flexible printed circuit board in the thickness direction where a correction lens is disposed is designated as the “front” side, and the opposite side of the flexible printed circuit board from where the correction lens is disposed is designated as the “back” side. The “front/back” side of the flexible printed circuit board as determined by conditions during use is not intended. 
     [Camera Image Stabilization Mechanism] 
       FIG.  1    is a schematic cross-sectional view illustrating an example of a camera image stabilization mechanism provided with an image stabilization module according to one embodiment of the present disclosures.  FIG.  2    is a schematic axonometric view illustrating the image stabilization module according to the embodiment of the present disclosures.  FIG.  3    is a schematic axonometric view illustrating a flexible printed circuit board according to the embodiment of the present disclosures. An image stabilization mechanism  60  embedded in digital cameras, smartphones, and the like has an image stabilization module  70  that is arranged perpendicularly to an optical axis between an unshown optical system and an imaging device  80 . The image stabilization module  70  is provided with a flexible printed circuit board  1  according to the embodiment of the present disclosures, and correct the blurring of an image of a photography subject resulting from camera shake. In the image stabilization mechanism  60 , the imaging device  80  implemented as an image sensor such as a CCD (charge-coupled device) is secured on a substrate  82  to face a correction lens  7 . The imaging device  80  takes and converts the image of a photography subject created by the optical system into electrical signals. 
     As illustrated in  FIG.  3   , planar coils  31 ,  32 ,  33 , and  34  are laminated on the front surface of a base film  2 , and unshown planar coils  41 ,  42 ,  43 , and  44  are laminated on the back surface of the base film  2 . The substrate  82  of  FIG.  1    has an insulating layer (not shown) laminated thereon to prevent damage and short-circuit caused by contact of the planar coils  31 ,  32 ,  33 ,  34  and the planar coils  41 ,  42 ,  43 ,  44  with other members or the like. The insulating layer preferably covers all the exterior surface of the substrate  82 , except for openings and cutouts that are intentionally provided. The insulating layer may be formed by use of a solder resist, a coverlay, or the like, and is preferably formed by curing a photosensitive dry film in particular. 
     &lt;Image Stabilization Module&gt; 
     The image stabilization module  70  includes the flexible printed circuit board  1  and permanent magnets  51 ,  52 ,  53 , and  54 . 
     The four permanent magnets  51 ,  52 ,  53 , and  54  are disposed around a magnet holder  6 . The magnet holder  6  supports the correction lens  7  at the center thereof. The permanent magnets  51 ,  52 ,  53 , and  54  are spaced apart from each other in directions perpendicular to the optical axis of the unshown optical system. Also, the permanent magnets  51  and  52  are disposed in an opposing fashion, and so are the permanent magnets  53  and  54 . The permanent magnets  51 ,  52 ,  53 , and  54  are magnetized with the N pole and the S pole aligned in the directions perpendicular to the optical axis direction. In the permanent magnet  51  and the permanent magnet  52  of  FIG.  1   , the magnetic pole  92  of the permanent magnet  51  and the magnetic pole  93  of the permanent magnet  52  facing each other are the same. For example, the magnetic pole  92  and the magnetic pole  93  may be the N pole, in which case the magnetic pole  91  of the permanent magnet  51  and the magnetic pole  94  of the permanent magnet  52  are the S pole. Each strand of the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  is wound in a spiral shape in a plane parallel to the longitudinal directions of the permanent magnets  51 ,  52 ,  53 ,  54 . The planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  are disposed in an opposing relationship to the permanent magnets  51 ,  52 ,  53 ,  54 , respectively, in a non-contact manner. The planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  consistently stay within the magnetic fields of the four permanent magnets  51 ,  52 ,  53 ,  54 , respectively. 
     Applying current to the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  causes interaction to occur between the magnetic fields of the permanent magnets  51 ,  52 ,  53 , and  54  and the magnetic fields created by the currents flowing through the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44 . For example, applying current to the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  causes an electromagnetic force, i.e., a Lorentz force, to be generated by the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  in accordance with Fleming&#39;s left-hand rule. As a result, the permanent magnets  51 ,  52 ,  53 , and  54  drive the correction lens  7  at the center of the magnet holder  6 . The image stabilization module  70  uses the magnetic force exerted by the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  to drive the correction lens  7  in the direction in which camera shake is canceled, thereby performing image stabilization. In this manner, the image stabilization module  70  is provided with the noted flexible printed circuit board  1 , so that the variation of Lorentz force occurring upon movement of the permanent magnets  51 ,  52 ,  53 , and  54  of the image stabilization module  70  is reduced. The image stabilization module  70  can thus improve the accuracy of camera image stabilization. 
     &lt;Flexible Printed Circuit Board&gt; 
     The flexible printed circuit board  1  illustrated in  FIG.  3    includes the flexible, insulating base film  2 , and includes the planar coils  31 ,  32 ,  33 , and  34 , the planar coils  41 ,  42 ,  43 , and  44 , and the like provided on the front surface of the base film  2 . 
     (Base Film) 
     The base film  2  is comprised mainly of synthetic resin, and is flexible and electrically insulating. The base film  2  is a substrate layer on which the planar coils  31 ,  32 ,  33 , and  34 , the planar coils  41 ,  42 ,  43 , and  44 , and the like are formed. The term “comprised mainly of” refers to a component accounting for the highest content on a weight basis, and may refer to a component with a content of 50 wt % or more, for example. The base film  2  has a square cutout  9 , and has a square ring shape. 
     Examples of the above-noted synthetic resin include polyimide, polyethylene terephthalate, liquid crystal polymer, fluorine resin, and the like. 
     The lower limit of the average thickness of the base film  2  is preferably 5 μm and more preferably 10 μm. The upper limit of the average thickness of the base film  2  is preferably 50 μm and more preferably 40 μm. Use of an average thickness of the base film  2  that does not reach the lower limit may create a risk that the insulation strength of the base film  2  is insufficient. Use of an average thickness of the base film  2  that exceeds the upper limit may create a risk that the flexible printed circuit board  1  is unnecessarily thick, or is not sufficiently flexible. In the instant specification, the average thickness refers to the average value of thicknesses measured at any 10 selected points. 
     (Planar Coil) 
     As illustrated in  FIG.  3   , the base film  2  having a square ring shape has four pairs of planar coils, i.e., the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44 , laminated on the front surface and the back surface along the respective edges. The planar coils may alternatively be laminated only on the front surface of the base film  2 . 
     The planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  are formed by patterning a conductive material. A metal such as copper, nickel, and silver is preferred as the material for forming the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44 . Among these, copper, which is relatively inexpensive and has low electrical resistance, is particularly preferable. 
     The four planar coils  31 ,  32 ,  33 , and  34  are fixedly attached to the front side of the base film  2 , and the unshown four planar coils  41 ,  42 ,  43 , and  44  are fixedly attached to the back side of the base film  2 . The permanent magnets  51  and  52  are reciprocally movable in the X direction relative to the planar coils  31 ,  32 ,  41 , and  42 , and the permanent magnets  53  and  54  are reciprocally movable in the Y direction relative to the planar coils  33 ,  34 ,  43 , and  44 . As described above, image stabilization corrects camera shake by causing the correction lens  7  to displace the image of a photography subject imaged by the optical system in the X and Y directions on the imaging surface of the imaging device  80  and by causing this displacement to match camera shake occurring during the image capturing, i.e., with vibration on the imaging surface, thereby to cancel the displacement. 
       FIG.  4    is a cross-sectional view taken along the line A-A in the flexible printed circuit board  1  of  FIG.  3   . The planar coils  31  and  41  are such that the number of turns  31   b  and  41   b  on the inside of a center point P of a coil width W of the planar coil in a plan view is greater than the number of turns  31   a  and  41   a  on the outside. As a result, the noted flexible printed circuit board  1  can reduce the variation of Lorentz force occurring upon movement of the permanent magnets  51 ,  52 ,  53 , and  54  in the peripheral area of the center portion of the coil provided in the image stabilization module  70 . Accordingly, the flexible printed circuit board provides highly accurate image stabilization when the range of shake stays within the predetermined peripheral area of the center portion because a stable Lorentz force is obtained regardless of a displacement of the magnet. Further, the flexible printed circuit board can improve its maximum Lorentz force, and can thus be used for a high-performance weighty optical configuration having a plurality of lenses and the like. Moreover, Lorentz force can be maintained by a relatively small electric current, compared with a conventional optical configuration having the same weight, which enables the reduction of power consumption. 
     In the flexible printed circuit board  1 , the lower limit of the proportion of the number of turns on the inside to the total number of turns is preferably 55%, and more preferably 60%. The fact that the proportion of the number of turns on the inside in the total number of turns is within the noted range makes it possible to reduce the variation of Lorentz force occurring upon a movement of the magnet in the image stabilization module. 
     The upper limit of the average thickness of the turns constituting the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  is preferably 100 μm, and more preferably 80 μm. Use of an average thickness exceeding the upper limit may create a risk of going against the demand for reduction in the thickness of the flexible printed circuit board  1 . The lower limit of the average thickness of the turns is preferably 10 μm, and more preferably 20 μm. Use of an average thickness failing to reach the lower limit may create a risk of bringing about a large electrical resistance. 
     The average intervals between adjacent turns among the turns on the inside are preferably equal. The fact that the average intervals between adjacent turns among the turns on the inside are equal makes it easy to form the planar coil. Further, as illustrated with respect to a flexible printed circuit board  20  of  FIG.  5   , when the average intervals between adjacent turns among turns  231   b  and  241   b  on the inside are the same, the average intervals between adjacent turns among turns  231   a  and  241   a  on the outside do not have to be the same. In the flexible printed circuit board  20  also, the planar coils  231  and  241  are such that the number of turns  231   b  and  241   b  on the inside of a center point P of a coil width W is greater than the number of turns  231   a  and  241   a  on the outside. 
     The lower limit of the average intervals between adjacent turns is preferably 5 μm, and more preferably 8 μm. Further, the upper limit of the average intervals is preferably 120 μm, and more preferably 110 μm. Use of average intervals failing to reach the lower limit may create a risk of making it difficult to form the turns. Conversely, use of average intervals exceeding the upper limit may create a risk of making it difficult to provide a desired winding density. It may be noted that the average interval refers to the average value of intervals between any 3 relevant points. 
     The lower limit of the average line width of the turns constituting the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44  is preferably 10 μm, and more preferably 20 μm. Further, the upper limit of the average line width of the turns is preferably 125 μm, and more preferably 115 μm. Use of an average line width of the turns failing to reach the lower limit may create a risk of making it difficult to form the turns. Also, use of an average line width of the turns failing to reach the lower limit may create a risk of failing to provide the turns with sufficient thickness (i.e., the height in the thickness direction of the base film  2 ). Conversely, use of an average line width of the turns exceeding the upper limit may create a risk of making it difficult to provide a desired winding density. It may be noted that the average line width refers to the average value of widths at any 3 relevant points. 
     The lower limit of the ratio of the average line width of the turns on the outside to the average line width of the turns on the inside is preferably 1.0, and more preferably 1.2. Further, the upper limit of the ratio of average line widths is preferably 5.0, and more preferably 4.0. Use of the ratio of average line widths less than 1 may create a risk of bringing about an increase in the resistance of the planar coils  31 ,  32 ,  33 , and  34  and the planar coils  41 ,  42 ,  43 , and  44 . Conversely, use of the ratio of average line widths exceeding 5 may create a risk of making it difficult to provide a desired winding density. 
     [Method of Making Flexible Printed Circuit Board] 
     The method of manufacturing the noted flexible printed circuit board includes laminating planar coils on at least one surface of an insulating base film. In the step of laminating planar coils, coil patterns are laminated on at least one surface of the base film. Any known methods may be used as the method of laminating coil patterns, and a subtractive method, a semi-additive method, or the like may be used. Use of the semi-additive method in particular allows coil patterns with high winding density to be efficiently obtained. 
     Other Embodiments 
     The embodiments disclosed herein should be regarded as examples only and as non-limiting in all aspects. The scope of the present disclosures is defined by the claims without being limited to the configurations of the disclosed embodiments, and is intended to include all modifications within the spirit and equivalents of the scope of the claims. 
     In the above-described embodiment, the average intervals between adjacent turns among the turns on the inside are the same. Alternatively, the average intervals between adjacent turns among the turns on the inside may be different as in a flexible printed circuit board  10  of  FIG.  6   . The fact that the average intervals between adjacent turns among the turns on the inside are different makes it easy to adjust the density of the turns on the inside. The flexible printed circuit board  10  is such that the average intervals between adjacent turns among turns  131   b  and  141   b  on the inside are different. In the flexible printed circuit board  10  also, the planar coils  131  and  141  are such that the number of turns  131   b  and  141   b  on the inside of a center point P of a coil width W is greater than the number of turns  131   a  and  141   a  on the outside. 
     In the above-described embodiment, the average line width of turns on the inside and the average line width of turns on the outside are the same. Alternatively, the average line width of turns on the inside and the average line width of turns on the outside may be different as in a flexible printed circuit board  150  of  FIG.  7   . The fact that the average line width of turns on the inside and the average line width of turns on the outside are different allows an increase in resistance to be reduced. The flexible printed circuit board  150  is such that the average intervals between adjacent turns among turns  331   b  and  341   b  on the inside are different. The planar coils  331  and  341  of the flexible printed circuit board  150  are such that the number of turns  331   b  and  341   b  on the inside of a center point P of a coil width W is greater than the number of turns  331   a  and  341   a  on the outside. Further, the ratio of the average line width of the turns  331   a  and  341   a  on the outside to the average line width of the turns  331   b  and  341   b  on the inside is greater than or equal to 1 and less than or equal to 5. 
     Further, as in a flexible printed circuit board  170  of  FIG.  8   , the average line width of turns on the inside and the average line width of turns on the outside may be different, and, also, the average intervals between adjacent turns within the same area may be different. In the flexible printed circuit board  170 , the average line width of turns  531   b  and  541   b  on the inside and the average line width of turns  531   a  and  541   a  on the outside are different, and also, the average intervals between adjacent turns are different within the inside area and different within the outside area. The planar coils  531  and  541  of the flexible printed circuit board  170  are such that the number of turns  531   b  and  541   b  on the inside of a center point P of a coil width W is greater than the number of turns  531   a  and  541   a  on the outside. Further, the ratio of the average line width of the turns  531   a  and  541   a  on the outside to the average line width of the turns  531   b  and  541   b  on the inside is greater than or equal to 1 and less than or equal to 5. 
     Further, as in a flexible printed circuit board  160  of  FIG.  9   , the average line width of adjacent turns within the same area may be different. The planar coils  431  and  441  of the flexible printed circuit board  160  are such that the number of turns  431   b  and  441   b  on the inside of a center point P of a coil width W is greater than the number of turns  431   a  and  441   a  on the outside. Further, the ratio of the average line width of the turns  431   a  and  441   a  on the outside to the average line width of the turns  431   b  and  441   b  on the inside is greater than or equal to 1 and less than or equal to 5. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
           1 ,  10 ,  20 ,  150 ,  160 ,  170  flexible printed circuit board 
           2  base film 
           6  magnet holder 
           7  correction lens 
           9  cutout 
           31 ,  32 ,  33 ,  34 ,  41 ,  42 ,  43 ,  44 ,  131 ,  141 ,  231 ,  241 ,  331 ,  341 ,  431 ,  441 ,  531 ,  541  planar coil 
           31   a ,  31   b ,  41   a ,  41   b ,  131   a ,  131   b ,  141   a ,  141   b ,  231   a ,  231   b ,  241   a ,  241   b ,  331   a ,  331   b ,  341   a ,  341   b ,  431   a ,  431   b ,  441   a ,  441   b ,  531   a ,  531   b ,  541   a ,  541   b  turns 
           51 ,  52 ,  53 ,  54  permanent magnet 
           60  image stabilization mechanism 
           70  image stabilization module 
           80  imaging device 
           82  substrate 
           91 ,  92 ,  93 ,  94  magnetic pole 
         P center point 
         W coil width