Patent Publication Number: US-2007097552-A1

Title: Hard disk drive and flexible printed circuit ribbon thereof

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a hard disk drive. More particularly, the present invention relates to the actuator of a hard disk drive for moving a read/write head to a desired position over a disk of the drive.  
      2. Description of the Related Art  
      A hard disk drive (HDD) reproduces data from a disk or records data on the disk using a read/write head. To this end, the read/write head is moved by means of an actuator to a desired position above a recording surface of the disk while the disk is rotated.  FIG. 1  is a perspective view of an essential portion of an HDD having a conventional actuator. Referring to  FIG. 1 , the HDD includes a data storage disk  50 , a spindle motor  55  for rotating the disk  50  at a constant angular speed (Ω), a read/write head  38 , and an actuator  30  for moving the read/write head  38  to a desired position over the disk  50 . The actuator  30  includes a swing arm  32  rotatably supported by a pivot  31 , a suspension  35  installed on a leading end of the swing arm  32  for supporting and elastically biasing the read/write head  38  toward a surface of the disk  50 , and a voice coil motor (VCM) for rotating the swing arm  32 . The VCM includes a VCM coil  41  disposed on a rear end of the swing arm  32 , and magnets  75  disposed above (not shown) and below the VCM coil  41  so as to face the VCM coil  41 . The VCM coil  41  is coupled to a coil support  45  provided on the rear end of the swing arm  32 . Reference numeral  71  denotes a yoke supporting the magnet  75  disposed below the VCM coil  41 . The VCM rotates the swing arm  32 , in a direction according to Fleming&#39;s left-hand rule, due to the interaction of the magnetic field induced by the current flowing through the VCM coil  41  and the magnetic field formed by the magnets  75 .  
      In operation, when the HDD is powered on and the disk  50  is rotated, the VCM rotates the swing arm  32  counterclockwise, for example, to move the read/write head  38  to a position above a recording surface of the disk  50 . The read/write head  38 , which is loaded above the disk  50  in this way, is maintained a predetermined distance from the surface of the disk  50  by a lift force generated by the rotation of the disk  50 . In this state, the read/write head  38  records data on the recording surface of the disk  50  or reproduces data from the recording surface of the disk  50 , as it traces a particular track (T) of the disk  50 .  
      On the other hand, when the HDD is powered off and the disk  50  is not rotated, the VCM rotates the swing arm  32  in the opposite direction, e.g., clockwise. Accordingly, the read/write head  38  is unloaded from the recording surface of the disk  50  and parked on a ramp  60  disposed radially outwardly of the disk  50 . In this unloading operation, an end (tab)  39  of the suspension  35  slides along the ramp  60  to a safe position, and then rests on a supporting surface of the ramp  60 .  
      In addition, a flexible printed circuit ribbon  20  is connected to one side of the swing arm  32  of the actuator  30  to supply power to the actuator  30  and to send/receive electrical signals to/from the actuator  30 . Specifically, one end of the flexible printed circuit ribbon  20  is connected to and supported by the actuator  30 . The other end of the flexible printed circuit ribbon  20  is connected to and supported by an upright leg  83  of a bracket  80  disposed close to the actuator  30 . The middle of the flexible printed circuit ribbon  20  is thus relatively free to bend.  
      The flexible printed circuit ribbon  20  includes a plurality of conductive line patterns through which different electrical signals are transmitted. For example, the conductive line patterns include a head signal line pattern through which electrical signals are sent to and received from the read/write head  38 , a ground line pattern for grounding the electronics, and a driving current line pattern through which current is supplied to the VCM. A printed circuit board (not shown) is disposed under the bracket  80 , and is connected with the conductive line patterns of the flexible printed circuit ribbon  20  through the bracket  80 .  
      Generally, the flexible printed circuit ribbon  20  has a length sufficient to allow the swing arm  32  to rotate without disturbing the swing arm  32 . That is, the flexible printed circuit ribbon  20  bends as the swing arm  32  rotates in one direction, e.g., in the clockwise direction, and is extended as the swing arm  32  rotates in the opposite (e.g., counterclockwise) direction. Nonetheless, a restoring force is exerted on the swing arm  32  by the flexible printed circuit ribbon  20  when the flexible printed circuit ribbon  20  is bent, due to the elasticity of the flexible printed circuit ribbon  20 . On the other hand, a tensile force is exerted by the flexible printed circuit ribbon  20  on the swing arm  32  when the flexible printed circuit ribbon  20  is extended.  
      Accordingly, these so-called bias forces, which are exerted on the swing arm  32  by the flexible printed circuit ribbon  20 , act to resist the rotation of the swing arm  32  when the swing arm  32  is loaded and unloaded. Therefore, the VCM has to exert a driving force on the swing arm  32  that is sufficient to overcome the bias forces produced by the flexible printed circuit ribbon  20 . In particular, the driving current supplied to the VCM coil  41  must be great enough to provide the required dynamic characteristics of the actuator  30  such as the ability of the actuator to provide a rapid response. Thus, the conventional actuator  30  consumes a relatively great amount of power and operates at a correspondingly low efficiency.  
      Furthermore, the flexible printed circuit ribbon  20  is preferably assembled in an upright position perpendicular to a base of the housing of the HDD. However, the side of the actuator  30  to which an end of the flexible printed circuit ribbon  20  is connected or the leg  83  of the bracket  80  to which the other end of the flexible printed circuit ribbon  20  is connected is not perpendicular to the base, due to manufacturing tolerances, for example. Similarly, one or the other end of the flexible printed circuit  20  ribbon is not fitted tightly against the actuator  30  or the leg  83  of the bracket  80 . Therefore, in any of these cases, the flexible printed circuit ribbon  20  assumes a twisted disposition.  
       FIG. 2  is a plan view of the flexible printed circuit ribbon  20  when it is twisted. The twist of the flexible printed circuit ribbon  20  negates the ideal relationship between the magnitude of the driving current supplied to the VCM and the angle over which the swing arm  32  is rotated by the VCM. That is, the read/write head  38  is not be precisely moved to a target track of the disk  50  when the flexible printed circuit ribbon  20  when it is twisted. Consequently, there is a time delay in moving the read/write head  38  to the target track of the disk  50 .  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a hard disk drive having an actuator that can be driven with a high degree of efficiency.  
      Another object of the present invention is to provide a hard disk drive which offers improved data seeking performance.  
      Still another object of the present invention is to provide a flexible printed circuit ribbon whose flexibility is enhanced.  
      According to an aspect of the present invention, there is provided a hard disk drive having at least one data storage disk, a spindle motor to which the disk is mounted, an actuator including a read/write head and an arm for moving the read/write head over the disk, a support member that is fixed in the hard disk drive, and a flexible printed circuit ribbon having a free bending portion. The flexible printed circuit ribbon is connected to the arm so as to move therewith and is to the fixed support member. The free bending portion is provided along the length of the ribbon between locations at which the ribbon is connected to the swing arm and the support member. The free bending portion is substantially thinner than other portions of the flexible printed circuit between those locations at which the ribbon is connected to the swing arm and the support member. Accordingly, the free bending portion imparts an increased flexibility to the ribbon to attenuate a force exerted on the arm by the flexible printed circuit ribbon.  
      The flexible printed circuit may include a first supported portion abutting and supported on the arm of actuator, a second supported portion abutting and supported on the fixed support member, and a connecting portion interconnecting the first and second supported portions. The connecting portion is freely suspended between the first and second supported portions so as to be free to bend and extend as the arm moves relative to the fixed support member. The free bending portion constitutes part of the connecting portion.  
      According to another aspect of the present invention, there is provided a flexible printed circuit ribbon including a conductive layer extending along the length of the ribbon, and a first protective layer and a second protective layer disposed over opposite sides of the conductive layer, respectively, and wherein the flexible printed circuit ribbon has first and second end portions to which the conductive layer extends, and a free bending portion located along the length of the ribbon between the first and second end portions. The free bending portion is substantially thinner than other portions of the flexible printed circuit ribbon located between the first and second end portions. Thus, the free bending portion imparts an increased flexibility to the ribbon.  
      According to either of these aspects of the present invention, the conductive layer preferably has substantially the same thickness over the entire length of the flexible printed circuit ribbon. At least one of the first and second protective layers is thinner at the free bending portion than at other portions of the flexible printed circuit ribbon. Alternatively, or in addition, at least one of the first and second protective layers has a discontinuity at the free bending portion. Preferably, the flexible printed circuit ribbon also has a respective bonding layer interposed between at least one of the first and second protective layers and the conductive layer for bonding the conductive layer to the protective layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detail description of the preferred embodiments thereof made with reference to the attached drawings in which:  
       FIG. 1  is a perspective view of essential parts of a conventional hard disk drive (HDD);  
       FIG. 2  is a plan view of a flexible printed circuit ribbon of the conventional HDD of  FIG. 1 ;  
       FIG. 3  is a perspective partially exploded view of an embodiment of an HDD according to the present invention;  
       FIG. 4  is a plan view of the HDD of  FIG. 3 ;  
       FIG. 5  is a perspective view of a main portion of an actuator of the HDD of  FIG. 3 ;  
       FIG. 6  is a plan view of a flexible printed circuit ribbon of the HDD of  FIG. 3  and depicted in  FIG. 5 ;  
       FIG. 7  are sectional views of the flexible printed circuit ribbon as taken along lines A-A′ and B-B′ of  FIG. 6 , respectively;  
       FIGS. 8 and 9  are graphs showing bias force with respect to angle of rotation of a swing arm according to the prior art and the present invention, respectively;  
       FIG. 10  is an explanatory diagram including a plan view of the swing arm of the HDD illustrating the angle of rotation of the actuator of an HDD;  
       FIG. 11  is a longitudinal sectional view of a segment of the flexible printed circuit ribbon depicted in  FIG. 3 ; and  
       FIG. 12  is a longitudinal sectional view of a segment of another form of a flexible printed circuit ribbon according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention will now be described more fully with reference to  FIGS. 3-12 .  
      Referring first to  FIGS. 3 and 4 , the HDD includes a data storage disk  150 , a spindle motor  155  for rotating the data storage disk  150  at a constant speed, a read/write head  138 , and an actuator  130  for moving the read/write head  138  to a desired position over the disk  150 . Although only one disk  150  is shown, a plurality of data storage disks  150  may be mounted to the spindle motor  155 . The spindle motor  155  is mounted to a base  112  of the HDD. The actuator  130  includes an actuator pivot  131  disposed on the base  112 , a swing arm  132 , a suspension  135 , the read/write head  138 , a coil support  145 , and a voice coil motor (VCM). The spindle motor  155  and the actuator  130  are accommodated in a space defined by the base  112  and a cover  111  that are coupled to each other. The base  112  and the cover  111  form a housing that protects the inner components of the HDD from foreign substances in the air outside the housing and muffles the noise produced during operation of the HDD.  
      The swing arm  132  is rotatably supported by the actuator pivot  131 , i.e., is rotatable about a central longitudinal axis of the pivot  131 . The suspension  135  is coupled to a leading end of the swing arm  132  to support the read/write head  138  and bias the read/write head  138  toward a surface of the disk  150 . The coil support  145  is provided on a rear end of the swing arm  132 .  
      The VCM drives the swing arm  132 . To this end, the VCM includes a VCM coil  141  wound around the coil support  145 , and magnets  175  disposed above and below the VCM coil  141  as facing the VCM coil  141 . The magnets  175  are supported by a yoke  171 . The VCM rotates the swing arm  132 , in a direction according to Fleming&#39;s left-hand rule, due to the interaction between a magnetic field induced by the flow of current through the VCM coil  141  and the magnetic field formed by magnets  175 .  
      When the HDD is powered on and the disk  150  is rotated, the VCM rotates the swing arm  132  in a predetermined direction (e.g., counterclockwise) to load the read/write head  138  on a recording surface of the disk  150 . The loaded read/write head  138  is maintained a predetermined distance from the recording surface of the disk  150  by a lift force generated by the rotation of the disk  150 . In this state, the read/write head  138  records data onto the recording surface of the disk or reproduces data from the recording surface of the disk  150  as it traces a particular track of the disk  150 . Note, the recording surface of the disk  150  refers to that portion of the surface of the disk where data can be effectively stored. Generally, the recording surface of the disk  150  does not occupy the entire surface of the disk  150 . Rather, the recording surface of the disk  150  only occupies a portion of the entire surface of the disk  150 . That is, an inner peripheral or central portion of the disk  150  is allocated for coupling with the spindle motor  155 , and an outer peripheral portion of the disk  150  is allocated for the parking of the read/write head  138 . Therefore, the recording surface of the disk  150  is delimited between inner and outer regions of the disk  150 , i.e., the recording surface of the disk  150  is delimited between two concentric circles ID and OD having diameters greater than the inner diameter of the disk and less than the outer diameter of the disk, respectively.  
      The HDD also includes a flexible printed circuit ribbon  120  connected to one side of the swing arm  132 . The flexible printed circuit ribbon  120  receives driving power and electric signals from a circuit board disposed beneath the base  112 , and transmits the driving power and electric signals to the actuator  130  for controlling the loading/unloading operations. In this respect, a fixed support member, such as a bracket  180  mounted to the base  112 , connects the flexible printed circuit ribbon  120  to the circuit board. Furthermore, electric signals can be transmitted to or received from the read/write head  138  through the flexible printed circuit ribbon  120 , such that data can be recorded on the disk  150  or read from the disk  150 . To these ends, the flexible printed circuit ribbon  120  includes a conductive pattern, e.g., in the form of a plurality of conductive lines, that transmits the electrical signals. For example, the ribbon  120  includes a conductive signal line for transmitting/receiving electric signals to/from the read/write head  138 , a ground line for grounding the electronic circuitry of the HDD, and a conductive driving current line for transmitting driving current to the VCM.  
      Referring to  FIG. 5 , the flexible printed circuit ribbon  120  includes a first supported portion  121  abutting the actuator  130 , a second supported portion  122  abutting the bracket  180 , and a connecting portion  125  extending freely between the first and second supported portions  121  and  122  so as to be relatively free to bend. The first supported portion  121  of the flexible printed circuit ribbon  120  is coupled to the actuator  130 . For example, the first supported portion  121  of the flexible printed circuit ribbon  120  is soldered to one side of the swing arm  132 . Also, the actuator  130  may include a pressing piece  133  extending from and obliquely to the side surface of the actuator to which the first supported portion  121  of the flexible printed circuit ribbon is coupled. In particular, the pressing piece  133  subtends and acute angle with the side surface of the swing arm  132 . The first supported portion  121  is bent between the pressing piece  133  and the side of the actuator  130  and is thereby urged against the actuator  130  by the elasticity of the flexible printed circuit ribbon  120 . The second supported portion  122  of the flexible printed circuit ribbon  120  is fixed to an upper portion (leg) of the bracket  180  by a screw, for example. To this, end the second supported portion  122  may have a through hole through which the screw extends. Also, the bracket  180  may have a central opening  180 ′ extending therethrough for facilitating the connection between the second supported portion  122  and the circuit board (not shown) which is disposed under the bracket  180 .  
      As mentioned above, the connecting portion  125  of the flexible printed circuit ribbon  120  is free to bend and extend. According to the present invention, the connecting portion  125  has enhanced flexibility to minimize the force exerted on the actuator  130  by the flexible printed circuit ribbon  120  especially when the ribbon is bending or unbending. More specifically, the connecting portion  125  has a free bending portion  125   a  that is substantially thinner than other portions of the flexible printed circuit ribbon. The term “substantially thinner” is used to exclude normal variations that may occur as the result of the manufacturing process, and connotes an ability of the free bending portion  125   a  to impart a measurably greater flexibility to the printed circuit ribbon than the ribbon would otherwise have without the free bending portion. In particular, the free bending portion  125   a  imparts a flexibility that has a discernible affect on the bias force exerted by the ribbon on the wing arm  132  as will be described in more detail later on.  
      The free bending portion  125   a  extends over a predetermined length of the connecting portion  125 . Thus, the free bending portion  125   a  is more flexible than the other portions of the flexible printed circuit ribbon  120  and consequently, is the first portion of the flexible printed circuit ribbon  120  to bend between the actuator  130  and the bracket  180 . The ratio of the over all length of the connecting portion  125  to the length of just the free bending portion  125   a , as well as the location of the free bending portion  125   a  within the connecting portion  125 , may be based on specific design specifications, for example, the relative positions of the actuator  130  and the bracket  180  and the distance between the actuator  130  and the bracket  180 . In general, the extent and location of the free bending portion  125   a  are determined to correspond to the location and length of a particular portion of the flexible printed circuit ribbon  120  where the flexible printed circuit ribbon  120  would experience the most bending if it had a uniform thickness as in the prior art.  
      Consequently, the flexible printed circuit ribbon  120  exerts very little restoring force even when it is twisted when assembled to the actuator  130  and the bracket  80 . Accordingly, the predetermined relationship between the magnitude of the driving current supplied to the VCM and the relative angular position to which the swing arm  132  is moved by the VCM is maintained even if the flexible printed circuit ribbon  120  is twisted. Thus, the read/write head  138  can be rapidly moved to a particular target track, i.e., twisting of the flexible printed circuit ribbon  120  hardly impacts the performance of the actuator  130 .  
       FIG. 6  is a plan view of the flexible printed circuit ribbon  120  depicted in  FIG. 5  when the flexible printed circuit ribbon  120  is twisted about 10 degrees, and  FIG. 7  illustrates the angle of twist θ between sections of the flexible printed circuit ribbon  120  at lines A-A′ and B-B′ of  FIG. 6 . Referring to  FIG. 7 , the angle of twist θ of the flexible printed circuit ribbon  120  can be defined by the maximum angle subtended between sections of the ribbon taken from one end of the connecting portion  125  adjacent to the first supported portion  121  (at line A-A′) and the other end of the connecting portion  125  adjacent to the second supported portion  122  (at line B-B′).  
      In the present invention shown in  FIG. 6 , the side of the flexible printed circuit ribbon  120  is observed from above only in a short region where the end of the connecting portion  125  at A-A′ is twisted. On the contrary, in the conventional art as shown in  FIG. 2 , the sides of the flexible printed circuit ribbon  20  are observed from above over most of the entire length of the flexible printed circuit ribbon  20 . This difference is due to the fact that the increased flexibility of the free bending portion  125   a  of the flexible printed circuit ribbon  120  relaxes the connecting portion  125  according to the present invention. Thus, most of the flexible printed circuit ribbon  120  can be kept in an upright position without twisting.  
       FIGS. 8 and 9  show experimental results of the relationship between bias force and angle of rotation of a swing arm, in the conventional art and the present invention, respectively. In these experiments, the bias force is the force exerted by the flexible printed circuit ribbon on the swing arm in the direction of its rotation while the swing arm is in a free state in which the driving current is not supplied to the VCM. The angle of rotation angle of the swing arm, as shown in  FIG. 10 , is measured with respect to an arbitrary reference line: the angle of rotation is about 41 degrees when the read/write head lies over the disk along the circle ID, and is about 18 degrees when the read/write lies over the disk along the circle OD. Reference character θ denotes the angle of twist of the flexible printed circuit ribbon as was described with reference to  FIG. 7 .  
      In both the conventional art and the present invention, the bias force does become more directly proportional to the angle of rotation of the swing arm, i.e., the plots become more linear, as the angle of twist θ of the flexible printed circuit ribbon becomes smaller. On the contrary, the relationships between the angle of rotation of the swing arm and the bias force become more exponential, i.e., the plots become more curved, as the angle of twist θ becomes greater. However, the present invention is advantageous over the conventional art as Table 1 below shows.  
      Table 1 offers a comparison between the present invention and the conventional art. In the table, the root mean square error represents the difference between an actual plot of the bias force vs. angle of rotation and a corresponding hypothetical linear plot derived by interpolating measurements of the bias force used to produce the actual plot. Except in the case in which the angle of twist θ is zero, the root mean square error is larger in the conventional art than in the present invention with respect to the same angles of twist θ. In the conventional art, the bias force is far from proportional to the angle of rotation of the swing arm. This means that it is difficult to precisely control the movement of the read/write head to a particular track on the disk. That is, as discussed in the background section, the performance of the conventional HDD is compromised by the relatively long time it takes to move the read/write head over the desired track. On the contrary, in the present invention, the bias force is nearly linear directly proportional to the angle of rotation of the swing arm over an entire range of angles of twist of the flexible printed circuit ribbon. Thus, the read/write head can be rapidly and precisely moved to a particular target track through the use of a controller configured with a simple algorithm that defines the generally linear relationship between the bias force and the angle of rotation of the swing arm.  
      Also, in Table 1 below, the maximum deviation denotes the difference between the maximum and minimum bias forces with respect to a given angel of rotation of the swing arm. The maximum deviation is relatively larger in the conventional art than in the present invention with respect to the same angles of twist θ. A large maximum deviation, as is present in the conventional art, means that it takes a correspondingly relatively large driving force to move the read/write head across the data zone between the inner circle ID and the outer circle OD, i.e., the operating efficiency of the conventional HDD in driving the swing arm of the actuator is significantly lower than that according to the present invention.  
      Moreover, tracking errors, in which the read/write head deviates from a particular track during the recording of data onto the track or the reproducing of data from the track, are much more likely to occur in the conventional HDD because of the relatively large bias force exerted on the swing arm. In order to prevent such tracking errors, a force must be applied to the swing arm in its direction of rotation to offset the bias force. However, any such applied force detracts from the efficiency of the HDD in driving the swing arm of the actuator.  
                                   TABLE 1                                   θ = 0   θ = 5   θ = 10   θ = 15                                                            Related art   Maximum   52   62   92   132           deviation           Root mean   0.375   1.188   2.502   6.344           square error       The present   Maximum   7   12   25   40       invention   deviation           Root mean   1.147   0.351   0.937   1.577           square error                  
 
      Referring now to  FIG. 11 , the flexible printed circuit ribbon  120  of the present invention includes a conductive layer  120   a , protective layers  120   c  on opposite sides of the conductive layer  120   a , and bonding layers  120   b  interposed between the conductive layer  120   a  and the protective layers  120   c , respectively. The conductive layer  120   a  is constituted by a conductive pattern, namely, a pattern of conductive lines, for transmitting/receiving data signals to/from the read/write head and transmitting/receiving driving signals to/from the actuator. The protective layers  120   c  envelop the conductive layer  120   a  to prevent short circuits between the conductive lines of the conductive layer  120   a  and to electrically insulate the conductive layer  120   a  from the surrounding environment. The bonding layers  120   b  may comprise an adhesive to bond the conductive layer  120   a  and the protective layers  120   c . In this case, the bonding layers  120   b  can be applied to both sides of the conductive layer  120   a . Alternatively, only one bonding layer  125  may be employed. The conductive layer  120   a  may be formed of a metal having a high electric conductivity, such as copper. The protective layers  120   c  may be formed of an insulating material such as polyimide.  
      The thickness of the conductive layer  120   a  is preferably uniformly over the entire length of the flexible printed circuit ribbon  120  because the conductive layer  120   a  is the layer that most affects the electrical performance of the flexible printed circuit ribbon  120 . At least one of the protective layers  120   c , on the other hand, may have thicknesses that varies over the length of the flexible printed circuit ribbon  120 : at least one of the protective layers  120   c  may be relatively thin at the free bending portion  125   a  for better flexibility, and relatively thick at all other portions of the flexible printed circuit ribbon  120 . In the embodiment shown in  FIG. 11 , both of the protective layers  120   c  are thinner at the free bending portion  125   a  than at all other portions of the flexible printed circuit ribbon  120 .  
      Another form of a flexible printed circuit ribbon  220  according to the present invention is shown in  FIG. 12 . Like the flexible printed circuit ribbon  120  shown in  FIG. 11 , the flexible printed circuit ribbon  220  includes a conductive layer  220   a  for transmitting electric signals, and protective layers  220   c  covering opposite sides of the conductive layer  220   a  to insulate the conductive layer  220   a . The flexible printed circuit ribbon  220  may further include one or more bonding layers  220   b  interposed between the conductive layer  220   a  and at least one of the protective layers  220   c  to bond the protective layers  220   c  to the conductive layer  220   a . The thickness of the conductive layer  220   a  is preferably uniform over the entire length of the flexible printed circuit ribbon  220  because the conductive layer  220   a  is the layer that most affects the electrical performance of the flexible printed circuit ribbon  220 . The flexible printed circuit ribbon  220   a  also has a free bending portion  225   a  that provides the flexible printed circuit ribbon  220  with a great amount of flexibility. For this, the free bending portion  225   a  is thinner than the other portions of the flexible printed circuit ribbon  220 .  
      However, the free bending portion  225   a  may be devoid of one or both of the protective layers  220   c . Furthermore, in the case in which the free bending portion  225   a  is devoid of one of the protective layers  220   c , the other protective layer  220   c  may be relatively thin at the free bending portion  225   a . That is, as shown in  FIG. 12 , one protective layer  220   c  (at the top of the figure) has a discontinuity at the free bending portion  225   a , and the other protective layer  220   c  (at the bottom of the figure) is relatively thin at the free bending portion  225   a.    
      In the HDD of the present invention, the flexible printed circuit ribbon, which transmits electrical signals to the actuator, has a free bending portion that imparts a higher degree of flexibility to the ribbon. Thus, the bias force exerted on the flexible printed circuit ribbon is minimized and consequently, the HDD has a correspondingly higher efficiency in driving the swing arm of the actuator. Furthermore, the relationship between the relative angular position of the swing arm (angle of rotation) and the magnitude of the current supplied to the VCM remains linear or approximately linear even when the flexible printed circuit ribbon is twisted when assembled in the HDD. Thus, regardless of the twisting of the flexible printed circuit ribbon, the read/write head can be rapidly moved to a target track. That is, the HDD can still have a high performance.  
      Finally, although the present invention has been particularly shown and described with reference to the preferred embodiments thereof, various changes in form and details, as it will be apparent to those of ordinary skill in the art that, may be made to the preferred embodiments without departing from the true spirit and scope of the present invention as defined by the following claims.