Abstract:
A flexible circuit for use in a head gimbal assembly having low stiffness and high robustness. The flexible interconnect circuit has interconnecting leads which connect the flexible interconnect circuit to a transduction head. To ensure the transducing head is able to follow the surface of the disc and properly read from and write to the disc, the interconnect leads are constructed with a varying width. The leads are widened at the points where breakage usually occurs and are narrowed at a middle portion to ensure the leads maintain the desired flexibility.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Patent Application Serial No. 60/133,185 filed on May 7, 1999, for “Head Gimbal Assembly Interconnecting Leads Having Improved Robustness and Lower Stiffness” by Adam Karl Himes, Michael Scott Bowers, and Paul Eugene Kupinski. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a head gimbal assembly in a disc drive system, and more particularly to the interconnecting leads which connect the transducing head to the flexible interconnect circuit. 
     Disc drives are well known in the art and comprise several discs, each disc having several concentric data tracks for storing data. A transducing head is used to read from or write to a data track on a disc. As the disc is spun, the head is positioned above a data track on a disc by moving an actuator arm on which the head is suspended. The arm is moved using a large scale actuation motor, such as a voice coil motor. 
     The head is mounted on the actuator arm using a head gimbal assembly. A standard head gimbal assembly comprises a load beam, a gimbal, a flexible interconnect circuit, and the head. The load beam provides the main support structure for the head gimbal assembly. A gimbal is attached under the load beam, and the head is attached to the gimbal. The gimbal is designed to allow the head to follow the surface of the disc more closely than if the head were mounted directly on the load beam. The flexible interconnect circuit is laid on top of the load beam and provides the circuitry to and from the head in the form of interconnect leads. The leads connect the flexible interconnect circuit to the head and thus allow electronic signals to pass between the two. 
     All parts of the head gimbal assembly are constructed to accommodate high precision movement so that the head can be placed above a desired data track. In addition, the design of the head gimbal assembly must allow the head to closely follow the surface of the disc as the disk is spinning. To allow the head to do so, the gimbal and flexible interconnect must be flexible. In particular, the leads connecting the head to the flexible interconnect circuit must have low stiffness. At the same time, the leads must be robust enough to survive the manufacturing process and day-to-day use. 
     Typically, prior art leads have a uniform width that is kept small to ensure flexibility, but the small width also causes the prior art leads to be fragile and easily breakable. Though it is possible to widen the leads to increase their robustness, when widened the leads become too stiff and prevent the head from moving in such a manner as to be able to follow the disc surface. If the leads are made narrower, the stiffness is reduced and the head is able to achieve the desired range of motion. However, as the leads are made to be more narrow, the leads also become less robust and experience a much higher incidence of breakage. When breakage occurs, it most often happens at the point where the lead must be bent to allow the lead to be attached to the head. 
     In an effort to strengthen the leads and improve their robustness and resistance to fatigue, an adhesive conformal coat may be selectively placed onto the leads. However, this process is time consuming and is difficult to control. Further-more, it does nothing to address the breakage that occurs at the areas where the leads are bent. 
     Thus, there is a need in the art for a head gimbal assembly having interconnecting leads which are robust enough to survive the manufacturing process and use while being flexible enough to allow the head gimbal assembly to function. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to head gimbal assembly interconnecting leads having improved robustness and lower stiffness. The interconnecting leads are shaped so that the width of the leads varies at points where the lead is likely to break. Specifically, the width of the lead at the point it becomes unsupported by the flex circuit is made wider. The lead is also made wider at the top of the head bond pad. To insure that the pitch stiffness remains low enough to allow the head to move over the disc surface, portions of the lead are made narrower. By widening some portions of the lead and narrowing other portions of the lead, it is possible to reach a point where the lead is made much more robust, yet retains the low stiffness required for the head gimbal assembly to function properly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of disc drive actuation system for positioning a slider over tracks of a disc. 
     FIG. 2 is an exploded perspective view of a portion of a head gimbal assembly. 
     FIG. 3 is a perspective view of a head gimbal assembly with variable width head leads. 
     FIG. 4 is a top view of a head gimbal assembly variable width head leads wherein the leads have not been assembled to a head. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a plan view of a disc actuation system  10  for positioning a head  12  over a selected track  16  of a disc  14 . An actuation system  10  includes a voice coil motor (VCM)  18  arranged to rotate actuator arm  20  around axis  22  on spindle  24 . The head suspension includes a load beam  26  connected to the actuator arm  20  at a head mounting block  28 . A gimbal  30  is connected to an end of the load beam  26 , and carries the head  12 . The head  12  is a transducing head for reading from or writing to the concentric tracks  16  of the disc  14 . The disc  14  rotates about an axis  34 , so that windage is encountered by the head  12  to keep it aloft a small distance above the surface of disc  14 . The head  12  must be able to closely follow the surface of the disc  14 , including any imperfections in the topography of the disc  14 , as the head  12  travels over the spinning disc  14 . 
     FIG. 2 is a perspective view of a greatly enlarged head gimbal assembly  40 . Visible is a flexible interconnect circuit  42  with traces  44  on a polyimide substrate  46 . The traces  44  terminate in leads  48  when they become unsupported by the substrate  46 . Also visible are the load beam  26 , the gimbal  30 , and the head  12 . On the head  12  are bond pads  50 . The head  12  is attached to the gimbal  30 , typically with the use of an adhesive or glue. The gimbal  30  is mounted on the underside of the load beam  26  in a conventional manner. The gimbal  30  allows more flexibility to be imparted to the head  12  than if the head  12  were attached directly to the load beam  26 . The flexible interconnect circuit  42  lays on the load beam  26 . As the leads  48  extend past the substrate  46 , they are bent to allow the leads  48  to be attached to the head  12  at the bond pads  50 . 
     The flexible interconnect circuit  42  is configured of a polyimide substrate and the traces  44  are made of,bold plated copper. The bond pads  50  are fabricated from gold. The traces  44  and leads  48  allow electronic signals to be passed between the flexible interconnect circuit  42  and the head  12 . 
     To allow the head  12  to be compliant enough to follow the disc surface as the disc spins, it is important for the leads  48  which connect the flexible interconnect circuit  42  and the transducing head  12  to be as compliant as possible to reduce the torque imparted by the air bearing. At the same time, the leads  48  must be robust enough so that they do not break or fracture during the assembly process or during use. Prior art leads were typically fabricated from gold plated copper and had a uniform width. The present invention results in leads  48  that are both robust and resistant to breaking or fatigue failure, yet also have a reduced stiffness so that the head  12  is able to follow the surface of a spinning disc more closely. 
     FIG. 3 is a perspective view of a portion of a head gimbal assembly. Visible in FIG. 3 is the flexible interconnect circuit  42 , traces  44 , leads  48 , the head  12 , and head bond pads  50 . The traces  44  run along the surface of the flexible interconnect circuit  42  and the leads  48  of the traces  44  are attached to the bond pads  50 . As can be seen, the leads  48  must be bent to allow to the leads  48  to connect the flexible interconnect circuit  42  to the head  12 . When the leads  48  are bent, a knuckle  60  is created at the point the lead  48  becomes unsupported by the flexible interconnect circuit  42 . A heel  62  is created at the point where the lead  48  is again bent so that it can meet the bond pad  50 . The leads  48  have a middle portion  64  between the knuckle  60  and the heel  62 . 
     To improve the robustness, and thus the resistance to breakage, the present invention makes the leads  48  wider at the knuckle  60  and heel  62 , yet correspondingly make the leads  48  narrower at the middle portion  64  between the knuckle  60  and heel  62 . This narrowing of the middle portion  64  decreases the stiffness of the flexible interconnect circuit, while widening the leads  48  at the knuckle  60  and heel  62  greatly increasing the resistance to fatigue and robustness of the leads  48  and results in less breakage. 
     FIG. 4 is a plan view of a portion of an unassembled head gimbal assembly  70 . The unassembled head gimbal assembly  70  comprises the flexible interconnect circuit  42 , traces  44 , and leads  48 . The leads  48  have not yet been bent to allow the ends of the leads  48  to be affixed to bond pads on the transducing head. The narrow middle portion  64  can be more clearly seen, as can the knuckle  60  and heel  62 . In a preferred embodiment, the leads  48  have a 0.003 inch width at the heel  62  and knuckle  60 . The leads  48  further have a width of 0.002 inches at the narrow portion  64  between the knuckle  60  and heel  62 . Such a design insures a low stiffness and high robustness of the leads  48 . 
     Other widths at points on the leads  48  are possible. The knuckle portion  60  can be made wider than 0.003 inches. In fact, the knuckle portion  60  can be as wide as will allow, up to the point where the leads  48  begin to overlap each other. Similarly, the heel portion  62  can be made as wide as will allow up to the point where the leads  48  no longer fit the bond pads  48 . The narrow portion  64  can likewise be made narrower than 0.002 inches, though it becomes increasingly more difficult to manufacture at smaller widths. In addition, if the narrow portion  64  becomes too narrow, the leads  48  become unacceptably fragile and difficult to handle without causing breakage. Thus, though disclosed as having a heel  62  and knuckle  60  portion with a width of 0.003 inches, other widths for the heel  62  and knuckle  60  are possible. Likewise, though disclosed as having a narrow portion  64  with a width of 0.002 inches, the narrow portion  64  can likewise have varying widths. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For instance, though discussed as being used in standard head gimbal assemblies, the present invention may be utilized in a variety of head gimbal assemblies. In addition, the present invention is useful for any flex on suspension to PCC (preamp chip carrier/printed circuit cable) interconnects with solder or flying leads.