Abstract:
An electrical circuit for a disk drive suspension has a first signal trace above a grounded support layer which is typically stainless steel, and a second signal trace below the support layer, the two signal traces typically being opposite polarities of a differential signal pair. The support layer between the two signal traces is windowed, and the two traces are physically separated by a distance that equals the nominal thickness of the support layer plus the thicknesses of the top and bottom dielectric layers that separate the signal traces from the support layer. The physical separation combined with the windowing reduces the impedance and increases the bandwidth of the differential pair. The impedance can be controlled by the percentage of windowing in the support layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application No. 61/883,339 dated Sep. 27, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of suspensions for disk drives. More particularly, this invention relates to the field of a suspension including a circuit trace constructed of circuit layers on opposite sides of a stainless steel layer, where the stainless steel layer have windows formed therein to allow passage of electric fields between the circuit layers. 
     2. Description of Related Art 
     Magnetic hard disk drives and other types of spinning media drives such as optical disk drives are well known.  FIG. 1  is an oblique view of an exemplary prior art hard disk drive and suspension for which the present invention is applicable. The prior art disk drive unit  100  includes a spinning magnetic disk  101  containing a pattern of magnetic ones and zeroes on it that constitutes the data stored on the disk drive. The magnetic disk is driven by a drive motor (not shown). Disk drive unit  100  further includes a disk drive suspension  105  to which a magnetic head slider (not shown) which defines a read/write head is mounted proximate a distal end of load beam  107 . The “proximal” end of a suspension or load beam is the end that is supported, i.e., the end nearest to base plate  12  ( FIG. 2 ) which is swaged or otherwise mounted to an actuator arm. The “distal” end of a suspension or load beam is the end that is opposite the proximal end, i.e., the “distal” end is the cantilevered end. 
     Suspension  105  is coupled to an actuator arm  103 , which in turn is coupled to a voice coil motor  108  that moves the suspension  105  arcuately in order to position the head slider over the correct data track on data disk  101  or other recording medium. The head slider is carried on a gimbal which allows the slider to pitch and roll so that it follows the proper data track on the disk, allowing for such variations as vibrations of the disk, inertial events such as bumping, and irregularities in the disk&#39;s surface. The gimbal and the flexible circuit are typically part of the suspension called the flexure. 
     Circuit integrated suspension (CIS) and trace suspension assembly (TSA) are known technologies for manufacturing flexures. These flexures typically have wiring trace impedances in the 50 to 100 ohm range. The wiring trace, also called the circuit trace or flexible circuit, electrically connects the slider and PZT components to control circuitry (not shown) of the hard disk drive. One portion of the wiring trace runs from slider contacts near the distal end of the suspension to the proximal end of the suspension. Another portion of the wiring trace runs from the PZTs to the proximal end of the suspension. 
       FIG. 2  is a side sectional view of a portion of a prior art suspension including a flexure circuit  10 . A supporting layer  12  such as a stainless steel layer is coated with a dielectric layer  14  such as polyimide; conductive signal traces  16  and  18  such as copper or copper alloy are metalized on the polyimide layer; and a cover coat or coverlayer  20  of an insulator such as polyimide is applied over the copper traces  16 ,  18  and the insulating dielectric layer  14 . 
     Circuit traces manufactured using CIS and TSA flexure technologies can have trace impedances in the 40-50 ohm range and a circuit bandwidth of over 6 GHz. However, the spacing between traces and the ground layer that can be manufactured using these technologies is limited. Consequently, flexures manufactured using these technologies are not expected to be able to meet future hard disk drive requirements that call for an impedance of less than 30 ohms along with a bandwidth of over 6 GHz. 
     Known techniques such as the use of interleaved traces or stacked traces can achieve impedances below 50 ohms. For example, a stack trace design can be constructed by adding an additional layer on top of the first circuit layer. Such designs have significant bandwidth limitations, however. For example, simulations have indicated that the stack trace design may be limited to a bandwidth of less than 3 GHz. In addition, simulations have indicated that the interleave trace design may be limited to a bandwidth of less than 7 GHz. Consequently, interleave trace designs and stack trace designs are not an optimal choice for future hard disk drive designs, such as designs for suspensions for 3.5 inch disks, that call for an impedance below 50 ohms along with a bandwidth of 8 GHz for a circuit having a length of 50 mm. 
     SUMMARY OF THE INVENTION 
     According to the invention, a suspension includes a circuit trace formed by circuit layers on each side of a conductive metal layer such as a stainless steel layer. The stainless steel layer is constructed with one or more windows, e.g., holes, such that traces of the circuit layers pass over the window(s). Consequently, when the traces carry a signal, an electric field generated by the signal will pass between the traces via the window(s). Due to the increased coupling of the electric fields between traces caused by the windowing, the traces exhibit a relatively high bandwidth and low impedance. 
     The disclosed circuit trace may be particularly beneficial for routing differential signals. The positive and negative traces of a differential signal pair are constructed such that they take complementary paths on opposite sides of the windowed stainless steel layer. Thus, the electric fields of differential signals carried by the differential trace pair will readily couple since the electric fields are able to pass through the window(s) in the stainless steel layer. 
     Exemplary embodiments of the invention will be further described below with reference to the drawings, in which like numbers refer to like parts. The drawing figures might not be to scale, and certain components may be shown in generalized or schematic form and identified by commercial designations in the interest of clarity and conciseness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an oblique view of a prior art disk drive having a dual stage actuated suspension. 
         FIG. 2  is a side sectional view of a portion of a prior art suspension including a circuit trace. 
         FIG. 3  is an exploded view of a suspension circuit according to an illustrative embodiment of the invention. 
         FIG. 4  is a top plan view of the circuit of  FIG. 3 . 
         FIG. 5  is a sectional view of the circuit of  FIG. 4  taken along section line  5 - 5 . 
         FIG. 6  is a sectional view of the circuit of  FIG. 4  taken along section line  6 - 6 . 
         FIG. 7  is an exploded view of a suspension circuit similar to  FIG. 3  with a larger window. 
         FIG. 8  is a graph comparing impedance and bandwidth of the circuit trace design of  FIG. 3  with impedance and bandwidth of prior art stacked trace and interleaved trace designs according to a simulation. 
         FIGS. 9A-9E  illustrate a process for manufacturing the circuit traces of  FIGS. 3-7  according to an illustrative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3  is an exploded view of a suspension circuit  110  according to an illustrative embodiment of the invention, showing only the conductive metal layer  112  such as stainless steel, windows  113  formed therein, and conductive signal traces  116  and  117  such as copper or copper alloy signal traces. A stacked trace design is employed whereby a first circuit layer and a corresponding circuit trace  116  are constructed above stainless steel layer  112  and a second circuit layer and corresponding circuit trace  117  are constructed below stainless steel layer  112 . The insulating layers, namely the dielectric layer  114  and the insulating coverlayers  20  and  21 , are omitted for clarity of illustration. Those layers are depicted in  FIGS. 5-6 . 
       FIG. 4  is a top plan view of the circuit of  FIG. 3  with dielectric layer  114  and coverlayer  20  omitted for clarity of illustration. Windows  113  in stainless steel layer  112  are preferably regularly located and evenly spaced underneath circuit trace  116 . The percentage of windowing, or the duty cycle of the windowing, can be defined as the length of run of circuit trace  116  below which there is a window  113 , i.e., below which there is no stainless steel  112 , divided by the total run length of the trace.  FIG. 4 , for example, shows slightly less than 50% windowing, i.e., with stainless steel under slightly more than half of the run length of circuit trace  116  along at least a particular section. 
       FIG. 5  is a sectional view of the circuit of  FIG. 4  taken along section line  5 - 5 . The figure shows stainless steel layer  112 , windows  113  formed in the stainless steel, polyimide or other dielectric  114 , circuit traces  116  and  117  above and below stainless steel layer  112 , respectively, and top and bottom protective insulating coverlayers  20  and  21 . Polyimide  114  fills windows  113 . 
       FIG. 6  is a sectional view of the circuit of  FIG. 4  taken along section line  6 - 6 . Window  113  located between top and bottom traces  116 ,  117  allows the electric fields between those traces to couple, thus decreasing the differential impedance of that differential trace pair. 
     Returning to  FIG. 3 , copper traces  116  and  117  are disposed on opposite sides of stainless steel layer  112 . Typically circuit  110  is part of the suspension&#39;s flexure, and typically stainless steel layer  112  is grounded such that layer  112  defines a ground plane. More generally, traces  116 ,  117  are located on opposite sides of some grounded suspension component, which could be metal support layer for the flexure, a load beam, or other grounded component. Additionally, the invention applies whether metal layer  112  is grounded or is held at some other constant potential such as a voltage, in which case metal layer  112  could be a voltage source plane. In addition, stainless steel layer  112  provides mechanical support for the circuit. Dielectric layer  114  immediately above and below stainless steel layer  112  as seen in  FIGS. 4-6  acts as a dielectric standoff, preventing copper signal traces  116  and  117  from shorting to the grounded stainless steel layer  112  and maintaining those copper signal traces at the desired distance away from each other and away from ground plane  112 . The distance between traces  116  and  117  both within the windowed area and within the non-windowed area substantially equals the sum of the respective nominal thicknesses of the stainless steel layer  112 , the insulating dielectric layer above layer  112 , and the insulating dielectric layer below layer  112 . That separation distance between the traces remains approximately constant as the traces pass throw the windowed areas and the non-windowed areas. The lengths and widths of windows  113 , and their duty cycles are chosen to achieve the desired differential impedance and bandwidth of the signal traces  116 ,  117 . The signal traces  116 ,  117  alternatingly extend over a number of windowed areas and a number of non-windowed areas. 
     Typically signal traces  116  and  117  will carry opposite polarities of a differential signal pair, although signal traces  116  and  117  could carry single-ended signals. In the case of single-ended signals, it might be desirable to offset signal traces  116  and  117  transversely such that the signal do not extend directly over and under each other, separating them by a greater distance from each other than if they extended directly over and under each other, thus reducing cross coupling between signals. 
     To reduce the complexity of  FIGS. 3-6 , only a single copper trace is shown for each circuit layer. It should be appreciated, however, that a circuit layer of a typical suspension will have multiple traces, and hence the circuit would include multiple configurations of such signal trace pairs. 
     The thickness of the stainless steel layer  112  provides a relatively wide spacing between the stacked traces  116 ,  117  of the circuit layers. Consequently, circuit  110  can exhibit a relatively low impedance, such as less than 30 ohms. 
     Windows  113 , i.e., holes, are formed in the stainless steel layer  112  to enable electric fields between the two circuit layers to couple. In  FIG. 3 , a window  113  is defined by the absence of stainless steel  112  between the two stainless steel sections. To reduce the complexity of  FIG. 3 , three windows are shown. It should be appreciated, however, that a circuit layer of a typical suspension may have many such windows. 
     Through the use of ground layer windowing as illustrated in  FIGS. 3-6 , the resulting circuit can exhibit a relatively high bandwidth and a relatively low impedance. For example, the circuit may exhibit a bandwidth of 6 GHz or higher along with an impedance of 50 ohms or lower. 
       FIG. 7  is an exploded view of a stainless steel layer  212  and traces  116 ,  117  above and below, with a relatively large window  213  formed in stainless steel layer  212 . The dielectric layer and coverlayers are omitted for clarify of illustration.  FIG. 7  can be thought of as the circuit of  FIG. 3  in which the percentage of windowing has been increased to over 90%, and to nearly 100%, or equal to 100%. 
       FIG. 8  is a graph comparing impedance and bandwidth of the circuit trace design of  FIGS. 3-7  with impedance and bandwidth of stacked trace and interleaved trace designs according to simulations. Of particular note is the improved performance for the double sided design of the present invention, highlighted via a dashed circle in the upper part of the figure, as compared to prior art stacked and/or interleaved traces, shown in smaller dashed circles toward the bottom of the figure, which exhibit low impedance but also relatively low bandwidth. As indicated, a circuit trace design as taught herein employing stainless steel windowing of 90% exhibits a bandwidth greater than 7 GHz and an impedance less than 30 ohms. Moreover, a circuit trace design as taught herein employing stainless steel windowing of 100% exhibits a bandwidth greater than 8 GHz and an impedance less than 30 ohms. 
     The percentage of windowing referred to above is inversely proportional to the percentage of stainless steel that lies between the two stacked traces. For example, in some implementations, stainless steel windowing of 100% means that there is no stainless steel between a pair of stacked traces for the entire run of the stacked traces. As another example, in some implementations, stainless steel windowing of 90% means that there is no stainless steel between a pair of stacked traces for 90% of the run of the stacked traces. Other percentages of windowing, such as 50%, 20%, and so on, may be employed in various implementations. 
       FIGS. 9A-9E  illustrate a process for manufacturing the circuits of  FIGS. 3-7  according to an illustrative embodiment of the invention. The individual process steps described below and variations on those steps are by themselves well known and therefore will not be described in detail. In  FIG. 9A  top side polyimide  114  is coated, imaged, or developed onto a top surface of stainless steel  112 . Next, in  FIG. 9B  one or more windows  113  are etched or otherwise formed into the stainless steel  112 . In  FIG. 9C  bottom side polyimide  115  is then coated, imaged, or developed onto a bottom surface of the stainless steel  112 . Polyimide  114  fills window(s)  113  in the stainless steel  112  such that the bottom side polyimide  115  and the top side polyimide  114  contact one another. In  FIG. 9D  a chromium and copper (Cr/Cu) composition is sputtered onto the top and bottom polyimide  114 ,  115  as a seed layer to enable subsequent plating, and copper signal traces  116 ,  117  are then electrodeposited onto the top polyimide layer  114  and  115 . In  FIG. 9E  polyimide coverlayers  20  and  21  are then coated, imaged, or developed onto the top and bottom surfaces, respectively, of the assembly. For any copper traces  116 ,  117  requiring gold plating such as where those traces will be exposed to air and used as contact pads, gold (not shown) is plated onto the copper traces. The resulting assembly may then be bonded to other suspension components to form a completed suspension. 
     It will be understood that terms such as “top,” “bottom,” “above,” and “below” as used within the specification and the claims herein are terms of convenience that denote the spatial relationships of parts relative to each other rather than to any specific spatial or gravitational orientation. Thus, the terms are intended to encompass an assembly of component parts regardless of whether the assembly is oriented in the particular orientation shown in the drawings and described in the specification, upside down from that orientation, or any other rotational variation. 
     Although the present invention has thus been described in detail with regard to the preferred embodiments and drawings thereof, it should be apparent to those skilled in the art that various adaptations and modifications of the present invention may be accomplished without departing from the spirit and the scope of the invention. Different materials and different process steps maybe employed to reach the same result. The percentage of windowing can be continuously varied from 0% to 100% for any of the embodiments shown and described. Other variations are possible. Accordingly, it is to be understood that the detailed description and the accompanying drawings as set forth hereinabove are not intended to limit the breadth of the present invention, which should be inferred only from the following claims and their appropriately construed legal equivalents.