Patent Publication Number: US-2005135015-A1

Title: Disc drives having flexible circuits with liquid crystal polymer dielectric

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This present application is a continuation of and claims priority to U.S. patent application Ser. No. 09/457,816, filed Dec. 9, 1999, which claims priority to U.S. Provisional Patent Application Ser. No. 60/116,781, filed Jan. 22, 1999 to Schulz et al., entitled “Flex on Suspension (FOS) With Liquid Crystal Polymer (LCP) Dielectric,” both of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      The invention relates to flexible circuits that are useful in suspension arm assemblies, for example, in disc drive units.  
      Flexible circuits can be useful for a variety of applications for connecting elements that can move relative to each other. For example, a transducer can be mounted on a moveable arm or the like such that the transducer can perform measurements at different location. The moveable arm generally includes a suspension assembly that provides for relative motion of the arm. In particular, disc drives for data storage include a head that carries transducers to facilitate reading and/or writing of data on the disc. Also, glide testers are disc drive units that are used to examine data storage discs for imperfections in the disc surface.  
      Disc drives are used for storing information, typically as magnetically encoded data, and more recently as optically encoded data, on a disc surface. The disc spin at high rotational velocities such that the head flies above the disc surface on a cushion of air. The suspension arm is used for radially accessing different data tracks on the rotating discs.  
      Generally, all hard drive discs are tested before shipment. During a glide test, the glide head or slider flies over a disc surface generally at a predetermined clearance from the disc surface, known as the glide height or fly height. If contact occurs between the glide head and a disc defect or asperity, forces on the glide head create responses that can be measured with transducers.  
      When any type of transducer head is flying above a spinning disc surface, heads experience undesirable radial forces, circumferential forces and yaw torques. The effects of these forces are preferably reduced or eliminated while necessarily allowing for roll and pitch movement of the transducer head. While the transducer head is experiencing these forces, an electrical connection must be maintained between the transducers and the signal processing components of the disc drive. A flexible circuit or flex cable provides the electrical connection between the head and the remaining portions of the disc drive unit.  
      The electrical connections preferably do not significantly effect the performance characteristics of the suspension assembly. As storage densities on disc recording media become higher, the performance characteristics of all of the disc drive components become more strict and tolerances are reduced. Therefore, as storage densities increase, it becomes even more important to reduce the effects on performance due to a flexible circuit.  
     SUMMARY OF THE INVENTION  
      In a first aspect, the invention pertains to a suspension assembly having a load beam and a flexible circuit. The flexible circuit is positioned on the load beam and includes an electrically conductive element and a dielectric liquid crystal substrate laminated to the conductive element.  
      In a second aspect, the present invention pertains to a device for supporting a transducing element. The device includes a flexible circuit coupled to the transducing element. The flexible circuit includes an electrically conductive element and a dielectric crystal substrate laminated to the conductive element.  
      In a further aspect, the invention pertains to a method for producing a flexible circuit comprising joining a dielectric liquid crystal polymer substrate to an electrically conductive element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic side view of a suspension assembly with a flexible circuit connecting a transducer head with external electronics.  
       FIG. 2  is a top view of the suspension assembly of  FIG. 1 , where the components have been made transparent to reveal hidden structure.  
       FIG. 3  is a sectional view of the suspension assembly of  FIG. 2  taken along line  3 - 3 .  
       FIG. 4  is a top view of an alternative embodiment of a suspension assembly.  
       FIG. 5  is a side view of the suspension assembly of  FIG. 4  where the insert displays an amplified view of the connection between a flexible circuit and a transducer head.  
       FIG. 6  is a top view of a flexible circuit where a cover layer is displayed as transparent to reveal electrical filaments between the cover and a substrate.  
       FIG. 7  is a perspective view of the suspension assembly of  FIG. 6 .  
       FIG. 8  is a schematic, top perspective view of a glide tester, where a disc is shown with phantom lines such that structure below the disc is visible.  
       FIG. 9  is a schematic, perspective view of a glide head with a PZT transducer mounted on a wing extending from the glide head&#39;s top surface.  
       FIG. 10  is a schematic, top view of a disc drive system.  
       FIG. 11  is a schematic, perspective view of a slider/transducer head with a magnetic sensor. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS  
      Improved flexible circuits of the invention incorporate dielectric liquid crystal polymers as a flexible substrate for the flexible circuit. Liquid crystal polymers have reduced sensitivity to temperature and humidity changes relative to polyimide films that have been used traditionally to form substrates for flexible circuits. Also, the liquid crystal polymers are relatively inexpensive. Preferred liquid crystal polymers are thermoplastic, so that they can be hot bonded or welded to metal electrically conductive elements. For precise positioning of the transducer, the flexible circuit preferably has sufficient mobility and other mechanical characteristics.  
      Flexible circuits provide for electrical connections between components that move relative to each other. Thus, the components can move relative to each other without interruption of the transmission of electrical signal. The flexible circuits include a dielectric substrate and one or more electrical conductors. The dielectric substrate electrically insulates the conductors from other components to prevent a short circuit and provides desired mechanical properties to the flexible circuit. The flexible circuit can optionally include a thin polymer cover to protect the electrical conductor(s).  
      In the improved embodiments described herein, the dielectric substrate is formed from solidified polymer liquid crystals. The polymer compositions are selected to have the desired dielectric properties. The liquid crystal polymers generally form ordered/crystalline domains without producing long range order of a crystal. The ordering within the polymer liquid crystal material can add desirable strength to the material without sacrificing needed flexibility.  
      The liquid crystal materials are frozen into a partial glass form such that they can form a solid substrate as a support for the flexible circuit. The underlying crystalline domains of the liquid crystal polymers are frozen into the final material. The polymers are no longer a flowable liquid in this state. The solidified liquid crystal material can have desired levels of strength, flexibility and insensitivity to environmental fluctuations. The liquid crystal polymer materials substitute for polyimide polymers in standard flexible circuits.  
      For use in disc drive units, a flexible circuit provides electrical connection between a transducer head and the disc drive circuitry. The transducer head floats or flies above the disc surface while the disc is spinning at high speeds. The suspension arm holding the head moves such that the transducer head accesses different tracks along the disc surface. Thus, the transducer head moves in one or more dimensions relative to the fixed base of the disc drive. To obtain desirable performance characteristics of the disc drive unit, the flexible circuit should not negatively influence to motion of the transducer head. Disc drive units can be used for reading from and/or writing to a disc surface or for glide testing to identify disc imperfections.  
      A variety of suspension assembly designs can provide for the desired degrees of motion of the transducer head relative to the fixed disc drive base. The transducer head, or slider, is generally supported by a gimbal that provides for tilting of the head relative to a load beam. The load beam is supported by a suspension arm that connects to an actuator for moving the arm to a desired position over the disc.  
      Suspension Assembly and Flexible Circuit  
      The flexible circuits generally are used in suspension assemblies that provides for the motion of a transducer or other electrical component relative to a fixed base in electrical contact with the transducer. The flexibility of the flexible circuit accommodates the relative motion of electrical elements while providing continuous electrical contact between the elements. In preferred embodiments, the flexible circuit does not significantly alter the mechanical movement of the suspension assembly or the mechanical effects of the flexible circuit are consistent over the range of operating conditions.  
      The suspension assembly can have any of a variety of configurations. A first embodiment of a suspension assembly is depicted in  FIG. 1 . A similar suspension assembly is described in U.S. Pat. No. 5,796,556 to Boutaghou, incorporated herein by reference. Suspension assembly  100  includes a load beam  102  and a flexible circuit  104 . A transducer head  106  is supported near distal end  108  of lead beam  102 . Post  110  is used to support flexible circuit  104  and transducer head  106 . Distal end  112  of flexible circuit  104  is attached to transducer head  106 . Transducer head  106  includes transducer  114 . Transducer head  106  is located near medium  116 , such as a magnetic disc, such that electrical responses of transducer  114  reflect conditions on medium  116 . Flexible circuit  104  is connected to external circuit  118 . Load beam  102  can be connected to an arm  120  of an actuator assembly.  
      The connection between flexible circuit  104  and transducer head  106  is depicted in more detail in  FIGS. 2 and 3 . Flexible circuit  104  includes a polymeric substrate  140  that provides flexible support for flexible circuit  104 . A gimbal insert  142  is attached to polymeric substrate  140 . Adhesive  144  can be used to secure gimbal insert  142  with transducer head  106  at top surface  146 . Gimbal insert  142  provides additional support for flexible circuit  104  at transducer head  106  and includes the attachment point for connecting the transducer head to the load beam.  
      Gimbal insert  142  has a center portion and two arms that extend along the edges of flexible circuit  104  that extend beyond load beam  102 . The center portion of gimbal insert  142  extends beyond polymeric substrate  140  and is secured to load beam  102  at post  110 . Gimbal insert  142  is further welded to load beam  102  at welds  146 . Gimbal insert  142  is preferably made from a metal, such as iron chromium alloy (FeCr). Gimbal insert  142  is somewhat flexible to provide for pitch and role of head  106 .  
      Flexible circuit  104  further includes electrical traces  150 . Electrical traces  150  are electrically conductive and provide for electrical communication between transducer  114  and external circuit  118 . The number of electrical traces can vary to provide the desired electrical connections between head  106  and external circuit  118 . Generally, there are a plurality of electrical traces  150 .  
      An alternative embodiment of a suspension assembly is depicted in  FIGS. 4 and 5 . A similar suspension assembly is described in U.S. Pat. No. 4,991,045 to Oberg, incorporated herein by reference. Suspension assembly  200  includes a base plate  202 , a load beam  204 , a spring  206 , a gimbal  208 , transducer head  210 , and flexible circuit  212 . Base plate  202  is connected to an arm of an actuator, which moves suspension assembly  200  to a desired orientation. Spring  206  connects load beam  204  with base plate  202 . Spring  206  can be, for example, a steel spring. Gimbal  208  connects transducer head  210  to load beam  204 . Gimbal can be made from stainless steel and can provide for roll and pitch of transducer head  210  when the head is near a rapidly moving surface, such as a spinning disc of a disc drive.  
      Flexible circuit  212  provides for electrical connection between transducer head  210  and external circuit  220 . Flexible circuit  212  includes electrical filaments  222 ,  224  that are electrically conductive. Electrical filaments  222 ,  224  are connected to a dielectric, polymer substrate  226 . While flexible circuit  212  is depicted with two electrical filaments  222 ,  224 , a different number of electrical filaments can be included to accommodate a desired number of distinct electrical connections between the transducer head and the external circuits.  
      This embodiment of flexible circuit is sometimes referred to as a pigtail due to the bend in the circuit between the transducer head and the load beam. In particular, flexible circuit  212  connects with side  228  of transducer head  210 . From the transducer head, flexible circuit  212  bends to orient along load beam  204  and twists to lay flat along load beam  204 .  
      In general, a flexible circuit  250  includes one or more electrical filaments  252  bonded to a dielectric polymer substrate  254 , as depicted in  FIGS. 6 and 7 . Flexible circuit  250  can optionally include a polymer cover  256 . Flexible circuit  250  has a structure suitable for its intended use. For example, electrical filaments  252  are located along substrate  254  such that filaments  252  can make necessary electrical connections between a transducer head or similar mobile electrical element and an external circuit, which can be used, for example, to evaluate a signal from the transducer.  
      Flexible circuit  250  can include any number of electrical filaments  252 , and generally includes at least two electrical filaments  252 . When a plurality of electrical filaments  252  are used, the different electrical filaments  252  generally are electrically insulated from each other by dielectric substrate  254  and by air and/or an electrically insulating cover  256 . Electrical filaments  252  can be made from any electrically conducting material. Preferred materials for electrically conducting filaments include conductive metals, especially including copper.  
      Substrate  254  has an appropriate shape for supporting electrical filaments  252  and for attachment to elements that guide flexible filament  250  from a transducer head or the like to external circuits. For example, in the embodiment of the suspension assembly shown in  FIGS. 4 and 5 , the flexible circuit bends to reverse direction and twists to lay flat along a load beam. In this embodiment, the dielectric substrate generally widens for attachment to the top of the load beam so that the attachment is secure.  
      In the preferred embodiments, dielectric substrate  254  includes liquid crystal polymers in a partly ordered glass. Substrate  254  preferably has a thickness less than about 0.001 in and more preferably from about 0.0001 in to about 0.0005 in. In preferred embodiments, the liquid crystal polymers are ordered in planes defined by the top surface of substrate  254 .  
      A variety of suitable liquid crystal polymers are available commercially. For example, liquid crystal polymers are available under the tradename Vectra® from Celanese Speciality Operations, Celanese Corporation, Summit, N.J. Vectra® brand polymers are primarily aromatic co-polyesters formed from p-hydroxybenzoic acid and hydroxy naphthoic acid monomers. These polymers are available with fillers, such as glass fiber fillers, graphite flakes, carbon fibers, and the like. In addition, Xydar™ brand liquid crystal polymers are available from Dartco Manufacturing Co., Augusta, Ga. Xydar™ polymers are polyesters based on terephthalic acid, p,p′-dihydroxybiphenol and p-hydroxybenzoic acid, i.e., 4,4′-dihydroxydiphenyl-p-hydroxybenzoic acid terephthalic acid polymers. Versions of these polymers with fillers are also available.  
      Cover  256  provides protection to electrical filaments  252  from accidental contact, short circuiting and corrosion. Cover  256  can be formed from any reasonable polymer including the liquid crystal polymers used to form substrate  254 . Other suitable polymers include, for example, polyimides, such as Kapton™ from DuPont, Wilmington, Del., epoxies and polyurethanes. Cover  256  preferably has a thickness less than about 0.001, and more preferably from about 0.0002 to about 0.0005.  
      The polymer substrate can be formed using a variety of approaches including conventional approaches. In particular, the polymer substrate can be formed by extrusion, calendering, solvent casting, melt casting, molding and the like.  
      Liquid crystals can be advantageously used in flexible circuits. In particular, liquid crystal films have no moisture adsorption or very little moisture adsorption. In contrast, conventional polyimide films used to form substrates of flexible circuits absorb significant amounts of moisture. Absorption of moisture can lead to dimensional instability, which can produce torque bias on a transducer head during changing temperature and humidity conditions. Also, liquid crystal polymers are relatively inexpensive. In addition, preferred liquid crystal polymers are thermoplastic, so that no adhesive is needed to secure the electrical filaments to the substrate.  
      Electrical filaments  252  can be attached to substrate  254  with an adhesive, such as an acrylic adhesive. Preferred liquid crystals substrates are sufficiently thermoplastic that the electrical filaments  252  can be secured to substrate  254  without an adhesive. Preferably, the filaments are treated to eliminate oxidation at the surface, for example, by applying a chemical antioxidant or by covering the surface with a corrosion resistant material such as sputtered chromium or nickel. The corrosion resistant material can be removed prior to connecting the flexible circuit to other elements to reduce electrical resistance.  
      In preferred embodiments, electrical filaments  252  are applied to substrate  254  by welding, hot bonding or roll lamination, at suitable temperatures and pressures for the particular materials. To increase adhesion, electrical filaments  252  can be applied to substrate at a temperature at or above its softening temperature and/or with calendering. Nevertheless, liquid crystal polymer films oriented in the plane of the film tend to have relatively poor strength along their thickness. The liquid crystal polymer preferably is selected to have desired amounts of z-axis strength.  
      Cover  256  can be formed as a coating directly on the substrate  254 —electrical filament  252  combination. Alternatively, cover  256  can be laminated to substrate  254  using calendering or the like. Heat can be applied during the lamination process, if desired to increase the binding of cover  256 . In addition, cover  256  can be coextruded with substrate  254 , where the electrical filaments  252  are fed between substrate  254  and cover  256  during the extrusion process so that filaments  252  are at their proper location within the complete flexible circuit  250 . Some preferred polymers for the cover can be applied as a liquid by spraying, screen printing or other approaches. The liquid is then cured, preferably using radiation, such as ultraviolet light. The radiation cure can be performed with an imaging system, such that the polymer is cured only at select locations with any uncured polymer being removed subsequent to the curing step.  
      The thermoplastic characteristics of the liquid crystal polymers can also be used to weld the flexible circuit to other components when assembling the flexible circuit into a suspension assembly. To perform the welding, the polymer is heated above the softening temperature for the formation of a fast, adhesiveless bond. Good adhesion can be obtained with smooth, untreated metal surfaces. The thermoplastic character also provides for the recycling of scrap material into new product.  
      Preferred liquid crystal polymers have a coefficient of thermal expansion similar to copper (16.5 ppm/° C.). In particular, the liquid crystal polymers preferably have a coefficient of thermal expansion from about 15 ppm/° C. to about 19 ppm/° C. Therefore, the flexible circuit has greater dimensional stability. In addition, liquid crystal polymers have a coefficient of humidity expansion close to zero, preferably less than about 4 ppm/% relative humidity and more preferably less than about 3 ppm/% relative humidity, in contrast with polyimides that have a coefficient of humidity expansion of about 8 ppm/% relative humidity. Also, liquid crystal polymers generally have a larger elastic modulus than polyimides, e.g., a representative comparison of 1100 kpsi vs. 800 kpsi, such that the substrate distorts less under stress. Preferably, the liquid crystal polymers have a elastic modulus greater than about 600 kpsi and more preferably from about 900 kpsi to about 1300 kpsi.  
      Also, preferred liquid crystal polymers have a reduced dielectric constant of about 2.8 relative to corresponding values for polyimides of about 3.3. Preferred liquid crystal polymers have a dielectric constant from about 2.6 to about 3.0. Having a substrate with a reduced dielectric constant leads to improved rise times and signal propagation at high data transfer rates. Furthermore, preferred liquid crystal polymers have a lower electrical dissipation factor. At 50% relative humidity, the liquid crystal polymer preferably has an electrical dissipation factor less than about 0.5% and more preferably less than about 0.3%.  
      Assembly of Flexible Circuits  
      Once the flexible circuit has been formed, the flexible circuit is integrated into the suspension assembly. To perform the integration of the components, the electrical connections of the flexible circuit are connected appropriately to complete the circuit. In addition, the flexible circuit can be secured to elements of the suspension assembly. Consistent with these two integration steps, the flexible circuit must be oriented with respect to the rest of the suspension assembly and points of attachment. The orientation and fastening of the flexible circuit preferably is performed such that the flexible circuit does not interfere significantly with the operation of the suspension assembly.  
      The order and approach of assembling the flexible circuit with the suspension assembly may depend on the particular construction of the various components. In some embodiments, the flexible circuit can be welded, soldered, or otherwise attached having an electrical connection with electrical contacts in the transducer head. The flexible circuit generally includes one electrical filament for each electrical contact in the transducer head. Thus, the flexible circuit provides an electrical connection with these individual electrical contacts of the transducer head and a circuit external to the suspension assembly. The electrical filaments at the end of the flexible circuit are positioned to align with the electrical contacts on the transducer head.  
      Generally, the flexible circuit is secured to a load beam or the like. As shown in  FIGS. 1 and 2 , the flexible circuit is welded at particular points to the load beam. These welds help to guide the flexible circuit as well as to prevent the flexible circuit from interfering with the operation of the suspension assembly. Similarly, in  FIGS. 4 and 5  the flexible circuit is laminated to the load beam. In each of these embodiments, the flexible circuit must be shaped to conform to the intended orientation of the flexible circuit with respect to the load beam.  
      In the embodiment shown in  FIGS. 4 and 5 , the flexible circuit can be laminated to the load beam prior to attachment to the transducer head. In particular, the flexible circuit can be laminated to the load beam during the formation of the flexible circuit. In addition, the flexible circuit can be laminated or otherwise welded to other components of the suspension assembly such as an arm attached to an actuator.  
      The attachment of the flexible circuit allows for the motion of the suspension assembly relative to fixed external circuits. For example, a bend or other free section of flexible circuit can be located at the juncture of moveable sections of the suspension assembly relative to fixed portions of the apparatus. The other end of the flexible circuit is in electrical contact with external circuits. Generally, an electrical filament in the flexible circuit provides for electrical connection between particular contacts with the external circuit and corresponding electrical contacts in the transducer head. For example, electrical resistance in the transducer can be measured using two electrical filaments to from a single closed circuit.  
      Flexible Circuits For Disc Drives  
      In some preferred embodiments, flexible circuits are used in disc drive units. One form of disc drive unit is used to measure asperities or other imperfections on a disc surface. These disc drive units can be referred to as asperity detection units or glide testers. In these embodiments, the transducer on the transducer head is used to measure disc imperfections. In other disc drive embodiments, the transducers are used to read and/or write data to/from the disc surface. Disc drives for reading/writing data can be based on, for example, purely magnetic storage or magneto-optical data storage.  
      Referring to  FIG. 8 , asperity detection unit or glide tester  300  includes a glide spinstand  302 , an arm assembly drive  304 , a suspension/glide head assembly  306 , and a controller  308 . Glide spinstand  302  includes a spindle motor  320  and disc  322 , shown in phantom lines. Spindle motor  320  supports and spins disc  322 . Arm assembly drive  304  has a motorized drive  324  that positions the suspension/glide head assembly  306 .  
      Suspension/glide head assembly  306  has a support arm  330  that connects with motorized drive  324  and an arm extension  332  that extends over disc  322 . Motorized drive  324  moves support arm  330  either by lateral motion or by rotational motion to alter the radial position of suspension/glide head assembly  306  along a disc  322  mounted on glide spinstand  302 .  
      Suspension/glide head assembly  306  generally also includes suspension  334 , gimbal/load beam  336  and glide head  338 . Suspension  334  connects with arm extension  332 . Glide head  338  is connected to suspension  334  by way of gimbal/load beam  336 . Suspension  334  and gimbal/load beam  336  can have a variety of designs including conventional structures.  
      Controller  308  is connected to arm assembly drive  304 , suspension/glide head assembly  306  and spindle motor  320 . In preferred embodiments, controller  308  correlates the position of suspension/glide head assembly  306  with the rotational speed of the spindle motor to maintain an approximately constant linear speed of slider  338  relative to the disc surface. Also, controller  308  correlates impact information detected by glide head  338  with a defect location on disc  322 . Glide head  338  is electrically connected to controller  308  by way of flexible circuit  340 .  
      Glide heads generally also include a transducer. The transducer can be a piezoelectric (PZT) transducer, a conductivity transducer, a thermal transducer, or other transducers suitable for contact based asperity detection. An enlarged view of an embodiment of a glide head  360  with a PZT transducer is shown in  FIG. 9 . Glide head  360  has a top surface  362  including wing  364 . In the embodiment shown, PZT transducer  366  is supported by wing  364 . PZT transducer  366  is connected to a measuring circuit  368  by way of electrically conductive wires  370  in flexible circuit  372 . Air-bearing surface  374  is located on the opposite side of glide head  360  from top surface  362 .  
       FIG. 10  depicts an embodiment of a disc drive system  400  including drive unit  402 , actuator assembly  404  and controller  406 . Drive unit  402  includes disc  408  and spindle  410  connected to a spindle motor. In the embodiment shown, actuator assembly  404  includes actuator  412 , support arm  414 , load beam  416  and gimbal/head assembly  418 . Actuator  412  controls the position of gimbal/head assembly  418  over disc  408  by rotating or laterally moving support arm  414 . Load beam  416  is located at the end of support arm  414  and gimbal/head assembly  418  is located at the end of load beam  416 . Controller  406  instructs actuator  412  regarding the position of support arm  414  over disc  408  and drive unit  402  regarding the control of the spindle motor. Gimbal/head assembly  418  is connected to controller  406  by way of flexible circuit  420 .  
      Gimbal/head assembly  418  includes a slider/head which, in operation, flies just above the disc surface.  FIG. 11  depicts an embodiment of a slider  440 . Slider  440  includes an air bearing surface  442  that is contoured to achieve the desired aerodynamic performance of slider  440 . In this embodiment, transducer  450  is located at or near the rear edge of slider  440  although the transducer can be located at other positions on the slider. Transducer  450  includes two electrical contacts that connect with flexible circuit  420 .  
      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.