Abstract:
Methods and devices are described that provide improved electromagnetic interference (EMI) protection for suspension assemblies. In one embodiment, a ground line is provided among the traces that are used in the suspension. Alternatively, a top ground plane is provided on top of the conductive traces with an interposed insulative layer. The ground line and/or the top ground plane provide EMI protection for read and write signals traveling through the traces (e.g., read and write traces) of the suspension assembly. The ground line and/or the top ground plane reduce the interaction between the read and the write traces, thus minimizing cross talk.

Description:
FIELD OF THE INVENTION 
   The present invention pertains to suspension designs for use in hard disk drives and the like. More particularly, the present invention pertains to electromagnetic interference (EMI) protection in suspension assemblies for hard disk drives. 
   BACKGROUND OF THE INVENTION  
   Hard disk drives are common information storage devices essentially consisting of a series of rotatable disks that are accessed by magnetic reading and writing elements. These data transferring elements, commonly known as transducers, are typically carried by and embedded in a slider body that is held in a close relative position over discrete data tracks formed on a disk to permit a read or write operation to be carried out. In order to properly position the transducer with respect to the disk surface, an air bearing surface (ABS) formed on the slider body experiences a fluid air flow that provides sufficient lift force to “fly” the slider and transducer above the disk data tracks. The high speed rotation of a magnetic disk generates a stream of air flow or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The air flow cooperates with the ABS of the slider body which enables the slider to fly above the spinning disk. In effect, the suspended slider is physically separated from the disk surface through this self-actuating air bearing. The ABS of a slider is generally configured on the slider surface facing the rotating disk, and greatly influences its ability to fly over the disk under various conditions. 
   As shown in  FIG. 1  an ABS design known for a common catamaran slider  5  may be formed with a pair of parallel rails  2  and  4  that extend along the outer edges of the slider surface facing the disk. Other ABS configurations including three or more additional rails, with various surface areas and geometries, have also been developed. The two rails  2  and  4  typically run along at least a portion of the slider body length from the leading edge  6  to the trailing edge  8 . The leading edge  6  is defined as the edge of the slider that the rotating disk passes before running the length of the slider  5  towards a trailing edge  8 . As shown, the leading edge  6  may be tapered despite the large undesirable tolerance typically associated with this machining process. The transducer or magnetic element  7  is typically mounted at some location along the trailing edge  8  of the slider as shown in  FIG. 1 . The rails  2  and  4  form an air bearing surface on which the slider flies, and provide the necessary lift upon contact with the air flow created by the spinning disk. As the disk rotates, the generated wind or air flow runs along underneath, and in between, the catamaran slider rails  2  and  4 . As the air flow passes beneath the rails  2  and  4 , the air pressure between the rails and the disk increases thereby providing positive pressurization and lift. Catamaran sliders generally create a sufficient amount of lift, or positive load force, to cause the slider to fly at appropriate heights above the rotating disk. In the absence of the rails  2  and  4 , the large surface area of the slider body  5  would produce an excessively large air bearing surface area. In general, as the air bearing surface area increases, the amount of lift created is also increased. Without rails, the slider would therefore fly too far from the rotating disk thereby foregoing all of the described benefits of having a low flying height. 
   As illustrated in  FIG. 2 , a head gimbal assembly  40  often provides the slider with multiple degrees of freedom such as vertical spacing, or pitch angle and roll angle which describe the flying height of the slider. As shown in  FIG. 2 , a suspension  74  holds the HGA  40  over the moving disk  76  (having edge  70 ) and moving in the direction indicated by arrow  80 . In operation of the disk drive shown in  FIG. 2 , an actuator  72  moves the HGA over various diameters of the disk  76  (e.g., inner diameter (ID), middle diameter (MD) and outer diameter (OD)) over arc  75 . 
   A preamplifier is typically connected to the head to supply write currents to the write head and receive currents from the read head. The suspension provides two functions: mechanical support and electrical connection between the head and the preamplifier. Rather than using physical wires to connect the head to the preamplifier, metal traces on the suspension are quite often used. 
   In the art, there are typically two types of wireless suspensions. In the first type, such as trace suspension assemblies (TSAs) and circuit integrated suspension (CISs), traces are built though a subtractive process (e.g., an etching operation) or through an additive process (e.g., a plating or deposition process) on the stainless steel flexure, with an insulative layer between the trace and the flexure. After the traces are set in place, the flexure can then be welded to other parts of the suspension. In the second type, such as flex suspension assemblies (FSAs) and flex on suspension (FOS), the traces are built on an insulation layer and then covered with another insulation layer to form a flex circuit. This circuit is then attached to the suspension with adhesive. Alternatively, an additional metal layer called a ground plane can be attached to the flex circuit before it is adhered to the suspension. 
   Typically four traces are supplied for a single head: one pair for the connection between the read head and the pre amplifier and one pair for the connection between the write head and the preamplifier. In the art, it is known that the stainless steel flexure, the suspension, and the ground plane (if one is provided) may provide some protection against electromagnetic interference (EMI). If this protection is not sufficient, then EMI can cause errors to occur in the data stream. Thus, there is a need for an improved system for controlling EMI in a suspension assembly. 
   SUMMARY OF THE INVENTION 
   According to an embodiment of the present invention, a suspension assembly is provided with one or more additional traces to provide EMI protection for the read and write traces. In an alternative embodiment, a separate ground plane coupled near the read and write traces provides the EMI protection. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a slider device with a read/write head that is known in the art. 
       FIG. 2  is a perspective view of a disk drive device that is known in the art. 
       FIG. 3  is cross-sectional view of a wireless suspension according to an embodiment of the present invention. 
       FIGS. 4   a  and  4   b  are top views of a suspension assembly according to an embodiment of the present invention. 
       FIG. 5  is a cross-sectional view of a wireless suspension according to another embodiment of the present invention. 
       FIG. 6  is a cross-sectional view of a wireless suspension according to another embodiment of the present invention. 
       FIG. 7  is a cross-sectional view of a wireless suspension according to another embodiment of the present invention. 
       FIG. 8  is an illustration of cross talk signal induced by a write signal. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 3 , a cross section perpendicular to the traces of a suspension assembly constructed according to an embodiment of the present invention is shown. In  FIG. 3 , a variety of traces  13   a ,  13   b  are provided in a suspension assembly  11 . Because the traces are electrical signal conductors, their interconnection with the head and preamplifier can lead to adverse effects. First, the electrical impedance of the traces can affect the amount and speed of the current traveling to the head in the write traces  13   a  and the magnitude of the read-back signal transmitted back to the read input of the pre-amplifier. Second, the read trace  13   b  can sense the write current going though the adjacent write trace and produce current through the read head (i.e., cross talk). Third, the read trace can pick up stray EM fields in the drive and produce noise in the read back signal, which generates errors in the data retrieval process. 
   Referring to  FIG. 3 , the flexure  21  may be made of stainless steel (for example). The flexure  21  and/or metal plane  17  may serve as ground planes for the traces  13   a  and  13   b . The main function of the ground plane in this embodiment is impedance control, though it may also provide some benefits in noise and cross talk reduction. An insulation layer  22  is provided on top of the metal plane  17  to provide electrical insulation between the flexure  21  with ground plane  17  and the traces  13   a ,  13   b . According to an embodiment of the present invention, a ground line  24  is provided between the first pair of traces  13   a  and the second pair of traces  13   b . The ground line may be provided in a manner similar to the traces (e.g., in TSA and FSA suspensions). In this embodiment the suspension assembly is provided with a top insulation layer  15  that covers each of the traces  13   a  and  13   b  along with the ground line  24 . Ground line  24  may be coupled to ground plane  17 , flexure  21  or a separate grounding source. 
   Referring to  FIGS. 4   a–b , top views of a suspension assembly are shown. In  FIG. 4   a , the suspension assembly  31  includes a slider  33  and a flex circuit  35 . In  FIG. 4   b , a portion of the flex circuit  35  of  FIG. 4   a  is shown. The flex circuit  35  includes a pair of read traces  37   a  and a pair of write traces  37   b  with a ground line  39  positioned between the two pairs of traces  37   a ,  37   b . As described above, the ground line  39  provides EMI protection for the two pairs of traces  37   a ,  37   b.    
   Referring to  FIG. 5 , a cross section of another suspension assembly is shown. In this embodiment, the top insulation layer  15  is etched or deposited in such a manner that the ground line  24  is exposed. A top ground plane  26  can then be provided on top of the top insulation layer  15  and the ground line  24 . 
   Referring to  FIG. 6 , a cross section, parallel to the traces, of another suspension assembly is shown constructed according to an embodiment of the present invention. In this embodiment a preamp ground connection  30  is provided on an insulation layer  29  and a support base  28 . The ground connection  30  can be coupled to the ground line  24  with solder  31 . The ground connection  30  may be coupled to the ground line  24  using a variety of other methods including ultrasonic bonding. In the suspension assembly of  FIG. 6 , a section of the ground line  24  is isolated from the top insulation layer  15 , bottom insulation layer  22 , top ground plane  26  and bottom ground plane  21  and  27 . 
   Referring to  FIG. 7 , a similar cross section of a suspension assembly to that presented in  FIG. 6  is shown. In this example, a preamp ground is not provided. Instead, the ground line  24  is connected to the stainless steel structure of the flexure  21  with a conductive adhesive  32 , or other conductive material. 
   Referring to  FIG. 8 , an example of cross talk induced by a write signal is shown. In this example, the coefficient of “forward” cross talk  41  (in seconds/meter) represents the cross talk in the read trace  13   b  traveling in the same direction as the write signal  40  in the write trace  13   a , which induced thecross talk. The magnitude of the cross talk is equal to the magnitude of the write signal multiplied by the forward cross talk coefficient and the length of the trace, from the preamplifier  43  to the transducer  44 , and divided by the rise time of the signal. The coefficient of “backward” cross talk  42  is a unitless coefficient that represents the cross talk in the read trace  13   b  traveling in the opposite direction as the write signal  40 , which induced the cross talk. The magnitude of the cross talk signal is equal to the magnitude of the write signal multiplied by this coefficient. 
   Using a standard wireless suspension design, the use of a ground line according to embodiments of the present invention may result in a significant reduction in cross talk between the pairs of traces  13   a  and  13   b . As shown in Table I, the cross talk coefficients in the forward and backward directions are shown for three examples—a wireless suspension with a ground plane only, a ground plane and ground line (e.g., ground line  24 ), a ground plane, a ground line, and a top ground plane (e.g., top ground plane  26 ). 
                           TABLE I               Wireless Suspension   Fwd. Cross Talk C   Bkwd Cross Talk C                   Bottom Ground Plane 27   −2.5 × 10 −11     3.5 × 10 −3         Only       Bottom Ground Plane 27 and   −2.0 × 10 −11     2.8 × 10 −3         Ground Line 24       Ground Plane 27, Ground   −1.6 × 10 −17     3.7 × 10 −9         Line 24, and Top Ground       Plane 26                    
From Table I, the cross talk coefficient is reduced by approximately 20% when a ground line is used. A much more significant reduction is seen when a top ground plane is used with a ground line and bottom ground plane. It is noted that the suspension designs of the present invention may have a significant effect on reducing noise pick up in the pairs of traces  13   a  and  13   b . Furthermore, the grounding system of the present invention may reduce electrostatic discharge (ESD) damage to the suspension. This is due, in part, to the presence of the top ground plane preventing friction induced charging on the insulation layer (e.g., layer  15 ) in the suspension. Also, the grounding system of the present invention may provide a wide range of impedance in the pairs of traces  13   a  and  13   b  in part because the ground planes increase the capacitance of the traces and lower the impedance beyond what single ground can provide.
 
   While the present invention has been described with reference to the aforementioned applications, this description of the preferred embodiments is not meant to be construed in a limiting sense. It shall be understood that all aspects of the present invention are not limited to the specific depictions, configurations or dimensions set forth herein which depend upon a variety of principles and variables. Various modifications in form and detail of the disclosed apparatus, as well as other variations of the present invention, will be apparent to a person skilled in the art upon reference to the present disclosure. It is therefore contemplated that the appended claims shall cover any such modifications or variations of the described embodiments as falling within the true spirit and scope of the present invention.