Abstract:
Inspection windows are cut or formed into the tail section of the flexure circuit tail in a hard disk drive Head Gimbal Assembly (HGA), or CIS, to enable visual inspection of the alignment of the CIS to the head preamp circuit, or FPC. The holes are made in the steel backing and base polyimide, and are positioned between adjacent conductive pads. In addition to facilitating visual inspection, the windows also enable rework of solder. Additionally, solder wicking holes may also be provided in the conductive pads and/or the polyimide and steel backing.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application Ser. No. 60/854,486, filed Oct. 25, 2006, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to manufacturing of hard disk drives (HDD) and, more particularly, to assembly, manufacturing and inspection of hard disk drive components. 
         [0004]    2. Description of Related Art 
         [0005]      FIGS. 1   a  and  1   b  depict a hard drive suitable for implementing embodiments of the subject invention.  FIG. 1   a  depicts the hard drive  100  with the cover removed, while  FIG. 1   b  depicts an enlarged image of the preamp area of the hard drive  100 . The hard drive  100  uses rotating platters (disks)  110  to store data. Each platter is rotated by a spindle (not shown) and has a smooth magnetic coated surface on which digital data is stored. Information is written to the disk by applying a magnetic field from a read-write head (not shown) that is attached to an actuator arm  120 . For reading, the read-write head detects the magnetic flux emanating from the magnetic bits that were written onto the platter. Since the signals from the read/write head is very faint, a preamp  130  is provided in close proximity to the head. The preamp  130  is a chip that is mounted on a substrate  140 . The substrate  140  is mounted onto a carrier plate  150 , that connects to the actuator arm assembly  120 . The flexible circuit loop  160  is connected to the substrate  140 , to transfer signals between the preamp  130  and the associated electronics (not shown). The associated electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. 
         [0006]    The read/write signal transmission between the pre-amp  130  and the read-write heads, is also done over a flexible circuit, generally referred to in the industry as “flexure” with a family of traces. The flexure is not shown in  FIG. 1   a , but part of it,  170 , is shown in  FIG. 1   b . The flexure  170  is connected, e.g., soldered, to a Flexible Printed Circuit (FPC)  180 , which in turn connects to the pre-amp  130 . 
         [0007]    The flexure technology is well known in the art as a suspension component to propagate the signal between the circuit board and the magnetic head when writing and reading to the disk. The characteristics required for flexures used in hard disk drives are low stiffness, high electrical conductivity, and high accuracy. In general, there are two types of manufacturing processes for flexures: additive and subtractive. In the additive type manufacturing an insulating layer (e.g., a polyimide) is provided over a base layer (e.g., stainless steel) and a conductor layer (e.g., copper) is deposited on top of the insulating layer. 
         [0008]    This type of flexure is generally referred to as ‘CIS’ (Circuit Integrated Suspension). In the subtractive type of manufacturing, a tri-laminate sheet having a base layer (e.g., stainless), an insulating layer (e.g., polyimide) and a conductor layer (e.g., copper) is the starting material. Using techniques such as etching the various elements of the flexure are defined. This type of flexure is generally referred to as ‘ILS’ (Integrated Laminate Suspension) or ‘TSA’ (Trace Suspension Assembly). 
         [0009]    In this document, the term Flexible Printed Circuit (FPC) refers to the dynamic flex circuit connected to the pre-amp; while flexure (also referred to as CIS (Circuit Integrated Suspension)) refers to the connecting circuit used to transmit the signals between the read/write heads and the pre-amp. 
         [0010]    In the prior art it is known to solder or bond copper conductors of the FPC, to copper conductors of the flexure tail. Due to the small sizes of the conductors, it is very critical that the conductors of the FPC and the flexure be aligned properly. However, in the prior art once the flexure is laid upon the FPC, it is sometimes not possible to visually inspect the alignment of the conductors. Poor alignment may result in shorts (solder bridging) between adjacent conductors or opens. Additionally, once the two parts are bonded or soldered, there is no possibility to inspect the integrity of the bonding and/or to make any repairs. 
         [0011]      FIG. 2   a  is a top view of the prior art CIS over FPC in proper alignment, while  FIG. 2   b  is a semi-exploded view of the CIS and FPC before soldering. In  FIG. 2   a , all layers are shown as transparent to enable better understanding of the layout. In  FIG. 2   b , the CIS copper layer  315  is exploded away from its base substrate  310  for illustration purposes.  FIG. 2   c  illustrates a top view and a corresponding cross-section view of the prior art assembly, while  FIG. 2   d  is a cross-section at line A-A of  FIG. 2   a , illustrating the prior art layout. As can be seen from these Figures, the proper alignment of the CIS and FPC cannot be verified, and errors cannot be corrected. 
         [0012]    Other background information can be found in, e.g., U.S. Pat. Nos. 5,955,176 and 6,399,899. 
       SUMMARY 
       [0013]    Various embodiments of the subject invention facilitate visual inspection of mated FPC/flexure parts and enable rework of solder bridging between adjacent conductors should they occur. The various embodiments of the invention are applicable to both CIS, ILS and TSA flexure circuits. According to various embodiments of the invention, small windows (or openings) are cut or formed into the base polyimide between adjacent conductors to facilitate visual inspection and rework of solder. This will allow visual inspection and also allow the rework of pieces that would otherwise be scrapped. 
         [0014]    According to aspects of the invention, a flexible circuit is provided, comprising: a steel backing layer; an insulating layer provided over the steel backing layer; a conductive circuitry provided over the polyimide layer; and inspection cutouts provided in the steel backing layer and in the insulating layer, the cutouts located to enable visual inspection of selected areas of the conductive circuitry. The steel backing layer may comprise stainless steel. The insulating layer may comprise polyimide. The conductive circuitry may comprise copper. The flexible circuit may further comprise solder-wicking holes provided in the conductive circuitry. The conductive circuitry may comprise conductive pads and the insulating layer may be slightly narrower than the conductive pads. 
         [0015]    According to other aspects of the invention, a combination flexible circuit and flexible printed circuit (FPC) is provided, comprising: a flexible circuit comprising a steel backing layer, an insulating layer provided over the steel backing, and conductive pads provided over the insulating layer; an FPC comprising FPC insulating layer and FPC conductive pads; wherein selected conductive pads are soldered to corresponding FPC conductive pads, and wherein inspection cutouts are provided in the steel backing layer and in the insulating layer to enable visual inspection of at least some of the conductive pads that are soldered to corresponding FPC conductive pads. 
         [0016]    According to further aspects of the invention a method for manufacturing flexible circuit is provided, comprising: providing steel backing layer; providing an insulating layer over the steel backing layer; providing a plurality of contact pads over the insulating layer; and providing cutouts in the insulating layer to enable visual inspection of at least some of the plurality of contact pads. 
         [0017]    According to yet further aspects of the invention, a method for manufacturing a combination of flexible circuit and flexible printed circuit FPC is provided, comprising: providing a steel backing layer; providing an insulating layer over the steel backing layer; providing a plurality of contact pads over the insulating layer; providing cutouts in the insulating layer to enable visual inspection of at least some of the plurality of contact pads; providing an FPC comprising FPC insulating layer and FPC conductive pads; soldering at least selected conductive pads to corresponding FPC conductive pads; inspecting the quality of the soldering via the cutouts. 
         [0018]    According to further aspects of the invention, in the flexible circuit the conductive circuitry may comprise conductive pads and the cutouts may be sized so as to expose edges of the conductive pads. The cutouts may be rectangular, circular, or other suitable shape. 
         [0019]    According to further aspects of the invention, a method for manufacturing flexible circuit is provided, comprising: fabricating a combination of insulation layer sandwiched between a backing layer and a conductive pads layer; fabricating cut-out holes in the combination to enable visual inspection of the conductive pads layer from the backing layer side. The cut-out holes are cut in the backing layer and in the insulation layer. The method may further comprise fabricating solder-wicking holes in the conductive pads layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIGS. 1   a - 1   b  depict a hard drive suitable for implementing embodiments of the subject invention. 
           [0021]      FIG. 2   a  is a top view of the prior art CIS over FPC in proper alignment, while  FIG. 2   b  is an exploded view of the CIS and FPC before soldering. 
           [0022]      FIG. 2   c  illustrates a top view and a corresponding cross-section view of the prior art assembly, while  FIG. 2   d  is a cross-section at line A-A of  FIG. 2   a , illustrating the prior art layout. 
           [0023]      FIG. 3   a  is a top view of an CIS over FPC in proper alignment according to an embodiment of the invention, while  FIG. 3   b  is a semi-exploded view of the CIS and FPC according to an embodiment of the invention. 
           [0024]      FIG. 4  is a cross section along line B-B of  FIG. 3   a.    
           [0025]      FIG. 5   a  illustrates the flexure tail of the prior art, while  FIG. 5   b  illustrates an embodiment of the flexure tail according to the subject invention. 
           [0026]      FIGS. 6   a  and  6   b  depict another embodiment of the invention, incorporating Nitto Denko&#39;s dog-bone shaped contact pads. 
           [0027]      FIGS. 7   a  and  7   b  depict another embodiment wherein solder-wicking holes are used. 
           [0028]      FIGS. 8   a  and  8   b  depict yet another embodiment of the invention using exposed edge. 
           [0029]      FIG. 9  depicts an example wherein the window is circular or oval. 
           [0030]      FIG. 10  depicts an example of the CIS positioned over the FPC, wherein the CIS has alignment windows according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 3   a  is a top view of a CIS over FPC in proper alignment according to an embodiment of the invention, while  FIG. 3   b  is a semi-exploded view of the CIS and FPC according to an embodiment of the invention. As shown, the CIS flexure of this embodiment comprises a sandwich of steel upper layer  305  (e.g., stainless steel), an insulating substrate  310 , e.g., polyimide layer, and a plurality of conducting contacts  315 , e.g., copper traces. The contacts  315  need to be aligned with and connected to the FPC contacts  320 , which are generally also copper traces. The FPC contacts  320  are provided over the FPC substrate  325 . These described elements are similar to the elements of the prior art FPC and flexure. However, unlike the prior art, as shown in  FIGS. 3   a  and  3   b , windows or cut-outs  350  are made in the CIS substrate  310  corresponding to and aligned with windows or cut-outs in the CIS steel layer  305 . This enables visual inspection of the alignment of the flexure contacts  315  and FPC contacts  320 . Additionally, the openings  350  enable inspection after bonding or soldering is completed and making repairs if necessary. 
         [0032]      FIG. 4  is a cross section along line B-B of  FIG. 3   a , illustrating the opening and visual inspection. As can be seen in  FIGS. 3   b  and  4  (see measurement arrows A and B), the openings are made sufficiently wide so to expose part of the FPC contacts once the flexure is mated to the FPC. In this manner, verification of proper alignment can be easily made and access is provided for any necessary repairs. For example, one may check to see that measurement A equals measurement B, inferring that the copper to copper alignment exists. 
         [0033]      FIG. 5   a  illustrates the flexure of the prior art, wherein the polyimide layer  310  obstructs the view for any visual inspection.  FIG. 5   b , on the other hand, illustrates an embodiment of the flexure according to the subject invention enabling inspection and repair. Comparison of the flexures of  FIGS. 5   a  and  5   b  clearly reveals the difference between the two for a better understanding of the benefits of the inventive flexure. That is, in  FIG. 5   b , windows  350  are cut in the polyimide layer so that one can inspect the alignment of the contacting pads. 
         [0034]      FIGS. 6   a  and  6   b  depict another embodiment of the invention. This embodiment incorporates the “dog bone shaped” pad variation. This embodiment is essentially the same as the basic configuration illustrated in  FIGS. 3   a  and  3   b , except that the flexure incorporates Nitto Denko&#39;s unique “dog-bone” shaped contact pads  615 . In  FIGS. 6   a  and  6   b , elements similar to that of  FIGS. 3   a  and  3   b  are indicated with the same reference, except that they are in the 6xx series. As can be send from  FIG. 6   b , the windows enable validation of the alignment and access for repairs if necessary, just as with the previous embodiment. In this embodiment, the windows  650  are shaped in a manner enabling the dog-bone shape of the contact pads  615  to remain unexposed. However, other shapes are possible and some of the shapes may expose part of the contact pads. 
         [0035]      FIGS. 7   a  and  7   b  depict another embodiment wherein solder-wicking holes  760  are used. Elements in  FIGS. 7   a  and  7   b  which are similar to elements of other embodiments are indicated using same references, except that they are in the 7xx series. It should be appreciated that while this solder-wicking embodiment is illustrated in conjunction with the dog-bone feature, it may be implemented using any of the other embodiments implementing the invention and does not require the dog-bone shaped contact pads. Alternate shapes and sizes of holes  760  can be considered to be within the spirit and scope of the present invention. 
         [0036]    As shown in  FIGS. 7   a  and  7   b , small holes  760  are provided in the contact pads  715  to allow the solder to wick through to enhance the strength of the solder joint. In one embodiment, the solder-wicking holes are provided only in the contact pads  715 ; however, in the embodiment illustrated in  FIGS. 7   a  and  7   b  slightly larger holes are also provided in the substrate  710  and the steel-backing layer  705 . This provides space for excess solder and allows for inspection of the alignment and integrity of the solder. If steel backing is used, it is recommended to isolate the steel into small “islands” to help minimize potential shorting between the wicked solder and the steel. This embodiment is particularly beneficial for “double-sided” parts. Double-sided refers to flexures in which there are exposed conductors from both sides of the base polyimide substrate. By providing these steel “islands,” the conventional hot-bar may be used for soldering, while minimizing shorts. 
         [0037]      FIGS. 8   a  and  8   b  depict yet another embodiment of the invention using exposed edge. Elements in  FIGS. 8   a  and  8   b  which are similar to elements of other embodiments are indicated using same references, except that they are in the 8xx series. In this embodiment, the supporting base polyimide  810  is slightly narrower than the width of the copper pads  815 . The windows  850  in the metal and the polyimide are made large enough so as to visually expose the edges of contact pad  815  of the CIS. To prevent potential shorting of the copper pads  815  to steel edge  805 , the copper pad is flared at the extremities of the pad&#39;s length. The advantage of this variation is that the solder connection between FPC and flexure can be seen at the flexure pad edges. If steel backing is used, it is recommended to reduce the width of the steel support to help minimize the risk of shorting the reflowed solder to the steel “spine”. This variation is also beneficial for “double-sided” parts. 
         [0038]    In the various embodiments shown, the inspection window or cut out is shown square. However, other shapes, such as a circular hole, may be used.  FIG. 9  depicts an example wherein the inspection window  950  is circular or oval. 
         [0039]      FIG. 10  depicts an example of the CIS  10  positioned over an FPC  12 , wherein the CIS  10  has alignment windows  15  according to an embodiment of the invention. As illustrated in  FIG. 10 , the alignment of the CIS  10  and FPC  12  can be confirmed using the alignment windows  15 .