Patent Publication Number: US-7593190-B1

Title: Flexure design and assembly process for attachment of slider using solder and laser reflow

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of U.S. patent application Ser. No. 10/026,152, filed Dec. 21, 2001, which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates in general to data storage systems such as disk drives, and it particularly relates to a slider and a flexure to which the slider is attached. More specifically, the present invention provides a novel flexure design and assembly process for securing the slider to the flexure by means of solder bumps applied to the metalized slider surface. 
   BACKGROUND OF THE INVENTION 
   In a conventional magnetic storage system, a thin film magnetic head includes an inductive read/write element mounted on a slider. The magnetic head is coupled to a rotary actuator magnet and a voice coil assembly by a suspension and an actuator arm positioned over a surface of a spinning magnetic disk. In operation, a lift force is generated by the aerodynamic interaction between the magnetic head and the spinning magnetic disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height is maintained over a full radial stroke of the rotary actuator assembly above the surface of the spinning magnetic disk. 
   The suspension assembly includes a resilient load beam, and a flexure to which the slider with a magnetic read/write head is attached. The load beam generally directs the slider toward the air bearing surface (ABS) at a predetermined angle. The aerodynamic force generated by the ABS is reacted by the load beam to maintain the slider over the surface of the spinning magnetic disk at a predetermined flying height. 
   In a conventional magnetic disk drive, the slider is attached to the flexure by means of an adhesive connection at its interface surface with the flexure. A conventional method of attaching the slider to the suspension that is in common use in the industry typically involves creating a permanent adhesive bond between the slider and the suspension. The method of using an epoxy bonding technique is illustrated in  FIG. 6 . 
   A disadvantage of the epoxy bonding method emanates from the permanence of the bond in that any attempt to separate the slider from the suspension would typically necessitate breaking the bond and thus inducing a potential irreversible damage to the suspension-flexure assembly. 
   Various attempts have been made to alleviate the foregoing concern. Slider-suspension assembly technologies such as solder bumping, under-bump metallization, and flip chip are known in the industry for providing solder bonding process in lieu of epoxy bonding. 
   One such attempt is exemplified by U.S. Pat. No. 4,761,699 to Ainslie et al. that describes a slider-suspension assembly suitable for mechanically and electrically joining the two components using solder bonding. The bonding method uses simultaneous reflow of all solder bumps, which might necessitate global heating of the entire assembly, including the thin film read/write head. 
   While conventional methods may have addressed and resolved certain aspects of the foregoing concern, they are not completely satisfactory in that the use of discrete solder contact pads requires masking process steps in manufacturing of slider. In addition, global heating to reflow typical solder alloys could require temperature exposure that is incompatible with the temperature limitation of the read/write head. The need for a comprehensive solution has heretofore remained unsatisfied. 
   SUMMARY OF THE INVENTION 
   The present invention can be regarded as a method of manufacturing a head gimbal assembly for use in a data storage system. A slider is fabricated to have an air bearing surface and a backside that opposes the air bearing surface. A plurality of solder bumps is deposited on the backside. The slider is positioned to be adjacent to a flexure having at least a thermally conductive flexure tongue coated with an insulation layer, and a pattern of receptacles that extend through the insulation layer to the flexure tongue, such that the solder bumps are substantially aligned with the receptacles. A laser beam is directed to the flexure tongue to heat the flexure tongue sufficiently to melt the solder bumps. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention and the manner of attaining them, will become apparent, and the invention itself will be understood by reference to the following description and the accompanying drawings, wherein: 
       FIG. 1  is a fragmentary perspective view of a data storage system utilizing a read/write head of the present invention; 
       FIG. 2  is a perspective view of a head gimbal assembly (HGA) comprised of a suspension, and a slider to which the read/write head of  FIG. 1  is secured, for use in a head stack assembly; 
       FIG. 3  is an enlarged, fragmentary, side elevational view of the HGA of  FIG. 2 ; 
       FIG. 4  is a top plan view of a load beam that forms part of the HGA of  FIG. 2 ; 
       FIG. 5  is an isometric view of a flexure that forms part of the HGA of  FIG. 2 ; 
       FIG. 6  is an enlarged, side view of a conventional slider/suspension assembly, illustrating the method of epoxy bonding for securing the slider to the suspension; 
       FIG. 7  is a side view of another conventional slider/suspension assembly, illustrating the method of solder bonding for securing the slider to the suspension; 
       FIG. 8  is a side view of a preferred embodiment of the slider/suspension assembly, illustrating the solder reflow process according to the present invention; 
       FIG. 9  is a bottom plan view of the slider shown secured to a flexure tongue, and illustrating a solder bump pattern; 
       FIG. 10  is a side view of an alternate embodiment of the slider/suspension assembly, illustrating the solder reflow process according to the present invention; 
       FIG. 11  is a top plan view of a wafer, illustrating the metallization process of the slider at the wafer level, according to the present invention; 
       FIG. 12  is a perspective view of a slider bar that has been diced from the wafer of  FIG. 11 ; and 
       FIG. 13  is an exploded view of a slider/suspension assembly of the present invention. 
   

   Similar numerals in the drawings refer to similar elements. It should be understood that the sizes of the different components in the figures might not be in exact proportion, and are shown for visual clarity and for the purpose of explanation. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a disk drive  10  comprised of a head stack assembly  12  and a stack of spaced apart magnetic data storage disks or media  14  that are rotatable about a common shaft  15 . The head stack assembly  12  is rotatable about an actuator axis  16  in the direction of the arrow C. The head stack assembly  12  includes a number of actuator arms, only three of which  18 A,  18 B,  18 C are illustrated, which extend into spacings between the disks  14 . 
   The head stack assembly  12  further includes an E-shaped block  19  and a magnetic rotor  20  attached to the block  19  in a position diametrically opposite to the actuator arms  18 A,  18 B,  18 C. The rotor  20  cooperates with a stator (not shown) for rotating in an arc about the actuator axis  16 . Energizing a coil of the rotor  20  with a direct current in one polarity or the reverse polarity causes the head stack assembly  12 , including the actuator arms  18 A,  18 B,  18 C, to rotate about the actuator axis  16  in a direction substantially radial to the disks  14 . 
   A head gimbal assembly (HGA)  28  is secured to each of the actuator arms, for instance  18 A. With reference to  FIG. 2 , the HGA  28  is comprised of a suspension  33  and a read/write transducer or head  35 . The suspension  33  includes a resilient load beam  36  and a flexure  40  to which the head  35  is secured. 
   The head  35  is formed of a slider  47  secured to the free end of the load beam  36  by means of the flexure  40 , and a read/write (or data transducing) element  50  supported by the slider  47 . The slider  47  can be any conventional or available slider. The read/write element  50  is mounted at the trailing edge  55  of the slider  47  so that its forwardmost tip is generally flush with the ABS of the slider  47 , which is parallel to the surface of the disks  14 . The backside  58  of the slider  47 , opposite to the ABS, is attached to a tongue  56  of the flexure  40 , as illustrated in  FIG. 3 . 
   With reference to  FIG. 5 , the flexure  40  includes a tongue  56  that extends inwardly within a clearance  158  formed in a flexure body  180 . As shown in  FIG. 3 , the flexure  40  provides the slider  47  with sufficient flexibility in various degrees of movement for accommodating the uneven topology of the disk (or data medium) surface and other components and drive assembly tolerances, while the slider  47  is flying over the disk. The flexure  40  is sufficiently stiff in a direction parallel to the disk plane, to resist physical deflection that may be caused by the rapid movement of the actuator arms  18 A,  18 B,  18 C. 
   The slider  47  is part of the read-write head  35 , and is secured to the tongue  56  by means of the technique described herein, which will be described in more detail in connection with  FIGS. 8 through 13 . A plurality of conductive contact pads  161  are secured to a trailing edge  55  of the slider  47 , with only one contact pad  161  being shown in  FIG. 3  for the purpose of illustration only. These contact pads  161  are electrically connected to a read/write element  50  by means of conductive traces. 
   With reference to  FIGS. 3 and 4 , a dimple  268  is formed in the forward tip  265  the load beam  36 , and is urged against the backside of the tongue  56 , for contributing to the gimbaling action. Alternatively, the dimple  268  may be formed on the tongue  56 , and urged against the underside of the load beam  36 . 
   The load beam tip  265  is positioned above the flexure clearance  158  and at least part of the tongue  56 . In one embodiment, the tip  265  extends integrally in a body  266  having two stiffening rails  267  projecting substantially along the length of the body  266 . The body  266  includes datum features  292 ,  294 , and  296  ( FIG. 4 ). 
   Referring now to  FIG. 6 , it illustrates a conventional slider/suspension assembly  600  that is assembled using a known epoxy adhesive bonding process. The backside  58  of the slider  47  is secured to the stainless flexure tongue  56  of the flexure  40  by a thin film of epoxy adhesive layer  682  that is deposited between the slider backside  58  and the flexure tongue  56 . The thin film of adhesive layer  682  may be a combination of adhesives each contributing different characteristics to the connection such as mechanical strength or electrical conductivity. 
   A plurality of dielectric pads  64  are positioned forward and rearward of the slider  47  between the slider backside  58  and the flexure tongue  56 . The dielectric pads  64  provide the electrical insulation for the slider  47  from making electrical contacts with the stainless steel flexure tongue  56 . 
   Typically, the epoxy adhesive layer  682  is prepared with a hardener at room temperature. Upon applying the epoxy adhesive to the flexure tongue  56 , a curing process at an elevated temperature is carried out by subjecting the slider/suspension assembly  600  to a thermal source, such as in a curing oven. The post-cure epoxy adhesive layer  682  reaches its optimal mechanical strength and provides a rigid mechanical connection of the slider  47  to the flexure  40 . An electrical connection of the slider  47  to the flexure  40  is then made by means of an electrical trace that connects the terminal pad  62  mounted at the trailing edge  55  of the slider  47  to the copper trace layer  59 , which rests upon a dielectric layer  61  in turn resting upon the stainless steel flexure tongue  56 . Upon curing, the epoxy adhesive layer  682  then becomes permanent and irreversible. In the event that the read/write head  35  should fail any of the inspection criteria, the flexure  40  and possibly the suspension  33 , together with the defective read/write head  35  would be discarded. 
     FIG. 7  illustrates another conventional slider/suspension assembly  700  that utilizes a conventional solder bonding process. According to this process, a plurality of solder bumps  780  are placed directly between the backside  58  of the slider  47  and the flexure tongue  56 . Upon heating, the solder bumps  780  reflow to wet the interface surface between the slider backside  58  and the tongue  56 , thereby creating a solid mechanical connection between the slider  47  and the flexure  40 . 
   During the reflow processing of the solder bumps  680 , the slider/suspension assembly  700  is usually subjected to heating to an elevated temperature that might be incompatible with the thermal rating of the read/write element  50 , thus potentially adversely affecting the performance of the read/write element  50  in an adverse manner. The use of solder bumps  680  on discrete solder pads usually requires an additional masking process in manufacturing the slider, which further increases the manufacturing complexity and cost. 
     FIGS. 8 and 9  illustrate a preferred embodiment of the slider/suspension assembly  800  made according to the present invention. The backside  58  of the slider  47  is positioned adjacent to the tongue  56  of the flexure  40 . 
   In this exemplary embodiment, the flexure  40  is formed of a multi-layer (i.e., three-layer) material. The first layer is known as the metallic bond pad  862 . The metallic bond pad  862  is made of a material that is compatible with a fluxless solder process, and also promotes adhesion of the solder material to the slider/suspension assembly  800 . 
   An exemplary material of the metallic bond pad  862  is gold plated copper. In general, the metallic bond pad  862  could extend from the leading edge tip  850  to the trailing edge tip  852  of the flexure  40 , which are located beyond the respective leading edge surface  156  and trailing edge surface  55  of the slider  47 . 
   The second or intermediate layer of the flexure  40  is a polyimide insulator layer  864 , which provides electrical insulation of the slider electrical connection. The leading edge  866  of the polyimide insulator layer  864  is recessed inward at a substantial distance relative to the leading edge tip  850  of the flexure  40  such that it is aligned with the leading edge surface  156  of the slider  47 . The polyimide insulator surface  864  spans a distance from its leading edge  866  to the trailing edge tip  852  of the flexure  40 . Alternatively, the leading edge  866  of the polyimide insulator layer  864  could be made to coincide with the leading edge tip  850  of the flexure  40  without substantially departing from the flexure design according to the present invention. 
   The third layer of the flexure  40  is a stainless steel flexure tongue  868  that provides the necessary resiliency to the slider/suspension assembly  800  to withstand the external aerodynamic forces induced in the ABS region. The stainless steel flexure tongue  868  generally extends from the leading edge tip  850  to the trailing edge tip  852  of the flexure  40 , so that its length is approximately the same as the length of the metallic bond pad  862 . 
   Still with reference to  FIGS. 8 and 9 , a plurality of solder bump receptacles  900  are also incorporated into the flexure  40 . For the purpose of clarity, only two solder bump receptacles  900  are illustrated in  FIG. 8 . It should be understood that the preferred embodiment may include a different number of solder bump receptacles  900 . 
   An exemplary solder bump receptacle  900  is generally formed by a cylindrical depression or cutout through the metallic bond pad  862  and the polyimide insulator layer  864 . The radius  906  of the cylindrical surface of the solder bump receptacle  900  is approximately 75 microns such that its measure is greater than the depth  908  of the solder bump receptacle. The opening  902  of the solder bump receptacle  900  lies on top of the metallic bond pad  862 , while the bottom  904  of the solder bump receptacle  900  lies on the surface of the stainless steel flexure tongue  868  adjacent to the polyimide insulator layer  864 . 
   A plurality of solder bumps  872  are then placed into the solder bump receptacles  900 . An exemplary solder bump  872  is generally of a hemispherical shape with a radius  876  of approximately 80 microns that is approximately the same as the radius  906  of the solder bump receptacles  900 . The solder bumps  872  may be made of a conventional solder material such as a eutectic tin lead alloy. 
   The solder bumps  872  may be applied to the back side  58  of the slider  47  in a pattern as illustrated in  FIG. 9 . It should be understood that a different pattern could be used in accordance with the present invention. The pattern of the solder bump locations corresponds to the locations of the receptacles  900  on the flexure  40 . 
   The solder bumps  872  are positioned within the solder bump receptacles  900  in a manner such that the flat surface  874  of the solder bumps  872  is protruded above the opening  902  and the convex surface  876  of the solder bumps  872  rests upon the bottom  904  of the solder bump receptacles  900 . The flat surface  874  of the solder bumps  872  is generally parallel to the surface of the metallic bond pad  862 . The protrusion of the solder bumps  872  above the metallic bond pad  862  is enabled by the geometric feature of the solder bump receptacles  900  in that its radius  906  is greater than its depth  908 . 
   The distance of the protrusion of the solder bumps  872  above the metallic bond pad  862  is defined by the volume of the solder bumps  872 , which in general must be greater than the volume of the solder bump receptacles  900  in order to form a bond line  870  between the back side  58  of the slider  47  and the metallic bond pad  862 . The thickness of the solder bond line  870  can be controlled by adjusting either the volume of the solder bumps  872  or the volume of the solder bump receptacles  900  accordingly. 
   Increasing the volume of the solder bumps  872  would result in greater excess material to form a thicker bond line  870 . Similarly, using a smaller volume of the solder bump receptacles  900  would accomplish the same objective as increasing the volume of the solder bumps  872 . The ability of adjusting the bond line  870  thickness thus also provide a means to control the static attitude of the slider  47  after bonding. 
   With more specific reference to  FIG. 8 , the flexure  40  is pressed against the slider  47  by a suspension gram preload such that the backside  58  of the slider  47  is in direct physical contact with the flat surface  874  of the solder bumps  872  protruded above the metallic bond pad  862  of the flexure  40 . The slider leading edge surface  156  is disposed at a recess from the leading edge tip  850  of the flexure  40 . 
   In this embodiment, the slider leading edge is disposed at a recess from the leading edge tip  850  of the flexure  40 . An extension of the metallic flexure layer used for the slider bond pad on the flexure is provided. Conductive adhesive is applied to the extension and underlying flexure stainless steel layer to create an electrical ground path between the slider body and the suspension flexure. 
   Laser (or heat) energy  877  is delivered to the outer surface of the stainless steel flexure tongue  868 . The laser energy  877  heats the stainless steel flexure tongue  868  to the melting temperature of the material of the solder bumps  872 , whereupon the solder bumps  872  become liquefied and reflow to wet the back side  58  of the slider  47  and the metallic bond pad  862 . 
   As the solder bumps  872  become liquefied, the suspension gram preload continues to push the flexure  40  toward the back side  58  of the slider  47 . The molten material of the solder bumps  872  fills the volume of the solder bump receptacles  900  in the metallic bond pad  862  and the polyimide insulator layer  864 . 
   The excess material then is squeezed out of the solder bump receptacles  900  by the suspension gram preload to form a thin bond line  870  between the back side  58  of the slider  47  and the metallic bond pad  862 . Upon cooling, the bond line  870  forms a solid mechanical and electrical connection between the back side  58  of the slider  47  and the metallic bond pad  862 . 
   A slider is loaded into a fixture and clamped in place. Then a suspension is loaded into the same fixture such that the slider bond pads and receptacles align with the slider and solder bumps. While loading the suspension into the fixture, an arm is positioned to prevent contact between the suspension flexure and the slider. Once the suspension is clamped in place at the suspension baseplate, the arm is lowered so that the flexure tongue comes in contact with the slider and/or solder bumps thereby applying the gram preload to the slider. 
   In the design of the magnetic read/write head  35 , careful consideration is given to the electrical grounding of the slider  47 . This electrical grounding is necessary to prevent the buildup of static charges that could discharge to the surface of the data medium through the transducer poles from the transducer pole to the data medium. Such a discharge could result in the failure of the read/write head  35  and loss of data stored on the recording medium of the data medium. 
   To address this issue, reference the preferred embodiment of  FIG. 8  incorporates a grounding path achieved by connecting the bond pad layer  862  of the flexure  40  to the stainless steel layer  868  of the flexure  40  with an electrically conductive adhesive  978 . By applying a conductive adhesive between the copper grounding cap  978  and the stainless steel flexure  868 , a grounding path between the slider  47  and the flexure  40  is established, thereby preventing harmful discharge of static electricity. 
     FIG. 10  illustrates an alternative embodiment of the present invention. The solder bump receptacles  900  in the embodiment of  FIGS. 8 and 9  are now replaced by solder bump cylindrical holes  920  through the metallic bond pad  862 , the polyimide insulator layer  864 , and the stainless steel flexure  868 . Laser energy  877  is now delivered directly to the solder bumps  872  to enable more rapid heating and faster reflow of the solder material of the solder bumps  872 . The copper ground cap  978  of the preferred embodiment of  FIGS. 8 and 9  is omitted in the alternative embodiment of  FIG. 10 . The electrical ground path is now established through contact between the solder material of the solder bumps  872  and the stainless steel flexure  868 . 
   The advantage of using solder bump bonding for attaching the slider  47  to the flexure  40  in accordance with the present invention is afforded by the ability to reflow the solder bond line  870 . This allows for the removal of the read/write head  35  while the solder is molten, in the event that the read/write head  35  fails any of several inspection criteria. The suspension  33  can then be prepared to accept another head through the same or similar solder ball bonding process. A large portion of the suspension  33  cost is thus saved through the suspension recovery and reuse, hence providing a substantial economic advantage. Furthermore, the application of laser energy  877  to the stainless steel surface  868  ensures that the local heating would not adversely affect the thermal compatibility of the read/write element  50 . 
   Another advantage of the present invention is the placement and simplicity since no patterning is required for the metallization process in the manufacturing flow which results in an improved manufacturing efficiency. 
   Referring now to  FIG. 11 , the trailing edge surface  55  of the slider  47  corresponds to the front side  1086  of the wafer  1084 , which is also the surface upon which the thin film read/write elements  50  are formed. The wafer  1084  is comprised of a plurality of slider bars  1088  adjoining one another lengthwise. 
   With further reference to  FIG. 12 , each slider bar  1088  is comprised of a plurality of sliders  47  that are positioned adjacent to one another on their sides, in such a manner that their trailing edge surfaces  55  form the front side  1086  of the wafer  1084 . A thin film read/write element  50  together with a plurality of electrical contact pads  51  are initially formed at the wafer  1084  level on the front side  1086 . 
   The read/write element  55  is positioned in the center of the trailing edge surface  55  of the slider  47 . Two electrical contact pads  51  are placed on either side of the read/write element  55 . Upon the formation of the read/write elements  55  and the electrical contact pads  51  on the trailing edge surfaces  55  of the sliders  47  at the wafer  1084  level, the slider bars  1088  are diced from the wafer  1084 . 
   The slider bars  1088  are then subjected to further processing. Initially, the ABS surfaces of the sliders  47  are formed on the front side  1090  of the slider bar  1088 . The subsequent operation is the metallization of the back side surfaces  58  of the sliders  47  on the back side of the slider bar  1088 . 
   The metallization step is comprised of three layers of material deposited onto the back side of the slider bar  1088 , namely; a metal adhesion layer, a wetting layer, and a corrosion resistance layer. The metal adhesion layer is generally constituted of chromium or titanium, which promotes the adhesion of the solder material. The wetting layer is typically made of copper and is placed between the metal adhesion layer and the corrosion resistance layer. 
   The corrosion resistance layer is made of gold plated material which is known to be chemically inert. The copper wetting layer protects the gold corrosion resistance layer from fusing into the metal adhesion layer, which would may otherwise result in a brittle failure of the gold corrosion protection layer. 
   After the metallization step at the slider bar  1088  level is complete, a pattern of solder bumps  872 , only three of which are shown for the purpose of clarity, is applied onto the metalized back side of the slider bar  1088 . The slider bar  1088  then undergoes a dicing process to form sliders  47 . 
   After the sliders  47  have been sliced from the slider bar  1088 , the individual slider  47  further undergoes an assembly process to form the read/write head  35 . Referring now to  FIG. 13 , the slider/suspension assembly  800  is comprised of the slider  47 , the metallic bond pad (or copper trace)  862 , the polyimide insulator layer  864 , and the stainless steel flexure tongue  868 . 
   The slider  47  includes a plurality of solder bumps  872  formed on its back side  58 . The metallic bond pad  862  generally is formed by copper traces  863  and includes a feature wherein a plurality of circular holes  867  are formed. The polyimide insulator  864  generally includes a feature wherein a plurality of circular holes  871  are formed. The stainless steel flexure tongue  868  is generally a rectangular tab formed within a clearance  158  of the flexure  40 . 
   The slider/suspension assembly  800  may be formed by the following manufacturing sequence:
     1. The metallic bond pad (8620 features and underlying polyimide insulator layer  864  are formed by masking and selectively etching the different layers from laminated sheet material.   2. The bonded assembly of the metallic bond pad  862  and the polyimide insulator  864  is then bonded to the stainless steel flexure tongue  868  to form the flexure  40 .   3. The slider  47  is then positioned onto the flexure  40  by aligning the solder bumps  872  with the solder bump receptacles  900  so that the crowns of the solder bumps  872  rest upon the stainless steel flexure tongue  868 .   4. A laser or heat energy  877  is directed to the stainless steel flexure tongue  868  to melt the solder bumps  872 . The liquefied solder bumps  872  begin to reflow to form a bond line  870  between the back side  58  of the slider  47  and metallic bond pad  862 . Upon cooling, the bond line  870  forms a solid mechanical and electrical connection between the slider  47  and the flexure  40 , which collectively form the slider/suspension  800 .   

   It should be understood that the geometry, compositions, and dimensions of the elements described herein can be modified within the scope of the invention and are not intended to be the exclusive; rather, they can be modified within the scope of the invention. Other modifications can be made when implementing the invention for a particular application or environment.