Abstract:
The method of bonding flying leads is capable of efficiently supersonic-bonding the flying leads to pads of a board and improving bonding reliability therebetween. The method comprises the steps of: mechanically processing the board so as to form projections, which act as margins for deformation, in boding faces of the pads, on each of which the flying lead will be bonded, positioning the flying leads to correspond to the pads; and applying supersonic vibrations to a bonding tool so as to deform and crush the projections, whereby the flying leads are respectively bonded to the pads.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method of bonding flying leads, more precisely relates to a method of bonding flying leads to pads of a board by using supersonic vibrations.  
         [0002]     A carriage assembly of a magnetic disk drive unit is shown in  FIG. 8 . The carriage assembly includes a plurality of carriage arms  10 , whose number corresponds to that of magnetic disks, and suspensions  12 , on which magnetic heads are mounted, are attached to front ends of the carriage arms  10 . Base ends of the carriage arms  10  are attached to an actuator shaft  14 , and the carriage arms  10  are turned about the shaft  14  and moved parallel to surfaces of the magnetic disks.  
         [0003]     The magnetic heads mounted on the suspensions  12  are electrically connected to a signal transmission circuit by several manners.  FIG. 8  shows a connecting structure using a so-called long tail suspension board, wherein an end of a suspension board of the suspension  12  is extended to a location of attaching a flexible board  16 , which is attached to side faces of the base ends of the carriage arms  10 , and ends of the suspension board is formed into flying leads.  
         [0004]     In the connecting structure using the long tail suspension board, pads of the flexible board  16  and the flying leads  18  (see  FIG. 9 ) of the long suspension board are correctly positioned, then the flying leads  18  are bonded to the pads by a supersonic bonding tool. In  FIG. 9 , the flying leads  18  are supersonic-bonded to the pads  17  of the flexible board  16  by the bonding tool  20 .  
         [0005]     The supersonic bonding method has been used for bonding a semiconductor chip to a circuit board by flip-chip connection, bonding wires to leads, etc. To securely perform the supersonic bonding, several ideas have been proposed. For example, Japanese Patent Gazette No. 10-150137 discloses a method of bonding wires, wherein a leadframe is pressed by a vibration restraining member so as to prevent resonance of the leadframe; Japanese Patent Gazette No. 2005-136399 discloses a method of forming bonding-electrodes, wherein an electrically conductive material is applied to electrodes of a circuit board so as to broaden a bonding area; Japanese Patent Gazettes No. 08-146451 and No. 10-189657 disclose methods of bonding two members, wherein an anisotropic conductive film is provided between the members, and supersonic waves are applied in the direction for mutual contact; Japanese Patent Gazette No. 05-63038 discloses a method of bonding two members, wherein their bonding faces are made rough; and Japanese Patent Gazette No. 2005-93581 discloses a method of bonding two members, wherein non-conductive adhesive is applied to bonding faces.  
         [0006]     In the connecting structure using the long tail suspension board shown in  FIG. 8 , a plurality of the flying leads  18  are arranged parallel with minute separations. The flying leads  18  may be supersonic-bonded, one by one, to the pads  17 , but it is efficient to simultaneously supersonic-bond a plurality of the flying leads  18  as shown in  FIG. 9 .  
         [0007]     In  FIG. 9 , the bonding tool  20  contacts and bonds two flying leads  18 . However, in case that the bonding tool  20  contacts a plurality of the flying leads  18 , a working face of the bonding tool  20  is a flat face. So, if asperities exist in the bonding face, bonding strength differs at every bonding point, so that bonding reliability must be lowered.  
         [0008]      FIG. 10  is a sectional view of the flying lead  18  and the pad  17 , which have been mutually bonded. The boundary includes three kinds of parts: (1) completely bonded parts, wherein projections are crushed and oxide film is broken, so that the bonding faces are actually bonded each other; (2) poorly bonded parts, wherein the oxide film exists between the bonding faces; and (3) nonbonded parts “A”, wherein the bonding faces are not bonded each other.  
         [0009]     Outer surfaces of the flying leads  18  and the pads  17  are gold-plated, so that they are connected by gold-gold bonding. The gold plated layers are capable of absorbing the asperities formed in the bonding faces of the flying leads  18  and the pads  17 . However, thickness of the gold layers are about 3 μm, so all of the asperities cannot be fully absorbed.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention was conceived to solve the above described problems.  
         [0011]     An object of the present invention is to provide a method of bonding flying leads, which is capable of efficiently supersonic-bonding the flying leads to pads of a board and improving bonding reliability therebetween.  
         [0012]     To achieve the object, the method of bonding flying leads to pads of a board comprises the steps of: mechanically processing the board so as to form projections, which act as margins for deformation, in boding faces of the pads, on each of which the flying lead will be bonded, positioning the flying leads to correspond to the pads; and applying supersonic vibrations to a bonding tool so as to deform and crush the projections, whereby the flying leads are respectively bonded to the pads.  
         [0013]     In the method, each of the projections may be formed by pressing a press tool onto the bonding face so as to form a cavity in the boding face, so that the projection is formed along an edge of the cavity. By pressing the press tool onto the bonding face, the projections can be easily formed in the bonding faces of the pads.  
         [0014]     In the methods, each of the projections may be formed by pressing a press tool onto a reverse face of the board toward the pad, so that the projection is formed in the boding face.  
         [0015]     In the method, the mechanical processing step may be performed by: driving a wedge section formed at a front end of a press tool into each of the bonding faces so as to form the projection around the wedge section; and separating the wedge section from the press tool so as to leave the wedge section in the pad. In this case, the flying leads can be bonded to the projections of the pads, further a function of bonding the flying leads to the wedge sections can be used, so that bonding reliability between the flying leads and the pads can be improved.  
         [0016]     In the method, the press tool may double as the bonding tool, the flying leads may be positioned to correspond to the pads after forming the cavities in the pads by the press tool, and supersonic vibrations may be applied to the press tool so as to bond the flying leads to the pads. In this case, forming the projections and bonding the flying leads to the pads can be continuously performed. The working efficiency of this improved method can be higher than that of the conventional assembling method.  
         [0017]     By employing the method of the present invention, the projections act as margins for deformation, so the flying leads can be securely bonded to the pads of the board, and bonding strength can be higher. Since the projections are formed in the board by the mechanical processing, the projections can be formed in the assembling step so that the flying leads can be securely and efficiently bonded to the pads. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Embodiments of the present invention will now be described by way of examples and with reference to the accompanying drawings, in which:  
         [0019]      FIG. 1  is an explanation view showing a process of forming projections in pads by the method of a first embodiment;  
         [0020]      FIGS. 2A and 2B  are explanation views showing a process of bonding flying leads to the pads formed by the method of the first embodiment;  
         [0021]      FIG. 3  is an explanation view showing a process of forming projections in pads by the method of a second embodiment;  
         [0022]      FIG. 4  is an explanation view showing a process of forming projections in pads by the method of a third embodiment;  
         [0023]      FIGS. 5A and 5B  are explanation views showing a process of bonding flying leads to the pads formed by the method of the third embodiment;  
         [0024]      FIG. 6  is an explanation view showing a process of forming projections in pads by the method of a fourth embodiment;  
         [0025]      FIG. 7  is an explanation view showing a process of bonding flying leads to the pads formed by the method of the fourth embodiment;  
         [0026]      FIG. 8  is a perspective view of the conventional carriage assembly having the long tail suspension board;  
         [0027]      FIG. 9  is an explanation view showing the conventional method of bonding the flying leads to the pads; and  
         [0028]      FIG. 10  is a sectional view of the boundary of the flying lead and the pad, which are mutually bonded. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0029]     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0030]     In the following embodiments, flying leads  18 , which are formed in a long tail suspension board, are bonded to a flexible board  16  when a carriage assembly is assembled.  
         [0031]     In the method of the following embodiments, projections are formed in bonding faces of pads  17  of the flexible board  16  by mechanical process. When the flying leads  18  are bonded to the pads  17  by supersonic bonding, the projections are crushed so that the flying leads  18  can be bonded to the pads  17 . At that time, the projections act as margins for deformation.  
       First Embodiment  
       [0032]     A first embodiment of the present invention will be explained with reference to  FIGS. 1, 2A  and  2 B.  
         [0033]      FIG. 1  shows a process of forming the projections, which act as the margins for deformation when the supersonic bonding is performed, in the pads  17  of the flexible board  16 . Press tools  30 , whose lower ends are formed into circular cones having flat tips, are respectively pressed onto the pads  17  in the thickness direction, so that the projections  17   a  can be formed. By pressing the press tools  30  onto bonding faces, on which the flying leads  18  will be bonded, in the thickness direction, cavities  17   b  are respectively formed in the bonding faces of the pads  17 . Thus, the projections  17   a  rise from edges of the cavities  17   b.    
         [0034]     In  FIG. 2A , the flying leads  18  are bonded to the pads  17  having the projections  17   a  by applying supersonic vibrations to a bonding tool  20 . The flying leads  18 , which are formed at an end of the long tail suspension board, are positioned above the pads  17  so as to respectively correspond to the pads  17 , then the flying leads  18  are pressed onto the pads  17  by the bonding tool  20  so that the flying leads  18  can be bonded to the pads  17 .  
         [0035]     In the present embodiment, surfaces of the pads  17  and the flying leads  18  are plated with gold, so the pads  17  and the flying leads  18  are electrically connected by gold-gold bonding.  
         [0036]     In  FIG. 2B , the flying leads  18  are respectively bonded to the pads  17 . When the bonding tool  20  applies supersonic vibrations to the flying leads  18  for bonding, the projections  17   a  formed in the boding faces of the pads  17  are crushed. Therefore, bonding strength between the flying leads  18  and the pads  17  is greater than that of the conventional method, in which the bonding faces of the pads are flat, so that the flying leads  18  can be securely bonded to the pads  17 . When supersonic vibrations are applied, the projections  17   a  of the pads  17  are rubbed with the flying leads  18 , so that the projections  17   a  are crushed and deformed. Therefore, the projections  17   a  act as the margins for deformation.  
         [0037]     By forming the margins for deformation, which will be deformed by supersonic vibrations, the flying leads  18  can be securely bonded to the pads  17 . Further, a plurality of the flying leads  18  are simultaneously bonded with the pads  17 , so bonding efficiency can be improved and variation of the bonding strength can be prevented.  
         [0038]     In the present embodiment, the projections  17   a  is formed by forming the cavities  17   b  in the pads  17 . In comparison with another example, in which the projections are merely formed in the pads, contact areas between the flying leads  18  and the pads  17  can be broader so that the bonding strength therebetween can be greater.  
       Second Embodiment  
       [0039]     A second embodiment of the present invention will be explained with reference to  FIG. 3 .  
         [0040]     In the present embodiment, the press tool  30  is used for not only forming the projections  17   a  in the pads  17  but also supersonic-bonding the flying leads  18  to the pads as the bonding tool. The projections  17   a  are formed in the pads  17  by the press tool  30  as well as the first embodiment.  
         [0041]     In  FIG. 3 , the projections  17   a  are formed in the pads  17  by the press tool  30 , then the flying leads  18  of the long tail suspension board are positioned to correspond to the pads  17 . Further, supersonic vibrations are applied to the press tool  30 , so that the flying lead  18  is pressed onto and bonded to the corresponding pad  17 .  
         [0042]     Since supersonic vibrations are applied to the press tool  30  to bond the flying leads  18  to the pads  17 , the step of bonding the flying leads  18  to the pads  17  can be performed in succession to the step of forming the projections  17   a  in the pads  17 . Therefore, working efficiency can be improved.  
         [0043]     Note that, the process of forming the projections  17   a , which act as margins for deformation, in the pads  17  is not limited to the above described embodiment. They may be formed by, for example, plating surfaces of the pads  17  when cable patterns are formed in the flexible board  16 . However, in case that the projections are formed by plating when the cable patterns are formed in the flexible board  16 , number of manufacturing steps must be increased, so that manufacturing cost must be increased. On the other hand, by using the press tool  30  for forming the projections  17   a , the projections  17   a  can be formed in the assembling step, so that manufacturing cost can be reduced.  
       Third Embodiment  
       [0044]     A third embodiment of the present invention will be explained with reference to  FIGS. 4, 5A  and  5 B.  
         [0045]     In the present embodiment, the projections  17   a  are formed in the pads  17  by pressing the press tool  30  onto a reverse face of a plastic base member  16   a  of the board  16  toward the pads  17 , so that the projections  17   a  can be formed in the boding faces of the pads  17 . In  FIG. 4 , the board  16  mounted on a flexible supporting stage  40  with the pads  17  extending downward. The press tool  30  is pressed onto the reverse face of the base member  16   a  so as to form the projections  17   a  in the pads  17 . The press tool  30  is positioned to correspond to the pad  17 , then pressed onto the base member  16   a . And the base member  16   a  and the pad  17  is deformed as shown by dotted lines, so that the projection  17   a  is formed in the bonding face of the pad  17 , to which the flying lead  18  will be bonded.  
         [0046]     In  FIG. 5A , the flying leads  18  are supersonic-bonded to the pads  17 , each of which has the projection  17   a  in the upper face of the bonding face. In each of the pads  17 , a center part of the bonding face, which is slightly upwardly expanded, is the projection  17   a.    
         [0047]     In the present embodiment too, the projections  17   a  act as the margins for deformation when the flying leads  18  are supersonic-bonded to the pads  17 . When the flying leads  18  are supersonic-bonded to the pads  17  by the bonding tool  20 , the projections  17   a  are crushed so that the flying leads  18  can be securely bonded to the pads  17 . In  FIG. 5B , the flying leads  18  have been securely bonded to the pads  17 .  
       Fourth Embodiment  
       [0048]     A fourth embodiment of the present invention will be explained with reference to  FIGS. 6 and 7 .  
         [0049]     In the present embodiment, wedge sections  32   a  are formed at lower ends of press tools  32 . The wedge sections  32   a  are respectively driven into the pads, and the wedge sections  32   a  are left in the board  16  so that the projections  17   a  are formed in the bonding faces of the pads  17 , to which the flying leads  18  will be bonded.  
         [0050]     In  FIG. 6 , the press tools  32  having the wedge sections  32   a  at the lower ends are moved downward, and the wedge sections  32   a  are driven into the pads  17 . The wedge sections  32   a  can be easily separated from the press tools  32  by breaking the press tools  32 . The press tools  32  are moved downward until lower ends of the wedge sections  32   a  reach the plastic base member  16   a . By driving the wedge sections  32   a  into the pads  17 , the projections  17   a  rise in the bonding faces of the pads  17 , to which the flying leads  18  will be bonded.  
         [0051]     In  FIG. 7 , the flying leads  18  are bonded to the pads  17  by applying supersonic vibrations to the bonding tool  20 . The flying leads  18  are pressed onto the pads  17  with applying supersonic vibrations to the flying leads  18 , so that the projections  17   a  are crushed and the flying leads  18  can be securely bonded to the pads  17 . In case that upper ends of the wedge sections  32   a  are slightly projected upward from the upper faces of the pads  17 , the projected parts of the wedge sections  32   a  are bonded to the flying leads  18  together with the pads  17 .  
         [0052]     In the present embodiment, the wedge sections  32   a  form the projections  17   a , which act as the deformation margins when the supersonic bonding is performed, and increase bonding strength between the flying leads  18  and the pads  17 . The bonding strength of the present embodiment using the wedge sections  32   a  is greater than that of the former embodiments, in each of which the flying leads  18  are bonded to the pads  17  only. Note that, in the present embodiment, the wedge sections  32   a  are left in bonded parts between the flying leads  18  and the flexible board  16 , so surfaces of the wedge sections  32   a  may be plated with a corrosion-resisting metal, e.g., gold.  
         [0053]     The invention may be embodied in other specific forms without departing from the spirit of essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.