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: positioning the flying leads to correspond to the pads arranged parallel; and applying supersonic vibrations to a bonding tool so as to respectively bond the flying leads to the pads. And widths of the flying leads are wider than those of the pads, and supersonic vibrations are applied to the bonding tool, which is pressing the flying leads onto the pads, so that the flying leads are respectively supersonic-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 .  
         [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, a first method of bonding flying leads to pads of a board comprises the steps of: positioning the flying leads to correspond to the pads arranged parallel; and applying supersonic vibrations to a bonding tool so as to respectively bond the flying leads to the pads, wherein widths of the flying leads are wider than those of the pads, and supersonic vibrations are applied to the bonding tool, which is pressing the flying leads onto the pads, so that the flying leads are respectively supersonic-bonded to the pads.  
         [0013]     With this method, the widths of the flying leads are made wider than those of the pads, so edges of the pads bite the flying leads when the bonding tool supersonic-bonds the flying leads to the pads. Therefore, the flying leads can be securely supersonic-bonded to the pads.  
         [0014]     A second method comprises the steps of: positioning the flying leads to correspond to the pads arranged parallel; and applying supersonic vibrations to a bonding tool so as to respectively bond the flying leads to the pads, wherein one side face of each flying lead, which is pressed by the bonding tool, is coated with an insulating layer, and supersonic vibrations are applied to the bonding tool, which is pressing the flying leads onto the pads through the insulating layers, so that the flying leads are respectively supersonic-bonded to the pads.  
         [0015]     With this method, the insulating layers, which are formed on the one side faces of the flying leads, act as buffer layers capable of absorbing variation of thickness of the pads and asperities in bonding faces. Therefore, the supersonic bonding can be securely performed.  
         [0016]     A third method comprises the steps of: positioning the flying leads to correspond to the pads arranged parallel; and applying supersonic vibrations to a bonding tool so as to respectively bond the flying leads to the pads, wherein an end section of each flying lead is folded in the thickness direction, and supersonic vibrations are applied to the bonding tool, which is pressing the folded flying leads, in the thickness direction, onto the pads, so that the flying leads are respectively supersonic-bonded to the pads.  
         [0017]     With this method, the folded end sections of the flying leads have elasticity, so they are capable of absorbing variation of thickness of the pads and asperities in bonding faces. Therefore, the supersonic bonding can be securely performed.  
         [0018]     A fourth method comprises the steps of: positioning the flying leads to correspond to the pads arranged parallel; and applying supersonic vibrations to a bonding tool so as to respectively bond the flying leads to the pads, wherein each flying lead is coated with a plated layer and has an elliptic cross-sectional shape, and supersonic vibrations are applied to the bonding tool, which is pressing the flying leads onto the pads, so that the flying leads are respectively supersonic-bonded to the pads.  
         [0019]     With this method, the plated layers have the elliptic cross-sectional shapes, so the plated layers are crushed when the supersonic bonding is performed. Thus, the crushed plated layers are capable of absorbing variation of thickness of the pads and asperities in bonding faces, and the supersonic bonding can be securely performed.  
         [0020]     By employing the methods of the present invention, variation of thickness of the pads and asperities in bonding faces can be absorbed, so that the flying leads can be securely supersonic-bonded to the pads. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Embodiments of the present invention will now be described by way of examples and with reference to the accompanying drawings, in which:  
         [0022]      FIGS. 1A and 1B  are explanation views showing the method of a first embodiment, wherein flying leads are supersonic-bonded to pads by a bonding tool;  
         [0023]      FIG. 2A  is a sectional view of the flying lead used in a second embodiment;  
         [0024]      FIG. 2B  is a sectional view of the conventional flying lead;  
         [0025]      FIG. 3  is an explanation view showing the method of the second embodiment, wherein the flying leads are supersonic-bonded to the pads by the bonding tool;  
         [0026]      FIG. 4  is a side view of the flying lead used in a third embodiment;  
         [0027]      FIG. 5  is an explanation view showing the method of the third embodiment, wherein the flying leads are supersonic-bonded to the pads by the bonding tool;  
         [0028]      FIG. 6  is a sectional view of the flying lead, which is used in a fourth embodiment, seen from an end;  
         [0029]      FIG. 7  is an explanation view showing the method of the fourth embodiment, wherein the flying leads are supersonic-bonded to the pads by the bonding tool;  
         [0030]      FIG. 8  is a perspective view of the conventional carriage assembly having the long tail suspension board;  
         [0031]      FIG. 9  is an explanation view showing the conventional method of bonding the flying leads to the pads; and  
         [0032]      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  
       [0033]     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0034]     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.  
       First Embodiment  
       [0035]     A first embodiment of the present invention will be explained with reference to  FIGS. 1A and 1B .  
         [0036]     In  FIG. 1A , the flying leads  18  of the long tail suspension board is positioned to correspond to the flexible board  16 , which is attached to an carriage arm  10 . The flying leads  18  respectively correspond to pads  17  of the flexible board  16 .  
         [0037]     In the present embodiment, width of each pad  17 , which is exposed in a surface of the flexible board  16 , is narrower than that of each flying lead  18 , and both side edges of each flying lead  18  locate on the outer sides of both edges of each pad  17  when the flying leads  18  are positioned to respectively correspond to the pads  17 .  
         [0038]     The flying leads  18  are arranged parallel to correspond to the pads  17 , which are arranged parallel at regular intervals in the flexible board  16 . Surfaces of the pads  17  and the flying leads  18  are plated with gold.  
         [0039]     In  FIG. 1B , a bonding tool  20  presses the flying leads  18 , and supersonic vibrations are applied to the bonding tool  20  so as to supersonic-bond the flying leads  18  to the pads  17 .  
         [0040]     In the present embodiment, the width of each flying lead  18  is wider than that of each pad  17 , so the both edges of each pad  17  bite a bonding face (bottom face) of each flying lead  18  when the supersonic vibrations are applied to the flying lead  18  via the bonding tool  20 . Therefore, the flying leads  18  can be securely bonded to the pads  17 .  
         [0041]     Since the both edges of each pad  17  bite the bonding face of each flying lead  18 , the flying leads  18  can be securely bonded to the pads  17  even if flatness of the pads  17  are irregular. Irregularity of the flatness and thickness of the pads  17  can be absorbed, so that secure super sonic bonding can be performed. Even if the bonding tool  20 , whose bottom working face is flat, simultaneously presses a plurality of the flying leads  18  onto the pads  17 , the super sonic bonding can be securely performed.  
         [0042]     In the present embodiment, the biting function of the edges of the pads  17  is used for the supersonic bonding. Thus, the flying leads  18  can be further securely bonded to the pads  17  by increasing rigidity of the pads  17 . The pads  17  and cable patterns of the flexible board  16  are simultaneously formed by etching a electrically conductive layer, e.g., copper layer, formed on a surface of the flexible board  16 . After exposing the pads  17  made of the copper layer, a rigid layer, e.g., nickel plated layer, is formed as a base layer, then the base layer is plated with gold, so that the rigidity of the pads  17  can be increased.  
       Second Embodiment  
       [0043]     A second embodiment of the present invention will be explained with reference to  FIGS. 2A, 2B  and  3 .  
         [0044]      FIG. 2B  shows a structure of the conventional flying lead  18 . The pad  17  and the flying lead  18  are seen from a side or in the width direction. In the conventional flying lead  18 , both sides of a conductive section  180  of the flying lead  18  are covered with insulating layers  18   b  and  18   c , e.g., polyimide films, and a conductive lead section  18   a , which correspond to the pad  17 , is not covered with the insulating layers  18   b  and  18   c , so that both side faces of the conductive lead section  18   a , which strides over the pad  17  in the longitudinal direction, are exposed.  
         [0045]      FIG. 2A  shows a structure of the flying lead  18  of the present embodiment. An upper side face of the flying lead  18 , which will be pressed by the bonding tool  20  for the supersonic boding, is entirely covered with the insulating layer  18   b ; a lower side face of the conductive lead section  18   a , which corresponds to the bonding face of the pad  17 , is not covered with the insulating layer  18   c . Namely, the lower side face is exposed.  
         [0046]     In  FIG. 3 , the flying lead  18  of the present embodiment (see  FIG. 2A ) is supersonic-bonded to the pad  17  by the bonding tool  20 . Since the upper side face of the flying lead  18 , which contacts the working face of the bonding tool  20 , is covered with the insulating layer  18   b , the insulating layer  18   b  located between the bonding tool  20  and the conductive lead section  18   a  acts as a buffer when the bonding tool  20  presses the flying lead  18  onto the pad  17  with applying supersonic vibrations. Even if flatness of the bonding faces of the pads  17  and the flying leads  18  and thickness of the pads  17  and the lead sections  18   a  are varied, the variations can be absorbed so that the flying leads  18  can be securely bonded to the pads  17 . The surfaces of the pads  17  and the exposed faces of the lead sections  18   a  are plated with gold, so the lead sections  18   a  are gold-gold-bonded to the pads  17 .  
       Third Embodiment  
       [0047]     A third embodiment of the present invention will be explained with reference to  FIGS. 4 and 5 .  
         [0048]     In  FIG. 4 , the flying lead  18  to be bonded to the pad  17  is seen from the side. In the present embodiment, the lead section  18   a , which is extended from the end of the long tail suspension board, is folded to form into a U-shape. A symbol  18   d  stands for a folded section, and a lower side part  18   e  and an upper side part  18   f  of the lead section  18   a  are mutually faced. A length of the lower side part  18   e  of the lead section  18   a  is almost equal to that of the pad  17 . A small clearance is formed between the lower side part  18   e  and the upper side part  18   f , so that the folded section  18   d  has spring function.  
         [0049]     In  FIG. 5 , the U-shaped flying lead  18  is positioned to correspond to the pad  17 , and the bonding tool  20  presses the flying lead  18  downward. Simultaneously, supersonic vibrations are applied to the bonding tool  20  so as to supersonic-bond the flying lead  18  to the pad  17 .  
         [0050]     As shown in the drawing, the lower side part  18   e  and the upper side part  18   f  of the flying lead  18  are clamped between the bonding tool  20  and the pad  17 , so the spring function of the folded section  18   d  works. As described above, the folded section  18   d  has the spring function. So, even if flatness of the bonding faces of the pads  17  and the flying leads  18  are varied, the variations can be absorbed so that the flying leads  18  can be securely bonded to the pads  17 .  
       Fourth Embodiment  
       [0051]     A fourth embodiment of the present invention will be explained with reference to  FIGS. 6 and 7 .  
         [0052]      FIG. 6  is a sectional view of the flying leads  18  to be bonded to the pads  17  seen from longitudinal ends. In the present embodiment, the lead section  18   a  of each flying lead  18  is coated with a gold plated layer  19 , and it has an elliptic cross-sectional shape. In  FIG. 6 , the flying leads  18  are positioned above the pads  17  and respectively corresponded to the pads  17 .  
         [0053]     In  FIG. 7 , the bonding tool  20  presses a plurality of the flying leads  18  downward. Simultaneously, supersonic vibrations are applied to the bonding tool  20  so as to supersonic-bond the flying leads  18  to the pads  17 .  
         [0054]     By forming the cross-sectional shape of each flying lead  18  coated with the gold plated layer  19  into the elliptical shape, a transverse center part of each flying lead  18 , which is the thickest projected part in section, contacts the bonding tool  20  and the pad  17  when the bonding tool  20  presses each flying lead  18  onto each pad  17 . Further, by gradually increasing a pressing force of the bonding tool  20  with applying supersonic vibrations to each flying lead  18 , the gold plated layer  19  of each projected parts is gradually crushed so that the flying lead  18  and the pad  17  are bonded with gradually broadening the boding area therebetween.  
         [0055]     By forming the cross-sectional shape of each lead section  18   a  coated with the gold plated layer  19  into the elliptical shape, the bonding tool  20  securely presses and crushes each flying lead  18 . Even if flatness of the bonding faces of the pads  17  and the flying leads  18  and thickness of the pads  17  and the lead sections  18   a  are varied, the variations can be absorbed so that the flying leads  18  can be securely bonded to the pads  17 .  
         [0056]     Note that, in the above described embodiments, the flying leads  18  of the long tail suspension board are supersonic-bonded to the pads  17  of the flexible board  16  in the assembling step of the carriage assembly. However, the present invention is not limited to the above described embodiments. For example, the method of the present invention may be applied for supersonic-bonding flying leads to pads of a circuit board. A plurality of the flying leads need not be bonded simultaneously. The flying leads may be bonded to the pads one by one.  
         [0057]     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.