Abstract:
This invention provides a bonded structure and a method of forming the bonded structure for joining a lead array to the conducting bonding pads of an integrated circuit element. The invention uses an anisotropic conductive film with tape automated bonding to form the bonded structure. The invention provides a low cost method of tape automated bonding which uses lower temperature and pressure in the bonding process and provides a bond which is automatically encapsulated after the bonding has been completed. The lower temperature and pressure improve the dimensional stability of the elements of the bonded structure and the automatic encapsulation provides improved reliability.

Description:
This is a division of patent application Ser. No. 08/579,511, filing date Dec. 27, 1995, U.S. Pat. No. 6,008,072 A New Tape Automated Bonding Method And Bonded Structure, assigned to the same assignee as the present invention. 
    
    
     RELATED PATENT APPLICATIONS 
     (1) (E83-0002), Ser. No. 08/239,375, filed May 6, 1994, entitled “Composite Bump Bonding” assigned to the same assignee. 
     (2) (E83-0004), Ser. No. 08/239,380, filed May 6, 1994, entitled “Composite Bump Flip Chip Bonding” assigned to the same assignee. 
     (3) (E83-0003A), U.S. Pat. No. 5,393,697, to Chang et al, Feb. 28, 1995, entitled “Composite Bump Structure and Methods of Fabrication,” filed May 6, 1994. 
     (4) (E83-0003B), Ser. No. 08/387,095, filed Feb. 13, 1995 entitled “Composite Bump Structure and Methods of Fabrication,” assigned to the same assignee. 
     (5) (E84-0010), Ser. No. 08/494,582, filed Jun. 23, 1995 entitled “Connection Construction and Method of Manufacturing the Same,” assigned to the same assignee. 
     (6) (ERSO-84-0052), Ser. No. 08/579,511, filed Dec. 27, 1995, entitled “A New Tape Automated Bonding Method And Bonded Structure” assigned to the same assignee. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to the use of an anisotropic conductive film, having conductive particles in an insulating adhesive, with tape automated bonding to form interconnections between a lead array and an integrated circuit element. The use of an anisotropic conductive film provides a bonding process with reduced temperature and pressure and a reliable encapsulated bonded structure. 
     (2) Description of Related Art 
     In conventional tape automated bonding high temperatures are required to form the bond between the inner lead ends of a lead array and the bonding pads on an integrated circuit element. The difference in thermal expansion between the elements of the assembly can distort the lead array and make subsequent processing, such as bonding the outer lead ends, difficult. This invention provides a method to avoid this problem by using an anisotropic conductive film in combination with tape automated bonding. The anisotropic conductive film comprises conductive particles in an insulating adhesive. 
     U.S. Pat. No. 4,963,002 to Tagusa et al. describes a connection construction using conductive particles and both conductive and insulating adhesives. Japanese Patent 3-62927 to Fujimoto describes a composite conductive particle, an adhesive layer, and flip chip bonding. 
     U.S. Pat. No. 5,001,542 to Tsukagoshi et al. describes a board made of glass, synthetic resin, metal ceramic or their composite materials. One variation of the disclosed board has a center part removed for a film carrier or TAB structure. Tsukagoshi et al. use a combination of deformable conducting particles, rigid conducting particles, and an adhesive to form the electrical bond. The Tape Automated bonding structure and method of this Invention uses a different, and more flexible, lead frame. This Invention uses an anisotropic conducting film using an adhesive with a different type conducting particles than Tsukagoshi et al. The method used to form the bond are different from Tsukagoshi et al. 
     U.S. Pat. No. 4,470,657 to Tsukagoshi et al. describes the use of an anisotropic conductive film using an adhesive with conductive particles of multiple sizes. Tape Automated Bonding is not described in this patent. 
     U.S. Pat. No. 4,731,282 to Tsukagoshi et al. describes insulating adhesives. 
     SUMMARY OF THE INVENTION 
     A diagram of a conventional TAB, or tape automated bonding setup is shown in FIG. 1. A lead array having leads  22  of a metal such as copper are formed on a dielectric layer  23  such as polyimide. The inner end of the leads contact conductive bonding pads  21 , of a material such as gold, formed on an integrated circuit element  20 . Heat energy at a chosen temperature and pressure, for a chosen time is delivered by a thermode  10  which is brought into contact with the leads of the lead array and a bond is formed between the inner ends of the leads  22  and the conductive bonding pads  21 . 
     In conventional tape automated bonding high temperatures, often between 450° C. and 550° C., are required to form the bond between the inner lead end of the lead array and the bonding pad on the integrated circuit element. The difference in thermal expansion between the copper lead  22  and the dielectric layer  23  which will distort the lead array and make subsequent processing, such as bonding the outer lead ends, difficult. Frequently the hardness of the gold bump can transmit damaging forces from the thermode to the integrated circuit element. The planarity of the thermode with respect to the bonding pads becomes very important and increases the cost of the equipment used in the bonding process. In conventional tape automated bonding it is important to encapsulate the bond as soon as it has been formed to prevent damage due for example to moisture and corrosion. 
     It is an object of this invention to provide a low cost method of tape automated bonding which uses lower temperature and pressure in the bonding process and provides a bond which is automatically encapsulated after the bonding has been completed. 
     It is a further object of this invention to provide a bonded structure formed using a low cost method of tape automated bonding which uses lower temperature and pressure in the bonding process and which is automatically encapsulated after the bonding has been completed. 
     These objectives are achieved using an anisotropic conductive film in the bonding process. The anisotropic film comprises conductive particles in an insulating adhesive. Using the anisotropic conductive film conductive particles  31  are placed between the leads  22  of the lead array and the conductive bonding pads  21  formed on the integrated circuit element  20 , see FIGS. 2A and 2B. The conductive particles compensate for deviations in planarity between the thermode and the conductive bonding pads so the bonding pressure can be substantially reduced, between about 20 and 40 kilograms/cm 2 . When the bond is formed the insulating adhesive  32  forms an encapsulation of the bond which protects the bond from moisture, corrosion, and mechanical damage. The bonding temperatures required are between about 150° C. and 180° C. and the insulating adhesive forms an adhesive bond to keep the electrical bond in place. 
     The lead array can be of the window type such as that shown in FIG. 2C. A lead array having leads  22  formed of a material such as copper are formed on a dielectric layer  23  formed of a material such as polyimide. The dielectric layer has an inner perimeter  73  bounding a region  75  with no dielectric material. The inner lead ends  74  of the leads extend into the region of no dielectric material. In this type of lead array an isolation film comprising a metal foil layer  27  formed on a polyimide layer  25  is used in the bonding process, see FIG.  2 A. 
     The lead array can also be of the type shown in FIG. 2D where there is no window and the leads  22  of the lead array are formed entirely on the polyimide layer  23 . In this type of lead frame the isolation film is not required, as shown in FIG.  2 B. 
     The insulating adhesive in the anisotropic conductive film can be a thermoplastic material, a thermosetting material, or a type of material which is both thermoplastic and thermosetting. The conductive particles can be metal spheres, graphite particles, or composite particles having a polymer body covered by a conductive metal coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagram of a conventional tape automated bonding set up. 
     FIG. 2A shows a cross section of a bonded structure formed using an anisotropic conductive film, a lead array with a window, and tape automated bonding. 
     FIG. 2B shows a cross section of a bonded structure formed using an anisotropic conductive film, a lead array without a window, and tape automated bonding. 
     FIG. 2C shows a top view of a lead array with a window. 
     FIG. 2D shows a top view of a lead array without a window. 
     FIG. 3A shows a cross section of an anisotropic conductive film placed on an integrated circuit element covering the conductive bonding pads. 
     FIG. 3B shows a cross section view of a thermode in contact with an anisotropic film placed on an integrated circuit element. 
     FIG. 3C shows a cross section of an anisotropic conductive film on an integrated circuit element which has been pre heated. 
     FIG. 3D shows a cross section view of a lead array with a window placed on an anisotropic conductive film placed on an integrated circuit element and preheated. 
     FIG. 3E shows the structure of FIG. 3D with an isolation film placed on the lead array and a thermode applied to the isolation film. 
     FIG. 4A shows a cross section view of an anisotropic conductive film with protective backing placed on a lead array without a window. 
     FIG. 4B shows a thermode in contact with an anisotropic conductive film with a protective backing placed on a lead array without a window. 
     FIG. 4C shows a cross section view of an anisotropic conductive film on a lead array after pre heating and removal of the protective backing. 
     FIG. 4D shows a cross section view of a bonding set up for tape automated bonding using a lead array without a window and applying heat to the lead array. 
     FIG. 4E shows a cross section view of a bonding set up for tape automated bonding using a lead array without a window and applying heat to the integrated circuit element. 
     FIG. 5A shows a cross section view of the anisotropic conductive film. 
     FIG. 5B shows graphite conductive particles for the anisotropic conductive film. 
     FIG. 5C shows metal spheres as conductive particles for the anisotropic conductive film. 
     FIG. 5D shows composite particles with a polymer body covered by a conductive metal coating as conductive particles for the anisotropic conductive film. 
     FIG. 5E shows composite particles with a first polymer body, covered by a conductive metal coating, and covered by a second polymer layer as conductive particles for the anisotropic conductive film. 
     FIG. 6A shows a cross section view of an anisotropic conductive film with a protective layer on both sides of the film. 
     FIG. 6B shows a cross section view of an anisotropic conductive film with a dielectric layer on one side of the film. 
     FIG. 7 shows a top view of a lead array without a window. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Refer now to FIGS. 2A,  2 C,  3 A- 3 E,  5 A- 5 E,  6 A, and  6 B, there is shown an embodiment of a method of forming a bonded structure using an anisotropic conductive film, a lead array with a window, and tape automated bonding. FIG. 3A shows a cross section view of an integrated circuit element  20  having conductive bonding pads  21 , in this example gold, formed thereon. An anisotropic conductive film  30  is cut to the proper size and placed over the integrated circuit element  20  so that the anisotropic film contacts the gold bonding pads  21 . The anisotropic conductive film  30  comprises conductive particles  31  in an insulating adhesive  32  formed on a protective layer  51 . 
     The anisotropic conductive film  30 , comprising conductive particles  31  and an insulating adhesive  32 , without the dielectric layer is shown in FIG.  5 A. Some examples of the conductive particles are shown in FIGS. 5B-5E. The conductive particles can be graphite  44  as shown in FIG. 5B, metal spheres  41  as shown in FIG. 5C, a composite particle having a polymer body  41  covered by a conductive metal coating  42  as shown in FIG. 5D, or a composite particle having a polymer body  41  covered by a conductive metal coating covered by a polymer coating  43  as shown in FIG.  5 E. Some examples of the anisotropic conductive film are shown in FIGS. 6A and 6B. As shown in FIG. 6A the anisotropic conductive film  30  comprising conductive particles  31  in an insulating adhesive  32  can have a protective layer  51  and  52  on both sides of the anisotropic conductive film. As shown in FIG. 6B the anisotropic conductive film  30  comprising conductive particles  31  in an insulating adhesive  32  can have a protective layer  51  on only one side of the anisotropic conductive film. Referring now to FIG. 5A, when the anisotropic conductive film is compressed in the direction of the arrow  75  the film will conduct in the direction of the arrow but remains an insulator in directions orthogonal to the arrow  75 . 
     Refer now to FIG. 3B, there is shown a cross section view of a thermode  10  brought into contact with the protective layer  51  on one side of the anisotropic conductive film  30 . The thermode  10  can deliver heat energy at a chosen temperature and pressure to objects in contact with the thermode. The thermode preheats the anisotropic conductive film to a temperature of between about 95° C. and 120° C. at a pressure of between about 5 and 10 kilograms/cm 2  for between about 3 and 5 seconds. This preheat cycle causes the insulating adhesive to flow slightly completely covering the gold bonding pads  21 , as shown in FIG.  3 C. As shown in FIG. 3C, the protective layer  51  is then removed from the anisotropic conductive film. 
     Next, as shown in FIG. 3D, a lead array comprising metal conducting leads  22  formed on a first dielectric layer  23  is placed on the anisotropic conductive film  30  so that the inner ends of the metal conducting leads  22  are over the gold bonding pads  21  of the integrated circuit element  20 . In this example the metal conducting leads  22  are copper and the first dielectric layer  23  is polyimide. A top view of the lead array is shown in FIG.  2 C. As shown in FIG. 2C, the first dielectric layer  23  has an inner perimeter  73  bounding a center region  75  having no dielectric material. In this lead array the inner ends of the leads  22  extend into the region having no dielectric material. This extension of the inner lead ends into the region of no dielectric material can also be seen in FIG.  3 D. 
     Next, as shown in FIG. 3E, an isolation layer comprising a metal foil layer  27  formed on a second dielectric layer  25 . In this example the second dielectric layer  25  is polyimide and the metal foil layer  27  is aluminum. The isolation layer is placed so that the isolation layer covers the window or region of the lead array with no first dielectric material, the second dielectric material  25  of the isolation layer contacts the first dielectric material  23  around the entire inner perimeter of the lead array bounding the window, and the second dielectric layer  25  of the isolation layer separates the metal foil  27  of the isolation layer from the lead array. 
     Next, as shown in FIG. 3E, the thermode  10  is brought into contact with the metal foil  27  of the isolation layer. The thermode delivers heat energy at a temperature of between about 150° C. and 180° C. and a pressure of between about 20 and 40 kilograms/cm 2  for between about 5 and 20 seconds to isolation layer, lead array, gold bonding pad, and integrated circuit element assembly shown in FIG. 3E thereby forming the completed bonded structure shown in FIG.  2 A. During the bonding the temperature of the thermode will rise to between about 280° C. and 300° C. 
     An embodiment of the completed bonded structure formed by the method of the preceding embodiment is shown in FIG.  2 A. The gold bonding pads  21  are formed on the integrated circuit element  20 . The inner lead ends of the copper leads  22  of the lead array are positioned so that one inner lead end overlays each gold bonding pad  21  forming a bonding pair. When the assembly is compressed during the bonding process described above a number of conductive particles  31  in the anisotropic conductive film are between the inner lead end and gold bonding pad of each bonding pair and the conductive particles  31  make electrical contact with the lead end and gold bonding pad of each bonding pair. The second dielectric layer  25  of the isolation layer contacts the insulating adhesive  32  of the anisotropic conductive film and the first dielectric layer  23  of the lead array at the inner perimeter of the window. The insulating adhesive reflows during the time heat and pressure is delivered to the assembly by the thermode and encapsulates the bonded structure and holds the entire assembly together after curing. The insulating adhesive can be a material such as epoxy and can be thermoplastic, thermosetting, or a mixture of thermoplastic and thermosetting. The isolation layer described in this example is an aluminum foil  27  formed on a layer of polymer  25  but can also be a layer of silicone rubber formed on a layer of polymer or other such materials. 
     Refer now to FIGS. 2B,  2 D,  4 A- 4 E,  5 A- 5 E,  6 A, and  6 B, there is shown another embodiment of a method of forming a bonded structure using an anisotropic conductive film, a lead array without a window, and tape automated bonding. FIG. 4A shows a cross section view of an anisotropic conductive film  30  with a protective backing  51  on one side of the conductive film placed on a lead array comprising a first dielectric layer  23  and metal leads  22 . FIG. 2D shows a top view of the lead array for this embodiment. The metal leads  22  are formed on the first dielectric layer  23  which in this embodiment has no window without dielectric material. The inner ends  74  of the leads are formed on the first dielectric layer  23 . 
     The anisotropic conductive film  30 , comprising conductive particles  31  and an insulating adhesive  32 , without the protective layer is shown in FIG.  5 A. Some examples of the conductive particles are shown in FIGS. 5B-5E. The conductive particles can be graphite  44  as shown in FIG. 5B, metal spheres  41  as shown in FIG. 5C, a composite particle having a polymer body  41  covered by a conductive metal coating  42  as shown in FIG. 5D, or a composite particle having a polymer body  41  covered by a conductive metal coating covered by a polymer coating  43  as shown in FIG.  5 E. Some examples of the anisotropic conductive film are shown in FIGS. 6A and 6B. As shown in FIG. 6A the anisotropic conductive film  30  comprising conductive particles  31  in an insulating adhesive  32  can have a protective layer  51  and  52  on both sides of the anisotropic conductive film. As shown in FIG. 6B the anisotropic conductive film  30  comprising conductive particles  31  in an insulating adhesive  32  can have a protective layer  51  on only one side of the anisotropic conductive film. Referring now to FIG. 5A, when the anisotropic conductive film is compressed in the direction of the arrow  75  the conduction will occur in the direction of the arrow but the film remains an insulator in directions orthogonal to the arrow  75 . 
     Refer again to FIG. 4A, the anisotropic conductive film  30  is cut to the proper size and placed on the lead array. The anisotropic conductive film has a protective layer  51  on one side of the anisotropic conductive film and is placed on the lead array so that the insulating adhesive contacts and covers the inner lead ends of the lead array. The lead array comprises metal leads  22  formed on a first dielectric layer  23 . In this example the leads  22  of the lead array are copper, the first dielectric material in the first dielectric layer  23  is polyimide, and the insulating adhesive is a thermoplastic material, a thermosetting material, or a combination of thermoplastic and thermosetting materials. 
     Next, as shown in FIG. 4B, the thermode  10  is brought into contact with the protective layer on the anisotropic conductive film and the assembly is pre heated to a temperature of between about 95° C. and 120° C. at a pressure of between about 5 and 10 kilograms/cm 2  for between about 3 and 5 seconds. As shown in FIG. 4C, the pre heating attaches the insulating adhesive to the inner lead ends of the lead array but does not cause the insulating adhesive to flow. As shown in FIG. 4C, the protective layer is then removed. 
     Next, as shown in FIG. 4D, the lead array anisotropic conductive film assembly of FIG. 4C is placed on the integrated circuit element and positioned so that one inner lead end overlays each gold bonding pad  21  forming a bonding pair. When the assembly is compressed during the bonding process a number of conductive particles  31  in the anisotropic conductive film will be between the inner lead end and gold bonding pad of each bonding pair and the conductive particles  31  will make electrical contact with the lead end and gold bonding pad of each bonding pair. 
     As shown in FIG. 4D the a thermode  10  is then brought into contact with the first dielectric layer  23  of the lead array and the thermode heats the assembly to a temperature of between about 150° C. and 180° C. at a pressure of between about 20 and 40 kilograms/cm 2  for between about 5 and 20 seconds thereby forming the completed bonded structure shown in FIG.  2 B. 
     An alternative bonding method is shown in FIG.  4 E. Again the lead array—anisotropic conductive film assembly of FIG. 4C is placed on the integrated circuit element and positioned so that one inner lead end overlays each gold bonding pad  21  forming a bonding pair. When the assembly is compressed during the bonding process a number of conductive particles  31  in the anisotropic conductive film will be between the inner lead end and gold bonding pad of each bonding pair and the conductive particles  31  will make electrical contact with the lead end and gold bonding pad of each bonding pair. In this method, as shown in FIG. 4E, a thermode  10  is then brought into contact with the back side of the integrated circuit element  20 , the side opposite that on which the conductive bonding pads  21  are formed, and the thermode heats the assembly to a temperature of between about 150° C. and 180° C. at a pressure of between about 20 and 40 kilograms/cm 2  for between about 5 and 20 seconds thereby forming the completed bonded structure shown in FIG.  2 B. 
     An embodiment of the completed bonded structure formed by the method of the preceding embodiment is shown in FIG.  2 B. The gold bonding pads  21  are formed on the integrated circuit element  20 . The inner lead ends of the copper leads  22  of the lead array are positioned so that one inner lead end overlays each gold bonding pad  21  forming a bonding pair. When the assembly is compressed during the bonding process described above a number of conductive particles  31  in the anisotropic conductive film are between the inner lead end and gold bonding pad of each bonding pair and the conductive particles  31  make electrical contact with the lead end and gold bonding pad of each bonding pair. The insulating adhesive ref lows during the time heat and pressure is delivered to the assembly by the thermode and encapsulates the bonded structure and holds the entire assembly together after curing. The insulating adhesive can be a material such as epoxy and can be thermoplastic, thermosetting, or a mixture of thermoplastic and thermosetting. 
     In the method and structure of this embodiment the lead array can have an area array of inner lead ends as shown in FIG.  7 . The inner lead ends  74  of the leads  22  in the lead array are formed on a first dielectric layer  23  without a window. In this example the leads  22  and the inner lead ends  74  are copper. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.