Patent Publication Number: US-2011068469-A1

Title: Semiconductor package with pre-formed ball bonds

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 12/204,810 filed on Sep. 5, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to wire bonding and more particularly to a method of forming a ball bond. 
     Ball bonding is widely employed in the semiconductor packaging industry to form electrical connections between an integrated circuit (IC) die and a die carrier such as a lead frame or a substrate. 
     Conventional ball bonding processes typically use a combination of heat, pressure and ultrasonic energy to form an intermetallic connection or weld between a wire and a connection pad. However, as the connection pad is typically subjected to a number of stressors such as impact force, contact power, contact force, bond power and bond force during the ball bonding process, the connection pad consequently is susceptible to cracking. Reliability becomes an issue when a crack extends into underlying metallization and silicone oxide structures. Thus, a need exists for a ball bonding method that reduces the number of stressors on a bonding site during ball bonding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. It is to be understood that the drawings are not to scale and have been simplified for ease of understanding the invention. 
         FIG. 1  is an enlarged cross-sectional view illustrating a step of bringing a bonding ball towards a first surface in accordance with an embodiment of the present invention; 
         FIG. 2  is an enlarged cross-sectional view illustrating a preformed bonding ball in accordance with an embodiment of the present invention; 
         FIG. 3  is an enlarged cross-sectional view illustrating a ball bond in accordance with an embodiment of the present invention; 
         FIG. 4  is an enlarged cross-sectional view illustrating a preformed bonding ball in accordance with another embodiment of the present invention; and 
         FIG. 5  is an enlarged partial top plan view of a semiconductor package in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout. 
     The present invention provides a method of forming a ball bond including the step of forming a bonding ball at an end of a bonding wire. The bonding ball is preformed to a substantially ball bond shape at a preform location remote from a bonding site. The preformed bonding ball is subsequently bonded to the bonding site. 
     The present invention also provides a method of forming a ball bond including the step of forming a bonding ball at an end of a bonding wire. The bonding ball is deformed against a first surface. The deformed bonding ball is subsequently removed from the first surface and bonded to a second surface. 
     The present invention further provides a semiconductor package including a die carrier having a plurality of bonding sites. An integrated circuit (IC) die is attached to the die carrier, the IC die having a plurality of first connection pads and a plurality of second connection pads. Indentation marks are formed on the first connection pads. A plurality of wire bonds electrically connects the bonding sites on the die carrier to the second connection pads on the IC die. 
     A method of forming a ball bond  10  will now be described below with reference to  FIGS. 1 through 3 . The ball bonding process described below may be performed using currently available wire bonders such as, for example, an ASM Eagle 60-Series wire bonder, an ASM Twin Eagle wire bonder, a K&amp;S Maxum Ultra wire bonder, a K&amp;S Maxum Elite wire bonder and a K&amp;S Model 8090 wire bonder. 
     Referring now to  FIG. 1 , a step of bringing a bonding ball  12  towards a first surface  14  is shown. The bonding ball  12  is formed at an end of a bonding wire  16  near the tip of a capillary bonding tool  18 . As indicated by the arrow in  FIG. 1 , the bonding ball  12  is moved downwards by the capillary bonding tool  18  to contact the first surface  14 . In the embodiment shown, the first surface  14  is a surface of a preform plate  20 . The preform plate  20  is remote from a bonding site, as is described below. In one embodiment, the preform plate  20  may be adjacent the bonding site. 
     The bonding ball  12  may be a free air ball (FAB) formed in a known manner, for example, by applying a high voltage electric charge to the bonding wire  16  to melt the bonding wire  16  at the tip of the capillary bonding tool  18 . Accordingly, further description of the formation of the bonding ball  12  is not required for a complete understanding of the present invention. The bonding ball  12  may have a diameter of between about 45 microns (μm) and about 65 μm. It should however be understood that the present invention is not limited by the diameter of the bonding ball  12 . The bonding ball  12  may have a larger or smaller diameter depending on, for example, the type of bonding wire  16  used and the method by which the bonding ball  12  is formed. 
     The bonding wire  16  may be made of gold (Au), copper (Cu), aluminum (Al) or other electrically conductive materials as are known in the art and commercially available and may have a diameter of between about 15 μm and about 60 μm, although wires of other diameters may also be used and the invention should not be limited to a particular wire diameter. As is known by those of skill in the art, various size wires are available for ball bonding, with the wire size being selected based on, among other things, the spacing between bonding sites. 
     The capillary bonding tool  18  may be made from ceramic, tungsten or ruby materials, as are typically used. Such a bonding tool is well known in the art and therefore, further description of the capillary bonding tool  18  is not required for a complete understanding of the present invention. 
     In the present embodiment, the preform plate  20 , and thus also the first surface  14  against which the bonding ball  12  is subsequently contacted, is made of a harder material than the bonding wire  16 . For instance, if copper (Cu) bonding wire  16  is used, the preform plate  20  may be made of ceramic with a mirror polished finishing. In one embodiment, the preform plate  20  may have a thickness of between about 500 μm and about 1000 μm. Nonetheless, it should be understood by those of skill in the art that the present invention is not limited by the hardness of the first surface  14  or thickness of the preform plate  20 . In alternative embodiments, the first surface  14  may be made of a material of the same or lesser hardness than the bonding wire  16 . The thickness of the preform plate  20  may depend on, for example, the material from which the preform plate  20  is made and/or the shaping forces involved. 
     Referring now to  FIG. 2 , a preformed bonding ball  22  is shown. The preformed bonding ball  22  is formed by deforming the bonding ball  12  of  FIG. 1  against the first surface  14 , thereby mechanically preforming or preconditioning the bonding ball  12  to a substantially ball bond shape. More particularly, the preformed bonding ball  22  is formed by pressing the bonding ball  12  against the first surface  14 , thus squashing the bonding ball  12  between the first surface  14  and the capillary bonding tool  18  into a substantially ball bond shape. 
     In one embodiment, the bonding ball  12  may be deformed to a predetermined ball bond diameter D of, for example, between about 40 μm and about 80 μm and a predetermined ball bond height H of, for example, between about 10 μm and about 30 μm. It should however be understood by those of skill in the art that the present invention is not limited by the dimensions of the preformed bonding ball  22 . Rather, the dimensions of the preformed bonding ball  22  may depend on, among other things, the subsequent ball bond requirements, the wire type employed and the deformation parameters such as, for example, the impact force applied. 
     Although the first surface  14  is shown as being a planar surface in the embodiment shown, it should be understood that the present invention is not limited to deformation against planar surfaces. The preformed bonding ball  22  may in alternative embodiments be formed by deforming the bonding ball  12  against a nonplanar surface (see, for example,  FIG. 4  described below). 
     The preformed bonding ball  22  is subsequently removed from the first surface  14  by the capillary bonding tool  18  and moved to enable it to be brought into contact with a second surface or bonding site  24  as shown in  FIG. 3 . The second surface or bonding site  24  is remote or spaced from the first surface  14 . In one embodiment, the bonding site  24  is adjacent the first surface  14 . 
     Referring now to  FIG. 3 , a ball bond  10  is shown. The ball bond  10  is formed by bonding the preformed bonding ball  22  of  FIG. 2  to the second surface or bonding site  24 . In the embodiment shown, the bonding site  24  is a surface of a connection pad  26  of an integrated circuit (IC) die  28 . 
     As the bonding ball  12  of  FIG. 1  is at least partially, more preferably wholly, preformed to the shape of the ball bond  10 , the capillary bonding tool  18  is able to bond the preformed bonding ball  22  directly to the bonding site  24  with reduced pressure, ultrasonic energy and time, as compared to conventional ball bonding processes, since pressure, ultrasonic energy and time are mainly or only required for forming a metallurgical weld between the preformed bonding ball  22  and the connection pad  26 , and less or not also to flatten the bonding ball  12 . In some embodiments, typical ball bonding stressors such as impact force, contact power and contact force may be substantially eliminated during the step of forming the ball bond  10  on the connection pad  26 . In this manner, pressure exerted on the connection pad  26  during the bonding step is reduced and consequently reliability is improved. Further advantageously, pressure exerted on the connection pad  26  is also further reduced because the preformed bonding ball  22  disperses pressure on the connection pad  26  due to its increased diameter D. In one embodiment, the ball bond  10  is formed by applying the following parameters to an ASM Eagle 60: Standby Power=0, Search Time=0, Search Speed=82, Contact time=3, Contact Power=0, Contact Force=85, Bond Time=25, Bond Power=40 and Bond Force=21. However, it should be understood that the present invention is not limited to a particular set of bonding parameters. Rather, the optimum bonding parameters are dependent on wire type, pad metallization and device configurations. 
     The connection pad  26  may be of sensitive pad structure such as, for example, a bond over active (BOA) pad structure, a bond pad formed over a low-κ structure or a diamond shaped via design. In one embodiment, the connection pad  26  may have a thickness of less than 0.13 μm. The connection pad  26  may be made of aluminum (Al) or other electrically conductive material as is known in the art. It should however be understood by those of skill in the art that the present invention is not limited by the type or thickness of the connection pad  26  or the material with which the connection pad  26  is made. Rather, these would depend on the application in which the present invention is put to use. 
     The IC die  28  may be a processor, such as a digital signal processor (DSP), a special function circuit, such as a memory address generator, or a circuit that performs any other type of function as is known in the art and commercially available. The semiconductor device  10  is not limited to a particular technology such as CMOS, or derived from any particular wafer technology. 
     Once the ball bond  10  is formed, the bonding wire  16  is either cut to form a stud bump (not shown), or run to a corresponding bonding site (not shown) on a lead frame or substrate and bonded thereto. 
     The process described above with reference to  FIGS. 1 through 3  is repeated for subsequent bonds. 
     Referring now to  FIG. 4 , a preformed bonding ball  30  in accordance with another embodiment of the present invention is shown. The preformed bonding ball  30  is formed by bringing a bonding ball (not shown) formed at an end of a bonding wire  32  near the tip of a capillary bonding tool  34  towards a first surface  36 , as indicated by the arrow in  FIG. 4 , and deforming the bonding ball against the first surface  36 . In the embodiment shown, the first surface  36  is a surface of a preform plate  38  having a protrusion  40  formed thereon. 
     Since the elements of the bonding ball, the bonding wire  32  and the capillary bonding tool  34  in the present embodiment are the same as in the previous embodiment, further detailed description of these elements is not required for a complete understanding of the present invention and will therefore be omitted. 
     In the embodiment shown, the bonding ball is brought down directly onto the protrusion  40  and is consequently deformed according to the contour of the first surface  36 . 
     Referring now to  FIG. 5 , an enlarged partial top plan view of a semiconductor package  50  is shown. The semiconductor package  50  includes a die carrier  52  having a plurality of bonding sites (not shown). An integrated circuit (IC) die  54  is attached to the die carrier  52 . The IC die  54  includes a plurality of first connection pads  56  and a plurality of second connection pads  58 . A plurality of wire bonds  60  electrically connect the bonding sites on the die carrier  52  to the second connection pads  58  on the IC die  54 . Each of the wire bonds  60  includes a ball bond  62  formed on respective ones of the second connection pads  58  on the IC die  52 . The ball bonds  62  are formed using the method described above, for example, with reference to  FIGS. 1 through 3 , except that preformed bonding balls are first deformed (shaped) using the first connection pads  56  by pressing an FAB against the first connection pads  56  on the IC die  54 . Subsequently preformed ball bonds are bonded to the second connection pads  58  on the IC die  54 . 
     In one embodiment of the invention, the first connection pads  56  have indentation marks  64  that are formed when the FAB is pressed against a surface thereof, which forms an indentation in the first connection pads  56 . The indentation marks  64  have a diameter that is substantially the same as a diameter of the preformed ball bonds and a depth of between about 100 nanometers (nm) to 700 nm depending on, amongst other things, the type of wire used and the wire bonder employed. 
     The die carrier  52  may be a substrate or a lead frame. Accordingly, the bonding sites on the die carrier  52  may be pad surfaces of a substrate or lead frame fingers. As substrates, lead frames and their respective bonding sites are known to those of ordinary skill in the art, detailed descriptions thereof are not necessary for a full understanding of the invention. 
     The IC die  54  may be as described above with reference to  FIG. 3 . 
     In the embodiment shown, the bonding balls are preformed so that they have substantially the shape as the ball bond  62 . Consequently, the indentation marks  64  on the first connection pads  56  are of a substantially same shape as the corresponding surfaces of the ball bonds  62  in contact with the second connection pads  58 . The similarity in shape suggests that substantially reduced stresses or no further stresses are exerted on the second connection pads  58  by the capillary bonding tool during the ball bonding step. 
     Although ball bonds are described in the above embodiments as being formed on connection pads of an IC die, those of ordinary skill in the art will understand that the present invention is not limited to ball bond formation on IC dice. In alternative embodiments, the ball bonds may be formed, for example, on the connection pads of a substrate. 
     As is evident from the foregoing discussion, the present invention provides a method of forming a ball bond that requires less pressure exertion on a bonding site during the ball bonding step because the bonding ball is preformed to substantially the shape of a ball bond before bonding to the bonding site. Thus, pressure exerted on the bonding site during the ball bonding step is largely for forming a metallurgical weld between the preformed bonding ball and the bonding site, and less or not also for flattening the bonding ball. Consequently, pressure exerted on the bonding site during the ball bonding step can be reduced and wire bonding integrity is thus enhanced. Advantageously, the present invention can be applied to sensitive bond pad applications such as, for example, Bond Over Active (BOA) circuitry, Low-κ structures, bond pads with a thickness of less than 0.13 μm, Diamond shaped Via designs, etc. 
     The description of the preferred embodiments of the present invention have been presented for purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.